diff --git a/Dockerfile.sandbox b/Dockerfile.sandbox
index f0db1c9e5..597b33445 100644
--- a/Dockerfile.sandbox
+++ b/Dockerfile.sandbox
@@ -24,6 +24,7 @@ RUN apt-get update \
         curl \
         git \
         libclang-rt-22-dev \
+        libpolly-22-dev \
         libzstd-dev \
         llvm-22-dev \
         ninja-build \
diff --git a/packages/compiler/src/backend/native-codegen.ts b/packages/compiler/src/backend/native-codegen.ts
index 6e91560dd..6aab5880a 100644
--- a/packages/compiler/src/backend/native-codegen.ts
+++ b/packages/compiler/src/backend/native-codegen.ts
@@ -2,7 +2,7 @@ import { execFile } from "node:child_process";
 import { createHash } from "node:crypto";
 import { constants } from "node:fs";
 import { createRequire } from "node:module";
-import { access, chmod, mkdir, open, readFile, rename, rm, stat } from "node:fs/promises";
+import { access, chmod, mkdir, readFile, rename, rm, stat } from "node:fs/promises";
 import { dirname, join } from "node:path";
 import { promisify } from "node:util";
 import {
@@ -27,6 +27,7 @@ import {
   type NativeTargetSpec,
 } from "./targets.js";
 import { compilerReleaseVersion } from "../library/sidecar.js";
+import { writeTextParts } from "../text-parts.js";
 
 import {
   NativeCodegenError,
@@ -87,39 +88,13 @@ export interface NativeCodegenOptions {
   importBitcode?: { paths: readonly string[]; digests: readonly string[] };
 }
 
-/** Writes LLVM text in large batches. Programs past the engine's string
- * limit arrive as many parts, and writing each part separately is several
- * times slower than writing the same bytes in a few large chunks. */
+/** Writes LLVM text in large batches; see `writeTextParts`. */
 export async function writeLlvmSource(
   path: string,
   llvm: string | readonly string[],
   mode = 0o666,
 ): Promise<void> {
-  const file = await open(path, "w", mode);
-  try {
-    const write = async (text: string): Promise<void> => {
-      const bytes = Buffer.from(text, "utf8");
-      for (let offset = 0; offset < bytes.length;) {
-        const { bytesWritten } = await file.write(bytes, offset, bytes.length - offset);
-        offset += bytesWritten;
-      }
-    };
-    if (typeof llvm === "string") return await write(llvm);
-    let batch: string[] = [];
-    let length = 0;
-    for (const part of llvm) {
-      batch.push(part);
-      length += part.length;
-      if (length >= 16 * 1024 * 1024) {
-        await write(batch.join(""));
-        batch = [];
-        length = 0;
-      }
-    }
-    if (batch.length > 0) await write(batch.join(""));
-  } finally {
-    await file.close();
-  }
+  await writeTextParts(path, llvm, mode);
 }
 
 function parseJsonObject(text: string): Record<string, unknown> | null {
diff --git a/packages/compiler/src/index.ts b/packages/compiler/src/index.ts
index 0b8d48430..0759a01cb 100644
--- a/packages/compiler/src/index.ts
+++ b/packages/compiler/src/index.ts
@@ -103,7 +103,8 @@ import {
 } from "./library/sidecar.js";
 import { type IrModule, type SrcLoc } from "./ir/ir.js";
 import { moduleRuntimeFeatures } from "./ir/runtime-features.js";
-import { serializeModule } from "./ir/serialize.js";
+import { serializeModuleParts } from "./ir/serialize.js";
+import { writeTextParts } from "./text-parts.js";
 import { validateModule } from "./ir/validate.js";
 import { loadProgram } from "./frontend/program-node.js";
 import { runFrontend, type FrontendFactory } from "./frontend/pipeline.js";
@@ -206,7 +207,13 @@ export {
 } from "./compat/fetch-profile.js";
 export { LIB_FN_SIGS } from "./ir/builtin-signatures.js";
 export { validateModule } from "./ir/validate.js";
-export { deserializeModule, IR_VERSION, serializeModule } from "./ir/serialize.js";
+export {
+  deserializeModule,
+  deserializeModuleBytes,
+  IR_VERSION,
+  serializeModule,
+  serializeModuleParts,
+} from "./ir/serialize.js";
 export {
   resolveLibraryFences,
   type LibraryFenceDecl,
@@ -670,7 +677,7 @@ async function prepareExecutableInput(
 
   if (outputKind === "ir") {
     await mkdir(dirname(opts.outPath), { recursive: true });
-    await writeFile(opts.outPath, serializeModule(mod, true));
+    await writeTextParts(opts.outPath, serializeModuleParts(mod));
     return { ok: true, artifact: { kind: "ir", path: opts.outPath } };
   }
 
@@ -692,7 +699,7 @@ async function prepareExecutableInput(
     }
     if (outputKind === "llvm") {
       await mkdir(dirname(opts.outPath), { recursive: true });
-      await writeFile(opts.outPath, llvm);
+      await writeTextParts(opts.outPath, llvm);
     } else {
       try {
         await emitNativeArtifact({
@@ -766,7 +773,7 @@ async function prepareExecutableInput(
   let irPath: string | undefined;
   if (opts.emitIr) {
     irPath = defaultSourcePaths.ir;
-    await writeFile(irPath, serializeModule(mod, true));
+    await writeTextParts(irPath, serializeModuleParts(mod));
   }
 
   const nativeFeatures = executableNativeFeatures(
@@ -1448,7 +1455,7 @@ async function emitSemanticLibraryHit(
   let irPath: string | undefined;
   if (opts.emitIr) {
     irPath = join(opts.outDir, `${stem}.lib.ir.json`);
-    await writeFile(irPath, serializeModule(mod, true));
+    await writeTextParts(irPath, serializeModuleParts(mod));
   }
   await compileLibraryNative(profile, llvmPath, archivePath, opts.sanitize ?? false, hit.native);
   timing("native-archive");
@@ -1724,7 +1731,7 @@ async function compileLibraryTracked(
           ? Buffer.byteLength(ll)
           : ll.reduce((bytes, part) => bytes + Buffer.byteLength(part), 0),
     });
-    await writeFile(llvmPath, ll);
+    await writeTextParts(llvmPath, ll);
     timing("llvm-write");
   } catch (err) {
     if (!(err instanceof LlvmUnsupportedError)) throw err;
@@ -1734,7 +1741,7 @@ async function compileLibraryTracked(
   let irPath: string | undefined;
   if (opts.emitIr) {
     irPath = join(opts.outDir, `${stem}.lib.ir.json`);
-    await writeFile(irPath, serializeModule(mod, true));
+    await writeTextParts(irPath, serializeModuleParts(mod));
   }
 
   const nativeFeatures = libraryNativeFeatures(mod, profile.emission);
diff --git a/packages/compiler/src/ir/serialize.test.ts b/packages/compiler/src/ir/serialize.test.ts
index be933514e..8345ab10a 100644
--- a/packages/compiler/src/ir/serialize.test.ts
+++ b/packages/compiler/src/ir/serialize.test.ts
@@ -1,6 +1,12 @@
 import { expect, test } from "vitest";
 import { F64, VOID, type IrStmt, type IrModule } from "./ir.js";
-import { IR_VERSION, deserializeModule, serializeModule } from "./serialize.js";
+import {
+  IR_VERSION,
+  deserializeModule,
+  deserializeModuleBytes,
+  serializeModule,
+  serializeModuleParts,
+} from "./serialize.js";
 
 function numbers(values: number[]): IrModule {
   const loc = { file: "numbers.ts", start: 0, end: 0 };
@@ -124,3 +130,71 @@ test("compact artifacts keep JSON's circular-data refusal", () => {
   expect(() => serializeModule(mod)).toThrow(TypeError);
   expect(() => serializeModule(mod, true)).toThrow(TypeError);
 });
+
+/** A module whose strings contain every byte the function splitter must not
+ * mistake for structure. */
+function structuralStrings(): IrModule {
+  const mod = numbers([NaN, -0, 1.5, Infinity]);
+  const tricky = 'q"\\"],[{"functions":[]}\\, \n\t😀é\\';
+  mod.sourceFile = tricky + '"functions":[';
+  const base = mod.functions[0]!;
+  mod.functions = [
+    base,
+    { ...base, name: tricky, loc: { file: tricky, start: -0, end: 2 } },
+    { ...base, name: "]", body: [] },
+    { ...base, name: "\\" },
+  ];
+  mod.globals = [{ id: "%g.functions", name: "functions", type: F64, mutable: false }];
+  return mod;
+}
+
+test("compact parts are exactly the single-root JSON encoding", () => {
+  const mod = structuralStrings();
+  const parts = serializeModuleParts(mod);
+  expect(parts.length).toBe(2 + mod.functions.length * 2 - 1);
+  const sentinels = (_key: string, value: unknown): unknown =>
+    typeof value !== "number"
+      ? value
+      : Number.isNaN(value)
+        ? { $nonfinite: "nan" }
+        : value === Infinity
+          ? { $nonfinite: "inf" }
+          : value === -Infinity
+            ? { $nonfinite: "-inf" }
+            : Object.is(value, -0)
+              ? { $nonfinite: "-0" }
+              : value;
+  expect(parts.join("")).toBe(JSON.stringify(mod, sentinels));
+  expect(serializeModule(mod, true)).toBe(parts.join(""));
+});
+
+test("byte decoding splits functions without being misled by string contents", () => {
+  const mod = structuralStrings();
+  for (const text of [serializeModule(mod, true), serializeModule(mod)]) {
+    const decoded = deserializeModuleBytes(Buffer.from(text, "utf8"));
+    expect(decoded).toEqual(mod);
+    expect(decoded).toEqual(deserializeModule(text));
+    expect(Object.is(decoded.functions[1]!.loc.start, -0)).toBe(true);
+  }
+  mod.functions = [];
+  expect(deserializeModuleBytes(Buffer.from(serializeModule(mod, true)))).toEqual(mod);
+  expect(deserializeModuleBytes(Buffer.from(serializeModule(mod)))).toEqual(mod);
+});
+
+test("byte decoding keeps the whole-document errors and version refusal", () => {
+  const text = serializeModule(structuralStrings(), true);
+  for (const broken of [text.slice(0, -1), text.slice(0, text.length - 40), text + "]", "[]", ""]) {
+    let expected: unknown;
+    try {
+      deserializeModule(broken);
+    } catch (error) {
+      expected = error;
+    }
+    expect(expected).toBeInstanceOf(Error);
+    expect(() => deserializeModuleBytes(Buffer.from(broken))).toThrow();
+  }
+  const older = text.replace(`"irVersion":${IR_VERSION}`, `"irVersion":${IR_VERSION - 1}`);
+  expect(() => deserializeModuleBytes(Buffer.from(older))).toThrow("IR version mismatch");
+  const sentinel = text.replace('"nan"', '"future"');
+  expect(() => deserializeModuleBytes(Buffer.from(sentinel))).toThrow("Invalid IR number sentinel");
+});
diff --git a/packages/compiler/src/ir/serialize.ts b/packages/compiler/src/ir/serialize.ts
index b156300ea..9455191d4 100644
--- a/packages/compiler/src/ir/serialize.ts
+++ b/packages/compiler/src/ir/serialize.ts
@@ -1,6 +1,12 @@
 /* IR ↔ JSON. Numeric literals preserve JavaScript's complete number domain:
- * JSON's null/zero spellings must not erase NaN, infinities or negative zero. */
-import type { IrModule } from "./ir.js";
+ * JSON's null/zero spellings must not erase NaN, infinities or negative zero.
+ *
+ * Compiler-scale artifacts are larger than one host string can hold (V8 caps
+ * strings at 2^29 - 24 UTF-16 units, and the self-hosted compiler's IR is
+ * already close to it). The compact encoding is therefore produced as ordered
+ * text parts and decoded from bytes one function at a time; neither direction
+ * materializes the complete document as a single string. */
+import type { IrFunction, IrModule } from "./ir.js";
 
 // Version 15 expands the native Error prefix with filesystem metadata.
 // Older modules embed incompatible subclass offsets and must be rejected.
@@ -22,52 +28,65 @@ function ordinaryNumbers(root: unknown): boolean {
   return visit(root);
 }
 
-/** Compiler artifacts use compact JSON to keep large graphs below the host's
- * string size limit. API consumers can retain the readable default. */
-export function serializeModule(mod: IrModule, compact = false): string {
-  const replacer = (_key: string, value: unknown): unknown => {
-    if (typeof value !== "number") return value;
-    if (!Number.isFinite(value)) {
-      return { $nonfinite: Number.isNaN(value) ? "nan" : value > 0 ? "inf" : "-inf" };
-    }
-    // JSON.stringify(-0) prints "0", silently losing the sign a numLit's
-    // f64 semantics depend on (String(-0) is "0" but 1/-0 is -Infinity) —
-    // the same sentinel mechanism carries it.
-    if (Object.is(value, -0)) {
-      return { $nonfinite: "-0" };
-    }
-    return value;
-  };
-  if (!compact) return JSON.stringify(mod, replacer, 2);
+function encodeNumber(_key: string, value: unknown): unknown {
+  if (typeof value !== "number") return value;
+  if (!Number.isFinite(value)) {
+    return { $nonfinite: Number.isNaN(value) ? "nan" : value > 0 ? "inf" : "-inf" };
+  }
+  // JSON.stringify(-0) prints "0", silently losing the sign a numLit's
+  // f64 semantics depend on (String(-0) is "0" but 1/-0 is -Infinity) —
+  // the same sentinel mechanism carries it.
+  if (Object.is(value, -0)) {
+    return { $nonfinite: "-0" };
+  }
+  return value;
+}
+
+/** The compact encoding as ordered parts: the module header up to the
+ * function array, one part per function (with separate comma parts), and the
+ * header's remainder. Concatenating the parts yields exactly
+ * `serializeModule(mod, true)`; writers stream them to disk in batches. */
+export function serializeModuleParts(mod: IrModule): string[] {
   // A replacer reads through live native views. Serializing the complete
   // function array through one view refreshes every function capsule for
   // each element, making large compiler artifacts quadratic. The sentinel
   // replacer is key-independent, so encode each function as its own root.
-  const header = JSON.stringify({ ...mod, functions: [] }, replacer);
+  const header = JSON.stringify({ ...mod, functions: [] }, encodeNumber);
   const slot = '"functions":[]';
   const offset = header.indexOf(slot);
-  const functions = mod.functions
-    .map(
-      (fn) => (ordinaryNumbers(fn) ? JSON.stringify(fn) : JSON.stringify(fn, replacer)) ?? "null",
-    )
-    .join(",");
-  return (
-    header.slice(0, offset) + '"functions":[' + functions + "]" + header.slice(offset + slot.length)
-  );
+  const parts: string[] = [header.slice(0, offset) + '"functions":['];
+  for (let index = 0; index < mod.functions.length; index++) {
+    const fn = mod.functions[index]!;
+    if (index > 0) parts.push(",");
+    parts.push(
+      (ordinaryNumbers(fn) ? JSON.stringify(fn) : JSON.stringify(fn, encodeNumber)) ?? "null",
+    );
+  }
+  parts.push("]" + header.slice(offset + slot.length));
+  return parts;
 }
 
-export function deserializeModule(json: string): IrModule {
-  const mod = JSON.parse(json, (_key, value: unknown) => {
-    if (typeof value === "object" && value !== null && "$nonfinite" in value) {
-      const tag = (value as { $nonfinite: string }).$nonfinite;
-      if (tag === "inf") return Infinity;
-      if (tag === "-inf") return -Infinity;
-      if (tag === "-0") return -0;
-      if (tag === "nan") return NaN;
-      throw new Error(`Invalid IR number sentinel: ${String(tag)}`);
-    }
-    return value;
-  }) as IrModule;
+/** Compiler artifacts use compact JSON; API consumers can retain the
+ * readable default. Both return one string, so modules whose encoding
+ * exceeds the host's string limit must use `serializeModuleParts`. */
+export function serializeModule(mod: IrModule, compact = false): string {
+  if (!compact) return JSON.stringify(mod, encodeNumber, 2);
+  return serializeModuleParts(mod).join("");
+}
+
+function decodeNumber(_key: string, value: unknown): unknown {
+  if (typeof value === "object" && value !== null && "$nonfinite" in value) {
+    const tag = (value as { $nonfinite: string }).$nonfinite;
+    if (tag === "inf") return Infinity;
+    if (tag === "-inf") return -Infinity;
+    if (tag === "-0") return -0;
+    if (tag === "nan") return NaN;
+    throw new Error(`Invalid IR number sentinel: ${String(tag)}`);
+  }
+  return value;
+}
+
+function checkedVersion(mod: IrModule): IrModule {
   if (mod.irVersion !== IR_VERSION) {
     throw new Error(
       `IR version mismatch: file has ${String(mod.irVersion)}, compiler expects ${IR_VERSION}`,
@@ -75,3 +94,150 @@ export function deserializeModule(json: string): IrModule {
   }
   return mod;
 }
+
+export function deserializeModule(json: string): IrModule {
+  return checkedVersion(JSON.parse(json, decodeNumber) as IrModule);
+}
+
+const QUOTE = 0x22;
+const BACKSLASH = 0x5c;
+const COMMA = 0x2c;
+const COLON = 0x3a;
+const OPEN_BRACKET = 0x5b;
+const CLOSE_BRACKET = 0x5d;
+const OPEN_BRACE = 0x7b;
+const CLOSE_BRACE = 0x7d;
+const FUNCTIONS_KEY = '"functions"';
+
+function isJsonSpace(byte: number): boolean {
+  return byte === 0x20 || byte === 0x0a || byte === 0x0d || byte === 0x09;
+}
+
+function skipSpace(bytes: Buffer, index: number): number {
+  while (index < bytes.length && isJsonSpace(bytes[index]!)) index++;
+  return index;
+}
+
+/** `index` is an opening quote; returns the index after its closing quote. */
+function skipString(bytes: Buffer, index: number): number {
+  const length = bytes.length;
+  index++;
+  while (index < length) {
+    const byte = bytes[index]!;
+    if (byte === BACKSLASH) index += 2;
+    else if (byte === QUOTE) return index + 1;
+    else index++;
+  }
+  return length;
+}
+
+interface FunctionSpans {
+  /** Offsets of the function array's brackets. */
+  open: number;
+  close: number;
+  /** Byte range of each element. */
+  starts: number[];
+  ends: number[];
+}
+
+/** Element ranges of the array starting at `open`, or null when the bytes
+ * are not well-formed enough to split (the whole-document parse then reports
+ * the exact syntax error). */
+function elementSpans(bytes: Buffer, open: number): FunctionSpans | null {
+  const length = bytes.length;
+  const starts: number[] = [];
+  const ends: number[] = [];
+  let index = skipSpace(bytes, open + 1);
+  if (index < length && bytes[index] === CLOSE_BRACKET) return { open, close: index, starts, ends };
+  while (index < length) {
+    const start = index;
+    let depth = 0;
+    while (index < length) {
+      const byte = bytes[index]!;
+      if (byte === QUOTE) {
+        index = skipString(bytes, index);
+        if (depth === 0) break;
+        continue;
+      }
+      if (byte === OPEN_BRACE || byte === OPEN_BRACKET) depth++;
+      else if (byte === CLOSE_BRACE || byte === CLOSE_BRACKET) {
+        if (depth === 0) break;
+        depth--;
+        if (depth === 0) {
+          index++;
+          break;
+        }
+      } else if (depth === 0 && (byte === COMMA || isJsonSpace(byte))) break;
+      index++;
+    }
+    if (index === start || index >= length) return null;
+    starts.push(start);
+    ends.push(index);
+    index = skipSpace(bytes, index);
+    if (index >= length) return null;
+    if (bytes[index] === CLOSE_BRACKET) return { open, close: index, starts, ends };
+    if (bytes[index] !== COMMA) return null;
+    index = skipSpace(bytes, index + 1);
+  }
+  return null;
+}
+
+/** Locate the root object's `"functions"` array. */
+function functionSpans(bytes: Buffer): FunctionSpans | null {
+  const length = bytes.length;
+  let depth = 0;
+  let index = 0;
+  while (index < length) {
+    const byte = bytes[index]!;
+    if (byte === QUOTE) {
+      const start = index;
+      index = skipString(bytes, index);
+      if (depth === 1 && index - start === FUNCTIONS_KEY.length) {
+        let key = true;
+        for (let offset = 1; offset < FUNCTIONS_KEY.length - 1; offset++) {
+          if (bytes[start + offset] !== FUNCTIONS_KEY.charCodeAt(offset)) {
+            key = false;
+            break;
+          }
+        }
+        if (key) {
+          let next = skipSpace(bytes, index);
+          if (next < length && bytes[next] === COLON) {
+            next = skipSpace(bytes, next + 1);
+            if (next < length && bytes[next] === OPEN_BRACKET) return elementSpans(bytes, next);
+          }
+        }
+      }
+      continue;
+    }
+    if (byte === OPEN_BRACE || byte === OPEN_BRACKET) depth++;
+    else if (byte === CLOSE_BRACE || byte === CLOSE_BRACKET) depth--;
+    index++;
+  }
+  return null;
+}
+
+/** Decode a UTF-8 IR artifact (compact or readable) without materializing
+ * it as one string: the header and each function are parsed separately.
+ * Split points are ASCII structural bytes, so every range is valid UTF-8. */
+export function deserializeModuleBytes(bytes: Buffer): IrModule {
+  const spans = functionSpans(bytes);
+  if (spans === null) return deserializeModule(bytes.toString("utf8"));
+  const mod = checkedVersion(
+    JSON.parse(
+      bytes.toString("utf8", 0, spans.open + 1) + bytes.toString("utf8", spans.close),
+      decodeNumber,
+    ) as IrModule,
+  );
+  const functions: IrFunction[] = [];
+  for (let index = 0; index < spans.starts.length; index++) {
+    functions.push(
+      JSON.parse(
+        bytes.toString("utf8", spans.starts[index]!, spans.ends[index]!),
+        decodeNumber,
+      ) as IrFunction,
+    );
+  }
+  mod.functions = functions;
+  return mod;
+}
diff --git a/packages/compiler/src/native/cache.test.ts b/packages/compiler/src/native/cache.test.ts
index d5ac6bc6a..3ffdf2335 100644
--- a/packages/compiler/src/native/cache.test.ts
+++ b/packages/compiler/src/native/cache.test.ts
@@ -38,6 +38,20 @@ test("cache hits require an intact payload and digest", () => {
   expect(cache.read("object", key)).toBeNull();
 });
 
+test("text parts publish one digest-checked entry and oversized reads are misses", () => {
+  const cache = new NativeCache(directory());
+  const key = contentDigest("parts identity");
+  const parts = ['{"a":', '"\u00e9é😀"', ",", "", '"b":1}'];
+  cache.writeParts("frontend", key, parts);
+  const bytes = cache.read("frontend", key);
+  expect(bytes?.toString("utf8")).toBe(parts.join(""));
+  expect(readFileSync(join(cache.root, "frontend", key + ".sha256"), "utf8").trim()).toBe(
+    contentDigest(parts.join("")),
+  );
+  expect(cache.read("frontend", key, bytes!.length)).toEqual(bytes);
+  expect(cache.read("frontend", key, bytes!.length - 1)).toBeNull();
+});
+
 test("a failed cache publication does not fail the build or replace unrelated files", () => {
   const cache = new NativeCache(directory());
   const occupied = join(cache.root, "object");
diff --git a/packages/compiler/src/native/cache.ts b/packages/compiler/src/native/cache.ts
index 22741a0c4..f81311f52 100644
--- a/packages/compiler/src/native/cache.ts
+++ b/packages/compiler/src/native/cache.ts
@@ -11,6 +11,12 @@ import {
 } from "node:fs";
 import { join } from "node:path";
 import { resolveBuildCacheRoot } from "../backend/cache-root.js";
+import { writeTextPartsSync } from "./text-file.js";
+
+/** UTF-8 bytes one native compiler string can hold (the runtime's
+ * SCR_STR_MAX_CAP). Exceeding it traps rather than throwing, so payloads
+ * decoded as one string are bounded by it up front. */
+export const NATIVE_STRING_BYTES = 0x7fffffff;
 
 export function contentDigest(value: string | Uint8Array): string {
   const hash = createHash("sha256");
@@ -29,9 +35,11 @@ export class NativeCache {
     mkdirSync(this.root, { recursive: true, mode: 0o700 });
   }
 
-  read(family: string, key: string): Buffer | null {
+  /** Entries larger than `maxBytes` are misses. */
+  read(family: string, key: string, maxBytes = Infinity): Buffer | null {
     try {
       const path = join(this.root, family, key);
+      if (maxBytes < Infinity && statSync(path).size > maxBytes) return null;
       const bytes = readFileSync(path);
       if (contentDigest(bytes) !== readFileSync(path + ".sha256", "utf8").trim()) return null;
       return bytes;
@@ -77,6 +85,31 @@ export class NativeCache {
     }
   }
 
+  /** Publish text parts without joining them into one string. */
+  writeParts(family: string, key: string, parts: readonly string[]): void {
+    let stage: string | null = null;
+    try {
+      const directory = join(this.root, family);
+      mkdirSync(directory, { recursive: true, mode: 0o700 });
+      stage = mkdtempSync(join(directory, ".write-"));
+      writeTextPartsSync(join(stage, "payload"), parts);
+      const hash = createHash("sha256");
+      for (const part of parts) hash.update(part);
+      writeFileSync(join(stage, "digest"), hash.digest("hex") + "\n");
+      const path = join(directory, key);
+      renameSync(join(stage, "payload"), path);
+      renameSync(join(stage, "digest"), path + ".sha256");
+    } catch {
+      /* Cache failures never change a build's result. */
+    } finally {
+      try {
+        if (stage !== null) rmSync(stage, { recursive: true, force: true });
+      } catch {
+        /* Cleanup is best effort too. */
+      }
+    }
+  }
+
   prune(): void {
     try {
       const configured = Number(process.env["SCRIPTC_CACHE_MAX_MB"] ?? "4096");
diff --git a/packages/compiler/src/native/compiler.ts b/packages/compiler/src/native/compiler.ts
index 984eb4339..57d2ab4d6 100644
--- a/packages/compiler/src/native/compiler.ts
+++ b/packages/compiler/src/native/compiler.ts
@@ -10,6 +10,7 @@ import {
   writeFileSync,
 } from "node:fs";
 import { basename, dirname, join, resolve } from "node:path";
+import { writeTextPartsSync } from "./text-file.js";
 import type {
   AnalyzeOptions,
   AnalyzeResult,
@@ -320,7 +321,7 @@ export class NativeCompiler {
       const stem = basename(entry).replace(/\.(ts|mts|cts|js|mjs|cjs)$/, "");
       const llvmPath = join(options.outDir, `${stem}.ll`);
       if (outputKind === "ir") {
-        writeFileSync(stagedOutput, prepared.ir!);
+        writeTextPartsSync(stagedOutput, prepared.ir!);
         renameSync(stagedOutput, output);
         return { ok: true, artifact: { kind: "ir", path: output } };
       }
@@ -464,7 +465,7 @@ export class NativeCompiler {
       let irPath: string | undefined;
       if (options.emitIr) {
         irPath = join(options.outDir, `${stem}.ir.json`);
-        writeFileSync(irPath, prepared.ir!);
+        writeTextPartsSync(irPath, prepared.ir!);
       }
       renameSync(stagedOutput, output);
       if (target.platform === "darwin" && (optimization !== "dev" || options.strip))
@@ -614,7 +615,7 @@ export class NativeCompiler {
       let irPath: string | undefined;
       if (options.emitIr) {
         irPath = join(options.outDir, `${stem}.lib.ir.json`);
-        writeFileSync(irPath, prepared.ir!);
+        writeTextPartsSync(irPath, prepared.ir!);
       }
       let sidecarPath: string | undefined;
       if (prepared.sidecarJson !== null) {
diff --git a/packages/compiler/src/native/prepare.test.ts b/packages/compiler/src/native/prepare.test.ts
index 4295e88ef..7e22450da 100644
--- a/packages/compiler/src/native/prepare.test.ts
+++ b/packages/compiler/src/native/prepare.test.ts
@@ -6,6 +6,7 @@ import { nativeCodegenTarget } from "../backend/targets.js";
 import { runFrontend, type FrontendFactory } from "../frontend/pipeline.js";
 import { loadProgram } from "../frontend/program-node.js";
 import { ts7Executable } from "../frontend/ts7/rpc-api.js";
+import { deserializeModule } from "../ir/serialize.js";
 import { NativeCache } from "./cache.js";
 import { prepareNativeExecutable } from "./prepare.js";
 import type { NativeToolchain } from "./toolchain.js";
@@ -55,7 +56,8 @@ function fixture() {
     loads++;
     return runFrontend(path, loadProgram, npmStatic, externalTypes);
   };
-  const prepare = () => prepareNativeExecutable(entry, options, null, toolchain, frontend, cache);
+  const prepare = (extra: { emitIr?: boolean } = {}) =>
+    prepareNativeExecutable(entry, { ...options, ...extra }, null, toolchain, frontend, cache);
   return { root, entry, dependency, cache, options, prepare, loads: () => loads };
 }
 
@@ -88,3 +90,24 @@ test("structurally invalid cached features cause a rebuild even with a valid dig
   expect(f.prepare().ok).toBe(true);
   expect(f.loads()).toBe(2);
 });
+
+test("serialized IR travels as bounded parts through the frontend cache", () => {
+  const f = fixture();
+  const first = f.prepare({ emitIr: true });
+  expect(first.ok, JSON.stringify(first)).toBe(true);
+  if (!first.ok) return;
+  expect(Array.isArray(first.ir)).toBe(true);
+  const mod = deserializeModule(first.ir!.join(""));
+  expect(mod.functions.length).toBeGreaterThan(0);
+  const warm = f.prepare({ emitIr: true });
+  expect(f.loads()).toBe(1);
+  expect(warm).toEqual(first);
+  const family = join(f.cache.root, "frontend");
+  const key = readdirSync(family).find((name) => /^[0-9a-f]{64}$/.test(name))!;
+  const payload = JSON.parse(readFileSync(join(family, key), "utf8"));
+  expect(payload.schema).toBe("scriptc.native-frontend.v3");
+  expect(payload.input.llvm).toBe("");
+  expect(payload.input.ir).toBeNull();
+  expect(payload.llvm).toBe(first.llvm);
+  expect(payload.ir).toEqual(first.ir);
+});
diff --git a/packages/compiler/src/native/prepare.ts b/packages/compiler/src/native/prepare.ts
index 24f339dae..69f317f28 100644
--- a/packages/compiler/src/native/prepare.ts
+++ b/packages/compiler/src/native/prepare.ts
@@ -25,15 +25,18 @@ import { provenanceSources } from "../frontend/provenance-registry.js";
 import { prepareLibrary } from "../library/prepare.js";
 import type { LibraryProfile } from "../library/library-profile.js";
 import type { IrModule } from "../ir/ir.js";
-import { deserializeModule, serializeModule } from "../ir/serialize.js";
-import { contentDigest, type NativeCache } from "./cache.js";
+import { deserializeModuleBytes, serializeModuleParts } from "../ir/serialize.js";
+import { contentDigest, NATIVE_STRING_BYTES, type NativeCache } from "./cache.js";
+import { batchTextParts } from "./text-file.js";
 import type { NativeToolchain } from "./toolchain.js";
 import { nativeFileIdentity } from "./file-identity.js";
 
 export interface NativeExecutableInput {
   ok: true;
   llvm: string;
-  ir: string | null;
+  /** Compact serialized IR as bounded parts (`batchTextParts`): compiler-scale
+   * modules exceed one native string. */
+  ir: string[] | null;
   sidecarJson: string | null;
   features: NativeLinkFeatures;
   sources: [string, string][];
@@ -42,10 +45,62 @@ export interface NativeExecutableInput {
   warnings?: ScrDiagnostic[];
 }
 
+/** The two large artifacts follow the input metadata as separate members so
+ * an entry can be written as parts instead of one JSON string; the stored
+ * input carries empty placeholders (`""`, `null`) for them. */
 interface FrontendCacheEntry {
-  schema: "scriptc.native-frontend.v2";
+  schema: "scriptc.native-frontend.v3";
   probes: FrontendInputSnapshot;
   input: NativeExecutableInput;
+  llvm: string;
+  ir: string[] | null;
+}
+
+function frontendCacheParts(probes: FrontendInputSnapshot, input: NativeExecutableInput): string[] {
+  const metadata = JSON.stringify({
+    schema: "scriptc.native-frontend.v3",
+    probes,
+    input: { ...input, llvm: "", ir: null },
+  });
+  const parts = [metadata.slice(0, metadata.length - 1) + ',"llvm":', JSON.stringify(input.llvm)];
+  if (input.ir === null) parts.push(',"ir":null}');
+  else {
+    parts.push(',"ir":[');
+    for (let index = 0; index < input.ir.length; index++) {
+      if (index > 0) parts.push(",");
+      parts.push(JSON.stringify(input.ir[index]!));
+    }
+    parts.push("]}");
+  }
+  return parts;
+}
+
+/** Publish an entry only when a native reader can decode it as one string;
+ * UTF-16 length never exceeds the UTF-8 byte length. */
+function publishFrontendCache(
+  cache: NativeCache,
+  key: string,
+  probes: FrontendInputSnapshot,
+  input: NativeExecutableInput,
+): void {
+  const parts = frontendCacheParts(probes, input);
+  let length = 0;
+  for (const part of parts) length += part.length;
+  if (length <= NATIVE_STRING_BYTES) cache.writeParts("frontend", key, parts);
+}
+
+function readFrontendCache(
+  cache: NativeCache,
+  key: string,
+  exclusions: FrontendInputExclusions,
+): NativeExecutableInput | null {
+  // The entry is decoded as one string; larger entries are misses.
+  const bytes = cache.read("frontend", key, NATIVE_STRING_BYTES);
+  if (bytes === null) return null;
+  const cached: unknown = JSON.parse(bytes.toString("utf8"));
+  if (!validCachedInput(cached) || !frontendInputsStillMatch(cached.probes, exclusions))
+    return null;
+  return { ...cached.input, llvm: cached.llvm, ir: cached.ir };
 }
 
 function implementationIdentity(toolchain: NativeToolchain): string {
@@ -106,7 +161,7 @@ function validCachedInput(value: unknown): value is FrontendCacheEntry {
   const entry = value as Partial<FrontendCacheEntry>;
   const input = entry.input;
   if (
-    entry.schema !== "scriptc.native-frontend.v2" ||
+    entry.schema !== "scriptc.native-frontend.v3" ||
     !validFrontendInputSnapshot(entry.probes) ||
     input === undefined ||
     input === null ||
@@ -115,8 +170,9 @@ function validCachedInput(value: unknown): value is FrontendCacheEntry {
     return false;
   return (
     input.ok === true &&
-    typeof input.llvm === "string" &&
-    (input.ir === null || typeof input.ir === "string") &&
+    typeof entry.llvm === "string" &&
+    (entry.ir === null ||
+      (Array.isArray(entry.ir) && entry.ir.every((part) => typeof part === "string"))) &&
     (input.sidecarJson === null || typeof input.sidecarJson === "string") &&
     validFeatures(input.features) &&
     Array.isArray(input.sources) &&
@@ -144,7 +200,7 @@ function preparedFromIr(
 ): { ok: true; mod: IrModule; sourceTexts: Map<string, string>; warnings: ScrDiagnostic[] } {
   if ((options.outputKind ?? "exe") !== "exe" || options.strip !== true)
     throw new Error(`${BOOTSTRAP_IR_INPUT} requires a stripped executable build`);
-  const mod = deserializeModule(readFileSync(path, "utf8"));
+  const mod = deserializeModuleBytes(readFileSync(path));
   if (resolve(mod.sourceFile) !== resolve(entry))
     throw new Error(`${BOOTSTRAP_IR_INPUT} was lowered from a different entry`);
   return { ok: true, mod, sourceTexts: new Map(), warnings: [] };
@@ -194,14 +250,10 @@ export function prepareNativeExecutable(
             .sort(([a], [b]) => a.localeCompare(b)),
         }),
       );
-      const bytes = cache.read("frontend", key);
-      if (bytes !== null) {
-        const cached: unknown = JSON.parse(bytes.toString("utf8"));
-        if (validCachedInput(cached) && frontendInputsStillMatch(cached.probes, exclusions)) {
-          const input: NativeExecutableInput = cached.input;
-          timing("frontend-cache-hit");
-          return input;
-        }
+      const input = readFrontendCache(cache, key, exclusions);
+      if (input !== null) {
+        timing("frontend-cache-hit");
+        return input;
       }
     } catch {
       key = null;
@@ -217,7 +269,10 @@ export function prepareNativeExecutable(
         );
   if (!prepared.ok) return prepared;
   if (irInput !== "") timing("ir-input");
-  const ir = outputKind === "ir" || options.emitIr ? serializeModule(prepared.mod, true) : null;
+  const ir =
+    outputKind === "ir" || options.emitIr
+      ? batchTextParts(serializeModuleParts(prepared.mod))
+      : null;
   timing("ir-serialize");
   const llvm =
     outputKind === "ir"
@@ -251,8 +306,7 @@ export function prepareNativeExecutable(
     probes.stable &&
     frontendInputsStillMatch(probes, exclusions)
   ) {
-    const saved: FrontendCacheEntry = { schema: "scriptc.native-frontend.v2", probes, input };
-    cache.write("frontend", key, JSON.stringify(saved));
+    publishFrontendCache(cache, key, probes, input);
   }
   timing("frontend-cache-publish");
   return input;
@@ -307,14 +361,10 @@ export function prepareNativeLibrary(
             .sort(([a], [b]) => a.localeCompare(b)),
         }),
       );
-      const bytes = cache.read("frontend", key);
-      if (bytes !== null) {
-        const cached: unknown = JSON.parse(bytes.toString("utf8"));
-        if (validCachedInput(cached) && frontendInputsStillMatch(cached.probes, exclusions)) {
-          const input: NativeExecutableInput = cached.input;
-          timing("frontend-cache-hit");
-          return input;
-        }
+      const input = readFrontendCache(cache, key, exclusions);
+      if (input !== null) {
+        timing("frontend-cache-hit");
+        return input;
       }
     } catch {
       key = null;
@@ -350,7 +400,7 @@ export function prepareNativeLibrary(
   const input: NativeExecutableInput = {
     ok: true,
     llvm,
-    ir: options.emitIr ? serializeModule(prepared.mod, true) : null,
+    ir: options.emitIr ? batchTextParts(serializeModuleParts(prepared.mod)) : null,
     sidecarJson: prepared.sidecarJson,
     features: executableLinkFeatures(prepared.mod, false),
     sources: [...prepared.sourceTexts],
@@ -363,8 +413,7 @@ export function prepareNativeLibrary(
     probes.stable &&
     frontendInputsStillMatch(probes, exclusions)
   ) {
-    const saved: FrontendCacheEntry = { schema: "scriptc.native-frontend.v2", probes, input };
-    cache.write("frontend", key, JSON.stringify(saved));
+    publishFrontendCache(cache, key, probes, input);
   }
   timing("frontend-cache-publish");
   return input;
diff --git a/packages/compiler/src/native/text-file.ts b/packages/compiler/src/native/text-file.ts
new file mode 100644
index 000000000..6f8990339
--- /dev/null
+++ b/packages/compiler/src/native/text-file.ts
@@ -0,0 +1,41 @@
+import { closeSync, openSync, writeSync } from "node:fs";
+
+/** Native compiler strings stop at 2^31 - 1 UTF-8 bytes, and exceeding that
+ * traps instead of throwing. Large artifacts therefore travel as bounded
+ * parts; this is the part size that batching aims for. */
+export const TEXT_BATCH_LENGTH = 16 * 1024 * 1024;
+
+/** Join consecutive small parts into batches of about TEXT_BATCH_LENGTH
+ * UTF-16 units, so callers hold and write few, still-bounded strings. */
+export function batchTextParts(parts: readonly string[]): string[] {
+  const batches: string[] = [];
+  let batch: string[] = [];
+  let length = 0;
+  for (const part of parts) {
+    batch.push(part);
+    length += part.length;
+    if (length >= TEXT_BATCH_LENGTH) {
+      batches.push(batch.join(""));
+      batch = [];
+      length = 0;
+    }
+  }
+  if (batch.length > 0) batches.push(batch.join(""));
+  return batches;
+}
+
+/** Write text parts to `path` in order without joining them first. Each part
+ * is one write; pass batched parts (`batchTextParts`) for many small ones. */
+export function writeTextPartsSync(path: string, parts: readonly string[]): void {
+  const fd = openSync(path, "w");
+  try {
+    for (const part of parts) {
+      const bytes = Buffer.from(part, "utf8");
+      for (let offset = 0; offset < bytes.length;) {
+        offset += writeSync(fd, bytes, offset, bytes.length - offset);
+      }
+    }
+  } finally {
+    closeSync(fd);
+  }
+}
diff --git a/packages/compiler/src/text-parts.ts b/packages/compiler/src/text-parts.ts
new file mode 100644
index 000000000..a4519b0ba
--- /dev/null
+++ b/packages/compiler/src/text-parts.ts
@@ -0,0 +1,37 @@
+import { open } from "node:fs/promises";
+
+/** Writes compiler text artifacts (LLVM, serialized IR) in large batches.
+ * Programs past the engine's string limit arrive as many parts, and writing
+ * each part separately is several times slower than writing the same bytes in
+ * a few large chunks. Batches stay far below the limit. */
+export async function writeTextParts(
+  path: string,
+  text: string | readonly string[],
+  mode = 0o666,
+): Promise<void> {
+  const file = await open(path, "w", mode);
+  try {
+    const write = async (chunk: string): Promise<void> => {
+      const bytes = Buffer.from(chunk, "utf8");
+      for (let offset = 0; offset < bytes.length;) {
+        const { bytesWritten } = await file.write(bytes, offset, bytes.length - offset);
+        offset += bytesWritten;
+      }
+    };
+    if (typeof text === "string") return await write(text);
+    let batch: string[] = [];
+    let length = 0;
+    for (const part of text) {
+      batch.push(part);
+      length += part.length;
+      if (length >= 16 * 1024 * 1024) {
+        await write(batch.join(""));
+        batch = [];
+        length = 0;
+      }
+    }
+    if (batch.length > 0) await write(batch.join(""));
+  } finally {
+    await file.close();
+  }
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index f0c431d54..69dae17d5 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -10493,24 +10493,6 @@
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/missing-read-lambda-results.ts": {
-   "order": [
-    "<repo>/tests/corpus/missing-read-lambda-results.ts"
-   ],
-   "diags": []
-  },
-  "<repo>/tests/corpus/missing-read-narrowed-storage.ts": {
-   "order": [
-    "<repo>/tests/corpus/missing-read-narrowed-storage.ts"
-   ],
-   "diags": []
-  },
-  "<repo>/tests/corpus/missing-read-number-conversions.ts": {
-   "order": [
-    "<repo>/tests/corpus/missing-read-number-conversions.ts"
-   ],
-   "diags": []
-  },
   "<repo>/tests/corpus/module-fallback-functions/main.js": {
    "order": [
     "<repo>/tests/corpus/module-fallback-functions/platform.js",
@@ -11427,18 +11409,6 @@
    ],
    "diags": []
   },
-  "<repo>/tests/corpus/typed-array-nan-coded-reads.ts": {
-   "order": [
-    "<repo>/tests/corpus/typed-array-nan-coded-reads.ts"
-   ],
-   "diags": []
-  },
-  "<repo>/tests/corpus/typed-array-present-reads.ts": {
-   "order": [
-    "<repo>/tests/corpus/typed-array-present-reads.ts"
-   ],
-   "diags": []
-  },
   "<repo>/tests/corpus/typed-array-unknown.ts": {
    "order": [
     "<repo>/tests/corpus/typed-array-unknown.ts"
@@ -14560,6 +14530,12 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/native-worker-arena-reuse.ts": {
+   "order": [
+    "<repo>/tests/corpus/native-worker-arena-reuse.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/native-worker-buffer-consumers.ts": {
    "order": [
     "<repo>/tests/corpus/native-worker-buffer-consumers.ts"
@@ -15773,6 +15749,42 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/4598-map-set-clear-reuse.ts": {
+   "order": [
+    "<repo>/tests/corpus/4598-map-set-clear-reuse.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/missing-read-lambda-results.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-lambda-results.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/missing-read-narrowed-storage.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-narrowed-storage.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/missing-read-number-conversions.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-number-conversions.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/typed-array-nan-coded-reads.ts": {
+   "order": [
+    "<repo>/tests/corpus/typed-array-nan-coded-reads.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/typed-array-present-reads.ts": {
+   "order": [
+    "<repo>/tests/corpus/typed-array-present-reads.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/4601-cycle-holders.ts": {
    "order": [
     "<repo>/tests/corpus/4601-cycle-holders.ts"
diff --git a/packages/runtime/src/scr_alloc.c b/packages/runtime/src/scr_alloc.c
index 9e08c9594..32352d9dd 100644
--- a/packages/runtime/src/scr_alloc.c
+++ b/packages/runtime/src/scr_alloc.c
@@ -6,9 +6,10 @@
  * and the out-of-line entry points the LLVM backend calls for acyclic
  * records and class instances.
  *
- * Freed blocks are recycled within their class and never returned to the
- * operating system; the reservation itself costs address space only, and
- * pages are committed as the bump pointers first touch them. */
+ * Freed blocks are recycled within their class; the reservation itself
+ * costs address space only, and pages are committed as the bump pointers
+ * first touch them. Only an exiting worker thread returns pages, those
+ * holding nothing but free blocks (scr_sa_trim, Linux). */
 #include "scr_runtime.h"
 
 /* Defined in every build: the emitted fast paths read it (see the layout
@@ -84,6 +85,15 @@ static char *scr_sa_extend(char *start, char *lim, char *end) {
 #ifdef SCR_WORKERS
 #include <pthread.h>
 #include <stdatomic.h>
+#if defined(__linux__) && defined(MADV_DONTNEED)
+#define SCR_SA_TRIM 1
+#include <stdio.h>
+#include <time.h>
+#include <unistd.h>
+#if defined(__GLIBC__)
+#include <malloc.h>
+#endif
+#endif
 
 /* Per-thread arenas (contract in scr_runtime.h). The region is reserved
  * once; arena i spans [region + i * arena_span, + arena_span) and is split
@@ -123,6 +133,13 @@ static bool scr_sa_reserve_region(void) {
   return atomic_load_explicit(&scr_sa_region, memory_order_acquire) != 0;
 }
 
+#ifdef SCR_SA_TRIM
+/* The arena's carved bytes when the current thread claimed it (see
+ * scr_sa_trim). */
+static size_t scr_sa_claim_carved[SCR_SA_ARENAS];
+static size_t scr_sa_carved(void);
+#endif
+
 /* Moves every block other threads freed into this arena's free lists. */
 static void scr_sa_drain_remote(void) {
   ScrSaBlock *b = atomic_exchange_explicit(&scr_sa_remote[scr_sa_arena], NULL, memory_order_acquire);
@@ -161,14 +178,287 @@ static bool scr_sa_reserve(void) {
         scr_sa.shift = scr_sa_region_shift;
       }
       scr_sa_drain_remote();
+#ifdef SCR_SA_TRIM
+      scr_sa_claim_carved[i] = scr_sa_carved();
+#endif
       return true;
     }
   }
 }
 
+#ifdef SCR_SA_TRIM
+/* Returning an exited thread's free memory (Linux). When a script thread
+ * exits, every page of its arena that holds only free blocks goes back to
+ * the system (MADV_DONTNEED, after which it reads as zero), so other
+ * threads can use that memory while the arena is parked. The free blocks on
+ * the kept pages are relinked in address order; the returned runs of pages
+ * become zeroed carve regions (`ScrSaHoles`), which the next thread to claim
+ * the arena carves before the slice's own frontier: the bump fast paths
+ * already rely on carved memory being zero. A thread that carved less than
+ * SCR_SA_TRIM_MIN, or whose arena holds less than that in free blocks,
+ * leaves the arena as it is. */
+#define SCR_SA_TRIM_MIN ((size_t)16 << 20)
+typedef struct {
+  char *lo, *hi;
+} ScrSaRun;
+typedef struct {
+  ScrSaRun *runs; /* zeroed regions left to carve, carved from the end */
+  size_t n, cap;
+  char *bump, *lim; /* the frontier while a region is carved (lim != NULL) */
+} ScrSaHoles;
+static ScrSaHoles scr_sa_holes[SCR_SA_ARENAS][SCR_SA_NCLASS];
+
+/* Bytes carved from the calling thread's slices: up to each frontier. */
+static size_t scr_sa_carved(void) {
+  size_t carved = 0;
+  for (unsigned c = 0; c < SCR_SA_NCLASS; c++) {
+    if (!scr_sa.lim[c]) continue;
+    const ScrSaHoles *h = &scr_sa_holes[scr_sa_arena][c];
+    char *top = h->lim ? h->bump : scr_sa.bump[c];
+    carved += (size_t)(top - ((char *)scr_sa.base + ((size_t)c << scr_sa.shift)));
+  }
+  return carved;
+}
+
+/* For the slow path of class c, whose bump room is below sz: moves the
+ * bump pointer to the next zeroed region, or back to the frontier. */
+static void scr_sa_next_hole(unsigned c, size_t sz) {
+  ScrSaHoles *h = &scr_sa_holes[scr_sa_arena][c];
+  if ((size_t)(scr_sa.lim[c] - scr_sa.bump[c]) >= sz || (!h->n && !h->lim)) return;
+  if (h->n) {
+    if (!h->lim) {
+      h->bump = scr_sa.bump[c];
+      h->lim = scr_sa.lim[c];
+    }
+    ScrSaRun r = h->runs[--h->n];
+    scr_sa.bump[c] = r.lo;
+    scr_sa.lim[c] = r.hi;
+  } else {
+    scr_sa.bump[c] = h->bump;
+    scr_sa.lim[c] = h->lim;
+    h->bump = h->lim = NULL;
+  }
+}
+
+static bool scr_sa_push_run(ScrSaHoles *h, char *lo, char *hi) {
+  if (h->n == h->cap) {
+    size_t cap = h->cap ? 2 * h->cap : 64;
+    ScrSaRun *runs = realloc(h->runs, cap * sizeof *runs);
+    if (!runs) return false;
+    h->runs = runs;
+    h->cap = cap;
+  }
+  h->runs[h->n++] = (ScrSaRun){lo, hi};
+  return true;
+}
+
+/* With SCR_SA_STATS=1: carved, free and returned bytes of the parking thread. */
+static _Thread_local size_t scr_sa_trim_stats[3];
+
+/* Returns the pages of class c that hold only free blocks (see above). */
+static void scr_sa_trim_class(unsigned c, size_t page) {
+  ScrSaHoles *h = &scr_sa_holes[scr_sa_arena][c];
+  size_t sz = (size_t)(c + 1) * SCR_SA_STEP;
+  char *start = (char *)scr_sa.base + ((size_t)c << scr_sa.shift);
+  /* Close the region being carved: its rest is still zero. */
+  if (h->lim) {
+    char *lo = scr_sa.bump[c], *hi = scr_sa.lim[c];
+    scr_sa.bump[c] = h->bump;
+    scr_sa.lim[c] = h->lim;
+    h->bump = h->lim = NULL;
+    if ((size_t)(hi - lo) >= sz && !scr_sa_push_run(h, lo, hi)) return;
+  }
+  char *top = scr_sa.bump[c];
+  size_t nblocks = (size_t)(top - start) / sz;
+  if (!nblocks) return;
+  size_t words = (nblocks + 63) / 64;
+  uint64_t *bits = mmap(NULL, words * 8, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
+  if (bits == MAP_FAILED) return;
+  size_t nfree = 0;
+  for (ScrSaBlock *b = scr_sa.free[c]; b; b = b->next) {
+    size_t k = (size_t)((char *)b - start) / sz;
+    bits[k / 64] |= (uint64_t)1 << (k % 64);
+    nfree++;
+  }
+  /* Uncarved zeroed regions count as free; they are rebuilt below. */
+  for (size_t r = 0; r < h->n; r++) {
+    for (char *q = h->runs[r].lo; q + sz <= h->runs[r].hi; q += sz) {
+      size_t k = (size_t)(q - start) / sz;
+      bits[k / 64] |= (uint64_t)1 << (k % 64);
+    }
+  }
+  scr_sa_trim_stats[0] += nblocks * sz;
+  scr_sa_trim_stats[1] += nfree * sz;
+#define SCR_SA_BIT(k) ((bits[(k) / 64] >> ((k) % 64)) & 1)
+  /* Whole pages below the frontier whose blocks are all free. */
+  uintptr_t first = ((uintptr_t)start + page - 1) & ~(uintptr_t)(page - 1);
+  uintptr_t last = (uintptr_t)top & ~(uintptr_t)(page - 1);
+  h->n = 0;
+  char *run = NULL;
+  for (uintptr_t pg = first; pg <= last; pg += page) {
+    bool empty = false;
+    if (pg < last) {
+      size_t k0 = (size_t)((char *)pg - start) / sz;
+      size_t k1 = (size_t)((char *)pg + page - 1 - start) / sz;
+      if (k1 >= nblocks) k1 = nblocks - 1;
+      empty = true;
+      for (size_t k = k0; k <= k1 && empty; k++) empty = SCR_SA_BIT(k);
+    }
+    if (empty && !run) run = (char *)pg;
+    if (!empty && run) {
+      (void)madvise(run, (size_t)((char *)pg - run), MADV_DONTNEED);
+      scr_sa_trim_stats[2] += (size_t)((char *)pg - run);
+      /* Blocks wholly inside the returned pages carve from it later. */
+      char *lo = start + ((size_t)(run - start) + sz - 1) / sz * sz;
+      char *hi = start + ((size_t)((char *)pg - start) / sz) * sz;
+      if (hi > lo) (void)scr_sa_push_run(h, lo, hi);
+      for (char *q = lo; q < hi; q += sz) {
+        size_t k = (size_t)(q - start) / sz;
+        bits[k / 64] &= ~((uint64_t)1 << (k % 64));
+      }
+      /* A block starting in the returned pages but ending on a kept one is
+       * dropped: relinking it would fault its page back in. */
+      if (hi < (char *)pg && hi >= run) {
+        size_t k = (size_t)(hi - start) / sz;
+        bits[k / 64] &= ~((uint64_t)1 << (k % 64));
+      }
+      run = NULL;
+    }
+  }
+#undef SCR_SA_BIT
+  /* The remaining free blocks, in address order. */
+  ScrSaBlock *head = NULL, **tail = &head;
+  for (size_t w = 0; w < words; w++) {
+    for (uint64_t m = bits[w]; m; m &= m - 1) {
+      size_t k = w * 64 + (size_t)__builtin_ctzll(m);
+      ScrSaBlock *b = (ScrSaBlock *)(start + k * sz);
+      *tail = b;
+      tail = &b->next;
+    }
+  }
+  *tail = NULL;
+  scr_sa.free[c] = head;
+  munmap(bits, words * 8);
+  /* The next owner carves the returned regions before the frontier. */
+  if (h->n) {
+    h->bump = scr_sa.bump[c];
+    h->lim = scr_sa.lim[c];
+    ScrSaRun r = h->runs[--h->n];
+    scr_sa.bump[c] = r.lo;
+    scr_sa.lim[c] = r.hi;
+  }
+}
+
+/* Whether the calling thread carved at least SCR_SA_TRIM_MIN since it
+ * claimed its arena: a short-lived thread reusing the arena's free blocks
+ * leaves them for the next one. */
+static bool scr_sa_trim_worth(void) {
+  return scr_sa_carved() - scr_sa_claim_carved[scr_sa_arena] >= SCR_SA_TRIM_MIN;
+}
+
+/* Returns the free pages of the calling thread's arena (see above). */
+static void scr_sa_trim(void) {
+  scr_sa_drain_remote();
+  size_t free_bytes = 0;
+  for (unsigned c = 0; c < SCR_SA_NCLASS; c++) {
+    size_t n = 0;
+    for (ScrSaBlock *b = scr_sa.free[c]; b && n * (c + 1) * SCR_SA_STEP < SCR_SA_TRIM_MIN;
+         b = b->next)
+      n++;
+    free_bytes += n * (c + 1) * SCR_SA_STEP;
+    if (free_bytes >= SCR_SA_TRIM_MIN) break;
+  }
+  const char *stats = getenv("SCR_SA_STATS");
+  const char *off = getenv("SCR_SA_TRIM");
+  bool trim = free_bytes >= SCR_SA_TRIM_MIN && !(off && off[0] == '0');
+  memset(scr_sa_trim_stats, 0, sizeof scr_sa_trim_stats);
+  if (!trim && stats && stats[0] == '1')
+    for (unsigned c = 0; c < SCR_SA_NCLASS; c++) {
+      if (!scr_sa.lim[c]) continue;
+      size_t sz = (size_t)(c + 1) * SCR_SA_STEP;
+      char *start = (char *)scr_sa.base + ((size_t)c << scr_sa.shift);
+      scr_sa_trim_stats[0] += (size_t)(scr_sa.bump[c] - start);
+      for (ScrSaBlock *b = scr_sa.free[c]; b; b = b->next) scr_sa_trim_stats[1] += sz;
+    }
+  if (trim) {
+    long page = sysconf(_SC_PAGESIZE);
+    for (unsigned c = 0; c < SCR_SA_NCLASS; c++)
+      if (scr_sa.lim[c]) scr_sa_trim_class(c, page > 0 ? (size_t)page : 4096);
+  }
+#if defined(__GLIBC__)
+  /* Large blocks: the system allocator's free pages too. */
+  if (trim) (void)malloc_trim(0);
+#endif
+  if (stats && stats[0] == '1')
+    fprintf(stderr, "scr_sa: park arena %d trim %d carved %zu MB free %zu MB returned %zu MB\n",
+            scr_sa_arena, (int)trim, scr_sa_trim_stats[0] >> 20, scr_sa_trim_stats[1] >> 20,
+            scr_sa_trim_stats[2] >> 20);
+}
+#endif
+
+#ifdef SCR_SA_TRIM
+/* Trims run on a detached helper thread: the parent of a worker joins it
+ * when it handles the worker's exit, which must not wait for the trim (at
+ * the end of a program every worker exits at once, and the process ends
+ * right after). The helper takes over the exiting thread's claim on the
+ * arena, so no thread claims it during the trim. */
+static _Atomic(unsigned) scr_sa_trimming; /* helpers running */
+
+static void scr_sa_release(unsigned i) {
+  scr_sa_parked[i] = scr_sa;
+  memset(&scr_sa, 0, sizeof scr_sa);
+  scr_sa_arena = -1;
+  scr_sa_refused = true;
+  atomic_fetch_and_explicit(&scr_sa_claimed, ~(1u << i), memory_order_release);
+}
+
+static void *scr_sa_trim_main(void *arg) {
+  unsigned i = (unsigned)(uintptr_t)arg;
+  scr_sa_arena = (int)i;
+  scr_sa = scr_sa_parked[i];
+  scr_sa_trim();
+  scr_sa_release(i);
+  atomic_fetch_sub_explicit(&scr_sa_trimming, 1, memory_order_release);
+  return NULL;
+}
+
+void scr_sa_wait_trims(void) {
+  while (atomic_load_explicit(&scr_sa_trimming, memory_order_acquire)) {
+    struct timespec pause = {0, 1000000};
+    nanosleep(&pause, NULL);
+  }
+}
+#else
+void scr_sa_wait_trims(void) {}
+#endif
+
 void scr_sa_thread_park(void) {
   if (scr_sa_arena < 0) return;
   unsigned i = (unsigned)scr_sa_arena;
+#ifdef SCR_SA_TRIM
+  if (scr_sa_trim_worth()) {
+    scr_sa_parked[i] = scr_sa;
+    atomic_fetch_add_explicit(&scr_sa_trimming, 1, memory_order_relaxed);
+    pthread_attr_t attr;
+    pthread_t helper;
+    bool started = pthread_attr_init(&attr) == 0;
+    if (started) {
+      started = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) == 0 &&
+                pthread_create(&helper, &attr, scr_sa_trim_main, (void *)(uintptr_t)i) == 0;
+      pthread_attr_destroy(&attr);
+    }
+    if (started) {
+      /* Later frees on this thread take the foreign path into the arena's
+       * remote list; the helper releases the claim. */
+      memset(&scr_sa, 0, sizeof scr_sa);
+      scr_sa_arena = -1;
+      scr_sa_refused = true;
+      return;
+    }
+    atomic_fetch_sub_explicit(&scr_sa_trimming, 1, memory_order_relaxed);
+    scr_sa_trim();
+  }
+#endif
   scr_sa_parked[i] = scr_sa;
   /* Later frees on this thread (thread-exit destructors) take the foreign
    * path into the parked arena's remote list. */
@@ -241,6 +531,9 @@ __attribute__((cold, noinline)) void *scr_sa_slow(size_t n, bool zero) {
     /* Carve from the slice, opening it or raising its limit as needed;
      * fresh pages are zero. */
     size_t sz = (size_t)(c + 1) * SCR_SA_STEP;
+#ifdef SCR_SA_TRIM
+    if (scr_sa.lim[c]) scr_sa_next_hole(c, sz);
+#endif
     char *start = (char *)scr_sa.base + ((size_t)c << scr_sa.shift);
     char *end = start + ((size_t)1 << scr_sa.shift);
     char *p = scr_sa.lim[c] ? scr_sa.bump[c] : start;
@@ -279,6 +572,7 @@ void *scr_mem_realloc(void *p, size_t n) {
 void *scr_mem_realloc(void *p, size_t n) { return realloc(p, n); }
 #ifdef SCR_WORKERS
 void scr_sa_thread_park(void) {}
+void scr_sa_wait_trims(void) {}
 void scr_sa_free_foreign(void *p) { free(p); }
 #endif
 #endif
diff --git a/packages/runtime/src/scr_alloc.test.c b/packages/runtime/src/scr_alloc.test.c
index ae0c92370..9978c819e 100644
--- a/packages/runtime/src/scr_alloc.test.c
+++ b/packages/runtime/src/scr_alloc.test.c
@@ -217,6 +217,91 @@ static void check_threads(void) {
 }
 #endif
 
+#if defined(SCR_WORKERS) && SCR_SMALL_ALLOC && defined(__linux__)
+#include <sys/mman.h>
+#include <unistd.h>
+
+/* An exiting thread returns the pages that hold only free blocks; the next
+ * thread to claim the arena carves the returned range again, zeroed, before
+ * growing the slice. */
+enum { TRIM_BLOCKS = 1 << 20, TRIM_KEEP = 4096, TRIM_SIZE = 48 };
+static unsigned char **trim_blocks;
+static unsigned char *trim_kept[TRIM_BLOCKS / TRIM_KEEP];
+static uintptr_t trim_lo, trim_hi;
+static size_t trim_outside;
+
+static size_t resident_pages(uintptr_t lo, uintptr_t hi) {
+  size_t page = (size_t)sysconf(_SC_PAGESIZE);
+  lo &= ~(uintptr_t)(page - 1);
+  size_t n = (hi - lo + page - 1) / page, count = 0;
+  unsigned char *vec = malloc(n);
+  assert(vec && mincore((void *)lo, n * page, vec) == 0);
+  for (size_t i = 0; i < n; i++) count += vec[i] & 1;
+  free(vec);
+  return count;
+}
+
+static void *trim_allocate(void *arg) {
+  (void)arg;
+  trim_lo = UINTPTR_MAX;
+  trim_hi = 0;
+  for (size_t i = 0; i < TRIM_BLOCKS; i++) {
+    trim_blocks[i] = scr_mem_alloc(TRIM_SIZE);
+    assert(trim_blocks[i] && owned(trim_blocks[i]));
+    fill(trim_blocks[i], TRIM_SIZE, (unsigned char)i);
+    if ((uintptr_t)trim_blocks[i] < trim_lo) trim_lo = (uintptr_t)trim_blocks[i];
+    if ((uintptr_t)trim_blocks[i] + TRIM_SIZE > trim_hi) trim_hi = (uintptr_t)trim_blocks[i] + TRIM_SIZE;
+  }
+  for (size_t i = 0; i < TRIM_BLOCKS; i++) {
+    if (i % TRIM_KEEP == 0) trim_kept[i / TRIM_KEEP] = trim_blocks[i];
+    else scr_mem_free(trim_blocks[i]);
+  }
+  scr_sa_thread_park();
+  return NULL;
+}
+
+static void *trim_reuse(void *arg) {
+  (void)arg;
+  trim_outside = 0;
+  for (size_t i = 0; i < TRIM_BLOCKS; i++) {
+    unsigned char *p = scr_mem_calloc(TRIM_SIZE);
+    assert(p && owned(p));
+    for (size_t k = 0; k < TRIM_SIZE; k++) assert(p[k] == 0);
+    fill(p, TRIM_SIZE, (unsigned char)i);
+    if ((uintptr_t)p < trim_lo || (uintptr_t)p >= trim_hi) trim_outside++;
+    trim_blocks[i] = p;
+  }
+  for (size_t i = 0; i < TRIM_BLOCKS; i++) scr_mem_free(trim_blocks[i]);
+  scr_sa_thread_park();
+  return NULL;
+}
+
+static void check_trim(void) {
+  trim_blocks = malloc(TRIM_BLOCKS * sizeof *trim_blocks);
+  assert(trim_blocks);
+  pthread_t t;
+  assert(pthread_create(&t, NULL, trim_allocate, NULL) == 0);
+  assert(pthread_join(t, NULL) == 0);
+  scr_sa_wait_trims(); /* a helper thread returns the pages */
+  /* One page per kept block stays, give or take the pages at run ends. */
+  size_t total = (trim_hi - trim_lo) / (size_t)sysconf(_SC_PAGESIZE);
+  size_t resident = resident_pages(trim_lo, trim_hi);
+  assert(resident < total / 8);
+  for (size_t k = 0; k < TRIM_BLOCKS / TRIM_KEEP; k++)
+    verify(trim_kept[k], TRIM_SIZE, (unsigned char)(k * TRIM_KEEP));
+  assert(pthread_create(&t, NULL, trim_reuse, NULL) == 0);
+  assert(pthread_join(t, NULL) == 0);
+  scr_sa_wait_trims();
+  /* Nearly every block came from the returned range: the slice did not grow. */
+  assert(trim_outside < 4 * TRIM_BLOCKS / TRIM_KEEP + 256);
+  for (size_t k = 0; k < TRIM_BLOCKS / TRIM_KEEP; k++) {
+    verify(trim_kept[k], TRIM_SIZE, (unsigned char)(k * TRIM_KEEP));
+    scr_mem_free(trim_kept[k]); /* remote free into the parked arena */
+  }
+  free(trim_blocks);
+}
+#endif
+
 int main(void) {
 #if SCR_SMALL_ALLOC
   check_slice_growth(); /* first: its class must be untouched */
@@ -227,6 +312,9 @@ int main(void) {
   check_churn();
 #if defined(SCR_WORKERS) && SCR_SMALL_ALLOC
   check_threads();
+#ifdef __linux__
+  check_trim();
+#endif
   printf("allocator checks passed: size-class workers\n");
 #else
   printf("allocator checks passed: %s\n", SCR_SMALL_ALLOC ? "size-class" : "system");
diff --git a/packages/runtime/src/scr_map.c b/packages/runtime/src/scr_map.c
index 064204975..804396c3f 100644
--- a/packages/runtime/src/scr_map.c
+++ b/packages/runtime/src/scr_map.c
@@ -518,15 +518,51 @@ static __attribute__((noinline)) void scr_map_leave_dense(ScrMap *m) {
 }
 
 /* Drop tombstones, preserving insertion order. Only legal when no iteration
- * is active (the forEach desugar's indices would shift). */
-static void scr_map_compact(ScrMap *m) {
-  size_t w = 0;
+ * is active (the forEach desugar's indices would shift).
+ *
+ * Index tables are sized by the most entries a map has held, so a map that
+ * shrank keeps rebuilding (and clearing) a table sized for its peak. A
+ * bucket table more than eight times what the survivors need shrinks to
+ * twice that, leaving room to double before it grows again. With `reindex`
+ * the direct index also follows the surviving keys: it shrinks to cover the
+ * largest one, and keys that became sparse move back to hashing (at twice
+ * the spread that admits them, so churn cannot flip modes). The append path
+ * passes false: it holds a slot of the current direct index. */
+static void scr_map_compact(ScrMap *m, bool reindex) {
+  size_t w = 0, top = 0;
+  bool dense = m->ndense != 0;
   for (size_t r = 0; r < m->nentries; r++) {
-    if (m->entries[r].hash) m->entries[w++] = m->entries[r];
+    if (!m->entries[r].hash) continue;
+    if (dense) {
+      size_t k = (size_t)scr_map_slot_to_f64(m->entries[r].key);
+      if (k > top) top = k;
+    }
+    m->entries[w++] = m->entries[r];
   }
   m->nentries = w;
-  if (m->nbuckets > 0) scr_map_rebuild_buckets(m, m->nbuckets);
-  else if (m->ndense) scr_map_fill_dense(m);
+  if (m->nbuckets > 0) {
+    size_t want = 16;
+    while (want < 2 * (w + 1)) want *= 2;
+    scr_map_rebuild_buckets(m, m->nbuckets / 8 > want ? 2 * want : m->nbuckets);
+    return;
+  }
+  if (!dense) return;
+  if (reindex) {
+    if (w == 0) {
+      free(m->dense);
+      m->dense = NULL;
+      m->ndense = 0;
+      return;
+    }
+    size_t limit = scr_map_dense_limit(w);
+    if (top / 2 >= limit) {
+      scr_map_leave_dense(m);
+      return;
+    }
+    size_t need = scr_map_dense_size(top);
+    if (need < m->ndense) scr_map_dense_alloc(m, need);
+  }
+  scr_map_fill_dense(m);
 }
 
 static void scr_map_grow_entries(ScrMap *m) {
@@ -548,7 +584,7 @@ static void scr_map_reserve_append(ScrMap *m) {
   if (m->ndense) {
     /* Directly indexed: the table is sized by keys, not entry count. */
     if (m->nentries < m->ecap) return;
-    if (m->iter_depth == 0 && m->nlive <= m->nentries / 2) scr_map_compact(m);
+    if (m->iter_depth == 0 && m->nentries > 0 && m->nlive <= m->nentries / 2) scr_map_compact(m, false);
     if (m->nentries == m->ecap) scr_map_grow_entries(m);
     return;
   }
@@ -557,7 +593,7 @@ static void scr_map_reserve_append(ScrMap *m) {
        m->nbuckets >= 2 * (m->nentries + 1))) return;
   if (m->iter_depth == 0 && m->nentries > 0 &&
       (m->nbuckets == 0 ? m->nlive < m->nentries : m->nlive <= m->nentries / 2)) {
-    scr_map_compact(m);
+    scr_map_compact(m, false);
   }
   if (m->nentries == m->ecap) scr_map_grow_entries(m);
   if (m->nbuckets == 0 && m->nentries < SCR_MAP_LINEAR_LIMIT) return;
@@ -701,10 +737,29 @@ void scr_map_trace_v(void *m, ScrTraceVisit visit, void *ctx) {
 
 double scr_map_size(const ScrMap *m) { return (double)m->nlive; }
 
+/* Storage clear() keeps regardless of how many entries it removed. */
+#define SCR_MAP_CLEAR_KEEP 256
+
+/* clear() costs time proportional to the entries it walks (live entries
+ * and tombstones appended since the last reset), never to the peak size of
+ * the index tables:
+ *  - the direct index is zero exactly at the absent keys (deletes clear
+ *    their slots), so only the live entries' slots are reset;
+ *  - the bucket table is reset only while it is proportional to the
+ *    entries that pointed into it;
+ *  - storage far larger than the entries just removed is freed, returning
+ *    the map to its initial empty state the way V8 reallocates a cleared
+ *    table. Regrowth is amortized against the insertions that need it.
+ * A map without entries has empty tables already: clearing it does no
+ * work and keeps its storage for the next fill. */
 void scr_map_clear(ScrMap *m) {
-  for (size_t e = 0; e < m->nentries; e++) {
+  size_t walked = m->nentries;
+  if (walked == 0) return;
+  uint32_t *dense = m->ndense ? m->dense : NULL;
+  for (size_t e = 0; e < walked; e++) {
     if (!m->entries[e].hash) continue;
     m->entries[e].hash = 0;
+    if (dense) dense[(size_t)scr_map_slot_to_f64(m->entries[e].key)] = 0;
     scr_map_release_key(m, m->entries[e].key);
     scr_map_release_val(m, m->entries[e].val);
   }
@@ -714,12 +769,29 @@ void scr_map_clear(ScrMap *m) {
      * entries so its indices stay stable — entries added by the callback
      * after the clear append past them and ARE visited (Node-exact). */
     m->nentries = 0;
+    if (m->ecap > SCR_MAP_CLEAR_KEEP && m->ecap / 8 > walked) {
+      free(m->entries);
+      m->entries = NULL;
+      m->ecap = 0;
+    }
+  }
+  /* Without either index, a map is a linear scan; dead entries kept for an
+   * active iteration are skipped by every later rebuild. */
+  if (m->ndense > SCR_MAP_CLEAR_KEEP && m->ndense / 32 > walked) {
+    free(m->dense);
+    m->dense = NULL;
+    m->ndense = 0;
   }
-  if (m->ndense) memset(m->dense, 0, m->ndense * sizeof *m->dense);
   if (m->nbuckets) {
-    bool compact = scr_map_compact_buckets(m->nbuckets);
-    memset(m->buckets, compact ? 0 : 0xff,
-           m->nbuckets * (compact ? sizeof(uint32_t) : sizeof(size_t)));
+    if (m->nbuckets > SCR_MAP_CLEAR_KEEP && m->nbuckets / 8 > walked) {
+      free(m->buckets);
+      m->buckets = NULL;
+      m->nbuckets = 0;
+    } else {
+      bool compact = scr_map_compact_buckets(m->nbuckets);
+      memset(m->buckets, compact ? 0 : 0xff,
+             m->nbuckets * (compact ? sizeof(uint32_t) : sizeof(size_t)));
+    }
   }
 }
 
@@ -733,13 +805,26 @@ bool scr_map_has_str(const ScrMap *m, const ScrStr *key) {
   return scr_map_find_str(m, key) != SCR_MAP_EMPTY;
 }
 
+/* Tombstones beyond this share of the entries are compacted on delete, so
+ * iteration and later rebuilds cost O(live) rather than O(peak). V8 shrinks
+ * an ordered hash table at the same occupancy. Small maps keep theirs. */
+#define SCR_MAP_SHRINK_MIN 64
+
 static bool scr_map_delete_found(ScrMap *m, size_t e) {
   if (e == SCR_MAP_EMPTY) return false;
+  uint64_t key = m->entries[e].key, val = m->entries[e].val;
   m->entries[e].hash = 0; /* bucket slot stays: probe chains intact */
   m->nlive--;
-  if (m->ndense) m->dense[(size_t)scr_map_slot_to_f64(m->entries[e].key)] = 0;
-  scr_map_release_key(m, m->entries[e].key);
-  scr_map_release_val(m, m->entries[e].val);
+  if (m->ndense) m->dense[(size_t)scr_map_slot_to_f64(key)] = 0;
+  /* Each such compaction follows deletes of three quarters of the entries,
+   * so it is amortized constant per delete. It runs before the releases:
+   * they may drop the last reference to this map. */
+  if (__builtin_expect(m->iter_depth == 0 && m->nentries >= SCR_MAP_SHRINK_MIN &&
+                       m->nlive < m->nentries / 4, 0)) {
+    scr_map_compact(m, true);
+  }
+  scr_map_release_key(m, key);
+  scr_map_release_val(m, val);
   return true;
 }
 
@@ -1166,9 +1251,14 @@ ScrMap *scr_map_clone(const ScrMap *source, bool keys_only) {
   out->entries = malloc(count * sizeof(ScrMapEntry));
   if (!out->entries) scr_map_oom();
   out->ecap = count;
+  size_t top = 0;
   for (size_t i = 0; i < source->nentries; i++) {
     const ScrMapEntry *entry = &source->entries[i];
     if (!entry->hash) continue;
+    if (source->ndense) {
+      size_t k = (size_t)scr_map_slot_to_f64(entry->key);
+      if (k > top) top = k;
+    }
     ScrMapEntry *copy = &out->entries[out->nentries++];
     *copy = *entry;
     if (out->key_kind == SCR_MAP_KEY_STR) scr_str_retain(scr_map_slot_to_ptr(copy->key));
@@ -1177,9 +1267,10 @@ ScrMap *scr_map_clone(const ScrMap *source, bool keys_only) {
     else if (out->val_kind == SCR_MAP_VAL_REF) out->val_retain(scr_map_slot_to_ptr(copy->val));
   }
   out->nlive = count;
-  if (source->ndense) {
-    /* Same keys, so the same table size; entry indices are compacted. */
-    scr_map_dense_alloc(out, source->ndense);
+  /* Size the copy's direct index for its own keys, not for the source's
+   * peak; keys that are sparse for their count hash instead. */
+  if (source->ndense && top / 2 < scr_map_dense_limit(count)) {
+    scr_map_dense_alloc(out, scr_map_dense_size(top));
     scr_map_fill_dense(out);
     return out;
   }
@@ -1245,7 +1336,7 @@ void scr_map_iter_exit(ScrMap *m) {
    * active they are safe to drop now (bounds memory under forEach-heavy
    * add/delete workloads). */
   if (m->iter_depth == 0 && m->nlive <= m->nentries / 2 && m->nentries >= 16) {
-    scr_map_compact(m);
+    scr_map_compact(m, true);
   }
 }
 
diff --git a/packages/runtime/src/scr_map.test.c b/packages/runtime/src/scr_map.test.c
index d947515bb..6c6e22e01 100644
--- a/packages/runtime/src/scr_map.test.c
+++ b/packages/runtime/src/scr_map.test.c
@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 
 #define MODEL_CAP 20000
 
@@ -227,10 +228,253 @@ static void modes(void) {
   scr_map_release(set);
 }
 
+/* clear() and delete must cost time proportional to the entries they
+ * touch, not to the largest size the map ever had. Each case grows the
+ * index tables to about a million slots, then times many small rounds
+ * against the same rounds on a fresh map: a clear that resets the whole
+ * table (megabytes per call), or iteration over the tombstones of the old
+ * peak, is hundreds of times slower and exceeds the bound early. The bound
+ * is relative, so machine speed and sanitizers do not matter. */
+#define BIG_KEYS (1 << 20)
+#define SMALL_ROUNDS 50000
+
+typedef void (*Round)(ScrMap *map, size_t round, void *ctx);
+
+static double cpu_seconds(void) {
+  return (double)clock() / CLOCKS_PER_SEC;
+}
+
+/* CPU seconds for SMALL_ROUNDS rounds; stops early once over budget. */
+static double timed_rounds(ScrMap *map, Round round, void *ctx, double budget) {
+  double start = cpu_seconds();
+  for (size_t r = 0; r < SMALL_ROUNDS; r++) {
+    round(map, r, ctx);
+    if (r % 1024 == 1023 && cpu_seconds() - start > budget) break;
+  }
+  return cpu_seconds() - start;
+}
+
+static void expect_proportional(const char *what, ScrMap *fresh, ScrMap *grown, Round round, void *ctx) {
+  double base = timed_rounds(fresh, round, ctx, INFINITY);
+  double budget = 20 * base + 0.05;
+  double spent = timed_rounds(grown, round, ctx, budget);
+  if (spent > budget) {
+    fprintf(stderr, "%s: cost follows the peak size (%.3f s, fresh map %.3f s)\n", what, spent, base);
+    exit(1);
+  }
+}
+
+/* keys: 0 directly indexed integers, 1 sparse integers (hashed). */
+static double cost_key(unsigned keys, size_t i) {
+  return keys == 0 ? (double)i : (double)i * 1000003.0;
+}
+
+/* clear() an empty map, add up to six keys, clear() again. */
+static void number_round(ScrMap *map, size_t round, void *ctx) {
+  unsigned keys = *(unsigned *)ctx;
+  scr_map_clear(map);
+  size_t n = round % 7;
+  for (size_t i = 0; i < n; i++) scr_map_set_f64_f64(map, cost_key(keys, round + i), (double)i);
+  assert(scr_map_size(map) == (double)n);
+  scr_map_clear(map);
+}
+
+static void clear_cost_numbers(unsigned keys) {
+  ScrMap *map = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t i = 0; i < BIG_KEYS; i++) scr_map_set_f64_f64(map, cost_key(keys, i), (double)i);
+  assert(keys == 0 ? map->ndense >= BIG_KEYS : map->nbuckets >= 2 * BIG_KEYS);
+  scr_map_clear(map);
+  assert(scr_map_size(map) == 0 && scr_map_iter_count(map) == 0 && !scr_map_has_f64(map, cost_key(keys, 1)));
+  ScrMap *fresh = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  expect_proportional(keys == 0 ? "dense clear" : "hashed clear", fresh, map, number_round, &keys);
+  scr_map_release(fresh);
+  /* Storage sized for the old peak is gone; the map is reusable. */
+  assert(map->ndense <= 256 && map->nbuckets <= 256 && map->ecap <= 256);
+  for (size_t i = 0; i < 100; i++) scr_map_set_f64_f64(map, cost_key(keys, 99 - i), (double)i);
+  size_t j = 0;
+  for (size_t i = 0; i < (size_t)scr_map_iter_count(map); i++) {
+    if (!scr_map_iter_live(map, (double)i)) continue;
+    assert(scr_map_iter_key_f64(map, (double)i) == cost_key(keys, 99 - j));
+    assert(scr_map_iter_val_f64(map, (double)i) == (double)j);
+    j++;
+  }
+  assert(j == 100 && !scr_map_has_f64(map, cost_key(keys, 100)));
+  scr_map_release(map);
+}
+
+static ScrStr *key_str(size_t i) {
+  char buf[32];
+  int len = snprintf(buf, sizeof buf, "k%zu", i);
+  return scr_str_new(buf, (size_t)len);
+}
+
+enum { STRING_KEYS = 1 << 18 };
+
+static void string_round(ScrMap *map, size_t round, void *ctx) {
+  ScrStr **keys = ctx;
+  scr_map_clear(map);
+  size_t n = round % 7;
+  for (size_t i = 0; i < n; i++) scr_map_set_str_f64(map, keys[(round + i) % STRING_KEYS], (double)i);
+  assert(scr_map_size(map) == (double)n);
+  scr_map_clear(map);
+}
+
+static void clear_cost_strings(void) {
+  ScrStr **keys = malloc(STRING_KEYS * sizeof *keys);
+  assert(keys);
+  for (size_t i = 0; i < STRING_KEYS; i++) keys[i] = key_str(i);
+  ScrMap *map = scr_map_new(SCR_MAP_KEY_STR, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t i = 0; i < STRING_KEYS; i++) scr_map_set_str_f64(map, keys[i], (double)i);
+  assert(map->nbuckets >= 2 * STRING_KEYS);
+  scr_map_clear(map);
+  ScrMap *fresh = scr_map_new(SCR_MAP_KEY_STR, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  expect_proportional("string clear", fresh, map, string_round, keys);
+  scr_map_release(fresh);
+  assert(map->nbuckets <= 256 && map->ecap <= 256);
+  for (size_t i = 0; i < 10; i++) scr_map_set_str_f64(map, keys[i], (double)i);
+  for (size_t i = 0; i < 20; i++) assert(scr_map_has_str(map, keys[i]) == (i < 10));
+  scr_map_release(map);
+  for (size_t i = 0; i < STRING_KEYS; i++) scr_str_release(keys[i]);
+  free(keys);
+}
+
+/* Walk the entries the way the forEach desugar does. */
+static void iterate_round(ScrMap *map, size_t round, void *ctx) {
+  (void)round;
+  size_t count = 0;
+  for (size_t i = 0; i < (size_t)scr_map_iter_count(map); i++) count += scr_map_iter_live(map, (double)i);
+  assert(count == *(size_t *)ctx);
+}
+
+/* Deleting almost everything compacts the entries and index tables, so
+ * iteration costs O(live), and keys left far apart move back to hashing
+ * instead of keeping a huge direct index. */
+static void delete_shrinks(void) {
+  ScrMap *map = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  ScrMap *fresh = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t i = 0; i < BIG_KEYS; i++) scr_map_set_f64_f64(map, (double)i, (double)i);
+  for (size_t i = 0; i + 10 < BIG_KEYS; i++) assert(scr_map_delete_f64(map, (double)i));
+  for (size_t i = BIG_KEYS - 10; i < BIG_KEYS; i++) scr_map_set_f64_f64(fresh, (double)i, (double)i);
+  assert(scr_map_size(map) == 10);
+  size_t live = 10;
+  expect_proportional("iteration after deletes", fresh, map, iterate_round, &live);
+  assert(scr_map_iter_count(map) <= 4 * 10 + 64);
+  assert(!map->ndense); /* ten keys near 2^20: hashed */
+  for (size_t i = BIG_KEYS - 10; i < BIG_KEYS; i++) assert(scr_map_has_f64(map, (double)i));
+  assert(!scr_map_has_f64(map, 0) && !scr_map_has_f64(map, BIG_KEYS - 11));
+  scr_map_release(fresh);
+  scr_map_release(map);
+
+  /* Small survivors keep a direct index sized for them. */
+  map = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t i = 0; i < BIG_KEYS; i++) scr_map_set_f64_f64(map, (double)i, (double)i);
+  for (size_t i = BIG_KEYS; i-- > 20;) assert(scr_map_delete_f64(map, (double)i));
+  assert(scr_map_size(map) == 20 && map->ndense && map->ndense <= 64);
+  for (size_t i = 0; i < 40; i++) assert(scr_map_has_f64(map, (double)i) == (i < 20));
+  scr_map_release(map);
+
+  /* A copy indexes its own keys, not the source's peak (here a quarter of
+   * the entries survive, too many to compact the source yet). */
+  map = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t i = 0; i < 4096; i++) scr_map_set_f64_f64(map, (double)i, (double)i);
+  for (size_t i = 1024; i < 4096; i++) assert(scr_map_delete_f64(map, (double)i));
+  assert(map->ndense == 4096 && scr_map_iter_count(map) == 4096);
+  ScrMap *copy = scr_map_clone(map, false);
+  assert(copy->ndense == 1024 && scr_map_size(copy) == 1024);
+  double out;
+  for (size_t i = 0; i < 1100; i++) {
+    assert(scr_map_get_f64_f64(copy, (double)i, &out) == (i < 1024));
+    if (i < 1024) assert(out == (double)i);
+  }
+  scr_map_release(copy);
+  scr_map_release(map);
+}
+
+/* Grow-and-drain cycles against the ordered model: the map repeatedly
+ * grows large and is drained by random deletes, deletes in insertion
+ * order, clear, or deletes during an iteration that also appends, so
+ * compaction on delete, index shrinking, the move back to hashing and
+ * clear's storage release all run under the Map contract. */
+static double drain_key(unsigned flavor, size_t cycle, size_t i) {
+  switch (flavor) {
+  case 0: return (double)(next() % 4000); /* dense */
+  case 1: return (double)(cycle * 3000 + i); /* a moving dense window */
+  default: return (double)(next() % 64) * 1000003.0 + (double)(next() % 3); /* hashed */
+  }
+}
+
+static void fuzz_drain(unsigned flavor) {
+  static Model model;
+  model.n = 0;
+  ScrMap *map = scr_map_new(SCR_MAP_KEY_F64, SCR_MAP_VAL_F64, NULL, NULL, NULL);
+  for (size_t cycle = 0; cycle < 16; cycle++) {
+    size_t grow = 1 + next() % 3000;
+    for (size_t i = 0; i < grow; i++) {
+      double k = drain_key(flavor, cycle, i), v = (double)(next() % 1000);
+      scr_map_set_f64_f64(map, k, v);
+      model_set(&model, k, v);
+    }
+    check_same(map, &model);
+    size_t keep = next() % 40;
+    switch (next() % 4) {
+    case 0:
+      while (model.n > keep) {
+        double k = model.key[next() % model.n];
+        assert(scr_map_delete_f64(map, k) && model_delete(&model, k));
+      }
+      break;
+    case 1:
+      while (model.n > keep) {
+        double k = model.key[0];
+        assert(scr_map_delete_f64(map, k) && model_delete(&model, k));
+      }
+      break;
+    case 2:
+      scr_map_clear(map);
+      model.n = 0;
+      if (next() % 2) scr_map_clear(map);
+      break;
+    default: {
+      /* Delete the visited entry with probability 3/4 and sometimes append
+       * a fresh key, which the same iteration must still visit. */
+      size_t cursor = 0, added = 0;
+      scr_map_iter_enter(map);
+      for (size_t i = 0; i < (size_t)scr_map_iter_count(map); i++) {
+        if (!scr_map_iter_live(map, (double)i)) continue;
+        double k = scr_map_iter_key_f64(map, (double)i);
+        assert(cursor < model.n && same_value_zero(k, model.key[cursor]));
+        if (added < 200 && next() % 8 == 0) {
+          double fresh = 1e7 + (double)(cycle * 1000 + added++);
+          scr_map_set_f64_f64(map, fresh, 1);
+          model_set(&model, fresh, 1);
+        }
+        if (next() % 4) {
+          assert(scr_map_delete_f64(map, k) && model_delete(&model, k));
+        } else {
+          cursor++;
+        }
+      }
+      assert(cursor == model.n);
+      scr_map_iter_exit(map);
+      break;
+    }
+    }
+    check_same(map, &model);
+    for (size_t i = 0; i < 64; i++) probe(map, &model, drain_key(flavor, cycle, i));
+    for (size_t i = 0; i < model.n; i++) probe(map, &model, model.key[i]);
+  }
+  scr_map_release(map);
+}
+
 int main(void) {
   modes();
   iterate_while_mutating();
   for (unsigned flavor = 0; flavor < 5; flavor++) fuzz(flavor);
+  clear_cost_numbers(0);
+  clear_cost_numbers(1);
+  clear_cost_strings();
+  delete_shrinks();
+  for (unsigned flavor = 0; flavor < 3; flavor++) fuzz_drain(flavor);
   puts("number-key maps passed");
   return 0;
 }
diff --git a/packages/runtime/src/scr_message.c b/packages/runtime/src/scr_message.c
index 655d97219..5b27bfc4e 100644
--- a/packages/runtime/src/scr_message.c
+++ b/packages/runtime/src/scr_message.c
@@ -465,12 +465,14 @@ static ScrMessage *scr_message_try_publish(const ScrDyn *value) {
 #ifdef SCR_RC_AUDIT
         scr_str_live_forget();
 #endif
+        scr_lsan_immortal(text);
       } else if (d->rc != SIZE_MAX) {
         scr_cyc_on_dead(d); /* leave the candidate and tenured buffers */
         d->rc = SIZE_MAX;
 #ifdef SCR_RC_AUDIT
         scr_dyn_live_forget();
 #endif
+        scr_lsan_immortal(d);
       }
     }
     message = scr_message_alloc(1, sizeof(*message));
diff --git a/packages/runtime/src/scr_publish.c b/packages/runtime/src/scr_publish.c
index 819ba3f68..7ef5e50e9 100644
--- a/packages/runtime/src/scr_publish.c
+++ b/packages/runtime/src/scr_publish.c
@@ -282,6 +282,7 @@ void scr_publish(void *root, ScrPubFn fn) {
 #ifdef SCR_RC_AUDIT
       scr_pub_audit_forget(ctx.log[i].kind);
 #endif
+      scr_lsan_immortal(ctx.log[i].obj);
     }
     if (trace) {
       uint64_t t2 = scr_hrtime_ns();
diff --git a/packages/runtime/src/scr_runtime.h b/packages/runtime/src/scr_runtime.h
index a83dffdae..4cff50b52 100644
--- a/packages/runtime/src/scr_runtime.h
+++ b/packages/runtime/src/scr_runtime.h
@@ -476,8 +476,9 @@ void scr_cyc_free(void *obj); /* frees the block, header included */
  * of SCR_SA_ARENAS equal slices of a single process-wide reservation, so
  * the owning arena of any block is address arithmetic. A thread claims an
  * arena on its first allocation and parks it (free lists and bump pointers
- * intact, memory never unmapped) when it exits; the next thread to claim
- * it continues from there. Script objects stay on their thread, so a free
+ * intact) when it exits; the next thread to claim it continues from there.
+ * On Linux an exiting thread that carved much first returns the pages of
+ * its arena that hold only free blocks (scr_alloc.c). Script objects stay on their thread, so a free
  * normally hits the caller's own arena inline. A block freed by another
  * thread goes to the owning arena's lock-free remote list, which the owner
  * drains on its slow path; published (immortal) objects are never freed,
@@ -577,6 +578,8 @@ _Static_assert(offsetof(ScrSaState, span) == 8 && offsetof(ScrSaState, shift) ==
 void scr_sa_free_foreign(void *p);
 /* The calling script thread is done with its arena (worker exit). */
 void scr_sa_thread_park(void);
+/* Waits until the arenas of exited threads are trimmed (tests). */
+void scr_sa_wait_trims(void);
 #define SCR_SA_FREE_FOREIGN(p) scr_sa_free_foreign(p)
 #else
 #define SCR_SA_FREE_FOREIGN(p) free(p)
@@ -2094,6 +2097,26 @@ void scr_map_live_forget(void);
 void scr_union_live_forget(void);
 #endif
 
+/* For the same reason LeakSanitizer must not report a published object
+ * whose last reference is dropped (scr_message.c, scr_publish.c). Ignored
+ * chunks are leak-scan roots, so everything an immortal object reaches is
+ * covered and the verdict on the rest of the heap is unchanged. Weak: ASan
+ * runtimes without a leak checker leave it NULL; other builds compile the
+ * hook away. */
+#if defined(__has_feature)
+#if __has_feature(address_sanitizer)
+#define SCR_LSAN_IMMORTAL 1
+void __lsan_ignore_object(const void *p) __attribute__((weak));
+#endif
+#endif
+static inline void scr_lsan_immortal(const void *obj) {
+#ifdef SCR_LSAN_IMMORTAL
+  if (__lsan_ignore_object) __lsan_ignore_object(obj);
+#else
+  (void)obj;
+#endif
+}
+
 /* ── closures ───────────────────────────────────────────────────────
  * A closure is a function pointer plus the boxes of its captured bindings.
  * A box is one shared, refcounted variable cell: every function that
diff --git a/scripts/sandbox-bootstrap.sh b/scripts/sandbox-bootstrap.sh
index 54c06c8b1..ac0a4558a 100644
--- a/scripts/sandbox-bootstrap.sh
+++ b/scripts/sandbox-bootstrap.sh
@@ -30,6 +30,7 @@ sudo env DEBIAN_FRONTEND=noninteractive apt-get install --quiet=2 --yes --no-ins
   clang-22 \
   cmake \
   libclang-rt-22-dev \
+  libpolly-22-dev \
   libzstd-dev \
   llvm-22-dev \
   ninja-build \
diff --git a/tests/corpus/4598-map-set-clear-reuse.ts b/tests/corpus/4598-map-set-clear-reuse.ts
new file mode 100644
index 000000000..0a4095139
--- /dev/null
+++ b/tests/corpus/4598-map-set-clear-reuse.ts
@@ -0,0 +1,147 @@
+// Map and Set clear(), reuse and iteration after clear and after mass
+// deletion, for number, string and object keys. The sizes cross the
+// runtime's storage thresholds: maps grow far beyond their later size, so
+// clear() and delete release or shrink their index tables, number keys move
+// between the direct index and hashing, and every later operation must
+// still follow insertion order exactly.
+
+class Point {
+  readonly x: number;
+  readonly y: number;
+  constructor(x: number, y: number) {
+    this.x = x;
+    this.y = y;
+  }
+}
+
+function summarize<K>(label: string, all: K[], show: (k: K) => string): void {
+  const head = all.slice(0, 4).map(show).join(",");
+  const tail = all.slice(-3).map(show).join(",");
+  console.log(`${label}: size=${all.length} head=[${head}] tail=[${tail}]`);
+}
+
+// ── number keys: dense table grown large, then cleared and reused ──────────
+const nums = new Map<number, number>();
+for (let i = 0; i < 70000; i++) nums.set(i, i * 2);
+console.log("nums grown", nums.size, nums.get(69999), nums.has(70000));
+nums.clear();
+console.log("nums cleared", nums.size, nums.has(0), nums.get(5), [...nums.keys()].length);
+nums.clear(); // empty clear
+nums.clear();
+for (const k of [5, 3, 9, 3, 1000000, 0.5, -0, 7]) nums.set(k, k + 1);
+summarize("nums reuse", [...nums.keys()], (k) => String(Object.is(k, -0) ? "-0" : k));
+console.log("nums -0 stored as", Object.is([...nums.keys()][5], -0) ? "-0" : "+0");
+console.log("nums values", [...nums.values()].join(","));
+// many small fill/clear rounds after the large one
+let roundSum = 0;
+for (let round = 0; round < 2000; round++) {
+  nums.clear();
+  for (let j = 0; j < round % 9; j++) nums.set(round * 10 + j, j);
+  for (const [k, v] of nums) roundSum += k * 3 + v;
+  roundSum += nums.size;
+}
+console.log("nums rounds", roundSum, nums.size);
+
+// ── number keys: mass deletion, then iteration and lookups ────────────────
+const ids = new Set<number>();
+for (let i = 0; i < 50000; i++) ids.add(i);
+for (let i = 0; i < 49990; i++) ids.delete(i); // survivors are far from zero
+summarize("ids after deletes", [...ids], (k) => String(k));
+console.log("ids lookups", ids.has(49989), ids.has(49990), ids.has(49999), ids.has(0));
+ids.add(3);
+ids.add(49989);
+summarize("ids re-added", [...ids], (k) => String(k));
+for (let i = 49999; i >= 20; i--) ids.delete(i); // now small keys only
+summarize("ids small", [...ids], (k) => String(k));
+for (let i = 0; i < 100; i++) ids.add(i);
+summarize("ids regrown", [...ids], (k) => String(k));
+
+// ── stack-like add/delete like a recursion guard, cleared on every reset ──
+const guard = new Set<number>();
+let maybe = 0;
+function visit(depth: number, id: number): void {
+  if (guard.has(id)) {
+    maybe++;
+    return;
+  }
+  guard.add(id);
+  if (depth > 0) {
+    visit(depth - 1, id * 3 + 1);
+    visit(depth - 1, (id * 7) % 1000);
+  }
+  guard.delete(id);
+}
+for (let check = 0; check < 300; check++) {
+  guard.clear();
+  visit(check % 50 === 0 ? 12 : 3, check * 100003);
+}
+console.log("guard", maybe, guard.size, [...guard].length);
+
+// ── string keys ──────────────────────────────────────────────────────────
+const words = new Map<string, number>();
+for (let i = 0; i < 20000; i++) words.set(`w${i}`, i);
+words.clear();
+console.log("words cleared", words.size, words.has("w1"), words.get("w2"));
+for (const w of ["delta", "alpha", "charlie", "alpha", "bravo"]) words.set(w, (words.get(w) ?? 0) + 1);
+summarize("words reuse", [...words.entries()], ([k, v]) => `${k}=${v}`);
+for (let i = 0; i < 5000; i++) words.set(`x${i}`, i);
+for (let i = 0; i < 4990; i++) words.delete(`x${i}`);
+summarize("words after deletes", [...words.keys()], (k) => k);
+const tags = new Set<string>(["a", "b", "c"]);
+tags.clear();
+tags.add("z");
+tags.add("a");
+console.log("tags", [...tags].join(","), tags.has("b"));
+
+// ── object keys ──────────────────────────────────────────────────────────
+const points: Point[] = [];
+for (let i = 0; i < 3000; i++) points.push(new Point(i, -i));
+const owners = new Map<Point, string>();
+for (const p of points) owners.set(p, `p${p.x}`);
+owners.clear();
+console.log("owners cleared", owners.size, owners.has(points[0]!));
+owners.set(points[10]!, "ten");
+owners.set(points[2]!, "two");
+owners.set(points[10]!, "TEN");
+summarize("owners reuse", [...owners.entries()], ([p, s]) => `${p.x}:${s}`);
+const seen = new Set<Point>(points);
+for (let i = 0; i < 2995; i++) seen.delete(points[i]!);
+summarize("seen after deletes", [...seen], (p) => `${p.x}/${p.y}`);
+
+// ── clear and delete during iteration (Node's live iteration) ────────────
+const live = new Map<number, string>();
+for (let i = 0; i < 400; i++) live.set(i, `v${i}`);
+const visited: number[] = [];
+for (const [k] of live) {
+  visited.push(k);
+  if (k === 2) {
+    live.clear();
+    live.set(1000, "after");
+    live.set(1001, "after");
+  }
+  if (k === 1000) live.set(1002, "late");
+}
+console.log("live visited", visited.join(","), "size", live.size);
+
+const churn = new Set<number>();
+for (let i = 0; i < 300; i++) churn.add(i);
+let walked = 0;
+churn.forEach((k) => {
+  walked++;
+  churn.delete(k);
+  if (k < 100) churn.add(k + 1000);
+});
+summarize("churn after forEach", [...churn], (k) => String(k));
+console.log("churn walked", walked);
+
+const strs = new Map<string, number>();
+for (let i = 0; i < 500; i++) strs.set(`s${i}`, i);
+const order: string[] = [];
+strs.forEach((_v, k) => {
+  order.push(k);
+  if (k === "s1") {
+    strs.clear();
+    strs.set("fresh", 1);
+  }
+});
+console.log("strs visited", order.join(","), [...strs.keys()].join(","));
diff --git a/tests/corpus/native-worker-arena-reuse.ts b/tests/corpus/native-worker-arena-reuse.ts
new file mode 100644
index 000000000..7a4f22e8f
--- /dev/null
+++ b/tests/corpus/native-worker-arena-reuse.ts
@@ -0,0 +1,47 @@
+import { Worker, isMainThread, parentPort, workerData } from "node:worker_threads";
+
+// Workers that hold and then drop tens of megabytes of small objects, and later workers that
+// allocate as much again once the first ones have exited (in scriptc on Linux, the first ones
+// return those pages when they exit and the later ones reuse them).
+type Item = { value: number; next: Item | undefined; tags: number[]; label: string };
+
+if (isMainThread) {
+  for (let wave = 0; wave < 2; wave++) {
+    const results = await Promise.all(
+      [1, 2, 3].map(
+        (k) =>
+          new Promise<number>((resolve, reject) => {
+            let result = -1;
+            const worker = new Worker(new URL(import.meta.url), { workerData: wave * 10 + k });
+            worker.once("message", (value: unknown) => {
+              result = value as number;
+            });
+            worker.once("error", reject);
+            worker.once("exit", () => resolve(result));
+          }),
+      ),
+    );
+    console.log("wave", wave, results.join(" "));
+  }
+} else {
+  const round = workerData as number;
+  let items: Item[] = [];
+  for (let i = 0; i < 300_000; i++)
+    items.push({ value: i * round, next: undefined, tags: [i & 7, round, i % 13], label: "item" + (i % 97) });
+  // A few items survive the rest, linked together.
+  const kept: Item[] = [];
+  for (let i = 0; i < items.length; i += 4096) kept.push(items[i]!);
+  for (let i = 1; i < kept.length; i++) kept[i - 1]!.next = kept[i];
+  let sum = 0;
+  for (const item of items) sum = (sum + item.value + item.tags[2]! + item.label.length) % 1_000_003;
+  items = [];
+  let walk = 0;
+  for (let item: Item | undefined = kept[0]; item !== undefined; item = item.next)
+    walk = (walk + item.tags[0]! + item.tags[1]! + item.label.length) % 1_000_003;
+  // Fresh objects after the drop.
+  const fresh: Item[] = [];
+  for (let i = 0; i < 100_000; i++) fresh.push({ value: i, next: undefined, tags: [], label: "" });
+  let empty = 0;
+  for (const item of fresh) if (item.next === undefined && item.tags.length === 0 && item.label === "") empty++;
+  parentPort!.postMessage((sum + walk + empty) % 1_000_003);
+}
diff --git a/tests/fixtures/publish-leaks/dropped.ts b/tests/fixtures/publish-leaks/dropped.ts
new file mode 100644
index 000000000..51178b8ed
--- /dev/null
+++ b/tests/fixtures/publish-leaks/dropped.ts
@@ -0,0 +1,41 @@
+import { publish } from "@scriptc/threads";
+
+// Published objects are immortal: retains and releases skip them and
+// nothing frees them, so dropping the last reference to one is not a leak.
+// Every graph below becomes unreachable once its function returns.
+
+class Item {
+  value: number;
+  label: string;
+  next: Item | null;
+  self: Item | undefined;
+  constructor(value: number, label: string, next: Item | null) {
+    this.value = value;
+    this.label = label;
+    this.next = next;
+  }
+}
+
+// `this` is published before its constructor returns.
+class SelfPublishing {
+  a: number;
+  constructor(a: number) {
+    this.a = a;
+    publish(this);
+  }
+}
+
+function dropTyped(): number {
+  let head: Item | null = null;
+  for (let i = 0; i < 100; i++) head = new Item(i, `item-${i}`, head);
+  head!.self = head!;
+  publish(head);
+  const labels = publish([`first-${head!.value}`, `second-${head!.next!.value}`]);
+  const index = publish(new Map<string, Item>([[`key-${head!.value}`, head!]]));
+  const own = new SelfPublishing(labels.length);
+  return head!.value + labels.length + index.size + own.a;
+}
+
+let total = 0;
+for (let round = 0; round < 3; round++) total += dropTyped();
+console.log(total);
diff --git a/tests/fixtures/self-hosting/serialize.ts b/tests/fixtures/self-hosting/serialize.ts
index 0146da1a4..4654ea415 100644
--- a/tests/fixtures/self-hosting/serialize.ts
+++ b/tests/fixtures/self-hosting/serialize.ts
@@ -1,9 +1,19 @@
 import { readFileSync } from "node:fs";
-import { deserializeModule, serializeModule } from "../../../packages/compiler/src/ir/serialize.js";
+import {
+  deserializeModule,
+  deserializeModuleBytes,
+  serializeModule,
+  serializeModuleParts,
+} from "../../../packages/compiler/src/ir/serialize.js";
 import { validateModule } from "../../../packages/compiler/src/ir/validate.js";
 
 try {
-  const mod = deserializeModule(readFileSync(process.argv[2]!, "utf8"));
+  // "bytes" exercises the compiler-scale path: per-function decoding and
+  // compact parts, as the native compiler uses for its own IR.
+  const bytes = process.argv[3] === "bytes";
+  const mod = bytes
+    ? deserializeModuleBytes(readFileSync(process.argv[2]!))
+    : deserializeModule(readFileSync(process.argv[2]!, "utf8"));
   const errors = validateModule(mod);
   if (errors.length > 0) {
     console.log(JSON.stringify(errors));
@@ -15,7 +25,10 @@ try {
       const statement = mod.functions[0]!.body[0];
       if (statement?.kind === "exprStmt" && statement.expr.kind === "numLit") statement.expr.value = NaN;
     }
-    console.log(serializeModule(mod, process.argv[3] === "compact"));
+    if (bytes) {
+      for (const part of serializeModuleParts(mod)) process.stdout.write(part);
+      process.stdout.write("\n");
+    } else console.log(serializeModule(mod, process.argv[3] === "compact"));
   }
 } catch (error) {
   if (error instanceof Error) console.log(error.name + ": " + error.message);
diff --git a/tests/harness/large-artifacts.test.ts b/tests/harness/large-artifacts.test.ts
new file mode 100644
index 000000000..207ce80a2
--- /dev/null
+++ b/tests/harness/large-artifacts.test.ts
@@ -0,0 +1,30 @@
+import { expect, test } from "vitest";
+import { occurrences, withoutFirstLine, withoutRange } from "./large-artifacts.js";
+
+test("byte-level artifact edits match their string equivalents", () => {
+  const text =
+    "x declare void @scr_runtime_abi_v3()\ndeclare void @scr_runtime_abi_vX()\n" +
+    "declare void @scr_runtime_abi_v12()\n  call void @scr_runtime_abi_v12()\n" +
+    "é\n  call void @scr_runtime_abi_v12()\n";
+  const bytes = Buffer.from(text);
+  const declaration = /^declare void @scr_runtime_abi_v\d+\(\)\n/m;
+  const call = /^ {2}call void @scr_runtime_abi_v\d+\(\)\n/m;
+  const edited = withoutFirstLine(
+    withoutFirstLine(
+      bytes,
+      "declare void @scr_runtime_abi_v",
+      /^declare void @scr_runtime_abi_v\d+\(\)\n$/,
+    ),
+    "  call void @scr_runtime_abi_v",
+    /^ {2}call void @scr_runtime_abi_v\d+\(\)\n$/,
+  );
+  expect(edited.toString()).toBe(text.replace(declaration, "").replace(call, ""));
+  expect(withoutFirstLine(bytes, "missing", /^missing\n$/)).toBe(bytes);
+
+  const line = "  call void @scr_runtime_abi_v12()\n";
+  const found = occurrences(bytes, line);
+  expect(found).toHaveLength(text.split(line).length - 1);
+  expect(withoutRange(bytes, found[0]!, Buffer.byteLength(line)).toString()).toBe(
+    text.replace(line, ""),
+  );
+});
diff --git a/tests/harness/large-artifacts.ts b/tests/harness/large-artifacts.ts
new file mode 100644
index 000000000..c21dc11ae
--- /dev/null
+++ b/tests/harness/large-artifacts.ts
@@ -0,0 +1,31 @@
+/** Byte-level helpers for compiler-scale text artifacts (the self-hosted
+ * compiler's LLVM and IR). They avoid decoding a complete artifact into one
+ * JavaScript string, which V8 caps at 2^29 - 24 UTF-16 units. */
+
+/** Offsets of every occurrence of `needle`. */
+export function occurrences(bytes: Buffer, needle: string): number[] {
+  const found: number[] = [];
+  const width = Buffer.byteLength(needle);
+  for (let at = bytes.indexOf(needle); at >= 0; at = bytes.indexOf(needle, at + width))
+    found.push(at);
+  return found;
+}
+
+/** `bytes` without `length` bytes at `start`. */
+export function withoutRange(bytes: Buffer, start: number, length: number): Buffer {
+  return Buffer.concat([bytes.subarray(0, start), bytes.subarray(start + length)]);
+}
+
+/** Remove the first complete line that starts with `prefix` and, including
+ * its newline, matches `line` (anchored), like `text.replace(/^...\n/m, "")`. */
+export function withoutFirstLine(bytes: Buffer, prefix: string, line: RegExp): Buffer {
+  for (let from = 0; ;) {
+    const at = bytes.indexOf(prefix, from);
+    if (at < 0) return bytes;
+    const newline = bytes.indexOf(10, at);
+    const stop = newline < 0 ? bytes.length : newline + 1;
+    if ((at === 0 || bytes[at - 1] === 10) && line.test(bytes.toString("utf8", at, stop)))
+      return withoutRange(bytes, at, stop - at);
+    from = at + 1;
+  }
+}
diff --git a/tests/harness/publish-leaks.test.ts b/tests/harness/publish-leaks.test.ts
new file mode 100644
index 000000000..4ebfbfd28
--- /dev/null
+++ b/tests/harness/publish-leaks.test.ts
@@ -0,0 +1,68 @@
+/* Published objects (@scriptc/threads publish() and the zero-copy delivery
+ * of deeply frozen plain-data graphs) are immortal by design: nothing frees
+ * them, and the RC audit stops counting them. LeakSanitizer must agree, so
+ * the runtime marks every published object as an intentional leak-scan
+ * root. The sanitized lanes otherwise run with detect_leaks=0, so this pins
+ * the contract with LeakSanitizer explicitly on: programs that drop their
+ * last reference to a published graph exit cleanly. Linux only (Apple's
+ * ASan has no LeakSanitizer); runs in the SCRIPTC_SAN=1 lane. */
+import { execFile } from "node:child_process";
+import { createHash } from "node:crypto";
+import { mkdirSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+import { promisify } from "node:util";
+import { describe, expect, test } from "vitest";
+import { compile } from "@scriptc/compiler";
+
+const execFileAsync = promisify(execFile);
+const repoRoot = join(import.meta.dirname, "../..");
+const cacheDir = join(repoRoot, "node_modules/.cache/scriptc-tests");
+const sanitize = process.env["SCRIPTC_SAN"] === "1";
+
+const programs = [
+  "tests/fixtures/publish-leaks/dropped.ts",
+  // Zero-copy delivery of deeply frozen plain-data graphs (scr_message.c).
+  "tests/corpus/native-worker-published-graphs.ts",
+  // Typed graphs published on both threads, including a refused publish.
+  "tests/corpus/native-worker-publish-classes.ts",
+];
+
+describe.skipIf(!sanitize || process.platform !== "linux")(
+  "published objects under LeakSanitizer",
+  () => {
+    test.each(programs)("%s reports no leaks", { timeout: 300_000 }, async (program) => {
+      const file = join(repoRoot, program);
+      const key = createHash("sha256")
+        .update(file)
+        .update(readFileSync(file))
+        .update("san")
+        .digest("hex")
+        .slice(0, 16);
+      const outDir = join(cacheDir, `publish-leaks-${key}`);
+      mkdirSync(outDir, { recursive: true });
+      const result = await compile(file, {
+        outPath: join(outDir, "program"),
+        outDir,
+        sanitize: true,
+        backend: "llvm",
+      });
+      if (!result.ok) {
+        throw new Error(
+          "program failed to compile:\n" +
+            result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"),
+        );
+      }
+      const run = await execFileAsync(result.binaryPath, [], {
+        env: { ...process.env, ASAN_OPTIONS: "detect_leaks=1" },
+        maxBuffer: 16 * 1024 * 1024,
+      }).catch((error: { code?: number; stdout?: string; stderr?: string }) => error);
+      expect(run.stderr).toBe("");
+      expect("code" in run ? run.code : 0).toBe(0);
+      const expected = await execFileAsync(process.execPath, [file], {
+        env: { ...process.env, SCRIPTC_TEST_ENV: "from-harness" },
+        maxBuffer: 16 * 1024 * 1024,
+      });
+      expect(run.stdout).toBe(expected.stdout);
+    });
+  },
+);
diff --git a/tests/harness/self-hosting-native-driver.test.ts b/tests/harness/self-hosting-native-driver.test.ts
index 87ba5a4c8..b57d0b6cf 100644
--- a/tests/harness/self-hosting-native-driver.test.ts
+++ b/tests/harness/self-hosting-native-driver.test.ts
@@ -24,6 +24,7 @@ import {
   nativeBootstrapPlan,
   runNativeBootstrapChecks,
 } from "../../scripts/ci-native-bootstrap.mjs";
+import { withoutFirstLine } from "./large-artifacts.js";
 import { bootstrapStep } from "./self-hosting-timing.js";
 
 const root = join(import.meta.dirname, "../..");
@@ -435,17 +436,20 @@ test(
             import assert from 'node:assert/strict';
             import { readFileSync } from 'node:fs';
             import { isDeepStrictEqual } from 'node:util';
-            import { deserializeModule, validateModule } from ${JSON.stringify(pathToFileURL(join(root, "packages/compiler/src/index.ts")).href)};
+            import { deserializeModuleBytes, validateModule } from ${JSON.stringify(pathToFileURL(join(root, "packages/compiler/src/index.ts")).href)};
+            import { occurrences, withoutRange } from ${JSON.stringify(pathToFileURL(join(root, "tests/harness/large-artifacts.ts")).href)};
+            // Both artifacts are compared as bytes and per-function documents:
+            // either can exceed V8's maximum string length.
             if (${ir}) {
-              const expected = deserializeModule(readFileSync(process.argv[1], 'utf8'));
-              const actual = deserializeModule(readFileSync(process.argv[2], 'utf8'));
+              const expected = deserializeModuleBytes(readFileSync(process.argv[1]));
+              const actual = deserializeModuleBytes(readFileSync(process.argv[2]));
               assert.ok(actual.functions.length > 1000);
               assert.deepEqual(validateModule(actual), []);
               assert.ok(isDeepStrictEqual(actual, expected), 'native self-lowering must match the Node seed');
             }
             if (${llvm}) {
-              const seedLlvm = readFileSync(process.argv[3], 'utf8');
-              let nativeLlvm = readFileSync(process.argv[4], 'utf8');
+              const seedLlvm = readFileSync(process.argv[3]);
+              let nativeLlvm = readFileSync(process.argv[4]);
               if (${sanitize}) {
                 // The Node sanitizer builds runtime sources directly; the native
                 // executable path also emits the runtime ABI check. Pin that
@@ -453,12 +457,13 @@ test(
                 const declaration = 'declare void @' + ${JSON.stringify(RUNTIME_ABI_MARKER)} + '()\\n';
                 const call = '  call void @' + ${JSON.stringify(RUNTIME_ABI_MARKER)} + '()\\n';
                 for (const line of [declaration, call]) {
-                  assert.equal(seedLlvm.split(line).length, 1);
-                  assert.equal(nativeLlvm.split(line).length, 2);
-                  nativeLlvm = nativeLlvm.replace(line, '');
+                  assert.equal(occurrences(seedLlvm, line).length, 0);
+                  const found = occurrences(nativeLlvm, line);
+                  assert.equal(found.length, 1);
+                  nativeLlvm = withoutRange(nativeLlvm, found[0], Buffer.byteLength(line));
                 }
               }
-              assert.ok(seedLlvm === nativeLlvm, 'native LLVM emission must match the Node seed');
+              assert.ok(seedLlvm.equals(nativeLlvm), 'native LLVM emission must match the Node seed');
             }
           `,
                 seedIr,
@@ -506,12 +511,18 @@ test(
           );
           // Executable output adds a runtime ABI check; textual LLVM emission
           // omits it. Compare all other output without changing either mode.
-          const withoutAbiCheck = (path: string): string =>
-            readFileSync(path, "utf8")
-              .replace(/^declare void @scr_runtime_abi_v\d+\(\)\n/m, "")
-              .replace(/^  call void @scr_runtime_abi_v\d+\(\)\n/m, "");
+          const withoutAbiCheck = (path: string): Buffer =>
+            withoutFirstLine(
+              withoutFirstLine(
+                readFileSync(path),
+                "declare void @scr_runtime_abi_v",
+                /^declare void @scr_runtime_abi_v\d+\(\)\n$/,
+              ),
+              "  call void @scr_runtime_abi_v",
+              /^  call void @scr_runtime_abi_v\d+\(\)\n$/,
+            );
           expect(
-            withoutAbiCheck(seedLlvm) === withoutAbiCheck(rebuiltLlvm),
+            withoutAbiCheck(seedLlvm).equals(withoutAbiCheck(rebuiltLlvm)),
             "native compiler generations must emit identical LLVM",
           ).toBe(true);
         };
diff --git a/tests/harness/self-hosting-serialization.test.ts b/tests/harness/self-hosting-serialization.test.ts
index 3051c86f1..4139cd648 100644
--- a/tests/harness/self-hosting-serialization.test.ts
+++ b/tests/harness/self-hosting-serialization.test.ts
@@ -178,6 +178,26 @@ for (const backend of ["llvm"] as const) {
       expect(deserializeModule(many.native.stdout.toString())).toEqual(
         deserializeModule(many.oracle.stdout.toString()),
       );
+      // Byte decoding and compact parts, from both encodings.
+      for (const input of [serializeModule(manyFunctions), serializeModule(manyFunctions, true)]) {
+        const parts = run(input, ["bytes"]);
+        expect(parts.native.status, parts.native.stdout.toString()).toBe(0);
+        expect(parts.native.stdout.toString().trimEnd()).not.toContain("\n");
+        expect(deserializeModule(parts.native.stdout.toString())).toEqual(manyFunctions);
+        expect(deserializeModule(parts.native.stdout.toString())).toEqual(
+          deserializeModule(parts.oracle.stdout.toString()),
+        );
+      }
+      const truncated = run(serializeModule(manyFunctions, true).slice(0, -1), ["bytes"]);
+      expect(truncated.native.status).toBe(1);
+      expect(truncated.oracle.status).toBe(1);
+      const olderBytes = run(
+        serializeModule({ ...manyFunctions, irVersion: IR_VERSION - 1 }, true),
+        ["bytes"],
+      );
+      expect(olderBytes.native.status).toBe(1);
+      expect(olderBytes.native.stdout).toEqual(olderBytes.oracle.stdout);
+      expect(olderBytes.native.stdout.toString()).toContain("IR version mismatch");
       const print = numeric.functions[0]!.body[1]!;
       expect(print.kind).toBe("exprStmt");
       if (print.kind !== "exprStmt" || print.expr.kind !== "intrinsic")