diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 3adb750ed..fdc855624 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -289,8 +289,9 @@ import { LlvmUnsupportedError } from "./unsupported.js";
 import { LlWalkers } from "./walkers.js";
 import { isObjectArm, NULLABLE_ABSENT, NULLABLE_NULL, NullableUnions } from "./nullable-unions.js";
 import { emitNullablePresent, emitUnionPeek, emitUnionTag } from "./union-repr.js";
-import { literalCapWord } from "./string-key-hash.js";
+import { literalCapWord, literalCapWord32 } from "./string-key-hash.js";
 import { NUMBER_MAP_ENTRY_DECL } from "./map-number-lookup.js";
+import { STRING_READ_HELPERS } from "./string-reads.js";
 import {
   emitConcatInputs,
   emitMixedConcat,
@@ -1770,8 +1771,11 @@ export class LlEmitter {
       // the runtime ABI’s static table, retain/release skip rc == SIZE_MAX.
       // 64-bit (little-endian) targets split the capacity word: the high
       // half carries the precomputed Map key hash (SCR_STR_HASH_CACHE).
+      // Bit 31 marks all-ASCII literals (SCR_STR_ASCII_BIT) on every target.
       const capWord =
-        this.sizeType === "i64" ? literalCapWord(Buffer.from(text, "utf8")) : String(lit.len);
+        this.sizeType === "i64"
+          ? literalCapWord(Buffer.from(text, "utf8"))
+          : literalCapWord32(Buffer.from(text, "utf8"));
       out.push(
         `@${lit.sym} = internal global { ${this.sizeType}, ${this.sizeType}, ${this.sizeType}, [${lit.len + 1} x i8] } ` +
           `{ ${this.sizeType} -1, ${this.sizeType} ${lit.len}, ${this.sizeType} ${capWord}, [${lit.len + 1} x i8] c"${llStrBytes(text)}" }`,
@@ -2643,6 +2647,14 @@ export class LlEmitter {
         ``,
       );
     }
+    // String length and charCodeAt read proven-ASCII strings inline: bit 31
+    // of the capacity word (SCR_STR_ASCII_BIT, the low half's sign bit on
+    // little-endian targets) makes UTF-16 units bytes. Unproven strings and
+    // anything but an in-range index take the runtime's UTF-16 mapping.
+    for (const helper of STRING_READ_HELPERS) {
+      if (!this.decls.has(helper.decl)) continue;
+      defs.push(...helper.define(this.sizeType, FN_ATTRS), ``);
+    }
     // Ordering also decides inline when the first bytes differ: byte order
     // is the plain comparison's order, and the UTF-16 order whenever both
     // bytes are ASCII. Callers only test the sign of the result.
diff --git a/packages/compiler/src/backend/llvm/expr-containers.ts b/packages/compiler/src/backend/llvm/expr-containers.ts
index 0957c992f..8a58fe6fb 100644
--- a/packages/compiler/src/backend/llvm/expr-containers.ts
+++ b/packages/compiler/src/backend/llvm/expr-containers.ts
@@ -30,6 +30,7 @@ import {
 } from "./map-number-lookup.js";
 import { emitBorrowedInput } from "./borrowed-inputs.js";
 import { emitStringSliceRead } from "./string-slices.js";
+import { STRING_CHAR_CODE_AT_DECL, STRING_UTF16_LEN_DECL } from "./string-reads.js";
 import { emitDenseArrayNumber, emitDenseArrayPush } from "./dense-array-access.js";
 import { emitUnionWiden } from "./expr-records.js";
 
@@ -189,16 +190,19 @@ export function emitStrIntrinsic(
   };
   const method = e.method;
   switch (method) {
-    case "length":
-      return call("scr_str_utf16_len", "double (ptr)", `ptr ${r.name}`, "double", false);
-    case "charCodeAt":
-      return call(
-        "scr_str_char_code_at",
-        "double (ptr, double)",
-        `ptr ${r.name}, double ${args[0]!.name}`,
-        "double",
-        false,
-      );
+    case "length": {
+      // sc_str_utf16_len (string-reads.ts) answers proven-ASCII strings inline.
+      host.declare(STRING_UTF16_LEN_DECL);
+      const t = B.tmp();
+      B.line(`${t} = call double @sc_str_utf16_len(ptr ${r.name})`);
+      return { name: t, type: e.type };
+    }
+    case "charCodeAt": {
+      host.declare(STRING_CHAR_CODE_AT_DECL);
+      const t = B.tmp();
+      B.line(`${t} = call double @sc_str_char_code_at(ptr ${r.name}, double ${args[0]!.name})`);
+      return { name: t, type: e.type };
+    }
     case "charAt":
       return call(
         "scr_str_char_at",
diff --git a/packages/compiler/src/backend/llvm/expr-primitives.test.ts b/packages/compiler/src/backend/llvm/expr-primitives.test.ts
index 3c773f239..8e929f851 100644
--- a/packages/compiler/src/backend/llvm/expr-primitives.test.ts
+++ b/packages/compiler/src/backend/llvm/expr-primitives.test.ts
@@ -80,8 +80,8 @@ test.each([32, 64] as const)(
     const zero = /^define internal [^\n]*@sc_[bf]+_length_0\([^]*?^}/m.exec(llvm)![0];
     const one = /^define internal [^\n]*@sc_[bf]+_length_1\([^]*?^}/m.exec(llvm)![0];
     expect(zero).toContain(`getelementptr inbounds i${bits}`);
-    expect(zero).not.toContain("@scr_str_utf16_len");
-    expect(one).toContain("@scr_str_utf16_len");
+    expect(zero).not.toContain("_str_utf16_len");
+    expect(one).toContain("@sc_str_utf16_len");
   },
 );
 
diff --git a/packages/compiler/src/backend/llvm/string-key-hash.test.ts b/packages/compiler/src/backend/llvm/string-key-hash.test.ts
index 1fab9278b..5ca347a61 100644
--- a/packages/compiler/src/backend/llvm/string-key-hash.test.ts
+++ b/packages/compiler/src/backend/llvm/string-key-hash.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, test } from "vitest";
-import { literalCapWord, stringKeyHash32 } from "./string-key-hash.js";
+import { literalCapWord, literalCapWord32, stringKeyHash32 } from "./string-key-hash.js";
 
 // Shared with packages/runtime/test/test_map.c (hash_vectors): the runtime
 // computes these for heap strings, the compiler bakes them into literals.
@@ -26,12 +26,24 @@ describe("string key hash", () => {
     expect(stringKeyHash32(Buffer.from("ab"))).not.toBe(stringKeyHash32(Buffer.from("abc")));
   });
 
-  test("literal capacity word keeps cap = len in the low half", () => {
+  test("literal capacity word keeps cap = len in the low 31 bits", () => {
     for (const [text, hash] of VECTORS) {
       const bytes = Buffer.from(text, "utf8");
       const word = BigInt.asUintN(64, BigInt(literalCapWord(bytes)));
-      expect(word & 0xffffffffn).toBe(BigInt(bytes.length));
+      expect(word & 0x7fffffffn).toBe(BigInt(bytes.length));
       expect(word >> 32n).toBe(BigInt(hash));
     }
   });
+
+  test("literal capacity words carry the proven-ASCII bit exactly", () => {
+    for (const [text] of VECTORS) {
+      const bytes = Buffer.from(text, "utf8");
+      const ascii = bytes.every((byte) => byte < 0x80);
+      const word = BigInt.asUintN(64, BigInt(literalCapWord(bytes)));
+      expect((word & 0x80000000n) !== 0n).toBe(ascii);
+      const word32 = Number(literalCapWord32(bytes)) >>> 0;
+      expect(word32 & 0x7fffffff).toBe(bytes.length);
+      expect((word32 & 0x80000000) !== 0).toBe(ascii);
+    }
+  });
 });
diff --git a/packages/compiler/src/backend/llvm/string-key-hash.ts b/packages/compiler/src/backend/llvm/string-key-hash.ts
index dc047185a..28afb643f 100644
--- a/packages/compiler/src/backend/llvm/string-key-hash.ts
+++ b/packages/compiler/src/backend/llvm/string-key-hash.ts
@@ -57,10 +57,28 @@ export function stringKeyHash32(bytes: Uint8Array): number {
   return folded === 0 ? 1 : folded;
 }
 
+/** Bit 31 of every capacity word: the string is proven all ASCII, so the
+ * runtime and inline string reads map UTF-16 units to bytes by identity
+ * (SCR_STR_ASCII_BIT in scr_runtime.h). */
+export const STRING_ASCII_BIT = 0x80000000;
+
+function isAscii(bytes: Uint8Array): boolean {
+  for (const byte of bytes) if (byte >= 0x80) return false;
+  return true;
+}
+
 /** The capacity word of an immortal literal on 64-bit targets: cap = len in
- * the low half, the precomputed key hash in the high half, printed as the
- * signed i64 LLVM expects. */
+ * bits 0-30, the proven-ASCII bit, the precomputed key hash in the high
+ * half, printed as the signed i64 LLVM expects. */
 export function literalCapWord(bytes: Uint8Array): string {
-  const word = BigInt(bytes.length) | (BigInt(stringKeyHash32(bytes)) << 32n);
+  const ascii = isAscii(bytes) ? BigInt(STRING_ASCII_BIT) : 0n;
+  const word = BigInt(bytes.length) | ascii | (BigInt(stringKeyHash32(bytes)) << 32n);
   return BigInt.asIntN(64, word).toString();
 }
+
+/** The capacity word of an immortal literal on 32-bit targets: cap = len
+ * plus the proven-ASCII bit, printed as the signed i32 LLVM expects. */
+export function literalCapWord32(bytes: Uint8Array): string {
+  const word = bytes.length + (isAscii(bytes) ? STRING_ASCII_BIT : 0);
+  return String(word | 0);
+}
diff --git a/packages/compiler/src/backend/llvm/string-lifetimes.test.ts b/packages/compiler/src/backend/llvm/string-lifetimes.test.ts
index 17ceb8fea..bf54116ee 100644
--- a/packages/compiler/src/backend/llvm/string-lifetimes.test.ts
+++ b/packages/compiler/src/backend/llvm/string-lifetimes.test.ts
@@ -277,9 +277,11 @@ const methodCases: {
   args: IrExpr[];
   result: IrType;
   target: string;
+  /** Sites enter through the inlined sc_str_* wrapper (string-reads.ts). */
+  inline?: boolean;
 }[] = [
-  { method: "length", args: [], result: F64, target: "utf16_len" },
-  { method: "charCodeAt", args: [num(1)], result: F64, target: "char_code_at" },
+  { method: "length", args: [], result: F64, target: "utf16_len", inline: true },
+  { method: "charCodeAt", args: [num(1)], result: F64, target: "char_code_at", inline: true },
   { method: "charAt", args: [num(1)], result: STRING, target: "char_at" },
   { method: "indexOf", args: [ref("argument"), num(1)], result: F64, target: "index_of" },
   { method: "includes", args: [ref("argument")], result: BOOL, target: "includes" },
@@ -314,7 +316,7 @@ const methodCases: {
 
 test.each(methodCases)(
   "$method passes borrowed string inputs through its runtime ABI",
-  ({ method, args, result, target }) => {
+  ({ method, args, result, target, inline }) => {
     const f = fn(
       "method",
       ["receiver", "argument"],
@@ -322,7 +324,7 @@ test.each(methodCases)(
     );
     for (const bits of [32, 64] as const) {
       const ir = body(mod(f), "sc_bf_method", bits);
-      expect(ir).toContain(`@scr_str_${target}(`);
+      expect(ir).toContain(`@${inline ? "sc" : "scr"}_str_${target}(`);
       expect(ir).not.toContain("@scr_str_retain_v");
       // Throwing methods release their owned result on the unwind path.
       if (method !== "normalize" && method !== "repeat")
@@ -381,7 +383,7 @@ test("boxed string lengths borrow initialized payloads and keep checked TDZ read
   expect(ir).toMatch(/br i1 %t\d+, label %lazy\.release\d+/);
   expect(ir).toContain("@scr_box_release");
   expect(ir).not.toContain("@scr_box_get_ref");
-  expect(ir).toContain("@scr_str_utf16_len");
+  expect(ir).toContain("@sc_str_utf16_len");
   inspect.locals[0]!.tdz = true;
   const checked = body(mod(inspect), "sc_f_inspect");
   expect(checked).toContain("@scr_box_get_ref");
diff --git a/packages/compiler/src/backend/llvm/string-reads.ts b/packages/compiler/src/backend/llvm/string-reads.ts
new file mode 100644
index 000000000..fe70d8a1b
--- /dev/null
+++ b/packages/compiler/src/backend/llvm/string-reads.ts
@@ -0,0 +1,84 @@
+/* Inline UTF-16 reads of proven-ASCII strings.
+ *
+ * Strings are UTF-8, so `length` and `charCodeAt` need a UTF-16 mapping in
+ * general. Bit 31 of a string's capacity word (SCR_STR_ASCII_BIT in
+ * scr_runtime.h) proves every byte is ASCII, where units are bytes. Sites
+ * call the internal sc_str_* helpers below, which LLVM inlines: the proven
+ * case is a header load and, for charCodeAt, one byte load; every other
+ * string or index defers to the runtime function, which keeps the complete
+ * semantics (surrogate halves, out-of-range NaN, fractional, negative and
+ * NaN indices). On little-endian targets the bit is the sign bit of the
+ * capacity word's low 32 bits on both i64 and i32 layouts.
+ */
+
+export const STRING_UTF16_LEN_DECL = "declare double @scr_str_utf16_len(ptr)";
+export const STRING_CHAR_CODE_AT_DECL = "declare double @scr_str_char_code_at(ptr, double)";
+
+/** Load the proven-ASCII flag of `%s` into `%ascii`. */
+function asciiTest(): string[] {
+  return [
+    `  %cw.p = getelementptr inbounds %ScrStr, ptr %s, i32 0, i32 2`,
+    `  %cw = load i32, ptr %cw.p`,
+    `  %ascii = icmp slt i32 %cw, 0`,
+  ];
+}
+
+function loadLength(sz: string): string[] {
+  return [
+    `  %len.p = getelementptr inbounds %ScrStr, ptr %s, i32 0, i32 1`,
+    `  %len = load ${sz}, ptr %len.p`,
+  ];
+}
+
+export interface StringReadHelper {
+  decl: string;
+  define(sz: string, attrs: string): string[];
+}
+
+export const STRING_READ_HELPERS: readonly StringReadHelper[] = [
+  {
+    decl: STRING_UTF16_LEN_DECL,
+    define: (sz, attrs) => [
+      `define internal double @sc_str_utf16_len(ptr %s) ${attrs} {`,
+      `entry:`,
+      ...asciiTest(),
+      `  br i1 %ascii, label %fast, label %slow`,
+      `fast:`,
+      ...loadLength(sz),
+      `  %n = uitofp ${sz} %len to double`,
+      `  ret double %n`,
+      `slow:`,
+      `  %r = call double @scr_str_utf16_len(ptr %s)`,
+      `  ret double %r`,
+      `}`,
+    ],
+  },
+  {
+    decl: STRING_CHAR_CODE_AT_DECL,
+    define: (sz, attrs) => [
+      `define internal double @sc_str_char_code_at(ptr %s, double %i) ${attrs} {`,
+      `entry:`,
+      ...asciiTest(),
+      `  br i1 %ascii, label %bounds, label %slow`,
+      `bounds:`,
+      ...loadLength(sz),
+      `  %lenf = uitofp ${sz} %len to double`,
+      // NaN fails both ordered comparisons; -0 passes and truncates to 0.
+      `  %lo = fcmp oge double %i, 0.0`,
+      `  %hi = fcmp olt double %i, %lenf`,
+      `  %in = and i1 %lo, %hi`,
+      `  br i1 %in, label %read, label %slow`,
+      `read:`,
+      `  %k = fptoui double %i to ${sz}`,
+      `  %data = getelementptr inbounds %ScrStr, ptr %s, i32 1`,
+      `  %p = getelementptr inbounds i8, ptr %data, ${sz} %k`,
+      `  %byte = load i8, ptr %p`,
+      `  %unit = uitofp i8 %byte to double`,
+      `  ret double %unit`,
+      `slow:`,
+      `  %r = call double @scr_str_char_code_at(ptr %s, double %i)`,
+      `  ret double %r`,
+      `}`,
+    ],
+  },
+];
diff --git a/packages/compiler/src/backend/llvm/switch-dispatch.test.ts b/packages/compiler/src/backend/llvm/switch-dispatch.test.ts
index 614ec75ba..e49b9e36e 100644
--- a/packages/compiler/src/backend/llvm/switch-dispatch.test.ts
+++ b/packages/compiler/src/backend/llvm/switch-dispatch.test.ts
@@ -81,7 +81,7 @@ test("string dispatch uses target-width byte lengths and checks equality inside
     const body = emit(STRING, ["", "a", "é", "😀", "tail", "é"].map(string), bits);
     expect(body).toContain(`load i${bits}, ptr`);
     expect(body).toContain(`switch i${bits}`);
-    expect(body).not.toContain("scr_str_utf16_len");
+    expect(body).not.toContain("_str_utf16_len");
     expect(body.match(/call zeroext i1 @sc_str_eq/g)).toHaveLength(5);
     const dispatch = body.split("\n").find((line) => line.includes(`switch i${bits}`))!;
     expect(dispatch).toContain(`i${bits} 2, label`);
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index da1c24f82..6fa10a6cb 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -15460,6 +15460,24 @@
     "<repo>/tests/corpus/4453-int32-closed-locals.ts"
    ],
    "diags": []
+  },
+  "<repo>/tests/corpus/4476-string-ascii-header-fact.ts": {
+   "order": [
+    "<repo>/tests/corpus/4476-string-ascii-header-fact.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/4477-buffer-write-ascii-runs.ts": {
+   "order": [
+    "<repo>/tests/corpus/4477-buffer-write-ascii-runs.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/4478-string-large-index-reads.ts": {
+   "order": [
+    "<repo>/tests/corpus/4478-string-large-index-reads.ts"
+   ],
+   "diags": []
   }
  }
 }
diff --git a/packages/runtime/src/scr_bytes.c b/packages/runtime/src/scr_bytes.c
index 3a87bd7c2..59a7fe152 100644
--- a/packages/runtime/src/scr_bytes.c
+++ b/packages/runtime/src/scr_bytes.c
@@ -2736,7 +2736,28 @@ double scr_bytes_write_str(ScrBytes *b, const ScrStr *s, const ScrStr *enc,
   bool utf16 = scr_enc_is(enc, "utf16le");
   if (utf16 || scr_enc_is(enc, "latin1") || scr_enc_is(enc, "ascii")) {
     size_t i = 0, written = 0, width = utf16 ? 2 : 1;
+    const uint64_t high = UINT64_C(0x8080808080808080);
     while (i < s->len && budget - written >= width) {
+      /* ASCII runs encode byte-for-unit: widen (or copy) eight bytes per
+       * step while the budget holds them all, else one code point below. */
+      if (s->len - i >= 8 && budget - written >= 8 * width) {
+        uint64_t w;
+        memcpy(&w, s->data + i, 8);
+        if (!(w & high)) {
+          uint8_t *d = b->data + o + written;
+          if (utf16) {
+            for (int k = 0; k < 8; k++) {
+              d[2 * k] = (uint8_t)s->data[i + k];
+              d[2 * k + 1] = 0;
+            }
+          } else {
+            memcpy(d, s->data + i, 8);
+          }
+          i += 8;
+          written += 8 * width;
+          continue;
+        }
+      }
       uint32_t cp = scr_bytes_next_cp((const uint8_t *)s->data, &i);
       uint32_t first = cp, second = 0;
       if (cp > 0xffff) {
diff --git a/packages/runtime/src/scr_regex.c b/packages/runtime/src/scr_regex.c
index 5a2c670cf..e86ed3046 100644
--- a/packages/runtime/src/scr_regex.c
+++ b/packages/runtime/src/scr_regex.c
@@ -416,6 +416,7 @@ typedef struct ScrRegexInput {
 } ScrRegexInput;
 
 static bool scr_text_ascii(const ScrStr *s) {
+  if (scr_str_known_ascii(s)) return true;
   size_t i = 0;
   for (; s->len - i >= sizeof(uint64_t); i += sizeof(uint64_t)) {
     uint64_t word;
diff --git a/packages/runtime/src/scr_runtime.h b/packages/runtime/src/scr_runtime.h
index 448034e1d..4af9a4f99 100644
--- a/packages/runtime/src/scr_runtime.h
+++ b/packages/runtime/src/scr_runtime.h
@@ -649,40 +649,113 @@ typedef struct ScrVt {
  * owned heap strings are ever mutated, and every such path clears the hash
  * (scr_str_hash_forget); immortal strings are never written, so they always
  * hash on demand.
+ *
+ * Bit 31 of the capacity word (on every target) is the proven-ASCII fact:
+ * when set, every byte is below 0x80, so UTF-16 indices equal byte offsets
+ * and .length == len. A clear bit means "not known", never "non-ASCII":
+ * constructors set it only where it is cheap, and string reads that prove
+ * ASCII set it later. Capacity therefore stops at 2^31 - 1 bytes, which
+ * still holds every string V8 can represent (2^29 - 24 UTF-16 units, at
+ * most three UTF-8 bytes each). Generated code reads the bit for inline
+ * charCodeAt/length (llvm/strings.ts); emitted literals and runtime
+ * statics set it in their capacity word. scr_str_hash_forget clears it
+ * with the hash: every in-place mutation already forgets cached facts.
  */
 #if SIZE_MAX > UINT32_MAX && defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #define SCR_STR_HASH_CACHE 1
-#define SCR_STR_MAX_CAP ((size_t)UINT32_MAX)
 #else
 #define SCR_STR_HASH_CACHE 0
-#define SCR_STR_MAX_CAP (SIZE_MAX - sizeof(size_t) * 3 - 1)
 #endif
+#define SCR_STR_MAX_CAP ((size_t)0x7fffffff)
 typedef struct ScrStr {
   size_t rc;
   size_t len;
 #if SCR_STR_HASH_CACHE
-  uint32_t cap;
+  uint32_t cap : 31;
+  uint32_t ascii : 1;
   uint32_t hash;
 #else
-  size_t cap;
+  size_t cap : 31;
+  size_t ascii : 1;
 #endif
   char data[];
 } ScrStr;
 
-/* The capacity word of a static string: cap == len, plus the precomputed
- * 32-bit key hash where the header caches one (0 leaves it to be computed). */
+/* The capacity word of a static string: cap == len, the proven-ASCII bit,
+ * plus the precomputed 32-bit key hash where the header caches one (0
+ * leaves it to be computed). */
+#define SCR_STR_ASCII_BIT ((size_t)1 << 31)
 #if SCR_STR_HASH_CACHE
-#define SCR_STR_CAP_WORD(len, hash32) ((size_t)(len) | ((size_t)(uint32_t)(hash32) << 32))
+#define SCR_STR_CAP_WORD(len, hash32, ascii)                                  \
+  ((size_t)(len) | ((ascii) ? SCR_STR_ASCII_BIT : 0) |                        \
+   ((size_t)(uint32_t)(hash32) << 32))
 #else
-#define SCR_STR_CAP_WORD(len, hash32) ((size_t)(len))
+#define SCR_STR_CAP_WORD(len, hash32, ascii) ((size_t)(len) | ((ascii) ? SCR_STR_ASCII_BIT : 0))
 #endif
 
+/* Forget every cached content fact (key hash, proven ASCII) before a
+ * uniquely owned string's bytes change. */
 static inline void scr_str_hash_forget(ScrStr *s) {
 #if SCR_STR_HASH_CACHE
   s->hash = 0;
-#else
-  (void)s;
 #endif
+  s->ascii = 0;
+}
+
+/* True only when every byte is proven below 0x80 (see above). */
+static inline bool scr_str_known_ascii(const ScrStr *s) { return s->ascii; }
+
+/* Record a proven-ASCII fact. Immortal strings may live in read-only
+ * memory and are shared across threads, so they are never written. */
+static inline void scr_str_note_ascii(ScrStr *s) {
+  if (s->rc != SIZE_MAX) s->ascii = 1;
+}
+
+/* memcpy for the short, variable-length copies of string construction
+ * (identifiers, keys, number text): fixed-size overlapping moves instead of
+ * a libc call. Longer spans keep the platform's bulk copy. */
+static inline void scr_copy_bytes(char *dst, const char *src, size_t n) {
+  if (n <= 16) {
+    if (n >= 8) {
+      uint64_t a, b;
+      memcpy(&a, src, 8);
+      memcpy(&b, src + n - 8, 8);
+      memcpy(dst, &a, 8);
+      memcpy(dst + n - 8, &b, 8);
+    } else if (n >= 4) {
+      uint32_t a, b;
+      memcpy(&a, src, 4);
+      memcpy(&b, src + n - 4, 4);
+      memcpy(dst, &a, 4);
+      memcpy(dst + n - 4, &b, 4);
+    } else if (n) {
+      dst[0] = src[0];
+      dst[n / 2] = src[n / 2];
+      dst[n - 1] = src[n - 1];
+    }
+    return;
+  }
+  memcpy(dst, src, n);
+}
+
+/* Whether a byte span is all ASCII: 32 bytes per step, stopping at the
+ * first chunk with a high bit. */
+static inline bool scr_bytes_all_ascii(const char *p, size_t n) {
+  const uint64_t high = UINT64_C(0x8080808080808080);
+  size_t i = 0;
+  for (; n - i >= 32; i += 32) {
+    uint64_t w[4];
+    memcpy(w, p + i, sizeof w);
+    if ((w[0] | w[1] | w[2] | w[3]) & high) return false;
+  }
+  uint64_t acc = 0;
+  for (; n - i >= 8; i += 8) {
+    uint64_t w;
+    memcpy(&w, p + i, 8);
+    acc |= w;
+  }
+  for (; i < n; i++) acc |= (unsigned char)p[i];
+  return (acc & high) == 0;
 }
 
 ScrStr *scr_str_new(const char *bytes, size_t len); /* returns +1 */
diff --git a/packages/runtime/src/scr_string.c b/packages/runtime/src/scr_string.c
index 92d5e4d74..7c56d245c 100644
--- a/packages/runtime/src/scr_string.c
+++ b/packages/runtime/src/scr_string.c
@@ -200,7 +200,9 @@ static ScrSidx *scr_sidx_claim(ScrSidx *table, unsigned *clock, unsigned *live)
  * receivers still shed their point buffer after proving identity mapping.
  * Both tiers remain fixed-size and allocation-free until a non-ASCII sparse
  * receiver actually needs checkpoints. */
-static ScrSidx *scr_sidx(const ScrStr *s) {
+static SCR_TL ScrSidx *scr_sidx_last; /* most recent hit; a slot, never freed */
+
+static ScrSidx *scr_sidx_lookup(const ScrStr *s) {
   if (s->len >= SCR_SIDX_MIN_BYTES) {
     for (int i = 0; i < SCR_SIDX_N; i++) {
       if (scr_sidx_sparse_tab[i].s == s) return &scr_sidx_sparse_tab[i];
@@ -234,6 +236,27 @@ static ScrSidx *scr_sidx(const ScrStr *s) {
   return e;
 }
 
+/* Sequential reads of one receiver (a scan over a source text) hit the
+ * same entry call after call. The memo is a pointer to a static slot, so a
+ * cleared, moved or reassigned entry simply fails the owner test. */
+static ScrSidx *scr_sidx(const ScrStr *s) {
+  ScrSidx *last = scr_sidx_last;
+  /* An in-place append can carry a cursor-tier receiver across the sparse
+   * threshold; only the lookup performs that transfer, so the memo must
+   * also match the receiver's current tier. */
+  bool sparse = (uintptr_t)last - (uintptr_t)scr_sidx_sparse_tab < sizeof scr_sidx_sparse_tab;
+  if (last && last->s == s && sparse == (s->len >= SCR_SIDX_MIN_BYTES)) return last;
+  return scr_sidx_last = scr_sidx_lookup(s);
+}
+
+/* The index entry for a receiver that is not proven ASCII, or NULL when it
+ * is: proven-ASCII strings map units to bytes by identity, and every mapper
+ * below checks the header bit before touching the entry. The bit only ever
+ * turns on during a read, so "ascii or entry" holds for the whole call. */
+static ScrSidx *scr_sidx_for(const ScrStr *s) {
+  return s->ascii ? NULL : scr_sidx(s);
+}
+
 /* In-place concat changes only the suffix. Keep every exact prefix anchor
  * (including the former end, which is now an ordinary boundary), but remove
  * the sole fact that described the old complete string. A threshold-crossing
@@ -268,10 +291,57 @@ static ScrStr *scr_str_alloc(size_t len, size_t cap) {
   return s;
 }
 
+/* scr_copy_bytes that also reports whether the bytes are all ASCII. Short
+ * spans test the words they already moved; longer ones scan after the bulk
+ * copy, stopping at the first non-ASCII chunk. */
+static bool scr_copy_bytes_ascii(char *dst, const char *src, size_t n) {
+  const uint64_t high = UINT64_C(0x8080808080808080);
+  if (n <= 16) {
+    if (n >= 8) {
+      uint64_t a, b;
+      memcpy(&a, src, 8);
+      memcpy(&b, src + n - 8, 8);
+      memcpy(dst, &a, 8);
+      memcpy(dst + n - 8, &b, 8);
+      return ((a | b) & high) == 0;
+    }
+    if (n >= 4) {
+      uint32_t a, b;
+      memcpy(&a, src, 4);
+      memcpy(&b, src + n - 4, 4);
+      memcpy(dst, &a, 4);
+      memcpy(dst + n - 4, &b, 4);
+      return ((a | b) & UINT32_C(0x80808080)) == 0;
+    }
+    if (n == 0) return true;
+    unsigned char c0 = (unsigned char)src[0], c1 = (unsigned char)src[n / 2],
+                  c2 = (unsigned char)src[n - 1];
+    dst[0] = (char)c0;
+    dst[n / 2] = (char)c1;
+    dst[n - 1] = (char)c2;
+    return ((c0 | c1 | c2) & 0x80) == 0;
+  }
+  memcpy(dst, src, n);
+  return scr_bytes_all_ascii(src, n);
+}
+
 ScrStr *scr_str_new(const char *bytes, size_t len) {
   ScrStr *s = scr_str_alloc(len, len);
-  memcpy(s->data, bytes, len);
+  /* Proving ASCII while copying lets every later UTF-16 index, length and
+   * slice skip the index cache. */
+  bool ascii = scr_copy_bytes_ascii(s->data, bytes, len);
   s->data[len] = '\0';
+  s->ascii = ascii;
+  return s;
+}
+
+/* scr_str_new for bytes already known to be ASCII (slices of an ASCII
+ * string): no scan. */
+static ScrStr *scr_str_new_ascii(const char *bytes, size_t len) {
+  ScrStr *s = scr_str_alloc(len, len);
+  scr_copy_bytes(s->data, bytes, len);
+  s->data[len] = '\0';
+  s->ascii = 1;
   return s;
 }
 
@@ -328,7 +398,9 @@ void scr_str_release(ScrStr *s) {
   if (!s || s->rc == SIZE_MAX) return; /* NULL: an uninitialized `let` local */
   if (--s->rc == 0) {
     scr_short_forget(s);
-    scr_sidx_purge(s); /* the address may be recycled by the next malloc */
+    /* The address may be recycled by the next malloc. A proven-ASCII string
+     * owns no index entry (scr_sidx_finish). */
+    if (!s->ascii) scr_sidx_purge(s);
 #ifdef SCR_RC_AUDIT
     scr_live_strings--;
 #endif
@@ -354,20 +426,25 @@ ScrStr *scr_str_concat(ScrStr *a, ScrStr *b) {
    * concat chains (`a + b + c`, template literals), where each intermediate
    * result reaches the next concat as a sole-reference temp. Any string
    * with rc > 1 might be aliased and is copied, never mutated. */
+  bool ascii = a->ascii && b->ascii;
   if (a->rc == 1 && a != b && a->cap >= newlen) {
     size_t oldlen = a->len;
     scr_short_forget(a);
     scr_str_hash_forget(a);
-    memcpy(a->data + a->len, b->data, b->len);
+    scr_copy_bytes(a->data + a->len, b->data, b->len);
     a->len = newlen;
     a->data[newlen] = '\0';
+    a->ascii = ascii;
     /* A cached UTF-16 length for a is stale now; checkpoints and the exact
      * old prefix remain valid. Its old terminal point is no longer an END
      * fact (u16len is invalidated below), but remains an excellent ordinary
      * checkpoint for accesses around the append boundary. The next mapper
      * lazily continues from oldlen rather than scanning the unchanged prefix
-     * again. */
-    scr_sidx_concat_append(a, oldlen);
+     * again. An ASCII string that stays ASCII needs none of this: every
+     * mapper answers it by identity before consulting an entry, and the
+     * append that first adds non-ASCII bytes invalidates against its own
+     * old length, which bounds every older fact as well. */
+    if (!ascii) scr_sidx_concat_append(a, oldlen);
     a->rc = 2; /* +1 for the returned reference, beside the caller's borrow */
     return a;
   }
@@ -386,9 +463,10 @@ ScrStr *scr_str_concat(ScrStr *a, ScrStr *b) {
   if (newcap > SCR_STR_MAX_CAP && newlen <= SCR_STR_MAX_CAP) newcap = SCR_STR_MAX_CAP;
   ScrStr *s = scr_str_take_spare(newlen);
   if (!s) s = scr_str_alloc(newlen, newcap);
-  memcpy(s->data, a->data, a->len);
-  memcpy(s->data + a->len, b->data, b->len);
+  scr_copy_bytes(s->data, a->data, a->len);
+  scr_copy_bytes(s->data + a->len, b->data, b->len);
   s->data[newlen] = '\0';
+  s->ascii = ascii;
   return s;
 }
 
@@ -406,10 +484,12 @@ ScrStr *scr_str_concat_mixed(ScrStr *head, ScrStr *const *parts, const double *n
   const size_t limit = SCR_STR_MAX_CAP;
   size_t extra = 0;
   bool aliased = false;
+  bool ascii = !head || head->ascii; /* formatted numbers are ASCII */
   for (size_t i = 0; i < count; i++) {
     if (parts[i]) {
       lens[i] = parts[i]->len;
       aliased |= parts[i] == head;
+      ascii &= parts[i]->ascii;
     } else {
       lens[i] = scr_f64_to_str(nums[i], text[i]);
     }
@@ -439,17 +519,18 @@ ScrStr *scr_str_concat_mixed(ScrStr *head, ScrStr *const *parts, const double *n
     if (newcap > SCR_STR_MAX_CAP) newcap = SCR_STR_MAX_CAP;
     out = scr_str_take_spare(newlen);
     if (!out) out = scr_str_alloc(newlen, newcap);
-    if (base) memcpy(out->data, head->data, base);
+    if (base) scr_copy_bytes(out->data, head->data, base);
     at = base;
   }
   for (size_t i = 0; i < count; i++) {
-    memcpy(out->data + at, parts[i] ? parts[i]->data : text[i], lens[i]);
+    scr_copy_bytes(out->data + at, parts[i] ? parts[i]->data : text[i], lens[i]);
     at += lens[i];
   }
   out->len = newlen;
   out->data[newlen] = '\0';
+  out->ascii = ascii;
   if (out == head) {
-    scr_sidx_concat_append(head, base);
+    if (!ascii) scr_sidx_concat_append(head, base); /* see scr_str_concat */
     head->rc = 2; /* +1 for the returned reference, beside the caller's borrow */
   }
   return out;
@@ -477,12 +558,15 @@ ScrStr *scr_str_concat_parts(ScrStr *const *parts, size_t count) {
   if (nonempty == 1) return scr_str_retain(only);
   ScrStr *out = scr_str_alloc_raw(len, len);
   size_t offset = 0;
+  bool ascii = true;
   for (size_t i = 0; i < count; i++) {
     const ScrStr *part = parts[i];
-    memcpy(out->data + offset, part->data, part->len);
+    scr_copy_bytes(out->data + offset, part->data, part->len);
     offset += part->len;
+    ascii &= part->ascii;
   }
   out->data[len] = '\0';
+  out->ascii = ascii;
   return out;
 }
 
@@ -529,7 +613,7 @@ int scr_str_cmp_u16(ScrStr *a, ScrStr *b) {
  */
 typedef struct { size_t rc; size_t len; size_t cap; char data[2]; } ScrChar1;
 #define SCR_A_WORD(c) (((uint64_t)(c) << 16) | ((uint64_t)(c) << 8) | (uint64_t)(c))
-#define SCR_A(c) {SIZE_MAX, 1, SCR_STR_CAP_WORD(1, SCR_KEY_HASH32_SHORT(1, SCR_A_WORD(c))), {(char)(c), 0}}
+#define SCR_A(c) {SIZE_MAX, 1, SCR_STR_CAP_WORD(1, SCR_KEY_HASH32_SHORT(1, SCR_A_WORD(c)), 1), {(char)(c), 0}}
 #define SCR_A8(c) \
   SCR_A(c), SCR_A(c + 1), SCR_A(c + 2), SCR_A(c + 3), \
   SCR_A(c + 4), SCR_A(c + 5), SCR_A(c + 6), SCR_A(c + 7)
@@ -540,7 +624,7 @@ static const ScrChar1 scr_ascii1[128] = {
   SCR_A8(96),  SCR_A8(104), SCR_A8(112), SCR_A8(120),
 };
 static const struct { size_t rc; size_t len; size_t cap; char data[1]; }
-    scr_lit_empty = {SIZE_MAX, 0, SCR_STR_CAP_WORD(0, SCR_KEY_HASH32_SHORT(0, 0)), ""};
+    scr_lit_empty = {SIZE_MAX, 0, SCR_STR_CAP_WORD(0, SCR_KEY_HASH32_SHORT(0, 0), 1), ""};
 
 static ScrStr *scr_str_empty(void) { return (ScrStr *)&scr_lit_empty; }
 
@@ -562,6 +646,13 @@ static ScrStr *scr_str_from_span(const char *bytes, size_t len) {
   return scr_str_new(bytes, len);
 }
 
+/* scr_str_from_span for a span of a proven-ASCII string: the result is
+ * ASCII without another scan. */
+static ScrStr *scr_str_from_span_ascii(const char *bytes, size_t len) {
+  if (len <= 4) return scr_str_from_span(bytes, len);
+  return scr_str_new_ascii(bytes, len);
+}
+
 /* A scalar fromCharCode needs neither the variadic argument array nor an
  * encoding buffer. Reuse the same empty/ASCII/tiny-span storage policy as
  * character indexing. A lone surrogate keeps the existing U+FFFD policy. */
@@ -790,10 +881,21 @@ static void scr_sidx_rebuild_points(const ScrStr *s, ScrSidx *e) {
   e->points_complete = true;
 }
 
+/* Completing a scan that proves a heap string ASCII records the fact in its
+ * header and drops the entry: a proven-ASCII string never owns one (every
+ * mapper answers it by identity first, and a release of it skips the
+ * purge). Callers re-check the header bit before touching the entry again.
+ * Immortal strings cannot record the bit and keep the entry. */
 static void scr_sidx_finish(const ScrStr *s, ScrSidx *e) {
   if (e->indexed_cb != s->len) return;
   e->u16len = e->indexed_cu;
   if (e->u16len == s->len) { /* proven all ASCII: identity needs no index */
+    if (s->rc != SIZE_MAX) {
+      ((ScrStr *)s)->ascii = 1;
+      bool sparse = (uintptr_t)e - (uintptr_t)scr_sidx_sparse_tab < sizeof scr_sidx_sparse_tab;
+      scr_sidx_clear(e, sparse ? &scr_sidx_sparse_live : NULL);
+      return;
+    }
     free(e->points);
     e->points = NULL;
     e->npoints = 0;
@@ -837,10 +939,11 @@ static void scr_sidx_materialize_identity_prefix(const ScrStr *s, ScrSidx *e) {
  * historical single word-wise scan. `u16len == byte len` proves ASCII and
  * restores identity mapping with zero retained checkpoint memory. */
 static size_t scr_sidx_len(const ScrStr *s, ScrSidx *e) {
+  if (s->ascii) return s->len;
   if (e->u16len != SCR_U16_UNKNOWN) return e->u16len;
   while (e->indexed_cb < s->len) scr_sidx_extend_one(s, e);
   scr_sidx_finish(s, e);
-  return e->u16len;
+  return s->ascii ? s->len : e->u16len;
 }
 
 /* Step the cursor back one char (cb must be > 0 and on a boundary). */
@@ -970,7 +1073,7 @@ static void scr_sidx_ascii_run(const ScrStr *s, ScrSidx *e) {
  * Same contract as a from-scratch scan. */
 static size_t scr_u16_to_byte_c(const ScrStr *s, ScrSidx *e, size_t u16,
                                  bool *mid) {
-  if (e->u16len == s->len) { /* all ASCII: identity mapping */
+  if (s->ascii || e->u16len == s->len) { /* all ASCII: identity mapping */
     *mid = false;
     return u16 < s->len ? u16 : s->len;
   }
@@ -983,7 +1086,7 @@ static size_t scr_u16_to_byte_c(const ScrStr *s, ScrSidx *e, size_t u16,
   scr_sidx_extend_to_u16(s, e, u16);
   /* Extending a far-end lookup can just have proved identity. Do not
    * materialize an index that the identity fast path will never consult. */
-  if (e->u16len == s->len) {
+  if (s->ascii || e->u16len == s->len) {
     *mid = false;
     return u16 < s->len ? u16 : s->len;
   }
@@ -1037,7 +1140,7 @@ static size_t scr_u16_to_byte_c(const ScrStr *s, ScrSidx *e, size_t u16,
  * the closest sparse anchor or hot cursor. */
 static size_t scr_byte_to_u16_c(const ScrStr *s, ScrSidx *e,
                                  size_t byte_off) {
-  if (e->u16len == s->len) return byte_off; /* all ASCII */
+  if (s->ascii || e->u16len == s->len) return byte_off; /* all ASCII */
   if (byte_off >= e->ascii_cb && byte_off < e->ascii_end) {
     e->cu = e->ascii_cu + (byte_off - e->ascii_cb);
     e->cb = byte_off;
@@ -1051,7 +1154,7 @@ static size_t scr_byte_to_u16_c(const ScrStr *s, ScrSidx *e,
   }
   /* As above, a completed all-ASCII scan is its own index. In particular,
    * do not rebuild points that scr_sidx_finish() deliberately discarded. */
-  if (e->u16len == s->len) return byte_off;
+  if (s->ascii || e->u16len == s->len) return byte_off;
   scr_sidx_materialize_identity_prefix(s, e);
   ScrSidxPoint near = scr_sidx_near_byte(s, e, byte_off);
   size_t cu = near.cu, cb = near.cb;
@@ -1098,7 +1201,7 @@ static const char *scr_byte_find(const char *hay, size_t hay_len,
 /* Search backwards from a clamped UTF-16 position. A low-surrogate position
  * maps to its scalar's first byte, so that scalar remains searchable. */
 double scr_str_last_index_of_from(ScrStr *s, ScrStr *needle, double position) {
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   size_t len16 = scr_sidx_len(s, e);
   size_t start16 = isnan(position) || position >= (double)len16 ? len16
                    : position <= 0 ? 0 : (size_t)trunc(position);
@@ -1121,16 +1224,32 @@ double scr_str_last_index_of(ScrStr *s, ScrStr *needle) {
 }
 
 double scr_str_utf16_len(ScrStr *s) {
+  if (s->ascii) return (double)s->len;
   return (double)scr_sidx_len(s, scr_sidx(s));
 }
 
 double scr_str_char_code_at(ScrStr *s, double i) {
+  /* A sequential scan of a non-ASCII receiver stays inside the bounded
+   * ASCII span cached at its last resolved position. Its unit offset is an
+   * integer, so trunc(i) - ascii_cu == trunc(i - ascii_cu) for i at or past
+   * it; NaN and anything before the span fail the comparison. */
+  ScrSidx *last = scr_sidx_last;
+  if (last && last->s == s && !s->ascii) {
+    double rel = i - (double)last->ascii_cu;
+    if (rel >= 0 && rel < (double)(last->ascii_end - last->ascii_cb)) {
+      size_t k = (size_t)rel;
+      last->cu = last->ascii_cu + k;
+      last->cb = last->ascii_cb + k;
+      return (double)(unsigned char)s->data[last->cb];
+    }
+  }
   double idx = scr_to_integer_or_infinity(i);
   if (!(idx >= 0)) return NAN; /* negative or -Infinity */
   /* UTF-16 length <= byte length always, so this also fences the cast. */
   if (idx >= (double)s->len) return NAN;
+  if (s->ascii) return (double)(unsigned char)s->data[(size_t)idx];
   ScrSidx *e = scr_sidx(s);
-  if (e->u16len == s->len) return (double)(unsigned char)s->data[(size_t)idx];
+  if (s->ascii || e->u16len == s->len) return (double)(unsigned char)s->data[(size_t)idx];
   bool mid;
   size_t off = scr_u16_to_byte_c(s, e, (size_t)idx, &mid);
   if (off >= s->len) return NAN; /* idx >= length */
@@ -1144,7 +1263,7 @@ double scr_str_char_code_at(ScrStr *s, double i) {
 double scr_str_index_of(ScrStr *s, ScrStr *needle, double fromIndex) {
   double pos = scr_to_integer_or_infinity(fromIndex);
   if (needle->len > s->len) return -1.0;
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   /* Per spec, the empty needle is found at the clamped fromIndex itself —
    * even when that index is between the halves of an astral pair. */
   if (needle->len == 0) {
@@ -1173,7 +1292,7 @@ static ScrStr *scr_str_slice_units(ScrStr *s, ScrSidx *e,
 /* Normalize once, then share slice's extraction without another cache
  * lookup, length query or floating-point boundary conversion. */
 ScrStr *scr_str_substring(ScrStr *s, double start, double end) {
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   double len16 = (double)scr_sidx_len(s, e);
   double a = scr_to_integer_or_infinity(start);
   double b = scr_to_integer_or_infinity(end);
@@ -1208,7 +1327,7 @@ bool scr_str_starts_with_from(ScrStr *s, ScrStr *needle, double position) {
   double pos = scr_to_integer_or_infinity(position);
   if (pos <= 0) return scr_str_starts_with(s, needle);
   if (pos >= (double)s->len || needle->len > s->len) return false;
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   bool mid;
   size_t start_byte = scr_u16_to_byte_c(s, e, (size_t)pos, &mid);
   return !mid && needle->len <= s->len - start_byte &&
@@ -1221,8 +1340,8 @@ bool scr_str_ends_with_from(ScrStr *s, ScrStr *needle, double end_position) {
   if (end_position >= (double)s->len) return scr_str_ends_with(s, needle);
   /* A second receiver lookup can evict the first receiver's entry. Resolve
    * the needle before borrowing the haystack entry for both conversions. */
-  size_t needle16 = scr_sidx_len(needle, scr_sidx(needle));
-  ScrSidx *e = scr_sidx(s);
+  size_t needle16 = scr_sidx_len(needle, scr_sidx_for(needle));
+  ScrSidx *e = scr_sidx_for(s);
   size_t len16 = scr_sidx_len(s, e);
   size_t end16 = scr_str_clamp_u16_position(end_position, len16);
   if (needle16 > end16) return false;
@@ -1245,7 +1364,7 @@ static size_t scr_slice_boundary(double v, size_t len16) {
 }
 
 ScrStr *scr_str_slice(ScrStr *s, double start, double end) {
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   size_t len16 = scr_sidx_len(s, e);
   size_t from = scr_slice_boundary(scr_to_integer_or_infinity(start), len16);
   size_t to = scr_slice_boundary(scr_to_integer_or_infinity(end), len16);
@@ -1254,7 +1373,7 @@ ScrStr *scr_str_slice(ScrStr *s, double start, double end) {
 
 void scr_str_slice_range(ScrStr *s, double start, double end, bool substring,
                          ScrStringSlice *out) {
-  ScrSidx *e = scr_sidx(s);
+  ScrSidx *e = scr_sidx_for(s);
   size_t length = scr_sidx_len(s, e);
   double a = scr_to_integer_or_infinity(start), b = scr_to_integer_or_infinity(end);
   size_t from, to;
@@ -1288,6 +1407,11 @@ double scr_str_slice_char_code_at(ScrStr *s, const ScrStringSlice *range, double
 static ScrStr *scr_str_slice_units(ScrStr *s, ScrSidx *e,
                                     size_t from, size_t to) {
   if (from >= to) return scr_str_empty();
+  if (s->ascii) {
+    /* Identity mapping; callers clamp both bounds to the length. */
+    if (from == 0 && to == s->len) return scr_str_retain(s);
+    return scr_str_from_span_ascii(s->data + from, to - from);
+  }
   if (from == 0 && to == e->u16len) return scr_str_retain(s);
 
   bool from_mid, to_mid;
@@ -1303,7 +1427,8 @@ static ScrStr *scr_str_slice_units(ScrStr *s, ScrSidx *e,
     /* Unit boundaries already prove the exact output length. Preserve it
      * instead of decoding a potentially megabyte-sized copy on .length.
      * Index checkpoints remain lazy and owned by the result's cache slot. */
-    if (content_len >= SCR_SIDX_MIN_BYTES) scr_sidx(result)->u16len = to - from;
+    if (content_len >= SCR_SIDX_MIN_BYTES && !result->ascii)
+      scr_sidx(result)->u16len = to - from;
     return result;
   }
 
@@ -1350,6 +1475,7 @@ ScrStr *scr_str_repeat(ScrStr *s, double count) {
     filled += chunk;
   }
   r->data[total] = '\0';
+  r->ascii = s->ascii;
   return r;
 }
 
@@ -1388,8 +1514,9 @@ ScrStr *scr_str_char_at(ScrStr *s, double i) {
   double idx = scr_to_integer_or_infinity(i);
   if (!(idx >= 0)) return scr_str_empty();
   if (idx >= (double)s->len) return scr_str_empty(); /* len16 <= len */
+  if (s->ascii) return (ScrStr *)&scr_ascii1[(unsigned char)s->data[(size_t)idx]];
   ScrSidx *e = scr_sidx(s);
-  if (e->u16len == s->len) { /* all ASCII */
+  if (s->ascii || e->u16len == s->len) { /* all ASCII */
     return (ScrStr *)&scr_ascii1[(unsigned char)s->data[(size_t)idx]];
   }
   bool mid;
@@ -1434,7 +1561,7 @@ ScrStr *scr_str_trim_end(ScrStr *s) {
  * lone surrogate (empty-separator split of an astral char, a pad fill
  * truncated mid-pair). */
 static const struct { size_t rc; size_t len; size_t cap; char data[4]; }
-    scr_lit_fffd = {SIZE_MAX, 3, SCR_STR_CAP_WORD(3, SCR_KEY_HASH32_SHORT(3, 0xEFBFBD)), "\xEF\xBF\xBD"};
+    scr_lit_fffd = {SIZE_MAX, 3, SCR_STR_CAP_WORD(3, SCR_KEY_HASH32_SHORT(3, 0xEFBFBD), 0), "\xEF\xBF\xBD"};
 
 /* Split owns a fresh array and appends only present string values. Fill an
  * available dense slot directly; ordinary push handles growth and the sparse
@@ -1583,15 +1710,14 @@ ScrArr *scr_str_split(ScrStr *s, ScrStr *sep) {
 static ScrStr *scr_pad_impl(ScrStr *s, double maxLength, ScrStr *fill,
                              bool at_start) {
   double target = scr_to_integer_or_infinity(maxLength);
-  ScrSidx *e = scr_sidx(s);
-  size_t len16 = scr_sidx_len(s, e);
+  size_t len16 = scr_sidx_len(s, scr_sidx_for(s));
   if (!(target > (double)len16) || fill->len == 0) return scr_str_retain(s);
   /* Reject pad sizes malloc could not satisfy before the double→size_t
    * conversion can overflow (each unit is at most 3 bytes here: BMP chars
    * and the U+FFFD stand-in; astral chars are 4 bytes for 2 units). */
   if (target > (double)((SIZE_MAX - sizeof(ScrStr) - 1) / 4)) scr_oom();
   size_t pad16 = (size_t)target - len16;
-  size_t fill16 = scr_sidx_len(fill, scr_sidx(fill));
+  size_t fill16 = scr_sidx_len(fill, scr_sidx_for(fill));
   size_t reps = pad16 / fill16, rem16 = pad16 % fill16;
   /* The truncated prefix of fill: whole chars while they fit in rem16
    * units; a final astral char that doesn't fit contributes U+FFFD. */
@@ -1936,7 +2062,7 @@ double scr_string_to_number(ScrStr *s) {
 ScrStr *scr_f64_to_scrstr(double x) {
   char buf[32];
   size_t len = scr_f64_to_str(x, buf);
-  return scr_str_new(buf, len);
+  return scr_str_new_ascii(buf, len); /* number text is always ASCII */
 }
 
 /* ── encodeURIComponent / encodeURI ───────────────────────────────────
@@ -2011,9 +2137,9 @@ ScrStr *scr_str_to_well_formed(ScrStr *s) { return scr_str_retain(s); }
 /* Immortal interned booleans (same layout trick the emitter uses for
  * string literals). */
 static const struct { size_t rc; size_t len; size_t cap; char data[5]; }
-    scr_lit_true = {SIZE_MAX, 4, 4, "true"};
+    scr_lit_true = {SIZE_MAX, 4, SCR_STR_CAP_WORD(4, 0, 1), "true"};
 static const struct { size_t rc; size_t len; size_t cap; char data[6]; }
-    scr_lit_false = {SIZE_MAX, 5, 5, "false"};
+    scr_lit_false = {SIZE_MAX, 5, SCR_STR_CAP_WORD(5, 0, 1), "false"};
 
 ScrStr *scr_bool_to_scrstr(bool b) {
   return b ? (ScrStr *)&scr_lit_true : (ScrStr *)&scr_lit_false;
@@ -2032,8 +2158,9 @@ ScrStr *scr_str_cp_at(ScrStr *s, double i) {
   double idx = scr_to_integer_or_infinity(i);
   if (!(idx >= 0)) return scr_str_empty();
   if (idx >= (double)s->len) return scr_str_empty(); /* len16 <= len */
+  if (s->ascii) return (ScrStr *)&scr_ascii1[(unsigned char)s->data[(size_t)idx]];
   ScrSidx *e = scr_sidx(s);
-  if (e->u16len == s->len) { /* all ASCII */
+  if (s->ascii || e->u16len == s->len) { /* all ASCII */
     return (ScrStr *)&scr_ascii1[(unsigned char)s->data[(size_t)idx]];
   }
   bool mid;
diff --git a/tests/corpus/4476-string-ascii-header-fact.ts b/tests/corpus/4476-string-ascii-header-fact.ts
new file mode 100644
index 000000000..3f47451d5
--- /dev/null
+++ b/tests/corpus/4476-string-ascii-header-fact.ts
@@ -0,0 +1,117 @@
+// UTF-16 reads (length, charCodeAt, charAt, slice, substring, indexOf,
+// lastIndexOf, startsWith/endsWith at a position, padStart) over strings of
+// every provenance: literals, concatenations, in-place appends, slices,
+// numbers, repeat, join and split pieces. Native strings record a
+// proven-ASCII fact in their header; appending a non-ASCII character to a
+// uniquely owned ASCII string must drop it, and every index form
+// (fractional, -0, negative, NaN, infinite, out of range) keeps Node's
+// semantics on both representations, including surrogate halves. (Reads
+// never split a surrogate pair into a string: scriptc substitutes U+FFFD.)
+
+function codes(s: string): string {
+  const out: number[] = [];
+  for (let i = 0; i < s.length; i++) out.push(s.charCodeAt(i));
+  return out.join(",");
+}
+
+function probe(label: string, s: string): void {
+  const idx = [0, -0, 0.5, 1.9999, -0.5, -1, NaN, Infinity, -Infinity, s.length - 1, s.length, 2 ** 32];
+  const units: string[] = [];
+  for (const i of idx) units.push(String(s.charCodeAt(i)));
+  const chars: string[] = [];
+  for (const i of idx) chars.push(JSON.stringify(s.charAt(i)));
+  console.log(label, s.length, units.join(" "), chars.join(" "));
+  console.log(
+    "  slice",
+    JSON.stringify(s.slice(1, -1)),
+    JSON.stringify(s.slice(-3)),
+    JSON.stringify(s.substring(5, 2)),
+    JSON.stringify(s.slice(2, 2)),
+  );
+  console.log(
+    "  search",
+    s.indexOf("b"),
+    s.indexOf("b", 3),
+    s.lastIndexOf("b"),
+    s.indexOf(""),
+    s.indexOf("", 99),
+    s.startsWith("b", 1),
+    s.endsWith("b", 2),
+  );
+  console.log("  pad", JSON.stringify(s.padStart(s.length + 3, "-")), JSON.stringify(s.padEnd(s.length + 2, "\u00e9")));
+}
+
+// Literals: ASCII, BMP and astral.
+probe("lit-ascii", "abcabc");
+probe("lit-bmp", "ab\u00e9cb\u00e9");
+probe("lit-astral", "ab\u{1F600}cbd");
+
+// Concatenations of every pairing.
+const n = 7;
+const left = "ab" + String(n);
+const right = "\u00e9b" + String(n);
+probe("cat-aa", left + "cb");
+probe("cat-an", left + right);
+probe("cat-na", right + left);
+probe("cat-astral", left + "\u{1F600}b");
+
+// Uniquely owned strings appended in place after an ASCII read.
+let grow = "";
+for (let i = 0; i < 6; i++) grow += String.fromCharCode(97 + i);
+console.log("grow", grow.length, grow.charCodeAt(3), codes(grow));
+grow += "\u00e9";
+console.log("grow+bmp", grow.length, grow.charCodeAt(6), grow.charCodeAt(7), codes(grow));
+grow += "\u{1F600}b";
+console.log("grow+astral", grow.length, grow.charCodeAt(7), grow.charCodeAt(8), grow.charCodeAt(9), codes(grow));
+probe("grow", grow);
+
+let chain = "x";
+for (let i = 0; i < 40; i++) {
+  chain += i % 13 === 12 ? "\u03a9" : String(i % 10);
+  if (chain.charCodeAt(chain.length - 1) > 127) console.log("non-ascii at", chain.length - 1, chain.charCodeAt(chain.length - 1));
+}
+console.log("chain", chain.length, chain.indexOf("\u03a9"), chain.lastIndexOf("\u03a9"), chain.slice(10, 16));
+
+// Template literals mixing numbers and strings.
+const id = 42;
+const key = `${id},${id + 1}|${right}`;
+probe("template", key);
+
+// Slices: ASCII slices of non-ASCII strings and the reverse.
+const mixed = "\u00e9abcb\u00e9xyz\u{1F600}!";
+const inner = mixed.slice(1, 5);
+probe("slice-ascii-of-mixed", inner);
+probe("slice-mixed", mixed.slice(0, 7));
+probe("substring-astral", mixed.substring(7, 13));
+console.log("astral halves", mixed.charCodeAt(9), mixed.charCodeAt(10), mixed.slice(9, 11) === "\u{1F600}");
+
+// Numbers, repeat, join, split pieces.
+probe("number", String(-1234.5e-7) + "b");
+probe("repeat", "ab".repeat(4));
+probe("repeat-bmp", "a\u00e9".repeat(3));
+probe("join", ["ab", "cb", "d"].join(""));
+probe("join-bmp", ["ab", "\u00e9", "cb"].join("-"));
+const pieces = "abcdefgh,b\u00e9cdefgh,abcdefgb".split(",");
+for (const p of pieces) console.log("piece", p.length, p.charCodeAt(1), p.charCodeAt(p.length - 1), codes(p));
+
+// Builder-made strings (join, JSON.stringify) are proven ASCII by their
+// first read, then appended in place with non-ASCII text and read again.
+let built = ["ab", "cd", "ef", String(n)].join("");
+console.log("built", built.length, built.charCodeAt(4), built.indexOf("7"), built.slice(2, 5));
+built += "\u00e9" + built;
+console.log("built+bmp", built.length, built.charCodeAt(7), built.charCodeAt(8), built.lastIndexOf("e"), built.slice(5, 9));
+let json = JSON.stringify({ k: [1, 2, 3], s: "x" });
+console.log("json", json.length, json.charCodeAt(2), json.indexOf("s"));
+json += "\u{1F600}";
+console.log("json+astral", json.length, json.charCodeAt(json.length - 2), json.charCodeAt(json.length - 1), json.slice(-4));
+
+// Interleaved reads of several receivers (index cache residency).
+const a1 = "alpha\u00e9" + String(n);
+const a2 = "beta" + String(n);
+const a3 = "\u{1F600}gamma";
+let mix = 0;
+for (let i = 0; i < 8; i++) {
+  mix = mix * 31 + a1.charCodeAt(i % a1.length) + a2.charCodeAt(i % a2.length) * 3 + a3.charCodeAt(i % a3.length) * 7;
+  mix %= 1000003;
+}
+console.log("interleaved", mix);
diff --git a/tests/corpus/4477-buffer-write-ascii-runs.ts b/tests/corpus/4477-buffer-write-ascii-runs.ts
new file mode 100644
index 000000000..865338db6
--- /dev/null
+++ b/tests/corpus/4477-buffer-write-ascii-runs.ts
@@ -0,0 +1,55 @@
+// Buffer#write with utf16le, latin1 and ascii encodings: long ASCII runs,
+// runs broken by BMP and astral characters, writes truncated by an offset
+// and length budget (including odd budgets that split a UTF-16 unit or a
+// surrogate pair), and the Uint16Array view a scanner builds from text.
+
+function hex(buf: Buffer): string {
+  return buf.toString("hex");
+}
+
+const texts = [
+  "",
+  "abc",
+  "abcdefgh",
+  "abcdefghijklmnopq",
+  "const x = 1;\nconst y = 2;\n",
+  "abcdefg\u00e9hijklmnopqrstu",
+  "\u00e9abcdefghijklmnop",
+  "abcdefghijklmn\u{1F600}opqrstuvwxyz0123",
+  "\u4e2d\u6587abcdefghijklmnopqrstuvwxyz",
+];
+
+function show(enc: string, text: string, n: number, buf: Buffer): void {
+  console.log(enc, text.length, n, hex(buf.subarray(0, n)));
+}
+
+for (const text of texts) {
+  const size = text.length * 2 + 4;
+  const u16 = Buffer.alloc(size);
+  show("utf16le", text, u16.write(text, "utf16le"), u16);
+  const l1 = Buffer.alloc(size);
+  show("latin1", text, l1.write(text, "latin1"), l1);
+  const as = Buffer.alloc(size);
+  show("ascii", text, as.write(text, "ascii"), as);
+  for (const budget of [1, 3, 7, 15, 16, 17, 33]) {
+    const a = Buffer.alloc(40);
+    const b = Buffer.alloc(40);
+    const c = Buffer.alloc(40);
+    const wa = a.write(text, 2, budget, "utf16le");
+    const wb = b.write(text, 2, budget, "latin1");
+    const wc = c.write(text, 2, budget, "ascii");
+    console.log("  budget", budget, wa, hex(a.subarray(0, 2 + wa)), wb, hex(b.subarray(0, 2 + wb)), wc, hex(c.subarray(0, 2 + wc)));
+  }
+}
+
+function toCodeUnits(text: string): Uint16Array {
+  const chars = new Uint16Array(text.length + 4);
+  Buffer.from(chars.buffer).write(text, "utf16le");
+  return chars;
+}
+
+const source = "function f(a) {\n  return a + 1; // caf\u00e9 \u{1F600}\n}\n".repeat(40);
+const units = toCodeUnits(source);
+let ok = units.length === source.length + 4;
+for (let i = 0; i < source.length; i++) if (units[i] !== source.charCodeAt(i)) ok = false;
+console.log("code units", units.length, ok, units[source.length], units[source.length + 3]);
diff --git a/tests/corpus/4478-string-large-index-reads.ts b/tests/corpus/4478-string-large-index-reads.ts
new file mode 100644
index 000000000..0a85415f9
--- /dev/null
+++ b/tests/corpus/4478-string-large-index-reads.ts
@@ -0,0 +1,85 @@
+// Sequential and random UTF-16 reads over large strings (well past 64 KiB):
+// an all-ASCII text, a text with sparse non-ASCII and astral characters, a
+// line-start scan in the style of a compiler scanner, alternating scans of
+// two large receivers, and a uniquely owned large ASCII string that turns
+// non-ASCII through an in-place append. Every result must match Node.
+
+function lineStarts(text: string): number[] {
+  const result: number[] = [];
+  let lineStart = 0;
+  let pos = 0;
+  while (pos < text.length) {
+    const ch = text.charCodeAt(pos);
+    pos++;
+    if (ch === 13) {
+      if (pos < text.length && text.charCodeAt(pos) === 10) pos++;
+    } else if (ch !== 10 && ch !== 0x2028 && ch !== 0x2029) {
+      continue;
+    }
+    result.push(lineStart);
+    lineStart = pos;
+  }
+  result.push(lineStart);
+  return result;
+}
+
+function summary(label: string, text: string): void {
+  const starts = lineStarts(text);
+  let sum = 0;
+  for (let i = 0; i < text.length; i += 997) sum = (sum * 33 + text.charCodeAt(i)) % 1000000007;
+  let back = 0;
+  for (let i = text.length - 1; i >= 0; i -= 1009) back = (back * 31 + text.charCodeAt(i)) % 1000000007;
+  console.log(label, text.length, starts.length, starts[starts.length - 1], starts[1000] ?? -1, sum, back);
+}
+
+const lines: string[] = [];
+for (let i = 0; i < 6000; i++) lines.push(`const value${i} = compute(${i}, "text") + ${i * 3};`);
+const ascii = lines.join("\n");
+summary("ascii", ascii);
+
+const mixedLines: string[] = [];
+for (let i = 0; i < 6000; i++) {
+  const note = i % 97 === 0 ? " // caf\u00e9 \u{1F600}" : i % 501 === 0 ? "\u2028" : "";
+  mixedLines.push(`let item${i} = [${i}, ${i + 1}];${note}`);
+}
+const mixed = mixedLines.join(i_crlf());
+summary("mixed", mixed);
+
+function i_crlf(): string {
+  return "\r\n";
+}
+
+// Alternate between the two large receivers and a short one.
+let alt = 0;
+const short = "s\u00e9q";
+for (let i = 0; i < 20000; i += 7) {
+  alt = (alt * 7 + ascii.charCodeAt(i) + mixed.charCodeAt(i) * 3 + short.charCodeAt(i % 3)) % 1000000007;
+}
+console.log("alternate", alt);
+
+// Search and slice far into both.
+const far = mixed.indexOf("item5999");
+console.log("far", far, mixed.slice(far, far + 12), mixed.lastIndexOf("\u{1F600}"), mixed.lastIndexOf("caf\u00e9"));
+const astralAt = mixed.indexOf("\u{1F600}");
+console.log("astral", astralAt, mixed.charCodeAt(astralAt), mixed.charCodeAt(astralAt + 1), mixed.charAt(astralAt - 1));
+console.log("ascii far", ascii.indexOf("value5999"), ascii.slice(-20), ascii.substring(100000, 100010).length);
+
+// A large uniquely owned ASCII string read, then appended in place.
+let big = "";
+for (let i = 0; i < 9000; i++) big += "line" + String(i) + "\n";
+console.log("big", big.length, big.charCodeAt(70000), big.charCodeAt(big.length - 1));
+big += "\u00e9nd";
+console.log("big+bmp", big.length, big.charCodeAt(big.length - 3), big.charCodeAt(big.length - 1), big.slice(-6));
+big += "\u{1F600}";
+console.log("big+astral", big.length, big.charCodeAt(big.length - 2), big.charCodeAt(big.length - 1), big.indexOf("\u00e9"));
+summary("big", big);
+
+// A large builder-made string proven ASCII by its length, then appended in
+// place: the proof must not survive the append.
+const rows: number[][] = [];
+for (let i = 0; i < 5000; i++) rows.push([i, i * 2, i % 7]);
+let doc = JSON.stringify(rows);
+console.log("doc", doc.length, doc.charCodeAt(doc.length - 1), doc.indexOf("[4999"));
+doc += "\u00e9\u{1F600}";
+console.log("doc+", doc.length, doc.charCodeAt(doc.length - 3), doc.charCodeAt(doc.length - 2), doc.charCodeAt(doc.length - 1), doc.slice(-8));
+summary("doc", doc);