diff --git a/tests/harness/self-hosting-emission.test.ts b/tests/harness/self-hosting-emission.test.ts
index 2105d5b4e..d245c136d 100644
--- a/tests/harness/self-hosting-emission.test.ts
+++ b/tests/harness/self-hosting-emission.test.ts
@@ -1,14 +1,15 @@
-import { spawnSync } from "node:child_process";
 import { mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { expect, test } from "vitest";
-import { analyze, compile, compileC, deserializeModule, serializeModule } from "@scriptc/compiler";
+import { compile, compileC, deserializeModule, serializeModule } from "@scriptc/compiler";
 import { emitLlvmModule } from "../../packages/compiler/src/backend/llvm/emitter.js";
 import { scalarizeNumericRecords } from "../../packages/compiler/src/ir/scalar-records.js";
 import { everyStmtList } from "../../packages/compiler/src/ir/traverse.js";
 import { type IrModule } from "../../packages/compiler/src/ir/ir.js";
+import { analyzeInChild, compileInChild } from "./self-hosting-compiler-process.js";
+import { spawnResult } from "./spawn-result.js";
 import {
   moduleUsesCopying,
   moduleUsesDynInvoke,
@@ -53,8 +54,8 @@ interface Emission {
   block: string;
 }
 
-test("the production layout and metadata stage lowers statically", () => {
-  const { coverage } = analyze(entry, { dynamic: false });
+test("the production layout and metadata stage lowers statically", async () => {
+  const coverage = await analyzeInChild(entry, { dynamic: false });
   expect(coverage.preflightFailed).toBe(false);
   expect(coverage.diagnostics).toEqual([]);
   expect(coverage.stats.statementsTotal).toBeGreaterThan(1200);
@@ -72,7 +73,7 @@ for (const backend of ["llvm"] as const) {
     const executable = (name: string): string =>
       join(dir, name + (process.platform === "win32" ? ".exe" : ""));
     try {
-      const built = await compile(entry, {
+      const built = await compileInChild(entry, {
         outDir: dir,
         outPath: executable("stage"),
         backend,
@@ -83,14 +84,23 @@ for (const backend of ["llvm"] as const) {
       if (!built.ok)
         throw new Error(built.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
       expect(built.backend).toBe(backend);
-      const run = (mod: IrModule, request: EmissionRequest, name: string, status = 0): string => {
+      const run = async (
+        mod: IrModule,
+        request: EmissionRequest,
+        name: string,
+        status = 0,
+      ): Promise<string> => {
         const input = join(dir, "input.json");
         const config = join(dir, "config.json");
         writeFileSync(input, serializeModule(mod));
         writeFileSync(config, JSON.stringify(request));
         const args = [input, config];
-        const oracle = spawnSync(process.execPath, ["--import", "tsx", entry, ...args], options);
-        const native = spawnSync(built.binaryPath, args, options);
+        const oracle = await spawnResult(
+          process.execPath,
+          ["--import", "tsx", entry, ...args],
+          options,
+        );
+        const native = await spawnResult(built.binaryPath, args, options);
         for (const result of [oracle, native]) {
           expect(result.error, name).toBeUndefined();
           expect(result.signal, `${name}: ${result.stderr}`).toBeNull();
@@ -106,7 +116,7 @@ for (const backend of ["llvm"] as const) {
       };
 
       for (const item of emissionCases()) {
-        const text = run(item.module, item.request, item.name);
+        const text = await run(item.module, item.request, item.name);
         const emitted = JSON.parse(text) as Emission;
         const definitions = [
           ...emitted.declarations,
@@ -143,12 +153,12 @@ for (const backend of ["llvm"] as const) {
           loc: { file: invalid.sourceFile, start: 0, end: 0 },
         },
       ];
-      expect(run(invalid, emissionRequest(), "missing base", 1)).toContain(
+      expect(await run(invalid, emissionRequest(), "missing base", 1)).toContain(
         "undeclared base class Missing",
       );
       const absent = emissionRequest();
       absent.classObjects = ["Missing"];
-      expect(run(emissionModule(), absent, "missing class object", 1)).toContain(
+      expect(await run(emissionModule(), absent, "missing class object", 1)).toContain(
         "class object for unknown class Missing",
       );
 
@@ -176,7 +186,7 @@ for (const backend of ["llvm"] as const) {
         },
       ];
       for (const { request, message } of invalidRequests) {
-        expect(run(emissionModule(), request, message, 1)).toContain(message);
+        expect(await run(emissionModule(), request, message, 1)).toContain(message);
       }
       const unsupported = emissionModule();
       unsupported.unions = [
@@ -186,7 +196,7 @@ for (const backend of ["llvm"] as const) {
         },
       ];
       expect(
-        run(
+        await run(
           unsupported,
           { ...emissionRequest(), joinUnions: ["containers"] },
           "unsupported union join",
@@ -225,7 +235,7 @@ for (const backend of ["llvm"] as const) {
               return true;
             },
           });
-        const native = JSON.parse(run(mod, request, source)) as Emission;
+        const native = JSON.parse(await run(mod, request, source)) as Emission;
         // Splice the native-generated sections into the real program. The
         // full emitter still runs on Node; these type and helper definitions
         // are supplied by the compiled production stage under test.
@@ -256,8 +266,8 @@ for (const backend of ["llvm"] as const) {
           regex: moduleUsesRegex(mod),
           copying: moduleUsesCopying(mod),
         });
-        const oracle = spawnSync(process.execPath, [sourcePath], options);
-        const program = spawnSync(outPath, [], options);
+        const oracle = await spawnResult(process.execPath, [sourcePath], options);
+        const program = await spawnResult(outPath, [], options);
         for (const result of [oracle, program]) {
           expect(result.error, source).toBeUndefined();
           expect(result.signal, `${source}: ${result.stderr}`).toBeNull();
diff --git a/tests/harness/self-hosting-frontend-services.test.ts b/tests/harness/self-hosting-frontend-services.test.ts
index 8216c3aa0..d294f5cdd 100644
--- a/tests/harness/self-hosting-frontend-services.test.ts
+++ b/tests/harness/self-hosting-frontend-services.test.ts
@@ -1,12 +1,12 @@
-import { execFileSync, spawnSync } from "node:child_process";
 import { mkdirSync, mkdtempSync, readFileSync, realpathSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { analyze, compile } from "@scriptc/compiler";
 import { expect, test } from "vitest";
 import { ts7Executable } from "../../packages/compiler/src/frontend/ts7/rpc-api.js";
 
 import { npmFetchCases } from "../../packages/compiler/test/npm-fetch-cases.js";
+import { analyzeInChild, compileInChild } from "./self-hosting-compiler-process.js";
+import { spawnResult } from "./spawn-result.js";
 
 const root = join(import.meta.dirname, "../..");
 const entry = join(root, "tests/fixtures/self-hosting/frontend-services.ts");
@@ -69,17 +69,23 @@ for (const backend of ["llvm"] as const) {
     );
     try {
       const object = join(directory, "process.o");
-      execFileSync("clang", [
-        "-std=c11",
-        "-Wall",
-        "-Wextra",
-        "-Werror",
-        ...(sanitize ? ["-fsanitize=address"] : []),
-        "-c",
-        join(nativeSources, "ts7-process.c"),
-        "-o",
-        object,
-      ]);
+      const clang = await spawnResult(
+        "clang",
+        [
+          "-std=c11",
+          "-Wall",
+          "-Wextra",
+          "-Werror",
+          ...(sanitize ? ["-fsanitize=address"] : []),
+          "-c",
+          join(nativeSources, "ts7-process.c"),
+          "-o",
+          object,
+        ],
+        { cwd: root, timeout: 120_000 },
+      );
+      expect(clang.error).toBeUndefined();
+      expect(clang.status, clang.stderr.toString()).toBe(0);
       const profile = join(directory, "ffi.json");
       writeFileSync(
         profile,
@@ -88,13 +94,13 @@ for (const backend of ["llvm"] as const) {
           libraries: [object],
         }),
       );
-      const { coverage } = analyze(entry, { dynamic: false, ffiProfilePath: profile });
+      const coverage = await analyzeInChild(entry, { dynamic: false, ffiProfilePath: profile });
       expect(coverage.preflightFailed, JSON.stringify(coverage.diagnostics)).toBe(false);
       expect(coverage.diagnostics).toEqual([]);
       expect(coverage.stats.statementsFailed).toBe(0);
       expect(coverage.stats.statementsIsland).toBe(0);
       expect(coverage.stats.functionsSkipped).toBe(0);
-      const built = await compile(entry, {
+      const built = await compileInChild(entry, {
         backend,
         dynamic: false,
         optimization: "dev",
@@ -108,25 +114,25 @@ for (const backend of ["llvm"] as const) {
       const request = join(directory, "request.json");
       writeFileSync(request, JSON.stringify(input(directory)));
       const expected = join(directory, "node.json");
-      const node = spawnSync(
+      const node = await spawnResult(
         process.execPath,
         ["--import", "tsx", oracle, ts7Executable(), request, expected],
-        { encoding: "utf8", timeout: 45_000 },
+        { cwd: root, timeout: 45_000 },
       );
-      expect(node.error, node.stderr).toBeUndefined();
-      expect(node.status, node.stderr).toBe(0);
-      expect(node.stdout).toBe("");
-      expect(node.stderr).toBe("");
+      expect(node.error, node.stderr.toString()).toBeUndefined();
+      expect(node.status, node.stderr.toString()).toBe(0);
+      expect(node.stdout.toString()).toBe("");
+      expect(node.stderr.toString()).toBe("");
       const report = join(directory, "native.json");
-      const run = spawnSync(built.binaryPath, [ts7Executable(), request, report], {
-        encoding: "utf8",
+      const run = await spawnResult(built.binaryPath, [ts7Executable(), request, report], {
+        cwd: root,
         timeout: 45_000,
       });
-      expect(run.error, run.stderr).toBeUndefined();
-      expect(run.signal, run.stderr).toBeNull();
-      expect(run.status, run.stderr).toBe(0);
-      expect(run.stdout).toBe(node.stdout);
-      expect(run.stderr).toBe(node.stderr);
+      expect(run.error, run.stderr.toString()).toBeUndefined();
+      expect(run.signal, run.stderr.toString()).toBeNull();
+      expect(run.status, run.stderr.toString()).toBe(0);
+      expect(run.stdout.toString()).toBe(node.stdout.toString());
+      expect(run.stderr.toString()).toBe(node.stderr.toString());
       const result = JSON.parse(readFileSync(report, "utf8"));
       expect(result).toEqual(JSON.parse(readFileSync(expected, "utf8")));
       expect(result.fetch).toEqual(
diff --git a/tests/harness/self-hosting-native-toolchain.test.ts b/tests/harness/self-hosting-native-toolchain.test.ts
index 43a230682..b00a46da7 100644
--- a/tests/harness/self-hosting-native-toolchain.test.ts
+++ b/tests/harness/self-hosting-native-toolchain.test.ts
@@ -1,10 +1,10 @@
-import { spawnSync } from "node:child_process";
 import { createHash } from "node:crypto";
 import { existsSync, mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { expect, test } from "vitest";
 import { analyzeInChild, compileInChild } from "./self-hosting-compiler-process.js";
+import { spawnResult } from "./spawn-result.js";
 import { nativeCodegenTarget } from "../../packages/compiler/src/backend/targets.js";
 import type { NativeLinkFeatures } from "../../packages/compiler/src/backend/native-link-info.js";
 import type { RuntimePackManifest } from "../../packages/compiler/src/backend/runtime-pack-core.js";
@@ -143,7 +143,7 @@ for (const backend of ["llvm"] as const) {
         default_target: target.helper.defaultTarget,
         data_layout: target.helper.defaultDataLayout,
       };
-      const check = (
+      const check = async (
         status: number,
         message: string,
         action = "stage",
@@ -153,11 +153,11 @@ for (const backend of ["llvm"] as const) {
           input,
           JSON.stringify({ toolchain, stage, features, action, version: helperVersion }),
         );
-        const options = { cwd: root, timeout: 30_000, maxBuffer: 1024 * 1024 };
+        const options = { cwd: root, timeout: 30_000 };
         const results = [];
         for (const native of [false, true]) {
           rmSync(stage, { recursive: true, force: true });
-          const result = spawnSync(
+          const result = await spawnResult(
             native ? built.binaryPath : process.execPath,
             native ? [input] : ["--import", "tsx", entry, input],
             native ? { ...options, env: { ...process.env, PATH: "" } } : options,
@@ -171,24 +171,24 @@ for (const backend of ["llvm"] as const) {
         expect(results[1]!.stdout).toEqual(results[0]!.stdout);
         expect(results[1]!.stderr).toEqual(results[0]!.stderr);
       };
-      check(0, "base.o");
+      await check(0, "base.o");
       expect(readFileSync(join(stage, artifact.path))).toEqual(bytes);
       // A later replacement of the installed input cannot change staged bytes.
       writeFileSync(join(pack, artifact.path), Buffer.from("changed"));
       expect(readFileSync(join(stage, artifact.path))).toEqual(bytes);
-      check(1, "hash mismatch");
+      await check(1, "hash mismatch");
       expect(existsSync(join(stage, artifact.path))).toBe(false);
       writeFileSync(join(pack, artifact.path), bytes);
       rmSync(join(pack, "license.txt"));
-      check(1, "license payload is incomplete");
+      await check(1, "license payload is incomplete");
       writeFileSync(join(pack, "license.txt"), "license");
       rmSync(join(pack, artifact.path));
-      check(1, "artifact is missing");
+      await check(1, "artifact is missing");
       writeFileSync(join(pack, artifact.path), bytes);
       writeFileSync(toolchain, JSON.stringify({ ...config, compiler_version: "other" }));
-      check(1, "version mismatch");
+      await check(1, "version mismatch");
       writeFileSync(toolchain, JSON.stringify(config));
-      check(0, "1 1.2.3 22.1.8", "helper");
+      await check(0, "1 1.2.3 22.1.8", "helper");
       for (const [field, value] of Object.entries({
         protocol_version: "2",
         llvm_version: "0",
@@ -199,7 +199,7 @@ for (const backend of ["llvm"] as const) {
         data_layout: "other",
         host_triple: null,
       })) {
-        check(1, field, "helper", { ...version, [field]: value });
+        await check(1, field, "helper", { ...version, [field]: value });
       }
     } finally {
       rmSync(directory, { recursive: true, force: true });
diff --git a/tests/harness/spawn-result.ts b/tests/harness/spawn-result.ts
new file mode 100644
index 000000000..3de656fe6
--- /dev/null
+++ b/tests/harness/spawn-result.ts
@@ -0,0 +1,44 @@
+import { spawn } from "node:child_process";
+
+export interface SpawnResult {
+  error: Error | undefined;
+  signal: NodeJS.Signals | null;
+  status: number | null;
+  stdout: Buffer;
+  stderr: Buffer;
+}
+
+/** spawnSync's result without blocking the worker: a test that runs child
+ * processes back to back with spawnSync starves Vitest's worker RPC, which
+ * fails the run with an unhandled timeout even when every test passes. */
+export function spawnResult(
+  file: string,
+  args: readonly string[],
+  options: { cwd: string; timeout: number; env?: NodeJS.ProcessEnv },
+): Promise<SpawnResult> {
+  return new Promise((resolve) => {
+    const stdout: Buffer[] = [];
+    const stderr: Buffer[] = [];
+    let error: Error | undefined;
+    const child = spawn(file, args, {
+      cwd: options.cwd,
+      env: options.env,
+      timeout: options.timeout,
+      stdio: ["ignore", "pipe", "pipe"],
+    });
+    child.stdout.on("data", (chunk: Buffer) => stdout.push(chunk));
+    child.stderr.on("data", (chunk: Buffer) => stderr.push(chunk));
+    child.on("error", (cause) => {
+      error = cause;
+    });
+    child.on("close", (status, signal) =>
+      resolve({
+        error,
+        signal,
+        status,
+        stdout: Buffer.concat(stdout),
+        stderr: Buffer.concat(stderr),
+      }),
+    );
+  });
+}