diff --git a/scripts/quality-gate/coverage.test.ts b/scripts/quality-gate/coverage.test.ts
index 95501fab..82b0382a 100644
--- a/scripts/quality-gate/coverage.test.ts
+++ b/scripts/quality-gate/coverage.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, test } from 'bun:test'
-import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { createHash } from 'node:crypto'
+import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import {
@@ -13,6 +14,7 @@ import {
   parseChangedLinesFromDiff,
   parseLcov,
   prefixRelativeLcovSourcePaths,
+  runCommand,
 } from './coverage'
 
 describe('coverage gate helpers', () => {
@@ -403,3 +405,52 @@ describe('coverage gate helpers', () => {
     expect(failures).toEqual(['new-suite: branches coverage 80% is below minimum 85%'])
   })
 })
+
+describe('coverage subprocess output', () => {
+  test('captures large synchronous reports to regular files without losing artifacts or exit status', async () => {
+    const root = mkdtempSync(join(tmpdir(), 'cc-haha-coverage-output-'))
+    const script = join(root, 'reporter.ts')
+    const logPath = join(root, 'logs', 'coverage.log')
+    const summary = '\nRan 1 test across 1 file. [1.00ms]\n'
+    const lcov = 'SF:src/example.ts\nDA:1,1\nLF:1\nLH:1\nend_of_record\n'
+    const rounds = 128
+    const chunkSize = 64 * 1024
+    try {
+      writeFileSync(script, `
+        import { fstatSync, writeSync, writeFileSync } from 'node:fs'
+        // A synchronous reporter must not depend on a nonblocking pipe's
+        // available buffer capacity. Assert the child receives disk-backed FDs.
+        if (!fstatSync(1).isFile() || !fstatSync(2).isFile()) {
+          throw new Error('synchronous coverage reporter requires regular output files')
+        }
+        const stdoutChunk = Buffer.alloc(${chunkSize}, 'o')
+        const stderrChunk = Buffer.alloc(${chunkSize}, 'e')
+        for (let round = 0; round < ${rounds}; round++) {
+          writeSync(1, stdoutChunk)
+          writeSync(2, stderrChunk)
+        }
+        writeFileSync('lcov.info', ${JSON.stringify(lcov)})
+        writeSync(2, ${JSON.stringify(summary)})
+        process.exit(7)
+      `)
+      const command = [process.execPath, '--no-env-file', script]
+      const result = await runCommand(command, root, logPath)
+      expect(result.exitCode).toBe(7)
+      expect(parseBunTestFileCount(result.output)).toBe(1)
+      expect(result.output.length).toBe(rounds * chunkSize * 2 + summary.length)
+      const expected = createHash('sha256')
+      for (let round = 0; round < rounds; round++) {
+        expected.update(Buffer.alloc(chunkSize, 'o'))
+        expected.update(Buffer.alloc(chunkSize, 'e'))
+      }
+      expected.update(summary)
+      expect(createHash('sha256').update(result.output).digest('hex'))
+        .toBe(expected.digest('hex'))
+      expect(readFileSync(logPath, 'utf8').slice(`$ ${command.join(' ')}\n`.length))
+        .toBe(result.output)
+      expect(hasUsableLcov(readFileSync(join(root, 'lcov.info'), 'utf8'))).toBe(true)
+    } finally {
+      rmSync(root, { recursive: true, force: true })
+    }
+  })
+})
diff --git a/scripts/quality-gate/coverage.ts b/scripts/quality-gate/coverage.ts
index 0123a345..164e930e 100644
--- a/scripts/quality-gate/coverage.ts
+++ b/scripts/quality-gate/coverage.ts
@@ -1,6 +1,6 @@
 #!/usr/bin/env bun
 
-import { existsSync, mkdirSync, mkdtempSync, readFileSync, readdirSync, rmSync, statSync, writeFileSync } from 'node:fs'
+import { closeSync, existsSync, mkdirSync, mkdtempSync, openSync, readFileSync, readdirSync, rmSync, statSync, writeFileSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { dirname, join, relative, sep, win32 } from 'node:path'
 import { rootBunTestFilter } from '../pr/bun-test-filter'
@@ -448,29 +448,35 @@ export function hasUsableCoverageSummary(summary: CoverageSummary) {
   return Object.values(summary).some((coverage) => coverage.total > 0)
 }
 
-async function runCommand(command: string[], cwd: string, logPath: string) {
+export async function runCommand(command: string[], cwd: string, logPath: string) {
   const started = Date.now()
   const sandboxHome = mkdtempSync(join(tmpdir(), 'cc-haha-coverage-test-'))
+  const header = `$ ${command.join(' ')}\n`
+  let logFd: number | undefined
   try {
+    mkdirSync(dirname(logPath), { recursive: true })
+    logFd = openSync(logPath, 'w')
+    writeFileSync(logFd, header)
+    // Bun's synchronous text coverage reporter can fail while writing a large
+    // report to a pipe, before it emits LCOV or its test-count summary. Give
+    // both streams a regular file descriptor, with no pipe buffer to exhaust.
     const proc = Bun.spawn(command, {
       cwd,
       env: createSandboxedTestEnvironment(sandboxHome),
-      stdout: 'pipe',
-      stderr: 'pipe',
+      stdout: logFd,
+      stderr: logFd,
     })
-    const [stdout, stderr, exitCode] = await Promise.all([
-      new Response(proc.stdout).text(),
-      new Response(proc.stderr).text(),
-      proc.exited,
-    ])
-    mkdirSync(dirname(logPath), { recursive: true })
-    writeFileSync(logPath, `$ ${command.join(' ')}\n${stdout}${stderr}`)
+    const exitCode = await proc.exited
+    closeSync(logFd)
+    logFd = undefined
+    const output = readFileSync(logPath, 'utf8').slice(header.length)
     return {
       exitCode,
       durationMs: Date.now() - started,
-      output: `${stdout}${stderr}`,
+      output,
     }
   } finally {
+    if (logFd !== undefined) closeSync(logFd)
     rmSync(sandboxHome, { recursive: true, force: true })
   }
 }