diff --git a/packages/adapter-utils/src/server-utils.test.ts b/packages/adapter-utils/src/server-utils.test.ts
index 2735bb5f907..1aafda0e3ed 100644
--- a/packages/adapter-utils/src/server-utils.test.ts
+++ b/packages/adapter-utils/src/server-utils.test.ts
@@ -1,8 +1,10 @@
-import { spawn } from "node:child_process";
+import { execFile as execFileCallback, spawn } from "node:child_process";
 import { randomUUID } from "node:crypto";
 import fs from "node:fs/promises";
 import os from "node:os";
 import path from "node:path";
+import { pathToFileURL } from "node:url";
+import { promisify } from "node:util";
 import { describe, expect, it } from "vitest";
 import { CONNECTION_INTENT_AGENT_GUIDANCE } from "@paperclipai/shared";
 import {
@@ -44,6 +46,8 @@ import {
   WATCHDOG_DEFAULT_MANDATE,
 } from "./server-utils.js";
 
+const execFile = promisify(execFileCallback);
+
 it("keeps Pi draft models empty and preserves saved native Pi settings", () => {
   expect(normalizePaperclipRunnerAdapterConfig("paperclip_runner", { provider: "acpx", acpxAgent: "pi", model: "" })).toMatchObject({ provider: "acpx", acpxAgent: "pi", model: "" });
   const saved = { provider: "acpx", acpxAgent: "pi", model: "openrouter/provider/model", piThinkingLevel: "max", acpxPermissionMode: "approve-reads", lifecycleMode: "warm" };
@@ -653,6 +657,86 @@ describe("adapter skill snapshots", () => {
 });
 
 describe("runChildProcess", () => {
+  it("does not crash when the child closes stdin before a delayed write", async () => {
+    const tempDir = await fs.mkdtemp(
+      path.join(os.tmpdir(), "paperclip-stdin-epipe-"),
+    );
+    const harnessPath = path.join(tempDir, "stdin-epipe-harness.ts");
+    const stdinClosedMarkerPath = path.join(tempDir, "stdin-closed");
+    const releaseChildMarkerPath = path.join(tempDir, "release-child");
+    const serverUtilsUrl = pathToFileURL(
+      path.join(import.meta.dirname, "server-utils.ts"),
+    ).href;
+    const tsxCli = path.resolve(
+      import.meta.dirname,
+      "../../../cli/node_modules/tsx/dist/cli.mjs",
+    );
+
+    try {
+      await fs.writeFile(
+        harnessPath,
+        [
+          "import fs from 'node:fs/promises';",
+          `import { runChildProcess } from ${JSON.stringify(serverUtilsUrl)};`,
+          `const stdinClosedMarkerPath = ${JSON.stringify(stdinClosedMarkerPath)};`,
+          `const releaseChildMarkerPath = ${JSON.stringify(releaseChildMarkerPath)};`,
+          "void (async () => {",
+          "const result = await runChildProcess(",
+          "  'stdin-epipe-regression',",
+          "  process.execPath,",
+          "  [",
+          "    '-e',",
+          "    \"const fs = require('node:fs'); fs.closeSync(0); fs.writeFileSync(process.argv[1], 'closed'); const deadline = Date.now() + 3000; const timer = setInterval(() => { if (fs.existsSync(process.argv[2])) { clearInterval(timer); process.exit(23); } if (Date.now() >= deadline) { clearInterval(timer); process.exit(24); } }, 10);\",",
+          "    stdinClosedMarkerPath,",
+          "    releaseChildMarkerPath,",
+          "  ],",
+          "  {",
+          "    cwd: process.cwd(),",
+          "    env: {},",
+          "    stdin: 'payload after child closed stdin',",
+          "    timeoutSec: 5,",
+          "    graceSec: 1,",
+          "    onLog: async () => {},",
+          "    onSpawn: async () => {",
+          "      const deadline = Date.now() + 2_000;",
+          "      while (Date.now() < deadline) {",
+          "        try {",
+          "          await fs.access(stdinClosedMarkerPath);",
+          "          setTimeout(() => {",
+          "            void fs.writeFile(releaseChildMarkerPath, 'release');",
+          "          }, 100);",
+          "          return;",
+          "        } catch {",
+          "          await new Promise((resolve) => setTimeout(resolve, 10));",
+          "        }",
+          "      }",
+          "      throw new Error('child did not confirm closing stdin');",
+          "    },",
+          "  },",
+          ");",
+          "if (result.exitCode !== 23) {",
+          "  throw new Error(`unexpected exit code: ${result.exitCode}`);",
+          "}",
+          "process.stdout.write(JSON.stringify({ exitCode: result.exitCode, timedOut: result.timedOut }));",
+          "})().catch((error) => {",
+          "  console.error(error);",
+          "  process.exitCode = 1;",
+          "});",
+        ].join("\n"),
+        "utf8",
+      );
+
+      const completed = await execFile(process.execPath, [tsxCli, harnessPath], {
+        cwd: process.cwd(),
+        timeout: 10_000,
+      });
+      expect(JSON.parse(completed.stdout)).toEqual({ exitCode: 23, timedOut: false });
+      expect(completed.stderr).not.toMatch(/Unhandled 'error' event|write EPIPE/);
+    } finally {
+      await fs.rm(tempDir, { recursive: true, force: true });
+    }
+  });
+
   it("does not arm a timeout when timeoutSec is 0", async () => {
     const result = await runChildProcess(
       randomUUID(),
diff --git a/packages/adapter-utils/src/server-utils.ts b/packages/adapter-utils/src/server-utils.ts
index d69258d5537..7746040ec8e 100644
--- a/packages/adapter-utils/src/server-utils.ts
+++ b/packages/adapter-utils/src/server-utils.ts
@@ -4933,10 +4933,30 @@ export async function runChildProcess(
 
         const stdin = child.stdin;
         if (opts.stdin != null && stdin) {
-          void spawnPersistPromise.finally(() => {
-            if (child.killed || stdin.destroyed) return;
-            stdin.write(opts.stdin as string);
-            stdin.end();
+          let stdinFailureHandled = false;
+          const handleStdinFailure = (err: unknown) => {
+            if (!err || stdinFailureHandled) return;
+            stdinFailureHandled = true;
+            const code = (err as NodeJS.ErrnoException).code;
+            if (code === "EPIPE" || code === "ERR_STREAM_DESTROYED") return;
+            onLogError(err, runId, "failed to write child process stdin");
+          };
+
+          stdin.on("error", handleStdinFailure);
+          void spawnPersistPromise.then(() => {
+            if (
+              child.exitCode !== null ||
+              child.signalCode !== null ||
+              child.killed ||
+              stdin.destroyed
+            )
+              return;
+            try {
+              stdin.write(opts.stdin as string, handleStdinFailure);
+              stdin.end();
+            } catch (err) {
+              handleStdinFailure(err);
+            }
           });
         }