diff --git a/plugins/devin/scripts/lib/fs.mjs b/plugins/devin/scripts/lib/fs.mjs
index 237eb01..e4267a7 100644
--- a/plugins/devin/scripts/lib/fs.mjs
+++ b/plugins/devin/scripts/lib/fs.mjs
@@ -34,9 +34,63 @@ export function isProbablyText(buffer) {
   return true;
 }
 
-export function readStdinIfPiped() {
-  if (process.stdin.isTTY) {
+// Prompts are text typed or piped by a person or a script; 8 MiB is far above any
+// realistic prompt and still small enough to hold in memory safely.
+export const MAX_STDIN_BYTES = 8 * 1024 * 1024;
+const STDIN_CHUNK_BYTES = 64 * 1024;
+
+function isNoDataError(e) {
+  return e.code === "EAGAIN" || e.code === "ENXIO";
+}
+
+export function readStdinIfPiped(deps = {}) {
+  const {
+    fstatImpl = fs.fstatSync,
+    readSyncImpl = fs.readSync,
+    isTTY = process.stdin.isTTY,
+    chunkSize = STDIN_CHUNK_BYTES,
+    maxBytes = MAX_STDIN_BYTES
+  } = deps;
+  if (isTTY) {
     return "";
   }
-  return fs.readFileSync(0, "utf8");
+  try {
+    const stats = fstatImpl(0);
+    if (!stats.isFIFO() && !stats.isFile()) {
+      return "";
+    }
+  } catch (e) {
+    if (isNoDataError(e)) {
+      return "";
+    }
+    throw e;
+  }
+  // Read incrementally so bytes already received survive an EAGAIN from a
+  // non-blocking FIFO whose writer is still open.
+  const chunks = [];
+  let total = 0;
+  for (;;) {
+    // Ask for one byte past the cap so overflow is detected without truncating.
+    const length = Math.min(chunkSize, maxBytes + 1 - total);
+    const buffer = Buffer.alloc(length);
+    let bytesRead;
+    try {
+      bytesRead = readSyncImpl(0, buffer, 0, length, null);
+    } catch (e) {
+      if (isNoDataError(e)) {
+        break;
+      }
+      throw e;
+    }
+    if (bytesRead === 0) {
+      break;
+    }
+    total += bytesRead;
+    if (total > maxBytes) {
+      throw new Error(`stdin input exceeds the ${maxBytes} byte limit.`);
+    }
+    chunks.push(buffer.subarray(0, bytesRead));
+  }
+  // Decode once so multi-byte characters split across chunks stay intact.
+  return Buffer.concat(chunks).toString("utf8");
 }
diff --git a/tests/fs.test.mjs b/tests/fs.test.mjs
new file mode 100644
index 0000000..fe4268b
--- /dev/null
+++ b/tests/fs.test.mjs
@@ -0,0 +1,236 @@
+import assert from "node:assert/strict";
+import { execFileSync, spawnSync } from "node:child_process";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+
+import { MAX_STDIN_BYTES, readStdinIfPiped } from "../plugins/devin/scripts/lib/fs.mjs";
+
+const FS_MODULE = fileURLToPath(new URL("../plugins/devin/scripts/lib/fs.mjs", import.meta.url));
+
+test("readStdinIfPiped returns empty string when isTTY is true", () => {
+  const result = readStdinIfPiped({ isTTY: true });
+  assert.equal(result, "");
+});
+
+const FIFO_STATS = { isFIFO: () => true, isFile: () => false };
+const FILE_STATS = { isFIFO: () => false, isFile: () => true };
+
+function errnoError(code, message) {
+  const error = new Error(`${code}: ${message}`);
+  error.code = code;
+  return error;
+}
+
+// Fake readSync that serves the given steps in order: a Buffer or string is
+// copied into the caller's buffer (at most `length` bytes, the rest is kept for
+// the next call), an Error is thrown, and an exhausted script reports EOF.
+function scriptedRead(steps) {
+  const queue = steps.map((step) => (typeof step === "string" ? Buffer.from(step) : step));
+  return (fd, buffer, offset, length) => {
+    assert.equal(fd, 0);
+    if (queue.length === 0) {
+      return 0;
+    }
+    const step = queue.shift();
+    if (step instanceof Error) {
+      throw step;
+    }
+    const n = Math.min(length, step.length);
+    step.copy(buffer, offset, 0, n);
+    if (n < step.length) {
+      queue.unshift(step.subarray(n));
+    }
+    return n;
+  };
+}
+
+test("readStdinIfPiped reads content from FIFO", () => {
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => FIFO_STATS,
+    readSyncImpl: scriptedRead(["hello from pipe"])
+  });
+  assert.equal(result, "hello from pipe");
+});
+
+test("readStdinIfPiped reads content from regular file", () => {
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => FILE_STATS,
+    readSyncImpl: scriptedRead(["hello from file"])
+  });
+  assert.equal(result, "hello from file");
+});
+
+test("readStdinIfPiped returns empty string for non-FIFO/non-file (socket)", () => {
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => ({ isFIFO: () => false, isFile: () => false }),
+    readSyncImpl: () => { throw new Error("should not be called"); }
+  });
+  assert.equal(result, "");
+});
+
+for (const code of ["EAGAIN", "ENXIO"]) {
+  test(`readStdinIfPiped returns empty string when fstat throws ${code}`, () => {
+    const result = readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => { throw errnoError(code, "no data"); },
+      readSyncImpl: () => { throw new Error("should not be called"); }
+    });
+    assert.equal(result, "");
+  });
+
+  test(`readStdinIfPiped returns empty string when the first read throws ${code}`, () => {
+    const result = readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => FIFO_STATS,
+      readSyncImpl: scriptedRead([errnoError(code, "no data")])
+    });
+    assert.equal(result, "");
+  });
+
+  test(`readStdinIfPiped keeps bytes already read when a later read throws ${code}`, () => {
+    const result = readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => FIFO_STATS,
+      readSyncImpl: scriptedRead(["PROMPT ", "DATA\n", errnoError(code, "no more data yet")])
+    });
+    assert.equal(result, "PROMPT DATA\n");
+  });
+}
+
+test("readStdinIfPiped throws non-EAGAIN/ENXIO errors from fstat", () => {
+  assert.throws(
+    () => readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => { throw errnoError("EPERM", "operation not permitted"); },
+      readSyncImpl: () => { throw new Error("should not be called"); }
+    }),
+    { code: "EPERM" }
+  );
+});
+
+test("readStdinIfPiped throws non-EAGAIN/ENXIO errors from read, even after data", () => {
+  assert.throws(
+    () => readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => FIFO_STATS,
+      readSyncImpl: scriptedRead(["partial", errnoError("EACCES", "permission denied")])
+    }),
+    { code: "EACCES" }
+  );
+});
+
+test("readStdinIfPiped joins multiple chunks and honors chunkSize", () => {
+  const lengths = [];
+  const inner = scriptedRead(["0123456789abcdefghij"]);
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => FIFO_STATS,
+    chunkSize: 6,
+    readSyncImpl: (fd, buffer, offset, length, position) => {
+      lengths.push(length);
+      return inner(fd, buffer, offset, length, position);
+    }
+  });
+  assert.equal(result, "0123456789abcdefghij");
+  assert.ok(lengths.every((length) => length <= 6), `unexpected read sizes: ${lengths}`);
+  assert.ok(lengths.length >= 4);
+});
+
+test("readStdinIfPiped decodes multi-byte UTF-8 split across chunks", () => {
+  const text = "h\u00e9llo \u2713 \u{1F600}\n";
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => FIFO_STATS,
+    chunkSize: 1,
+    readSyncImpl: scriptedRead([text])
+  });
+  assert.equal(result, text);
+});
+
+test("readStdinIfPiped accepts input of exactly maxBytes", () => {
+  const result = readStdinIfPiped({
+    isTTY: false,
+    fstatImpl: () => FIFO_STATS,
+    chunkSize: 4,
+    maxBytes: 10,
+    readSyncImpl: scriptedRead(["0123456789"])
+  });
+  assert.equal(result, "0123456789");
+});
+
+test("readStdinIfPiped throws a clear error when input exceeds maxBytes", () => {
+  assert.throws(
+    () => readStdinIfPiped({
+      isTTY: false,
+      fstatImpl: () => FIFO_STATS,
+      chunkSize: 4,
+      maxBytes: 10,
+      readSyncImpl: scriptedRead(["0123456789X"])
+    }),
+    /stdin input exceeds the 10 byte limit/
+  );
+});
+
+test("MAX_STDIN_BYTES is a positive default cap", () => {
+  assert.ok(Number.isInteger(MAX_STDIN_BYTES) && MAX_STDIN_BYTES >= 1024 * 1024);
+});
+
+// End-to-end tests without mocks: fd 0 is the read end of a FIFO whose
+// O_NONBLOCK flag is set and whose writer stays open, which is what the Claude
+// Code Bash tool hands to child processes. `payload` is written before the
+// child starts; `options` is the argument passed to readStdinIfPiped.
+function readFromOpenFifo(payload, options = {}) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "devin-stdin-"));
+  const fifo = path.join(dir, "stdin");
+  let readFd;
+  let writeFd;
+  try {
+    execFileSync("mkfifo", [fifo]);
+    readFd = fs.openSync(fifo, fs.constants.O_RDONLY | fs.constants.O_NONBLOCK);
+    writeFd = fs.openSync(fifo, fs.constants.O_WRONLY | fs.constants.O_NONBLOCK);
+    if (payload !== undefined) {
+      const bytes = Buffer.from(payload, "utf8");
+      assert.equal(fs.writeSync(writeFd, bytes), bytes.length);
+    }
+    const script = `import(${JSON.stringify(FS_MODULE)}).then((m) => process.stdout.write(JSON.stringify(m.readStdinIfPiped(${JSON.stringify(options)}))));`;
+    const result = spawnSync(process.execPath, ["-e", script], {
+      stdio: [readFd, "pipe", "pipe"],
+      encoding: "utf8",
+      timeout: 10000
+    });
+    assert.equal(result.status, 0, result.stderr);
+    return JSON.parse(result.stdout);
+  } finally {
+    if (readFd !== undefined) fs.closeSync(readFd);
+    if (writeFd !== undefined) fs.closeSync(writeFd);
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+}
+
+const skipWin32 = { skip: process.platform === "win32" };
+
+test("readStdinIfPiped returns empty string for a non-blocking FIFO with no data", skipWin32, () => {
+  assert.equal(readFromOpenFifo(undefined), "");
+});
+
+test("readStdinIfPiped returns bytes already in a non-blocking FIFO whose writer is still open", skipWin32, () => {
+  assert.equal(readFromOpenFifo("PROMPT DATA\n"), "PROMPT DATA\n");
+});
+
+test("readStdinIfPiped returns multi-byte UTF-8 from an open non-blocking FIFO", skipWin32, () => {
+  assert.equal(readFromOpenFifo("h\u00e9llo \u2713\n"), "h\u00e9llo \u2713\n");
+});
+
+test("readStdinIfPiped reads a payload spanning several chunks from an open non-blocking FIFO", skipWin32, () => {
+  // 7-byte chunks split the 2- and 3-byte characters at awkward offsets; the
+  // payload stays far below the OS pipe buffer so the writer never hits EAGAIN.
+  const payload = `${"line of text \u2713 h\u00e9llo\n".repeat(8)}end\n`;
+  assert.ok(Buffer.byteLength(payload) > 150 && Buffer.byteLength(payload) < 1024);
+  assert.equal(readFromOpenFifo(payload, { chunkSize: 7 }), payload);
+});