diff --git a/packages/harness/src/first-output.mjs b/packages/harness/src/first-output.mjs
index 76c124dc6..a64807ac2 100644
--- a/packages/harness/src/first-output.mjs
+++ b/packages/harness/src/first-output.mjs
@@ -96,7 +96,8 @@ if (process.argv[1] && resolve(process.argv[1]) === fileURLToPath(import.meta.ur
     if (bad) { process.stderr.write('model event stream failed\n'); process.exitCode = 1; return; }
     if (signal) {
       process.stderr.write(`model child killed by ${signal}\n`);
-      process.exitCode = 128 + (constants.signals[signal] ?? 0);
+      const number = constants.signals[signal];
+      process.exitCode = Number.isInteger(number) && number > 0 ? 128 + number : 1;
       return;
     }
     if (code !== 0) { process.exitCode = code ?? 1; return; }
diff --git a/packages/harness/tests/first-output.test.mjs b/packages/harness/tests/first-output.test.mjs
index 6a8c83103..a28a7349c 100644
--- a/packages/harness/tests/first-output.test.mjs
+++ b/packages/harness/tests/first-output.test.mjs
@@ -3,6 +3,7 @@ import assert from 'node:assert/strict';
 import fs from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+import { spawn } from 'node:child_process';
 import { firstOutputMarker } from '../src/first-output.mjs';
 test('stdout marker contains a fixed token only, never output text; emitted once after nonempty output', t => {
   const root = fs.mkdtempSync(join(tmpdir(), 'p2-output-')); t.after(() => fs.rmSync(root, { recursive: true, force: true }));
@@ -14,3 +15,23 @@ test('stdout marker contains a fixed token only, never output text; emitted once
   observe('other output'); assert.equal(fs.statSync(file).mtimeMs, before);
   assert.equal(fs.readFileSync(file, 'utf8').includes('fx331-private-output'), false);
 });
+
+// A signalled model child keeps the shell status 128+n; a signal name with no known number fails 1.
+test('a signalled model child exits 128 plus its signal number, an unmapped signal exits 1', async t => {
+  const root = fs.mkdtempSync(join(tmpdir(), 'p2-signal-')); t.after(() => fs.rmSync(root, { recursive: true, force: true }));
+  const fake = join(root, 'fake-spawn.cjs');
+  fs.writeFileSync(fake, "const cp = require('node:child_process'); const { EventEmitter } = require('node:events');\n"
+    + "cp.spawn = () => { const child = new EventEmitter(); child.stdout = new EventEmitter(); child.stdout.setEncoding = () => {}; child.kill = () => {};\n"
+    + "  setImmediate(() => child.emit('close', null, process.env.FAKE_SIGNAL)); return child; };\n"
+    + "require('node:module').syncBuiltinESMExports();\n");
+  const entry = new URL('../src/first-output.mjs', import.meta.url).pathname;
+  for (const [signal, expected] of [['SIGKILL', 137], ['SIGTERM', 143], ['SIGUNKNOWN', 1]]) {
+    const child = spawn(process.execPath, ['--require', fake, entry, 'claude', root, 'unused'], { env: { PATH: process.env.PATH, FAKE_SIGNAL: signal }, stdio: ['ignore', 'pipe', 'pipe'] });
+    let stdout = '', stderr = '';
+    child.stdout.on('data', x => { stdout += x; }); child.stderr.on('data', x => { stderr += x; });
+    const code = await new Promise(done => child.on('close', done));
+    assert.equal(code, expected, signal);
+    assert.equal(stdout, '', signal);
+    assert.equal(stderr, `model child killed by ${signal}\n`, signal);
+  }
+});