diff --git a/action.yml b/action.yml
index a32ba38e..14bbfcc6 100644
--- a/action.yml
+++ b/action.yml
@@ -180,6 +180,7 @@ runs:
         ZERO_INPUT_ADD_DIR: ${{ inputs.add-dir }}
         ZERO_INPUT_WORKTREE: ${{ inputs.worktree }}
         ZERO_INPUT_OUTPUT_FORMAT: ${{ inputs.output-format }}
+        ACTION_PATH: ${{ github.action_path }}
         RUNNER_TEMP: ${{ runner.temp }}
       run: |
         set -euo pipefail
@@ -247,10 +248,9 @@ runs:
         echo "exit-code=${code}" >> "${GITHUB_OUTPUT}"
         echo "output-file=${output_file}" >> "${GITHUB_OUTPUT}"
 
-        # Best-effort one-line summary: last non-empty line of the captured output.
-        summary="$(grep -v '^[[:space:]]*$' "${output_file}" | tail -n 1 || true)"
-        # Keep the summary single-line and bounded for the step output.
-        summary="$(printf '%s' "${summary}" | cut -c1-280)"
+        # Best-effort one-line summary. Structured output ends with lifecycle
+        # metadata, so parse final/error events instead of tailing stdout.
+        summary="$(node "${ACTION_PATH}/scripts/action-summary.mjs" "${ZERO_INPUT_OUTPUT_FORMAT}" "${output_file}" || true)"
         {
           echo "summary<<ZERO_EOF"
           printf '%s\n' "${summary}"
diff --git a/scripts/action-summary.mjs b/scripts/action-summary.mjs
new file mode 100644
index 00000000..6392aa53
--- /dev/null
+++ b/scripts/action-summary.mjs
@@ -0,0 +1,83 @@
+#!/usr/bin/env node
+
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import { pathToFileURL } from 'node:url';
+
+const SUMMARY_LIMIT = 280;
+const STRUCTURED_FORMATS = new Set(['json', 'stream-json']);
+
+function normalizeSummary(value) {
+  return String(value ?? '')
+    .replace(/\s+/g, ' ')
+    .trim()
+    .slice(0, SUMMARY_LIMIT);
+}
+
+function truncateSummary(value) {
+  return String(value ?? '').slice(0, SUMMARY_LIMIT);
+}
+
+function lastNonEmptyLine(contents) {
+  let last = '';
+  for (const line of String(contents ?? '').split(/\r?\n/)) {
+    if (line.trim()) {
+      last = line;
+    }
+  }
+  return last;
+}
+
+function summarizeStructured(contents) {
+  let summary;
+
+  for (const line of String(contents ?? '').split(/\r?\n/)) {
+    if (!line.trim()) {
+      continue;
+    }
+
+    let event;
+    try {
+      event = JSON.parse(line);
+    } catch {
+      continue;
+    }
+
+    if (event?.type === 'final' && typeof event.text === 'string') {
+      summary = event.text;
+    } else if (event?.type === 'error' && typeof event.message === 'string') {
+      summary = event.message;
+    }
+  }
+
+  if (summary !== undefined) {
+    return normalizeSummary(summary);
+  }
+  return normalizeSummary(lastNonEmptyLine(contents));
+}
+
+export function summarizeOutput(format, contents) {
+  const normalizedFormat = String(format ?? 'text').toLowerCase();
+  if (!STRUCTURED_FORMATS.has(normalizedFormat)) {
+    return truncateSummary(lastNonEmptyLine(contents));
+  }
+  return summarizeStructured(contents);
+}
+
+function isMainModule() {
+  return (
+    process.argv[1] !== undefined &&
+    pathToFileURL(resolve(process.argv[1])).href === import.meta.url
+  );
+}
+
+if (isMainModule()) {
+  const [format, outputFile] = process.argv.slice(2);
+  let contents = '';
+  try {
+    contents = readFileSync(outputFile, 'utf8');
+  } catch {
+    contents = '';
+  }
+  process.stdout.write(`${summarizeOutput(format, contents)}\n`);
+}
diff --git a/scripts/action-summary.test.mjs b/scripts/action-summary.test.mjs
new file mode 100644
index 00000000..3b14c4ed
--- /dev/null
+++ b/scripts/action-summary.test.mjs
@@ -0,0 +1,148 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { summarizeOutput } from './action-summary.mjs';
+
+const scriptPath = fileURLToPath(new URL('./action-summary.mjs', import.meta.url));
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+
+function jsonl(events) {
+  return events.map((event) => JSON.stringify(event)).join('\n');
+}
+
+test('stream-json final followed by run_end returns final text', () => {
+  const output = jsonl([
+    { type: 'final', text: 'Done. Tests passed.' },
+    { type: 'run_end', status: 'success', exitCode: 0 },
+  ]);
+
+  assert.equal(summarizeOutput('stream-json', output), 'Done. Tests passed.');
+});
+
+test('stream-json error followed by run_end returns error message', () => {
+  const output = jsonl([
+    { type: 'error', code: 'provider_error', message: 'provider request failed' },
+    { type: 'run_end', status: 'error', exitCode: 3 },
+  ]);
+
+  assert.equal(summarizeOutput('stream-json', output), 'provider request failed');
+});
+
+test('stream-json final then error returns error message', () => {
+  const output = jsonl([
+    { type: 'final', text: 'partial answer' },
+    { type: 'error', code: 'incomplete', message: 'run stopped with work unfinished' },
+    { type: 'run_end', status: 'incomplete', exitCode: 4 },
+  ]);
+
+  assert.equal(summarizeOutput('stream-json', output), 'run stopped with work unfinished');
+});
+
+test('json final followed by done returns final text', () => {
+  const output = jsonl([
+    { type: 'final', text: 'Review complete.' },
+    { type: 'done', exit_code: 0 },
+  ]);
+
+  assert.equal(summarizeOutput('json', output), 'Review complete.');
+});
+
+test('json error followed by done returns error message', () => {
+  const output = jsonl([
+    { type: 'error', code: 'exec_failed', message: 'command failed' },
+    { type: 'done', exit_code: 1 },
+  ]);
+
+  assert.equal(summarizeOutput('json', output), 'command failed');
+});
+
+test('json final then error returns error message', () => {
+  const output = jsonl([
+    { type: 'final', text: 'partial answer' },
+    { type: 'error', code: 'incomplete', message: 'run stopped with work unfinished' },
+    { type: 'done', exit_code: 4 },
+  ]);
+
+  assert.equal(summarizeOutput('json', output), 'run stopped with work unfinished');
+});
+
+test('text output returns last non-empty line', () => {
+  const output = '\nfirst line\n\n  final streamed line  \n';
+
+  assert.equal(summarizeOutput('text', output), '  final streamed line  ');
+});
+
+test('unknown format falls back to text behavior', () => {
+  const output = 'alpha\n  beta  \n';
+
+  assert.equal(summarizeOutput('xml', output), '  beta  ');
+});
+
+test('malformed structured output falls back safely', () => {
+  const output = '{not-json}\nraw fallback\n';
+
+  assert.equal(summarizeOutput('stream-json', output), 'raw fallback');
+});
+
+test('long multiline final becomes one-line 280-char summary', () => {
+  const text = `first line\n${'x'.repeat(400)}`;
+  const summary = summarizeOutput('stream-json', jsonl([{ type: 'final', text }]));
+
+  assert.equal(summary.length, 280);
+  assert.equal(summary.includes('\n'), false);
+  assert.equal(summary, `first line ${'x'.repeat(400)}`.slice(0, 280));
+});
+
+test('cli reads output file and prints summary', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'zero-action-summary-'));
+  try {
+    const outputFile = join(dir, 'zero-output.jsonl');
+    writeFileSync(
+      outputFile,
+      jsonl([
+        { type: 'final', text: 'CLI summary' },
+        { type: 'run_end', status: 'success', exitCode: 0 },
+      ]),
+    );
+
+    const result = spawnSync(process.execPath, [scriptPath, 'stream-json', outputFile], {
+      encoding: 'utf8',
+    });
+
+    assert.equal(result.status, 0);
+    assert.equal(result.stderr, '');
+    assert.equal(result.stdout, 'CLI summary\n');
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('cli runs when invoked with relative script path', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'zero-action-summary-'));
+  try {
+    const outputFile = join(dir, 'zero-output.jsonl');
+    writeFileSync(
+      outputFile,
+      jsonl([
+        { type: 'final', text: 'relative CLI summary' },
+        { type: 'run_end', status: 'success', exitCode: 0 },
+      ]),
+    );
+
+    const result = spawnSync(process.execPath, ['scripts/action-summary.mjs', 'stream-json', outputFile], {
+      cwd: repoRoot,
+      encoding: 'utf8',
+    });
+
+    assert.equal(result.status, 0);
+    assert.equal(result.stderr, '');
+    assert.equal(result.stdout, 'relative CLI summary\n');
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});