fix(#925): context monitor echoes the invoking hook event name (#927)

Read `data.hook_event_name` from the stdin payload and fall back to the
Gemini/non-Gemini heuristic only when the field is absent or blank.
Fixes Claude Code rejecting output with "expected Stop but got PostToolUse"
when the monitor is called by Stop, SubagentStop, or PreCompact hooks.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/925-context-monitor-hook-event-name.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 926
+---
+**`gsd-context-monitor.js` now echoes the actual invoking hook event name** — instead of hardcoding `hookEventName: "PostToolUse"` (or `"AfterTool"` for Gemini), the hook reads `data.hook_event_name` from the stdin payload and falls back to the runtime heuristic only when the field is absent or blank; this fixes Claude Code rejecting hook output with `"expected Stop but got PostToolUse"` when the monitor is invoked by the Stop, SubagentStop, or PreCompact hooks registered in PR #821. (#925)

hooks/gsd-context-monitor.js

@@ -182,7 +182,8 @@ process.stdin.on('end', () => {
 
     const output = {
       hookSpecificOutput: {
-        hookEventName: process.env.GEMINI_API_KEY ? "AfterTool" : "PostToolUse",
+        hookEventName: (data.hook_event_name && data.hook_event_name.trim())
+          || (process.env.GEMINI_API_KEY ? "AfterTool" : "PostToolUse"),
         additionalContext: message
       }
     };

tests/bug-925-context-monitor-hook-event-name.test.cjs

@@ -0,0 +1,205 @@
+/**
+ * Regression test for bug #925
+ *
+ * hooks/gsd-context-monitor.js hardcodes `hookEventName: "PostToolUse"` (or
+ * "AfterTool" for Gemini) regardless of which hook event invoked it. Since
+ * PR #821 the same script is also registered under Stop, SubagentStop, and
+ * PreCompact in hooks/hooks.json. Claude Code rejects output whose
+ * hookSpecificOutput.hookEventName doesn't echo the triggering event:
+ *
+ *   "expected Stop but got PostToolUse"
+ *
+ * Fix: derive hookEventName from the parsed stdin payload's `hook_event_name`
+ * field (already available in the data object), falling back to the
+ * Gemini / non-Gemini heuristic for runtimes that don't send it.
+ */
+
+'use strict';
+
+const { test, describe } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { execFileSync } = require('node:child_process');
+
+const MONITOR_PATH = path.join(__dirname, '..', 'hooks', 'gsd-context-monitor.js');
+
+/**
+ * Write a bridge metrics file and invoke the context monitor with the given
+ * payload fields. Returns the parsed stdout object (or null if the hook
+ * produced no output).
+ *
+ * remainingPct must be <= 35 to cross the WARNING threshold so the hook
+ * actually emits output.
+ */
+function runMonitor({ hookEventName, sessionId, remainingPct = 30, usedPct = 70, env = {} }) {
+  const bridgePath = path.join(os.tmpdir(), `claude-ctx-${sessionId}.json`);
+  fs.writeFileSync(bridgePath, JSON.stringify({
+    session_id: sessionId,
+    remaining_percentage: remainingPct,
+    used_pct: usedPct,
+    timestamp: Math.floor(Date.now() / 1000),
+  }));
+
+  const payload = { session_id: sessionId, cwd: os.tmpdir() };
+  if (hookEventName !== undefined) {
+    payload.hook_event_name = hookEventName;
+  }
+
+  let stdout = '';
+  try {
+    stdout = execFileSync(process.execPath, [MONITOR_PATH], {
+      input: JSON.stringify(payload),
+      encoding: 'utf-8',
+      timeout: 5000,
+      env: { ...process.env, ...env },
+    });
+  } catch (e) {
+    stdout = e.stdout || '';
+  } finally {
+    try { fs.unlinkSync(bridgePath); } catch { /* noop */ }
+    try {
+      fs.unlinkSync(path.join(os.tmpdir(), `claude-ctx-${sessionId}-warned.json`));
+    } catch { /* noop */ }
+  }
+
+  if (!stdout) return null;
+  return JSON.parse(stdout);
+}
+
+function makeSessionId(suffix) {
+  return `test-925-${suffix}-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+}
+
+// ─── hookEventName echoing ────────────────────────────────────────────────────
+
+describe('bug #925: context monitor echoes the invoking hook event name', () => {
+  test('hookEventName is "Stop" when payload contains hook_event_name: "Stop"', () => {
+    const out = runMonitor({ hookEventName: 'Stop', sessionId: makeSessionId('stop') });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold (remaining=30)');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'Stop',
+      `Expected hookEventName "Stop" but got "${out.hookSpecificOutput?.hookEventName}". ` +
+      'The hook must echo the hook_event_name from stdin, not hardcode "PostToolUse".'
+    );
+  });
+
+  test('hookEventName is "SubagentStop" when payload contains hook_event_name: "SubagentStop"', () => {
+    const out = runMonitor({ hookEventName: 'SubagentStop', sessionId: makeSessionId('subagent-stop') });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'SubagentStop',
+      `Expected hookEventName "SubagentStop" but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+
+  test('hookEventName is "PreCompact" when payload contains hook_event_name: "PreCompact"', () => {
+    const out = runMonitor({ hookEventName: 'PreCompact', sessionId: makeSessionId('precompact') });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'PreCompact',
+      `Expected hookEventName "PreCompact" but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+
+  test('hookEventName is "PostToolUse" when payload contains hook_event_name: "PostToolUse"', () => {
+    const out = runMonitor({ hookEventName: 'PostToolUse', sessionId: makeSessionId('posttools') });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'PostToolUse',
+      `Expected hookEventName "PostToolUse" but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+});
+
+// ─── Fallback behaviour (no hook_event_name in payload) ──────────────────────
+
+describe('bug #925: context monitor falls back to heuristic when hook_event_name absent', () => {
+  test('falls back to "PostToolUse" when hook_event_name is absent (non-Gemini)', () => {
+    const env = { ...process.env };
+    delete env.GEMINI_API_KEY;
+    const out = runMonitor({
+      hookEventName: undefined,
+      sessionId: makeSessionId('fallback-non-gemini'),
+      env: { GEMINI_API_KEY: '' }, // ensure unset
+    });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'PostToolUse',
+      `Expected fallback "PostToolUse" for non-Gemini but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+
+  test('falls back to "AfterTool" when hook_event_name is absent and GEMINI_API_KEY is set', () => {
+    const out = runMonitor({
+      hookEventName: undefined,
+      sessionId: makeSessionId('fallback-gemini'),
+      env: { GEMINI_API_KEY: 'fake-key-for-test' },
+    });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'AfterTool',
+      `Expected fallback "AfterTool" for Gemini but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+
+  test('falls back to "PostToolUse" when hook_event_name is an empty string (non-Gemini)', () => {
+    const out = runMonitor({
+      hookEventName: '',
+      sessionId: makeSessionId('fallback-empty'),
+      env: { GEMINI_API_KEY: '' },
+    });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'PostToolUse',
+      `Expected fallback "PostToolUse" for empty hook_event_name but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+
+  test('falls back to "PostToolUse" when hook_event_name is whitespace-only (non-Gemini)', () => {
+    // trim() makes "   " → "" which is falsy, so the || fallback fires
+    const out = runMonitor({
+      hookEventName: '   ',
+      sessionId: makeSessionId('fallback-whitespace'),
+      env: { GEMINI_API_KEY: '' },
+    });
+    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'PostToolUse',
+      `Expected fallback "PostToolUse" for whitespace-only hook_event_name but got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+  });
+});
+
+// ─── Critical threshold also echoes the event name ───────────────────────────
+
+describe('bug #925: critical threshold warning also uses correct hookEventName', () => {
+  test('CRITICAL warning emitted under Stop also echoes "Stop"', () => {
+    const out = runMonitor({
+      hookEventName: 'Stop',
+      sessionId: makeSessionId('critical-stop'),
+      remainingPct: 20,
+      usedPct: 80,
+    });
+    assert.ok(out, 'hook must emit output at critical threshold (remaining=20)');
+    assert.strictEqual(
+      out.hookSpecificOutput?.hookEventName,
+      'Stop',
+      `Expected hookEventName "Stop" at critical threshold, got "${out.hookSpecificOutput?.hookEventName}".`
+    );
+    assert.match(
+      out.hookSpecificOutput?.additionalContext || '',
+      /CONTEXT CRITICAL/,
+      'Output should be a CRITICAL warning at remaining=20'
+    );
+  });
+});