diff --git a/.changeset/925-context-monitor-hook-event-name.md b/.changeset/925-context-monitor-hook-event-name.md
new file mode 100644
index 000000000..8a3dd76d3
--- /dev/null
+++ b/.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)
diff --git a/hooks/gsd-context-monitor.js b/hooks/gsd-context-monitor.js
index 5aa93f67f..991535f40 100644
--- a/hooks/gsd-context-monitor.js
+++ b/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
       }
     };
diff --git a/tests/bug-925-context-monitor-hook-event-name.test.cjs b/tests/bug-925-context-monitor-hook-event-name.test.cjs
new file mode 100644
index 000000000..9257d5936
--- /dev/null
+++ b/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'
+    );
+  });
+});