diff --git a/.changeset/curious-cats-parade.md b/.changeset/curious-cats-parade.md
new file mode 100644
index 000000000..6269c9eed
--- /dev/null
+++ b/.changeset/curious-cats-parade.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 317
+---
+context-monitor hook no longer performs a redundant stat syscall before each file read on the PostToolUse hot path
diff --git a/hooks/gsd-context-monitor.js b/hooks/gsd-context-monitor.js
index 51f23dee3..4d3fac96d 100644
--- a/hooks/gsd-context-monitor.js
+++ b/hooks/gsd-context-monitor.js
@@ -54,30 +54,33 @@ process.stdin.on('end', () => {
     }
 
     // Check if context warnings are disabled via config.
-    // Quick sentinel check: skip config read entirely for non-GSD projects (#P2.5).
+    // Collapsed existsSync+readFileSync into a single read guarded by try/catch
+    // (ENOENT or parse error → use defaults, same as old "planningDir absent" branch).
     const cwd = data.cwd || process.cwd();
-    const planningDir = path.join(cwd, '.planning');
-    if (fs.existsSync(planningDir)) {
-      try {
-        const configPath = path.join(planningDir, 'config.json');
-        const config = JSON.parse(fs.readFileSync(configPath, 'utf8'));
-        if (config.hooks?.context_warnings === false) {
-          process.exit(0);
-        }
-      } catch (e) {
-        // Ignore config read/parse errors (config may not exist in .planning/)
+    try {
+      const configPath = path.join(cwd, '.planning', 'config.json');
+      const config = JSON.parse(fs.readFileSync(configPath, 'utf8'));
+      if (config.hooks?.context_warnings === false) {
+        process.exit(0);
       }
+    } catch (e) {
+      // Missing or unparseable config → proceed with defaults (context warnings enabled)
     }
 
     const tmpDir = os.tmpdir();
     const metricsPath = path.join(tmpDir, `claude-ctx-${sessionId}.json`);
 
-    // If no metrics file, this is a subagent or fresh session -- exit silently
-    if (!fs.existsSync(metricsPath)) {
-      process.exit(0);
+    // If no metrics file, this is a subagent or fresh session -- exit silently.
+    // Collapsed existsSync+readFileSync: ENOENT → exit 0 (identical to old !existsSync branch),
+    // other errors rethrow to the outer catch (swallowed → exit 0, as before).
+    let metricsRaw;
+    try {
+      metricsRaw = fs.readFileSync(metricsPath, 'utf8');
+    } catch (e) {
+      if (e && e.code === 'ENOENT') process.exit(0);
+      throw e;
     }
-
-    const metrics = JSON.parse(fs.readFileSync(metricsPath, 'utf8'));
+    const metrics = JSON.parse(metricsRaw);
     const now = Math.floor(Date.now() / 1000);
 
     // Ignore stale metrics
@@ -98,13 +101,13 @@ process.stdin.on('end', () => {
     let warnData = { callsSinceWarn: 0, lastLevel: null };
     let firstWarn = true;
 
-    if (fs.existsSync(warnPath)) {
-      try {
-        warnData = JSON.parse(fs.readFileSync(warnPath, 'utf8'));
-        firstWarn = false;
-      } catch (e) {
-        // Corrupted file, reset
-      }
+    // Collapsed existsSync+readFileSync: ENOENT or parse error → keep default warnData
+    // (same as old "file absent" branch). firstWarn tracks whether we read a valid sentinel.
+    try {
+      warnData = JSON.parse(fs.readFileSync(warnPath, 'utf8'));
+      firstWarn = false;
+    } catch (e) {
+      // Missing or corrupted sentinel → firstWarn stays true, warnData stays at defaults
     }
 
     warnData.callsSinceWarn = (warnData.callsSinceWarn || 0) + 1;
diff --git a/tests/perf-317-context-monitor-fs.test.cjs b/tests/perf-317-context-monitor-fs.test.cjs
new file mode 100644
index 000000000..2ad5e9e87
--- /dev/null
+++ b/tests/perf-317-context-monitor-fs.test.cjs
@@ -0,0 +1,280 @@
+/**
+ * Behavior-lock tests for perf #317 — context-monitor hook fs I/O collapse
+ *
+ * The fix collapses each `if (existsSync(p)) { readFileSync(p) }` pattern
+ * into a single `readFileSync` guarded by try/catch treating ENOENT as the
+ * "file absent" branch. These tests lock the observable behavior so that
+ * the optimized code is proved equivalent across all three files:
+ *   1. metrics file (early-exit path when absent)
+ *   2. config.json (defaults when absent)
+ *   3. warn sentinel (first-warn vs debounce)
+ */
+
+'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');
+const tmpDir = os.tmpdir();
+
+/**
+ * Spawn the context-monitor hook with the given options.
+ *
+ * @param {object} opts
+ * @param {string}  opts.sessionId     - session ID embedded in stdin payload
+ * @param {string}  [opts.cwd]         - cwd in payload (defaults to tmpDir)
+ * @param {boolean} [opts.writeMetrics] - if true, write a bridge file before spawn
+ * @param {number}  [opts.remaining]   - remaining_percentage for bridge file
+ * @param {number}  [opts.usedPct]     - used_pct for bridge file
+ * @param {boolean} [opts.writeWarn]   - if true, write a warn sentinel before spawn
+ * @param {object}  [opts.warnData]    - content for warn sentinel (defaults to first-warn-like data)
+ * @returns {{ exitCode: number, stdout: string }}
+ */
+function runMonitorRaw(opts) {
+  const {
+    sessionId,
+    cwd = tmpDir,
+    writeMetrics = false,
+    remaining = 20,
+    usedPct = 80,
+    writeWarn = false,
+    warnData = null,
+  } = opts;
+
+  const metricsPath = path.join(tmpDir, `claude-ctx-${sessionId}.json`);
+  const warnPath = path.join(tmpDir, `claude-ctx-${sessionId}-warned.json`);
+
+  if (writeMetrics) {
+    fs.writeFileSync(metricsPath, JSON.stringify({
+      session_id: sessionId,
+      remaining_percentage: remaining,
+      used_pct: usedPct,
+      timestamp: Math.floor(Date.now() / 1000),
+    }));
+  }
+
+  if (writeWarn) {
+    const wd = warnData ?? { callsSinceWarn: 0, lastLevel: null };
+    fs.writeFileSync(warnPath, JSON.stringify(wd));
+  }
+
+  const input = JSON.stringify({ session_id: sessionId, cwd });
+  let stdout = '';
+  let exitCode = 0;
+
+  try {
+    stdout = execFileSync(process.execPath, [MONITOR_PATH], {
+      input,
+      encoding: 'utf-8',
+      timeout: 5000,
+    });
+  } catch (e) {
+    exitCode = e.status ?? 1;
+    stdout = e.stdout || '';
+  } finally {
+    try { fs.unlinkSync(metricsPath); } catch { /* already absent */ }
+    try { fs.unlinkSync(warnPath); } catch { /* already absent */ }
+  }
+
+  return { exitCode, stdout };
+}
+
+// ─── 1. Metrics file absent → early exit 0, no stdout ────────────────────────
+
+describe('perf #317: metrics file absent (exercises ENOENT early-exit path)', () => {
+  test('exits 0 with empty stdout when metrics file does not exist', () => {
+    // This is the "subagent / fresh session" path. The original code did:
+    //   if (!existsSync(metricsPath)) process.exit(0)
+    // The fix collapses to try/catch ENOENT → process.exit(0).
+    // Both branches must produce: exit code 0, zero bytes on stdout.
+    const sessionId = `test-317-no-metrics-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const { exitCode, stdout } = runMonitorRaw({ sessionId, writeMetrics: false });
+
+    // Non-vacuous: assert the exact signature of the early-exit branch
+    assert.strictEqual(exitCode, 0,
+      'hook must exit 0 when metrics file is absent (subagent/fresh-session path)');
+    assert.strictEqual(stdout, '',
+      'hook must produce NO stdout when metrics file is absent — empty stdout is the ' +
+      'unique signature of the early-exit branch; any output would mean the hook ' +
+      'continued past the metrics-absent guard, proving the ENOENT branch is not taken');
+  });
+
+  test('a distinct session with a present metrics file DOES produce output (proves the absent-file test is not vacuous)', () => {
+    // If the absent-file test passed vacuously (e.g. the hook never emits output
+    // for ANY session), this companion test would fail — locking non-vacuousness.
+    const sessionId = `test-317-has-metrics-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const { stdout } = runMonitorRaw({
+      sessionId,
+      writeMetrics: true,
+      remaining: 20,  // below CRITICAL_THRESHOLD=25 → will emit
+      usedPct: 80,
+    });
+    assert.ok(stdout.length > 0,
+      'hook must emit JSON output when metrics ARE present and remaining <= CRITICAL_THRESHOLD; ' +
+      'this proves the absent-file test above is non-vacuous');
+    const parsed = JSON.parse(stdout);
+    assert.ok(
+      parsed?.hookSpecificOutput?.additionalContext,
+      'output must contain hookSpecificOutput.additionalContext'
+    );
+  });
+});
+
+// ─── 2. config.json absent → uses defaults, still emits warning ──────────────
+
+describe('perf #317: config.json absent (exercises config-missing → defaults path)', () => {
+  test('emits warning using defaults when .planning/config.json is absent', () => {
+    // Original code: existsSync(planningDir) guards the config read.
+    // Fix collapses to: try { config = JSON.parse(readFileSync(configPath)) } catch { defaults }
+    // When config.json is missing, the hook should proceed with defaults
+    // (context_warnings not disabled) and emit the same warning.
+    //
+    // We point cwd at a temp dir that has NO .planning/config.json.
+    const sessionId = `test-317-no-config-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const testCwd = fs.mkdtempSync(path.join(tmpDir, 'gsd-317-no-config-'));
+
+    try {
+      // Metrics present, below warning threshold → should warn
+      const { exitCode, stdout } = runMonitorRaw({
+        sessionId,
+        cwd: testCwd,
+        writeMetrics: true,
+        remaining: 20,
+        usedPct: 80,
+      });
+
+      assert.strictEqual(exitCode, 0, 'hook should exit 0 (not crash) when config.json absent');
+      assert.ok(stdout.length > 0,
+        'hook should still emit a warning when config.json is absent (defaults apply)');
+      const parsed = JSON.parse(stdout);
+      assert.ok(
+        parsed?.hookSpecificOutput?.additionalContext,
+        'warning output must contain additionalContext'
+      );
+    } finally {
+      try { fs.rmSync(testCwd, { recursive: true, force: true }); } catch { /* tolerate */ }
+    }
+  });
+
+  test('respects context_warnings=false when config.json IS present', () => {
+    // Proves the config read actually works (not just always-defaults).
+    const sessionId = `test-317-config-disabled-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const testCwd = fs.mkdtempSync(path.join(tmpDir, 'gsd-317-config-disabled-'));
+    const planningDir = path.join(testCwd, '.planning');
+    fs.mkdirSync(planningDir, { recursive: true });
+    fs.writeFileSync(
+      path.join(planningDir, 'config.json'),
+      JSON.stringify({ hooks: { context_warnings: false } })
+    );
+
+    // Write metrics so the hook would warn if config_warnings wasn't false
+    const metricsPath = path.join(tmpDir, `claude-ctx-${sessionId}.json`);
+    fs.writeFileSync(metricsPath, JSON.stringify({
+      session_id: sessionId,
+      remaining_percentage: 20,
+      used_pct: 80,
+      timestamp: Math.floor(Date.now() / 1000),
+    }));
+
+    let exitCode = 0;
+    let stdout = '';
+    try {
+      stdout = execFileSync(process.execPath, [MONITOR_PATH], {
+        input: JSON.stringify({ session_id: sessionId, cwd: testCwd }),
+        encoding: 'utf-8',
+        timeout: 5000,
+      });
+    } catch (e) {
+      exitCode = e.status ?? 1;
+      stdout = e.stdout || '';
+    } finally {
+      try { fs.unlinkSync(metricsPath); } catch { /* noop */ }
+      try { fs.rmSync(testCwd, { recursive: true, force: true }); } catch { /* tolerate */ }
+    }
+
+    assert.strictEqual(exitCode, 0, 'hook should exit 0 when context_warnings=false');
+    assert.strictEqual(stdout, '',
+      'hook should produce NO output when context_warnings=false in config.json');
+  });
+});
+
+// ─── 3. Warn sentinel absent vs present (debounce behavior) ──────────────────
+
+describe('perf #317: warn sentinel absent/present (exercises sentinel ENOENT path)', () => {
+  test('emits warning on first call when warn sentinel is absent', () => {
+    // Original: !existsSync(warnPath) → firstWarn=true → emit immediately.
+    // Fix: try { warnData = JSON.parse(readFileSync(warnPath)) } catch { /* keep defaults */ }
+    // When sentinel absent, warnData stays at default { callsSinceWarn:0, lastLevel:null }
+    // and firstWarn=true → hook emits immediately.
+    const sessionId = `test-317-first-warn-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const { exitCode, stdout } = runMonitorRaw({
+      sessionId,
+      writeMetrics: true,
+      remaining: 30,
+      usedPct: 70,
+      writeWarn: false,  // sentinel absent
+    });
+
+    assert.strictEqual(exitCode, 0);
+    assert.ok(stdout.length > 0,
+      'hook should emit warning on first call (sentinel absent = firstWarn path)');
+    const parsed = JSON.parse(stdout);
+    assert.ok(parsed?.hookSpecificOutput?.additionalContext,
+      'first-warn output must contain additionalContext');
+  });
+
+  test('debounces when warn sentinel is present and callsSinceWarn is below threshold', () => {
+    // Original: existsSync(warnPath) → readFileSync → warnData loaded → debounce check.
+    // Fix: try { warnData = JSON.parse(readFileSync(warnPath)) } catch { defaults }
+    // When sentinel present with recent warn, hook exits 0 with no output.
+    const sessionId = `test-317-debounced-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const { exitCode, stdout } = runMonitorRaw({
+      sessionId,
+      writeMetrics: true,
+      remaining: 30,
+      usedPct: 70,
+      writeWarn: true,
+      warnData: {
+        // callsSinceWarn=1 (below DEBOUNCE_CALLS=5), same level → debounce fires
+        callsSinceWarn: 1,
+        lastLevel: 'warning',
+      },
+    });
+
+    assert.strictEqual(exitCode, 0,
+      'hook must exit 0 during debounce window');
+    assert.strictEqual(stdout, '',
+      'hook must emit NO output during debounce window (sentinel present, callsSinceWarn < 5)');
+  });
+
+  test('severity escalation (WARNING → CRITICAL) bypasses debounce even with sentinel present', () => {
+    // Even if callsSinceWarn is low, escalating from warning to critical must fire immediately.
+    // This tests the `severityEscalated` bypass path.
+    const sessionId = `test-317-escalated-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const { exitCode, stdout } = runMonitorRaw({
+      sessionId,
+      writeMetrics: true,
+      remaining: 20,   // CRITICAL (below 25)
+      usedPct: 80,
+      writeWarn: true,
+      warnData: {
+        callsSinceWarn: 1,      // below DEBOUNCE_CALLS → would normally debounce
+        lastLevel: 'warning',   // previous level was warning → escalation to critical
+      },
+    });
+
+    assert.strictEqual(exitCode, 0);
+    assert.ok(stdout.length > 0,
+      'severity escalation (warning→critical) must bypass debounce and emit warning');
+    const parsed = JSON.parse(stdout);
+    const msg = parsed?.hookSpecificOutput?.additionalContext;
+    assert.ok(msg, 'escalation output must contain additionalContext');
+    assert.match(msg, /CONTEXT CRITICAL/,
+      'escalated message must say CONTEXT CRITICAL');
+  });
+});