fix(hooks): stop pre/post Bash dispatcher from echoing the input event (#2240)

runHooks() returned the unmodified raw stdin (the PreToolUse/PostToolUse
input event) on stdout whenever no sub-hook produced additionalContext.
Claude Code parses a hook's stdout as JSON and validates it against the
hook-output schema, so echoing the input object
({session_id, hook_event_name, tool_name, tool_input, ...}) fails with
"Hook JSON output validation failed — (root): Invalid input" on nearly
every Bash command.

Track whether a sub-hook deliberately set stdout (string / {stdout}, e.g.
GateGuard) via a rawModified flag and emit '' in the pass-through case
instead of the echoed input. Preserves GateGuard pass-through and
block-no-verify's exit-2 blocking.

Update the three dispatcher tests that codified the buggy echo behavior to
expect empty stdout, and add a regression test for a plain pass-through
command.

Fixes #2239

Co-authored-by: WOZCODE <contact@withwoz.com>

scripts/hooks/bash-hook-dispatcher.js

@@ -122,6 +122,12 @@ function normalizeHookResult(previousRaw, output) {
 
 function runHooks(rawInput, hooks) {
   let currentRaw = rawInput;
+  // Track whether a sub-hook deliberately produced stdout (a string or
+  // {stdout}) versus currentRaw still being the untouched input event.
+  // Echoing the unmodified input event back to stdout fails Claude Code's
+  // hook-output JSON schema validation ("(root): Invalid input"), so in the
+  // pass-through case we must emit nothing instead.
+  let rawModified = false;
   let stderr = '';
   let additionalContext = '';
 
@@ -132,6 +138,9 @@ function runHooks(rawInput, hooks) {
 
     try {
       const result = normalizeHookResult(currentRaw, hook.run(currentRaw));
+      if (result.raw !== currentRaw) {
+        rawModified = true;
+      }
       currentRaw = result.raw;
       if (result.stderr) {
         stderr += result.stderr.endsWith('\n') ? result.stderr : `${result.stderr}\n`;
@@ -140,7 +149,12 @@ function runHooks(rawInput, hooks) {
         additionalContext = combineAdditionalContext(additionalContext, result.additionalContext);
       }
       if (result.exitCode !== 0) {
-        return { output: currentRaw, stderr, additionalContext, exitCode: result.exitCode };
+        return {
+          output: rawModified ? currentRaw : '',
+          stderr,
+          additionalContext,
+          exitCode: result.exitCode,
+        };
       }
     } catch (error) {
       stderr += `[Hook] ${hook.id} failed: ${error.message}\n`;
@@ -150,7 +164,9 @@ function runHooks(rawInput, hooks) {
   return {
     output: additionalContext
       ? buildPreToolUseAdditionalContext(additionalContext)
-      : currentRaw,
+      : rawModified
+        ? currentRaw
+        : '',
     stderr,
     additionalContext,
     exitCode: 0,

tests/hooks/bash-hook-dispatcher.test.js

@@ -53,6 +53,16 @@ function runTests() {
     assert.strictEqual(result.stdout, '', 'Blocking hook should not pass through stdout');
   })) passed++; else failed++;
 
+  if (test('pre dispatcher emits no stdout for a plain command (regression: issue #2239)', () => {
+    // A pass-through command (no sub-hook adds context) must NOT echo the
+    // input event back to stdout — Claude Code validates hook stdout against
+    // the hook-output schema and the input event fails as "(root): Invalid input".
+    const input = { tool_input: { command: 'ls -la' } };
+    const result = runScript(preDispatcher, input, { ECC_HOOK_PROFILE: 'standard' });
+    assert.strictEqual(result.status, 0);
+    assert.strictEqual(result.stdout, '', `Pass-through must emit empty stdout, got: ${result.stdout}`);
+  })) passed++; else failed++;
+
   if (test('pre dispatcher still honors per-hook disable flags', () => {
     const input = { tool_input: { command: 'git push origin main' } };
 
@@ -69,7 +79,7 @@ function runTests() {
       ECC_DISABLED_HOOKS: 'pre:bash:git-push-reminder',
     });
     assert.strictEqual(disabled.status, 0);
-    assert.strictEqual(disabled.stdout, JSON.stringify(input), 'Disabled hook should pass through original input');
+    assert.strictEqual(disabled.stdout, '', 'Disabled hook should emit no stdout (echoing the input event fails hook-output schema validation)');
     assert.ok(!disabled.stderr.includes('Review changes before push'), 'Disabled hook should not emit reminder');
   })) passed++; else failed++;
 
@@ -78,7 +88,7 @@ function runTests() {
     const result = runScript(preDispatcher, input, { ECC_HOOK_PROFILE: 'minimal' });
     assert.strictEqual(result.status, 0);
     assert.strictEqual(result.stderr, '', 'Strict-only reminders should stay disabled in minimal profile');
-    assert.strictEqual(result.stdout, JSON.stringify(input));
+    assert.strictEqual(result.stdout, '', 'Pass-through must emit no stdout, not echo the input event');
   })) passed++; else failed++;
 
   if (test('post dispatcher writes both bash audit and cost logs in one pass', () => {
@@ -91,7 +101,7 @@ function runTests() {
         USERPROFILE: homeDir,
       });
       assert.strictEqual(result.status, 0);
-      assert.strictEqual(result.stdout, JSON.stringify(payload));
+      assert.strictEqual(result.stdout, '', 'Post dispatcher pass-through must emit no stdout, not echo the input event');
 
       const auditLog = fs.readFileSync(path.join(homeDir, '.claude', 'bash-commands.log'), 'utf8');
       const costLog = fs.readFileSync(path.join(homeDir, '.claude', 'cost-tracker.log'), 'utf8');