diff --git a/hooks/ponytail-runtime.js b/hooks/ponytail-runtime.js
index 37d682f..7b5a662 100644
--- a/hooks/ponytail-runtime.js
+++ b/hooks/ponytail-runtime.js
@@ -43,10 +43,10 @@ function writeHookOutput(event, mode, context = '') {
   if (isCodex) {
     const output = { systemMessage: `PONYTAIL:${mode.toUpperCase()}` };
     if (context) {
-      output.hookSpecificOutput = {
-        hookEventName: event,
-        additionalContext: context,
-      };
+      // ponytail: Codex CLI expects additionalContext at top level, not nested
+      // under hookSpecificOutput. The hookSpecificOutput wrapper is for Claude
+      // Code's SubagentStart. See issue #505.
+      output.additionalContext = context;
     }
     process.stdout.write(JSON.stringify(output));
     return;
diff --git a/tests/hooks.test.js b/tests/hooks.test.js
index 5389850..bc2ab20 100644
--- a/tests/hooks.test.js
+++ b/tests/hooks.test.js
@@ -61,9 +61,10 @@ assert.equal(fs.readFileSync(codexState, 'utf8'), 'ultra');
 let output = JSON.parse(result.stdout);
 assert.equal(output.systemMessage, 'PONYTAIL:ULTRA');
 assert.match(
-  output.hookSpecificOutput.additionalContext,
+  output.additionalContext,
   /PONYTAIL MODE ACTIVE — level: ultra/,
 );
+assert.equal(output.hookSpecificOutput, undefined, 'Codex should not use hookSpecificOutput (#505)');
 
 result = run(
   'ponytail-mode-tracker.js',
@@ -85,7 +86,7 @@ assert.equal(result.status, 0, result.stderr);
 assert.equal(fs.readFileSync(codexState, 'utf8'), 'lite');
 output = JSON.parse(result.stdout);
 assert.match(
-  output.hookSpecificOutput.additionalContext,
+  output.additionalContext,
   /PONYTAIL MODE ACTIVE — level: lite/,
 );
 
@@ -229,8 +230,8 @@ result = run('ponytail-subagent.js', { HOME: subHome, USERPROFILE: subHome, PLUG
 assert.equal(result.status, 0, result.stderr);
 output = JSON.parse(result.stdout);
 assert.equal(output.systemMessage, 'PONYTAIL:FULL');
-assert.equal(output.hookSpecificOutput.hookEventName, 'SubagentStart');
-assert.match(output.hookSpecificOutput.additionalContext, /PONYTAIL MODE ACTIVE — level: full/);
+assert.ok(output.additionalContext, 'Codex SubagentStart should have additionalContext at top level (#505)');
+assert.match(output.additionalContext, /PONYTAIL MODE ACTIVE — level: full/);
 
 // SubagentStart scoping (issue #506): PONYTAIL_SUBAGENT_MATCHER limits the
 // injection to agent types whose name matches the regex. Unset keeps the