fix(codex): drop oversized message ids on Responses input replay

Codex assigns assistant message items server-side ids that can run
400+ chars (base64 encrypted blobs), but the Responses API caps
input[].id at 64 chars and rejects the whole request with a
non-retryable HTTP 400. Once a session captures one of these long
ids, every subsequent turn replays it and 400s forever, since the
history persists it in codex_message_items.

Add a 64-char length guard at both replay sites — the history-to-
input converter and the final preflight gate — so oversized ids are
dropped while short ids (msg_...) are kept for prefix-cache hits.
Mirrors the existing pattern for reasoning items, which already
strip their id before replay because store=False means the API
can't resolve ids server-side anyway.

Fixes #27038

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

agent/codex_responses_adapter.py

@@ -288,6 +288,13 @@ _RESPONSES_BUILTIN_TOOL_TYPES = {
 
 _RESPONSE_MESSAGE_STATUSES = {"completed", "incomplete", "in_progress"}
 
+# The Responses API rejects input[].id longer than this with a non-retryable
+# HTTP 400 ("string too long"). Codex-issued assistant message ids are
+# server-assigned base64 blobs that can run 400+ chars, while Hermes-minted
+# ids (msg_...) stay well under this cap and are worth keeping for
+# prefix-cache hits. Drop only the oversized ones on replay.
+_MAX_RESPONSES_ITEM_ID_LENGTH = 64
+
 
 def _normalize_responses_message_status(value: Any, *, default: str = "completed") -> str:
     """Normalize a Responses assistant message status for replay.
@@ -464,7 +471,9 @@ def _chat_messages_to_responses_input(
                         }
                         item_id = raw_item.get("id")
                         if isinstance(item_id, str) and item_id.strip():
-                            replay_item["id"] = item_id.strip()
+                            stripped_id = item_id.strip()
+                            if len(stripped_id) <= _MAX_RESPONSES_ITEM_ID_LENGTH:
+                                replay_item["id"] = stripped_id
                         phase = raw_item.get("phase")
                         if isinstance(phase, str) and phase.strip():
                             replay_item["phase"] = phase.strip()
@@ -718,7 +727,9 @@ def _preflight_codex_input_items(raw_items: Any) -> List[Dict[str, Any]]:
             }
             item_id = item.get("id")
             if isinstance(item_id, str) and item_id.strip():
-                normalized_item["id"] = item_id.strip()
+                stripped_id = item_id.strip()
+                if len(stripped_id) <= _MAX_RESPONSES_ITEM_ID_LENGTH:
+                    normalized_item["id"] = stripped_id
             phase = item.get("phase")
             if isinstance(phase, str) and phase.strip():
                 normalized_item["phase"] = phase.strip()

tests/agent/test_codex_responses_adapter.py

@@ -3,9 +3,11 @@ from types import SimpleNamespace
 import pytest
 
 from agent.codex_responses_adapter import (
+    _chat_messages_to_responses_input,
     _format_responses_error,
     _normalize_codex_response,
     _preflight_codex_api_kwargs,
+    _preflight_codex_input_items,
 )
 
 
@@ -139,6 +141,128 @@ def test_normalize_codex_response_in_progress_message_still_incomplete():
     assert finish_reason == "incomplete"
 
 
+# ---------------------------------------------------------------------------
+# Replayed assistant message items with an oversized server-assigned ``id``
+# (Codex issues 400+ char base64 blobs) must never reach the API — the
+# Responses endpoint caps input[].id at 64 chars and rejects the whole
+# request with a non-retryable HTTP 400, permanently bricking the session
+# (every subsequent turn replays the same bad id). Short ids (msg_...) are
+# still worth keeping for prefix-cache hits, so this is a length guard, not
+# a blanket strip.
+# ---------------------------------------------------------------------------
+
+_OVERSIZED_ITEM_ID = "x" * 408
+_VALID_ITEM_ID = "msg_abc123"
+
+
+def test_chat_messages_to_responses_input_drops_oversized_message_id():
+    messages = [
+        {
+            "role": "assistant",
+            "content": "pong",
+            "codex_message_items": [
+                {
+                    "type": "message",
+                    "role": "assistant",
+                    "status": "completed",
+                    "content": [{"type": "output_text", "text": "pong"}],
+                    "id": _OVERSIZED_ITEM_ID,
+                    "phase": "final_answer",
+                }
+            ],
+        }
+    ]
+
+    items = _chat_messages_to_responses_input(messages)
+
+    message_item = next(item for item in items if item.get("type") == "message")
+    assert "id" not in message_item
+    assert message_item["phase"] == "final_answer"
+    assert message_item["content"] == [{"type": "output_text", "text": "pong"}]
+
+
+def test_chat_messages_to_responses_input_keeps_short_message_id():
+    messages = [
+        {
+            "role": "assistant",
+            "content": "pong",
+            "codex_message_items": [
+                {
+                    "type": "message",
+                    "role": "assistant",
+                    "status": "completed",
+                    "content": [{"type": "output_text", "text": "pong"}],
+                    "id": _VALID_ITEM_ID,
+                }
+            ],
+        }
+    ]
+
+    items = _chat_messages_to_responses_input(messages)
+
+    message_item = next(item for item in items if item.get("type") == "message")
+    assert message_item["id"] == _VALID_ITEM_ID
+
+
+def test_preflight_codex_input_items_drops_oversized_message_id():
+    items = _preflight_codex_input_items(
+        [
+            {
+                "type": "message",
+                "role": "assistant",
+                "status": "completed",
+                "content": [{"type": "output_text", "text": "pong"}],
+                "id": _OVERSIZED_ITEM_ID,
+                "phase": "final_answer",
+            }
+        ]
+    )
+
+    assert "id" not in items[0]
+    assert items[0]["phase"] == "final_answer"
+
+
+def test_preflight_codex_input_items_keeps_short_message_id():
+    items = _preflight_codex_input_items(
+        [
+            {
+                "type": "message",
+                "role": "assistant",
+                "status": "completed",
+                "content": [{"type": "output_text", "text": "pong"}],
+                "id": _VALID_ITEM_ID,
+            }
+        ]
+    )
+
+    assert items[0]["id"] == _VALID_ITEM_ID
+
+
+def test_preflight_codex_api_kwargs_drops_oversized_message_id_end_to_end():
+    kwargs = _preflight_codex_api_kwargs(
+        {
+            "model": "gpt-5.5",
+            "instructions": "You are Hermes.",
+            "input": [
+                {"role": "user", "content": "ping"},
+                {
+                    "type": "message",
+                    "role": "assistant",
+                    "status": "completed",
+                    "content": [{"type": "output_text", "text": "pong"}],
+                    "id": _OVERSIZED_ITEM_ID,
+                    "phase": "final_answer",
+                },
+            ],
+            "tools": [],
+            "store": False,
+        }
+    )
+
+    message_item = next(item for item in kwargs["input"] if item.get("type") == "message")
+    assert "id" not in message_item
+
+
 # ---------------------------------------------------------------------------
 # _preflight_codex_api_kwargs — built-in (provider-executed) tools must pass
 # through validation.  Regression guard for the xAI native web_search