fix(anthropic): strip Responses-only kwargs before Messages SDK call (#31673) (#42155)

A Responses-API-shaped payload carrying instructions=/input=/store=/
parallel_tool_calls= can reach the native Anthropic messages.stream() /
messages.create() call under a rare api_mode-flip race (e.g. a concurrent
auxiliary vision call mutating a shared agent between the kwargs build and
the stream dispatch). The Anthropic SDK rejects these with a non-retryable
TypeError that kills the whole turn and propagates the entire fallback chain.

Add sanitize_anthropic_kwargs() at both Anthropic dispatch sites: it drops
the Responses-only keys in place and logs a WARNING (with #31673 breadcrumb)
when one is present, so the underlying race stays visible in the wild
instead of being silently papered over.

agent/anthropic_adapter.py

@@ -2301,3 +2301,43 @@ def build_anthropic_kwargs(
         kwargs["extra_headers"] = {"anthropic-beta": ",".join(betas)}
 
     return kwargs
+
+
+# Keys that belong exclusively to the OpenAI Responses / Codex API shape.
+# The Anthropic Messages SDK (``messages.create()`` / ``messages.stream()``)
+# raises ``TypeError: ... got an unexpected keyword argument`` on any of them.
+_RESPONSES_ONLY_KWARGS = frozenset(
+    {"instructions", "input", "store", "parallel_tool_calls"}
+)
+
+
+def sanitize_anthropic_kwargs(api_kwargs: Any, *, log_prefix: str = "") -> Any:
+    """Drop Responses-API-only keys before an Anthropic Messages SDK call.
+
+    Defensive boundary guard for #31673: under rare api_mode-flip races
+    (e.g. a concurrent auxiliary call mutating a shared agent between the
+    kwargs build and the stream dispatch), a Responses-shaped payload
+    carrying ``instructions=`` can reach ``messages.stream()`` /
+    ``messages.create()``. The Anthropic SDK rejects it with a
+    non-retryable ``TypeError`` that nukes the whole turn and propagates
+    the entire fallback chain.
+
+    Mutates ``api_kwargs`` in place and returns it. When a foreign key is
+    present we log a WARNING so the underlying race stays visible in the
+    wild instead of being silently papered over.
+    """
+    if not isinstance(api_kwargs, dict):
+        return api_kwargs
+    leaked = _RESPONSES_ONLY_KWARGS.intersection(api_kwargs)
+    if leaked:
+        for _key in leaked:
+            api_kwargs.pop(_key, None)
+        logger.warning(
+            "%sStripped Responses-only kwarg(s) %s from an Anthropic Messages "
+            "call (api_mode flip race — see #31673). The call will proceed; "
+            "this breadcrumb means a kwargs build ran under a Responses "
+            "api_mode while dispatch ran under anthropic_messages.",
+            log_prefix,
+            sorted(leaked),
+        )
+    return api_kwargs

agent/chat_completion_helpers.py

@@ -2024,6 +2024,14 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
         # Per-attempt diagnostic dict for the retry block to consume.
         _diag = agent._stream_diag_init()
         request_client_holder["diag"] = _diag
+        # Defensive: strip Responses-only kwargs (instructions, input, ...)
+        # that can leak in under an api_mode-flip race. The Anthropic SDK
+        # raises a non-retryable TypeError on them, killing the turn. See
+        # #31673 / sanitize_anthropic_kwargs().
+        from agent.anthropic_adapter import sanitize_anthropic_kwargs
+        sanitize_anthropic_kwargs(
+            api_kwargs, log_prefix=getattr(agent, "log_prefix", "")
+        )
         # Use the Anthropic SDK's streaming context manager
         with agent._anthropic_client.messages.stream(**api_kwargs) as stream:
             # The Anthropic SDK exposes the raw httpx response on