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Python: Fix hosted MCP replay producing orphan function_call_output (#5581)

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* Python: Fix hosted MCP replay producing orphan function_call_output

Resolves part of #5546. After a turn ran a hosted MCP / Foundry-toolbox-MCP
tool, the next turn's replayed input array carried a function_call_output
with an mcp_* call_id and no matching function_call, and the Responses API
returned a 400.

Two layers covered here:

* Chat-client serialize layer (packages/openai): adds mcp_server_tool_call
  and mcp_server_tool_result cases to _prepare_message_for_openai and
  _prepare_content_for_openai. Pairs are coalesced via a post-pass into a
  single mcp_call input item carrying both arguments and output. Orphan
  results are dropped (debug-logged) rather than serialized as orphan
  function_call_output, which is what the Responses API rejected.

* Host read layer (packages/foundry_hosting): _item_to_message and
  _output_item_to_message now route custom_tool_call_output whose
  call_id.startswith("mcp_") to Content.from_mcp_server_tool_result.
  Non-mcp_ call_ids continue to produce Content.from_function_result.
  Symmetric with the host write-side choice for hosted-MCP results.

Two further fixes (agentserver SDK additions, host write-side single-item
emission) remain tracked on the issue and depend on an SDK release.

* Python: Fix pyright unknown-type in _stringify_mcp_output

cast(Sequence[Any], output) after the isinstance check so pyright stops
flagging the loop variable as unknown. Also normalizes a couple of
em-dashes in docstrings I introduced in the prior commit.

* Python: Harden _stringify_mcp_output for dict-shaped MCP outputs

Address Copilot review on PR #5581. Today the helper falls back to
str() for any non-string, non-text-attribute entry, which produces
Python repr (single-quoted dicts) for the canonical MCP raw-JSON
text-content shape `{"type": "text", "text": "..."}` and any other
dict-shaped output.

Three small changes:

* List-entry path: prefer plain string entries, then `.text` attribute
  (Content objects), then `entry["text"]` for Mapping entries in the
  canonical MCP shape, then JSON-encode anything else.
* Final fallback: `json.dumps(output, default=str)` so Mappings and
  scalars produce valid JSON rather than Python repr.
* Two new unit tests covering the dict-with-text shape and the
  non-text-dict JSON fallback.

* Python: Suppress mypy redundant-cast on _stringify_mcp_output narrowing

The cast is needed by pyright (reportUnknownVariableType) but mypy
considers it redundant after the preceding isinstance narrowing.
Pyright's behavior is correct for the strict-mode reporting we run,
so keep the cast and silence mypy on the line.
This commit is contained in:
Evan Mattson
2026-04-30 21:01:52 +00:00
committed by GitHub
parent 6cd81286a9
commit 317ef4491e
4 changed files with 332 additions and 1 deletions
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python/packages/openai/tests/openai/test_openai_chat_client.py
+133
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@@ -5133,4 +5133,137 @@ def test_prepare_messages_for_openai_filters_none_fc_id() -> None:
assert fc_item["id"].startswith("fc_")
# region: hosted MCP round-trip (issue #5546)
def test_prepare_messages_for_openai_serializes_mcp_server_tool_call_as_mcp_call_input_item() -> None:
"""A Message containing only an mcp_server_tool_call Content should produce
a top-level mcp_call input item, not be silently dropped (which today's
_prepare_content_for_openai default branch does).
"""
client = OpenAIChatClient(model="test-model", api_key="test-key")
messages = [
Message(
role="assistant",
contents=[
Content.from_mcp_server_tool_call(
call_id="mcp_abc123",
tool_name="search",
server_name="api_specs",
arguments='{"q": "cats"}',
)
],
),
]
result = client._prepare_messages_for_openai(messages)
mcp_items = [item for item in result if isinstance(item, dict) and item.get("type") == "mcp_call"]
assert len(mcp_items) == 1, f"expected exactly one mcp_call item; got result={result}"
item = mcp_items[0]
assert item["id"] == "mcp_abc123"
assert item["server_label"] == "api_specs"
assert item["name"] == "search"
assert item["arguments"] == '{"q": "cats"}'
assert "output" not in item or item["output"] is None
def test_prepare_messages_for_openai_coalesces_mcp_call_and_result_into_single_item() -> None:
"""An mcp_server_tool_call followed by an mcp_server_tool_result with the
same call_id (in same or separate Messages) must produce ONE mcp_call
input item carrying both arguments and output. Two items would let the
Responses API see an orphaned output and reject the request.
"""
client = OpenAIChatClient(model="test-model", api_key="test-key")
messages = [
Message(
role="assistant",
contents=[
Content.from_mcp_server_tool_call(
call_id="mcp_abc123",
tool_name="search",
server_name="api_specs",
arguments='{"q": "cats"}',
)
],
),
Message(
role="tool",
contents=[
Content.from_mcp_server_tool_result(
call_id="mcp_abc123",
output=[Content.from_text(text="found 10 cats")],
)
],
),
]
result = client._prepare_messages_for_openai(messages)
mcp_items = [item for item in result if isinstance(item, dict) and item.get("type") == "mcp_call"]
assert len(mcp_items) == 1, f"expected one coalesced mcp_call item carrying both arguments and output; got {result}"
item = mcp_items[0]
assert item["id"] == "mcp_abc123"
assert item["arguments"] == '{"q": "cats"}'
assert item.get("output") == "found 10 cats"
# And no orphaned function_call_output should appear anywhere in the input.
fco_items = [item for item in result if isinstance(item, dict) and item.get("type") == "function_call_output"]
assert fco_items == [], f"unexpected orphan function_call_output items: {fco_items}"
def test_prepare_messages_for_openai_drops_orphan_mcp_server_tool_result() -> None:
"""When an mcp_server_tool_result has no matching mcp_server_tool_call in
the message list, it must be dropped, NOT serialized as a
function_call_output. An orphan function_call_output is what triggers the
Responses API 400 reported in #5546.
"""
client = OpenAIChatClient(model="test-model", api_key="test-key")
messages = [
Message(
role="tool",
contents=[
Content.from_mcp_server_tool_result(
call_id="mcp_orphan_id",
output=[Content.from_text(text="dangling output")],
)
],
),
]
result = client._prepare_messages_for_openai(messages)
fco_items = [item for item in result if isinstance(item, dict) and item.get("type") == "function_call_output"]
assert fco_items == [], f"orphan mcp_server_tool_result must not serialize as function_call_output; got {fco_items}"
mcp_items = [item for item in result if isinstance(item, dict) and item.get("type") == "mcp_call"]
assert mcp_items == [], f"orphan mcp_server_tool_result must not synthesize a stand-alone mcp_call; got {mcp_items}"
def test_stringify_mcp_output_extracts_text_from_dict_entries() -> None:
"""A list of dicts in the canonical MCP text-content shape
(`{"type": "text", "text": "..."}`, e.g. from raw-JSON-decoded MCP
responses) must unwrap to plain text rather than Python `repr`.
"""
result = OpenAIChatClient._stringify_mcp_output([{"type": "text", "text": "found 10 cats"}])
assert result == "found 10 cats"
def test_stringify_mcp_output_falls_back_to_json_for_non_text_dict_entries() -> None:
"""Dict entries that are not in the canonical text-content shape must
serialize as JSON, not Python `repr`. Python `repr` for a dict uses
single quotes and would not round-trip through any JSON-aware consumer.
"""
result = OpenAIChatClient._stringify_mcp_output([{"type": "image", "url": "https://example.com/x"}])
# Valid JSON: starts with `{`, contains the keys, no Python-repr single quotes.
assert result.startswith("{")
assert '"url"' in result
assert "'" not in result
# endregion
# endregion