fix(client): validate JSON-null structuredContent against outputSchema

SEP-2106 allows structuredContent to be JSON null. The client presence
check used `is None`, which also matches an omitted field, so a tool
that advertised a null-capable outputSchema and returned null was
rejected as missing structured content.

Use model_fields_set so omitted still fails closed, explicit null is
schema-validated, and falsy JSON values (0, false, "") stay checked.

Fixes #3345

Author: epistemedeus

src/mcp/client/session.py

@@ -1141,7 +1141,9 @@ async def validate_tool_result(self, name: str, result: types.CallToolResult) ->
         if output_schema is not None:
             from jsonschema import exceptions as jsonschema_exceptions
 
-            if result.structured_content is None:
+            # SEP-2106 allows JSON null. Pydantic maps both omitted and explicit
+            # null to None; model_fields_set is the presence check (not falsy).
+            if result.structured_content is None and "structured_content" not in result.model_fields_set:
                 raise RuntimeError(f"Tool {name} has an output schema but did not return structured content")
             validator = self._output_schema_validator(name, output_schema)
             # `best_match` picks the same error the previous `jsonschema.validate()` call raised,

tests/client/test_session_promotions.py

@@ -66,6 +66,49 @@ async def test_validate_tool_result_raises_on_schema_mismatch() -> None:
             await client.session.validate_tool_result("t", CallToolResult(content=[], structured_content={"x": "no"}))
 
 
+@pytest.mark.anyio
+async def test_validate_tool_result_accepts_explicit_json_null_when_the_schema_allows_it() -> None:
+    """SEP-2106: JSON null is a legal structuredContent value. A parsed result that
+    includes `"structuredContent": null` must be validated against the schema, not
+    rejected as a missing field. Pydantic stores both omitted and JSON null as None;
+    `model_fields_set` is how the client tells them apart.
+    """
+    server = _make_server({"type": "null"})
+    parsed = CallToolResult.model_validate({"content": [], "structuredContent": None})
+    async with Client(server) as client:
+        await client.session.validate_tool_result("t", parsed)
+
+
+@pytest.mark.anyio
+async def test_validate_tool_result_rejects_explicit_json_null_when_the_schema_does_not_allow_it() -> None:
+    """An explicit JSON null that fails the advertised object schema is a schema mismatch,
+    not a missing structuredContent field.
+    """
+    server = _make_server({"type": "object", "properties": {"x": {"type": "integer"}}, "required": ["x"]})
+    parsed = CallToolResult.model_validate({"content": [], "structuredContent": None})
+    async with Client(server) as client:
+        with pytest.raises(RuntimeError, match="Invalid structured content returned by tool t"):
+            await client.session.validate_tool_result("t", parsed)
+
+
+@pytest.mark.anyio
+async def test_validate_tool_result_still_rejects_omitted_structured_content() -> None:
+    server = _make_server({"type": "null"})
+    async with Client(server) as client:
+        with pytest.raises(RuntimeError, match="Tool t has an output schema but did not return structured content"):
+            await client.session.validate_tool_result("t", CallToolResult(content=[]))
+
+
+@pytest.mark.anyio
+async def test_validate_tool_result_validates_falsy_non_null_structured_content() -> None:
+    """0 / false / "" are present JSON values and must be schema-checked, not treated as missing."""
+    server = _make_server({"type": "number"})
+    async with Client(server) as client:
+        await client.session.validate_tool_result("t", CallToolResult(content=[], structured_content=0))
+        with pytest.raises(RuntimeError, match="Invalid structured content returned by tool t"):
+            await client.session.validate_tool_result("t", CallToolResult(content=[], structured_content=False))
+
+
 @pytest.mark.anyio
 async def test_validate_tool_result_raises_on_an_unusable_output_schema() -> None:
     """A schema that isn't valid JSON Schema is reported as such, on every call."""