fix: strip discriminator after dereferencing schemas (#3682)

Author: jlowin

src/fastmcp/utilities/json_schema.py

@@ -73,6 +73,33 @@ def _strip_remote_refs(obj: Any) -> Any:
     return obj
 
 
+def _strip_discriminator(obj: Any) -> Any:
+    """Recursively remove OpenAPI ``discriminator`` keys from a schema.
+
+    Pydantic emits ``discriminator.mapping`` with values like
+    ``#/$defs/ClassName``.  After ``$defs`` are inlined and removed by
+    ``dereference_refs``, those mapping entries dangle.  The keyword is an
+    OpenAPI extension — the ``anyOf`` variants already carry ``const`` on
+    the discriminant field, so the mapping is redundant.
+
+    Only strips ``discriminator`` when it appears alongside ``anyOf`` or
+    ``oneOf``, which is where the OpenAPI keyword lives.  A property
+    *named* ``discriminator`` (inside ``properties``) is left alone.
+    """
+    if isinstance(obj, dict):
+        skip = "discriminator" in obj and ("anyOf" in obj or "oneOf" in obj)
+        # Keys that hold instance data, not sub-schemas — don't recurse.
+        _DATA_KEYS = {"default", "const", "examples", "enum"}
+        return {
+            k: (v if k in _DATA_KEYS else _strip_discriminator(v))
+            for k, v in obj.items()
+            if not (k == "discriminator" and skip)
+        }
+    if isinstance(obj, list):
+        return [_strip_discriminator(item) for item in obj]
+    return obj
+
+
 def dereference_refs(schema: dict[str, Any]) -> dict[str, Any]:
     """Resolve all $ref references in a JSON schema by inlining definitions.
 
@@ -135,6 +162,13 @@ def dereference_refs(schema: dict[str, Any]) -> dict[str, Any]:
         if "$defs" in dereferenced:
             dereferenced = {k: v for k, v in dereferenced.items() if k != "$defs"}
 
+        # Strip `discriminator` keys — they contain `mapping` values that
+        # point at `#/$defs/...` entries we just removed.  `discriminator`
+        # is an OpenAPI extension; after inlining, the `anyOf` variants
+        # already carry `const` on the discriminant field, making the
+        # mapping redundant.
+        dereferenced = _strip_discriminator(dereferenced)
+
         return dereferenced
 
     except JsonRefError:

tests/utilities/test_json_schema.py

@@ -197,6 +197,67 @@ def test_preserves_nested_siblings(self):
         assert country["default"] == "US"
         assert "$defs" not in result
 
+    def test_strips_discriminator_mapping_after_inlining(self):
+        """Discriminator.mapping refs dangle after $defs are inlined (#3679)."""
+        schema = {
+            "$defs": {
+                "IdentifyPerson": {
+                    "type": "object",
+                    "properties": {
+                        "action": {"const": "identify", "type": "string"},
+                        "name": {"type": "string"},
+                    },
+                    "required": ["action", "name"],
+                },
+                "PersonDelete": {
+                    "type": "object",
+                    "properties": {
+                        "action": {"const": "delete", "type": "string"},
+                    },
+                    "required": ["action"],
+                },
+            },
+            "anyOf": [
+                {"$ref": "#/$defs/IdentifyPerson"},
+                {"$ref": "#/$defs/PersonDelete"},
+            ],
+            "discriminator": {
+                "mapping": {
+                    "identify": "#/$defs/IdentifyPerson",
+                    "delete": "#/$defs/PersonDelete",
+                },
+                "propertyName": "action",
+            },
+        }
+        result = dereference_refs(schema)
+
+        assert "$defs" not in result
+        assert "discriminator" not in result
+        # The anyOf variants should be inlined with their const values intact
+        assert len(result["anyOf"]) == 2
+        actions = {v["properties"]["action"]["const"] for v in result["anyOf"]}
+        assert actions == {"identify", "delete"}
+
+    def test_preserves_property_named_discriminator(self):
+        """A field *named* 'discriminator' inside properties must survive."""
+        schema = {
+            "$defs": {
+                "Inner": {
+                    "type": "object",
+                    "properties": {
+                        "discriminator": {"type": "string"},
+                    },
+                },
+            },
+            "properties": {
+                "item": {"$ref": "#/$defs/Inner"},
+            },
+        }
+        result = dereference_refs(schema)
+
+        assert "$defs" not in result
+        assert "discriminator" in result["properties"]["item"]["properties"]
+
 
 class TestCompressSchema:
     """Tests for the compress_schema function."""