ceos/fastmcp
1
0
Fork 0
mirror of https://github.com/PrefectHQ/fastmcp.git synced 2026-08-22 05:24:18 +02:00
Code Issues Projects Releases Packages Wiki Activity

Fix compress_schema to preserve additionalProperties: false for MCP compatibility (#3102)

Browse source 
Changes:
- Changed default of prune_additional_properties from True to False in compress_schema
- Added test demonstrating MCP client compatibility requirement
- Updated existing tests to explicitly enable pruning when needed
- Added additionalProperties: false to manually constructed schemas in tool_transform
- Updated inline snapshots to reflect new behavior

Fixes #3008

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>
This commit is contained in:
Jeremiah Lowin 2026-02-06 18:52:21 -05:00 • committed by GitHub
commit 6a358902f2
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
5 changed files with 72 additions and 7 deletions
Download patch file Download diff file Expand all files Collapse all files

2
src/fastmcp/tools/tool_transform.py
View file

@ -685,6 +685,7 @@ class TransformedTool(Tool):
"type": "object",
"properties": new_props,
"required": list(new_required),
"additionalProperties": False,
}
if parent_defs:
@ -868,6 +869,7 @@ class TransformedTool(Tool):
"type": "object",
"properties": merged_props,
"required": list(final_required),
"additionalProperties": False,
}
if merged_defs:

6
src/fastmcp/utilities/json_schema.py
View file

@ -364,7 +364,7 @@ def _single_pass_optimize(
def compress_schema(
schema: dict[str, Any],
prune_params: list[str] | None = None,
prune_additional_properties: bool = True,
prune_additional_properties: bool = False,
prune_titles: bool = False,
) -> dict[str, Any]:
"""
@ -378,7 +378,9 @@ def compress_schema(
Args:
schema: The schema to compress
prune_params: List of parameter names to remove from properties
prune_additional_properties: Whether to remove additionalProperties: false
prune_additional_properties: Whether to remove additionalProperties: false.
Defaults to False to maintain MCP client compatibility, as some clients
(e.g., Claude) require additionalProperties: false for strict validation.
prune_titles: Whether to remove title fields from the schema
"""
# Dereference $ref - this inlines all definitions and removes $defs

8
tests/tools/tool/test_tool.py
View file

@ -30,6 +30,7 @@ class TestToolFromFunction:
"description": "Add two numbers.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"a": {"type": "integer"},
"b": {"type": "integer"},
@ -83,6 +84,7 @@ class TestToolFromFunction:
"description": "Fetch data from URL.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {"url": {"type": "string"}},
"required": ["url"],
"type": "object",
@ -117,6 +119,7 @@ class TestToolFromFunction:
"description": "Adds two numbers.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"x": {"type": "integer"},
"y": {"type": "integer"},
@ -153,6 +156,7 @@ class TestToolFromFunction:
"description": "Adds two numbers.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"x": {"type": "integer"},
"y": {"type": "integer"},
@ -192,6 +196,7 @@ class TestToolFromFunction:
"description": "Create a new user.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"user": {
"properties": {
@ -270,6 +275,7 @@ class TestToolFromFunction:
"name": "my_tool",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {"x": {"title": "X"}},
"required": ["x"],
"type": "object",
@ -302,6 +308,7 @@ class TestToolFromFunction:
"description": "Add two numbers.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"_a": {"type": "integer"},
"_b": {"type": "integer"},
@ -353,6 +360,7 @@ class TestToolFromFunction:
"description": "Add two numbers.",
"tags": set(),
"parameters": {
"additionalProperties": False,
"properties": {
"x": {"type": "integer"},
"y": {"type": "integer"},

5
tests/tools/tool_transform/test_schemas.py
View file

@ -383,6 +383,7 @@ class TestInputSchema:
"field2": {"type": "boolean"},
},
"required": [],
"additionalProperties": False,
}
)
@ -424,6 +425,7 @@ class TestInputSchema:
}
},
"required": ["used_param"],
"additionalProperties": False,
}
)
@ -464,6 +466,7 @@ class TestInputSchema:
}
},
"required": ["renamed_input"],
"additionalProperties": False,
}
)
@ -508,6 +511,7 @@ class TestInputSchema:
},
},
"required": IsList("param_b", "param_a", check_order=False),
"additionalProperties": False,
}
)
@ -530,5 +534,6 @@ class TestInputSchema:
}
},
"required": ["param_a"],
"additionalProperties": False,
}
)

58
tests/utilities/test_json_schema.py
View file

@ -228,13 +228,14 @@ class TestCompressSchema:
assert result["required"] == ["bar"]
def test_pruning_additional_properties(self):
"""Test pruning additionalProperties when False."""
"""Test pruning additionalProperties when explicitly enabled."""
schema = {
"type": "object",
"properties": {"foo": {"type": "string"}},
"additionalProperties": False,
}
result = compress_schema(schema)
# Must explicitly enable pruning now (default changed for MCP compatibility)
result = compress_schema(schema, prune_additional_properties=True)
assert "additionalProperties" not in result
def test_disable_pruning_additional_properties(self):
@ -263,7 +264,9 @@ class TestCompressSchema:
"unused_def": {"type": "number"},
},
}
result = compress_schema(schema, prune_params=["remove"])
result = compress_schema(
schema, prune_params=["remove"], prune_additional_properties=True
)
# Check that parameter was removed
assert "remove" not in result["properties"]
# Check that required list was updated
@ -296,7 +299,7 @@ class TestCompressSchema:
assert "title" not in result["properties"]["bar"]["properties"]["nested"]
def test_prune_nested_additional_properties(self):
"""Test pruning additionalProperties: false at all levels."""
"""Test pruning additionalProperties: false at all levels when explicitly enabled."""
schema = {
"type": "object",
"additionalProperties": False,
@ -313,7 +316,7 @@ class TestCompressSchema:
},
},
}
result = compress_schema(schema)
result = compress_schema(schema, prune_additional_properties=True)
assert "additionalProperties" not in result
assert "additionalProperties" not in result["properties"]["foo"]
assert (
@ -393,6 +396,51 @@ class TestCompressSchema:
)
assert "title" not in compressed["properties"]["normal_field"]
def test_mcp_client_compatibility_requires_additional_properties(self):
"""Test that compress_schema preserves additionalProperties: false for MCP clients.
MCP clients like Claude require strict JSON schemas with additionalProperties: false.
When tools use Pydantic models with extra="forbid", this constraint must be preserved.
Without this, MCP clients return:
"Invalid schema for function 'X': In context=('properties', 'Y'),
'additionalProperties' is required to be supplied and to be false"
See: https://github.com/jlowin/fastmcp/issues/3008
"""
# Schema representing a Pydantic model with extra="forbid"
schema = {
"type": "object",
"properties": {
"graph_table": {
"type": "object",
"properties": {
"name": {"type": "string"},
"columns": {"type": "array", "items": {"type": "string"}},
},
"required": ["name"],
"additionalProperties": False,
}
},
"required": ["graph_table"],
"additionalProperties": False,
}
# By default, compress_schema should NOT strip additionalProperties: false
# This is the new expected behavior for MCP compatibility
result = compress_schema(schema)
# Root level should preserve additionalProperties: false
assert result.get("additionalProperties") is False, (
"Root additionalProperties: false was removed, breaking MCP compatibility"
)
# Nested object should also preserve additionalProperties: false
graph_table = result["properties"]["graph_table"]
assert graph_table.get("additionalProperties") is False, (
"Nested additionalProperties: false was removed, breaking MCP compatibility"
)
class TestResolveRootRef:
"""Tests for the resolve_root_ref function.