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fix: remove dereference=True while calling compress_schema #3774

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fix: remove dereference=True while calling compress_schema #3774

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4 changes: 2 additions & 2 deletions src/fastmcp/tools/tool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -500,7 +500,6 @@ def from_function(
input_schema,
prune_params=prune_params,
prune_titles=True,
dereference=True,
)
Comment on lines 500 to 503
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@chatgpt-codex-connector chatgpt-codex-connector Bot Apr 6, 2026

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P1 Badge Keep input schemas dereferenced for client compatibility

Removing dereference=True here changes tool parameter schemas back to $ref/$defs form for common Pydantic inputs (e.g., user: UserInput), which regresses the compatibility behavior introduced for MCP clients that do not resolve JSON Schema refs. The codebase still documents this as required for clients like Copilot/Claude (src/fastmcp/utilities/json_schema.py) and existing tool-manager expectations assume inlined parameter objects, so this will make affected tools unusable in those clients rather than just changing formatting.

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output_schema = None
Expand Down Expand Up @@ -561,7 +560,8 @@ def from_function(
output_schema = base_schema

output_schema = compress_schema(
output_schema, prune_titles=True, dereference=True
output_schema,
prune_titles=True,
)

# Resolve root-level $ref to meet MCP spec requirement for type: object
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9 changes: 6 additions & 3 deletions tests/tools/test_tool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -194,15 +194,18 @@ def create_user(user: UserInput, flag: bool) -> dict:
"tags": set(),
"enabled": True,
"parameters": {
"properties": {
"user": {
"$defs": {
"UserInput": {
"properties": {
"name": {"type": "string"},
"age": {"type": "integer"},
},
"required": ["name", "age"],
"type": "object",
},
}
},
"properties": {
"user": {"$ref": "#/$defs/UserInput"},
"flag": {"type": "boolean"},
},
"required": ["user", "flag"],
Expand Down
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