Feat: Add support for form-encoded and multipart request bodies

[ChingEnLin]

Describe your changes

Add support for application/x-www-form-urlencoded and multipart/form-data request bodies

Key Changes and Features:

1. Included logic to automatically detect the appropriate content type for an operation's request body the OpenAPI conversion process (fastapi_mcp/openapi/convert.py)

   • Established priority order: application/x-www-form-urlencoded > multipart/form-data > application/json.
   • Added new utility functions (detect_form_encoded_content_type, detect_multipart_content_type) to fastapi_mcp/openapi/utils.py to facilitate detection.

2. Added utility function extract_form_field_names in fastapi_mcp/openapi/utils.py to extract the expected form field names directly from the OpenAPI schema's properties for application/x-www-form-urlencoded and multipart/form-data request bodies

3. Included the detected content_type and form_fields for each operation in the operation_map

   • When an API tool is executed (_execute_api_tool in fastapi_mcp/server.py), the request body (arguments) is now intelligently filtered:
     • For form-based requests, only the arguments corresponding to the extracted form_fields are included in the request payload, ensuring that httpx sends them correctly as data (form-urlencoded) or files (multipart).
     • For application/json requests (or fallback scenarios), all arguments are still sent as a JSON payload, maintaining existing behavior.

4. Refactored the internal _request method in fastapi_mcp/server.py to use a new helper _request_with_body.

   • _request_with_body dynamically chooses between json=, data=, or files= parameters for httpx.AsyncClient.request based on the detected content_type, ensuring correct encoding for different request body types.

Backward Compatibility:

• Extensive backward compatibility tests (tests/test_backward_compatibility.py) have been added to ensure that existing JSON-based API endpoints continue to function without any changes.
• GET, PUT, DELETE, and other HTTP methods, along with error handling and header forwarding, have been verified to behave identically.
• The system gracefully falls back to JSON behavior if no specific form-based content type is detected or if schema extraction for form fields fails.

This enhancement provides greater flexibility in integrating with diverse OpenAPI-defined APIs that utilize various request body formats.

Issue ticket number and link (if applicable)

#119

Screenshots of the feature / bugfix

Profile Update (Form-encoded)
Tool: update_profile_users_profile_post
   Arguments: {
      "name": "John Doe",
      "email": "john.doe@example.com",
      "bio": "Software developer passionate about AI and automation",
      "newsletter": true
}
   Result: {
  "success": true,
  "message": "Profile updated successfully",
  "user_id": "user_1",
  "profile": {
    "name": "John Doe",
    "email": "john.doe@example.com",
    "bio": "Software developer passionate about AI and automation",
    "newsletter": true
  }
}
  HTTP Request Type: application/x-www-form-urlencoded

Checklist before requesting a review

• Added relevant tests
• Run ruff & mypy
• All tests pass

[Copilot]

Pull Request Overview

This pull request introduces comprehensive support for form-encoded and multipart request bodies in OpenAPI specifications. The enhancement enables MCP servers to intelligently handle different content types beyond JSON, with automatic detection and proper HTTP client configuration.

• Implements content type detection with priority-based selection (form-encoded > multipart > JSON)
• Adds form field extraction from OpenAPI schemas for precise request body construction
• Refactors HTTP client handling to use appropriate encoding based on detected content types

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_openapi_conversion.py	Adds comprehensive tests for content type detection functionality and operation map structure verification
tests/test_mcp_execute_api_tool.py	Updates existing tests to accommodate new HTTP client behavior using generic request() method
tests/test_backward_compatibility.py	New extensive backward compatibility test suite ensuring JSON endpoints remain unchanged
fastapi_mcp/server.py	Implements form field filtering logic and new _request_with_body() method for content-type-specific HTTP requests
fastapi_mcp/openapi/utils.py	Adds utility functions for content type detection and form field extraction from schemas
fastapi_mcp/openapi/convert.py	Integrates content type detection into OpenAPI conversion process and stores metadata in operation map

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