feat: Add FastAPI-style dependency injection system for MCPServer

[gspeter-max]

Summary

Implements a FastAPI-style dependency injection system for MCPServer, allowing tools to declare dependencies via a Depends() marker. Dependencies are automatically resolved and injected, supporting nested dependencies, async providers, per-request caching, and testing overrides.

Problem

MCPServer lacked a built-in mechanism for injecting user-defined dependencies (database connections, auth services, configuration objects, etc.) into tool handlers. Developers had to rely on global variables or lifespan context, making testing difficult and limiting code organization.

Solution

Added a comprehensive dependency injection system with:

• Depends() class for declaring dependencies in function parameters
• DependencyResolver for automatic dependency graph resolution
• MCPServer.override_dependency() for testing
• Support for sync and async dependency functions
• Per-request caching to prevent redundant instantiation
• Full backward compatibility with existing code

Changes

Core Infrastructure

• src/mcp/server/mcpserver/utilities/dependencies.py (new): Depends class and find_dependency_parameters() function
• src/mcp/server/mcpserver/utilities/dependency_resolver.py (new): DependencyResolver class for resolving dependency graphs

Tool Integration

• src/mcp/server/mcpserver/tools/base.py: Added dependency_kwarg_names field to Tool class
• src/mcp/server/mcpserver/tools/tool_manager.py: Added dependency override support and resolver creation

Server Integration

• src/mcp/server/mcpserver/server.py: Added override_dependency() method to MCPServer

Prompt Support

• src/mcp/server/mcpserver/prompts/base.py: Added dependency support to Prompt class
• src/mcp/server/mcpserver/prompts/manager.py: Added dependency override support

Public API

• src/mcp/server/__init__.py: Exported Depends class

Tests

• tests/server/mcpserver/utilities/test_dependencies.py (new): 11 unit tests for the DI system
• tests/server/mcpserver/test_dependency_injection.py (new): 7 integration tests with tools

Example Usage

from mcp import MCPServer, Depends

def get_database():
    return Database()

server = MCPServer("my-server")

@server.tool()
def query_users(
    limit: int,
    db: Database = Depends(get_database),
) -> list[User]:
    return db.query("SELECT * FROM users LIMIT ?", limit)

Nested Dependencies

def get_config() -> Config:
    return Config(db_url="postgresql://localhost/mydb")

def get_database(config: Config = Depends(get_config)) -> Database:
    return Database(config.db_url)

@server.tool()
def find_user(
    repo: UserRepository = Depends(get_repository),
) -> User:
    return repo.find(user_id)

Testing with Overrides

def get_test_db() -> Database:
    return MockDatabase([...])

server.override_dependency(get_database, get_test_db)

Test Results

All tests passing:

• 11 unit tests for core DI functionality
• 7 integration tests with tools
• 100% backward compatibility maintained

Type Safety

• Full type hints provided
• Works with pyright static type checking
• Generic Depends[T] for return type tracking

Backward Compatibility

This implementation is 100% backward compatible:

• Existing tools without Depends() continue to work unchanged
• Context injection continues to work as before
• No breaking changes to public API

Fixes

Implements #1254 from the V2 checklist