Use Depends() for database connection and RequestContext injection into tool handlers

[dtehan-td]

Summary

Replace the multi-step wrapper chain (make_tool_wrapper, create_mcp_tool, execute_db_tool, asyncio.to_thread) with FastMCP v3 Depends() so database connections and request context are injected directly into handler signatures as hidden parameters.

Background

Every tool currently goes through ~130 lines of plumbing: make_tool_wrapper() strips internal params from the signature, create_mcp_tool() builds an async wrapper with asyncio.to_thread(), execute_db_tool() fetches the connection, and _fetch_request_context() reaches into FastMCP state for the RequestContext. FastMCP v3's Depends() (borrowed from FastAPI) handles all of this natively — parameters annotated with Depends() are automatically hidden from the MCP schema.

Proposed Approach

from fastmcp.dependencies import Depends

def get_db_connection(ctx: Context = CurrentContext()) -> Connection:
    tdconn = ctx.lifespan_context["tdconn"]
    return tdconn.engine.connect()

def get_request_context() -> RequestContext | None:
    ctx = get_context()
    return ctx.get_state_sync("request_context")

@mcp.tool
def base_readQuery(
    sql: str,
    conn: Connection = Depends(get_db_connection),
    rc: RequestContext | None = Depends(get_request_context),
) -> str:
    """Run a read-only SQL query."""
    ...

Files to Change

• app.py:323-450
• tools/utils/factory.py (can be deleted)
• All tools/*/handle_*.py handler files (signature change)

Value

• Eliminates factory.py entirely — create_mcp_tool, _fetch_request_context, and asyncio.to_thread wrapper are no longer needed.
• Eliminates make_tool_wrapper() in app.py.
• handle_* functions can be registered directly with @mcp.tool — no wrapping step.
• Sync handlers still run in a thread pool automatically (FastMCP default for sync tools).

Dependencies

Requires lifespan management (#9) to be in place first. Benefits from docstring Args: sections (#6) being completed beforehand. A phased approach — adopt Depends() for new tools first and migrate existing handlers incrementally — is recommended to reduce risk.