feat: implement claude code backend

src/beaver_gateway/mcp/client_pool.py

@@ -0,0 +1,48 @@
+"""Proxy ``FastMCP`` servers for user-declared external MCPs (``stdio``/``http``).
+
+Both flavours end up as a ``FastMCPProxy`` instance, built via
+``fastmcp.server.create_proxy``. The proxy lazily opens the underlying
+client transport when the first MCP request arrives, so we don't pay
+for connections that nothing routes to. From the aggregator app's
+point of view a proxy is indistinguishable from a regular ``FastMCP``
+namespace — same ``http_app`` surface, same mount semantics.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastmcp import Client
+from fastmcp.client.transports import StdioTransport, StreamableHttpTransport
+from fastmcp.server import create_proxy
+
+if TYPE_CHECKING:
+    from fastmcp import FastMCP
+
+    from beaver_gateway.mcp.types import HttpMcp, StdioMcp
+
+
+def build_stdio_proxy(spec: StdioMcp) -> FastMCP:
+    """Wrap a stdio subprocess MCP into a mountable ``FastMCPProxy``.
+
+    ``spec.command`` is a non-empty tuple; the first element is the
+    executable and the rest are CLI args. ``StdioTransport`` keeps the
+    subprocess alive across calls.
+    """
+    if not spec.command:
+        msg = f"stdio MCP {spec.name!r} has empty command"
+        raise ValueError(msg)
+    command, *args = spec.command
+    transport = StdioTransport(
+        command=command,
+        args=list(args),
+        env=spec.env,
+        cwd=str(spec.cwd) if spec.cwd is not None else None,
+    )
+    return create_proxy(Client(transport, name=spec.name))
+
+
+def build_http_proxy(spec: HttpMcp) -> FastMCP:
+    """Wrap a remote streamable-HTTP MCP into a mountable ``FastMCPProxy``."""
+    transport = StreamableHttpTransport(url=spec.url, auth=spec.auth)
+    return create_proxy(Client(transport, name=spec.name))