Ensure multi-client configurations use new ProxyClient (#1045)

* Validate elicitation responses are dicts

* Ensure advanced MCP features are forwarded through proxy

* Use proxy client in tests

src/fastmcp/client/elicitation.py

@@ -53,6 +53,11 @@ def create_elicitation_callback(
             if not isinstance(result, ElicitResult):
                 result = ElicitResult(action="accept", content=result)
             content = to_jsonable_python(result.content)
+            if not isinstance(content, dict | None):
+                raise ValueError(
+                    "Elicitation responses must be serializable as a JSON object (dict). Received: "
+                    f"{result.content!r}"
+                )
             return MCPElicitResult(**result.model_dump() | {"content": content})
         except Exception as e:
             return mcp.types.ErrorData(

src/fastmcp/client/transports.py

@@ -773,8 +773,6 @@ class MCPConfigTransport(ClientTransport):
     """
 
     def __init__(self, config: MCPConfig | dict):
-        from fastmcp.client.client import Client
-
         if isinstance(config, dict):
             config = MCPConfig.from_dict(config)
         self.config = config
@@ -792,9 +790,9 @@ class MCPConfigTransport(ClientTransport):
             composite_server = FastMCP()
 
             for name, server in self.config.mcpServers.items():
-                server_client = Client(transport=server.to_transport())
                 composite_server.mount(
-                    prefix=name, server=FastMCP.as_proxy(server_client)
+                    prefix=name,
+                    server=FastMCP.as_proxy(backend=server.to_transport()),
                 )
 
             self.transport = FastMCPTransport(mcp=composite_server)

tests/client/test_elicitation.py

@@ -51,7 +51,7 @@ async def test_elicitation_with_no_handler(fastmcp_server):
 
     async with Client(fastmcp_server) as client:
         with pytest.raises(ToolError, match="Elicitation not supported"):
-            await client.call_tool("ask_for_name", {})
+            await client.call_tool("ask_for_name")
 
 
 async def test_elicitation_accept_content(fastmcp_server):
@@ -64,7 +64,7 @@ async def test_elicitation_accept_content(fastmcp_server):
     async with Client(
         fastmcp_server, elicitation_handler=elicitation_handler
     ) as client:
-        result = await client.call_tool("ask_for_name", {})
+        result = await client.call_tool("ask_for_name")
         assert result.data == "Hello, Alice!"
 
 
@@ -77,7 +77,7 @@ async def test_elicitation_decline(fastmcp_server):
     async with Client(
         fastmcp_server, elicitation_handler=elicitation_handler
     ) as client:
-        result = await client.call_tool("ask_for_name", {})
+        result = await client.call_tool("ask_for_name")
         assert result.data == "No name provided."
 
 
@@ -600,3 +600,36 @@ class TestPatternMatching:
         async with Client(mcp, elicitation_handler=elicitation_handler) as client:
             result = await client.call_tool("pattern_match_tool", {})
             assert result.data == "Cancelled"
+
+
+async def test_elicitation_implicit_acceptance(fastmcp_server):
+    """Test that elicitation handler can return data directly without ElicitResult wrapper."""
+
+    async def elicitation_handler(message, response_type, params, ctx):
+        # Return data directly without wrapping in ElicitResult
+        # This should be treated as implicit acceptance
+        return response_type(name="Bob")
+
+    async with Client(
+        fastmcp_server, elicitation_handler=elicitation_handler
+    ) as client:
+        result = await client.call_tool("ask_for_name")
+        assert result.data == "Hello, Bob!"
+
+
+async def test_elicitation_implicit_acceptance_must_be_dict(fastmcp_server):
+    """Test that elicitation handler can return data directly without ElicitResult wrapper."""
+
+    async def elicitation_handler(message, response_type, params, ctx):
+        # Return data directly without wrapping in ElicitResult
+        # This should be treated as implicit acceptance
+        return "Bob"
+
+    async with Client(
+        fastmcp_server, elicitation_handler=elicitation_handler
+    ) as client:
+        with pytest.raises(
+            ToolError,
+            match="Elicitation responses must be serializable as a JSON object",
+        ):
+            await client.call_tool("ask_for_name")

tests/client/test_openapi.py

@@ -48,8 +48,7 @@ def run_server(host: str, port: int, **kwargs) -> None:
 
 
 def run_proxy_server(host: str, port: int, shttp_url: str, **kwargs) -> None:
-    client = Client(transport=StreamableHttpTransport(shttp_url))
-    app = FastMCP.as_proxy(client)
+    app = FastMCP.as_proxy(StreamableHttpTransport(shttp_url))
     app.run(host=host, port=port, **kwargs)
 
 

tests/server/test_import_server.py

@@ -297,7 +297,7 @@ async def test_import_with_proxy_tools():
     def get_data(query: str) -> str:
         return f"Data for query: {query}"
 
-    proxy_app = FastMCP.as_proxy(Client(api_app))
+    proxy_app = FastMCP.as_proxy(api_app)
     await main_app.import_server(proxy_app, "api")
 
     async with Client(main_app) as client:
@@ -321,7 +321,7 @@ async def test_import_with_proxy_prompts():
         """Example greeting prompt."""
         return f"Hello, {name} from API!"
 
-    proxy_app = FastMCP.as_proxy(Client(api_app))
+    proxy_app = FastMCP.as_proxy(api_app)
     await main_app.import_server(proxy_app, "api")
 
     async with Client(main_app) as client:
@@ -349,7 +349,7 @@ async def test_import_with_proxy_resources():
             "base_url": "https://api.example.com",
         }
 
-    proxy_app = FastMCP.as_proxy(Client(api_app))
+    proxy_app = FastMCP.as_proxy(api_app)
     await main_app.import_server(proxy_app, "api")
 
     # Access the resource through the main app with the prefixed key
@@ -376,7 +376,7 @@ async def test_import_with_proxy_resource_templates():
     def create_user(name: str, email: str):
         return {"name": name, "email": email}
 
-    proxy_app = FastMCP.as_proxy(Client(api_app))
+    proxy_app = FastMCP.as_proxy(api_app)
     await main_app.import_server(proxy_app, "api")
 
     # Instantiate the template through the main app with the prefixed key

tests/server/test_mount.py

@@ -777,9 +777,7 @@ class TestProxyServer:
             return f"Data for {query}"
 
         # Create proxy server
-        proxy_server = FastMCP.as_proxy(
-            Client(transport=FastMCPTransport(original_server))
-        )
+        proxy_server = FastMCP.as_proxy(FastMCPTransport(original_server))
 
         # Mount proxy server
         main_app = FastMCP("MainApp")
@@ -800,9 +798,7 @@ class TestProxyServer:
         original_server = FastMCP("OriginalServer")
 
         # Create proxy server
-        proxy_server = FastMCP.as_proxy(
-            Client(transport=FastMCPTransport(original_server))
-        )
+        proxy_server = FastMCP.as_proxy(FastMCPTransport(original_server))
 
         # Mount proxy server
         main_app = FastMCP("MainApp")
@@ -832,9 +828,7 @@ class TestProxyServer:
             return {"api_key": "12345"}
 
         # Create proxy server
-        proxy_server = FastMCP.as_proxy(
-            Client(transport=FastMCPTransport(original_server))
-        )
+        proxy_server = FastMCP.as_proxy(FastMCPTransport(original_server))
 
         # Mount proxy server
         main_app = FastMCP("MainApp")
@@ -856,9 +850,7 @@ class TestProxyServer:
             return f"Welcome, {name}!"
 
         # Create proxy server
-        proxy_server = FastMCP.as_proxy(
-            Client(transport=FastMCPTransport(original_server))
-        )
+        proxy_server = FastMCP.as_proxy(FastMCPTransport(original_server))
 
         # Mount proxy server
         main_app = FastMCP("MainApp")
@@ -914,7 +906,7 @@ class TestAsProxyKwarg:
     async def test_as_proxy_ignored_for_proxy_mounts_default(self):
         mcp = FastMCP("Main")
         sub = FastMCP("Sub")
-        sub_proxy = FastMCP.as_proxy(Client(transport=FastMCPTransport(sub)))
+        sub_proxy = FastMCP.as_proxy(FastMCPTransport(sub))
 
         mcp.mount(sub_proxy, "sub")
 
@@ -923,7 +915,7 @@ class TestAsProxyKwarg:
     async def test_as_proxy_ignored_for_proxy_mounts_false(self):
         mcp = FastMCP("Main")
         sub = FastMCP("Sub")
-        sub_proxy = FastMCP.as_proxy(Client(transport=FastMCPTransport(sub)))
+        sub_proxy = FastMCP.as_proxy(FastMCPTransport(sub))
 
         mcp.mount(sub_proxy, "sub", as_proxy=False)
 
@@ -932,7 +924,7 @@ class TestAsProxyKwarg:
     async def test_as_proxy_ignored_for_proxy_mounts_true(self):
         mcp = FastMCP("Main")
         sub = FastMCP("Sub")
-        sub_proxy = FastMCP.as_proxy(Client(transport=FastMCPTransport(sub)))
+        sub_proxy = FastMCP.as_proxy(FastMCPTransport(sub))
 
         mcp.mount(sub_proxy, "sub", as_proxy=True)
 

tests/tools/test_tool_transform.py

@@ -725,7 +725,7 @@ class TestProxy:
     def proxy_server(self, mcp_server: FastMCP) -> FastMCP:
         from fastmcp.client.transports import FastMCPTransport
 
-        proxy = FastMCP.as_proxy(Client(transport=FastMCPTransport(mcp_server)))
+        proxy = FastMCP.as_proxy(FastMCPTransport(mcp_server))
         return proxy
 
     async def test_transform_proxy(self, proxy_server: FastMCP):

tests/utilities/test_mcp_config.py

@@ -4,6 +4,7 @@ from pathlib import Path
 from fastmcp.client.auth.bearer import BearerAuth
 from fastmcp.client.auth.oauth import OAuthClientProvider
 from fastmcp.client.client import Client
+from fastmcp.client.logging import LogMessage
 from fastmcp.client.transports import (
     SSETransport,
     StdioTransport,
@@ -195,3 +196,91 @@ async def test_remote_config_with_oauth_literal():
     client = Client(config)
     assert isinstance(client.transport.transport, StreamableHttpTransport)
     assert isinstance(client.transport.transport.auth, OAuthClientProvider)
+
+
+async def test_multi_client_with_logging(tmp_path: Path):
+    """
+    Tests that logging is properly forwarded to the ultimate client.
+    """
+    server_script = inspect.cleandoc("""
+        from fastmcp import FastMCP, Context
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        async def log_test(message: str, ctx: Context) -> int:
+            await ctx.log(message)
+            return 42
+
+        if __name__ == '__main__':
+            mcp.run()
+        """)
+
+    script_path = tmp_path / "test.py"
+    script_path.write_text(server_script)
+
+    config = {
+        "mcpServers": {
+            "test_server": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+            "test_server_2": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+        }
+    }
+
+    MESSAGES = []
+
+    async def log_handler(message: LogMessage):
+        MESSAGES.append(message)
+
+    async with Client(config, log_handler=log_handler) as client:
+        result = await client.call_tool("test_server_log_test", {"message": "test 42"})
+        assert result.data == 42
+        assert len(MESSAGES) == 1
+        assert MESSAGES[0].data == "test 42"
+
+
+async def test_multi_client_with_elicitation(tmp_path: Path):
+    """
+    Tests that elicitation is properly forwarded to the ultimate client.
+    """
+    server_script = inspect.cleandoc("""
+        from fastmcp import FastMCP, Context
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        async def elicit_test(ctx: Context) -> int:
+            result = await ctx.elicit('Pick a number', response_type=int)
+            return result.data
+
+        if __name__ == '__main__':
+            mcp.run()
+        """)
+
+    script_path = tmp_path / "test.py"
+    script_path.write_text(server_script)
+
+    config = {
+        "mcpServers": {
+            "test_server": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+            "test_server_2": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+        }
+    }
+
+    async def elicitation_handler(message, response_type, params, ctx):
+        return response_type(value=42)
+
+    async with Client(config, elicitation_handler=elicitation_handler) as client:
+        result = await client.call_tool("test_server_elicit_test", {})
+        assert result.data == 42