huggingface/smolagents · error · ValueError

Unsupported transport: {transport}. Supported transports are

Error message

Unsupported transport: {transport}. Supported transports are 'streamable-http' and 'sse'.

What it means

When MCPClient is constructed with a dict of server_parameters, it reads the 'transport' key (defaulting to 'streamable-http') and only accepts 'streamable-http' or 'sse'. Any other value raises ValueError immediately, before any connection is attempted. This mirrors the transports supported by the underlying MCPAdapt library.

Source

Thrown at src/smolagents/mcp_client.py:114

                "To suppress this warning, explicitly set structured_output=True (new behavior) or structured_output=False (legacy behavior). "
                "See documentation at https://huggingface.co/docs/smolagents/tutorials/tools#structured-output-and-output-schema-support for more details.",
                FutureWarning,
                stacklevel=2,
            )
            structured_output = False

        try:
            from mcpadapt.core import MCPAdapt
            from mcpadapt.smolagents_adapter import SmolAgentsAdapter
        except ModuleNotFoundError:
            raise ModuleNotFoundError("Please install 'mcp' extra to use MCPClient: `pip install 'smolagents[mcp]'`")
        if isinstance(server_parameters, dict):
            transport = server_parameters.get("transport")
            if transport is None:
                transport = "streamable-http"
                server_parameters["transport"] = transport
            if transport not in {"sse", "streamable-http"}:
                raise ValueError(
                    f"Unsupported transport: {transport}. Supported transports are 'streamable-http' and 'sse'."
                )
        adapter_kwargs = adapter_kwargs or {}
        self._adapter = MCPAdapt(
            server_parameters, SmolAgentsAdapter(structured_output=structured_output), **adapter_kwargs
        )
        self._tools: list[Tool] | None = None
        self.connect()

    def connect(self):
        """Connect to the MCP server and initialize the tools."""
        self._tools: list[Tool] = self._adapter.__enter__()

    def disconnect(
        self,
        exc_type: type[BaseException] | None = None,
        exc_value: BaseException | None = None,
        exc_traceback: TracebackType | None = None,

View on GitHub (pinned to 30bb116109)

Solutions

Set transport to 'streamable-http' or 'sse', or omit it to get the 'streamable-http' default
For stdio servers (command/args config), use the MCPAdapt stdio parameters form rather than a dict with 'transport'
Double-check for typos and trailing whitespace in the transport value

Example fix

# before
client = MCPClient({"url": "http://localhost:8000/mcp", "transport": "http"})

# after
client = MCPClient({"url": "http://localhost:8000/mcp", "transport": "streamable-http"})

Defensive patterns

Strategy: validation

Validate before calling

SUPPORTED_TRANSPORTS = {"streamable-http", "sse"}
transport = server_parameters.get("transport", "streamable-http")
if transport not in SUPPORTED_TRANSPORTS:
    raise ValueError(f"Pick one of {SUPPORTED_TRANSPORTS}")
client = MCPClient(server_parameters)

Type guard

def is_valid_mcp_transport(params: dict) -> bool:
    t = params.get("transport", "streamable-http")
    return t in {"streamable-http", "sse"}

Try / catch

try:
    client = MCPClient(server_parameters)
except ValueError as e:
    if 'Unsupported transport' in str(e):
        server_parameters['transport'] = 'streamable-http'
        client = MCPClient(server_parameters)

Prevention

Omit 'transport' to use the streamable-http default
Validate config dicts against the supported set before constructing MCPClient
Don't reuse stdio-style MCP configs with the dict transport form

When it happens

Trigger: Passing server_parameters={'url': ..., 'transport': 'stdio'} or 'websocket', 'http', or a typo like 'sse ' to MCPClient.__init__.

Common situations: Copy-pasting stdio-based MCP server config (command/args style) into the dict form; protocol rename confusion (older MCP examples used 'http'); misspelled transport strings.

Related errors

AI-assisted analysis of huggingface/smolagents@30bb116109 (2026-08-28). Data as JSON: /api/errors/9ce7a574dc718cd3. Report an issue: GitHub.