langchain-ai/deepagents · error · ValueError

Server '{server_name}' has unsupported transport type '{serv

Error message

Server '{server_name}' has unsupported transport type '{server_type}'. Supported types: stdio, sse, http

What it means

MCP server configs support only three transport types: `stdio`, `sse`, and `http`. `_validate_server_config` resolves the effective type from the `type` field and raises this ValueError for anything else, since no transport implementation exists for other values.

Source

Thrown at libs/code/deepagents_code/mcp_tools.py:933

                f"Server '{server_name}' has type 'stdio' but also declares "
                "a 'url' field. Remove 'url' or set "
                '`"type": "http"` (or `"sse"`) for a remote server.'
            )
            raise ValueError(error_msg)

        if "args" in server_config and not isinstance(server_config["args"], list):
            error_msg = f"Server '{server_name}' 'args' must be a list"
            raise TypeError(error_msg)

        if "env" in server_config and not isinstance(server_config["env"], dict):
            error_msg = f"Server '{server_name}' 'env' must be a dictionary"
            raise TypeError(error_msg)
    else:
        error_msg = (
            f"Server '{server_name}' has unsupported transport type '{server_type}'. "
            "Supported types: stdio, sse, http"
        )
        raise ValueError(error_msg)

    auth = server_config.get("auth")
    if auth is not None:
        if auth != "oauth":
            msg = (
                f"Server '{server_name}' has unsupported auth value "
                f"{auth!r}. Only 'oauth' is supported."
            )
            raise ValueError(msg)
        if server_type == "stdio":
            msg = (
                f"Server '{server_name}' uses stdio transport; "
                "'auth: oauth' is only valid for http/sse transports."
            )
            raise ValueError(msg)
        header_names = {name.lower() for name in (server_config.get("headers") or {})}
        if "authorization" in header_names:
            msg = (

View on GitHub (pinned to a1af029e6e)

Solutions

1. Change `type` to one of the supported values: "stdio", "sse", or "http".
2. Use "http" for modern streamable-HTTP remote servers if your previous client called it 'streamable-http'.
3. Fix casing/typos in the type string (e.g. 'htttp' -> 'http').
4. Remove the `type` key for local subprocess servers so it defaults to stdio.

Example fix

// before
{"docs": {"type": "websocket", "url": "wss://mcp.example.com"}}
// after
{"docs": {"type": "http", "url": "https://mcp.example.com"}}

SUPPORTED_TYPES = {"stdio", "sse", "http"}

def validate_type(name: str, cfg: dict) -> None:
    stype = cfg.get("type", "stdio")
    if stype not in SUPPORTED_TYPES:
        raise ValueError(f"Server '{name}' has unsupported transport type '{stype}'. Supported: {sorted(SUPPORTED_TYPES)}")

def has_supported_type(cfg: dict) -> bool:
    return cfg.get("type", "stdio") in {"stdio", "sse", "http"}

try:
    tools = resolve_and_load_mcp_tools(config)
except ValueError as e:
    if "unsupported transport type" in str(e):
        print(f"Fix 'type' field: {e}")  # map 'websocket'/'streamable-http' etc. to http, fix typos/casing
    else:
        raise

When it happens

Trigger: A server entry has `"type"` set to a value outside {stdio, sse, http} — e.g. `"type": "websocket"`, `"type": "streamable"`, or a typo like `"type": "htttp"` — and is validated through `select_server`, `resolve_and_load_mcp_tools`, `_validate_mcp_config_servers`, or `_drop_invalid_mcp_config_servers`.

Common situations: Typoed type strings; copying types from other MCP clients that support extra transports (websocket, streamable-http); uppercase or mixed-case values like 'STDIO' when resolution is case-sensitive; spec drift after a library version change.

Related errors

• Server '{server_name}' uses {transport!r} transport; OAuth l
• {str(exc)}
• {field} references unset env var {name}. Set {name} in the e
• {field} contains a malformed '${{...}}' reference. Use '${VA
• {field} must be a string, got {type(value).__name__}

AI-assisted analysis of langchain-ai/deepagents@a1af029e6e (2026-08-29). Data as JSON: /api/errors/1f3061caf607975d. Report an issue: GitHub.