Kuberwastaken/claurst · error · anyhow::Error

MCP server ' ': unsupported transport type

Error message

MCP server '{}': unsupported transport type '{}'

What it means

McpClient::connect dispatches on config.server_type ("stdio", "sse", streamable-http). Any other string hits the catch-all arm and produces this error naming the server and the unrecognized transport type.

Solutions

  1. Set server_type to one of the supported values: "stdio", "sse", or the streamable-http value used by this library
  2. Check spelling/casing of the server_type field in the MCP config
  3. Consult docs for the exact accepted strings and update the config schema
  4. If you need a new transport, implement it in connect's match rather than inventing a server_type

Example fix

// before
{"name": "fs", "server_type": "streamable_http", ...}
// after
{"name": "fs", "server_type": "streamable-http", ...}
Defensive patterns

Strategy: validation

Validate before calling

const VALID: &[&str] = &["stdio", "sse", "streamable-http"];
assert!(VALID.contains(&cfg.server_type.as_str()), "bad server_type: {}", cfg.server_type);

Prevention

When it happens

Trigger: An McpServerConfig whose server_type field is misspelled or an unsupported value (e.g. "http", "websocket", "streamable_http", empty string) passed to McpClient::connect.

Common situations: Hand-written settings.json with `"type": "streamable_http"` instead of the accepted spelling; newer/older config schema mixing; copy-pasted config from a different MCP client using different transport names.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of Kuberwastaken/claurst@b0637c97ec (2026-09-10). Data as JSON: /api/errors/3c1895a75244954e. Report an issue: GitHub.

Appendix: source

Thrown at src-rust/crates/mcp/src/lib.rs:648

                                        protocol_version = %protocol_version_label,
                                        error = %message,
                                        "Streamable HTTP protocol version unsupported; trying fallback"
                                    );
                                    last_error = Some(e);
                                    continue;
                                }
                                return Err(e);
                            }
                        }
                    }
                    Err(last_error.unwrap_or_else(|| {
                        anyhow::anyhow!(
                            "MCP server '{}': no supported streamable HTTP protocol version found",
                            config.name
                        )
                    }))
                }
                other => Err(anyhow::anyhow!(
                    "MCP server '{}': unsupported transport type '{}'",
                    config.name,
                    other
                )),
            }
        }

        pub(crate) fn is_unsupported_protocol_error(message: &str) -> bool {
            // rmcp, servers, and gateways do not emit a single stable
            // protocol-negotiation error string, so match the known variants
            // conservatively to keep downgrade fallback working.
            let lower = message.to_ascii_lowercase();
            lower.contains("unsupported protocol version")
                || lower.contains("unsupported mcp-protocol-version")
                || (lower.contains("protocol version") && lower.contains("unsupported"))
                || (lower.contains("mcp-protocol-version") && lower.contains("bad request"))
        }

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