Hmbown/CodeWhale · error · anyhow::Error

Unsupported MCP transport '{transport}'. Supported values: s

Error message

Unsupported MCP transport '{transport}'. Supported values: sse

What it means

validate_mcp_transport only accepts the value "sse" (compared case-insensitively after trimming); any other explicit transport string in an MCP server config is rejected with this message. Omitting the transport field entirely is valid - validation returns Ok(()) for None. The message deliberately lists the single supported value.

Source

Thrown at crates/tui/src/mcp.rs:1380

    }
}

fn is_legacy_sse_transport(config: &McpServerConfig) -> bool {
    config
        .transport
        .as_deref()
        .map(|transport| transport.trim().eq_ignore_ascii_case("sse"))
        .unwrap_or(false)
}

pub fn validate_mcp_transport(transport: Option<&str>) -> Result<()> {
    let Some(transport) = transport else {
        return Ok(());
    };
    if transport.trim().eq_ignore_ascii_case("sse") {
        return Ok(());
    }
    anyhow::bail!("Unsupported MCP transport '{transport}'. Supported values: sse");
}

fn response_id_matches(id: Option<&serde_json::Value>, expected_id: &str) -> bool {
    let Some(id) = id else {
        return false;
    };
    if id.as_str() == Some(expected_id) {
        return true;
    }
    id.as_u64()
        .map(|id| id.to_string() == expected_id)
        .unwrap_or(false)
}

// === McpConnection - Async Connection Management ===

/// Manages a single async connection to an MCP server
pub struct McpConnection {

View on GitHub (pinned to 0c42157ee5)

Solutions

Remove the transport field from the server entry; the url field alone determines the HTTP/SSE behavior
If you keep the field, set it to exactly "sse" (case-insensitive)
For local servers, drop url/transport and use "command" instead (stdio subprocess, no transport field needed)

Example fix

// before
{"servers": {"api": {"url": "https://example.com/mcp", "transport": "http"}}}

// after
{"servers": {"api": {"url": "https://example.com/mcp"}}}

Defensive patterns

Strategy: validation

Validate before calling

fn validTransport(value: Option<&str>) -> bool {
    match value {
        None => true,
        Some(t) => t.trim().eq_ignore_ascii_case("sse"),
    }
}

Prevention

Omit the transport field unless you specifically mean SSE
Validate config files with the same rule before shipping: only "sse" is accepted
Rely on the url field to select remote transports; do not copy transport enums from other clients

When it happens

Trigger: An MCP server config entry with "transport": "http", "stdio", "streamable-http", or any other string besides "sse", passed through config load or validate_mcp_transport.

Common situations: Copying a config from a client that supports richer transport enums; manually setting transport when the URL scheme already implies it; believing 'streamable-http' is a selectable option here (this build only gates the SSE transport field).

Related errors

AI-assisted analysis of Hmbown/CodeWhale@0c42157ee5 (2026-08-20). Data as JSON: /api/errors/a93d0af07ab52bf2. Report an issue: GitHub.