What causes "MCP server '{}': HTTP {} from legacy SSE transport: {}" in Kuberwastaken/claurst?

Reconnect the MCP session so a fresh endpoint event updates the POST URL

MCP server ' ': HTTP from legacy SSE transport

Error message

MCP server '{}': HTTP {} from legacy SSE transport: {}

What it means

After the SSE stream is established, JSON-RPC messages are POSTed to the server's message endpoint. This guard accepts 2xx and 202 Accepted, but any other non-success status on a POST response is fatal and reported with the server name, status, and body.

Solutions

Reconnect the MCP session so a fresh endpoint event updates the POST URL
Refresh OAuth credentials (re-run auth) if the status is 401/403
Back off and retry if the status is 429, honoring any Retry-After header
Inspect the body for 5xx and check server/proxy health

Defensive patterns

Strategy: retry

Try / catch

match client.send(msg).await {
    Err(e) if e.to_string().contains("from legacy SSE transport: HTTP") => {
        // 401/403: refresh credentials; 429: back off; 5xx: reconnect session
    }
    other => other?,
}

Prevention

Reconnect the session after any server restart to refresh the POST endpoint
Refresh OAuth tokens before they expire during long sessions
Implement backoff on 429 and surface 5xx bodies to server operators

When it happens

Trigger: handle_legacy_sse_http_response (reached from send and the message-queue handlers) receives an HTTP response whose status is neither success nor 202 ACCEPTED.

Common situations: Message endpoint URL became stale after a server restart; auth token expired mid-session (401); server rate-limiting (429); endpoint path wrong in proxy config (404); server crash returning 5xx.

Understand the failure class

Background: "API error: {status}" and "HTTP 401/403/404/429/5xx" errors: non-2xx HTTP responses explained — this error's family across 27 libraries.

Related errors

AI-assisted analysis of Kuberwastaken/claurst@b0637c97ec (2026-09-10). Data as JSON: /api/errors/b68141bbd4896583. Report an issue: GitHub.

Appendix: source

Thrown at src-rust/crates/mcp/src/rmcp_backend.rs:445

    serde_json::from_str(data).map_err(|e| {
        anyhow::anyhow!(
            "MCP server '{}': failed to parse legacy SSE JSON payload: {}",
            server_name,
            e
        )
    })
}

async fn handle_legacy_sse_http_response(
    server_name: String,
    response: reqwest::Response,
    incoming_tx: mpsc::UnboundedSender<rmcp::service::RxJsonRpcMessage<RoleClient>>,
    background_tasks: Arc<StdMutex<Vec<JoinHandle<()>>>>,
) -> anyhow::Result<()> {
    let status = response.status();
    if !status.is_success() && status != reqwest::StatusCode::ACCEPTED {
        let body = response.text().await.unwrap_or_default();
        anyhow::bail!(
            "MCP server '{}': HTTP {} from legacy SSE transport: {}",
            server_name,
            status,
            body
        );
    }

    if status == reqwest::StatusCode::ACCEPTED {
        return Ok(());
    }

    if transport::is_event_stream_response(&response) {
        let server_name_for_task = server_name.clone();
        let task = tokio::spawn(async move {
            if let Err(e) = transport::process_sse_response(response, |_, data| {
                if data.trim().is_empty() {
                    return Ok(());
                }

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