What causes "legacy SSE endpoint event did not include a POST endpoint" in Kuberwastaken/claurst?

Verify the server actually implements the legacy SSE MCP handshake and emits an 'endpoint' event with a non-empty value

legacy SSE endpoint event did not include a POST endpoint

Error message

legacy SSE endpoint event did not include a POST endpoint

What it means

Legacy SSE MCP servers first send an 'endpoint' event telling the client where to POST JSON-RPC messages. If that event's payload is empty or whitespace-only after trimming, the client cannot construct the POST endpoint and bails. This protects against malformed servers that open an SSE stream but never advertise a message endpoint.

Solutions

Verify the server actually implements the legacy SSE MCP handshake and emits an 'endpoint' event with a non-empty value
If the server supports the streamable HTTP transport, switch the config type from 'sse' to 'http'
Check intermediate proxies/gateways for rewriting or dropping the endpoint event payload
Confirm the base_url is correct so relative endpoints can be resolved

Defensive patterns

Strategy: try-catch

Try / catch

match client.connect().await {
    Err(e) if e.to_string().contains("legacy SSE endpoint event") => {
        // fall back to streamable HTTP transport or surface server-compat error
    }
    other => other?,
}

Prevention

Prefer the streamable HTTP transport when the server supports it
Test MCP servers behind proxies for intact endpoint events
Pin to server versions known to implement the legacy SSE handshake correctly

When it happens

Trigger: resolve_legacy_endpoint is called with an empty or whitespace-only endpoint string, i.e. the server's SSE 'endpoint' event carried no endpoint data.

Common situations: Server behind a proxy that strips the endpoint event body; a non-MCP SSE endpoint mistakenly registered as an MCP server; old/broken SSE server implementations; URL path not URL-encoded when parsed upstream.

Related errors

AI-assisted analysis of Kuberwastaken/claurst@b0637c97ec (2026-09-10). Data as JSON: /api/errors/830d1bfb22e2f7b1. Report an issue: GitHub.

Appendix: source

Thrown at src-rust/crates/mcp/src/lib.rs:417

    pub(crate) fn bearer_header_value(token: &str) -> anyhow::Result<HeaderValue> {
        HeaderValue::from_str(&format!("Bearer {}", token))
            .map_err(|e| anyhow::anyhow!("invalid bearer token header: {}", e))
    }

    pub(crate) fn is_event_stream_response(response: &reqwest::Response) -> bool {
        response
            .headers()
            .get(CONTENT_TYPE)
            .and_then(|value| value.to_str().ok())
            .map(|value| value.contains("text/event-stream"))
            .unwrap_or(false)
    }

    pub(crate) fn resolve_legacy_endpoint(base_url: &str, endpoint: &str) -> anyhow::Result<String> {
        let endpoint = endpoint.trim();
        if endpoint.is_empty() {
            anyhow::bail!("legacy SSE endpoint event did not include a POST endpoint");
        }
        if let Ok(url) = url::Url::parse(endpoint) {
            return Ok(url.to_string());
        }
        let base = url::Url::parse(base_url)
            .map_err(|e| anyhow::anyhow!("invalid legacy SSE base URL '{}': {}", base_url, e))?;
        base.join(endpoint)
            .map(|url| url.to_string())
            .map_err(|e| anyhow::anyhow!("failed to resolve legacy SSE endpoint '{}': {}", endpoint, e))
    }

    #[cfg(test)]
    pub(super) fn route_incoming_value(
        server_name: &str,
        value: serde_json::Value,
        response_tx: &mpsc::UnboundedSender<JsonRpcResponse>,
        notification_tx: &mpsc::UnboundedSender<serde_json::Value>,
    ) -> anyhow::Result<()> {

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