"Transport error: Error decoding response body" when connecting to streamable http endpoint

[dingfeli]

Describe the bug
When connecting to certain endpoints with streamable http transport, the following error is thrown:

TransportError { error: DynamicTransportError { transport_name: "rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async_impl::client::Client>>", transport_type_id: TypeId(0x32b1c270444201caf977817085727746), error: TransportChannelClosed }, context: "send initialized notification" }.

This was preceded by the following:

rmcp::transport::worker: 128: worker quit with fatal: Client error: error decoding response body, when send initialized notification

The endpoint I used to discover this fault was https://www.javadocs.dev/mcp.
Some info about this endpoint:

• It uses streamable http
• It is open and does not require auth
• It uses https://github.com/softwaremill/chimp for the server
• It works with MCP inspector
• Server protocol version: 2025-03-26

To Reproduce
Steps to reproduce the behavior:

1. Go to the streamable http example and change the endpoint to "https://www.javadocs.dev/mcp": https://github.com/modelcontextprotocol/rust-sdk/blob/main/examples/clients/src/streamable_http.rs#L19.
2. Run the example with cargo run --example clients_streamable_http.

Expected behavior
Tools should be fetched

Logs
The following was outputted from the example:

2025-09-25T20:37:08.944863Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: checkout waiting for idle connection: ("https", www.javadocs.dev)
2025-09-25T20:37:08.944929Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: reqwest::connect: starting new connection: https://www.javadocs.dev/
2025-09-25T20:37:08.944964Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: Http::connect; scheme=Some("https"), host=Some("www.javadocs.dev"), port=None
2025-09-25T20:37:09.048052Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: connecting to 15.197.149.68:443
2025-09-25T20:37:09.129384Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: connected to 15.197.149.68:443
2025-09-25T20:37:09.314160Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: http1 handshake complete, spawning background dispatcher task
2025-09-25T20:37:09.314348Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: waiting for connection to be ready
2025-09-25T20:37:09.314628Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: connection is ready
2025-09-25T20:37:09.314689Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: checkout dropped for ("https", www.javadocs.dev)
2025-09-25T20:37:09.393272Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: put; add idle connection for ("https", www.javadocs.dev)
2025-09-25T20:37:09.393373Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: pooling idle connection for ("https", www.javadocs.dev)
2025-09-25T20:37:09.393452Z TRACE transport_worker{name="StreamableHttpClientWorker"}: reqwest::retry: shouldn't retry!
2025-09-25T20:37:09.394605Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: take? ("https", www.javadocs.dev): expiration = Some(90s)
2025-09-25T20:37:09.394687Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: reuse idle connection for ("https", www.javadocs.dev)
2025-09-25T20:37:09.475093Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: put; add idle connection for ("https", www.javadocs.dev)
2025-09-25T20:37:09.475187Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: pooling idle connection for ("https", www.javadocs.dev)
2025-09-25T20:37:09.475242Z TRACE transport_worker{name="StreamableHttpClientWorker"}: reqwest::retry: shouldn't retry!
2025-09-25T20:37:09.475841Z ERROR clients_streamable_http: client error: TransportError { error: DynamicTransportError { transport_name: "rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async_impl::client::Client>>", transport_type_id: TypeId(0x8a3d8550d26d791f0ae89dea6f935c32), error: TransportChannelClosed }, context: "send initialized notification" }
2025-09-25T20:37:09.475906Z ERROR rmcp::transport::worker: worker quit with fatal: Client error: error decoding response body, when send initialized notification
Error: Send message error Transport [rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async_impl::client::Client>>] error: Transport channel closed, when send initialized notification

Additional context
None.

[jokemanfire]

Is it due to incompatible specifications? @4t145

[jamesward]

The initialize MCP response via MCP Inspector is:

{
  "capabilities": {
    "tools": {
      "listChanged": false
    }
  },
  "serverInfo": {
    "name": "javadocs.dev",
    "version": "0.0.1"
  }
}

But the error message indicates maybe this is after that on an initialized notification response. However, there is no response to an initialized notification response that needs to be deserialized.

Also, checking my server logs I don't see a request being made to my server. So could this somehow be before the initialize is even sent?

Btw, this MCP server works fine in Claude Code, MCP inspector, claude.ai, etc.

[4t145]

2025-10-11T08:27:57.333511Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: checkout waiting for idle connection: ("https", www.javadocs.dev)
2025-10-11T08:27:57.333638Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: reqwest::connect: starting new connection: https://www.javadocs.dev/    
2025-10-11T08:27:57.333739Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: Http::connect; scheme=Some("https"), host=Some("www.javadocs.dev"), port=None
2025-10-11T08:27:57.340191Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: connecting to 7.0.117.205:443
2025-10-11T08:27:57.348354Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::connect::http: connected to 7.0.117.205:443
2025-10-11T08:27:57.897147Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: http1 handshake complete, spawning background dispatcher task
2025-10-11T08:27:57.897267Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: waiting for connection to be ready
2025-10-11T08:27:57.897449Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::client: connection is ready
2025-10-11T08:27:57.897502Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: checkout dropped for ("https", www.javadocs.dev)
2025-10-11T08:27:58.191350Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: put; add idle connection for ("https", www.javadocs.dev)
2025-10-11T08:27:58.191464Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: pooling idle connection for ("https", www.javadocs.dev)
2025-10-11T08:27:58.191745Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: rmcp::transport::common::reqwest::streamable_http_client: received json response: {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-03-26","capabilities":{"tools":{"listChanged":false}},"serverInfo":{"name":"javadocs.dev","version":"0.0.1"}}}
2025-10-11T08:27:58.192321Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: take? ("https", www.javadocs.dev): expiration = Some(90s)
2025-10-11T08:27:58.192390Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: reuse idle connection for ("https", www.javadocs.dev)
2025-10-11T08:27:58.496071Z TRACE transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: put; add idle connection for ("https", www.javadocs.dev)
2025-10-11T08:27:58.496183Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: hyper_util::client::legacy::pool: pooling idle connection for ("https", www.javadocs.dev)
2025-10-11T08:27:58.496351Z DEBUG transport_worker{name="StreamableHttpClientWorker"}: rmcp::transport::common::reqwest::streamable_http_client: received json response: {"jsonrpc":"2.0","method":"notifications/initialized"}
2025-10-11T08:27:58.496585Z ERROR clients_streamable_http: client error: TransportError { error: DynamicTransportError { transport_name: "rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async_impl::client::Client>>", transport_type_id: TypeId(0x8c7eda2fb63bf494b30f928606599e6a), error: TransportChannelClosed }, context: "send initialized notification" }

check this log: server response with a initialized notification

{"jsonrpc":"2.0","method":"notifications/initialized"}

, which is not supposed to be sent from a server accroding to specification. Client raises an decoding error while trying to decode this response as a server message.

This happens when client sending initialized notification, in this case, server are expected to respond with 202 ACCEPTED status code instead of an initialized notification.

https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server

If the input is a JSON-RPC response or notification:

    If the server accepts the input, the server MUST return HTTP status code 202 Accepted with no body.
    If the server cannot accept the input, it MUST return an HTTP error status code (e.g., 400 Bad Request). The HTTP response body MAY comprise a JSON-RPC error response that has no id.

[jamesward]

Thanks for investigating. I've filed a bug with the MCP library I'm using: softwaremill/chimp#53