Should the streamable HTTP transport respond with 202 Accepted instead of a 200?

[stevegeek]

When testing in LM Studio an MCP server built with the StreamableHTTPTransport I get an error about the body of the response to tool calls "code": "unrecognized_keys", "keys": [ "accepted" ]"

I tried modifying the response from send_response_to_stream (

ruby-sdk/lib/mcp/server/transports/streamable_http_transport.rb

Line 217 in f63f360

[200, { "Content-Type" => "application/json" }, [{ accepted: true }.to_json]]

) to a body-less 202 Accepted and that fixes the tool call (and only parse the response if its not nil.. for some reason it seems to be nil for "notification_cancelled" method call)

ie

        def send_response_to_stream(stream, response, session_id)
          if response
            message = JSON.parse(response)
            send_to_stream(stream, message)
          end
          [202, {}, []]
...

Looking at the spec I see https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server If the server accepts the input, the server MUST return HTTP status code 202 Accepted with no body. ... but Im not sure if thats the relavent section related to send_response_to_stream

Logs

MCP Responded with

127.0.0.1 - - [21/Aug/2025:05:27:41 +0000] "POST / HTTP/1.1" 200 - 0.0007
[MCP] Request: notifications/initialized (id: )
[MCP] Request body: {
  "method": "notifications/initialized",
  "jsonrpc": "2.0"
}
127.0.0.1 - - [21/Aug/2025:05:27:41 +0000] "POST / HTTP/1.1" 202 - 0.0002
127.0.0.1 - - [21/Aug/2025:05:27:41 +0000] "GET / HTTP/1.1" 200 - 0.0002
[MCP] Request: tools/list (id: 1)
[MCP] Request body: {
  "method": "tools/list",
  "params": {},
  "jsonrpc": "2.0",
  "id": 1
}
[MCP] Response: empty (id: )
[MCP] Response body: {
  "accepted": true
}
127.0.0.1 - - [21/Aug/2025:05:27:41 +0000] "POST / HTTP/1.1" 200 - 0.0003

then LM Studio reports this parse error:

Failed to load plugin tools: [ { "code": "invalid_union", "unionErrors": [ { "issues": [ { "code": "invalid_literal", "expected": "2.0", "path": [ "jsonrpc" ], "message": "Invalid literal value, expected \"2.0\"" }, { "code": "invalid_union", "unionErrors": [ { "issues": [ { "code": "invalid_type", "expected": "string", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "number", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" } ], "path": [ "id" ], "message": "Invalid input" }, { "code": "invalid_type", "expected": "string", "received": "undefined", "path": [ "method" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "accepted" ], "path": [], "message": "Unrecognized key(s) in object: 'accepted'" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_literal", "expected": "2.0", "path": [ "jsonrpc" ], "message": "Invalid literal value, expected \"2.0\"" }, { "code": "invalid_type", "expected": "string", "received": "undefined", "path": [ "method" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "accepted" ], "path": [], "message": "Unrecognized key(s) in object: 'accepted'" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_literal", "expected": "2.0", "path": [ "jsonrpc" ], "message": "Invalid literal value, expected \"2.0\"" }, { "code": "invalid_union", "unionErrors": [ { "issues": [ { "code": "invalid_type", "expected": "string", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "number", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" } ], "path": [ "id" ], "message": "Invalid input" }, { "code": "invalid_type", "expected": "object", "received": "undefined", "path": [ "result" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "accepted" ], "path": [], "message": "Unrecognized key(s) in object: 'accepted'" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_literal", "expected": "2.0", "path": [ "jsonrpc" ], "message": "Invalid literal value, expected \"2.0\"" }, { "code": "invalid_union", "unionErrors": [ { "issues": [ { "code": "invalid_type", "expected": "string", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "number", "received": "undefined", "path": [ "id" ], "message": "Required" } ], "name": "ZodError" } ], "path": [ "id" ], "message": "Invalid input" }, { "code": "invalid_type", "expected": "object", "received": "undefined", "path": [ "error" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "accepted" ], "path": [], "message": "Unrecognized key(s) in object: 'accepted'" } ], "name": "ZodError" } ], "path": [], "message": "Invalid input" } ]

[koic]

This issue was resolved in #114.