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Streamable HTTP answers an accepted notification with 202 and no body,
but some servers send a body anyway, such as {}. HttpMcpClient parsed
that body as a JSON-RPC response and failed with "Missing required
members: [jsonrpc]". For notifications/initialized that failed
McpRemoteClient.initialize, so the remote server contributed no tools.
A notification has no JSON-RPC response, so discard the body, as
StdioMcpClient does. A short body of known length is read so the
connection can be reused; anything else, including a stream, is closed
unread so a misbehaving server cannot block the client or make it
buffer without bound.
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What behavior changes?
HttpMcpClient.exchangeno longer parses the response body of a notification (a request without an id). It returnsnull, asStdioMcpClientdoes.text/event-streamor one of unknown length, is closed unread. A misbehaving server can't block the client or make it buffer without bound.Why is this change needed?
Fixes #1412. A server that accepts
notifications/initializedwith202and a{}body madeMcpRemoteClient.initializefail withMissing required members: [jsonrpc], so the remote server contributed no tools. The spec requires 202 with no body, but the client loses nothing by tolerating one.How was this validated?
New
HttpMcpClientTestcases:testNotificationIgnoresBodyOfAcceptedResponse: a202with{}returnsnull.initializeSucceedsWhenInitializedNotificationIsAcceptedWithABody: a fullinitializeagainst such a server succeeds, and the next request works.testNotificationDoesNotReadAStreamingResponseBody: a never-ending event stream answering a notification doesn't blockexchange. It times out if the body is read in full.The first two fail without the fix.
./gradlew :mcp:mcp-server:test :mcp:mcp-server:integ :mcp:mcp-server:spotlessCheckpasses: 255 unit and 125 integration tests.What should reviewers focus on?
HttpMcpClient.exchangeanddiscardBody: the notification branch sits after the session-404 and non-2xx handling, so error bodies still surface. Also the 8 KiB threshold for reading versus closing.Additional Links
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.