fix(client): keep the original error as the cause of "SSE stream disconnected" - #2948
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…onnected"
When an SSE response stream failed while being read, the transport passed
`new Error(`SSE stream disconnected: ${error}`)` to onerror, which dropped the
type and stack of the original error. The error now carries it as `cause`; the
message is unchanged.
The e2e test for `client-transport:http:body-stream-error-preserved` opened its
stream with `data: {}`, which is not a JSON-RPC message, so the first error
reaching onerror was the parse failure and the test could not pass whatever the
transport did. It now opens the stream with a comment frame, and the
requirement's knownFailures entry is removed.
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The
SSE stream disconnectederror thatStreamableHTTPClientTransportpasses toonerrornow carries the original stream error as itscause. The message is unchanged.Motivation and Context
StreamableHTTPClientTransportis reading it, the transport reportsnew Error(`SSE stream disconnected: ${error}`)toonerror.error.causeisundefined.TypeError: terminatedfrom another failure has to parse the message string.test/e2e/requirements.tslists the behaviour asclient-transport:http:body-stream-error-preserved(onerrorshould receive an error that preserves the original one, either as the instance itself or via.cause), with aknownFailuresentry describing exactly this wrapper.Error.causeonSdkError).The change:
packages/client/src/client/streamableHttp.ts: pass{ cause: error }when constructing theSSE stream disconnectederror.@modelcontextprotocol/client, patch.How Has This Been Tested?
client-transport:http:body-stream-error-preservedpass with the change and fail when only the source change is reverted.@modelcontextprotocol/clientunit tests: 997 passed.typecheckandlintpass for the client and e2e packages.Breaking Changes
None. An existing error gains a
cause; its message and class are the same, so anything matching on the message keeps working.Types of changes
Checklist
Additional context
The e2e test needed a one-line fix:
data: {}, then errors the stream, and asserts onerrors[0].{}is not a JSON-RPC message, so the transport reports the schema parse failure toonerrorfirst. The stream error arrives second.ZodError(fromdata: {})Error: SSE stream disconnected: TypeError: ...withcause === originalError: open), which produces no message and no error, soerrors[0]is the stream error.knownFailuresentry for the requirement is removed.Not included:
v1.xhas the same line (src/client/streamableHttp.ts). Not touched here; happy to send a separate PR againstv1.xif wanted.Failed to reconnect SSE stream: ...) are left as they are. The requirement only covers the body-stream error.