When a Forward implementation already wrote a complete non-SSE (JSON) error
response to the client and returned an error -- e.g. the case-400 passthrough
in GatewayService.handleErrorResponse -- the handler unconditionally called
ensureForwardErrorResponse, which detected the writer was already written and
appended a fallback `data: {"type":"error",...}` SSE frame. The client then
received a corrupted body: the upstream JSON immediately followed by a stray
`data:` line.
Add gatewayForwardErrorAlreadyCommunicated (and the OpenAI counterpart) to
detect this case -- writer size changed AND Content-Type is not
text/event-stream -- and skip the fallback. SSE streams that only flushed
keepalive pings or partial data still receive a protocol-compliant terminal
frame, so strict SDKs (Codex CLI) do not see a silent EOF.
Applied consistently across the Messages / ChatCompletions / Responses
gateway handlers and the OpenAI chat/images handlers. Added regression tests
covering JSON passthrough, mid-stream SSE 400, nil-error and no-write cases.
Case B: when a slot wait flushes SSE ping comments first (Writer.Written
becomes true), the previous ensureForwardErrorResponse short-circuited
on `c.Writer.Written()` and returned false without notifying the client.
Subsequent upstream errors (http2 timeout, stream INTERNAL_ERROR, etc.)
produced silent EOF; Codex CLI reported "stream closed before
response.completed" just like the user-slot timeout case.
Remove the Written() early return; coerce streamStarted to true when
Writer has already been written to, and let handleStreamingAwareError
walk the existing logic — which now (thanks to the previous commits)
emits a protocol-compliant response.failed for /responses paths and the
legacy `event: error` for others.
Update tests that previously asserted "do not override written response":
the new contract is to *append* an SSE terminal frame so the client sees
a clean close instead of EOF. recoverResponsesPanic inherits this fix.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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