The `openai-responses` client sent the session-affinity header as
`session_id`. Proxies such as nginx with `underscores_in_headers off`
silently drop headers with underscores, and Envoy can be configured to
reject them. Codex now sends `session-id`, and the
`openai-codex-responses` path already followed it (#4967).
With the default `openai` session-affinity format, `openai-responses`
now sends `session-id`. `x-client-request-id` and `prompt_cache_key` are
unchanged. The `openai-nosession` and `openrouter` formats behave as
before.
The compat schema description for `sessionAffinityFormat`
(`openai-responses`) and `docs/models-configuration.md` are updated to
match.
Not changed: `openai-completions` still sends `session_id`. I did not
check whether xAI or other backends on the Responses API depend on the
underscored name. Their `prompt_cache_key` is still sent, so prompt
caching should not depend on the header.
Testing:
- `test/openai-responses-compat.test.ts` now captures `session-id`. It
also checks that `session_id` is no longer sent, and the
explicit-override test uses the new name. `test/xai-responses.test.ts`
expects `session-id` and no `session_id`.
- Without the source change, these tests fail (3 failures); with it,
both files pass (50 tests).
- `npm run check` passes.
- The full `packages/ai` suite passes, except
`generate-models-strict.test.ts`. That failure is unrelated: the test
depends on live model catalog data.
Fixes#11639
Requested by @badlogic
Co-authored-by: triagent <triagent@localhost>
c5d3978350