[Bug]: Grok Build turns on ChatGPT-backed OpenAI models get almost no prompt-cache reuse (missing session_id) #6481

Client or integration

Other

Area

Proxy and routing

Summary

Grok Build (the grok CLI, wired by ocx through the managed [model_providers.opencodex] block with x-opencodex-grok = "1") gets almost no prompt-cache reuse when it uses an OpenAI model served by the ChatGPT Codex backend (for example gpt-6-astra). The same model used from Codex reuses the prefix normally.

From the last 400 request-log entries on one machine:

The Grok rows show cachedInputTokens of 0 or a fixed 3200/4992 regardless of conversation length, and conversationId is never set.

Cause: Grok sends its conversation identity as x-grok-conv-id / x-grok-session-id and as the body prompt_cache_key, but it never sends the session_id header that Codex clients send. The ChatGPT Codex backend only reuses a warmed prefix when session_id is present, and opencodex also reads that header for thread affinity and request-log conversation identity. Grok's request bodies are append-only across turns (checked by diffing consecutive captured bodies: tools identical, input a strict prefix), so the client is not the problem.

Reproduction

1. ocx with an OpenAI (ChatGPT) account and the Grok integration enabled.
2. In Grok Build, pick ocx-gpt-6-astra and run a multi-turn task (several tool calls).
3. Open Logs: every Grok/gpt-6-astra row is affinity: new_bind and cached input stays near zero.

A/B confirmation by replaying one captured Grok request body (≈16.7k input tokens) three times against /v1/responses with the Grok headers:

Version

2.76.0 (dev at 4b74668)

Operating system

macOS (aarch64)

Provider and model

openai (ChatGPT Codex backend, pool account) / gpt-6-astra, Grok Build 1.0.46

Logs or error output

surface=grok model=gpt-6-astra affinity=new_bind conversationId=<none> usage.inputTokens=42565 cachedInputTokens=3200

Screenshots and supporting files

Logs page with consecutive Grok gpt-6-astra rows at 2만+ tokens and cache c 3200 or none.

Redacted configuration

[model_providers.opencodex]
base_url = "http://127.0.0.1:10100/v1"
api_backend = "responses"
api_key = "opencodex-loopback"

[model_providers.opencodex.extra_headers]
x-opencodex-grok = "1"

Checks

• I searched existing issues and documentation.
• I removed secrets, tokens, account details, request credentials, and personal data.