fix: when using codex sub, send the custom agent prompts as a separate developer message (previously sent as user message but api allows for instructions AND developer messages)

[rekram1-node]

fixes: #11163

credit to @CarlosCommits for finding

[CarlosCommits]

Thanks for the credit for finding the issue, but I think with this new solution OAuth Codex behavior is still different from non-OAuth Codex:

OAuth Codex path

• isCodex detection: packages/opencode/src/session/llm.ts line 65
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/llm.ts#L65
• Base Codex prompt always sent via options.instructions: packages/opencode/src/session/llm.ts lines 114-115
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/llm.ts#L114-L115
• System bundle (agent.prompt + input.system + input.user.system) sent as system messages: packages/opencode/src/session/llm.ts lines 67-80 and 235-241
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/llm.ts#L67-L80
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/llm.ts#L235-L241

Non-OAuth Codex (Zen/opencode)

• isCodex is false, so system bundle uses agent.prompt if present, otherwise provider prompt: packages/opencode/src/session/llm.ts lines 70-72
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/llm.ts#L70-L72
• Provider prompt selection includes PROMPT_CODEX for gpt-5 models: packages/opencode/src/session/system.ts lines 18-20
  https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/system.ts#L18-L20

Resulting behavior:

• OAuth Codex keeps PROMPT_CODEX (via options.instructions) and appends custom agent prompt + env/AGENTS as system/developer messages.
• Non-OAuth Codex REPLACES the PROMPT_CODEX with agent.prompt when present.

So behavior still differs (append vs replace). Assumning parity is desired, we likely need to either:

1. For OAuth: set options.instructions to agent.prompt ?? PROMPT_CODEX and append input.system, or
2. For non-OAuth: always include base prompt even when agent prompt is present.

Also input.system is env + AGENTS.md: packages/opencode/src/session/prompt.ts line 605
https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/prompt.ts#L605

[rekram1-node]

yeah this is expected from what openai told me.

If you are using a codex sub the instructions field should always be that codex prompt and then anything custom should be sent as a developer message (system message) separately

If you arent using a codex sub the all that instructions stuff is moot

[rekram1-node]

does that make sense? Am i missing something?

[CarlosCommits]

I might be missing something. If OpenAI recommends PROMPT_CODEX for the Codex model, why only enforce it for OAuth Codex?

Why not apply the same rule to all Codex instances (including Zen/non‑OAuth)?

Right now behavior differs:

• 5.2 Codex via Zen + custom agent --> PROMPT_CODEX is replaced by agent prompt.
• 5.2 Codex via OAuth + custom agent --> PROMPT_CODEX remains and agent prompt is appended.

Is this intentional?

[rekram1-node]

sorry it is a requirement for oauth flow thats what im saying

[rekram1-node]

yes this is intentional (it’s a requirement from them they asked me to do)

and its only for the codex subscription

[CarlosCommits]

Oh that's interesting, thanks for clarifying. I guess that makes it more difficult to use oauth codex for custom agents, since there is a baked in default that we can't remove.