Summary

For Codex OAuth sessions using GPT-5, the same ~300-line prompt content is sent twice:

1. In the instructions API parameter (from codex_header.txt)
2. As the first user message (from codex.txt)

Evidence

The two files are effectively identical (only trailing whitespace differences):

$ diff packages/opencode/src/session/prompt/codex_header.txt packages/opencode/src/session/prompt/codex.txt
137c137
< - Use the \`edit\` tool to edit files
---
> - Use the \`edit\` tool to edit files 
191c191
< If the codebase has tests or the ability to build or run, consider using them to verify that your work is complete.
---
> If the codebase has tests or the ability to build or run, consider using them to verify that your work is complete. 

Code Path

• codex_header.txt is loaded via PROMPT_CODEX_INSTRUCTIONS at packages/opencode/src/session/system.ts:17
• codex.txt is loaded via PROMPT_CODEX at packages/opencode/src/session/system.ts:16

For Codex sessions (provider.id === "openai" && auth?.type === "oauth"):

1. codex_header.txt → set as options.instructions at llm.ts:103
2. codex.txt → included in the system array which becomes the first user message at llm.ts:191-197

Impact

Wasteful token usage—sending the same ~5K tokens twice per request.

Suggested Fix

Either:

• Differentiate the files: codex_header.txt should be lightweight operational guidance, codex.txt the full prompt
• Or skip one when both would be identical
• Or clarify if this duplication is intentional (caching/prioritization reasons)