Context window size at launch should approximate effect of `AGENTS.md`

What version of Codex is running?

0.63.0

What subscription do you have?

Pro

Which model were you using?

gpt-5.1-codex-max ex high

What platform is your computer?

macOS

What issue are you seeing?

Context window size displayed 100% at launch, then suddenly 95% after the first message - it seems the AGENTS.md consumed 5% but this wasn't immediately obvious at initial lunch without first sending a message

What steps can reproduce the bug?

Uploaded thread: 019aaa37-ec3e-73e2-a25c-35b7b598f82c

• Create a large AGENTS.md
• Start codex
• It says 100%
• Type 'Hello'
• Context window drops to 95% (my AGENTS.md is 40kb)

At launch:
<img width="1088" height="652" alt="Image" src="https://github.com/user-attachments/assets/a4a4b5cc-eb27-4caf-acc8-9d5957de23a0" />

After first message:
<img width="956" height="884" alt="Image" src="https://github.com/user-attachments/assets/48f39c18-ac01-47c4-af1d-c8dd08e3f03a" />

What is the expected behavior?

At launch the remaining context window size should be accurate, should reflect how much space has been consumed already by AGENTS.md

Additional information

_No response_