This has just been shared tonight by Tibo on Twitter
Here is how to enable a 1M-token context window in Codex for GPT-5.6 Sol.
Codex is tuned to use a context limit that balances performance and cost, but a larger context window is a common request, so here is how to configure it.
A larger context window lets Codex retain more code, tool output, and conversation history before summarizing older material. You need a model that supports it. GPT-5.6 Sol, for example, has a documented 1,050,000-token context window.
Open ~/.codex/config.toml and add or update these settings at the top level, before any [section] headers:
model = "gpt-5.6-sol"
model_context_window = 1000000
model_auto_compact_token_limit = 900000
The first setting selects the model. The second tells Codex to use a one-million-token context budget. The third starts automatic history compaction around 900,000 tokens, leaving some headroom.
Restart the Codex client and start a new session after saving.
To try the configuration for a single CLI session without changing your defaults:
codex -m gpt-5.6-sol
-c model_context_window=1000000
-c model_auto_compact_token_limit=900000
Have fun, but keep in mind that the default context limit has been tuned carefully for performance and cost.