Regression: GPT-5.4 / GPT-5.5 1M context setting is ignored in Codex IDE extension, stuck at 258k

This appears to be a regression of the same context-window handling issue that happened when GPT-5.4 was first released. The model supports 1M context, but Codex IDE still behaves as if the usable context is capped at ~258k.

Impact:

This makes long coding sessions with GPT-5.4 / GPT-5.5 much less useful, because Codex starts auto-summarizing far earlier than expected. The main benefit of using the 1M context models is effectively unavailable inside the IDE extension.

What version of the IDE extension are you using?

latest

What subscription do you have?

Pro

Which IDE are you using?

VS Code

What platform is your computer?

Windows

What issue are you seeing?

The 5.4 and 5.5 models should support a 1M context window, but Codex appears to remain capped at around 258k context even after adding or changing the relevant 1M context configuration.

Because of this, Codex starts auto-summarizing very early, even when the actual conversation/workspace context is nowhere near 1M tokens. This makes long coding sessions difficult because useful context gets compressed or lost much earlier than expected.

This looks very similar to the issue that happened when 5.4 was first released: the model supported a larger context window, but Codex did not actually use it until the extension/config handling was fixed later. Now the same problem seems to be happening again with 5.5.

What steps can reproduce the bug?

Uploaded thread: 019dd732-5c39-77e3-9c28-d245c0bea2b4

What is the expected behavior?

Codex should respect the configured 1M context setting for models that support it, such as 5.4 and 5.5.

Configure Codex to use a 5.4 or 5.5 model that supports 1M context.
Add or change the relevant configuration for 1M context.
Start a long coding session in Codex.
Observe that Codex still reports or behaves as if the usable context is around 258k.
Auto-summarization begins much earlier than expected.

Additional information

The larger context window is one of the main reasons to use these models for coding tasks. If Codex is still capped at 258k internally, then the advertised 1M context support is effectively unusable inside Codex.

Please check whether the IDE extension / Codex runtime is correctly reading and applying the 1M context configuration for 5.4 and 5.5 models.