[FEATURE] Support extended context window option for `opusplan`

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

opusplan mode is nice, but often gets interrupted for compaction, which requires manual intervention to compact then resume the task that was performed because the execution context is 200k tokens. This is often a deal breaker on long running tasks or big monorepos.

Both models, Opus and Sonnet separately support 1 million tokens, it would be advisable to be able to tune the context for either Opus or Sonnet. Moreover with 5 mcp servers, I get around already 10% of the context already taken.

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This issue is clearly a duplicate of #16409, but it's closed. The OP did not respond to the bot and it is incorrectly flagged as duplicate of #5628: #16409 is clearly about opusplan where #5628 isn't. So is this request.

Proposed Solution

Unsure how that would present, maybe via sone env var or by parameters like /model opusplan[sonnet-1m,opus-1m], /model opusplan[sonnet-1m].

Alternative Solutions

Manually handling the compaction, and manually resuming the work.

Priority

Critical - Blocking my work

Feature Category

API and model interactions

Use Case Example

Start to plan soe work on large monorepo that spans over multiple files.
At some point, the execution is interrupted because a compaction is needed.
This requires to look at what claude was doing previously, then run the compaction, then resume the work, hoping it matches what was planned.

This can happen several time during execution.

Additional Context

_No response_