fix(session): trigger auto-compaction at the effective input ceiling (#45168)

[reisi007]

Issue for this PR

Closes #45168

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Auto-compaction was gated by usable() = context - maxOutputTokens. maxOutputTokens is capped at OUTPUT_TOKEN_MAX (32000), so for opencode-go/hy3 (models.dev: context 256000, output 64000, no limit.input) the threshold came out to 224000. The provider actually pins input at 262144 - 65536 = 196608, so the measured token count plateaus around 198k and isOverflow() never returned true — the session kept re-sending the full ~196k context as raw, uncached input.

usable() now derives the input ceiling from the model's effective input ceiling and reserves a 20k buffer for the next response:

inputCeiling    = limit.input ?? (context - output)
effectiveCeiling = min(inputCeiling, context - output)   // always leaves room for a full response
usable          = effectiveCeiling - min(reserved, output)

For hy3 that is 192000 - 20000 = 172000, which the pinned count (198108) exceeds, so compaction fires before the provider caps input. Because this is based on the real input ceiling rather than context - maxOutputTokens, it works for any model whose real input cap sits below context - reserved. Both the V1 (processor/prompt) and V2 (SessionCompaction) call paths use this same function, so both are covered.

How did you verify your code works?

• Added packages/opencode/test/session/overflow.test.ts (pure unit tests on usable/isOverflow) reproducing compaction: auto-compaction never triggers on hy3 — token count plateaus below the isOverflow threshold, context pinned at 196608 raw input tokens #45168 with tencent/hy3 pinned at input 196608, plus anthropic/claude-opus (200k input cap), openai/gpt (400k/128k), google/gemini (1M/64k) and deepseek (1M/384k).
• Confirmed the new tests fail on the original overflow.ts (13 failures) and pass after the fix (22/22).
• Existing packages/opencode/test/session/compaction.test.ts still passes (55/55, no regressions).
• bun typecheck is clean.

Screenshots / recordings

N/A — this is a session/compaction logic change, not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
• Positive reactions are counted as thumbs-up, heart, celebration, or rocket reactions on the PR

PRs created within the last month are not affected by this cleanup.

If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.