fix: cap thinking.budget_tokens when >= max_tokens

[flwrenn]

Summary

• Cap thinking.budget_tokens to 80% of max_tokens when OpenCode sends budget_tokens >= max_tokens, which the Anthropic API rejects with a 400 error
• Affects custom models (e.g. claude-opus-4-6) not in OpenCode's built-in registry, where max_tokens and budget_tokens may be set to the same value
• Follows the same defensive-fix pattern as the existing haiku effort stripping in transformBody

Problem

When using claude-opus-4-6 (or other custom model IDs) through OpenCode, the Go binary may set thinking.budget_tokens equal to max_tokens. The Anthropic API requires strict inequality (max_tokens > thinking.budget_tokens) and rejects these requests:

opencode-claude-auth: API 400 for claude-opus-4-6: `max_tokens` must be greater than
`thinking.budget_tokens`. Please consult our documentation at
https://docs.claude.com/en/docs/build-with-claude/extended-thinking#max-tokens-and-context-window-size

Fix

In transformBody, after the existing model override logic, check if budget_tokens >= max_tokens and cap it to Math.floor(max_tokens * 0.8). The 0.8 ratio matches what OpenCode's own Anthropic provider uses when it correctly configures thinking for built-in models.

Tests

Three new test cases covering:

• budget_tokens == max_tokens (the observed failure case)
• budget_tokens > max_tokens
• budget_tokens < max_tokens (no-op, left unchanged)

[Copilot]

Pull request overview

This PR adds a defensive transformation to avoid Anthropic API 400s when thinking.budget_tokens is configured to be greater than or equal to max_tokens (a situation observed with custom model IDs outside OpenCode’s built-in model registry).

Changes:

• Parse and surface max_tokens in transformBody’s request-body shape.
• Cap thinking.budget_tokens to Math.floor(max_tokens * 0.8) when budget_tokens >= max_tokens to satisfy the API’s strict inequality requirement.
• Add three unit tests covering equal/greater-than/no-op scenarios for the cap logic.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/transforms.ts	Adds runtime guard to ensure max_tokens > thinking.budget_tokens by capping the budget when invalid.
src/transforms.test.ts	Adds targeted regression tests for the new capping behavior.

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[griffinmartin]

Thanks, will try to take a look at this tonight.