fix(provider): do not place Bedrock cachePoint after reasoning blocks

[jacktrnr]

Problem

On Amazon Bedrock (Claude with extended thinking), a prompt-cache breakpoint gets placed on the last couple of messages. The AI SDK emits message-level bedrock.cachePoint after every content part, so when an assistant message ends in a reasoning block, Bedrock rejects the request:

ValidationException: Cache point cannot be inserted after reasoning block.

The turn fails, and every retry rebuilds the same invalid request, so the session is wedged until that message ages out of history. In Amicode this surfaces as the cryptic undefined: prefix (the error is stringified as ${type}: ${message} and Bedrock returns a null type). It hit a live session this week.

Fix

Anchor the cache breakpoint on the last non-reasoning content part (or skip caching entirely for reasoning-only assistant messages), across both code paths that place the marker:

• packages/llm/src/cache-policy.ts — markMessageAt prefers the last text/non-reasoning part.
• packages/opencode/src/provider/transform.ts — applyBedrockCacheOptions moves the providerOptions.bedrock.cachePoint off a trailing reasoning block for Bedrock models.
• packages/llm/src/protocols/bedrock-converse.ts — comment marking the Converse lowering guard.

Provenance

Faithful cherry-pick of anomalyco/opencode#36532, which fixed upstream issue #36517. That PR was auto-closed unmerged by the cleanup bot (>1 month old, <2 reactions) — not rejected on merit — so it never landed here.

Verification

• 6 regression tests ported from #36532 and all green (commit e669df47b):
  • cache-policy.test.ts — anchors on text before trailing reasoning; skips reasoning-only messages
  • bedrock-converse.test.ts — no cachePoint after a trailing reasoning block (#36517)
  • transform.test.ts — bedrock cachePoint-after-reasoning describe (3 cases)
• Full suites pass: bun test → llm 43/43, opencode transform 402/402, 0 fail.
• bun typecheck passes in both packages/llm and packages/opencode.

Note

A sibling variant — "There is nothing available to cache" (empty/below-min-size segment, open upstream #45855 / #45842) — is a separate bug, not covered here.

cc @jeonghun-jj-lee — this is the Bedrock reasoning-block cache crash; small, tested, green. Good to get into local/amicode.

[coderabbitai]

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[jacktrnr]

Superseded by harmoniqs/amicode#1209, which lands this exact fix in the correct place — the engine overlay (packages/app-bundle/overlay/) that the Amicode binary is actually built from. This repo/fork is no longer the build source ("the fork is now archived; all engine development happens directly in the overlay"), so this merge is inert for the product. Kept for history; no action needed here.