ai-anthropic: 400 on signed thinking blocks after compaction rewrites history (missing block_binding drop_block)

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@tombeckenham ya está trabajando en esto.

Desde el 31/10/2026.

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Descripción

waiting-on: maintainer
What happens

With @tanstack/ai-anthropic 1.0.0, @tanstack/ai-harness 0.1.1 and withCompaction (summarizeOldest), the first model call after a compaction fails with a 400 from Anthropic (Claude Sonnet 5.5, reasoning: 'medium'):

messages.1.content.1: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block". That setting requires the `thinking-binding-controls-2026-08-01` value in ...

Compaction replaces the earlier messages with a summary, so the signed thinking blocks of the kept turns no longer match the conversation prefix they were bound to.

Expected

The adapter sends thinking.block_binding: { prefix_mismatch_behavior: "drop_block" } with the thinking-binding-controls-2026-08-01 beta whenever thinking is on (pi-ai does this), or drops thinking blocks before the rewritten prefix itself.

Workaround

A custom fetch on anthropicText(model, { fetch, midConversationChannels: true }) that adds the field and the beta header. With it, the same 5-question run with compaction completes. (Passing fetch turns midConversationChannels off, so it has to be set back on.)

Where

packages/ai-anthropic/src/text/reasoning.ts builds thinking without block_binding; checked on main at 3ce3085.

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