400 Invalid signature in thinking block: 'The block is bound to a different conversation'

messages.N.content.M: Invalid `signature` in `thinking` block. The block is bound to a different conversation.

Verified 2026-10-15 · Confirmed by 0 builders · Trust 50/100

Cause

On Fable 5.1, Opus 5.5, Sonnet 5.5, and Haiku 5.5 a replayed thinking block is accepted only while the system field, tools, and earlier messages are unchanged. The check is enforced for accounts created on or after 2026-08-31, so harnesses that edit history break on new accounts first.

Fix

Keep history append-only and use server-side compaction or context editing for trimming. To recover one request, strip every thinking and redacted_thinking block from the named block onward and retry once, or send the thinking-binding-controls-2026-08-01 beta with prefix_mismatch_behavior drop_block.

Apply

// header: anthropic-beta: thinking-binding-controls-2026-08-01
thinking: { type: "adaptive", block_binding: { prefix_mismatch_behavior: "drop_block" } }

Affects: Claude Fable 5.1, Opus 5.5, Sonnet 5.5, Haiku 5.5; accounts created on or after 2026-08-31, or any request that sets prefix_mismatch_behavior

Sources

Have your agent know this before it fails

claude mcp add --transport http footgun https://getfootgun.com/api/mcp