What
Claude Code has internal logic that recognizes a specific "Extra inputs are not permitted" schema error related to thinking blocks. The regular expression used to detect this error was updated to match block_binding.prefix_mismatch_behavior, replacing the older block_binding.mismatch_behavior, tracking a rename of that field in the thinking configuration.
Why
This keeps Claude Code's error handling in sync with the current field name, so it continues to correctly recognize this particular schema error instead of missing it.
Names in the bundleprefix_mismatch_behavior
Documented inapi/agents-and-tools/tool-use/computer-use-tool
block_binding.prefix_mismatch_behavior
compaction-thinking-blocks modified, high confidence
If a condition doesn't hold, nothing fails when you compact, and the API accepts the block on later requests either way. The failure comes on the first later request that sends the kept thinking where the API enforces the check: a 400 erro…see the edit
block_binding.prefix_mismatch_behavior
preserved-thinking modified, high confidence
No. Send the `thinking-binding-controls-2026-08-01` beta header and set `thinking.block_binding.prefix_mismatch_behavior`. Setting the field opts that request into enforcement regardless of account age. `"error"` rejects an edited history …see the edit
block_binding.prefix_mismatch_behavior
errors modified, high confidence
On Claude Fable 5.1 and Claude Opus 5.5, the API accepts a replayed thinking block only while the `system` prompt, `tools`, and messages that preceded it are unchanged. For new accounts created on or after August 31, 2026, and for any requ…see the edit
block_binding.prefix_mismatch_behavior
compaction-threshold modified, high confidence
On Claude Fable 5.1 and Claude Opus 5.5, remove the `thinking` and `redacted_thinking` blocks from any assistant turn you re-insert after the compaction block, or send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with th…see the edit
block_binding.prefix_mismatch_behavior
preserved-thinking modified, high confidence
The model check applies to every account. The API enforces the prefix check by default for accounts created on or after August 31, 2026, 00:00 UTC. On older accounts, it enforces the prefix check only on requests that set `thinking.block_b…see the edit
block_binding.prefix_mismatch_behavior
thinking modified, high confidence
The model check applies to every account. The API enforces the prefix check by default for accounts created on or after August 31, 2026, 00:00 UTC. On older accounts it enforces the check only on requests that set `thinking.block_binding.p…see the edit
block_binding.prefix_mismatch_behavior
thinking-troubleshooting modified, high confidence
On Claude Fable 5.1 and Claude Opus 5.5, the API accepts a replayed thinking block only while the `system` prompt, `tools`, and messages that preceded it are unchanged. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en…see the edit
block_binding.prefix_mismatch_behavior
Or, for the Project Glasswing model with the same capabilities: modified, high confidence
The API enforces the check for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but doesn't act on it unless the request sets `thinking.block_binding.prefix_mismatch_behavior`, wh…see the edit
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up prefix_mismatch_behavior, on Or, for the Project Glasswing model with the same capabilities:.
prefix_mismatch_behavior: drop_blockmodels/fable-5-1/migration-guide see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
block_binding.prefix_mismatch_behavior on compaction-thinking-blocks