## Compatibility
The whole hunk
from line 2, old and new numbered
/
lines
from line 2
22title: Compaction and preserved thinking
33url: https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks
44description: When thinking blocks in turns kept after on-demand compaction stay valid on models with preserved thinking, and how to check.
5featureMetadata:
6 status: beta
7 betaHeader: compact-2026-09-04
8 supportedModels:
9 - claude-fable-5-1
10 - claude-mythos-5-1
11 - claude-fable-5
12 - claude-mythos-5
13 - claude-mythos-preview
14 - claude-opus-5
15 - claude-opus-4-8
16 - claude-opus-4-7
17 - claude-opus-4-6
18 - claude-sonnet-5
19 - claude-sonnet-4-6
20 supportedPlatforms:
21 Claude API: beta
22 Claude Platform on AWS: beta
23 Amazon Bedrock: not available
24 Google Cloud: beta
25 Microsoft Foundry: beta
526---
627
7## Compatibility
8- Status: Beta
9- [Beta header](https://platform.claude.com/docs/en/api/beta-headers): `compact-2026-09-04`
10- Supported models: `claude-fable-5-1`, `claude-mythos-5-1`, `claude-fable-5`, `claude-mythos-5`, `claude-mythos-preview`, `claude-opus-5`, `claude-opus-4-8`, `claude-opus-4-7`, `claude-opus-4-6`, `claude-sonnet-5`, `claude-sonnet-4-6`
11- Platforms: Claude API (beta), Claude Platform on AWS (beta), Microsoft Foundry (beta); not available on Amazon Bedrock, Google Cloud
12
1328Skip this page unless you send thinking blocks back to a model with [preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) and keep turns after the compaction block. Kept turns are the turns that follow the block: recent turns you left out of the compaction request, as in [Compaction that keeps recent turns](https://platform.claude.com/docs/en/build-with-claude/compaction-keep-recent-turns), or turns that arrived while the summary was being written, as in [Compaction in the background](https://platform.claude.com/docs/en/build-with-claude/compaction-background).
1429
1530Models with preserved thinking check earlier thinking blocks against the conversation that produced them. A summary replaces part of that conversation, but the check accepts the swap when the API wrote the summary, so the thinking in kept turns can stay valid.
from line 37
2237* **The kept turns directly follow the summarized messages, and you send them unchanged.** Send each kept message exactly as it is in your history. Don't skip or add a message between the last summarized message and the first kept one. The first kept message must also have a different role from the last summarized message, and it can't be a mid-conversation `role: "system"` message. Otherwise, the API merges it into the last summarized message. One way to get the first kept message right is to compact exactly the `messages` of a request you already sent. The kept turns then start with Claude's reply to it.
2338* **`system` and the `tools` not marked `defer_loading: true` don't change.** They are the same on the compaction request as on the requests that produced the kept thinking, and they stay the same on the requests that follow. [Change the system prompt or tools](https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks#change-the-system-prompt-or-tools) covers how to change them safely.
2439
25If 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 error by default, or dropped thinking blocks if the request sets `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. In the Message Batches API, an item that leaves the field unset drops the blocks instead. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes both outcomes, and [When the API enforces the check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) says which requests are checked.
40If 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 error by default, or dropped thinking blocks if the request sets `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. In the Message Batches API, an item that leaves the field unset doesn't fail. Where the check applies by default, the API drops the blocks instead. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes both outcomes, and [When the API enforces the check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) says which requests are checked.
2641
2742## Compact again without breaking older thinking
2843