compaction-threshold changedbuild-with-claude/compaction-threshold
Nearest release: v2.1.314, published under an hour after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+4added
Lines−4removed
From line
1,656
where the diff opens
First seen
21 Sep 2026
this site's first read of the page
Recorded edits23to this page, all time
The whole hunk
from line 1656, old and new numbered
/
from line 1656
16561656* Keep the original messages in your list and let the API handle removing the compacted content
16571657* Manually drop the compacted messages and only include the compaction block onward
16581658
1659On Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, thinking blocks from before a `compaction` block aren’t carried forward, so the summary is all the model has of that earlier work. If you write your own `instructions`, tell the model what the summary must retain; see [Tell the model what to preserve in compaction summaries](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#tell-the-model-what-to-preserve-in-compaction-summaries).
1659On Claude 5.1 and later models, thinking blocks from before a `compaction` block aren’t carried forward, so the summary is all the model has of that earlier work. If you write your own `instructions`, tell the model what the summary must retain; see [Tell the model what to preserve in compaction summaries](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#tell-the-model-what-to-preserve-in-compaction-summaries).
16601660
16611661### Streaming
16621662
from line 2513
25132513 messages.append({"role": "assistant", "content": response.content})
25142514
25152515 # Return the text content
2516 return next(block.text for block in response.content if block.type == "text")
2516 return next((block.text for block in response.content if block.type == "text"), "")
25172517
25182518
25192519 # Run a long conversation
from line 2771
27712771 ```
27722772</CodeGroup>
27732773
2774On Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, thinking blocks in any assistant turn you reinsert after the compaction block fail the [prefix check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation), because they were produced when the full history was present. Strip the `thinking` and `redacted_thinking` blocks from those turns. Alternatively, send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). The preserved text and tool blocks can stay as they are. Letting the API summarize everything, without reinserting earlier turns, avoids this. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With any other thinking setting on those models, strip the blocks.
2774On Claude 5.1 and later models, thinking blocks in any assistant turn you reinsert after the compaction block fail the [prefix check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation), because they were produced when the full history was present. Strip the `thinking` and `redacted_thinking` blocks from those turns. Alternatively, send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). The preserved text and tool blocks can stay as they are. Letting the API summarize everything, without reinserting earlier turns, avoids this. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With any other thinking setting on those models, strip the blocks.
27752775
27762776Here’s an example that uses `pause_after_compaction` to preserve the prior exchange and the current user message (three messages total) instead of summarizing them. It keeps their text and tool blocks unchanged, removes the `thinking` and `redacted_thinking` blocks from the preserved assistant turn, and drops that turn if nothing else is left:
27772777
from line 2857
28572857 messages.append({"role": "assistant", "content": response.content})
28582858
28592859 # Return the text content
2860 return next(block.text for block in response.content if block.type == "text")
2860 return next((block.text for block in response.content if block.type == "text"), "")
28612861
28622862
28632863 # Run a long conversation
No line in this hunk matches that.