Critical: Conversation compaction fails for conversations started under previous Claude Code versions

Summary

Claude Code 2.0.57 cannot compact conversations that were started under previous versions, resulting in a 400 API error. This effectively freezes ongoing work in those conversations with no recovery path.

Error Message

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.1.content.160: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CVjFJq2S3oRkQTSqBVYaX"}

Impact

Severity: Critical (P1)

Conversations at 0% context cannot proceed without compaction
Compaction fails, making these conversations permanently unusable
All accumulated context and work-in-progress is lost
No user-side workaround exists (this is an API validation error)

Environment

Claude Code version: 2.0.57
Previous version: Unknown (conversation started before upgrade)
Platform: macOS (Darwin 24.5.0)

Root Cause Hypothesis

The compaction process modifies message structure in a way that changes thinking blocks from previous assistant turns. The current API enforces that thinking/redacted_thinking blocks must remain byte-identical to original responses, causing validation failure.

Expected Behavior

Conversations started under previous Claude Code versions should remain functional after upgrade, including compaction. This is a basic backward compatibility expectation for production software.

Suggested Fixes

Migration: Automatically migrate old conversation format on upgrade
Compaction handling: Modify compaction to preserve thinking blocks exactly as stored
Graceful degradation: If compaction fails, offer user options (export context, start fresh with summary) rather than hard failure

User Impact Statement

"Claude Code is not experimental software at this point. As a product owner, I would expect a clean transition from local file formats and/or API stability. Imagine if Microsoft Word didn't open previous documents!"

This breaks the fundamental contract of being able to continue previous work sessions.

@sid374

Update: Issue Persists in 2.0.57

Just encountered this again:

> /compact
  ⎿ Error: Error during compaction: Error: API Error: 400
    {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.101: `thinking` or `redacted_thinking`
    blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original
    response."},"request_id":"req_011CVnLgBPDzFuBq6FDWW27H"}

Related Issues

This appears to be part of a broader pattern affecting many users:

Issue	Title	Comments	Status
#7530	[BUG] Error during compaction	92	Open (assigned, oncall)
#10199	[BUG] API Error 400 - Thinking Block Modification Error	24	Open
#12225	[BUG] API Error 400: Thinking Block Modification Error (Not a Duplicate)	19	Open
#12973	Compaction fails with 400 error when thinking blocks present	2	Open
#13086	[Bug] Cannot modify thinking blocks during message compaction	1	Open
#12311	Auto-compact fails with Opus 4.5 extended thinking blocks	5	Open

Impact Escalation

This is now a production blocker. Conversations that hit context limits become completely unusable with no recovery path. The only workaround is:

Manually /export the conversation
Rewind (esc twice) to earlier state
Lose all accumulated context and work

This defeats the purpose of long-running agentic sessions.

Note on Claimed Fixes

In #7530, @sid374 mentioned fixes were shipped post-2.0.40. However, users continue reporting this in 2.0.55 and now 2.0.57. The fix appears incomplete or the root cause may be different for conversations that span version upgrades.

Correction: Workarounds Do NOT Work

The workarounds suggested in #7530 are not viable in practice:

/export fails - When the conversation is in this broken state, export also fails
Rewinding context fails - The esc rewind mechanism also fails when thinking blocks are corrupted

This means there is NO recovery path once a conversation hits this state. All accumulated context, work-in-progress, and session history is permanently lost.

This is not an edge case inconvenience - this is data loss affecting production workflows.

I got the same error when I tried to click /compact, with the Sonnet 4.5 model. When all the context is used up, the agent gets stuck on "prompt is too long" and becomes completely unresponsive.

same

Same issue for me on MacOS.

Claude Code claims to have used internal thinking blocks (I didn't use ultrathink)

/compact 
  ⎿ Error: Error during compaction: Error: API Error: 400 
    {"type":"error","error":{"type":"invalid_request_error","message":"me
    ssages.7.content.144: `thinking` or `redacted_thinking` blocks in the
     latest assistant message cannot be modified. These blocks must 
    remain as they were in the original 
    response."},"request_id":"req_011CVtGiUXiWBhpnjpfrJ2M5"}

From Claude Code Agent

This error happens because I use internal thinking blocks (even
  though they're not visible to you) and the API won't allow those to
   be modified during compaction.

Bumping;

Error: Error during compaction: Error: API Error: 400 
{
  "type":"error",
  "error": {
    "type":"invalid_request_error",
    "message":"messages.45.content.24: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},
  "request_id":"req_011CVwLyz8qE1ti3pD4z1QZU"
}

Closing as duplicate of #12311 (earliest compact-related thinking block issue with most activity). Please follow that issue for updates.

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.

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Critical: Conversation compaction fails for conversations started under previous Claude Code versions #12946

Summary

Error Message

Impact

Environment

Root Cause Hypothesis

Expected Behavior

Suggested Fixes

User Impact Statement

Update: Issue Persists in 2.0.57

Related Issues

Impact Escalation

Note on Claimed Fixes

Correction: Workarounds Do NOT Work

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Critical: Conversation compaction fails for conversations started under previous Claude Code versions #12946

Description

Summary

Error Message

Impact

Environment

Root Cause Hypothesis

Expected Behavior

Suggested Fixes

User Impact Statement

Activity

joshcreter commented on Dec 4, 2025

Update: Issue Persists in 2.0.57

Related Issues

Impact Escalation

Note on Claimed Fixes

joshcreter commented on Dec 4, 2025

Correction: Workarounds Do NOT Work

chenyanv commented on Dec 5, 2025

willyjie23 commented on Dec 7, 2025

mnadler commented on Dec 8, 2025

davidjpnelson commented on Dec 9, 2025

dicksontsai commented on Dec 9, 2025

github-actions commented on Dec 17, 2025

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