[BUG] /resume produces immediate 400 tool use concurrency error on first message

Status Fixed / completed

Maintainer reply None cached

Activity 5 comments · opened Mar 27, 2026 · closed Mar 27, 2026

Description

After using /resume to continue a previous conversation, the very first message sent triggers:

API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.

The error occurs immediately — no tool calls or compaction happened in the new session. The resumed conversation history is already corrupted from reconstruction.

Steps to Reproduce

Have a conversation with tool use (e.g., file reads, bash commands)
End the conversation
Start a new session and /resume the previous conversation
Send any message (e.g., "so")
Immediately get API Error: 400 due to tool use concurrency issues

Expected Behavior

The resumed conversation should have a valid message history and accept new messages.

Actual Behavior

The first message after /resume triggers a 400 error. /rewind does not fix it because the corruption is baked into the reconstructed history, not in a recent turn.

Additional Context

Platform: Linux (Arch Linux 6.18.13)
Interface: CLI (terminal)
This is distinct from the compaction-related 400 error (#37452). The corruption happens during /resume history reconstruction, not during context compaction.
/rewind cannot recover the conversation because the malformed tool_use/tool_result pairing exists in the resumed base history, not in a rewindable turn.

Screenshot

The previous session worked fine. After resuming and sending one message ("so"), the error appeared:

> so
⌐ API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.

Related Issues

#37452 — Similar 400 error but caused by context compaction
#21321 — Meta-issue for 400 concurrency errors
#23093 — /resume breaking
#9895 — Resume crashes

View original on GitHub ↗

5 Comments

lotz · 6 months ago

This is a clear regression in version 2.1.85. Downgrading to 2.1.84 (https://code.claude.com/docs/en/setup#advanced-installation-options) works with the same conversation. It's reproducible in multiple conversations that were working fine in 2.1.84 and now fail with this error in 2.1.85. I do not see any mismatched tool_use/tool_results in my context either, using some "repair" tools to check them.

github-actions[bot] · 6 months ago

Found 2 possible duplicate issues:

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dark942 · 6 months ago

Closing as duplicate of #39316 — same root cause (v2.1.85 regression). Adding my /resume-specific reproduction there.

1nv1s1b1e · 6 months ago

I met the question today as well :(