[BUG] Frequent "API Error: 400 due to tool use concurrency issues" in VSCode extension

Additional findings on pipe mode (-p) root cause

### Summary
The 400 error is specific to pipe mode (-p). Parallel tool calls work correctly in interactive mode but fail consistently with -p.

### Environment

• Claude Code version: 2.1.19
• OS: macOS (Darwin 25.2.0)
• Models tested: claude-sonnet-4-5-20250929, claude-opus-4-5-20251101

### Key findings

1. Root cause is -p flag, not any other flag

• Tested removing --dangerously-skip-permissions → still fails
• Tested removing --output-format stream-json → still fails
• Tested different models (sonnet/opus) → both fail
• Only common factor: -p (stdin pipe mode)

1. Works in interactive mode

Same parallel tool calls (Bash + Read, or Read + Read) succeed in interactive CLI but fail in pipe mode.

1. Error can occur AFTER successful tool execution

In some cases, the tool call completes successfully (returns valid results), but the 400 error triggers on the next turn when the model tries to continue.

1. Prompt instructions don't help

Adding explicit instructions like "Execute ALL tool calls ONE AT A TIME" doesn't prevent the error - model optimizes with parallel calls anyway.

### Minimal reproducer
```bash
# This triggers parallel Read calls and fails
echo "Read /etc/hosts and /etc/passwd and compare them" | claude -p

# Same request works fine in interactive mode
claude # then type the same prompt

Log pattern

// Assistant sends parallel tool calls
{"type":"tool_use","id":"toolu_1","name":"Read","input":{"file_path":"file1"}}
{"type":"tool_use","id":"toolu_2","name":"Read","input":{"file_path":"file2"}}

// Results return successfully
{"type":"tool_result","tool_use_id":"toolu_1","content":"..."}
{"type":"tool_result","tool_use_id":"toolu_2","content":"..."}

// Then error on next assistant turn
{"type":"assistant","error":"invalid_request","content":[{"text":"API Error: 400 due to tool use concurrency issues."}]}

Workaround attempts (none worked)

• ❌ Remove --output-format stream-json
• ❌ Remove --dangerously-skip-permissions
• ❌ Switch models (sonnet → opus)
• ❌ Add "execute sequentially" instructions to prompt

Suggested investigation

The issue appears to be in how pipe mode handles concurrent tool execution vs interactive mode. There may be a race condition or different code path for tool result aggregation in -p mode.
```

same here

I have the same issue :(

Same issue too :(:(

A temporary fix is to downgrade to 2.1.15

curl -fsSL https://claude.ai/install.sh | bash -s 2.1.15

I downgraded and it works again thanks

On Mon, 26 Jan 2026, 16:32 berna, @.***> wrote:

bbernag left a comment (anthropics/claude-code#20929) <https://github.com/anthropics/claude-code/issues/20929#issuecomment-3800516262> A temporary fix is to downgrade to 2.1.15 curl -fsSL https://claude.ai/install.sh | bash -s 2.1.15 — Reply to this email directly, view it on GitHub <https://github.com/anthropics/claude-code/issues/20929#issuecomment-3800516262>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AKGF5RMBJHBIR3GOS7WHRN34IY6QXAVCNFSM6AAAAACS334T2OVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQMBQGUYTMMRWGI> . You are receiving this because you commented.Message ID: @.***>

Yes same here. Downgraded and it's working

I'm using Claude Code 2.1.19 for VS Code; any way to download to 2.1.15?

I'm using Claude Code 2.1.19 for VS Code; any way to download to 2.1.15?

Yes, you can

<img width="788" height="207" alt="Image" src="https://github.com/user-attachments/assets/5bf6cc94-254e-4357-9787-c0515b2283a3" />

I am getting this error with claude code on cursor extension. i tried downgrading it doesn't help

Update: Issue resolved — it was a formatting mistake on my end. ALT + SHIFT + F is working again!

New to all of this, good to now issues are abound. Prettier Extension was working fine in VS Code. Installed Claude Code yesterday and now ALT + SHIFT + F isn't working at all. Uninstalled both Prettier and Claude Code, still not properly functioning. Each ALT + SHIFT + F gets Prettier! error in bottom right corner of VS Code.

Can confirm the same error.

Platform: Linux (6.17.0-8-generic)
Claude Code running in VSCode extension
Model: claude-opus-4-5-20251101

same error. Had to downgrade. to 2.1.15

Same error here: API Error: 400 due to tool use concurrency issues.

Took a look at the logs:

[ERROR] MCP server "claude-vscode" Failed to fetch tools: MCP error -32601: Method not found

messages.1: `tool_use` ids were found without `tool_result` blocks immediately after: toolu_01FgDfzzzUtV74HedBYfTbQs. Each `tool_use` block must have a corresponding `tool_result` block in the next message.

Same issue, downgrading extension from 2.1.19 to 2.1.15 helped (claude cli is latest)

claude --version
2.1.9 (Claude Code)

VScode:

Version: 1.108.2
Commit: c9d77990917f3102ada88be140d28b038d1dd7c7
Date: 2026-01-21T13:52:09.270Z
Electron: 39.2.7
ElectronBuildId: 12953945
Chromium: 142.0.7444.235
Node.js: 22.21.1
V8: 14.2.231.21-electron.0
OS: Darwin arm64 25.2.0