[BUG] API Error: 400 due to tool use concurrency issues.

Found 3 possible duplicate issues:

1. https://github.com/anthropics/claude-code/issues/20592
2. https://github.com/anthropics/claude-code/issues/20571
3. https://github.com/anthropics/claude-code/issues/20555

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🤖 Generated with Claude Code

We're seeing the exact same behavior after upgrading to v2.1.19.

try downgrading to 2.16/2.7, and this problem still persists.

try downgrading to 2.16/2.7, and this problem still persists.

So this is an issue on anthropic side

> try downgrading to 2.16/2.7, and this problem still persists. So this is an issue on anthropic side

2.15 works for me

How to downgrade ?

Hey, just wanted to let you know - I encountered the same issue when using the Claude Code extension (Antigravity).
Running the same claude code version (v2.1.19 ) in the terminal works without any issues... So pretty sure you can use the terminal as a temp workaround

How to downgrade ?

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

I was having this same issue. Downgraded to 2.1.15, and turned off Auto-Update. Seems to be working.

✅ WORKAROUND FOUND

The fix: Install VS Code extension version 2.1.17 while keeping CLI at 2.1.19

Steps:

1. In VS Code Extensions → "Claude Code for VS Code"
2. Click dropdown next to Uninstall → "Install Specific Version" → 2.1.17
3. Uncheck "Auto Update"
4. Restart VS Code

Result: Extension 2.1.17 + CLI 2.1.19 = ✅ Works

The bug is in the VS Code extension code (2.1.18/2.1.19), not the CLI or API.

I am also experiencing this just simply using claude.ai/code

I had to defer to just using CLI, downgrading didn't work

I had this issue using VS Code. Here is what solved it for me:

/compact

then

continue

Seeing this issue and diagnosed it as the following using Copilot CLI 😛

Bug Report: MCP Tool Names Exceed 64-Character API Limit

  Error:

    API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.2.content.0.tool_result.content.0.to
  ol_reference.tool_name: String should have at most 64 characters"}}

  Root Cause: Claude Code generates MCP tool names using the pattern:

    mcp__plugin_{plugin-name}_{server-name}__{tool-name}

  This naming convention can easily exceed the Anthropic API's 64-character limit for tool names when plugin names, server names,
  or tool names are descriptive.

  Example: A plugin with an MCP server named azure-devops-mcp-server exposing a tool repo_list_pull_requests_by_commits produces a
  combined name well over 64 characters.

  Suggested Fix Options:

    - Truncate or hash long tool names before sending to the API
    - Use a shorter naming convention for MCP tools from plugins
    - Validate tool name length during plugin installation and warn/error if names would exceed 64 characters
    - Document the limitation so plugin authors know to use short server names

  Current Workaround: Plugin authors must use very short MCP server names (e.g., ado instead of azure-devops-mcp-server) to stay
  under the limit.

Closing for now — inactive for too long. Please open a new issue if this is still relevant.

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.