API Error: 400 due to tool use concurrency issues on session start

Bug

Claude Code crashes with API Error: 400 due to tool use concurrency issues immediately on session start when it makes parallel tool calls (e.g., reading project files + running git commands simultaneously).

A related issue: starting with a direct command like claude "git status zeigen" results in Invalid tool parameters followed by the session hanging indefinitely ("Sautéing…").

Steps to reproduce

Scenario 1 — Concurrency crash:

1. Open a project directory with CLAUDE.md files present
2. Start a new session with claude
3. Send a simple greeting like "moin"
4. Claude Code attempts to read multiple files and run bash commands in parallel
5. → Crashes with: API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.

Scenario 2 — Invalid tool parameters:

1. Start with claude "git status zeigen"
2. → Invalid tool parameters error, then session hangs at "Sautéing…"

Expected behavior

Parallel tool calls should execute without error, or be serialized automatically if concurrency is not supported. Direct command invocation should work reliably.

Actual behavior

• Scenario 1: Session crashes immediately. /rewind recovers but the issue recurs on next session start with parallel tool calls. Reproducible across multiple new sessions.
• Scenario 2: Session hangs indefinitely after invalid tool parameters error.

Environment

• Claude Code: v2.1.89 (latest as of 2025-04-01)
• Model: Opus 4.6 (1M context) · Claude Max
• macOS: 26.3.1
• Node: v25.8.1
• npm: 11.11.1
• Hardware: Mac mini M4, 16 GB RAM
• Shell: zsh

Additional context

• Happens consistently when Claude Code's auto-start behavior triggers multiple tool calls at once (reading CLAUDE.md, running git status, etc.)
• The error seems to be a server-side rejection of concurrent tool use, not a client-side issue
• Multiple MCP servers are configured (n8n, Cloudflare, Airtable, Playwright, etc.) which may increase the chance of concurrent tool initialization conflicts