Claude Code Thinking Block Fix

Fix for Claude Code API Error 400 - Invalid signature in thinking block.

Problem

When using Claude Code with extended thinking mode, you may encounter this error:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.X.content.0: Invalid `signature` in `thinking`block"}}

This happens when thinking blocks in session files become corrupted.

Solution

This tool removes corrupted thinking and redacted_thinking blocks from Claude Code session files, allowing you to resume your conversation.

Installation

Quick Install (Recommended)

# Clone the repository
git clone https://github.com/a1exlism/claude-code-cmd-fix-thinking-error.git
cd claude-code-cmd-fix-thinking-error

# Run install script
./install.sh

The install script will optionally configure Claude Code hooks for automatic session checking.

Manual Install

# Copy the fix script
cp fix_claude_thinking_error.py ~/.claude/

# Copy the command file (optional, for /fix-thinking-error command)
mkdir -p ~/.claude/commands
cp commands/fix-thinking-error.md ~/.claude/commands/

Usage

When Error Occurs (In-Session Fix)

When you encounter the 400 error, use this workflow to fix it without leaving Claude Code:

# Step 1: Clear current corrupted session
/clear

# Step 2: Fix the session file
/fix-thinking-error

# Step 3: Resume the fixed session
/resume
# Then select the session you want to restore

Note: /clear clears the current conversation context, allowing /fix-thinking-error to run without triggering the 400 error.

Method 1: Direct Python Script

# Fix latest session (default)
python3 ~/.claude/fix_claude_thinking_error.py

# List all session files
python3 ~/.claude/fix_claude_thinking_error.py --list

# Fix all session files
python3 ~/.claude/fix_claude_thinking_error.py --all

# Fix specific file
python3 ~/.claude/fix_claude_thinking_error.py --file /path/to/session.jsonl

# Fix by index (from --list)
python3 ~/.claude/fix_claude_thinking_error.py --index 2

# Skip backup (use with caution)
python3 ~/.claude/fix_claude_thinking_error.py --no-backup

Method 2: Claude Code Command (after installation)

/fix-thinking-error           # Fix latest session (default)
/fix-thinking-error --list    # List all session files
/fix-thinking-error --all     # Fix all session files

Note: Using /fix-thinking-error inside Claude Code may generate new thinking blocks. If the error persists after fixing, run the script again from an external terminal.

Using Hooks for Automatic Fix

Claude Code hooks are user-defined shell commands that run automatically at different points in the lifecycle. You can configure a hook to automatically check for corrupted thinking blocks when resuming a session.

Add the following to ~/.claude/settings.json:

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "resume",
        "hooks": [
          {
            "type": "command",
            "command": "python3 ~/.claude/fix_claude_thinking_error.py --hook-mode --cwd \"$CLAUDE_PROJECT_DIR\""
          }
        ]
      }
    ]
  }
}

This hook runs on SessionStart (specifically when resuming) and checks session files for corrupted thinking blocks.

Options

Option	Description
--list / -l	List all session files
--file <path> / -f	Fix specific file
--index <n> / -i	Fix/restore file by index
--all / -a	Fix all session files
--no-backup	Skip backup creation (use with caution)
--auto-fix	Skip confirmation when using --all
--cwd <path> / -c	Filter sessions by project directory
--check	Check for corrupted thinking blocks
--hook-mode	Run as hook, output JSON format
--list-backups / -lb	List all backup files
--restore / -r	Restore session from backup
--delete / -d	Delete backup after restore

Filter by Project

When running multiple Claude Code instances, use --cwd to filter sessions by project:

# Fix only current project's sessions
python3 ~/.claude/fix_claude_thinking_error.py --cwd .

# List sessions for a specific project
python3 ~/.claude/fix_claude_thinking_error.py --cwd /path/to/project --list

# Fix all sessions (all projects)
python3 ~/.claude/fix_claude_thinking_error.py

The /fix-thinking-error command automatically uses --cwd to filter to the current project. Use --global to fix all projects.

Backup & Restore

List Backups

python3 ~/.claude/fix_claude_thinking_error.py --list-backups

Restore from Backup

# Restore latest backup
python3 ~/.claude/fix_claude_thinking_error.py --restore

# Restore specific backup by index
python3 ~/.claude/fix_claude_thinking_error.py --restore --index 2

# Restore and delete backup file
python3 ~/.claude/fix_claude_thinking_error.py --restore --delete

After Fixing

IMPORTANT: You MUST reload the session after fixing. The fix only modifies the file on disk, not the current session in memory.

# In Claude Code, reload the session
/resume
# Then select the current session to reload it

⚠️ DO NOT continue using the current session after fixing. The error will persist until you /resume to reload the fixed session data.

How It Works

1. Locates Claude Code session files in ~/.claude/projects/
2. Creates a timestamped backup (e.g., session.jsonl.bak.20250109_120000)
3. Removes thinking and redacted_thinking content blocks from messages
4. Writes the cleaned session file

File Structure

claude-thinking-fix/
├── README.md                           # This file
├── install.sh                          # Installation script
├── fix_claude_thinking_error.py        # Main fix script
└── commands/
    └── fix-thinking-error.md           # Claude Code command file

Requirements

• Python 3.6+
• No external dependencies (uses only standard library)

Platform Support

Platform	Status	Notes
Linux	✅ Supported	Native support
macOS	✅ Supported	Native support
WSL	✅ Supported	Recommended for Windows users
Windows	❌ Not tested	Contributions welcome

Reference

• Issue #10199 - BUG: API Error 400 - Thinking Block Modification Error

License

MIT License