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How to Fix Common Claude Code Errors: Installation, Auth, MCP, and Performance Issues

Cowrie
Cowrie
Dev @ Bswen

Problem

When I use Claude Code, I encounter various errors during installation, authentication, or MCP configuration. Here are the common errors I’ve seen:

Common errors
# Error 1: Installation
command not found: claude


# Error 2: Authentication
Authentication failed. Please run 'claude init' to authenticate.


# Error 3: MCP Server
tools.11.custom.name: String should match pattern '^[a-zA-Z0-9_-]{1,64}$'


# Error 4: Performance
Response time: 30+ seconds for simple queries

Environment

  • Claude Code (latest version)
  • Node.js v18+
  • macOS / Linux / Windows
  • Various MCP servers installed

What Happened?

I installed Claude Code and tried to use it, but ran into several issues. Let me show you what I encountered and how I fixed each one.

Issue 1: Command Not Found

After installing with npm install -g @anthropic-ai/claude-code, I got this error:

Terminal output
$ claude
zsh: command not found: claude

The npm global bin directory wasn’t in my PATH.

Issue 2: Authentication Failed

When I ran claude, the browser didn’t open for authentication:

Auth error
Authentication failed. Please try again.

Issue 3: MCP Server Validation Error

When adding an MCP server, I got a validation error:

MCP error
Error: tools.11.custom.name: String should match pattern '^[a-zA-Z0-9_-]{1,64}$'

Issue 4: Slow Performance

Claude Code was responding very slowly:

Performance issue
Response time: 30+ seconds for simple queries
Memory usage: 2GB+

How to Solve It?

Solution 1: Fix PATH for Installation

First, I checked my PATH:

Check PATH
echo $PATH
npm config get prefix

Then I added the npm global bin to my PATH:

Fix PATH (zsh)
echo 'export PATH="$PATH:$(npm config get prefix)/bin"' >> ~/.zshrc
source ~/.zshrc

For bash users:

Fix PATH (bash)
echo 'export PATH="$PATH:$(npm config get prefix)/bin"' >> ~/.bashrc
source ~/.bashrc

If you still get permission errors, use a user-level npm directory:

User-level npm directory
mkdir ~/.npm-global
npm config set prefix '~/.npm-global'
echo 'export PATH=~/.npm-global/bin:$PATH' >> ~/.profile
source ~/.profile

Solution 2: Fix Authentication

I cleared the current authentication and re-initialized:

Re-authenticate
claude logout
claude init

If the browser doesn’t open automatically:

Manual authentication
claude auth login --manual

Then check your authentication status:

Check auth status
claude auth status

Solution 3: Fix MCP Server Issues

The MCP error occurred because I used invalid characters in the server name. MCP server names must match the pattern ^[a-zA-Z0-9_-]{1,64}$ - only letters, numbers, underscores, and hyphens, max 64 characters.

Invalid vs valid server names
# BAD - contains exclamation mark
claude mcp add my-server! -- npx -y @anthropic-ai/server-filesystem


# GOOD - valid characters only
claude mcp add my_server -- npx -y @anthropic-ai/server-filesystem

For Windows path issues, use forward slashes:

Windows path fix
# BAD - backslashes cause issues
claude mcp add fs -- npx -y @anthropic-ai/server-filesystem C:\Users\name\Documents


# GOOD - use forward slashes
claude mcp add fs -- npx -y @anthropic-ai/server-filesystem C:/Users/name/Documents

To debug MCP issues:

MCP debug commands
# Enable MCP debug mode
claude --mcp-debug


# Test MCP server
claude mcp test <server_name>


# View MCP logs
tail -f ~/Library/Logs/Claude/mcp*.log  # macOS

Solution 4: Fix Performance Issues

I used /compact to compress the conversation context:

Compress context
/compact

I also created a .claudeignore file to limit file scope:

Create .claudeignore
echo "node_modules/" >> .claudeignore
echo "dist/" >> .claudeignore
echo "*.log" >> .claudeignore
echo ".git/" >> .claudeignore

To clear cache and logs:

Clear cache and logs
claude cache clear
claude logs clear

The Reason

I think the key reasons for these errors are:

  1. Command not found: npm installs global packages to a directory that may not be in your shell’s PATH by default, especially on fresh systems.

  2. Authentication failed: Token expiration, corrupted auth files, or network issues can break authentication.

  3. MCP validation: MCP tools have strict naming rules for security and compatibility - invalid characters cause API rejection.

  4. Slow performance: Large context windows from reading many files consume memory and slow down responses. Context compression and file filtering solve this.

Debug Commands Reference

Here’s a quick reference for debugging:

Debug commands
# Enable debug mode
claude --debug
claude --mcp-debug


# View configuration
claude config show


# Check cache
claude cache size
claude cache stats


# Clear all caches
claude cache clear


# Check auth
claude auth status

Quick Troubleshooting Checklist

Before asking for help, check these items:

Troubleshooting checklist
[ ] Is Claude Code installed? (claude --version)
[ ] Is PATH configured correctly? (echo $PATH)
[ ] Is authentication valid? (claude auth status)
[ ] Are MCP servers running? (claude mcp list)
[ ] Is context compressed? (/compact)
[ ] Is .claudeignore configured?
[ ] Is Node.js v18+ installed? (node --version)

Summary

In this post, I showed how to fix the most common Claude Code errors: PATH issues, authentication failures, MCP validation errors, and performance problems. The key point is using --debug for diagnostics and applying targeted fixes. Most issues fall into these four categories, so the checklist above should help you resolve most problems quickly.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

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