Key Guard

Key Guard

A security skill that ensures API keys stay local and are never sent to Claude.

When This Skill Applies

Activate whenever the user wants to:

• Call an external API (OpenAI, DeepL, Oxford Dictionary, etc.)
• Check if an API key is configured
• Read .env, *.key, secrets.*, or any credentials file
• View or edit a script (.sh, .bash, curl commands, config files) that may contain a hardcoded API key
• Debug why an API call is failing

Rules (ALWAYS follow these)

1. NEVER read .env or key files directly — do not use bash cat .env or file read tools on any file containing keys
2. NEVER read script or config files directly if they might contain hardcoded API keys — use read_file_masked instead
3. NEVER include a key value in your response, even partially
4. ALWAYS use the key-guard MCP server for anything key-related

How to Use the MCP Server

The key-guard MCP server exposes five tools:

Tool 1: list_keys

Discover all available key names — never values.

Call: list_keys()
Returns: { keys: ["KEY_A", "KEY_B", "KEY_C"] }

Tool 2: validate_key

Check if a key is configured without seeing it.

Call: validate_key({ key_name: "OPENAI_API_KEY" })
Returns: { exists: true, length: 51, preview: "sk-a****", message: "Key is set" }

Tool 2: call_api

Make an authenticated HTTP request locally. The key is injected by the MCP server — Claude only sees the API response.

Call: call_api({
  key_name: "OPENAI_API_KEY",
  url: "https://api.openai.com/v1/models",
  method: "GET"
})
Returns: { status: 200, data: { ... API response ... } }

Tool 3: read_file_masked

Read a script or config file with all key values replaced by {{KEY_NAME}} placeholders. Use this instead of reading files directly.

Call: read_file_masked({ file_path: "./call.sh" })
Returns: {
  content: "curl -H 'Authorization: Bearer {{OPENAI_API_KEY}}' https://..."
}

You can now safely view and suggest edits to the non-key parts.

Tool 4: write_file_with_keys

Write a file back after editing, with {{KEY_NAME}} placeholders substituted with real key values locally.

Call: write_file_with_keys({
  file_path: "./call.sh",
  content: "curl -H 'Authorization: Bearer {{OPENAI_API_KEY}}' https://api.openai.com/v1/chat/completions ..."
})
Returns: { success: true, message: "File written with keys substituted locally" }

Setup Instructions (tell the user if MCP is not running)

If the MCP server hasn't been registered yet:

# Clone the repo
git clone https://github.com/your-username/key-guard.git

# Copy .env.example to .env and fill in your keys
cp .env.example .env

# Register the MCP server (run once) — replace the path with your actual clone location
/mcp add key-guard node /path/to/key-guard/key-guard.js

# Or add directly to ~/.copilot/mcp-config.json for auto-load on restart:
# {
#   "mcpServers": {
#     "key-guard": {
#       "command": "node",
#       "args": ["/path/to/key-guard/key-guard.js"]
#     }
#   }
# }

Example Workflows

User: "Is my OpenAI key set up?"

1. Call validate_key({ key_name: "OPENAI_API_KEY" })
2. Report back: "Yes, your key is set (51 chars, starts with sk-a****)"

User: "Call the OpenAI API to get word definitions"

1. Call call_api({
     key_name: "OPENAI_API_KEY",
     url: "https://api.openai.com/v1/chat/completions",
     method: "POST",
     body: { model: "gpt-4o-mini", messages: [...] }
   })
2. Use the returned response — never the key itself

User: "Show me my .env file"

Do NOT read .env directly.
Instead, call validate_key for each expected key name and show:
- Which keys are configured
- Approximate length (as a sanity check)
Never show actual values.

User: "Edit my curl script to add a header"

1. Call read_file_masked({ file_path: "./call.sh" })
   → Claude sees "curl -H 'Authorization: Bearer {{OPENAI_API_KEY}}' ..."
2. Make the requested edit to the non-key parts
3. Call write_file_with_keys({ file_path: "./call.sh", content: "<edited content with {{OPENAI_API_KEY}} still in place>" })
   → MCP substitutes the real key before writing to disk