feat: add MCP server (psst-mcp) for Claude and agent integration

[djhenry]

Summary

Adds a Model Context Protocol (MCP) server so Claude Code, Claude Desktop, Cursor, Windsurf, and any MCP-capable agent can use psst secrets without ever seeing their values.

• New binary psst-mcp (installed alongside psst via npm)
• Pure stdio transport — no daemon, no ports
• Secrets masked as [REDACTED] in all command output
• Unlocks vault through OS keychain or PSST_PASSWORD — same auth as the CLI

Tools

Tool	Description
list_secrets	List secret names + tags. Never exposes values.
exec_command	Run a command with specific named secrets injected (least-privilege).
run_command	Run a command with all vault secrets injected.
list_environments	List available vault environments.

Setup

Claude Code — add to .claude/mcp.json or ~/.claude/mcp.json:

{
  "mcpServers": {
    "psst": {
      "command": "psst-mcp"
    }
  }
}

Claude Desktop — add to claude_desktop_config.json:

{
  "mcpServers": {
    "psst": {
      "command": "psst-mcp"
    }
  }
}

Then Claude can do things like:

list_secrets()                                           → STRIPE_KEY, DATABASE_URL
exec_command(["STRIPE_KEY"], ["curl", "-H", "Authorization: Bearer $STRIPE_KEY", "https://api.stripe.com"])
                                                         → stdout: [REDACTED response body]

How it works

The MCP server uses the Vault class directly (no subprocess shelling out to psst). Secret values are fetched from the vault, injected into the subprocess environment, and any that appear in stdout/stderr are masked before being returned to the agent. The agent only ever sees secret names and masked output — never values.

Test plan

• npm install -g psst-cli && psst-mcp starts without error
• list_secrets returns names but never values
• exec_command runs command with secret injected; value appears as [REDACTED] in output
• run_command runs command with all secrets; values masked in output
• Error cases return isError: true with a readable message (no vault, unlock failed, missing secret)
• Works with both local (.psst/) and global (~/.psst/) vaults
• Claude Code can call all four tools via MCP

🤖 Generated with Claude Code