Reading · eshlox · RSS Amplifier

TLDR: Claude in a sandbox can reach your API at http://host.docker.internal:3333. Network access to a port does not give access to env vars. Three options: exposed ports (zero setup), shared Docker network (hides DB from Claude), MCP proxy (full audit trail).

An isolated agent that cannot test anything is useless. Claude needs to hit endpoints and see error responses. The key insight: network access to a container’s port does NOT give access to that container’s environment variables.

Option 1: Exposed ports (recommended)

Your docker-compose.dev.yml already exposes ports. Claude uses host.docker.internal:

# Inside Claude's sandbox
curl http://host.docker.internal:3333/api/health

Zero setup. Works with Docker Desktop out of the box.

Only the API is visible to Claude. Database and Redis stay hidden:

services:
  api:
    networks: [dev-net, claude-shared]
  postgres:
    networks: [dev-net] # Claude cannot reach this
networks:
  claude-shared:
    name: claude-shared
    external: true

docker network create claude-shared

Option 3: MCP proxy (most secure)

A lightweight MCP server that mediates all requests with logging:

server.tool(
  "api_request",
  "Send request to dev API",
  {
    method: { type: "string", enum: ["GET", "POST", "PUT", "DELETE"] },
    path: { type: "string" },
  },
  async ({ method, path }) => {
    console.error(`[MCP] ${method} ${path}`);
    const res = await fetch(`http://api:3333${path}`, { method });
    return {
      content: [
        { type: "text", text: `HTTP ${res.status}\n${await res.text()}` },
      ],
    };
  },
);

Add to CLAUDE.md

## Local Development
API: http://host.docker.internal:3333
Frontend: http://host.docker.internal:3000
Check .env.example for required variables.
Never read .env files or access container env vars.

MCP server credentials

MCP config files (claude_desktop_config.json, .mcp.json) often contain API keys inline. This is a secret on disk that AI agents read automatically.

Fix: Use environment variable references:

{
  "mcpServers": {
    "github": {
      "command": "mcp-server-github",
      "env": { "GITHUB_TOKEN": "${GITHUB_TOKEN}" }
    }
  }
}

Set the variable before launching Claude:

export GITHUB_TOKEN=$(infisical secrets get GITHUB_TOKEN --env=dev --plain --silent)

Add MCP configs to .claudeignore:

claude_desktop_config.json
.mcp.json
**/mcp.json

Which option

Solo dev: Option 1. Want to hide DB: Option 2. Real money involved: Option 3.

Spot something wrong?

Found a typo, a broken link, or something that could be better? I would love to hear from you. Drop me a message and I will fix it.

Connecting AI sandboxes to dev containers