# GitHub API Token Injection
This tutorial shows how to configure mcp-js to inject a GitHub personal access token into outgoing `fetch()` requests, so that JavaScript code executed in the sandbox can call the GitHub API without ever seeing the token directly.
## How It Works
mcp-js supports **header injection rules** via the `--fetch-header` CLI flag. When a fetch request matches a rule's host (and optionally HTTP method), the server automatically injects the configured headers into the outgoing request. The injected headers are invisible to user-supplied JavaScript — they cannot be read or leaked by the sandbox code.
## Prerequisites
- Docker and Docker Compose
- A GitHub personal access token (classic or fine-grained)
## Step 1: Configure `docker-compose.yml`
Add a `--fetch-header` argument to the mcp-js service command. The format is:
```
host=<hostname>,header=<header-name>,value=<header-value>
```
Here is a full example using an environment variable for the token:
```yaml
services:
opa:
image: openpolicyagent/opa:latest
command: ["run", "--server", "--addr", "0.0.0.0:8181", "/policies"]
ports:
- "8181:8181"
volumes:
- ./policies:/policies:ro
mcp-js:
build: .
command:
- --http-port=3000
- --directory-path=/data/heaps
- --session-db-path=/data/sessions
- --opa-url=http://opa:8181
- --fetch-header=host=api.github.com,header=Authorization,value=Bearer ${GITHUB_TOKEN}
tmpfs:
- /data:uid=1000,gid=1000
ports:
- "3000:3000"
depends_on:
- opa
```
The `${GITHUB_TOKEN}` variable is read from your shell environment or from a `.env` file in the same directory as the compose file.
### Using a `.env` file
Create a `.env` file (make sure it is in `.gitignore`):
```bash
echo 'GITHUB_TOKEN=ghp_yourTokenHere' > .env
```
## Step 2: Start the Services
```bash
docker compose up --build -d
```
Verify the header injection rule was loaded by checking the logs:
```bash
docker compose logs mcp-js | grep "header injection"
```
You should see:
```
Loaded 1 fetch header injection rule(s)
```
## Step 3: Test the Token
Use the `/api/exec` endpoint to execute JavaScript that calls the GitHub API. Note that you must include a `User-Agent` header in the fetch request — GitHub requires it.
### List repositories for a user
```bash
curl -s -X POST http://localhost:3000/api/exec \
-H "Content-Type: application/json" \
-d '{
"code": "(async () => { const r = await fetch(\"https://api.github.com/users/r33drichards/repos?per_page=5&sort=updated\", { headers: { \"Accept\": \"application/vnd.github+json\", \"User-Agent\": \"mcp-js-test\" } }); const data = await r.json(); return JSON.stringify(data.map(repo => ({ name: repo.full_name, private: repo.private, url: repo.html_url })), null, 2); })()"
}'
```
Example response:
```json
{
"output": "[\n {\n \"name\": \"r33drichards/mcp-registry\",\n \"private\": false,\n \"url\": \"https://github.com/r33drichards/mcp-registry\"\n },\n {\n \"name\": \"r33drichards/mcp-js\",\n \"private\": false,\n \"url\": \"https://github.com/r33drichards/mcp-js\"\n },\n {\n \"name\": \"r33drichards/headlessmc-bot\",\n \"private\": false,\n \"url\": \"https://github.com/r33drichards/headlessmc-bot\"\n },\n {\n \"name\": \"r33drichards/mcp-tlaplus\",\n \"private\": false,\n \"url\": \"https://github.com/r33drichards/mcp-tlaplus\"\n },\n {\n \"name\": \"r33drichards/darwin\",\n \"private\": false,\n \"url\": \"https://github.com/r33drichards/darwin\"\n }\n]"
}
```
### Check the authenticated user
```bash
curl -s -X POST http://localhost:3000/api/exec \
-H "Content-Type: application/json" \
-d '{
"code": "(async () => { const r = await fetch(\"https://api.github.com/user\", { headers: { \"User-Agent\": \"mcp-js-test\" } }); const data = await r.json(); return JSON.stringify({ login: data.login, name: data.name }, null, 2); })()"
}'
```
## Alternative: JSON Config File
For multiple header rules or more complex configurations, use `--fetch-header-config` with a JSON file:
```json
[
{
"host": "api.github.com",
"methods": ["GET", "POST"],
"headers": {
"Authorization": "Bearer ghp_yourTokenHere",
"X-GitHub-Api-Version": "2022-11-28"
}
}
]
```
Mount the file and reference it in your compose command:
```yaml
command:
- --http-port=3000
- --opa-url=http://opa:8181
- --fetch-header-config=/config/headers.json
volumes:
- ./headers.json:/config/headers.json:ro
```
## Important Notes
- **User-provided headers take precedence.** If the JavaScript code explicitly sets an `Authorization` header in a fetch call, the injected header will not override it.
- **Wildcard hosts are supported.** You can use `*.github.com` to match all GitHub subdomains.
- **OPA policy must allow the domain.** The default policy already allows `api.github.com`. If you have a custom policy, make sure it permits requests to GitHub.
- **Never commit tokens.** Use environment variables or a `.env` file (added to `.gitignore`) to keep secrets out of version control.
