# Derive Client

Reverse-engineer a website's internal API by recording browser traffic into a HAR file...

- Repository: https://github.com/vercel-labs/agent-browser (45133 stars)
- Category: Engineering
- License: Apache-2.0
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add vercel-labs/agent-browser --skill derive-client -g -a claude-code -y`
Codex: `npx skills add vercel-labs/agent-browser --skill derive-client -g -a codex -y`
Cursor: `npx skills add vercel-labs/agent-browser --skill derive-client -g -a cursor -y`
Gemini CLI: `npx skills add vercel-labs/agent-browser --skill derive-client -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add vercel-labs/agent-browser`

## Description

Reverse-engineer a website's internal API by recording browser traffic into a HAR file, then generate a standalone client or CLI that calls the endpoints directly, with no browser needed after the first recording. Use when asked to "derive a client", "build a CLI for <site>", "reverse engineer this site's API", "record network requests", "turn this site into an API", or when the same site will be automated repeatedly and direct HTTP calls would beat driving the browser every time.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Derive an API client from a recorded session

Driving a browser is the right tool for the first visit and the wrong tool for the hundredth. This skill records a site's network traffic once while you use it, then turns the captured requests into a standalone client (script, CLI, or library) that talks to the site's internal API directly.

The recording alone contains everything needed: agent-browser embeds text response bodies (JSON/HTML/JS) in the HAR by default, so endpoint shapes can be studied offline after the browser is closed.

## Workflow

```
1. Record     Start HAR capture, drive the flows you want in the client
2. Identify   Find the real API endpoints among the noise
3. Extract    Pull request shapes, response schemas, and auth material
4. Generate   Write the client, one function per flow
5. Verify     Call every endpoint for real before declaring done
```

## 1. Record

```bash
agent-browser network har start          # embeds text response bodies by default
# ... drive the site: search, open a detail page, paginate, etc. ...
agent-browser network har stop /tmp/site.har
```

- Exercise **every flow the client should support**, and run each one at least twice with different inputs (two search terms, two detail pages). Diffing the recorded URLs reveals which parts are parameters.
- If the site needs login, log in **before** starting the HAR so credentials don't land in the recording unnecessarily. The session cookies are exported separately in step 3.
- `--content all` embeds binary bodies too (base64); `--content none` disables embedding. Per-body cap is 2 MB.

While the session is still open, `agent-browser network requests` and `network request <id>` give the same data interactively, but only the HAR survives navigation and browser close, so prefer it for anything multi-page.

## 2. Identify endpoints

Query the HAR with `jq`:

The rest: https://github.com/vercel-labs/agent-browser/blob/HEAD/skill-data/derive-client/SKILL.md