Building cdpx: A Driverless CDP Browser Engine for AI Agents

Playwright in Python or Go spent over 150MB on a Node.js subprocess before it touched a single DOM node. For an AI agent driving a browser, that driver is most of the cost. cdpx removes the driver entirely.

cdpx is a single Rust binary that speaks Chrome DevTools Protocol (CDP) over a raw WebSocket, injects a 4KB tree-walker into the page, and compresses interactive state into handles like @e1, @e2. It idles under 25MB RSS. No Node.js, no Python, no external process.

+------------------+        +------------------+        +------------------+
|   AI Agent       |        |   cdpx (Rust)    |        |   Chromium       |
|  stdio JSON-RPC  | <----> |   ~1.2 MB binary | <----> |  WebSocket :9222 |
|   or CLI         |  MCP   |   tokio runtime  |   CDP  |  remote-debug    |
+------------------+        +------------------+        +------------------+
                                   |
                                   | injects
                                   v
                         +------------------+
                         | dom_walker.min.js|
                         | 4KB tree walker  |
                         | + occlusion test |
                         +------------------+

The CDP Layer

cdpx skips driver bootstrap entirely. It connects tokio-tungstenite straight to Chromium's --remote-debugging-port WebSocket and issues CDP JSON-RPC commands. The crate pulls tokio, serde, clap, reqwest, which, url, and tempfile; the lockfile resolves to 148 transitive crates, and the release binary strips to 1.2MB.

The transport is one file: src/cdp.rs (229 lines). It holds an [Target.setAutoAttach] listener and routes raw CDP messages. Because there is no Runtime.enable on by default and no injected __playwright__binding__ global, the page sees no driver footprint that Cloudflare Turnstile or DataDome fingerprint.

Semantic Action-State Projection

Full DOM dumps are token garbage for an agent. cdpx injects dom_walker.min.js (4,001 bytes) which walks the tree, culls viewport-invisible and offscreen nodes, runs an occlusion hit-test, and emits only visible interactive elements. src/sasp.rs (97 lines) assigns monotonic handles and serializes the projection.

The per-turn output stays under 800 tokens. An open on Hacker News produces:

[Page: Hacker News] (url: https://news.ycombinator.com)
------------------------------------------------------------
@e1: link "Hacker News"
@e2: link "new"
@e3: link "past"
@e4: link "comments"

Hydration-Safe Actions

Next.js and Remix pages eat naive clicks: the handler is not attached yet when the driver clicks. cdpx runs actions in two phases:

1. Scroll into view and measure the center rect in-page.
2. Dispatch a trusted Input.dispatchMouseEvent sequence only after the DOM mutation and React fiber quiet periods settle.

src/action.rs (226 lines) owns this. The guard is not a fixed sleep; it waits for the mutation stream to go quiet, so dead clicks on rehydrating routes stop happening.

MCP over stdio

The same binary is an MCP server. cdpx --mcp speaks stdio JSON-RPC 2.0 and exposes three tools:

• browser_navigate(url) - navigates, settles hydration, returns the SASP map
• browser_act(ref, action, value?) - performs click or type on an @eN handle
• browser_snapshot() - re-emits the token-compressed element map

Any agent host that reads stdio servers - Claude Code, Antigravity, Codex - declares one entry:

{
  "mcpServers": {
    "cdpx": {
      "command": "cdpx",
      "args": ["--mcp"]
    }
  }
}

CLI

For one-off automation the same engine is a CLI:

cdpx open "https://github.com/login"
cdpx type @e2 "octocat"
cdpx click @e4

Flag --headed runs a visible browser; the default is headless.

Build numbers

The whole engine is 1,145 lines of Rust across six source files, plus one embedded 4KB script. Release profile compiles with opt-level = "z", lto = true, codegen-units = 1, panic = "abort", and strip = true to keep the binary at 1.2MB and RSS under 25MB.

cdpx is published on crates.io and the source lives at github.com/AkashPriyadarshii/cdpx.