browser/docs/agent.md

Agent mode

Looking for a step-by-step walkthrough instead of a reference? See agent-tutorial.md — it builds one end-to-end Hacker News scenario covering the REPL, recording, replay, --self-heal, and the MCP roundtrip.

lightpanda agent runs a browsing agent backed by Lightpanda's headless engine. It can act as:

• an LLM agent that drives the browser with tool calls (--provider),
• a scripted runner that replays a .lp script deterministically,
• a basic REPL for hand-driven PandaScript with no LLM at all,
• a one-shot task runner that prints a single answer to stdout (--task).

All four modes share the same browser tools (goto, click, fill, tree, markdown, search, ...). The same set is exposed over MCP via lightpanda mcp, so an agent script and an MCP client see the same surface — that is also the way to drive Lightpanda from an external LLM agent (Claude Code, etc.) without giving Lightpanda its own API key.

Quick start

# Interactive REPL — auto-detects an API key from your environment
./lightpanda agent

# Force a specific provider
./lightpanda agent --provider anthropic

# Basic REPL (no LLM, PandaScript only)
./lightpanda agent --no-llm

# Replay a recorded script
./lightpanda agent session.lp

# Replay then continue interactively, appending new commands to the file
./lightpanda agent -i session.lp

# One-shot: ask a question, capture the answer on stdout
./lightpanda agent --task "what is on the front page of hn?"

Providers and API keys

Provider	Flag	API key env
Anthropic	--provider anthropic	ANTHROPIC_API_KEY
OpenAI	--provider openai	OPENAI_API_KEY
Gemini	--provider gemini	GOOGLE_API_KEY or GEMINI_API_KEY
Ollama	--provider ollama	none (local)

Defaults: --model falls back to a sensible per-provider default; --base-url overrides the API endpoint (Ollama defaults to http://localhost:11434/v1).

Provider auto-detection

When --provider is omitted, lightpanda inspects the environment and picks one:

• No keys set → falls back to the basic REPL (PandaScript only). Natural language, LOGIN, ACCEPT_COOKIES, and --self-heal will reject. A one-line notice is printed so you know which mode you landed in.
• Exactly one key set → that provider is used. A one-line notice identifies the env var that won.
• Multiple keys set, on a TTY → a numbered prompt asks which to use.
• Multiple keys set, non-interactive → the agent fails fast and tells you to pass --provider explicitly. Ollama is never auto-detected (no env var to look at) — pass --provider ollama if you want it.

--no-llm is the explicit bypass: it forces the basic REPL even when an API key is present or --provider is set. Use it to test PandaScript without burning tokens, or to disable the LLM in a saved command without editing the existing flags. --no-llm wins over --provider.

PandaScript

PandaScript is a tiny, line-oriented DSL for browser actions. Each line is one command. Comments start with #. Strings are quoted with ', ", or '''…''' for values that mix both quote styles. Quoting rules are content-aware so that recorded scripts round-trip through the parser.

Commands are matched ALL CAPS only (CLICK, GOTO, TYPE, …). Lowercase or mixed-case input is forwarded to the LLM as natural language, so prompts like click the login button flow through without being misread as a CLICK command. In the REPL, TAB completion fills in the canonical caps form for you.

Command	Form	Notes
GOTO	GOTO <url>	Navigate. URL is unquoted.
CLICK	CLICK '<selector>'	CSS selector.
TYPE	TYPE '<selector>' '<value>'	Fills an input. $LP_* env refs auto-resolve.
WAIT	WAIT '<selector>'	Wait for selector to be present in the DOM.
SCROLL	SCROLL [x] [y]	Default (0, 0).
HOVER	HOVER '<selector>'
SELECT	SELECT '<selector>' '<value>'	<select> option by value.
CHECK	CHECK '<selector>' [true|false]	Check / uncheck. Default true.
EXTRACT	EXTRACT '<selector>'	Returns text content.
EVAL	EVAL '<js>' or EVAL '''…'''	Triple-quote for multi-line JS.
TREE	TREE	Print the semantic tree (not recorded).
MARKDOWN	MARKDOWN	Print page as markdown (not recorded).
LOGIN	LOGIN	LLM-driven: fills credentials from $LP_* env vars.
ACCEPT_COOKIES	ACCEPT_COOKIES	LLM-driven: dismiss the consent banner.

In the REPL, anything that does not parse as a PandaScript command is sent to the LLM as natural language. To leave the REPL, use the /quit slash command.

Example script

# Log into the demo and grab the dashboard title.
# Site-scoped vars (LP_<SITE>_<FIELD>) avoid collisions when you have
# credentials for several sites; the unprefixed form is the fallback.
GOTO https://demo-browser.lightpanda.io/
ACCEPT_COOKIES
TYPE '#email' '$LP_DEMO_USERNAME'
TYPE '#password' '$LP_DEMO_PASSWORD'
CLICK 'button[type="submit"]'
WAIT '.dashboard'
EXTRACT '.dashboard h1'

Recording

Interactive sessions can write back to a .lp file:

./lightpanda agent -i session.lp

State-mutating commands (GOTO, CLICK, TYPE, ...) are appended; read-only commands (TREE, MARKDOWN) and the natural-language turns that produced them are not. Natural-language turns are recorded as # <prompt> comments above the resulting tool calls so the script stays readable.

Replay and self-healing

./lightpanda agent script.lp replays without making any LLM call.

With --self-heal --provider <p>, a failed command (typically a stale selector after the page changed) triggers a short LLM turn that inspects the current page and emits a replacement command. The healed command runs, and the original script line is rewritten in place so the next replay succeeds deterministically.

Self-heal is constrained: at most one replacement per failure, capped LLM budget, no navigation away from the current page. It is meant to recover from selector drift, not to redesign the script.

REPL features

• Tab completion (case-insensitive): cycles through PandaScript keywords and /<tool> slash commands. The dim grey suffix shown after the cursor is the first match.
• Persistent history: stored in .lp-history in the working directory.
• Slash commands: /<tool> [args] calls a browser tool directly without going through the LLM. Args accept either a single positional value (for tools with one required field), key=value pairs, or a raw {json} blob. Two meta commands round out the set: /help lists tools (/help <tool> prints the JSON schema), and /quit exits the REPL.

  > /goto https://example.com
  > /findElement role=button name=Submit
  > /eval {"script": "document.title"}
  > /quit
  

• Stdout vs stderr: the final assistant answer and data-producing commands (EXTRACT, EVAL, MARKDOWN, TREE) write to stdout. Tool calls, progress, and errors go to stderr, so lightpanda agent --task ... > out.txt captures a clean answer.

One-shot mode (--task)

./lightpanda agent --provider gemini \
  --task "what is the top story on news.ycombinator.com?"

--task runs a single user turn, prints the final answer on stdout, and exits. Combine with --task-attachment <path> (repeatable) to feed local files to providers that accept attachments.

Driving Lightpanda from an external LLM agent

When the calling agent already has its own LLM (e.g. Claude Code), use lightpanda mcp rather than lightpanda agent. The MCP server exposes the same browser tools (goto, click, fill, ...) listed below, so the external agent does the planning while Lightpanda only drives the browser. No --provider or API key is required on the Lightpanda side.

{
  "mcpServers": {
    "lightpanda": {
      "command": "/path/to/lightpanda",
      "args": ["mcp"]
    }
  }
}

For sub-task delegation in the other direction — calling Lightpanda's own LLM-driven agent in a one-shot fashion — use --task on stdin instead.

Recording PandaScript over MCP

lightpanda mcp exposes three recording tools so an external agent can capture a session as a .lp script for later deterministic replay:

Tool	Args	Effect
record_start	{ path: string }	Begin appending state-mutating tool calls to path (relative, no ..). Errors if already on.
record_stop	{}	Close the recording and return {path, line_count}. Errors if no recording is active.
record_comment	{ text: string }	Write # <text> to the active recording — useful as a breadcrumb above LLM-driven steps.

While recording is active, every goto / click / fill / scroll / hover / selectOption / setChecked / waitForSelector / eval that succeeds is appended verbatim. Query-only tools (tree, markdown, findElement, consoleLogs, …) are not recorded. The resulting file replays without an LLM via ./lightpanda agent session.lp.

Replay + self-heal over MCP

Self-heal is a two-tool roundtrip: lightpanda runs steps and reports structured failures, the calling agent synthesizes a replacement, and lightpanda atomically rewrites the script.

Tool	Args	Effect
script_step	{ line: string }	Parse one PandaScript line and run it on the current session. Comments and blank lines are no-ops. Returns isError: true with a structured message on failure.
script_heal	{ path: string, replacements: [{original_line, replacement_lines}] }	Atomically rewrite the script in place. A <path>.bak of the original is written first; each original_line must match verbatim. The first replacement gets a # [Auto-healed] Original: … header.

Typical loop on the caller side: read the script, walk lines, call script_step per line, on failure ask the caller's LLM for a replacement, call script_heal with the patch, then continue. Lines executed via script_step are intentionally NOT auto-recorded — replay shouldn't double-record.

LOGIN, ACCEPT_COOKIES, and natural-language steps are rejected by script_step: those require an LLM and belong to the calling agent.

Browser tools

The agent and MCP server share the tool set defined in src/browser/tools.zig. Highlights:

• goto, search (Google with DuckDuckGo fallback on captcha)
• tree, markdown, links, interactiveElements, structuredData, detectForms, nodeDetails, findElement
• click, fill, hover, press, scroll, selectOption, setChecked, waitForSelector
• eval, consoleLogs, getUrl, getCookies, getEnv

Selectors prefer CSS over backendNodeId for the click-family tools, since node IDs are invalidated by any DOM mutation. The system prompt enforces this for the LLM.

Security notes

• The agent treats page content as untrusted data, not instructions. URLs surfaced by a page are not followed unless they match the user's task.
• $LP_* environment variable references in TYPE / fill values are resolved at execution time inside the subprocess, so credentials never enter the LLM context. Conventional naming for site-scoped values is LP_<SITE>_<FIELD> (e.g. LP_HN_USERNAME, LP_GH_TOKEN); the unprefixed LP_USERNAME / LP_PASSWORD form is the generic fallback.
• The getEnv tool only reads variables whose name starts with LP_. Everything else (provider API keys, system env, third-party secrets) reports "not set" so the model can't probe for it. The user controls what lives under LP_*. Note that getEnv returns the value to the model — fine for non-secret config like base URLs, but never call it on credentials (use $LP_* placeholders in fill values instead).
• --obey-robots, --http-proxy, --user-agent, and the rest of the browser-level CLI flags apply to agent the same way they apply to serve, fetch, and mcp.