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, /acceptCookies, 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 a slash command (/<tool> [args]), a # comment, or blank. There is no other syntax: anything that doesn't match those three forms is a parse error.

Slash commands accept any of:

• a single positional value, when the tool has exactly one required field — /goto 'https://example.com', /click selector='Login', /extract '{"karma":"#karma"}';
• key=value pairs — values may be bare or quoted; strings with whitespace must be quoted (/fill selector='#email' value='user@x.com');
• a raw {json} blob — handed straight to the tool (/findElement {"role":"button"}).

Quoting is content-aware: '…', "…", and triple-quoted '''…''' / """…""" for values that mix both quote styles or span multiple lines. Recorded scripts round-trip through the parser without escapes.

Two slash commands have no underlying tool — they trigger an LLM turn that the agent translates into actual tool calls:

Command	Notes
/login	LLM-driven: fills credentials from $LP_* env vars.
/acceptCookies	LLM-driven: dismiss the consent banner.

Both require an LLM. --no-llm rejects them.

In the REPL (and only the REPL), a line that isn't a slash command and doesn't start with # is sent to the LLM as a natural-language prompt. In .lp scripts and through MCP script_step, the same input is a parse error. To leave the REPL, use the /quit meta command.

Example script

# Log into the demo and grab the dashboard title and visible cards.
# 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/'
/acceptCookies
/fill selector='#email' value='$LP_DEMO_USERNAME'
/fill selector='#password' value='$LP_DEMO_PASSWORD'
/click selector='button[type="submit"]'
/waitForSelector '.dashboard'
/extract '{"title": ".dashboard h1", "cards": [".dashboard .card .name"]}'

/extract takes a JSON schema object — each value tells the extractor what to lift off the page, and the whole result is printed to stdout as a single JSON object. Supported value forms:

• "<sel>" — textContent.trim() of the first match (string or null).
• "" — the matched element's own text (only inside a fields block).
• ["<sel>"] — text of every match (string array).
• {"selector": "<sel>", "attr": "<name>"} — attribute of the first match.
• [{"selector": "<sel>", "fields": {…}}] — array of records, each fields value resolved relative to the matched element.

Use /extract '''…''' (or """…""") to spread a schema across multiple lines. The schema is parsed in Zig before the page-side walker runs, so a malformed schema fails with Error: invalid /extract schema JSON rather than a V8 stack trace. See agent-tutorial.md section 3 for a worked example against Hacker News.

Recording

Interactive sessions can write back to a .lp file:

./lightpanda agent -i session.lp

State-mutating commands (/goto, /click, /fill, /scroll, /hover, /selectOption, /setChecked, /waitForSelector, /press, /eval, /extract) are appended; read-only commands (/tree, /markdown, /links, /findElement, …) and the natural-language turns that produced them are not. Natural-language turns are recorded as # <prompt> comments above the resulting slash commands 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 /<tool> and meta slash commands. The dim grey suffix shown after the cursor is the first match.
• Persistent history: stored in .lp-history in the working directory.
• Meta slash commands: /help lists tools (/help <tool> prints the JSON schema), /quit exits the REPL, /verbosity <low|medium|high> tunes the log level. These are REPL-only and never recorded.

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

• Stdout vs stderr: the final assistant answer and data-producing slash 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 -a <path> / --attach <path> (repeatable) to feed local files to providers that accept attachments. Text files are inlined into the prompt (max 512 KiB each); binary files (image/*, audio/*, pdf) are base64-encoded inline (max 20 MiB each). Unsupported MIME types error out before any browser work runs.

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, /acceptCookies, and anything that isn't a slash command 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 /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.
• REPL prompts are persisted to .lp-history in the current working directory in plaintext (no encryption). Anything you type at the prompt — including natural-language context that accompanies a /login — lands in that file. Delete it or move out of sensitive directories if you don't want it retained.
• record_start and script_heal reject empty, absolute, and .. paths, but do not follow-up on symlinks. On a shared filesystem, a pre-existing symlink at the recording target would be written through to whatever it points at. Prefer a fresh directory you own when recording in untrusted environments.