browser-automation

$2c

Mode Selection Guide

Mode

When to use

How it works

Puppeteer (default)

User wants to browse a URL, scrape data, test UI — no need for their own browser

Launches a new headless Chrome, isolated from user's browser

CDP mode

User says "connect to my Chrome", "control my browser", "CDP", "remote debugging", or wants to operate their existing browser. Also use when the task implicitly requires login state (e.g., "check my orders", "open my dashboard", "look at my account")

Connects to user's Chrome via DevTools Protocol. Requires remote debugging enabled (chrome://inspect > "Allow remote debugging"). No extension needed

Bridge mode

User explicitly mentions "bridge", "extension", or has Midscene Chrome Extension installed and prefers to use it

Connects to user's Chrome via the Midscene Chrome Extension

CDP vs Bridge: Both control the user's real Chrome with login sessions preserved. CDP only needs a Chrome setting toggle; Bridge needs a Chrome Extension installed. If the user doesn't specify, prefer CDP mode as it has fewer prerequisites.

Precheck: detect available connection modes

Before using CDP or Bridge mode, run a quick precheck to verify the target is reachable. This avoids long timeouts when the user hasn't enabled remote debugging or installed the extension.

# CDP precheck (port 9222, 2s timeout) — returns "101" if available

curl -s --max-time 2 -o /dev/null -w "%{http_code}" -H "Upgrade: websocket" -H "Connection: Upgrade" -H "Sec-WebSocket-Version: 13" -H "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==" http://127.0.0.1:9222/devtools/browser

# Bridge precheck (port 3766, 2s timeout) — returns "200" or "400" if extension is listening

curl -s --max-time 2 -o /dev/null -w "%{http_code}" http://127.0.0.1:3766/socket.io/?EIO=4&transport=polling

How to use precheck results:

• CDP returns 101 → CDP mode is available, use --cdp

• Bridge returns 200 or 400 → Bridge extension is listening, use --bridge

• Both fail → Chrome may not be running. Try opening Chrome using a shell command appropriate for the current platform, wait 2-3 seconds, then re-run the precheck. If it still fails, fall back to Puppeteer mode or ask the user to check their Chrome settings.

• Both available and user didn't specify → prefer CDP

Prerequisites

Midscene requires models with strong visual grounding capabilities. The following environment variables must be configured — either as system environment variables or in a .env file in the current working directory (Midscene loads .env automatically):

MIDSCENE_MODEL_API_KEY="your-api-key"

MIDSCENE_MODEL_NAME="model-name"

MIDSCENE_MODEL_BASE_URL="https://..."

MIDSCENE_MODEL_FAMILY="family-identifier"

Example: Gemini (Gemini-3-Flash)

MIDSCENE_MODEL_API_KEY="your-google-api-key"

MIDSCENE_MODEL_NAME="gemini-3-flash"

MIDSCENE_MODEL_BASE_URL="https://generativelanguage.googleapis.com/v1beta/openai/"

MIDSCENE_MODEL_FAMILY="gemini"

Example: Qwen 3.5

MIDSCENE_MODEL_API_KEY="your-aliyun-api-key"

MIDSCENE_MODEL_NAME="qwen3.5-plus"

MIDSCENE_MODEL_BASE_URL="https://dashscope.aliyuncs.com/compatible-mode/v1"

MIDSCENE_MODEL_FAMILY="qwen3.5"

MIDSCENE_MODEL_REASONING_ENABLED="false"

# If using OpenRouter, set:

# MIDSCENE_MODEL_API_KEY="your-openrouter-api-key"

# MIDSCENE_MODEL_NAME="qwen/qwen3.5-plus"

# MIDSCENE_MODEL_BASE_URL="https://openrouter.ai/api/v1"

Example: Doubao Seed 2.0 Lite

MIDSCENE_MODEL_API_KEY="your-doubao-api-key"

MIDSCENE_MODEL_NAME="doubao-seed-2-0-lite"

MIDSCENE_MODEL_BASE_URL="https://ark.cn-beijing.volces.com/api/v3"

MIDSCENE_MODEL_FAMILY="doubao-seed"

Commonly used models: Doubao Seed 2.0 Lite, Qwen 3.5, Zhipu GLM-4.6V, Gemini-3-Pro, Gemini-3-Flash.

If the model is not configured, ask the user to set it up. See Model Configuration for supported providers.

CDP Mode (Connect to Existing Browser)

Use CDP mode to control the user's existing Chrome browser. The default CDP endpoint is ws://127.0.0.1:9222/devtools/browser (port 9222 is Chrome's standard remote debugging port). If the user specifies a different port, replace 9222 accordingly.

Add --cdp <ws-endpoint> to every command:

npx -y @midscene/web@1 connect --cdp ws://127.0.0.1:9222/devtools/browser --url https://example.com

npx -y @midscene/web@1 act --cdp ws://127.0.0.1:9222/devtools/browser --prompt "click the button"

npx -y @midscene/web@1 take_screenshot --cdp ws://127.0.0.1:9222/devtools/browser

npx -y @midscene/web@1 disconnect --cdp ws://127.0.0.1:9222/devtools/browser

Important notes for CDP mode

• The browser is managed externally — disconnect releases the connection but does NOT close the browser. There is no close command in CDP mode.

• In CDP mode, connect --url navigates the existing active tab instead of opening a new tab.

• connect without --url attaches to the current active tab without navigating.

• If connection fails, ask the user to enable remote debugging: open chrome://inspect in Chrome and turn on "Allow remote debugging".

Bridge Mode (Connect via Chrome Extension)

Use Bridge mode when the user explicitly mentions "bridge", "extension", or has the Midscene Chrome Extension installed. Add --bridge to every command:

npx -y @midscene/web@1 --bridge connect --url https://example.com

npx -y @midscene/web@1 --bridge act --prompt "click the button"

npx -y @midscene/web@1 --bridge take_screenshot

npx -y @midscene/web@1 --bridge disconnect

Important notes for Bridge mode

• The user must have Chrome open with the Midscene Extension installed and enabled.

• Install the extension from Chrome Web Store: https://chromewebstore.google.com/detail/midscenejs/gbldofcpkknbggpkmbdaefngejllnief

• Check that the "bridge mode" indicator in the extension shows "Listening" status.

• disconnect only closes the CLI-side bridge connection, not the browser or tabs.

• If the extension is not installed, guide the user to install it or suggest switching to CDP mode instead.

• See the Bridge Mode documentation.

Commands

Connect to a Web Page

npx -y @midscene/web@1 connect --url https://example.com

Take Screenshot

npx -y @midscene/web@1 take_screenshot

After taking a screenshot, read the saved image file to understand the current page state before deciding the next action.

Perform Action

Use act to interact with the page and get the result. It autonomously handles all UI interactions internally — clicking, typing, scrolling, hovering, waiting, and navigating — so you should give it complex, high-level tasks as a whole rather than breaking them into small steps. Describe what you want to do and the desired effect in natural language:

# specific instructions

npx -y @midscene/web@1 act --prompt "click the Login button and fill in the email field with 'user@example.com'"

npx -y @midscene/web@1 act --prompt "scroll down and click the Submit button"

# or target-driven instructions

npx -y @midscene/web@1 act --prompt "click the country dropdown and select Japan"

Assert Current Page State

Use assert to verify that the current page satisfies a natural language condition. It does not perform UI actions; it checks the visible page state and passes only when the assertion is true. Use this for validation, QA checks, and final state verification after act.

npx -y @midscene/web@1 assert --prompt "there is a login button visible"

npx -y @midscene/web@1 assert --prompt "the checkout page shows the order total and a Pay button"

In CDP or Bridge mode, pass the same connection flags you use for other commands:

npx -y @midscene/web@1 assert --cdp ws://127.0.0.1:9222/devtools/browser --prompt "the dashboard is loaded"

npx -y @midscene/web@1 --bridge assert --prompt "the profile page shows the user's avatar"

Use a Reference Image for Precise Targeting

When the user provides a screenshot, icon, logo, or reference image and wants an exact visual match, prefer tap --locate instead of a generic act --prompt. Pass --locate as JSON. The prompt describes the target, images supplies named reference images, and convertHttpImage2Base64: true is useful when the image URL may not be directly accessible to the model.

npx -y @midscene/web@1 tap --locate '{

  "prompt": "tap the area contains the image",

  "images": [

    {

      "name": "target image",

      "url": "https://github.githubassets.com/assets/GitHub-Mark-ea2971cee799.png"

    }

  ],

  "convertHttpImage2Base64": true

}'

The same locate JSON shape also works for other commands that accept a locate parameter.

Disconnect

Disconnect from the page but keep the browser running:

npx -y @midscene/web@1 disconnect

Close Browser

Close the browser completely when finished (Puppeteer mode only):

npx -y @midscene/web@1 close

Consume Report Files

The generated HTML report is recommended for human reading first. It includes step-by-step execution details and replay videos for each operation, which makes it much easier to understand what happened and troubleshoot problems.

If another skill or tool needs to consume the report, first convert it with report-tool from the same platform CLI package. Prefer Markdown for LLM-based workflows. Use JSON when the report needs to be processed programmatically.

npx -y @midscene/web@1 report-tool --action to-markdown --htmlPath ./midscene_run/report/.../index.html --outputDir ./output-markdown

npx -y @midscene/web@1 report-tool --action split --htmlPath ./midscene_run/report/.../index.html --outputDir ./output-data

Workflow Pattern

The browser persists across CLI calls via a background Chrome process. Follow this pattern:

• Connect to a URL to open a new tab

• Take screenshot to see the current state, make sure the page is loaded.

• Execute action using act to perform the desired action or target-driven instructions, and use assert when you need to verify the resulting page state.

• Close the browser when done (or disconnect to keep it for later)

• Report results — summarize what was accomplished, present key findings and data extracted during the task, and list any generated files (screenshots, logs, etc.) with their paths

Best Practices

• Always connect first: Navigate to the target URL with connect --url before any interaction.

• Inspect visible state: After navigation or actions that trigger page changes, take a screenshot and read it before deciding the next step.

• Use natural, specific prompts: Describe visible UI and desired outcomes, such as "click the blue Submit button in the contact form", not selectors like "#submit".

• **Batch related operations into a single act command**: For example, fill the email and password fields, then click Log In in one prompt. Use separate commands when you need to inspect the intermediate state.

• **Use assert for verification**: Check observable page state with assert --prompt "...", such as "the success toast is visible" or "the cart total is $42.00".

• **Prefer tap --locate when a reference image is provided**: If the user shares a screenshot, icon, or logo and wants that exact visual target, use tap --locate with a multimodal locate JSON object such as { "prompt": "...", "images": [...] } instead of relying only on act --prompt.

Example — Dropdown selection:

npx -y @midscene/web@1 act --prompt "click the country dropdown and select Japan"

npx -y @midscene/web@1 take_screenshot

Example — Form interaction:

npx -y @midscene/web@1 act --prompt "fill in the email field with 'user@example.com' and the password field with 'pass123', then click the Log In button"

npx -y @midscene/web@1 take_screenshot

Troubleshooting

Connection Failures

• Ensure Chrome/Chromium is installed on the system (Puppeteer downloads its own by default).

• Check that no firewall blocks local Chrome debugging ports.

API Key Errors

• Check .env file contains MIDSCENE_MODEL_API_KEY=<your-key>.

• Verify the key is valid for the configured model provider.

Timeouts

• Web pages may take time to load. After connecting, take a screenshot to verify readiness before interacting.

• For slow pages, wait briefly between steps.

@midscene/* Dependency Version Outdated

• Check local versions: npm ls @midscene/web @midscene/core @midscene/shared (or pnpm why @midscene/web).

• Check latest versions: npm view @midscene/web version, npm view @midscene/core version, npm view @midscene/shared version.

• Upgrade dependencies: npm i @midscene/web@latest @midscene/core@latest @midscene/shared@latest.

Screenshots Not Displaying

• The screenshot path is an absolute path to a local file. Use the Read tool to view it.