Browser Script Builder

Create self-contained JS scripts that run inside platform browser agents (linkedin-agent, reddit-agent, twitter-agent) via browser_run_code. Replaces 5-10 Claude browser tool calls with 2 calls (set params + run script), cutting token usage 10-20x.

Usage

/browser-script <platform> <action-description>
/browser-script linkedin "edit a comment to append link text"
/browser-script reddit "post a reply to a comment"
/browser-script linkedin "extract post data from search results"

Workflow

Step 1: Understand the browser flow

Before writing code, manually walk through the flow in the target browser agent to discover:

• What elements exist (use browser_snapshot)
• What role/name locators work (use getByRole)
• What API calls the page makes (use browser_network_requests)
• What timing/scrolling is needed

Step 2: Write the script

Create scripts/<action>_<platform>.js following this skeleton:

// action_platform.js — One-line description
// Runs inside {platform}-agent browser via browser_run_code.
//
// Params via window.__params:
//   { field1: "...", field2: "..." }
//
// Returns: { ok: true, ...data } or { ok: false, error: "error_code" }

async (page) => {
  const params = await page.evaluate(() => window.__params);
  if (!params || !params.requiredField) {
    return JSON.stringify({ ok: false, error: 'missing_params' });
  }

  try {
    // Step 1: Navigate
    await page.goto(params.url, { waitUntil: 'domcontentloaded', timeout: 30000 });
    await page.waitForTimeout(3000);

    // Step 2: Scroll to load content
    await page.evaluate(() => window.scrollBy(0, 1500));
    await page.waitForTimeout(2000);

    // Step 3: Find element (with retry after scroll)
    const element = page.getByRole('button', { name: /pattern/i }).first();
    try {
      await element.waitFor({ timeout: 10000 });
    } catch {
      await page.evaluate(() => window.scrollBy(0, 2000));
      await page.waitForTimeout(2000);
      try {
        await element.waitFor({ timeout: 5000 });
      } catch {
        return JSON.stringify({ ok: false, error: 'element_not_found' });
      }
    }

    // Step 4: Interact
    await element.click();
    await page.waitForTimeout(1000);

    // Step 5: Verify
    return JSON.stringify({ ok: true, result: 'done' });

  } catch (e) {
    return JSON.stringify({ ok: false, error: e.message });
  }
}

Step 3: Test interactively

Test in two calls:

// Set params
mcp__{platform}-agent__browser_run_code code:
  async (page) => {
    await page.evaluate(() => {
      window.__params = { url: "...", text: "..." };
    });
  }

// Run script
mcp__{platform}-agent__browser_run_code filename:
  ~/social-autoposter/scripts/the_script.js

If it fails, take a browser_snapshot to debug, fix the script, re-test.

Step 4: Update the shell script

Replace the Claude prompt instructions for this flow with the 2-call pattern. Example from engage.sh Phase D LinkedIn:

# Old: 8-line prompt telling Claude to navigate, find comment, click menu, edit, save
# New:
8. For LinkedIn: use the edit script via linkedin-agent browser:
   a. Set params: browser_run_code with code setting window.__params
   b. Run: browser_run_code with filename=$REPO_DIR/scripts/edit_linkedin_comment.js
   c. Parse JSON result: {ok:true} = success, {ok:false, error} = handle error

Design Rules (MUST follow)

1. Params via window.__params — never hardcode. Script must be reusable across different targets.

2. Return JSON always — {ok: true, ...data} or {ok: false, error: "code"}. Error codes are specific and actionable: comment_not_found, link_already_present, not_logged_in, save_failed, element_not_found.

3. Script owns all waits — waitForTimeout, waitFor, scroll-retry loops. Caller just runs and reads result.

4. keyboard.type() not fill() — React-based sites (LinkedIn, possibly Twitter) don't detect fill() changes. Always keyboard.type() for rich text editors. For full text replacement: click({clickCount: 3}) + Meta+a + keyboard.type(newText).

5. Scroll before find — social platforms lazy-load. Always scroll into view before locating elements.

6. Locate by role/name — page.getByRole('button', {name: /pattern/i}) not CSS class selectors. Survives redesigns.

7. One script = one complete action — "edit a comment", "scan notifications", "post a reply". Not a single click. Not an entire engagement loop.

8. Safety checks inside the script — dedup detection (link already present, already replied). Don't rely on the caller.

9. Generous timeouts — social sites are slow. Use 3s after navigation, 2s after scroll, 1s after click.

10. Two-phase element finding — first try with 10s timeout, then scroll more and retry with 5s. Return specific error if still not found.

When NOT to Script

• Content generation — Claude needs to decide what to write. Keep that in prompts.
• Project/topic matching — Claude picks which project fits. Keep in prompts.
• One-off investigations — manual browser_snapshot is fine.
• Flows that vary — if the steps change based on page state in unpredictable ways.

The split: Claude decides WHAT (pick project, write text, choose targets). Scripts execute HOW (navigate, click, type, save).

Platform Notes

LinkedIn

• Rich text editor uses React — fill() won't trigger change detection
• Edit textbox is the LAST textbox[name="Text editor for creating comment"] after clicking Edit
• Our comment menu: button[name=/View more options for Matthew/i]
• Official API cannot read or edit comments
• Internal Voyager API accessible via in-page fetch() for read-only operations (notifications)
• Agent: mcp__linkedin-agent__browser_run_code

Reddit

• old.reddit.com is simpler to automate than new Reddit
• Edit button visible on own comments without menu
• Agent: mcp__reddit-agent__browser_run_code

Twitter/X

• Tweets cannot be edited
• Agent: mcp__twitter-agent__browser_run_code

Existing Scripts

Script	Platform	Action	Params
scan_linkedin_notifications.js	LinkedIn	Read notifications via internal API	none (uses cookies)
edit_linkedin_comment.js	LinkedIn	Append text to our comment	{postUrl, appendText}

Reference

Full design doc: scripts/BROWSER_SCRIPTS.md