Browser Preview
You already know preview_serve and preview_stop. This skill fills the gap: what happens after preview_serve returns a URL โ how the user actually sees it.
What is Browser
The frontend has a right-side panel with two tabs: Workspace and Browser. Browser renders preview URLs inside an iframe. When you call preview_serve, the frontend automatically opens a Browser tab loading that URL.
Key facts:
- Each
preview_serve call creates one Browser tab
- URL format:
https://<host>/preview/{id}/
- Browser panel has a โฎ menu (top-right) showing "RUNNING SERVICES" list
- Browser tab can be closed by the user without stopping the backend service
- Backend service stopping โ Browser tab shows an error page
โ ๏ธ CRITICAL: Never Tell Users to Access localhost
The user's browser CANNOT access localhost or 127.0.0.1. These addresses point to the server container, not the user's machine. The preview architecture uses a reverse proxy:
User's Browser โ https://<host>/preview/{id}/path โ (reverse proxy) โ 127.0.0.1:{port}/path
Rules:
- NEVER tell the user to visit
http://localhost:{port} or http://127.0.0.1:{port} โ they cannot reach it
- ALWAYS direct users to the preview URL:
/preview/{id}/ (or the full URL https://<host>/preview/{id}/)
curl http://localhost:{port} is for your own server-side diagnostics only โ never suggest it to the user as a way to "test" the preview- When a preview is running, tell the user: "Check the Browser panel, or refresh the Browser panel"
- If you need to give the user a URL, use the
url field returned by preview_serve (format: /preview/{id}/)
โ ๏ธ Static Assets Must Use Relative Paths
Because previews are served under /preview/{id}/, absolute paths in HTML/JS/CSS will break. The reverse proxy strips the /preview/{id} prefix before forwarding to the backend, but the browser resolves absolute paths from the domain root.
Example of the problem:
<script src="/static/app.js"></script>
<script src="static/app.js"></script>
<script src="./static/app.js"></script>
Common patterns to fix:
| Broken (absolute) |
Fixed (relative) |
"/static/app.js" |
"static/app.js" or "./static/app.js" |
"/api/users" |
"api/users" or "./api/users" |
"/images/logo.png" |
"images/logo.png" or "./images/logo.png" |
url('/fonts/x.woff') |
url('./fonts/x.woff') |
fetch('/data.json') |
fetch('data.json') |
Check ALL places where paths appear:
- HTML
src, href attributes
- JavaScript
fetch(), XMLHttpRequest, dynamic imports
- CSS
url() references
- JavaScript string literals (e.g.,
'/static/' in template strings or concatenation)
- Framework config files (e.g.,
publicPath, base, assetPrefix)
โ ๏ธ Be thorough โ it's common to fix CSS url() but miss JS string literals like '/static/' (with single quotes). Search for ALL occurrences of absolute paths across all file types.
โ ๏ธ Do NOT Browse Filesystem to Debug Previews
Never look at workspace directories like preview/, output/, or random folders to understand preview state. Those are user data, not preview service state.
The only sources of truth:
- Registry file:
/data/previews.json (running services)
- History file:
/data/preview_history.json (all past services)
preview_serve / preview_stop tools- Port checks via
curl (server-side only, for your diagnostics)
Do NOT use ls/find on workspace directories to diagnose preview issues. Do NOT call unrelated tools like list_scheduled_tasks. Stay focused.
Step-by-Step: Diagnosing Browser Issues
When a user reports any Browser problem, follow this exact sequence:
Step 1: Read the registry (running services)
cat /data/previews.json 2>/dev/null || echo "NO_REGISTRY"
โ ๏ธ Your bash CWD is /data/workspace/. The registry is at /data/previews.json (absolute path, one level up). Always use the absolute path.
JSON structure:
{
"previews": [
{"id": "f343befc", "title": "My App", "dir": "/data/workspace/my-project", "command": "npm run dev", "port": 9080, "is_builtin": false}
]
}
Step 2: Branch based on registry state
If registry has entries โ Go to Step 3 (verify services)
If registry is empty or missing โ Go to Step 4 (check history)
Step 3: Registry has entries โ verify and fix
For each preview in the registry, check if the port is responding server-side (this is your diagnostic, not for the user):
curl -s -o /dev/null -w "%{http_code}" http://localhost:{port}
If port responds (200):
- The service IS running. Tell the user:
- "You have a running service: {title}"
- "Click the โฎ menu at the top-right of the Browser panel, then click it in the RUNNING SERVICES list to reopen"
- Preview URL:
/preview/{id}/
- If user says the โฎ menu is empty or doesn't show the service โ frontend lost sync. Fix by recreating:
preview_stop(id) then preview_serve(dir, title, command) using the info from the registry. This forces the frontend to re-register the tab.
If port does NOT respond:
Step 4: No running services โ check history first, then scan workspace
When there are no running services, use a two-tier lookup to find projects the user can preview:
Tier 1: Read preview history (preferred โ fast and accurate)
cat /data/preview_history.json 2>/dev/null || echo "NO_HISTORY"
JSON structure:
{
"history": [
{
"id": "f343befc",
"title": "Trading System",
"dir": "/data/workspace/my-project",
"command": "python main.py",
"port": 8000,
"is_builtin": false,
"created_at": 1709100000.0,
"last_started_at": 1709200000.0
}
]
}
History entries are never removed by preview_stop โ they persist across restarts. Entries are automatically pruned only when the project directory no longer exists.
If history has entries:
- List all history entries to the user with title, directory, and last started time
- Ask which one they want to restart
- Call
preview_serve with the dir, title, and command from the history entry
If user says a project is missing from history โ fall through to Tier 2.
Tier 2: Scan workspace (fallback โ when history is empty or incomplete)
find /data/workspace -maxdepth 2 \( -name "package.json" -o -name "index.html" -o -name "*.html" -o -name "app.py" -o -name "main.py" -o -name "vite.config.*" \) -not -path "*/node_modules/*" -not -path "*/skills/*" -not -path "*/memory/*" -not -path "*/prompt/*" -not -path "*/.git/*" 2>/dev/null
Then:
- List discovered projects with brief descriptions
- Ask the user which one to preview
- Call
preview_serve with the appropriate directory
Don't just say "no services running" and stop. Always check history first, then scan, and offer options.
Quick Reference
| User says |
You do |
| "tab disappeared" / "tab ไธ่งไบ" |
Step 1 โ 2 โ 3 or 4 |
| "blank page" / "็ฝๅฑ" |
Check port (server-side), if dead โ recreate; if alive โ check for absolute path issues |
| "not updating" / "ๅ
ๅฎนๆฒกๆดๆฐ" |
Suggest refresh button in Browser tab, or recreate preview |
| "port conflict" / "็ซฏๅฃๅฒ็ช" |
preview_stop old โ preview_serve new |
| "can't see service" / "โฎ menu empty" |
preview_stop + preview_serve to force re-register |
| "where's my project" / "what did I build" |
Read /data/preview_history.json and list entries |
| "resource load failed" / "JS/CSS 404" |
Check for absolute paths (/static/, /api/), fix to relative paths |
What You Cannot Do
- Cannot directly open/close/refresh Browser tabs (frontend UI)
- Cannot force-refresh the iframe
- Cannot read what the iframe displays
When you can't do something, tell the user the manual action (e.g., "click refresh in Browser tab"). If manual action doesn't work, recreate the preview with preview_stop + preview_serve.
Common Mistakes to Avoid
- โ Telling user to "visit http://localhost:18791/" โ user cannot access localhost
- โ Saying "refresh the page at localhost" โ meaningless to the user
- โ Only fixing CSS
url() paths but missing JS string literals with absolute paths
- โ Forgetting to check ALL file types (HTML, JS, CSS, config) for absolute paths
- โ
Always use
/preview/{id}/ as the user-facing URL
- โ
Always use
curl localhost:{port} only for your own server-side diagnostics
- โ
After fixing paths, call
preview_stop + preview_serve to restart, then tell user to check Browser panel
