preview‑sidecar

Your dev server just bound a port. This notices, opens a preview, and
stamps a dated screenshot receipt — without touching your agent CLI.

REF github.com/anthropics/claude-code#53997

ITEM Bring back local preview for agent-built apps

STATUS closed · not planned → built anyway

✂ – – – – – join early access – – – – – ✂

✓ RECEIVED — thanks, you're on the list.
We'll only email you about preview-sidecar. Never shared.

One line on your email: used only to reach you about preview-sidecar, never sold, never shared.

itemized

detect newly bound localhost port✓

auto-open preview pane (iframe)✓

one-click screenshot receipt✓

standalone CLI, zero deps✓

no Claude Code changes✓

before / after

BEFORE

$ npm run dev
  ready on http://localhost:5173
(you alt-tab, guess the port,
 open it yourself, no record
 of what it looked like)

AFTER

$ npm run dev
  ready on http://localhost:5173
[preview-sidecar] detected
  127.0.0.1:5173 at 17:29:17Z
  -> preview pane opened
  -> receipt: receipt-5173-*.png

real run, recorded

$ python3 demo.py
$ (throwaway dev server on 127.0.0.1:61125)
$ python3 sidecar.py watch --ports 61125 \
    --interval 0.3 --max-cycles 3 --no-open
[preview-sidecar] detected 127.0.0.1:61125
  at 2026-10-10T17:29:17Z -> preview-61125...html
$ python3 sidecar.py screenshot --port 61125
{
  "url": "http://127.0.0.1:61125",
  "bytes": 16208,
  "sha256": "6e13265f215eb1e3..."
}

Full transcript: demo.txt · full receipt: demo-receipt.json

Actual headless-Chrome screenshot of the throwaway dev server page captured by sidecar.py screenshot, reading Hello from dev server / Started by preview-sidecar demo.py

EXHIBIT A — real capture of 127.0.0.1:61125
sha256 6e13265f215e… · 2026‑10‑10T17:29:17Z

how to use it

python3 sidecar.py watch
python3 sidecar.py watch --ports 3000,5173
python3 sidecar.py screenshot --port 3000

Python 3.9+, no installs. watch polls a bounded port list with a plain TCP connect and opens a preview the moment one answers. screenshot needs a Chromium‑family browser already on the machine.

bill of goods

FREE · the CLI, forever$0

EARLY ACCESS · hosted receipt gallery$7/mo*

TEAM · PR auto-receipts, shared history$29/mo*

TOTAL DUE TODAY$0.00

*founding price, early access — there is no checkout yet, joining only reserves the rate. The CLI stays free no matter what you pick above.

honest limits

Watches a bounded candidate port list (≤200 ports), not every socket on the machine — it can't see which process owns a port.
Polling means a detection delay, and a port that opens and closes between polls is missed.
A dev server's own X‑Frame‑Options/CSP can block the preview iframe; every preview page also has a plain "open in new tab" link.
Screenshots need Chrome/Chromium/Edge already installed — not bundled, and a clear error if none is found.
Loopback only. No remote tunneling, no multi‑port orchestration — one detected port gets one preview.

Full README →

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