UI JSON Capture

Capturing paginated JSON from an authenticated research UI

Drive the user's logged-in browser over CDP, hook XMLHttpRequest, click through the result pages, and keep the JSON the page already received. No direct API calls, no credential handling, no fighting the export cap.

Decide the access route BEFORE touching anything

Systematic capture of a paid research corpus is the user's decision to make knowingly, not yours to slide into. Surface the trade-off once, in plain terms, then do what they decide.

LexisNexis, Bloomberg and Thomson Reuters terms prohibit systematic downloading; the export row cap is that prohibition's enforcement mechanism, and paging the XHR endpoint routes around it. The exposure — account termination, an institutional licence, a research programme — lands on the user, not on you.

Say it once and move on. Repeating it after they decide is not caution, it is nagging, and it wastes the turn they already spent answering.

Two routes make the question moot and are worth naming in the same breath, because they are often available and nobody thinks of them: these vendors sell API access as a contract line, and they grant academic bulk agreements. Either produces a citable, reproducible dataset. A scrape does not.

Check whether you need to page at all

A single response usually carries the facet counts. Before capturing 667 pages to count things, read the facets in page one. The Lex Machina response that answered "how many derivative suits are there" carried de-dcc-case_tag (12 terms), de-dcc-case_type (15 terms) and a filed_on year histogram — every count the question needed, from one ordinary page load.

Then check whether a filtered slice fits under the export cap. Lex Machina caps exports at 1,000 rows, but annual filings ran 932–1,510, so year-by-year exports were under the cap for every year but one. Fourteen sanctioned exports beat one unsanctioned scrape.

Page only when you need per-record fields across a population that will not slice under the cap.

Procedure

Load the browser-automation skill first — it owns port/prefix selection. Use the interactive browser (9222 / mcp__chrome-devtools__* on Linux), because the session must be logged in.

1. Find the JSON. With the results page open, list_network_requests filtered to xhr/fetch, then get_network_request on the one whose response is application/json. Often it is the same URL as the page, differing only by an X-Requested-With: XMLHttpRequest request header. Save the body with responseFilePath and read its shape — you need the array field, the offset field, the total, and the page size.
2. Install the hook — scripts/01-install-hook.js, after editing the three site-specific lines it flags (URL test, array field, offset field).
3. Smoke-test 4 pages before committing to the run. Confirm offsets advance and errors is empty.
4. Start the pager — scripts/02-run-pager.js. It returns immediately by design.
5. Poll and drain — scripts/03-status.js to check, scripts/04-dump.js to write a batch, passing filePath. Drain every few hundred pages.
6. Stop — scripts/05-stop.js, or let it end on the disabled Next button.
7. Assemble and prove it — scripts/assemble.py. Its non-zero exit is the gate.

python3 "${CLAUDE_SKILL_DIR}/scripts/assemble.py" \
  --batches scratch/capture --glob 'batch_*.json' --out data/raw/cases.jsonl \
  --expect-total 16670 --page-size 25 --id-field id --first-page scratch/page0.json

Capture Facts

• evaluate_script dies at the CDP protocolTimeout (~120 s) but the in-page loop keeps running. A 50-page awaited batch returned Runtime.callFunctionOn timed out while the page went on to capture 20 more. Awaiting a long loop therefore reports failure for work that succeeded — you lose the return value, not the data. Detach and poll; that is why 02-run-pager.js returns immediately.
• Route the payload to disk with filePath, never through the return value. 16,670 records was 258 MB. Returning that inline would blow the context window for zero benefit — the model never needs to see the rows, only the integrity counts.
• filePath is restricted to configured workspace roots. A path under a symlinked data directory resolving outside them fails with Access denied … not within any of the configured workspace roots. Write into the repo (e.g. scratch/), then move it.
• Page 1 is missed if the hook is installed after the page loaded. The hook only sees requests made after it is installed. Recover it from the responseFilePath you already saved in step 1 and splice it with --first-page; do not re-run the whole capture for one page.
• Scrolling and other UI events fire the same endpoint, so extra pages arrive unbidden — 6 captured against 2 clicks in one measured stretch. The dedupe key makes this harmless. Absent it you would silently double-count records.
• A saved "default column view" silently overrides cols= URL parameters. Two navigations in a row came back with the columns stripped and the old view restored. Set columns through the UI control, then read the URL the app rewrote — that is the authoritative parameter list.
• Drawer and checkbox UIs need a real click and two round-trips. getElementById fails on ids containing spaces, and the panel's contents do not exist in the DOM until after the click renders. Click, return, then read in a second call.
• Human-ish pacing is a dial, not a switch. The first run averaged 9 pages/min; raising the long-pause rate dropped it to 5 and the ETA to 81 minutes. Halving the pause rate and trimming the scroll count restored ~2× with jitter intact. The protective property is that pauses and jitter exist, not their exact frequency.

Red Flags — STOP if you catch yourself

Generalising to another site

Only four things are site-specific, all flagged inline in the scripts: the URL test for the results endpoint, the array field, the offset field, and the Next-button selector. Everything else — dedupe, detached loop, pacing, draining, integrity proof — transfers unchanged.

Sites differing structurally:

• Cursor pagination instead of numeric offsets (next_token): the dedupe key must be the cursor or a record-id hash, and assemble.py's gap check does not apply — pass --page-size 0 semantics by verifying ids only, and say in the report that offset-continuity was unavailable.
• Infinite scroll instead of a Next button: replace nextBtn() with a scroll-to-bottom and wait for the offset to advance.
• GraphQL POST endpoints: hook the same way, but key on the POST body's variables — the URL is constant across pages and a URL-based dedupe key will collapse every page into one.

Related

• browser-automation — port/prefix selection and CDP mechanics. Load it first.
• The Delaware Chancery capture this generalises from: 16,670 cases, 667 pages, 0 duplicates, 0 gaps, ids matching the vendor's own reported total exactly.