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Here are frequent issues and how to fix them. If something isn’t covered, email calder@predictiondata.dev.

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Download returns 404 or “file not found”

Polymarket
  • Check the date. Exports are generated daily. Use a date that exists in the list response. Polymarket uses the v1 API, which does not zero-pad months and days (e.g. 2025-11-8, not 2025-11-08). Kalshi uses the v2 API, which uses standard YYYY-MM-DD formatting.
  • Check the path. For asset ID: https://datasets.predictiondata.dev/polymarket/{export_type}/{asset_id}/{date}.csv.gz — use books, trades, or onchain/fills as the export type (not plain onchain). For slug: https://datasets.predictiondata.dev/polymarket/{export_type}/{market_slug}/{outcome}/{date}.csv.gz?slug=true&apikey=YOUR_KEY (e.g. .../books/will-the-seattle-seahawks-win-super-bowl-2026/YES/2026-2-9.csv.gz?slug=true&apikey=...).
  • List first. Call the list endpoint and use the same asset_id and date (or slug/outcome) that appear there. Don’t guess IDs or dates.
Kalshi
  • Use the exact path from the list response in GET /v2/datasets/{path}. Don’t modify or construct the path yourself.
  • The download URL returns a 302 redirect. If your client doesn’t follow redirects, you’ll get a redirect response instead of the file. Use curl -L or enable redirect following in your HTTP client.

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“Unauthorized” or authentication errors

  • Polymarket downloads: Auth is via the query parameter apikey=YOUR_API_KEY. If you forget it or typo the key, you’ll get 401/403.
  • Kalshi: Auth is the header x-auth-token: YOUR_API_KEY. Query params are not used for auth. Make sure the header is sent on both the list and the download request.
  • Wrong tier: Your key’s tier must allow the exchange (e.g. Kalshi tier for Kalshi data). If you get access denied, ask calder@predictiondata.dev to confirm your tier.

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List returns empty or no exports

  • Polymarket: Confirm the market/event/asset exists and has data for the date range. New markets may not have backfilled history yet. Try an event slug you know has markets (e.g. from the Polymarket site).
  • Kalshi: Same idea — check the ticker. Ensure you’re sending x-auth-token; missing or invalid tokens return a 401 error.
  • Date: Exports are usually produced once per day. There is no same-day data for “today” until the next run (e.g. after 2am UTC for Polymarket).

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Wrong or weird date format

  • Polymarket (v1 API): Dates are not zero-padded (e.g. 2025-11-8, not 2025-11-08). Use the exact date string from the list response.
  • Kalshi (v2 API): Standard YYYY-MM-DD format (e.g. 2025-11-08).
  • All times are UTC. The “day” of an export is the UTC date.

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Slug vs asset ID (Polymarket) — which to use?

  • Event slug — What you see in the URL for a Polymarket event (e.g. super-bowl-champion-2026-731). Use it to list all markets in that event.
  • Market slug — The URL-friendly name for a single market (e.g. will-the-seattle-seahawks-win-super-bowl-2026).
  • Asset ID — The internal CLOB token ID (long number). You get it from our list response. Use it in download URLs when you don’t want to pass slug + outcome.
For downloading, you can either:
  • Use asset_id + date in the path and apikey in the query, or
  • Use market_slug + outcome (YES/NO) + date in the path and both slug=true and apikey in the query. Don’t mix: if you use slug in the path, you must set slug=true.

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Kalshi download returns HTML or a redirect URL instead of the file

The Kalshi download endpoint returns a 302 redirect to a signed CloudFront URL. You must follow that redirect to get the file. In curl, use -L. In code, allow your HTTP client to follow redirects (often the default). If you only read the first response body, you’ll get the redirect page or URL, not the actual data.

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Rate limits / “too many requests”

We do not rate limit by request count or download volume for normal use. If you see rate-limiting errors, they may be from a proxy or from the CDN (e.g. after following the Kalshi redirect). If you’re doing very high volume (e.g. bulk backfill), reach out to calder@predictiondata.dev.

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How do I read the files?

CSV files are gzip-compressed (.csv.gz):
  • Command line: gunzip -k file.csv.gz or gzip -dc file.csv.gz > file.csv.
  • Python: pandas.read_csv('file.csv.gz') (pandas handles gzip automatically).
  • Other languages: Use a gzip library or decompress before parsing.
Parquet files (.parquet, Kalshi only) are zstd-compressed:
  • Python: pandas.read_parquet('file.parquet') or use pyarrow.
  • Other languages: Use any Parquet library (Arrow, DuckDB, etc.).

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Still stuck?

  • Double-check the Quickstart for the exact URLs and parameters.
  • Confirm your API key and tier with calder@predictiondata.dev.
  • For Polymarket, re-fetch the list response and use the same asset_id, date, and export_type (or slug + outcome) in your download URL.
  • For Kalshi, re-fetch the list response and use the exact path from the response.