{"id":2979,"date":"2026-06-18T10:00:00","date_gmt":"2026-06-18T10:00:00","guid":{"rendered":"https:\/\/oddspapi.io\/blog\/?p=2979"},"modified":"2026-06-05T15:31:57","modified_gmt":"2026-06-05T15:31:57","slug":"kalshi-vs-polymarket-api","status":"publish","type":"post","link":"https:\/\/oddspapi.io\/blog\/kalshi-vs-polymarket-api\/","title":{"rendered":"Kalshi API vs Polymarket API: A Python Developer&#8217;s Comparison"},"content":{"rendered":"<p>If you&#8217;re building anything on prediction-market data, you eventually hit the same fork in the road: <strong>Kalshi API vs Polymarket API<\/strong>. They look interchangeable from the outside \u2014 two venues quoting probabilities on the same events \u2014 but the moment you open an editor they could not be more different. Different auth models, different identifiers, different price formats, even a different <em>number<\/em> of APIs. Get the wrong mental model and your parser breaks on the first request.<\/p>\n<p>This is a developer-to-developer breakdown: how each API is actually shaped, the gotchas that bite, and a shortcut that gives you both venues \u2014 already converted to decimal odds \u2014 from a single HTTP call.<\/p>\n<h2>The 30-Second Answer<\/h2>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>Dimension<\/th>\n<th>Kalshi API<\/th>\n<th>Polymarket API<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Venue type<\/td>\n<td>CFTC-regulated US exchange<\/td>\n<td>Onchain (Polygon), crypto-settled<\/td>\n<\/tr>\n<tr>\n<td>Number of APIs<\/td>\n<td>One REST API<\/td>\n<td>Two: Gamma (catalog) + CLOB (order book)<\/td>\n<\/tr>\n<tr>\n<td>Read auth<\/td>\n<td>None (public market data)<\/td>\n<td>None (public reads)<\/td>\n<\/tr>\n<tr>\n<td>Market identifier<\/td>\n<td><code>ticker<\/code> (human-readable)<\/td>\n<td><code>token_id<\/code> (77-digit ERC-1155)<\/td>\n<\/tr>\n<tr>\n<td>Price format<\/td>\n<td>USD strings, 0\u20131 (<code>_dollars<\/code> fields)<\/td>\n<td>Share-price strings, 0\u20131<\/td>\n<\/tr>\n<tr>\n<td>Decimal odds?<\/td>\n<td>No \u2014 compute <code>1 \/ price<\/code><\/td>\n<td>No \u2014 compute <code>1 \/ price<\/code><\/td>\n<\/tr>\n<tr>\n<td>Trading auth<\/td>\n<td>API key + RSA signature<\/td>\n<td>EIP-712 wallet sig + L2 headers<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>Neither API hands you decimal odds, and neither lets you pull both venues in one request. If all you want is to <em>read<\/em> prices side by side, skip to the <a href=\"https:\/\/oddspapi.io\/blog\/free-odds-api-350-bookmakers\/\">OddsPapi section<\/a> \u2014 both venues come back pre-converted from one endpoint. If you&#8217;re going deeper (trading, raw depth, settlement), read on.<\/p>\n<h2>Kalshi API: One REST API, Ticker-Based<\/h2>\n<p>Kalshi is a CFTC-regulated exchange, so the API feels like a traditional financial data feed. There&#8217;s a single base URL, everything is keyed off a human-readable <code>ticker<\/code>, and market data reads need no authentication.<\/p>\n<p><strong>Base URL:<\/strong> <code>https:\/\/api.elections.kalshi.com\/trade-api\/v2<\/code><\/p>\n<h3>The ID hierarchy<\/h3>\n<p>Kalshi nests three levels, all string tickers:<\/p>\n<ul>\n<li><strong>Series<\/strong> \u2014 a recurring template (e.g. a weekly question type)<\/li>\n<li><strong>Event<\/strong> (<code>event_ticker<\/code>) \u2014 a single real-world question<\/li>\n<li><strong>Market<\/strong> (<code>ticker<\/code>) \u2014 one yes\/no contract inside an event<\/li>\n<\/ul>\n<h3>Fetching markets and prices<\/h3>\n<pre class=\"wp-block-code\"><code>import requests\n\nKALSHI = \"https:\/\/api.elections.kalshi.com\/trade-api\/v2\"\n\n# List open markets (no auth required for reads)\nr = requests.get(f\"{KALSHI}\/markets\", params={\"status\": \"open\", \"limit\": 100})\nmarkets = r.json()[\"markets\"]\n\nm = markets[0]\nprint(m[\"ticker\"])             # e.g. KXBTCD-25JUN06-T100000\nprint(m[\"title\"])\nprint(m[\"yes_bid_dollars\"])    # \"0.6550\"  -> implied probability of YES\nprint(m[\"yes_ask_dollars\"])    # \"0.6600\"\nprint(m[\"no_bid_dollars\"], m[\"no_ask_dollars\"])\nprint(m[\"last_price_dollars\"]) # last trade\n<\/code><\/pre>\n<p><strong>Current-API gotcha:<\/strong> Kalshi moved its price fields to a <code>_dollars<\/code> suffix. The values are <strong>strings<\/strong> in the 0\u20131 range (so <code>\"0.6550\"<\/code> = a 65.5% implied probability). Older tutorials reference integer-cent fields like <code>yes_bid<\/code>; on the live v2 API you want <code>yes_bid_dollars<\/code>, <code>yes_ask_dollars<\/code>, <code>no_bid_dollars<\/code>, <code>no_ask_dollars<\/code>, and <code>last_price_dollars<\/code>. Cast before doing math.<\/p>\n<h3>The order book<\/h3>\n<pre class=\"wp-block-code\"><code>ticker = m[\"ticker\"]\nob = requests.get(f\"{KALSHI}\/markets\/{ticker}\/orderbook\", params={\"depth\": 5}).json()\n\n# Shape: {\"orderbook_fp\": {\"yes_dollars\": [[price, size], ...],\n#                          \"no_dollars\":  [[price, size], ...]}}\nbook = ob[\"orderbook_fp\"]\nfor price, size in book[\"yes_dollars\"]:\n    print(float(price), float(size))   # both are strings -> cast\n<\/code><\/pre>\n<p>Each level is a <code>[price, size]<\/code> pair, both strings. To turn a Kalshi share price into decimal odds: <code>decimal = 1 \/ float(price)<\/code>. A YES contract at <code>0.655<\/code> is decimal <code>1.527<\/code>.<\/p>\n<h2>Polymarket API: Two Services, Token-Based<\/h2>\n<p>Polymarket splits its read API in two \u2014 and confusing them is the #1 reason people&#8217;s first script fails. We covered this in depth in the <a href=\"https:\/\/oddspapi.io\/blog\/polymarket-api-gamma-clob-json-format\/\">Polymarket API deep dive<\/a>; here&#8217;s the comparison-relevant version.<\/p>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>Service<\/th>\n<th>Base URL<\/th>\n<th>Purpose<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Gamma<\/td>\n<td><code>https:\/\/gamma-api.polymarket.com<\/code><\/td>\n<td>Catalog: <code>\/events<\/code>, <code>\/markets<\/code>, metadata, discovery<\/td>\n<\/tr>\n<tr>\n<td>CLOB<\/td>\n<td><code>https:\/\/clob.polymarket.com<\/code><\/td>\n<td>Order book: <code>\/book<\/code>, <code>\/price<\/code>, <code>\/midpoint<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<h3>Step 1 \u2014 discover via Gamma<\/h3>\n<pre class=\"wp-block-code\"><code>import requests, json\n\nGAMMA = \"https:\/\/gamma-api.polymarket.com\"\n\nr = requests.get(f\"{GAMMA}\/markets\",\n                 params={\"active\": \"true\", \"closed\": \"false\", \"limit\": 5})\nm = r.json()[0]\nprint(m[\"question\"])\nprint(m[\"conditionId\"])          # 0x... on-chain market hash\n\n# GOTCHA: these arrive as STRINGIFIED JSON, not native arrays\ntoken_ids = json.loads(m[\"clobTokenIds\"])   # double-parse required\noutcomes  = json.loads(m[\"outcomes\"])       # [\"Yes\", \"No\"]\nprices    = json.loads(m[\"outcomePrices\"])  # [\"0.505\", \"0.495\"]\n<\/code><\/pre>\n<p><strong>The classic Polymarket trap:<\/strong> <code>clobTokenIds<\/code>, <code>outcomes<\/code>, and <code>outcomePrices<\/code> are <em>strings containing JSON<\/em>. If you iterate them directly you&#8217;ll loop over individual characters. Always <code>json.loads()<\/code> a second time.<\/p>\n<h3>Step 2 \u2014 get the book via CLOB<\/h3>\n<pre class=\"wp-block-code\"><code>CLOB = \"https:\/\/clob.polymarket.com\"\ntoken_id = token_ids[0]   # the 77-digit ERC-1155 position id\n\nbook = requests.get(f\"{CLOB}\/book\", params={\"token_id\": token_id}).json()\n# {\"bids\": [{\"price\": \"0.50\", \"size\": \"...\"}], \"asks\": [...], ...}\n\nmid   = requests.get(f\"{CLOB}\/midpoint\", params={\"token_id\": token_id}).json()  # {\"mid\": \"0.505\"}\nprice = requests.get(f\"{CLOB}\/price\", params={\"token_id\": token_id, \"side\": \"buy\"}).json()\n<\/code><\/pre>\n<p>Every CLOB endpoint wants the <code>token_id<\/code> (a.k.a. <code>asset_id<\/code>), not the human-readable slug or the <code>conditionId<\/code>. Prices and sizes are strings in the 0\u20131 range; decimal odds = <code>1 \/ float(price)<\/code>.<\/p>\n<h2>Head-to-Head: The Differences That Actually Bite<\/h2>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>You want to\u2026<\/th>\n<th>Kalshi<\/th>\n<th>Polymarket<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Find a market<\/td>\n<td><code>GET \/markets<\/code> \u2192 <code>ticker<\/code><\/td>\n<td><code>GET \/markets<\/code> (Gamma) \u2192 <code>clobTokenIds<\/code><\/td>\n<\/tr>\n<tr>\n<td>Get best bid\/ask<\/td>\n<td>Read <code>yes_bid_dollars<\/code> \/ <code>yes_ask_dollars<\/code> off the market<\/td>\n<td><code>GET \/price<\/code> (CLOB) per token<\/td>\n<\/tr>\n<tr>\n<td>Get full depth<\/td>\n<td><code>GET \/markets\/{ticker}\/orderbook<\/code><\/td>\n<td><code>GET \/book<\/code> (CLOB) per token<\/td>\n<\/tr>\n<tr>\n<td>Parse prices<\/td>\n<td>String 0\u20131 in <code>_dollars<\/code> fields<\/td>\n<td>String 0\u20131, double-<code>json.loads<\/code> the arrays<\/td>\n<\/tr>\n<tr>\n<td>Both outcomes<\/td>\n<td>One market = one YES\/NO pair<\/td>\n<td>One token per outcome (separate ids)<\/td>\n<\/tr>\n<tr>\n<td>Decimal odds<\/td>\n<td>Compute <code>1\/price<\/code><\/td>\n<td>Compute <code>1\/price<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>The summary: <strong>Kalshi is one tidy REST API with readable tickers and prices attached to the market object<\/strong>. <strong>Polymarket is two APIs, opaque token IDs, and stringified JSON you have to parse twice<\/strong>. Neither gives you decimal odds, and \u2014 critically \u2014 neither lets you compare the two venues in a single call. You&#8217;re maintaining two clients, two ID schemes, two price-parsing paths, and writing your own odds converter, just to read a number both venues already know.<\/p>\n<h2>The Shortcut: Both Venues, Decimal Odds, One Call<\/h2>\n<p>This is where <a href=\"https:\/\/oddspapi.io\/blog\/what-is-oddspapi\/\">OddsPapi<\/a> earns its place in the stack. It aggregates <strong>350+ bookmakers and exchanges \u2014 including both Polymarket and Kalshi<\/strong> \u2014 behind one schema, and pre-converts every price into decimal, American, and fractional. One request returns both prediction markets <em>plus<\/em> the sharp sportsbook line (Pinnacle) for the same event, so you can compare apples to apples instead of stitching three feeds together.<\/p>\n<h3>Authentication<\/h3>\n<pre class=\"wp-block-code\"><code>import requests\n\nAPI_KEY = \"YOUR_API_KEY\"          # grab a free one at oddspapi.io\nBASE = \"https:\/\/api.oddspapi.io\/v4\"\n\n# apiKey is a QUERY PARAMETER, never a header\nparams = {\"apiKey\": API_KEY}\nprint(requests.get(f\"{BASE}\/sports\", params=params).status_code)  # 200\n<\/code><\/pre>\n<h3>Pull both prediction markets for one fixture<\/h3>\n<p>Here&#8217;s a real, live example: the 2026 FIFA World Cup match <strong>Brazil vs Morocco<\/strong>. One call returns Kalshi, Polymarket, and Pinnacle on the Full Time Result market.<\/p>\n<pre class=\"wp-block-code\"><code>fixture_id = \"id1000001666456928\"   # Brazil vs Morocco, World Cup 2026\n\nr = requests.get(f\"{BASE}\/odds\", params={\n    \"apiKey\": API_KEY,\n    \"fixtureId\": fixture_id,\n    \"bookmakers\": \"kalshi,polymarket,pinnacle\",\n})\nbooks = r.json()[\"bookmakerOdds\"]\n\n# 1X2 = market 101; outcomes 101=Home, 102=Draw, 103=Away\nLABELS = {\"101\": \"Brazil\", \"102\": \"Draw\", \"103\": \"Morocco\"}\n\nfor slug in (\"kalshi\", \"polymarket\", \"pinnacle\"):\n    outcomes = books[slug][\"markets\"][\"101\"][\"outcomes\"]\n    quote = {LABELS[oid]: o[\"players\"][\"0\"][\"price\"] for oid, o in outcomes.items()}\n    print(f\"{slug:11} {quote}\")\n<\/code><\/pre>\n<p>Live output (captured June 2026):<\/p>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>Venue<\/th>\n<th>Brazil<\/th>\n<th>Draw<\/th>\n<th>Morocco<\/th>\n<th>Overround<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Kalshi<\/td>\n<td>1.639<\/td>\n<td>4.167<\/td>\n<td>5.882<\/td>\n<td><strong>2.01%<\/strong><\/td>\n<\/tr>\n<tr>\n<td>Polymarket<\/td>\n<td>1.639<\/td>\n<td>4.000<\/td>\n<td>5.882<\/td>\n<td>3.01%<\/td>\n<\/tr>\n<tr>\n<td>Pinnacle (sharp benchmark)<\/td>\n<td>1.641<\/td>\n<td>3.840<\/td>\n<td>5.950<\/td>\n<td>3.79%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>Already decimal, no <code>1\/price<\/code> needed. The interesting read: on this early World Cup line, <strong>both prediction markets quoted a tighter margin than Pinnacle<\/strong> \u2014 Kalshi&#8217;s 2.01% overround was the lowest of the three. (Early lines tighten by matchday; this is a snapshot of available prices, not a value claim.) The point is you got all three, normalized, from one request \u2014 no token IDs, no double-parsing, no converter.<\/p>\n<h3>Depth-of-book is there too<\/h3>\n<p>Because Kalshi and Polymarket are exchanges, OddsPapi exposes their ladder under <code>exchangeMeta<\/code> \u2014 the same depth you&#8217;d otherwise hit two separate APIs for:<\/p>\n<pre class=\"wp-block-code\"><code>brazil = books[\"polymarket\"][\"markets\"][\"101\"][\"outcomes\"][\"101\"][\"players\"][\"0\"]\nladder = brazil[\"exchangeMeta\"]   # null on sportsbooks; a ladder on exchanges\n\nfor level in ladder[\"back\"][:3]:\n    print(level[\"price\"], \"odds |\", level[\"size\"], \"liquidity\")\n# 1.639 odds | 35456.27 liquidity\n# 1.613 odds | 12432.34 liquidity\n# 1.587 odds | 21985.0  liquidity\n<\/code><\/pre>\n<p><strong>Parse defensively:<\/strong> <code>exchangeMeta<\/code> is <code>null<\/code> on sportsbooks and a <code>{back: [...], lay: [...]}<\/code> ladder on exchanges. Each level is <code>{cents, price, size, limit}<\/code>, best price first. Polymarket also ships a <code>bookmakerLayOutcomeId<\/code> (the opposite-side token, which drops straight into Polymarket&#8217;s CLOB); Kalshi omits it. Always check the field is truthy before indexing into it.<\/p>\n<h2>When to Use Which<\/h2>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>If you&#8217;re\u2026<\/th>\n<th>Use<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Reading\/comparing prices across both venues<\/td>\n<td>OddsPapi (one call, decimal odds, + Pinnacle anchor)<\/td>\n<\/tr>\n<tr>\n<td>Backtesting prediction-market accuracy<\/td>\n<td>OddsPapi free <a href=\"https:\/\/oddspapi.io\/blog\/historical-odds-csv-excel-backtesting\/\">historical odds<\/a><\/td>\n<\/tr>\n<tr>\n<td>Placing trades on Kalshi<\/td>\n<td>Kalshi API directly (RSA-signed key)<\/td>\n<\/tr>\n<tr>\n<td>Placing trades on Polymarket<\/td>\n<td>Polymarket CLOB directly (EIP-712 + L2 auth)<\/td>\n<\/tr>\n<tr>\n<td>Raw onchain settlement \/ token mechanics<\/td>\n<td>Polymarket CLOB + conditionId<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>For trading you&#8217;ll always go direct \u2014 each venue&#8217;s order-placement layer is venue-specific and wallet\/key bound. But for the 90% of work that&#8217;s <em>reading<\/em> \u2014 comparison, alerts, dashboards, modeling, arbitrage scanning \u2014 one OddsPapi call beats maintaining two clients and a converter.<\/p>\n<h2>FAQ<\/h2>\n<h3>Is the Kalshi API free to use?<\/h3>\n<p>Yes for market data. Reads against <code>\/markets<\/code>, <code>\/events<\/code>, and <code>\/markets\/{ticker}\/orderbook<\/code> need no authentication. Placing trades requires an account and an RSA-signed API key.<\/p>\n<h3>Is the Polymarket API free to use?<\/h3>\n<p>Yes for reads. Both Gamma (<code>gamma-api.polymarket.com<\/code>) and CLOB (<code>clob.polymarket.com<\/code>) serve catalog and order-book data unauthenticated. Trading requires an EIP-712 wallet signature plus L2 auth headers.<\/p>\n<h3>Do Kalshi and Polymarket return decimal odds?<\/h3>\n<p>No. Both return share\/contract prices in the 0\u20131 range as strings (a 0.65 price = 65% implied probability). Convert with <code>decimal = 1 \/ float(price)<\/code>. OddsPapi pre-converts to decimal, American, and fractional so you skip the math.<\/p>\n<h3>What&#8217;s the biggest difference between the two APIs?<\/h3>\n<p>Structure. Kalshi is one REST API keyed off readable <code>ticker<\/code> strings with prices on the market object. Polymarket is two services (Gamma for catalog, CLOB for books), keyed off 77-digit token IDs, and ships its arrays as stringified JSON you must parse twice.<\/p>\n<h3>Can I get both Kalshi and Polymarket from one API?<\/h3>\n<p>Yes \u2014 OddsPapi aggregates both (plus 350+ sportsbooks and exchanges) behind one schema. A single <code>\/v4\/odds<\/code> call returns both venues for the same event, already converted to decimal odds, with full depth-of-book under <code>exchangeMeta<\/code>.<\/p>\n<h2>Stop Maintaining Two Clients<\/h2>\n<p>Kalshi and Polymarket are great venues with awkward, mismatched APIs. If you&#8217;re trading, go direct. If you&#8217;re reading, comparing, or modeling, <a href=\"https:\/\/oddspapi.io\/blog\/free-odds-api-350-bookmakers\/\">get your free OddsPapi key<\/a> and pull both \u2014 plus the sharp line \u2014 from one endpoint.<\/p>\n<p><script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@type\": \"FAQPage\",\n  \"mainEntity\": [\n    {\"@type\": \"Question\", \"name\": \"Is the Kalshi API free to use?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Yes for market data. Reads against \/markets, \/events, and the orderbook endpoint need no authentication. Placing trades requires an account and an RSA-signed API key.\"}},\n    {\"@type\": \"Question\", \"name\": \"Is the Polymarket API free to use?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Yes for reads. Both Gamma and CLOB serve catalog and order-book data unauthenticated. Trading requires an EIP-712 wallet signature plus L2 auth headers.\"}},\n    {\"@type\": \"Question\", \"name\": \"Do Kalshi and Polymarket return decimal odds?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"No. Both return share or contract prices in the 0 to 1 range as strings. Convert with decimal = 1 \/ float(price). OddsPapi pre-converts to decimal, American, and fractional.\"}},\n    {\"@type\": \"Question\", \"name\": \"What's the biggest difference between the two APIs?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Structure. Kalshi is one REST API keyed off readable ticker strings with prices on the market object. Polymarket is two services keyed off 77-digit token IDs and ships its arrays as stringified JSON you must parse twice.\"}},\n    {\"@type\": \"Question\", \"name\": \"Can I get both Kalshi and Polymarket from one API?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Yes. OddsPapi aggregates both plus 350+ sportsbooks and exchanges behind one schema. A single \/v4\/odds call returns both venues for the same event, already converted to decimal odds, with full depth-of-book under exchangeMeta.\"}}\n  ]\n}\n<\/script><\/p>\n<p><!--\nFocus Keyphrase: kalshi vs polymarket api\nSEO Title: Kalshi API vs Polymarket API: A Python Developer's Comparison\nMeta Description: Kalshi API vs Polymarket API compared for developers: auth, endpoints, price formats & a one-call shortcut to read both in decimal odds with OddsPapi.\nSlug: kalshi-vs-polymarket-api\n--><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Kalshi API vs Polymarket API for developers: auth, endpoints, price formats &#038; a one-call shortcut to read both in decimal odds with 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