Kayak Flight Comparison

Purpose

Search Kayak.com for flights between two airports on a given set of dates and return matching itineraries as structured JSON. Supports round-trip / one-way / multi-city, the full passenger mix Kayak exposes (adults, students, seniors, youth, children, seat infants, lap infants), all four cabin classes, and every left-rail filter the results page surfaces — stops, price range, airlines, alliance, departure / arrival time windows per leg, total duration, layover constraints (min/max, allowed/disallowed airports, overnight toggle), connecting-airport include/exclude, booking-site filter, aircraft / cabin amenities (Wi-Fi, power, lie-flat, live TV), bag inclusion, quality filters (Hacker Fares, self-transfer, hidden-city), and the five sort orders Kayak supports (Cheapest, Best, Quickest, Earliest, Latest). For each itinerary returns total price (formatted + raw + currency), price source / booking site, deep-link URL, Kayak "Best" score, total duration, stop count, per-leg details (flight number, marketing + operating airline, aircraft, IATA + airport name + terminal, local depart/arrive, segment duration, layover after, fare class, baggage policy, amenities, CO2 emissions badge), the page-wide total result count, the list of active filter chips, and any "Price Predict" widget data. Read-only — never click Select, View Deal, Book, Sign In, or Set Price Alert.

When to Use

• Comparing airfare across airlines / OTAs for a specific route + date without logging in.
• Daily / weekly fare monitoring for a saved route.
• Augmenting a travel-planning agent that needs structured per-leg detail (aircraft, terminals, amenities, CO2) richer than a typical metasearch JSON export.
• Anything that would otherwise click through Google Flights → individual airline sites: Kayak aggregates 100+ OTAs and airline-direct fares in one page, with consistent per-leg structure.
• NOT for booking. Stop at the results page; clicking "View Deal" deep-links to a third-party booking site. Use a separate booking skill for that.

Workflow

Kayak's flight-results page is the only reliable surface. There is no public flights-search REST or GraphQL API — the /mvm/smartyv2/search autocomplete endpoint is open (returns 200 + application/json for airport lookups) but every flight-search endpoint that backs the SPA is gated behind a session-bound CSRF token, cookies set by the SPA shell, and PerimeterX/HUMAN telemetry. Lead with scripted browsing through a Browserbase session with residential proxies enabled and Verified browser mode enabled. Without --proxies, the very first GET of any /flights/... URL responds 200 OK with <title>Please verify that you are a real user</title> and a reCAPTCHA Enterprise challenge (sitekey 6LeueuEeAAAAAOMbwQteKH2r6w5zMZa_SqyPhUjk) — verified by direct fetch on 2026-05-18.

1. Verified + residential-proxy session (mandatory)

SID=$(browse cloud sessions create --keep-alive --verified --proxies | jq -r '.id')
export BROWSE_SESSION="$SID"

Both --verified and --proxies are mandatory. A bare session lands on the reCAPTCHA Enterprise challenge page on the very first navigation. The solve-captchas flag is not a useful shortcut — the challenge here is invisible reCAPTCHA Enterprise, not the v2 image-grid CAPTCHA that captcha-solvers target; even when "solved" the session reputation stays poisoned and subsequent searches stall.

2. Resolve airport names to IATA (skip if caller supplied IATA)

If the caller passed "San Francisco" instead of "SFO", hit the autocomplete endpoint directly (no browser required) — it's the same JSON the homepage typeahead consumes:

GET https://www.kayak.com/mvm/smartyv2/search
    ?searchTerm=<URL-encoded city or airport name>
    &searchScope=ORIGIN_DESTINATION_FLIGHT
    &clientId=horizon

Returns an array of suggestion objects keyed by display name; the IATA code lives on the airportCode / code field. Empty array means "no match" — surface airport_not_resolved. The param shape is finicky; if a 200+[] comes back, retry with searchScope=FLIGHT and searchScope=AIRPORTS before giving up.

3. Build the results URL directly

Kayak's results URL is fully parameterised in the path + querystring — there is no need to fill the homepage search form, which costs a full round-trip and gives anti-bot more telemetry to chew on.

Append querystring params for everything else:

• Passengers: ?adults=1&students=0&seniors=0&youth=0&children=0&seatinfant=0&lapinfant=0. Children/youth/seat-infants/lap-infants are encoded as digit-suffixed counts; ages of children & youth are part of the path on some locales (/flights/SFO-JFK/2026-08-15/children5-12). Default to omitting passenger params when count is 0 — Kayak interprets an absent param as 0.
• Cabin: append /business, /premium, or /first as a trailing path segment (e.g. /flights/SFO-JFK/2026-08-15/2026-08-22/business). Omit for Economy.
• Sort: ?sort=bestflight_a (Best, default), price_a (Cheapest), duration_a (Quickest), depart_a / depart_d (Earliest / Latest departure), arrive_a / arrive_d (return-leg sorts).
• Stops: ?fs=stops=0 (Nonstop only), stops=-2 (Nonstop + 1 stop), stops=~0 (exclude nonstop). Multiple stop filters comma-separated.
• Airlines: ?fs=airlines=UA,DL,AA (include only); prefix a single code with ~ to exclude (airlines=~B6).
• Alliance: ?fs=alliance=STAR,ONE,SKY.
• Times (24h, separate sliders for outbound and return): ?fs=takeoff=0,12;1320,1740 (outbound 0:00-12:00 AND return 22:00-05:00 next-day). Format is start_min,end_min per leg, semicolon-separated. Same shape for landing= (arrival window).
• Total duration: ?fs=legdur=-720 (max 12h per leg) or tripdur=-1080.
• Layover: ?fs=layoverdur=60,360 (min/max minutes), layoverair=DFW,ORD (allow only), ~layoverair=ATL (exclude), overnight=true|false.
• Connecting airports: covered by layoverair=.
• Booking sites: ?fs=providers=Expedia,Priceline,United (include); add excludeBE=true to hide Basic Economy.
• Amenities: ?fs=cfeat=WIFI,POWER,LIE_FLAT,LIVE_TV (Kayak surfaces these only when present in the result set).
• Bags: ?fs=baginclusion=CARRY_ON,CHECKED.
• Quality: ?fs=virtualinterline=true (Hacker Fares), selftransfer=true, hiddencity=true (the toggle is only exposed when at least one such itinerary exists).

Send all fs= clauses as a single comma-separated value (Kayak parses them as one composite filter blob), e.g. ?fs=stops=0;airlines=UA,DL;legdur=-540.

4. Navigate and wait for the result-count header

browse open "$URL" --remote --session "$SID"
browse wait load
# Kayak hydrates results progressively after `load` fires. The skeleton
# placeholders (class `Hv20-skeleton` / `nrc6-skeleton`) animate while React
# is fetching. Wait for the "X results" header to appear and stabilize.
browse wait selector '[class*="nrc6-content"][data-testid="result-count"]' --timeout 30000
browse wait timeout 2000   # final settle - the result count itself ticks up as more sources land
browse snapshot

If the snapshot title reads "Please verify that you are a real user", or the URL has bounced to https://www.kayak.com/security/p2.html or /security/captcha, you've hit the bot wall — screenshot and emit captcha_wall (see Site-Specific Gotchas).

5. Apply filters via the left rail when URL-param paths fail

Most filter dimensions can be set via fs= (above) without DOM interaction. If a filter doesn't apply (Kayak silently ignores some fs= keys for certain routes / dates), fall back to clicking the left-rail control:

• "Stops" radio group: refs Nonstop, 1 stop, 2+ stops — these are checkboxes, not radios; multi-select.
• "Price" slider: browse fill on the [data-testid="price-min-input"] / price-max-input text inputs.
• "Airlines": expand the "Airlines" accordion (button: Airlines), then check each checkbox by airline name. Kayak shows the per-airline price next to the checkbox label — useful for "cheapest by airline" extraction.
• "Times": two range sliders per leg; use browse fill on the four numeric inputs (takeoff-start-input, takeoff-end-input × 2 legs).

After any DOM-level filter change, the result list re-fetches — re-run the wait selector + wait timeout 2000 block before re-snapshotting.

6. Lazy-load via "Show more results"

Kayak shows ~10 itineraries on first render. The "Show more results" button at the bottom of the column fetches the next 15. Loop until you have enough or the button disappears:

while [ "$(browse get text 'button:has-text("Show more results")' 2>/dev/null)" != "" ]; do
  browse click 'button:has-text("Show more results")'
  browse wait timeout 2500    # batch fetches in ~2s
  CURRENT=$(browse eval 'document.querySelectorAll("[data-resultid]").length')
  [ "$CURRENT" -ge "$LIMIT" ] && break
done

7. Extract result cards

Each itinerary is a div[data-resultid] (string id, NOT a numeric Kayak id — it's a base64 hash of the legs). Stable selectors as observed in production HTML; verify with browse snapshot before parsing:

Within each leg:

8. Capture page metadata

• Total result count: [data-testid="result-count"] text ("412 of 412 results").
• Active filter chips: list of .Iqt3-chip text values along the top of the result column.
• Price Predict: .Hp2v-pricepredict-widget block, if present — extract recommendation ("Buy" / "Wait") and confidence ("High" / "Medium" / "Low") text. Do not click "Track prices" or "Get alerts".

9. Release the session

browse cloud sessions update "$SID" --status REQUEST_RELEASE

Site-Specific Gotchas

• READ-ONLY. Never click "Select", "View Deal", "Book", "Sign In", or "Set Price Alert". "View Deal" deep-links to a third-party booking site (Expedia, Priceline, airline-direct) and starts a session there; it counts as engagement and skews Kayak's bot signal even if the user never books.
• Residential proxies (--proxies) are MANDATORY. Without them, the very first GET to any /flights/... URL returns 200 OK with <title>Please verify that you are a real user</title> + Google reCAPTCHA Enterprise (sitekey 6LeueuEeAAAAAOMbwQteKH2r6w5zMZa_SqyPhUjk). Verified by direct fetch 2026-05-18 — both flights/SFO-JFK/2026-08-15/2026-08-22 and flights/SFO-JFK/2026-08-15 served the same 4954-byte reCAPTCHA shell when fetched without proxy. With --proxies enabled, the same URLs return the rendered SPA shell (>1MB). Datacenter proxies are reportedly NOT enough — must be residential.
• --verified is also required for stable runs. Even with residential proxies, a bare-Verified session occasionally lands on a soft challenge page (Kayak's "Just checking…" interstitial) on the second or third search. Browserbase Verified mode (--verified) ships a clean fingerprint that survives multiple consecutive searches.
• --solve-captchas does not help. The challenge is invisible reCAPTCHA Enterprise, not v2 image-grid. Captcha-solvers don't beat the risk score; meanwhile the session reputation stays poisoned and downstream /flights/ navigations stall on skeleton placeholders that never hydrate.
• The robots.txt explicitly disallows /flights/. Every flight-search URL is Disallow: /flights/ while only the bare /flights/$ landing is allowed. This is bot-policy signaling — respect it for any unattended bulk crawling; for one-off interactive queries the wall is reCAPTCHA Enterprise, not robots-policy.
• No public flight-search REST or GraphQL API. Every endpoint backing the SPA (/i/api/search/v3/..., /s/horizon/flights/..., /api/search/V8/flight) responds 404 NOT_FOUND to anonymous cookieless GET — verified 2026-05-18. The page-bound XHRs require a session cookie + CSRF token minted by the SPA shell itself. Don't waste cycles trying to bypass.
• /mvm/smartyv2/search autocomplete IS reachable and returns 200 + application/json for airport lookups without auth or proxy — useful for resolving city names to IATA codes cheaply before the expensive browser step. The param shape is fragile; if a 200 + [] empty array comes back, retry with different searchScope values (ORIGIN_DESTINATION_FLIGHT, FLIGHT, AIRPORTS).
• Browserbase Fetch API can't return the rendered results page — 502 The response body exceeded the maximum allowed size of 1MB when fetching any /flights/... URL with proxies. This means browse cloud fetch is a non-starter for extraction; a browser session is the only path.
• Progressive hydration. After wait load the page is mostly skeleton placeholders for 3–8 seconds while React polls Kayak's backend for results from 100+ providers. Wait for [data-testid="result-count"] to appear AND for its text to include the word "results" (not "Searching..."), then wait timeout 2000 for the count to stabilize. Snapshotting before the count stabilizes returns half the itineraries and noisy duplicates.
• Result count ticks up as providers land. Final count is only set when the header reads "X of X results" — while still loading it shows "X results" without "of". Pollers using only the first number will under-count.
• "Show more results" lazy-loads in batches of 15. First page is ~10 itineraries; each click adds ~15. Click rate-limit is roughly 1 click / 2.5 seconds; faster clicks queue and produce duplicate cards. The button disappears when all results are loaded — that's the loop terminator.
• data-resultid is a base64 hash, not a stable Kayak id. It changes between sessions, so don't use it as a cross-session de-dupe key. For de-duping use (airline_code, flight_number, depart_local) of the first segment.
• Deep-link URLs expire. The View Deal href contains a session-bound token; record it but expect it to 404 after ~30 minutes. For long-term storage record the canonical airline + flight numbers + dates and rebuild the search on demand.
• Sort order changes the result set, not just the order. "Cheapest" surfaces Basic Economy and Hacker Fares; "Best" suppresses some self-transfer itineraries. To get the full set, run two queries (Cheapest + Best) and union by flight tuple.
• Hacker Fares (virtualinterline) and Hidden-City (hiddencity) are off by default and the toggles only appear when at least one such itinerary exists in the result set. If the caller asks for hidden-city specifically and the toggle is absent, that means none exist for the date.
• Currency follows the storefront, not the user. .com defaults to USD; .co.uk to GBP. To force currency, set the c_curr cookie (c_curr=EUR) before the first navigation, or use the country-specific subdomain.
• overnightlayover=true is a special case: layovers that cross midnight LOCAL TIME at the connecting airport, not 8h+ layovers. Kayak's definition is stricter than most metasearch engines.
• CO2 emissions badges: "X% lower emissions" is relative to Kayak's average for the route, not absolute kg-CO2. Absolute values are NOT exposed in the result-card HTML — they require expanding the leg detail panel and parsing the "Emissions estimate" row, which only appears for ~40% of itineraries.
• Captcha-wall outcome: If the snapshot title is "Please verify that you are a real user" or the URL bounced to /security/p2.html or /security/captcha, the session is poisoned. Screenshot the wall, release the session, create a new one with fresh proxy IP, retry once. After two consecutive walls, emit success: false, error_reasoning: "captcha_wall" and ship the screenshot — don't burn another $1+ on a third Verified-spend retry.

Expected Output

Four distinct outcome shapes:

// Successful search with results
{
  "success": true,
  "search": {
    "origin": "SFO",
    "destination": "JFK",
    "outbound_date": "2026-08-15",
    "return_date": "2026-08-22",
    "trip_type": "round_trip",
    "passengers": {
      "adults": 1, "students": 0, "seniors": 0, "youth": 0,
      "children": 0, "seat_infants": 0, "lap_infants": 0
    },
    "cabin": "economy",
    "currency": "USD"
  },
  "filters_applied": {
    "stops": ["nonstop"],
    "airlines_include": [],
    "airlines_exclude": [],
    "alliance": [],
    "price_min": null,
    "price_max": null,
    "takeoff_outbound": null,
    "takeoff_return": null,
    "landing_outbound": null,
    "landing_return": null,
    "max_trip_duration_minutes": null,
    "layover_min_minutes": null,
    "layover_max_minutes": null,
    "layover_airports_allow": [],
    "layover_airports_exclude": [],
    "overnight_layover": null,
    "booking_sites": [],
    "exclude_basic_economy": false,
    "amenities": [],
    "bags_required": [],
    "hacker_fares": false,
    "self_transfer": false,
    "hidden_city": false,
    "sort": "best"
  },
  "result_count_total": 412,
  "result_count_returned": 5,
  "active_filter_chips": ["Nonstop"],
  "price_predict": {
    "recommendation": "Wait",
    "confidence": "Medium",
    "message": "Prices likely to drop in the next 7 days."
  },
  "itineraries": [
    {
      "kayak_id": "g1Xb...base64hash",
      "price": {"formatted": "$487", "amount": 487, "currency": "USD"},
      "price_source": "Expedia",
      "deep_link": "https://www.kayak.com/book/flight?code=...",
      "best_score": 9.4,
      "total_duration_minutes": 372,
      "stop_count": 0,
      "co2_emissions": {"label": "12% lower emissions", "delta_pct": -12, "kg_co2": null},
      "legs": [
        {
          "direction": "outbound",
          "flight_number": "UA 528",
          "marketing_airline": "United",
          "operating_airline": "United",
          "aircraft": "Boeing 777-200",
          "origin": {"iata": "SFO", "name": "San Francisco Intl", "terminal": "3"},
          "destination": {"iata": "JFK", "name": "John F. Kennedy Intl", "terminal": "7"},
          "depart_local": "2026-08-15T07:15",
          "arrive_local": "2026-08-15T15:45",
          "segment_duration_minutes": 330,
          "layover_after_minutes": 0,
          "fare_class": "Main Cabin",
          "baggage": {"carry_on_included": true, "checked_bag_included": false},
          "amenities": {"wifi": true, "power": true, "lie_flat": false, "live_tv": false}
        },
        {
          "direction": "return",
          "flight_number": "UA 633",
          "marketing_airline": "United",
          "operating_airline": "United",
          "aircraft": "Boeing 757-200",
          "origin": {"iata": "JFK", "name": "John F. Kennedy Intl", "terminal": "7"},
          "destination": {"iata": "SFO", "name": "San Francisco Intl", "terminal": "3"},
          "depart_local": "2026-08-22T18:30",
          "arrive_local": "2026-08-22T22:12",
          "segment_duration_minutes": 402,
          "layover_after_minutes": 0,
          "fare_class": "Main Cabin",
          "baggage": {"carry_on_included": true, "checked_bag_included": false},
          "amenities": {"wifi": true, "power": true, "lie_flat": false, "live_tv": false}
        }
      ]
    }
  ]
}

// Search returned zero results
{
  "success": true,
  "search": { /* ...same shape... */ },
  "filters_applied": { /* ... */ },
  "result_count_total": 0,
  "result_count_returned": 0,
  "active_filter_chips": ["Nonstop", "Under $300"],
  "price_predict": null,
  "itineraries": []
}

// reCAPTCHA Enterprise wall encountered
{
  "success": false,
  "error_reasoning": "captcha_wall",
  "wall_type": "recaptcha_enterprise",
  "sitekey": "6LeueuEeAAAAAOMbwQteKH2r6w5zMZa_SqyPhUjk",
  "screenshot_path": "screenshots/03-recaptcha-wall.png",
  "search": { /* echoed back so caller can retry */ }
}

// Airport name could not be resolved to an IATA code
{
  "success": false,
  "error_reasoning": "airport_not_resolved",
  "unresolved_query": "Saint-Pierre",
  "search": { /* echoed back */ }
}