Steam's Web API counts players by AppID and a Steam key. Trends API counts them by the name on the store page. source steam on get_growth or get_time_series takes that display name. The first Steam store search result wins, so the string must be unambiguous. The live list is type Steam Most Played. No Steam key. body is a JSON string and must be parsed a second time. Docs: the API reference. Steam notes: the Steam guide. Caps: pricing.
ISteamUserStats needs an AppID
Valve's Web API is the right tool for a known AppID, achievements, or a user profile. It is the wrong tool when the warehouse key is a game title sitting next to YouTube hype on one 0-100 scale. Do not put a Steam Web API key in the Trends API header.
source steam wants the store display name
POST https://api.trendsapi.ai/api
Authorization: Bearer <api_key>
Content-Type: application/json
{"mode":"get_growth","source":"steam","keyword":"Elden Ring","percent_growth":["3M"]}
A live pull on 2026-08-27 for Elden Ring on steam returned 3M growth of -9.52 percent (1.9 vs 2.1) with recent_volume 46694 against baseline_volume 51362. recent_date on that row was 2026-07-01. CS2 is the documented short form when the long Counter-Strike title is ambiguous.
First store result wins
If the name matches two store pages, the first hit is the series. Add a distinguishing word. Do not append (AppID) in parentheses and hope. If the inner payload is empty, the search missed. Change the string. Do not retry with a number.
Steam Most Played is the live board
{"mode":"get_top_trends","type":"Steam Most Played","limit":25}
No keyword. This is who is on the most-played list now. It is not the history for Elden Ring. sort rank_change is the climber view. Keep the board and the named series in different jobs.