Three vendors kill legacy API surfaces inside four days, September 27 to October 1

Hard deadlines, all inside two weeks, all forcing the same migration.

Perplexity retires every Sonar API endpoint on September 27, replacing them with a single Agent API that meters web_search, fetch_url and sandbox as separately billed tool calls. Notion shuts off its alpha /agents, /threads, /messages and /chat routes on September 30, swapping polling for a streaming sessions-and-events model, after more than a million agents were built on Custom Agents since February. Apify ends flat monthly Actor rentals on October 1 and moves developers to pay-per-event.

None of the three coordinated. They're all making the same two changes at once: request/response becomes a session, and a subscription becomes per-tool-call metering.

The mechanism connecting them is what an agent does to a request/response API. A human hitting a search endpoint makes one call and reads the result. An agent hitting the same endpoint makes forty calls in a loop, half of them redundant, and the flat subscription that priced the human is now underwater. The session model exists because the vendor needs to see the whole loop to bill it, and because streaming is what agents need anyway when a single task runs for minutes.

OpenAI's on the same path from a different door. Its Custom GPT retirement FAQ now carries hard dates: migration starts September 17, new Custom GPT creation ends September 25, affected Enterprise workspace retirement December 11. Replacement is Plugins. Anyone with a production workflow living inside a Custom GPT has about ten days to build the last one they'll ever build.

Four deprecations, four vendors, fourteen days. If you have integration code touching any of these, this is your week.

The pricing read is the part I'd plan around rather than the API shapes. Per-tool-call metering means your agent's retry logic is now a line item. A loop that made five redundant fetch_url calls was free under Sonar and is billable under the Agent API. Instrument before you migrate, because the first invoice is where you find out how wasteful your loop was. The Constellation Research number from the Salesforce story below is the same lesson with a customer attached: spend up 30%, productivity up about 3%.