A User-Agent shape rule has one honest signal behind it. `Vendor/Language version` means a programmatic client, and the official SDKs are programmatic clients, because that pattern is what they send by default [4]. The rule did what it was written to do. What made it hard to see is where it sits: the 403 is returned at the edge, before the request reaches the application at all [7], and browser traffic never carried the matching shape, so the audience most likely to complain in words was never blocked [6].
The suite itself is decent engineering and worth saying so. Rather than installing nine GUI clients and clicking through them, the team reproduces the characteristic HTTP request each client sends, fires it at the real user-facing endpoint, and checks the response field by field [10]. Its assertions covered streaming frames, tool-call assembly, `response_format`, and the Anthropic event sequence [11]. Each of those answers whether the server understands the request rather than whether the request is admitted [11]. The author's own diagnosis is the useful one: the test client's identity happened to be the safe one, so the suite kept passing for a reason that had nothing to do with test coverage [13]. The safest identity in the stack turned out to be curl's default, which nobody picked on purpose [20].
The check that finds it is two requests with the same body, path and key, differing only in the User-Agent; if the statuses disagree, something in front of the application is filtering on UA [14]. Count the matrix as described and it is small: one curl baseline against the API and one against a static asset, plus the three SDK defaults and the five crawler strings, is ten requests [22]. The crawler half runs against a static asset rather than the API [16], which is the right call, because the thing being tested is reachability of the file, not the endpoint.
Two caveats on transferring the numbers. The author offers their own `/llms.txt` answering 200 to all five crawler UAs as of the retest date, 2026-08-31, as a live target for validating a harness [17][18]. That is a statement about one edge configuration on one day, and it proves exactly one thing about your setup: that your harness sends the header you think it sends before you point it at your own host. Second, keep the real-SDK confirmation step, because a hand-typed `OpenAI/Python 1.68.2` is a guess pinned to a version, and the installed client moves without asking you [15][5]. Match the block on its signature too: status 403 plus a `server: cloudflare` header [19].
Duration is the one number missing from the post. It says the rule ran that way for a while, with the suite green throughout [21][2], which is the sort of gap that only exists because nothing downstream of the edge had anything to record.
