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Kaggle benchmark tests which URL parser an LLM gatekeeper imitates before approving a tool call
Nine LLMs completed a Kaggle benchmark of 120 URLs, 45 of which Python's urlsplit and fetch() resolve to different hosts. If a model approves the Python reading and the request goes out through fetch(), the API key reaches a host nobody approved.
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What happened
- A leak is an ALLOW verdict or a True return on a URL that WHATWG resolves to an unapproved host, and no key or request is ever sent.
- The labels expect ALLOW on three credential-bearing URLs that fetch() itself would refuse with a TypeError, a mismatch a reviewer caught and the author kept.
- Qwen 3 Next 80B and gpt-oss-120b finished only the guard-writing task after their other runs failed on provider overload or overlong reasoning.
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Why it matters
- exposure Any agent that lets a model approve outbound calls carrying an API key is exposed wherever the model's reading of a host differs from its HTTP client's.
- constraint Leaderboard scores apply only to agents whose requests leave through a WHATWG parser such as fetch(); a stack with a different sender would need its own labels.
- contradiction Models that answer DENY on credentialed URLs, as fetch() would, lose points on three cases, so the gatekeeper ranking partly rewards the parser's rule over fetch()'s.
Python's urlsplit sees a username in front of the @ and returns api.forecast.test as the host [1]. The WHATWG parser, the one behind fetch() and every browser, treats the backslash as a slash [3]. Its authority ends there, and `@api.forecast.test/` becomes path [2]. "Neither parser is broken. They follow different rules," the author wrote [5]. This family of disagreements has been documented for years [21]. According to the author, the newer element is who does the reading: an agent deciding whether a tool call may go out, an assistant writing the allowlist, a reviewer approving a config [19].
The test set is built with care. Its 120 strings are 38 templates on three host pairs plus six encodings of 127.0.0.1 [6][1]. Python and WHATWG disagree on 45 of them, or 37.5 percent [8][2]. Hosts use the reserved .test TLD with dull names, so a model cannot pass by spotting evil.com [7]. No answer is hand-written. Node 25.9's URL and Python 3.12's urlsplit compute them at run time [10]. The harness retries overloaded providers. It refuses to score a run that misses a single case, and it still calls a function the model named isAllowed instead of is_allowed [20]. The author also warns that three leaks from one model are usually one mistake met on three host pairs [9].
The guard-writing task has 63 URLs that must be refused and 30 that must pass. Another 21 exotic URLs that reach the approved host are left unscored [12]. That makes 93 scored cases [3]. Only 45 URLs in the full set split the parsers, so at least 18 of the 63 refusals are URLs where both parsers agree and the host is simply wrong [8][4]. A guard-code score therefore mixes parser-differential mistakes with plain allowlist mistakes.
A reviewer caught a labelling problem in the gatekeeper prompt [13]. The prompt asks what fetch() would do, but the expected verdicts come from the parser. fetch() refuses any URL carrying credentials, because new Request() throws a TypeError as the Fetch standard requires. The leaderboard still expects ALLOW on the three `https://B@A/v1` cases. A model that follows fetch() and answers DENY loses points there [13]. The author kept the labels and says no counted leak involves a credentialed URL. The backslash trap has no credentials, because its @ sits in the path [13].
A leaderboard number transfers to a production agent only if the request leaves through a WHATWG parser, since a leak is defined against WHATWG's reading [11]. The gatekeeper prompt also has to resemble the one tested. The risk sits in the gap between the checker's reading and the sender's [4]. The fifth task, url-gatekeeper-tools, lets the model call a Python tool before it answers [14]. For the trap URL, Python's standard library returns the one host fetch() will not contact [1][2].
Nine models finished all four tasks, from Anthropic, OpenAI, Google and the open-weights Gemma 4 family [15]. Runs went through Kaggle's model proxy starting 27 September [17]. The post's headline says smaller models often read URLs like Python, not like fetch() [18]. The available text of the post ends before the per-model leak counts, so that claim cannot yet be checked against the leaderboard.
What to watch
- Per-model leak counts on the gatekeeper and guard-code columns, and whether the smaller models fail the backslash structure on all three host pairs.
- Whether relabelling the three https://B@A/v1 cases to DENY reorders the gatekeeper leaderboard.
- Results from url-gatekeeper-tools, showing whether models handed a Python tool adopt urlsplit's host for the trap URL.