discovery.md: what should a client do when the card fetch returns a 429 from the recommended rate limit?
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- Más de una semana
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- Documentación
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- Bastante claro
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- documentation
Línea de trabajo
Read docs/discovery.md, especially the client-flow and Denial of Service sections, then compare them with the SEP text at the current PR head. Determine and document the client behavior for a card-fetch 429, Retry-After, other error responses, and possible inline catalog data, with the client flow's failure outcomes updated to match the decision.
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Descripción
One question on the discovery mechanics, from an operator's side.
docs/discovery.md reserves <streamable-http-url>/server-card and notes the resulting path-namespacing as a benefit (a server at https://host/mcp yields https://host/mcp/server-card). Under Denial of Service — the guidance restored in #21 — it recommends that servers rate-limit the card endpoint, and asks clients to respect Cache-Control and use ETags. The client flow has a single negative outcome, "Discovery unavailable for this domain", and it hangs off the catalog fetch (/.well-known/ai-catalog.json); the later step "Fetch Server Card from url" has no failure branch at all.
Why this is structural rather than hypothetical: a limit protecting an MCP endpoint is normally scoped by path prefix and placed ahead of application code. In my deployment, anonymous requests under the API prefix hit a per-client bucket in middleware before the application initializes and before any database query; authenticated requests are exempt elsewhere, but not under the MCP path itself, because resolving a bearer key there costs an HMAC and an indexed query per attempt. A card served under that prefix shares that bucket with all other anonymous traffic from the same client address by default — unless the operator carves it out, which the document does not tell them to do. A single first GET will not exhaust it, but a crawler fleet, a shared corporate egress, or a client that already made requests under the prefix can arrive with the bucket spent — and first contact is the one moment it holds neither a cache entry nor an ETag, so the document's mitigation is not yet available. A 403 from a bot filter, or a challenge page returned with 200, collapses the same way.
For comparison, RFC 9309 §2.3.1.3/§2.3.1.4 separates "unavailable" (4xx) from "unreachable" (5xx/network) for robots.txt, and Google's crawler documentation carves 429 out of its 4xx rule (https://developers.google.com/search/docs/crawling-indexing/robots/robots_txt#http-status-codes). In the SEP text at the current PR head and in docs/discovery.md including its client-flow section, I find no mention of 429 or Retry-After; the SEP text repeats the recommendation ("Servers SHOULD rate-limit discovery endpoints") without naming a status code either.
So, one question: what should a client do when the card fetch — not the catalog — returns a 429 produced by the rate limiting the same document recommends: treat the entry as unavailable, retry after Retry-After, or fall back to inline data in the catalog?
Florian Berger — Bolzano/Bozen (IT)
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