[finding] rest: `GET /api/v1/data/:object/:id` refuses the `$expand` alias that the list route folds — residual of #8039

[os-sam]

Observation from objectstack-ai/ats#13 (PR objectstack-ai/ats#48), cli 17.3.0 (@objectstack/rest 17.3.0).

What

The single-record route rejects the $-prefixed spelling:

GET /api/v1/data/ats_application/ID?$expand=candidate
400 {"error":{"code":"VALIDATION_ERROR","message":"The \"$expand\" query parameter is not supported by this endpoint. This endpoint will not silently ignore a parameter it does not understand — an ignored filter is indistinguishable from one that matched everything. Supported parameters: expand, select."}}

GET /api/v1/data/ats_application/ID?expand=candidate
200 — candidate expanded

while the list route's query-alias table maps expand to $expand (and filter to $filter, top, skip, …), so GET /api/v1/data/:object?$expand=… is the spelling the list surface teaches.

Why it is only a finding

The refusal is loud and its message names the accepted spelling, so nothing fails silently — this is the good failure shape. What remains is that the two routes disagree on whether the $ alias family is folded. #8039 (closed) fixed the canonical fields spelling on this route; $expand is the same class one alias over.

Suggested resolution (not a ruling)

Fold the same alias table on GET /:object/:id that the list route uses, or state in the 400 text that the $-prefixed family is list-only. Either removes the disagreement.

[os-zhuang]

Triage: closing not planned — outside the three admission classes (#16351, as refined by PR #16557), on the card's own section heading: "Why it is only a finding."

Against each class:

• (a) reproducible defect — ⛔ none. ⭐ "The refusal is loud and its message names the accepted spelling, so nothing fails silently — this is the good failure shape." The 400 tells the caller exactly what to write.
• (b) violation of a declared contract — ⛔ none, and the endpoint is in fact honouring a declared policy, quoted in its own error text: "This endpoint will not silently ignore a parameter it does not understand — an ignored filter is indistinguishable from one that matched everything." ⇒ It refuses rather than pretends. Nothing declares that the two routes must fold the same alias family.
• (c) metadata trap — ⛔ no. A URL query parameter is constructed and consumed in one breath; it is not 「由写它的人以外的人存储并再作者化的键」. ⇒ Same side of that boundary as objectui#8120 (QueryParams.$count), closed today on the same reasoning.

⚠️ The strongest argument against this close, stated because it is real: a named consumer hit it in a live repo (objectstack-ai/ats#13, PR ats#48, cli 17.3.0 / @objectstack/rest 17.3.0), and they hit it for a good reason — $expand is the spelling the list surface teaches, since the list route's alias table maps expand → $expand alongside filter/top/skip. ⇒ Learning the API from one route and applying it to its sibling is exactly what a consumer should be able to do. ⛔ But the cost of the disagreement is one refused request with a corrective message, which is the failure shape this repository deliberately buys.

承接者: ⛔ none. #8039 (closed) fixed the canonical fields spelling on this same route — so the precedent exists — but nothing open opens GET /:object/:id's parameter handling.

What I am uncertain about: whether the disagreement is deliberate. ⛔ Nobody has established that the $-prefixed family is meant to be list-only, and if it is, the second remedy below is not a workaround but the correct documentation of a real boundary.

⭐ Free rider, available to anyone touching that route's error path and needing no ruling: the card's second option — state in the 400 text that the $-prefixed family is list-only. One sentence, removes the disagreement from the consumer's point of view without changing any accept set, and it is strictly better than the current message, which lists the supported parameters but does not say why the sibling route's spelling is absent.

Reopen conditions (either, no approval needed):

1. A second consumer hits it, in this repo or a downstream one. Two independent reports make it a papercut with a rate rather than one confused request.
2. The $-prefix alias family is ruled list-only or route-uniform. Either ruling makes one of the two remedies correct rather than optional.

Reversibility: fully reversible; ⛔ nothing decays. Both spellings behave identically tomorrow, the working spelling (expand) keeps working, and the refusal keeps naming it.

⭐ Worth recording as the good shape: this is what an endpoint refusing an unknown parameter is supposed to look like — loud, specific, and naming the fix. ⛔ The finding is the inconsistency, ⛔ not the refusal.

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