[backryun]

Part of #8484. Type-only, no toolchain changes. First slice into open-sse/handlers — see the scope note at the bottom, because that area turned out not to be what the tracking issue said it was.

One untyped parameter, 12 diagnostics

export function transformResponseFromProvider(providerConfig, data, options = {}) {

No annotation, so TS infers options as {} from the default value. Every read of it fails:

rerank.ts: TS2339: Property 'top_n' does not exist on type '{}'.              x6
           TS2339: Property 'documents' does not exist on type '{}'.          x4
           TS2339: Property 'return_documents' does not exist on type '{}'.   x2

Six of those are top_n alone, because the DeepInfra and Voyage branches each spell it three times in one expression.

Fix

Declared the shape the JSDoc on handleRerank already documents:

type RerankDocument = string | { text?: string };

interface RerankResponseOptions {
  documents?: RerankDocument[];
  return_documents?: boolean;
  top_n?: number;
}

All fields optional — the parameter defaults to {}, and the existing suites call it with subsets ({ documents: ["a", "b"] }).

RerankDocument is a union rather than string[] because the Cohere-compatible API accepts both forms and both adapters already branch on it:

const text = typeof doc === "string" ? doc : doc?.text || "";

providerConfig and data are left unannotated. Only options produced errors — it is the one parameter with a default value, which is what handed TS the {} — so only options is touched.

Verification

280 → 268, zero new errors, on a line-number-agnostic diff of the full tsc error set (with the worktree path normalized out — TS embeds absolute paths in some messages, which otherwise reads as phantom regressions).

• npm run typecheck:core — clean
• eslint on both changed files — clean
• 43/43 across the 6 affected suites (rerank, rerank-providers-5332, rerank-voyage-7809, rerank-openrouter-6574, rerank-proxy-pinning-7350, media-cost-headers-handlers)

Tests — the union had a real hole

Existing coverage of these three options is genuinely good: top_n truncation, descending sort, return_documents: false omitting text, and Voyage's index remapping are all asserted on both adapters. My first read was that there was nothing worth adding.

That was wrong, and the distinction matters. The { text } document form is tested — but only through the request adapter:

• #5332 deepinfra request adapter → {queries,documents} (string + {text})
• #7809 voyage request adapter handles {text} object documents

Every response-adapter test passes plain strings. So typeof doc === "string" ? doc : doc?.text on the response side had no coverage — and that branch is precisely what makes RerankDocument a union rather than string. Nothing would have caught a later "simplification" to documents?: string[].

tests/unit/rerank-object-documents-response-path.test.ts, 5 tests:

test	pins
deepinfra, { text } documents	text resolved from objects, ranking order preserved
deepinfra, mixed array	["plain string", { text: "object form" }] both resolve
deepinfra, {} with no text	falls back to "", never undefined
voyage, { text } documents	same through the second adapter
voyage, empty { text: "" }	filtered index 1 maps back to original index 2

As with the other slices in this campaign these are preservation guards, not TDD red/green — there is no bug. I verified that by reverting rerank.ts to the parent commit and re-running: 5/5 pass there too.

Scope note on open-sse/handlers

#8484 previously described this area as "121 diagnostics — chatCore/stream, standalone, last, heaviest validation". Measured, that is wrong: only 39 are in chatCore.ts. The other 82 are peripheral handlers with no dependency on it — rerank.ts (12), designerWeb.ts (10), audioTranscription.ts (8), responsesHandler.ts (5), and so on. They are sliceable now rather than blocked behind the chatCore work.

They do not share a root cause with each other, though — audioTranscription is mostly TS2769 overloads, imageGeneration is TS2554 arity plus a Buffer → BlobPart mismatch. rerank.ts is the only clean single-cause cluster in the group, which is why it goes first. The rest slice per-file. Issue body updated with the breakdown.

[diegosouzapw]

Thanks @backryun — merged into release/v3.8.49 via the local merge-train (validated as one combined tree: full test:unit + test:vitest 274/274 on the 32-core box, tip d4b9ce6016). Your commit keeps its authorship. 🚀