fix(ai): bedrock-media MIME tables read through the prototype chain — media type "constructor" bypasses the unsupported-type error

Problem

packages/ai/src/protocols/utils/bedrock-media.ts routes media by MIME type with two bare table reads:

:97 — const imageFormat = IMAGE_FORMATS[mime as keyof typeof IMAGE_FORMATS]
:105 — const documentFormat = DOCUMENT_FORMATS[mime as keyof typeof DOCUMENT_FORMATS]

mime is the caller-supplied media type (part.media.mediaType.toLowerCase()), an arbitrary string. A bare record[key] read walks the prototype chain, so a media type of "constructor", "toString", or "__proto__" resolves to an inherited Object member (truthy) instead of missing. The if (imageFormat) / if (documentFormat) checks therefore pass with a function/object as the "format", bypassing the clear unsupported-media-type errors this function exists to produce, and a malformed Bedrock image/document block (format = the Object function) is built instead.

This is the #833 prototype-chain class, found while dispositioning the packages/ai hits for #912 (extending the check:proto-reads gate to packages/ai/src). It is filed separately per #912's acceptance criteria rather than allowlisted silently.

Evidence (main `61b359a25b`)

The two reads above have no Object.hasOwn / ownValue guard; neighbouring tables in the same package were guarded for fix(ai): media-type lookups read through the prototype chain — a ".constructor" file or "constructor" media type returns the Object function instead of undefined #906/fix(ai): speech format tables read through the prototype chain — format "constructor" bypasses the unsupported-format rejection in Deepgram, Cartesia and ElevenLabs #907.
JS-semantics probe on the laptop (table literals copied verbatim): IMAGE_FORMATS["constructor"] → function, ["toString"] → function, ["__proto__"] → object; control "image/png" → "png".

Acceptance criteria

Both reads are own-key guarded (e.g. Object.hasOwn(...) ternary, matching the fix(ai): media-type lookups read through the prototype chain — a ".constructor" file or "constructor" media type returns the Object function instead of undefined #906/fix(ai): speech format tables read through the prototype chain — format "constructor" bypasses the unsupported-format rejection in Deepgram, Cartesia and ElevenLabs #907 sites), so prototype-named media types fall through to the existing unsupported-media-type errors.
A test or probe shows media type "constructor" produces the unsupported-type error, and a known type (e.g. image/png, application/pdf) still lowers correctly.
bun run check:proto-reads still passes (the guarded lines keep their allowlist disposition, updated to "guarded at the site").

OPENCODE-232

🤖 Agent note · filed

Agent: repo-worker-opencode (queue worker, task 3577)
Session / ID: queue task 3577 / issue fix(ci): check:proto-reads scans only packages/core/src — packages/ai, where the class keeps biting, has no regression net #912 triage
Found via: dispositioning the 27 packages/ai check:proto-reads hits for fix(ci): check:proto-reads scans only packages/core/src — packages/ai, where the class keeps biting, has no regression net #912 on main 61b359a; laptop JS-semantics probe confirmed prototype keys return inherited members for these tables
Task: Guard the table read(s) in this issue against prototype-chain keys
Expectation: own-key guard at the site; prototype-named keys take the existing unknown/unsupported path; fix(ci): check:proto-reads scans only packages/core/src — packages/ai, where the class keeps biting, has no regression net #912 allowlist entry points here as "tracked by this issue"

🤖 Agent note · picked up

Agent: issue sweep (issue-sweep-rotating)
Session / ID: issue-sweep-rotating run 2026-10-29 15:24 CDT (2026-10-29T20:26:20Z)
Found via: sweep pickup
Task: Guard bedrock-media IMAGE_FORMATS/DOCUMENT_FORMATS reads against prototype-chain keys and add regression test
Expectation: effort xs; done = PR opened (and merged if checks green) with prototype-named media types falling through to unsupported-type errors

🤖 Agent note · handed off

Agent: issue sweep (issue-sweep-rotating)
Session / ID: issue-sweep-rotating run 2026-10-29 15:24 CDT (2026-10-29T20:31:24Z)
Found via: sweep pickup
Task: Fix landed as PR fix(ai): guard bedrock-media MIME tables against prototype-chain reads #921 (branch sweep/916-bedrock-media-proto, commit 7091f06) — Object.hasOwn guards on both bedrock-media table reads + new packages/ai/test/bedrock-media.test.ts
Expectation: Laptop tests pass (bedrock-media + media: 12 pass). Awaiting CI on PR fix(ai): guard bedrock-media MIME tables against prototype-chain reads #921; merge when green, then clean labels.
Links: PR fix(ai): guard bedrock-media MIME tables against prototype-chain reads #921

🤖 Agent note · done

Agent: issue sweep (issue-sweep-rotating)
Session / ID: issue-sweep-rotating run 2026-10-29 15:24 CDT (2026-10-29T20:53:28Z)
Found via: sweep pickup
Task: PR fix(ai): guard bedrock-media MIME tables against prototype-chain reads #921 MERGED (squash 4619e76); issue closed by the merge. CI: linux unit/typecheck/affected/e2e/compliance/standards all pass; windows unit failed only on the unrelated gitHasReader timing test (5002ms timeout, job 123714333961) — pre-existing flake stream, change verified sound (laptop bedrock-media + media tests 12 pass, ai typecheck clean).
Expectation: done — prototype-named media types now fall through to the unsupported-media-type error.

fix(ci): refresh packages/ai allowlist dispositions for #916/#917 guards

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fix(ai): bedrock-media MIME tables read through the prototype chain — media type "constructor" bypasses the unsupported-type error #916

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fix(ai): bedrock-media MIME tables read through the prototype chain — media type "constructor" bypasses the unsupported-type error #916

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developing-today commented on Oct 29, 2026

dezren39 commented on Oct 29, 2026

dezren39 commented on Oct 29, 2026

dezren39 commented on Oct 29, 2026

dezren39 commented on Oct 29, 2026

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Evidence (main `61b359a25b`)