test: accept native Vertex malformed-function terminal with empty content

[jpr5]

Problem

The Drift Tests run 38104122278 on main @ b9cff07 failed the live Vertex case k5-object (expected [ 'TARGET_COMPARED', 'NOT_TRIGGERED' ] to include 'FAILURE'). Because of that, k5-stream, k9-object and k9-stream were not attempted (earlier-case-failed).

Vertex returned HTTP 200 from gemini-2.5-flash:

"candidates":[{"content":{},"finishReason":"MALFORMED_FUNCTION_CALL","finishMessage":"Malformed function call: print(default_api.archive(cities=['Tokyo"}]

The comparator already approves a MALFORMED_FUNCTION_CALL terminal as NOT_TRIGGERED (reason malformed-function-terminal). The retained k5-stream capture passes this way. The difference is the shape: in object mode Vertex sent an empty content: {} object, while the stream capture has no content key. Any content present had to have role: "model" plus a parts array, so the check threw invalid content. This is a deterministic shape-handling bug in the test comparator, not a new upstream behavior class.

Change (test-only; no src/ change)

src/__tests__/drift/vertex-misbehavior.drift.ts:

• On the native leg only, when the candidate's finishReason is MALFORMED_FUNCTION_CALL, an empty content object (no keys) is treated the same as absent content.
• This covers object and stream mode on purpose: an empty content means no content in either mode.
• Any other content still needs role: "model" plus a parts array. The modeled (aimock) leg stays strict.
• The CI capture is retained byte-exact as a fixture with its sha256 (4c38ba91…477eb), with the same request hash as the existing k5 captures.
• withVertexSequence takes an optional list of served captures, so the recurrence test can serve the CI capture first.

Local red/green

Real surface: the captured raw bytes are replayed through the actual @google/genai SDK over an owned localhost HTTP server, then through compareVertexObservation. Separately, the CI capture runs through runVertexRecurrence, the live path that failed in run 38104122278, against a localhost origin.

RED (new tests, with the comparator's content check restored to the b9cff07 line):

× ... classifies the captured empty-content object as an approved non-trigger      AssertionError: invalid content
× ... rejects the captured malformed-function terminal without a finish message    (got 'invalid content' before reaching the terminal check)
× ... accepts empty content with a malformed-function terminal in stream mode      AssertionError: invalid content
× Vertex recurrence continues past the captured empty-content malformed-function terminal
    → expected [ … ] to have a length of 4 but got 1   (k5-object FAILURE; later cases earlier-case-failed)
Tests  4 failed | 6 passed | 72 skipped (82)

GREEN (after fix):

✓ Vertex retained native dispositions > 'k5-object' | 'k5-stream' | 'k9-object' | 'k9-stream' preserve genuine wire bytes and approved non-trigger
✓ ... classifies the captured empty-content object as an approved non-trigger     (NOT_TRIGGERED / malformed-function-terminal)
✓ ... accepts empty content with a malformed-function terminal in stream mode      (NOT_TRIGGERED / malformed-function-terminal)
✓ ... rejects the same empty content on the modeled leg
✓ ... rejects malformed-function content with 'non-model role'   ({role:"user"})
✓ ... rejects malformed-function content with 'non-array parts'  ({role:"model", parts:"x"})
✓ ... rejects malformed-function content with 'missing parts'    ({role:"model"})
✓ ... rejects empty content with a STOP terminal
✓ ... rejects empty content with a MAX_TOKENS terminal
✓ ... rejects the captured malformed-function terminal without a finish message   (invalid malformed-function terminal)
✓ Vertex recurrence continues past the captured empty-content malformed-function terminal
    (4 requests; all four NOT_TRIGGERED; no earlier-case-failed)
Whole file: Tests  78 passed | 4 skipped (82)   (the 4 skipped are the live-gated cases)

Mutation check: I replaced each guard clause with true, one at a time, and each mutant was caught:

• dropping origin === "native" fails the modeled-leg test
• dropping the MALFORMED_FUNCTION_CALL check fails the STOP and MAX_TOKENS tests
• dropping the empty-object check fails the role and parts tests
• dropping the existing Array.isArray(content.parts) check fails the 'non-array parts' and 'missing parts' tests

Live verification: not run locally

I could not run Vertex live locally. CI gets its token through GitHub workload identity for the main schedule/dispatch runs only. The local gcloud credentials need interactive re-authentication, and no Vertex credential is in the local secret store. The live confirmation is a manual Drift Tests run on main after merge.

Gates

pnpm typecheck, pnpm lint, pnpm format:check, pnpm build: pass. pnpm test: 377 files / 14073 passed, 3 skipped.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@copilotkit/aimock@524

commit: 1dab38b

[jpr5]

Merging with admin override over drift-live-pr. Its only failure is cohere.drift.ts:688 AssertionError: expected object, which also fails in the job's base-main leg, so it happens on main without this PR. This PR changes only vertex-misbehavior.drift.ts, and Vertex doesn't run live on PR legs. All other checks pass. The live Vertex confirmation is the post-merge Drift Tests run on main.