test: add Gemini thoughtSignature metadata preservation tests

[peterkc]

Context

While investigating #6018 (Gemini 3.1 Pro thought_signature errors on v5.8.0), I traced the full message pipeline to understand where thoughtSignature metadata could be lost. The pipeline works correctly on current main — the v5.9.0 fix ("preserve extra_content for Gemini 3 thought_signature support") resolves the issue.

However, the differentModel guard in message-v2.ts (line 600) — which strips provider-specific metadata when models differ — had zero test coverage for the positive case (same-model metadata preservation). These tests fill that gap to prevent regression.

Refs: #6018, diagnostic analysis

Implementation

No production code changes. Test-only PR adding regression coverage to message-v2.test.ts:

• Gemini model fixture (geminiModel): A Provider.Model matching the existing test mock pattern, with providerID: "google", id: "gemini-3.1-pro-preview", npm: "@ai-sdk/google"
• Metadata fixture (geminiThoughtSignature): { google: { thoughtSignature: "opaque-test-sig==" } } — the shape @ai-sdk/google emits via providerMetadata
• Test 1 — same-model tool call: Verifies callProviderMetadata with thoughtSignature is preserved on tool-call parts when differentModel evaluates false
• Test 2 — same-model reasoning: Verifies providerMetadata with thoughtSignature is preserved on reasoning parts

Both tests use the existing assistantInfo() helper with its meta parameter to set matching providerID/modelID, ensuring the differentModel guard evaluates correctly.

Data Flow Under Test

The differentModel guard at message-v2.ts:600 controls metadata flow:

Same model (differentModel = false) — Tests 1 & 2 verify this path:

  processor.ts        DB (ToolPart)       message-v2.ts        AI SDK            Gemini API
  providerMetadata -> part.metadata  ->  callProviderMetadata -> providerOptions -> thoughtSignature
  { google:           stored as-is       preserved by guard      maps to            on functionCall
    thoughtSignature                                              google.            part
    : "opaque..." }                                               thoughtSignature

Model switch (differentModel = true) — Test 3 verifies this path:

  processor.ts        DB (ToolPart)       message-v2.ts        AI SDK
  providerMetadata -> part.metadata  ->  (stripped)            (nothing sent)
                                         guard correctly        prevents cross-
                                         omits metadata         provider leakage

Test Matrix

#	Scenario	Model	differentModel	Expected	Status
1	Same-model tool call	google/gemini-3.1-pro-preview	false	callProviderMetadata preserved	New
2	Same-model reasoning	google/gemini-3.1-pro-preview	false	providerMetadata preserved	New
3	Cross-model switch	test/test-model vs other/other	true	metadata stripped	Existing (line 358)
4	Same-model text	google/gemini-3.1-pro-preview	false	providerMetadata preserved	Future
5	Sequential tool calls	google/gemini-3.1-pro-preview	false	each signature preserved	Future
6	Error/pending parts	google/gemini-3.1-pro-preview	false	callProviderMetadata preserved	Future

Rows 1-3 are covered by this PR. Rows 4-6 are natural follow-ups if the team wants broader coverage.

How to Test

cd packages/opencode && bun test test/session/message-v2.test.ts

• 21/21 pass (19 existing + 2 new)
• Existing differentModel regression test (line 358) continues to pass
• Typecheck passes (10/10 packages)

Additionally verified via E2E test against live gemini-3.1-pro-preview API — thoughtSignature flows through all streaming events (tool-input-start, tool-call, tool-result) and appears in response message providerOptions.

Get in Touch

Happy to iterate on these tests or expand coverage (rows 4-6 in the matrix). Available here on GitHub.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (1 files)

• packages/opencode/test/session/message-v2.test.ts