Groundedness check validates `evidence_refs` against the *untrimmed* evidence object, not the trimmed evidence actually sent to the model

[tarunkumartiwary1]

Summary

Same defect as packages/llm/llm-anthropic/src/anthropic-adapter.ts and packages/llm/llm-openai/src/openai-adapter.ts (filed separately), present in the Gemini adapter:

const contextLimit = getContextLimit(this.provider, this.model);
const { evidence: trimmedEvidence, trim } = trimEvidenceForContext(evidence, contextLimit);

const response = await withRetry(() =>
  this.client.models.generateContent({
    model: this.model,
    contents: buildUserPrompt(trimmedEvidence, question),   // model sees trimmedEvidence
    ...
  }),
);

...

const grounding = validateEvidenceGrounding(evidence, evidenceRefs);
//                                           ^^^^^^^^ should be trimmedEvidence

Why this matters

The prompt sent to Gemini (contents: buildUserPrompt(trimmedEvidence, question)) is built from the trimmed evidence object, which trimEvidenceForContext (packages/llm/llm-core/src/context.ts) may have shrunk by dropping evidence items, what_changed entries, or similar-incident entries to stay under the model's context budget. But the hallucination check afterward, validateEvidenceGrounding(evidence, evidenceRefs), is run against the original, untrimmed evidence.

Because validateEvidenceGrounding (packages/llm/llm-core/src/grounding.ts) only checks id membership against evidence.evidence, and trimmedEvidence.evidence is a subset of that, an evidence_refs entry that references an evidence item the model was never actually shown (because it was trimmed) can still be marked as "grounded". This defeats the purpose of grounded_evidence_refs / ungrounded_evidence_refs / groundedness_warning, exactly in the large-trace case where trimming (and therefore the risk of the model's answer drifting from what it was shown) is most likely.

Suggested fix

const grounding = validateEvidenceGrounding(trimmedEvidence, evidenceRefs);

Files:

packages/llm/llm-gemini/src/gemini-adapter.ts