Contract-Test LLM Endpoints Before a Model Migration

Changing an LLM endpoint is often treated like changing a configuration value. The HTTP request may remain valid, but the product behavior can still change.
A candidate might omit required fields, format tool arguments differently, weaken citations, or respond confidently when the workflow expects escalation.
That makes model migration a testing problem.
Start with a behavior contract
A useful contract describes observable product requirements rather than provider-specific language:
Test fixtures
↓
Request adapter
↓
Candidate endpoints
↓
Output normalizer
↓
Contract assertions
↓
Review report
Each fixture should contain an input, required output properties, and conditions that require human review.
Teams assembling candidate endpoints for this workflow can include VectorEngine in their evaluation set while keeping the test contract independent of the endpoint.
Disclosure: this article includes an external referral link for readers who want to explore the platform.
A lightweight TypeScript-style example
type ContractCase = {
input: string;
requiredFields: string[];
expectsCitation: boolean;
allowedTools: string[];
};

function validate(result: any, testCase: ContractCase) {
const missingFields = testCase.requiredFields.filter(
(field) => result[field] === undefined
);

return {
schemaPassed: missingFields.length === 0,
citationPassed:
!testCase.expectsCitation || result.citations?.length > 0,
toolPassed:
!result.tool || testCase.allowedTools.includes(result.tool.name),
missingFields,
};
}
The example checks only deterministic properties. Subjective requirements such as usefulness, tone, or factual support need evaluated datasets and, where appropriate, human review.
Treat failures as release evidence
Record failures by category:
schema mismatch;
invalid tool arguments;
unsupported citation;
unexpected refusal;
missing escalation;
material answer regression.
Do not hide these differences behind one aggregate score. A small number of critical failures may matter more than a higher average evaluation result.
Begin with ten to twenty representative fixtures from one important workflow. Expand the suite when production incidents or reviewer feedback reveal a new behavioral requirement.
A model change becomes easier to discuss when the team has a shared contract and a visible body of evidence.