Behaviour contract

What the behaviour contract includes

A behaviour contract is broader than a single prompt. It bundles the runtime inputs and controls that collectively shape model behaviour, so changes to any one element can alter outputs, tool use, or safety boundaries. That makes it a configuration object as much as a conversational one, and it should be treated as a governed unit rather than a loose collection of settings.

In practice, the contract may include instructions, model selection, decoding parameters, allowed tools, output rules, and rollout metadata. Those parts are interdependent: a safe prompt can behave differently under a different model, and a narrow output constraint can be undermined by a more permissive tool assumption.

Why it matters for runtime behaviour

The main value of the concept is that it captures the full behaviour-shaping surface, not just the text the user sees. If teams only version the prompt, they may miss a materially different runtime state caused by a model swap, a parameter tweak, or a newly enabled tool. The contract gives practitioners a way to reason about behaviour as an assembled system property.

This is especially important when behaviour changes have business, safety, or compliance implications. A contract-based view makes it easier to compare versions, explain why outputs changed, and separate intentional policy shifts from accidental drift.

How behaviour contracts fail in practice

Behaviour contracts fail when parts of the runtime state change without coordinated review. Common failure modes include prompt overrides, stale tool assumptions, silent model updates, and parameter drift that changes creativity, refusal behaviour, or consistency. The result is often unexpected output variation that looks like model instability but is actually configuration inconsistency.

Another risk is boundary mismatch. A system may be designed with one set of constraints, then deployed with different tool access, different rollout metadata, or different evaluation assumptions. In that case, the contract no longer matches the actual operating conditions, so the system can behave outside the intended envelope.

How to manage it as a control surface

The safest way to manage a behaviour contract is to treat it as a versioned, testable release artifact. The prompt, model, parameters, tool permissions, and rollout settings should be reviewed together, because each one can change the final behaviour. That also means behaviour changes should be assessed in terms of the complete contract, not only the visible instruction text.

Teams should also tie contract changes to explicit ownership and approval, especially where tool access or output constraints affect production decisions. A clear contract makes it easier to compare intended behaviour against observed behaviour, and to decide whether a change is an acceptable policy update or an operational regression.

Risk and Threat Considerations

Behaviour contracts can create exposure when a system’s effective runtime behaviour changes without equivalent governance, review, or testing. The risk is not just incorrect output, but unsafe tool use, policy bypass, or broader trust in a release that no longer matches its approved operating conditions.

Failure mechanism: A malicious or accidental change to the prompt, model, parameters, tool assumptions, or rollout metadata can widen the action surface or weaken output constraints, allowing behaviour that was never intended or validated.

Impact: The system may produce misleading, non-compliant, or unsafe actions at scale, and post-incident review becomes harder because the effective behaviour was altered by several interacting controls rather than a single obvious defect.

Practitioner Guidance

What to watch for: Treat any change to the prompt, model, parameters, tools, or rollout settings as a contract change, not a minor tuning adjustment. The practical mistake is assuming that only the prompt deserves review, when the runtime contract is what actually governs behaviour.

Governance implication: Assign one owner for the full contract and require version comparison across all behaviour-shaping inputs before release. That keeps evaluation aligned to the actual operating state instead of a partial representation of it.

Related resources from NHI Mgmt Group

• What happens when smart contract logic changes are made without covering the affected behaviour?
• Behaviour Contract Drift
• What is NHI behaviour monitoring and what does it detect?
• What breaks when agent behaviour drifts beyond approved scope?