Behavioral Contracts (Pillar 2)

What it is: A contract is a named set of invariants (rules the agent must always follow). Flakestorm runs your agent under each scenario in a chaos matrix and checks every invariant in every scenario. The result is a resilience score (0–100%) and a pass/fail matrix.

Why it matters: You need to know that the agent still obeys its rules when tools fail, the LLM is degraded, or context is poisoned — not just on the happy path.

Question answered: Does the agent obey its rules when the world breaks?

When to use it

You have hard rules: “always cite a source”, “never return PII”, “never fabricate numbers when tools fail”.
You want a single resilience score for CI that reflects behavior across multiple failure modes.
You run flakestorm contract run for contract-only checks, or flakestorm ci to include contract in the overall score.

Configuration

In flakestorm.yaml with version: "2.0" add contract and chaos_matrix:

contract:
  name: "Finance Agent Contract"
  description: "Invariants that must hold under all failure conditions"
  invariants:
    - id: always-cite-source
      type: regex
      pattern: "(?i)(source|according to|reference)"
      severity: critical
      when: always
      description: "Must always cite a data source"
    - id: never-fabricate-when-tools-fail
      type: regex
      pattern: '\\$[\\d,]+\\.\\d{2}'
      negate: true
      severity: critical
      when: tool_faults_active
      description: "Must not return dollar figures when tools are failing"
    - id: max-latency
      type: latency
      max_ms: 60000
      severity: medium
      when: always
  chaos_matrix:
    - name: "no-chaos"
      tool_faults: []
      llm_faults: []
    - name: "search-tool-down"
      tool_faults:
        - tool: market_data_api
          mode: error
          error_code: 503
    - name: "llm-degraded"
      llm_faults:
        - mode: truncated_response
          max_tokens: 20

Invariant fields

Field	Required	Description
`id`	Yes	Unique identifier for this invariant.
`type`	Yes	Same as run invariants: `contains`, `regex`, `latency`, `valid_json`, `similarity`, `excludes_pii`, `refusal_check`, `completes`, `output_not_empty`, `contains_any`, etc.
`severity`	No	`critical` \| `high` \| `medium` \| `low` (default `medium`). Weights the resilience score; any critical failure = automatic fail.
`when`	No	`always` \| `tool_faults_active` \| `llm_faults_active` \| `any_chaos_active` \| `no_chaos`. When this invariant is evaluated.
`negate`	No	If true, the check passes when the pattern does not match (e.g. “must NOT contain dollar figures”).
`description`	No	Human-readable description.
Plus type-specific	—	`pattern`, `value`, `values`, `max_ms`, `threshold`, etc., same as invariants.

Chaos matrix

Each entry is a scenario: a name plus optional tool_faults, llm_faults, and context_attacks. The contract engine runs your golden prompts under each scenario and verifies every invariant. Result: invariants × scenarios cells; resilience score is severity-weighted pass rate, and any critical failure fails the contract.

Resilience score

Formula: (Σ passed × severity_weight) / (Σ total × severity_weight) × 100.
Weights: critical = 3, high = 2, medium = 1, low = 1.
Automatic FAIL: If any invariant with severity critical fails in any scenario, the contract is considered failed regardless of the numeric score.

Commands

Command	What it does
`flakestorm contract run`	Run the contract across the chaos matrix; print resilience score and pass/fail.
`flakestorm contract validate`	Validate contract YAML without executing.
`flakestorm contract score`	Output only the resilience score (e.g. for CI: `flakestorm contract score -c flakestorm.yaml`).
`flakestorm ci`	Runs contract (if configured) and includes contract_compliance in the overall weighted score.

Stateful agents

If your agent keeps state between calls, each (invariant × scenario) cell should start from a clean state. Set reset_endpoint (HTTP) or reset_function (Python) in your agent config so Flakestorm can reset between cells. If the agent appears stateful and no reset is configured, Flakestorm warns but does not fail.

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