Fitness Functions

"An architectural fitness function provides an objective integrity assessment of some architectural characteristic(s)." -- Neal Ford, Rebecca Parsons & Patrick Kua, Building Evolutionary Architectures

Core Philosophy

A fitness function turns an architectural intention into an automated, objective check that the CI pipeline enforces on every push. It is the executable memory of an ADR: the ADR records why a boundary exists; the fitness function makes crossing that boundary fail the build. Without the gate, architectural decisions decay silently between reviews. The mechanism is uniform across stacks -- a check wired into CI as a gatekeeper -- but the tool is per-stack.

Non-Negotiable Constraints:

1. A fitness function is a GATE, not a report -- it exits non-zero and fails CI on violation; a check that only prints is not a fitness function
2. TRACEABLE -- every fitness function names the ADR or measurable quality attribute it enforces; no orphan checks
3. PROVE it gates -- pass against today's code AND fail against a deliberate violation before you declare it done; an untested gate is assumed broken
4. PER-STACK tool, uniform mechanism -- load references/<stack>.md; never hardcode a stack's tooling into the gate-wiring logic
5. CONTEXT BUDGET: keep utilization under 40%. At 60%, write progress to the target's architecture.md fitness section and tell the user to start a fresh session.

Workflow

PRE-FLIGHT
    [ ] Identify the target stack (dotnet|python|rust|policy) from architecture.md/the ADRs, or ASK
    [ ] Locate the CI workflow file (.github/workflows/*, .gitlab-ci.yml, etc.)
    [ ] Identify the ADR or quality attribute each fitness function will enforce

SELECT
    Load references/<stack>.md and choose the tool for the rule category (layering/dependency
    direction · coupling · dependency policy · coverage · package rules). For coupling-as-metric,
    reference dependency-mapper (Martin Ca/Ce/I/A/D) rather than re-deriving the math.

AUTHOR
    Write the check as an executable artifact in the target repo (a test, a contract, a policy
    file). It must read the real source/build graph and exit non-zero on violation.

WIRE
    Add the check to the CI workflow as a required step/job that blocks merge on failure.
    Comment the step with the ADR id / quality attribute it gates.

VERIFY
    1. Run it against current code -- must PASS (green)
    2. Introduce a deliberate violation -- must FAIL (non-zero) -- then revert. Observe both.

REPORT
    Each fitness function · the ADR/attribute it gates · CI step location · the violation proof.

Exit criteria: >= 1 fitness function authored in the target repo; wired into CI as a merge-blocking gate; verified GREEN today and verified to FAIL on a deliberate violation; each gate traces to a named ADR or quality attribute.

State Block

<fitness-functions-state>
phase: PRE-FLIGHT | SELECT | AUTHOR | WIRE | VERIFY | REPORT | COMPLETE
target_stack: dotnet | python | rust | policy
ci_workflow: [path to the CI file the gate is wired into]
fitness_functions: [count]
gates_on: [ADR ids / quality attributes enforced]
wired_as_ci_gate: true | false        # MUST be true to COMPLETE
verified_fails_on_violation: true | false   # MUST be true to COMPLETE
context_budget: under-40 | approaching-60 | checkpoint-now
status: in_progress | complete
</fitness-functions-state>

Output Template

See references/<stack>.md for the per-stack tool, a minimal check, the CI-wiring snippet, and the deliberate-violation proof: dotnet.md (NetArchTest), python.md (import-linter), rust.md (cargo-deny + dependency-direction test), policy.md (Conftest/OPA Rego).

Integration with Other Skills