(mess) Stuffing commit.

docs/getting-started.md

@@ -94,6 +94,48 @@ apophis replay --artifact reports/apophis/failure-2026-04-28T12-30-22Z.json
 
 Fix the bug in your handler. Re-run verify. The failure should now pass.
 
+## Behavioral vs Structural Contracts
+
+APOPHIS contracts should verify **behavior**, not structure. Fastify and `@fastify/swagger` already enforce status codes, required fields, and types. Behavioral contracts catch what schemas cannot:
+
+| Structural (avoid) | Behavioral (prefer) |
+|---|---|
+| `status:200` | `response_body(this) == request_body(this)` |
+| `response_body(this).id != null` | `response_code(GET /users/{response_body(this).id}) == 200` |
+| `response_body(this).name != null` | `response_body(GET /users/{id}).name == previous(response_body(this).name)` |
+
+**Good behavioral patterns (from the paper):**
+- **Constructor precondition**: Resource must not exist before creation
+  ```apostl
+  response_code(GET /users/{request_body(this).email}) == 404
+  ```
+- **Round-trip equality**: POST response matches the request body
+  ```apostl
+  response_body(this) == request_body(this)
+  ```
+- **Cross-route retrievability**: Creating a resource makes it readable via GET
+  ```apostl
+  response_code(GET /users/{response_body(this).id}) == 200
+  ```
+- **State-change verification**: DELETE causes subsequent GET to return 404
+  ```apostl
+  response_code(GET /users/{request_params(this).id}) == 404
+  ```
+- **Previous state preservation**: DELETE returns the last known state
+  ```apostl
+  response_body(this) == previous(response_body(GET /users/{request_params(this).id}))
+  ```
+- **Invariant over collections**: All resources satisfy a cross-resource constraint
+  ```apostl
+  for t in response_body(GET /tournaments) :-
+    response_body(GET /tournaments/{t.id}/players).length <= t.capacity
+  ```
+
+**Anti-patterns to avoid:**
+- Checking status codes (handled by schema validation)
+- Checking field existence (handled by schema validation)
+- Checking field types (handled by schema validation)
+
 ## Next Steps
 
 - Add more routes to your profile: `apophis verify --profile quick --routes "POST /users,PUT /users/:id"`