SKILL.md
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greenline renders its own frontmatter: quoted name and description, the description from the manifest override where one existed, no upstream activation flag
name: principle-boundary-discipline
name: "boundary-discipline"
description: "Apply when wiring validation, error handling, or framework adapters. Concentrate guards at system boundaries (CLI, config, network, external APIs); trust internal types and keep business logic in pure functions."
description: "Apply when wiring validation, error handling, or framework adapters. Concentrate guards at system boundaries (CLI, config, network, external APIs); trust internal types and keep business logic in pure functions."
greenline renders its own frontmatter: quoted name and description, the description from the manifest override where one existed, no upstream activation flag
disable-model-invocation: true
---
---
# Boundary Discipline
# Boundary Discipline
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Place validation, type narrowing, and error handling at system boundaries. Trust internal code unconditionally. Business logic lives in pure functions. The shell is thin and mechanical.
Place validation, type narrowing, and error handling at system boundaries. Trust internal code unconditionally. Business logic lives in pure functions. The shell is thin and mechanical.
**Why:** Scattered validation is noisy, redundant, and gives a false sense of safety. Keep logic out of framework wiring so it can be tested without the framework.
**Why:** Scattered validation is noisy, redundant, and gives a false sense of safety. Keep logic out of framework wiring so it can be tested without the framework.
**The pattern:**
**The pattern:**
- **At boundaries** (CLI args, config files, external APIs, network protocols): validate, return errors, handle defensively.
- **At boundaries** (CLI args, config files, external APIs, network protocols): validate, return errors, handle defensively.
- **Inside the system:** typed data, error propagation, no re-validation. Trust the types.
- **Inside the system:** typed data, error propagation, no re-validation. Trust the types.
- **Across the boundary.** Expose domain concepts, not the boundary's private representation. Keep general-purpose mechanism inside and special-purpose policy at the edge.
- **Across the boundary.** Expose domain concepts, not the boundary's private representation. Keep general-purpose mechanism inside and special-purpose policy at the edge.
**Applications:**
**Applications:**
Validation and error handling:
Validation and error handling:
- Validate config at parse time (the boundary), not inside business logic
- Validate config at parse time (the boundary), not inside business logic
- Parse raw data into domain types at the boundary
- Parse raw data into domain types at the boundary
- Do not re-export transport, storage, framework, or wire types through the public surface
- Do not re-export transport, storage, framework, or wire types through the public surface
- No redundant nil checks deep in call chains if the boundary already validated
- No redundant nil checks deep in call chains if the boundary already validated
Code organization:
Code organization:
- Business logic in pure functions with no framework dependencies
- Business logic in pure functions with no framework dependencies
- Parse functions: pure transforms from raw bytes to typed state
- Parse functions: pure transforms from raw bytes to typed state
- Prompt construction: structured state in, string out
- Prompt construction: structured state in, string out
- Scoring and assessment: pure transforms from state to results
- Scoring and assessment: pure transforms from state to results
**The tests:**
**The tests:**
- "Is this data crossing a system boundary right now?" If not, validation is redundant.
- "Is this data crossing a system boundary right now?" If not, validation is redundant.
- "Can this be a pure function that the shell just calls?" If yes, extract it.
- "Can this be a pure function that the shell just calls?" If yes, extract it.