Code Maintainability

Code Maintainability

Use this capability whenever you write, refactor, or review changed code, to keep it readable and cheap to change. Each concern below is one lens with two modes: a practice to uphold while you author the code, and a finding to raise while you review a change. Hold the line the same way in both modes — the standard does not soften because you wrote the code yourself.

Where the project's development or structure conventions own a rule — or a project-defined component, routing, or domain skill does — this capability applies the maintainability lens and defers to that owner by name rather than restating it. Rules gated on "if the project has such a convention" are conditional: skip them cleanly on a project that has none.

Guidelines:

• MUST defer each rule to its owning skill by name where one exists (the project's development, structure, component, routing, or domain conventions), summarizing rather than duplicating its wording.
• MUST treat a concern gated on a project-specific convention as inapplicable — not a violation — on a project that does not ship that convention.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Naming and Organization

See naming-and-organization.md for:

• Keeping file names on the project's established file-naming convention (e.g., kebab-case) and the co-located sibling files that convention requires (such as a paired style-module file)
• Placing components, helpers, and data-access modules in the correct directory tier (route-local before group-shared before global), with a decision flowchart for the tier
• Following the project's own routing convention, if it defines one, and co-locating a route's required sibling files (props/types, not-found, social-image)
• Matching identifier names and casing to the conventions in and around the changed file
• The fallback identifier vocabulary for a name nothing else governs, and how it yields to a project convention, an owning capability, or a platform/host API the value crosses into

Guidelines:

• MUST read naming-and-organization.md before naming a file, placing a module in a directory tier, adding a route's co-located sibling files, or naming an identifier that no project convention, owning capability, or platform API already governs.

Abstraction Boundaries

See abstraction-boundaries.md for:

• Deciding what belongs inside one function, class, or module, and why the cohesion scale is non-linear — functional far stronger than the rest, coincidental and logical far weaker than the intermediate levels
• Placing new shared logic at the lowest tier that has more than one caller (route-local before group-shared before global)
• Splitting the server / client boundary per the project's own component convention, if it defines one
• Keeping a domain-specific pipeline (such as a content-rendering chain) behind its single owning module, per the project's own domain convention, if it defines one
• Keeping tier imports pointed the right way, so shared code never depends on route-local code

Guidelines:

• MUST read abstraction-boundaries.md before deciding what belongs inside one function, class, or module, or before judging where a change put it.

Complexity and Readability

See complexity-and-readability.md for:

• Staying within the project's configured linter and complexity budget instead of silently bypassing it
• Giving magic numbers and strings a named constant or design token, reserving an inline lint-suppression directive (with a justifying comment) for the rare justified case
• Removing dead code (unused imports, unreachable branches, commented-out blocks)
• Why a comment is the fallback rather than the plan, and why a unit, a nullability, or a bound's inclusivity belongs in the type or the name before it belongs in a comment
• Why a boundary doc-comment that has to describe implementation details signals a shallow interface, not a well-documented one
• Deferring doc-comment, restating-comment, and comment-voice rules to the project's development conventions, and extracting a repeated inline type into a named alias in a statically-typed language

Guidelines:

• MUST read complexity-and-readability.md before adding a literal constant, a comment, or a doc-comment on a module or domain boundary, before leaving a function past the project's configured complexity or length budget, and before removing or keeping code that looks dead.

Scope Discipline

See scope-discipline.md for:

• Keeping the change matched to its stated goal — no drive-by refactors, per the project's development conventions
• Flagging pre-existing problems separately instead of bundling them into this change
• Justifying a new abstraction with two or more concrete call sites (YAGNI), and consolidating repeated logic only when it is truly the same concern (DRY without coupling unrelated callers)

Guidelines:

• MUST read scope-discipline.md before adding a new helper, prop, configuration option, or generic type parameter, before extracting or consolidating duplicated logic, and before touching a file the change's stated goal does not name.