Feature Impl

Feature Implementation Workflow

Implement new features by discovering and following existing codebase patterns before writing code. This skill prevents the most common implementation defects: duplicated utilities, convention violations, infrastructure resource conflicts, and bolt-on design.

Discipline model: Phases 0 and 3 are RIGID (produce artifacts, gate the next phase). Phases 1 and 2 are FLEXIBLE (adapt to the problem).

Arguments

$ARGUMENTS

Arguments should be a problem statement describing what to build. Examples:

• "sync users from identity provider to billing system"
• "add a webhook handler for build completion events"
• "create a scheduled job that reconciles X with Y"

If no arguments provided, ask the user for a problem statement before proceeding.

Phase 0: Research (MANDATORY — BLOCKING)

No code is written until this phase produces a research document.

Launch a separate Explore agent (thoroughness: "very thorough") with the problem statement. The agent must complete all substeps and return a structured summary.

0.1 Discover existing patterns

Find how similar things are built in this codebase. Use whatever code intelligence is available:

• If mache MCP tools are available: mcp__mache__search with keywords from the problem statement, mcp__mache__find_callers on framework entry points, mcp__mache__find_definition to locate implementations
• Otherwise: Grep for domain keywords (e.g., Reconciler, Handler, BackfillJob, Sync), Glob for structural patterns (e.g., **/cmd/*/main.go, **/iac/*/main.tf)

The goal is to find 2-3 implementations of the same kind of thing you're building — not by guessing names, but by searching the codebase.

0.2 Read reference implementations

Read 2-3 implementations discovered in 0.1 — discovered, not hardcoded. For each, extract:

0.3 Search for reusable utilities

Before planning to write ANY utility function, search for it:

• Search by function name pattern and by behavior description
• Check internal/ packages of sibling modules — these can't be imported across module boundaries
• If utility exists behind internal/: flag it as a cross-module duplication risk. Options:
  1. Extract to a shared package (preferred, but may be out of scope)
  2. Copy with // TODO(<ticket>): extract to shared package — duplicated from <source> comment
  3. Never copy silently

0.4 Check for infrastructure conflicts (if IAC is involved)

• Read sibling IAC directories (iac/, env/, modules/) in the same area
• Grep for exact resource names you plan to create:

  grep -r 'name = "<your-resource-name>"' <iac-directory>
  

  If this returns hits from another module, you have a collision. Decide: reference the existing resource, or use a distinct name.
• Read the upstream module's variables.tf (not just usage examples) to understand its actual interface
• Check: does the module create resources unconditionally, or can creation be made conditional?

0.5 Output a research document

GATE: This document must exist before Phase 1 begins. If empty, stop and investigate.

## Research Summary

### Reference Implementations
- [Name]: [path] — [key patterns noted]
- [Name]: [path] — [key patterns noted]

### Conventions Discovered
- Constructor pattern: [e.g., private type, interface return]
- Test context: [e.g., t.Context()]
- Error wrapping: [e.g., fmt.Errorf("doing X: %w", err)]
- Logging: [e.g., slog.InfoContext(ctx, ...)]
- Config: [e.g., env var library with struct tags]

### Reusable Utilities
- [Function/package]: [location, importable? or needs copying with TODO?]
- [None found]: [what was searched]

### Infrastructure (if applicable)
- Shared resources to reference: [name, owning module]
- New resources to create: [name, verified no collision]
- Module interfaces read: [module name, key variables]

### Risks
- [Any cross-module duplication, known fragility, or design tradeoffs]

Phase 1: Design

Using the research document:

1. Match patterns — which discovered pattern fits? Document the choice. If deviating from an existing pattern, document why explicitly.
2. Design safety modes first — if the feature has destructive operations (deletes, overwrites, state mutations), design dry-run mode and safety guards BEFORE writing the happy path. These are first-class concerns, not afterthoughts.
3. Map resource ownership — for IAC: shared resources are referenced (data source), new resources are created (resource block), collision risks use conditional creation or distinct names.
4. Define interfaces before structs — follow whatever visibility convention the research document recorded.

Phase 2: Implement

Write code following the research document and design decisions. The only rules:

• Follow the patterns discovered in Phase 0 (don't invent new ones)
• Import utilities found in Phase 0 (don't duplicate them)
• Reference shared resources found in Phase 0 (don't recreate them)
• When copying a utility from behind internal/, add the TODO comment from 0.3

Phase 3: Self-Review (MANDATORY — NON-OPTIONAL GATE)

GATE: All checks must pass before committing. No exceptions.

3.1 Run the repo's standards skill

Check if a language-specific standards skill exists (e.g., /go-standards, /python-standards, /ts-standards). If it does, invoke it on all new files. If it doesn't, proceed to the manual checks below.

3.2 Convention grep audit

Run targeted searches on your new files. These are language-agnostic patterns — adapt the regexes to your language:

Fix any violations found. Do not skip.

3.3 Cross-module duplication check

For every new utility function or helper you wrote:

• Search the codebase for the function name — does it already exist?
• If it does: import it, or justify with a TODO comment
• Pay special attention to functions you might have unconsciously copied from a reference implementation

3.4 Test hygiene

For every test file:

• Every declared variable must appear in at least one assertion
• Collection builders should pre-allocate capacity when the size is known
• Test data should use the patterns discovered in Phase 0 (context helpers, assertion style, etc.)

3.5 Build and test

Run the language-appropriate build, lint, and test commands:

# Examples — use whatever your project requires
go build ./... && go vet ./... && go test ./...
npm run build && npm test
cargo check && cargo test

3.6 IAC audit (if applicable)

• Format all infrastructure files (e.g., terraform fmt)
• Grep for resource name collisions with sibling modules (repeat the check from 0.4)
• Verify configurable values are variables with defaults — no hardcoded schedules, timeouts, or names that operators can't override
• Verify shared resources use data sources or variable inputs, not unconditional resource blocks that collide

Phase 4: Commit and PR

• Squash into logical commits (library, entrypoints, IAC — not your debugging history)
• Write a PR description that tells the reviewer:
  • How to review (commit order, what each commit contains)
  • What patterns were followed (reference the implementations from Phase 0)
  • Known limitations and tradeoffs (from the research document's Risks section)
• Call out anything you copied (with the TODO comments) so reviewers can decide if extraction is worth doing now

Error Handling

Phase 0 finds no matching patterns: This is a novel feature. Proceed, but document in the PR description that no existing pattern was found and explain the design choices from first principles.

Phase 0 finds utility behind internal/: Copy with a TODO comment. Do not import across module boundaries. Do not silently duplicate.

Phase 3 audit finds violations: Fix them. Do not skip. These are the exact issues that waste reviewer time and cause review cycles.

Code intelligence tools (mache) unavailable: Fall back to Grep + Glob + Read. The research phase is mandatory regardless of tooling.

Anti-Patterns This Skill Prevents

These are real defects found in production PRs that followed the "build first, read later" workflow:

Notes

• This skill is language-agnostic and repo-topology-agnostic. It works for monorepos, multi-repo dependency graphs, and single repos with workspaces.
• The key insight: most implementation defects come from building before reading. Phase 0 forces reading first.
• Phase 3 exists because self-review discipline erodes under time pressure. A checklist with explicit grep commands prevents skipping.
• If a repo-specific feature-impl-config skill exists, Phase 3.1 will invoke it for language-specific checks. Otherwise, the generic checks in 3.2 apply.