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.

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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:

Convention	What to look for
Type visibility	Do constructors return interfaces or concrete types? Are structs exported or private?
Error handling	How are external API errors wrapped? What's retryable vs fatal?
Test patterns	What context function? Table-driven? What assertion style? Fake/mock conventions?
Logging	Which logger? Structured fields or format strings?
Config	Env vars via library or manual os.Getenv? Global var or injected?
Safety	Dry-run modes? Guard conditions? Graceful shutdown?

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]

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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.

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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

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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:

What	Why	How
Hardcoded status/error codes	Should use named constants	grep -rn '[0-9][0-9][0-9]' <new_files> in error-handling contexts
Test context creation	Should use test-framework-provided context	Check test files for manual context creation vs framework helpers
Constructor return types	Should match convention from research doc	Check that constructors return the type pattern found in Phase 0
Dead variables in tests	Every declared variable must be asserted	Scan test files for variables that are written but never read in assertions

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

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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

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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.

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Anti-Patterns This Skill Prevents

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

Defect	How Phase 0 prevents it
Exported concrete type when convention is interface return	0.2 records constructor conventions before you write yours
Utility function duplicated from sibling module's internal/	0.3 searches for existing utilities and flags duplication
IAC secret/resource name collision with sibling module	0.4 greps for exact resource names before creating new ones
Magic numbers instead of named constants	0.2 records error-handling conventions; 3.2 greps for violations
Test context using wrong helper	0.2 records test patterns; 3.2 checks for violations
Safety mode bolted on after implementation	1.2 requires designing dry-run/guards before happy path
Dead test variables	3.4 checks every variable is asserted

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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.