Feature Dev

Feature Development

End-to-end feature implementation from documentation to working code. Automatically detects and applies the appropriate engineering role based on the project's tech stack.

1. Receive and Parse Documentation

Gather all input documentation provided by the user:

• Accepted formats: PRD, ARD (Architecture Decision Record), design doc, implementation plan, ticket/issue, or any structured feature specification
• Read every provided document thoroughly
• Extract and organize:
  • Goal: what the feature does (1–2 sentences)
  • Requirements: functional and non-functional
  • Acceptance criteria: how to verify the feature works
  • Implementation plan: ordered steps, architecture decisions, data models, API contracts
  • Constraints: performance, security, compatibility, dependencies
  • Out of scope: what this feature explicitly does NOT cover

Check for an existing implementation plan. Search the documentation, linked files, and project directory for an implementation plan (file named *plan*, *implementation*, or a plan section inside the PRD/ARD). If one exists — it is the authority. Proceed to Step 2 with the existing plan loaded.

If the documentation is ambiguous or incomplete, ask the user before proceeding.

2. Detect Tech Stack and Apply Role

Determine the project's tech stack and apply the appropriate engineering role:

1. Read project's AGENTS.md — look for tech stack declaration (language, framework, runtime)
2. Scan project files — check package.json, pom.xml, *.csproj, requirements.txt, go.mod, Cargo.toml, or equivalent to confirm
3. Role matching — activate the relevant Codex role overlays based on the detected stack and file ownership:
   • If multiple specializations match (e.g., fullstack) — apply all relevant roles
   • If no specialization role exists — fall back to base engineering principles
4. Announce the detected stack and applied role(s) to the user for confirmation

Examples:

• Next.js + TypeScript → frontend-engineer role applies
• Spring Boot → java-engineer role applies
• Python + FastAPI → python-engineer role applies
• Terraform / Docker / K8s → devops-engineer role applies
• Cloud architecture / landing zones / networking / multi-cloud → cloud-architect role applies
• GitHub Actions / CI/CD pipelines / deployment strategy → devops-architect role applies
• React Native / Flutter / iOS / Android → mobile-engineer role applies
• ETL / Spark / dbt / Airflow / data pipelines → data-engineer role applies
• SQL / database schema / migrations / query optimization → db-engineer role applies
• ARCHITECTURE.md / system design / component boundaries → system-architect role applies
• LLM / RAG / agents / memory / multi-agent / AI pipelines → ml-engineer role applies + consult context-engineering skill for context pipeline design

3. Analyze Codebase Context

Before writing any code, understand the existing codebase:

1. Project structure — directory layout, module boundaries, entry points
2. Existing patterns — naming conventions, error handling, logging, testing approach
3. Dependencies — installed packages, available libraries, version constraints
4. Related code — files and modules the new feature will interact with, extend, or modify
5. Test infrastructure — test framework, test file locations, existing test patterns

Map how the new feature fits into the existing architecture. Identify:

• Files to create (new modules, components, tests)
• Files to modify (integration points, routes, configs)
• Files to not touch (unrelated code — minimize blast radius)

4. Resolve Implementation Plan

If a plan already exists (from documentation, PRD, ARD, or plan skill output):

• Use it as-is. Do NOT rewrite, reorder, or simplify it
• Follow it STRICTLY — step by step, in the exact order specified
• Present the loaded plan to the user for confirmation before proceeding

If no plan exists:

• Create one from scratch based on the parsed requirements and codebase analysis

When creating a plan, break the feature into ordered, atomic implementation steps:

1. Number each step sequentially
2. Each step = one logical unit of work (one file or one cohesive change across tightly coupled files)
3. Order by dependency — implement foundations before consumers
4. Interleave test steps with implementation (do not defer all tests to the end)

Present the plan to the user:

Feature: [name]
Stack: [detected] | Role: [applied]
Plan source: [loaded from <file> | created from scratch]
Steps:
  1. [description] → [file(s)]
  2. [description] → [file(s)]
  ...
  N. [description] → [file(s)]

Wait for user approval before proceeding. The user may reorder, add, remove, or modify steps.

5. Implement

Execute the approved plan STRICTLY step by step. Do not skip steps, reorder steps, or combine steps.

If the plan needs correction (a step is blocked, wrong, or a new step is needed):

1. STOP implementation immediately
2. Explain to the user what happened and why the plan needs to change
3. Propose the specific correction (add/remove/modify steps)
4. Wait for user approval
5. Update the plan document if one exists as a file
6. Resume implementation from the corrected point

Never silently deviate from the plan. Every deviation requires explicit user approval.

For each step:

1. State what you are about to do (step number, file, change summary)
2. Write code following:
   • Project's existing patterns and conventions
   • Active role's guidelines (stack-specific best practices)
   • Documentation's architecture decisions and constraints
3. Verify the code compiles/parses without errors after each step
4. If a step introduces a new dependency — install it immediately

Rules:

• Minimal, focused changes — do not refactor unrelated code
• Follow existing code style (indentation, naming, imports)
• Add imports at the top of files
• Production-quality code — no TODOs, no placeholders, no stubs (unless the plan explicitly calls for them)
• If you encounter an unexpected issue — stop and consult the user

6. Write Tests

For each implemented component, write tests following the project's test infrastructure:

1. Unit tests — business logic, utilities, data transformations
2. Integration tests — API endpoints, database queries, service interactions
3. Component tests (frontend) — UI components with user interactions
4. Cover both happy path and edge cases (error handling, boundary values, empty states)
5. Run the tests and verify they pass

If the documentation specifies acceptance criteria, write tests that directly verify each criterion.

7. Verify

Run the full verification sequence:

1. Build/compile — project builds without errors or warnings
2. Lint — run the project's linter if configured
3. Test — run the full test suite (new + existing) to catch regressions
4. Acceptance check — review implementation against documentation's acceptance criteria

Checklist:

• All acceptance criteria from the documentation are met
• No new warnings or errors in build output
• All tests pass (new and existing)
• No unrelated files were modified
• Code follows project conventions and active role's guidelines

If any check fails — fix the issue and re-verify.

8. Summary

Present the completed work:

• Feature: what was implemented
• Stack / Role: detected tech stack and applied role(s)
• Files changed: list of created and modified files with brief descriptions
• Tests: number of tests added, pass status
• Acceptance criteria: status of each criterion (met / partially met / not met)
• Notes: deviations from original plan, trade-offs, follow-up items

Integration

• Precedes: run-tests skill, pre-commit skill, create-pr skill
• Planning: plan skill (produces the implementation plan this workflow executes)
• Skills: test-strategy skill (test strategy), code-review skill (review standards), context-engineering skill (context pipelines, RAG, agent harness, production checklists — for AI/LLM features), worktree-isolation skill (branch isolation via git worktree)