implement-feature

name	implement-feature
description	Use when implementing a feature or story, especially with acceptance criteria to verify. Triggers on /implement, build, create feature, add functionality.

Implement Feature

Plan sidecar: {output_dir}/*/plan.json (searches all feature plans, updates story status in place)

Structured feature implementation workflow with test-driven development, acceptance criteria confirmation, and continuous lightweight review.

Input

The user provides one or more of:

• A story ID (e.g., EPIC-1-S1) — looked up in the plan sidecar JSON
• A feature description
• A project management card URL or ID
• Context about constraints or approach

When to Use

• User says "implement", "add", "build", or "create" a feature
• User references a story ID from the plan sidecar
• User provides a project management card URL/ID
• Multi-step implementation that benefits from planning before coding

When NOT to use:

• Bug fixes — use systematic-debugging instead
• Single-line changes or trivial edits
• Pure research or exploration tasks

Step Skeleton

At startup, call execute-steps to register these steps. Execute them in order, updating status after each.

Step	Action	Condition	Mandatory
0	Devcontainer gate (skip if not in repo with .devcontainer/)	always	Yes
1	Load story from plan.json	skip if no plan context	No
2	Confirm acceptance criteria	always	Yes
3	Write failing test	always (TDD)	Yes
4	Implement to pass test	always	Yes
5	Per-step review	always	Yes
6	Commit + update AC status in plan.json	always	Yes
7	Repeat 3-6 for next AC	loop until all AC done	Yes
8	Update story status in plan.json	always	Yes

Phase 0: Devcontainer Gate

Before any other work, run the pre-flight gate to ensure /implement runs in the right place:

SHIELD_REPO=. python3 shield/scripts/devcontainer_gate.py

Behavior:

• Inside a devcontainer (SHIELD_IN_DEVCONTAINER=true): proceeds silently.
• Outside, but no .devcontainer/ in the repo: proceeds silently (no devcontainer set up).
• Outside, with .devcontainer/ present:
  • If .shield.json devcontainer.required = true: refuses to start; prints reopen instructions; exits.
  • If false: proceeds.
  • If ask (default): prompts [y/n/always/never]. y/always refuses + instructs reopen; n/never proceeds.
• Exit code 1 means refuse — /implement must not continue.

This is the same logic implemented in shield/scripts/devcontainer_gate.py (covered by test_devcontainer_gate.py).

Per-AC tracking: Step 6 updates each AC's status in plan.json immediately after commit — not just at the end (step 8). This means plan.json is the per-AC source of truth. On resume, the skill reads plan.json to determine which ACs are already done, then continues the loop from the next incomplete AC. steps.json tracks coarse workflow position; plan.json tracks per-AC completion.

Workflow

1. Load context (sidecar, PM card, or user description)
2. Confirm acceptance criteria with user
3. Plan implementation steps
4. Get user approval
5. TDD: write failing tests, implement, per-step review
6. Final verification against acceptance criteria

Phase 1: Load Context

From plan sidecar (preferred)

If plan sidecars exist in {output_dir}/*/plan.json:

1. Read all plan files found
2. Search for the story by ID across all plans
3. Extract: name, description, tasks, acceptance criteria
4. Show the story details to the user

If no plans found in {output_dir}/*/plan.json, check for legacy shield/docs/plans/<name>.json and suggest migration.

From project management card

If a PM card URL/ID is provided and a PM tool is configured:

1. Call pm_get_capabilities to verify the adapter is available
2. Fetch the card details
3. Extract: title, description, acceptance criteria, subtasks

From user description

If no sidecar or PM card, gather requirements by asking:

• What should this feature do?
• What inputs/outputs?
• How do we know it's done?

Phase 2: Confirm Acceptance Criteria

ALWAYS confirm acceptance criteria before starting implementation.

Present the criteria to the user:

Story: EPIC-1-S1 — IPAM Pool Hierarchy

Acceptance Criteria:
  1. Regional pools allocate /20 CIDRs
  2. No CIDR overlap across regions
  3. Rollback without data loss

Confirm before starting implementation:
  [a] Proceed as-is
  [b] Edit criteria (add/remove/modify)
  [c] Skip — implement without formal criteria

If the user edits criteria:

1. Update the plan sidecar JSON with the new criteria
2. Re-render the HTML plan document from the updated sidecar
3. Optionally sync changes to the PM card

Do NOT skip this step. The confirmed criteria are what /review will verify against.

Phase 3: Plan Implementation Steps

Explore the codebase to understand existing patterns, then create a plan:

1. Files to create or modify
2. Steps in execution order
3. Test strategy for each step
4. Which steps can run in parallel

If superpowers is available, delegate to superpowers:writing-plans for the plan structure.

Phase 4: Get User Approval

Show the plan summary. NEVER proceed without explicit approval.

Phase 5: TDD Implementation

For each step:

5a. Write failing test

Write a test that captures the expected behavior. Run it — confirm it FAILS.

5b. Implement

Write the minimal code to make the test pass.

5c. Per-step lightweight review

After each step passes its test:

• Check for obvious issues (logic bugs, style, missing edge cases)
• For the changed file's domain, consult the relevant skill(s):
  • *.tf / *.tfvars → shield/skills/terraform/*/SKILL.md
  • *.java / *.kt / *.py / *.ts / *.js / *.go → shield/skills/backend/*/SKILL.md
  • *.yaml (K8s manifests) → shield/skills/kubernetes/*/SKILL.md
  • .github/workflows/*.yml → shield/skills/github-actions/*/SKILL.md
• Use the LLM's judgment to pick which skills are applicable to the file. Skip skills that don't apply (e.g., spring-security on a controller file).
• This is NOT a full agent review — keep it focused on what changed in this step. Don't run a comprehensive multi-skill audit; that happens at /review.
• If the file's domain has no matching skill, fall back to general code-quality judgment.

5d. Commit

Commit the step immediately:

git add <specific files>
git commit -m "feat(<feature-name>): step N — <description>"

5e. Update sidecar status

If implementing from a sidecar story, update the story status:

• When starting: "status": "in-progress"
• When complete: "status": "in-review" Re-render HTML after status change.

5f. Update last_aligned_with on story close

When a story transitions to "status": "done" (or, in the in-review path, when its acceptance criteria are all met and the PR merges), update the plan-level last_aligned_with field to the current HEAD commit SHA:

last_aligned_with=$(git rev-parse HEAD)
# write into {plan_json}.last_aligned_with

This satisfies the schema 1.3 drift-accountability contract — /plan-review and /pm-sync surface the value so reviewers can compare plan and code as of the same commit. The field stays null until the first close.

5h. Promote feature-folder LLD drafts to canonical (milestone close)

After step 5f (last_aligned_with update), check whether the just-closed story was the last open story in its milestone. If yes, walk the milestone's touches_lld[] and promote each draft from docs/shield/{feature}/lld-{name}.md to docs/lld/{name}.md.

This step has six sub-steps, executed in order per component:

1. Look up registry entry. Read lld_components[] for the entry where name == <component>. Extract type and fork_blob_sha.

2. Locate the draft. draft_path = docs/shield/{feature}/lld-{name}.md.

   • If the draft is missing, auto-heal by invoking lld-docs in draft mode just-in-time; print a loud audit-trail warning (see "Just-in-time auto-heal" below).

3. Concurrency check. If docs/lld/{name}.md exists AND fork_blob_sha is non-null:

   • Compute current_sha = blob_sha(docs/lld/{name}.md) via shield/scripts/lld_blob_sha.py.
   • If current_sha != fork_blob_sha: fork drift detected. Invoke lld-docs in remerge mode (canonical changed since /plan drafted; merge canonical changes into draft). On clean merge, refresh fork_blob_sha = current_sha in plan.json and proceed. On conflict markers in the merged output, abort this component's promotion and surface a clear remediation message.

4. Append §14 Changelog row to the draft:

   | <milestone_id> | <YYYY-MM-DD> | <milestone.name> | <story_ids touching this component> |
   

   Insert at the bottom of the §14 table; preserve all existing rows.

5. Atomic promote. os.replace(draft_path, canonical_path) (writes via <canonical>.tmp first, then rename — same atomic contract as lld-docs skill's own writes).

6. Back-fill design_refs[] anchors. Walk all stories in plan.json; for each design_refs[] entry where doc == "lld" AND component == <this component> AND anchor_url is null:

   • Use shield/scripts/lld_anchor_heuristic.py select_anchor() with the story name and the template's slug allow-list (loaded from shield/schema/lld-sections-<type>.yaml).
   • Set anchor_url = "lld-{name}.md#{slug}".
   • Update label = "§{n} {title}" (look up the slug's number+title from the schema).

Just-in-time auto-heal for missing drafts

If step 5h sub-step 2 finds no draft for a component listed in touches_lld[], just-in-time invoke the lld-docs skill to draft. Print a loud multi-line warning to the run log AND include it in the run summary:

⚠️  DRAFT AUTO-GENERATED AT PROMOTION
    Component: <name>
    The /plan run did not produce docs/shield/{feature}/lld-{name}.md.
    /implement just-in-time-drafted the LLD; the design bypassed human review
    before promotion. Review docs/lld/{name}.md for content quality before
    next /plan-review.

Proceed with steps 3 through 6 normally.

Milestone-close detection

A milestone is closed iff every story whose milestone_id == <M> has status == "done". The just-closed story is the trigger; step 5h runs when its closure tips the milestone over to all-done.

If multiple stories close in the same /implement session (e.g. batch), step 5h runs after the LAST one, not after each — the implementation can short-circuit by checking the milestone state once per session-end.

Summary output

/implement prints one block per promoted LLD:

LLD promoted: <component> (<type>)
  Source draft:    docs/shield/{feature}/lld-{component}.md
  Canonical:       docs/lld/{component}.md
  Fork drift:      none | auto-healed | aborted
  Changelog row:   | <M> | <date> | <milestone name> | <story_ids> |
  Anchor backfill: <count> entries updated (<exact-match: X, heuristic: Y, fallback: Z>)

Failure modes

Failure	Behavior
Draft missing AND auto-heal lld-docs invocation fails	Abort this component's promotion; continue with remaining touches_lld[] entries; surface as run-end error.
Concurrency check produces conflict markers	Abort this component's promotion; print re-run /plan to refresh fork and the conflicting section IDs; continue with remaining entries.
Atomic rename fails	Abort this component's promotion; the draft and the canonical both remain in their pre-step state (the .tmp is cleaned up); surface error.
Anchor back-fill: plan.json write-back fails	Roll back the changelog row append (re-read draft from prior state) is NOT attempted — the promotion already happened. Surface "promotion succeeded but design_refs[] back-fill failed; re-run /implement to retry back-fill."

Phase 6: Verify Against Acceptance Criteria

After all steps complete:

1. Run all tests — every test must pass

2. Run linters/formatters — code must be clean

3. Check each acceptance criterion against the implementation:

   Criteria	Status	Evidence
   Regional pools allocate /20 CIDRs	Met	main.tf:42 — netmask_length = 20
   No CIDR overlap	Met	tests/overlap.tftest.hcl

4. Update sidecar — mark story as "in-review" if all criteria met

5. Invoke shield:summarize — produce an implementation summary

Phase 7: Offer Next Steps

• /review — run full agent-based code review with AC verification
• /pm-sync — update story status in the PM tool

Common Mistakes

Mistake	Fix
Skipping AC confirmation	Always confirm before implementing
Implementing before tests	Write test first, confirm it fails
No per-step review	Check each step before committing
Not updating sidecar	Update status and re-render HTML
Giant end-of-feature commit	Commit after each step
Skipping verification	Check every AC before claiming done