apply

Implement code based on the change artifacts. Supports two modes:

• Gherkin-driven (default): feature files drive implementation and testing
• Proposal-driven: when gherkin is skipped (technical changes), proposal drives testing

<decision_boundary>

Use for:

• Implementing code for a Beat change that has spec artifacts (gherkin or proposal) done
• TDD implementation driven by feature files or proposal risk points
• Resuming implementation of a partially completed change

NOT for:

• Creating spec artifacts like proposal, gherkin, or design.md (use /beat:design)
• Breaking down tasks or creating execution plans (use /beat:plan)
• Verifying implementation completeness (use /beat:verify)
• Exploring ideas or investigating a problem (use /beat:explore)

Trigger examples:

• "Implement the change" / "Start coding" / "Apply the plan" / "TDD the scenarios"
• Should NOT trigger: "design a feature" / "break down tasks" / "verify the implementation"

</decision_boundary>

Prerequisites (invoke before proceeding)

Superpower	When	Priority
using-git-worktrees	Verify isolation (should exist from design/plan; creates if not)	MUST
test-driven-development	At start, in TDD mode	MUST
systematic-debugging	When stuck (3 failed attempts)	SHOULD
subagent-driven-development	When tasks.md has multiple independent tasks	SHOULD

Invoke in order: worktrees first (verify isolation), then TDD (discipline). Debugging and subagent are conditional — only invoke when triggered. If a superpower is unavailable (skill not installed), skip and continue.

Rationalization Prevention

Thought	Reality
"The change is small, I don't need a worktree"	Worktrees protect against contamination. The worktree should already exist from design/plan — verify, don't skip.
"I'll write the test after the implementation, same result"	TDD is about design feedback, not just test coverage. Writing tests after loses the design signal.
"This is a refactor, TDD doesn't apply"	Refactors need tests most — they prove behavior is preserved. If testing.required is false, TDD is already skipped.
"I'll add @covered-by annotations at the end for all scenarios"	Annotations must be added per-scenario immediately after writing the test. Batching them leads to forgetting.
"The e2e test setup is too complex, I'll write a unit test instead"	The scenario is tagged @e2e for a reason. If e2e setup is genuinely blocked, announce the blocker and ask — don't silently downgrade.
"This @behavior test is obvious, a skeleton is enough"	Every test must be executable. A skeleton that doesn't run is not a test.
"These tasks are small, I'll combine them for efficiency"	Each task is bounded for a reason. Merging recreates the oversized-output problem that decomposition solved. One task = one subagent dispatch.
"The existing test is roughly correct, no need to update it"	If scenario steps changed, the test must reflect those changes. An old test passing does not mean the new behavior is correct.
"I'll create a new test file, it's faster"	If @covered-by already points to an existing test, creating a new file breaks traceability. Update the existing test.

Red Flags — STOP if you catch yourself:

• Writing implementation code before invoking using-git-worktrees
• Writing implementation code before writing a failing test (in TDD mode)
• Thinking "I'll set up the worktree after this first file"
• Skipping TDD because "the test would be trivial"
• Moving to the next scenario without adding @covered-by to the .feature file
• Skipping e2e test creation because "the e2e framework is complex to set up"
• Writing a test skeleton instead of an executable test
• Thinking "I'll add the annotations at the end after all scenarios are done"
• Dispatching a single subagent for multiple tasks to "save time"
• Creating a new test file for a scenario that already has @covered-by pointing to an existing test
• Modifying scenario steps without updating the corresponding e2e test

Process Flow

digraph apply {
    "Select change, read artifacts" [shape=box];
    "Invoke using-git-worktrees" [shape=box, style=bold];
    "testing.required false?" [shape=diamond];
    "No-test mode" [shape=box];
    "Invoke test-driven-development" [shape=box, style=bold];
    "Gherkin done?" [shape=diamond];
    "Gherkin-driven" [shape=box];
    "Proposal-driven" [shape=box];
    "tasks.md exists?" [shape=diamond];
    "Has Task N headings?" [shape=diamond];
    "Executor mode\nfollow tasks exactly" [shape=box];
    "Planner mode\nextract scenarios/criteria" [shape=box];
    "Implementation loop" [shape=box];
    "All tasks complete" [shape=doublecircle];

    "Select change, read artifacts" -> "Invoke using-git-worktrees";
    "Invoke using-git-worktrees" -> "testing.required false?";
    "testing.required false?" -> "No-test mode" [label="yes"];
    "testing.required false?" -> "Invoke test-driven-development" [label="no"];
    "No-test mode" -> "Gherkin done?";
    "Invoke test-driven-development" -> "Gherkin done?";
    "Gherkin done?" -> "Gherkin-driven" [label="yes"];
    "Gherkin done?" -> "Proposal-driven" [label="gherkin skipped"];
    "Gherkin-driven" -> "tasks.md exists?";
    "Proposal-driven" -> "tasks.md exists?";
    "tasks.md exists?" -> "Has Task N headings?" [label="yes"];
    "tasks.md exists?" -> "Planner mode\nextract scenarios/criteria" [label="no"];
    "Has Task N headings?" -> "Executor mode\nfollow tasks exactly" [label="yes"];
    "Has Task N headings?" -> "Planner mode\nextract scenarios/criteria" [label="no"];
    "Executor mode\nfollow tasks exactly" -> "Implementation loop";
    "Planner mode\nextract scenarios/criteria" -> "Implementation loop";
    "Implementation loop" -> "All tasks complete";
}

Input: Optionally specify a change name. If omitted, infer from context or prompt.

Steps

1. Select the change

   If no name provided:

   • Look for beat/changes/ directories (excluding archive/)
   • If only one exists, use it (announce: "Using change: ")
   • If multiple exist, use AskUserQuestion tool to let user select

2. Read status.yaml and verify readiness (schema: references/status-schema.md)

   Check that either:

   • gherkin has status: done → Gherkin-driven mode
   • gherkin has status: skipped AND proposal has status: done → Proposal-driven mode

   If neither condition is met: "Features or proposal are required before implementation. Run /beat:design first." STOP.

3. Read all artifacts and determine testing mode

   Read in order:

   • proposal.md (if exists) -- business context and risk points
   • features/*.feature (all files, if gherkin is done) -- implementation targets
   • design.md (if exists) -- technical decisions
   • tasks.md (if exists) -- implementation checklist

   Read beat/config.yaml (if exists, schema: references/config-schema.md).

   Determine testing mode:

   • If testing.required: false → no-test mode: skip TDD cycles for all scenarios, write implementation only
   • If testing.behavior is set → use that framework for @behavior scenarios (skip auto-detection)
   • If testing.e2e is set → use that framework for @e2e scenarios (skip auto-detection)
   • If testing.framework is set (legacy) → treat as testing.behavior
   • If testing is absent or testing.required: true → TDD mode (default): require tests for all scenarios

   Determine BDD feature paths (for running e2e tests):

   • Base: beat/features/ (unchanged features; .feature.orig files are invisible to BDD runners)
   • If status.yaml has gherkin.modified: add beat/changes/<name>/features/ (modified + new features)
   • Combine both paths when invoking BDD runner (e.g., npx cucumber-js beat/features beat/changes/<name>/features)

4. Determine implementation strategy

   If tasks.md exists: Use it as the implementation checklist.

   • Detailed format (contains ### Task N: headings with Steps): Enter executor mode — follow each step exactly as written, don't re-plan.
   • Simple format (only - [ ] checkboxes): Enter planner mode — plan each task's implementation yourself (existing behavior).

   If no tasks.md and Gherkin-driven: Extract each Scenario from feature files as a unit of work (planner mode).

   If no tasks.md and Proposal-driven: Extract success criteria and risk points from proposal.md as units of work (planner mode).

5. Show implementation overview

   ## Implementing: <change-name>
   
   ### Drive mode: gherkin-driven / proposal-driven
   ### Execution mode: executor / planner
   ### Tasks/Scenarios to implement:
   1. [source] <name>
   2. [source] <name>
   ...
   

6. Implement (loop)

   For each task (from tasks.md) or scenario (from features) or risk point (from proposal):

   a. Announce: "Working on: "

   b. Write automated test first (TDD mode only):

   • Skip this step if no-test mode
   • The test framework: use testing.behavior or testing.e2e from config (depending on scenario tag), or detect from codebase

   Generate vs Update (see references/testing-conventions.md for details):

   • @covered-by present + test file exists → Update: modify the existing test to reflect new scenario steps
   • @covered-by present + test file missing → Generate + WARNING (stale annotation)
   • @covered-by absent → Generate: create new test (existing flow)

   For @e2e scenarios (Gherkin-driven):

   • Generate (or update) e2e test or step definitions using testing.e2e framework from config, or auto-detect from codebase
   • If the project uses a BDD runner (Cucumber, pytest-bdd, etc.), generate step definitions that bind to the .feature file
   • If no BDD runner, generate a regular e2e test with @feature/@scenario annotations (same as @behavior)

   For @behavior scenarios (Gherkin-driven):

   • Generate a test file (using testing.behavior from config, or auto-detect) with annotation comments:

     @feature: <feature-filename>.feature
     @scenario: <exact scenario name>
     

     (Use the project language's comment syntax: // for JS/TS/Java/C#, # for Python/Ruby, etc.)
   • After writing the test, update the .feature file with a @covered-by annotation (placed between the tag and the scenario line):

     @behavior @happy-path
     # @covered-by: <relative path to test file>
     Scenario: <name>
     

   For proposal-driven units:

   • Generate test files covering the risk point using the project's test framework
   • No annotation conventions needed (no features to link to) For pytest-bdd projects: the @scenario decorator serves as the annotation — no separate # @feature / # @scenario comments needed (see references/testing-conventions.md).

   Follow the conventions in references/testing-conventions.md for annotation format, e2e test style, and Generate vs Update path.

   c. Write implementation code:

   • Follow design.md decisions if available
   • Keep changes minimal and focused on the scenario
   • In no-test mode: still write implementation, just without preceding test

   d. If using tasks.md: Mark task complete - [ ] -> - [x]

   e. Continue to next

   f. Scenario completion checklist (verify before moving to next scenario):

   For @e2e scenarios (TDD mode):

   • E2e test or step definition exists and is executable
   • Test references the scenario (@feature/@scenario annotations or BDD binding)
   • # @covered-by: <path> annotation added to .feature file (between tag and Scenario line)

   For @behavior scenarios (TDD mode):

   • Test file exists with @feature and @scenario comments
   • # @covered-by: <path> annotation added to .feature file (between tag and Scenario line)
   • Test is executable (not a skeleton)

   For all scenarios:

   • Implementation code handles the scenario's behavior
   • Task checkbox marked complete (if using tasks.md)

   Do NOT move to the next scenario until all applicable items are checked.

   Pause if:

   • Task/scenario is unclear -> ask for clarification
   • Implementation reveals design issue -> suggest updating artifacts
   • Error or blocker -> report and wait

7. E2E regression check (after all scenarios implemented)

   If @e2e scenarios exist and testing.e2e is configured (or auto-detected):

   • Run the full e2e test suite using the combined BDD feature paths
   • If any failures → report and pause. Do NOT advance to verify phase.
   • All passing → continue to completion.

   Skip if no @e2e scenarios or no-test mode.

8. On completion or pause, show status

   If all done: update status.yaml phase to verify

   ## Implementation Complete
   
   **Change:** <name>
   **Scenarios:** N/N implemented with tests
   
   Suggested next steps:
   - `/beat:verify` -- validate implementation against artifacts
   - `/beat:archive` -- sync features and archive the change
   

Testing Rule: Conditional TDD

In TDD mode (default when testing.required is true or unset):

• For every Scenario in every .feature file: there MUST be a corresponding automated test
• @e2e scenarios → e2e test or step definitions (using project's e2e framework)
• @behavior scenarios → test with @feature/@scenario annotations + @covered-by comment in .feature
• The test MUST be executable (not just a skeleton)
• The test framework: testing.behavior (for @behavior) or testing.e2e (for @e2e) from config, or auto-detect from codebase
• If the project has a BDD runner (Cucumber, pytest-bdd, etc.), generate step definitions that bind directly to .feature files

In proposal-driven mode (gherkin skipped):

• Tests are driven by proposal.md success criteria and risk points
• Each risk point should have corresponding test coverage
• No annotation conventions (no features to link to)

In no-test mode (testing.required: false):

• Tests are not required. Implementation code is written directly.
• Developers may still write tests voluntarily using testing.behavior/testing.e2e if specified.

Guardrails

• Never implement without reading artifacts first (features in gherkin-driven, proposal in proposal-driven)
• In TDD mode: always write test before implementation
• For @behavior scenarios: always add @covered-by annotation to .feature after writing the test
• For @behavior scenarios: always add @feature/@scenario annotations in the test file
• Keep each change scoped to one scenario/task
• Update task checkbox immediately after completing each task
• Pause on errors, blockers, or unclear requirements -- don't guess
• If implementation reveals issues with features/design, suggest updating artifacts