nowsprinting/unity-coding-skills/plan-feature

Guide for plan mode. This skill defines the orchestration workflow for test-first implementation planning.

Mode Check

This skill requires plan mode. Before doing anything else, check the current mode:

• ExitPlanMode is in the deferred tools list → not in plan mode → stop immediately and tell the user:

> "This skill (/plan-feature) requires plan mode. Enter plan mode first: use /plan or press Shift+Tab to toggle."

• ExitPlanMode is NOT in the deferred tools list (i.e., directly callable) → in plan mode → proceed.

Task Type Check

If the user's request is to investigate or fix a bug rather than implement a new feature, change a specification, or refactor, use ExitPlanMode immediately and guide the user to invoke the /fix-bug skill instead. The rest of this skill applies to feature implementation, spec changes, and refactoring only.

Plan Mode Workflow

Step 1: Initial Understanding

Launch Explore agents to understand the codebase relevant to the task.

TBD items: If the requirements or specifications explicitly contain the text "TBD" for any item, treat that item as non-existent — do not design or implement it. Only ask the user via AskUserQuestion if the TBD item is a prerequisite that cannot be deferred without blocking the overall design.

Step 2: Implementation Design (Plan Agent)

Launch a Plan agent to design the class/method structure. Include the following instruction in the Plan agent prompt:

Design the class/method seams with testability in mind:

- Prefer small, focused public interfaces

- Test through the same seam production code uses. Tests should exercise the same public/internal API that production callers go through. Do not promote a private method or widen visibility merely to reach it from a test — by default, private stays private.

- Sanctioned exception (reduce combinatorial coverage): When a method takes 3 or more parameters — a heuristic for the real trigger, which is that several independent conditions combine so exhaustive coverage cost explodes (this can also happen with fewer parameters that each take many values, and may not apply when the extra parameters don't drive branching) — extract the cohesive sub-logic (a pure computation or decision that depends on only 1–2 of those inputs) into its own unit. Prefer a standalone class or pure function when the logic stands on its own; fall back to an internal method on the same class only when it cannot be cleanly separated from instance state. Give the extracted unit internal (not public) visibility when it need not be reachable outside the assembly — the narrowed visibility is a consequence of the extraction, not the goal.

- Inject dependencies via interfaces so they can be replaced with test doubles

- Avoid hidden static/global state and new calls inside constructors for external dependencies

Naming: If any class or public method name explicitly specified by the user is a poor fit for what the spec describes, propose a more appropriate alternative using AskUserQuestion before finalizing the design. Accept the user's final choice without further challenge.

TBD items: Any item explicitly marked "TBD" in the requirements or spec must be excluded from the design. Skip it silently unless it is structurally required to complete the design (in which case, ask via AskUserQuestion).

The Plan agent output should include only:

• Class names and responsibilities
• Public and internal method signatures
• Dependency interfaces (if any)
• File placement: the file path for each new or modified class, and the assembly (.asmdef) it belongs to — assembly boundaries drive test references and InternalsVisibleTo, so the implementer must not have to re-derive placement
• Brief rationale for design decisions

Do NOT include test cases, manual tests, or any test design and verification instructions — those are the sole responsibility of the test-designer agent in Step 3.

Step 3: Test Case Design (test-designer Agent)

After Step 2, launch the test-designer agent with an Opus-class or more capable model set in the model parameter, so test design stays on a strong model even when the main session runs on a faster one. Use the following prompt structure:

## Requirements
[feature requirements]
 
## Implementation Design
[class names, public and internal method signatures, dependency interfaces, and design rationale from the Step 2 Plan agent]
 
## Existing Code Context
[relevant existing code structure from Step 1 Explore]
 
## Language Convention
[project language for test names and prose, e.g., Japanese]

Rules for assembling the prompt:

• Under Implementation Design, include only the design output — do NOT include any test cases or manual tests the Plan agent may have produced. Test design is the test-designer agent's sole responsibility.
• Do NOT add output format specifications. The test-designer agent's output format is self-contained; caller-supplied format overrides produce non-standard output.
• Always include `## Language Convention`. Resolve the project language from CLAUDE.md (look for the language specified for test method names / code comments). If CLAUDE.md does not specify a language, default to English.

The test-designer agent returns:

• Test Cases across all layers (Editor tests, Unit tests, Integration tests, Visual verification tests, Manual tests) — ready to paste into the plan file as one block
• Testability Assessment (TESTABILITY: PASS, WARN, or FAIL)

Handling the Testability Assessment

Loopback to Step 2 (on FAIL)

1. Extract the "Testability Issues" table from the test-designer agent output
2. Re-launch the Plan agent with:

• The previous design output
• The Testability Issues
• Instruction: "Revise the design to address the Testability Issues listed below"

1. Re-run Step 3 with the revised design
2. If still FAIL after one retry → Abort (see below)

Abort (second consecutive FAIL)

Use AskUserQuestion to present the user with three options:

• Proceed with the current design despite testability concerns
• Exit plan mode to revise the requirements
• Provide explicit design hints and re-run Step 2

Step 4: Review

Read the critical files identified in the plan. Verify that the Plan agent's design and the test-designer agent's test cases are consistent with each other and with the user's intent.

Step 5: Write the Plan File

Assemble the plan file with the following sections:

1. Context — why this change is needed
2. Implementation Design — from Step 2 Plan agent output
3. Test Cases — pasted verbatim as one block from the test-designer agent output (all 5 layers: Editor tests, Unit tests, Integration tests, Visual verification tests, Manual tests). Do not rewrite, translate, or clean up the output — the test-designer agent already applies the content restrictions in test-designing-guide.
4. Known Trade-offs — from TESTABILITY: WARN issues (if any)
5. Development Workflow — Read ${CLAUDE_SKILL_DIR}/assets/development-workflow-template.md and paste its full contents verbatim as the body of this section in the plan file, then add any project-specific steps per CLAUDE.md

Step 6: Call ExitPlanMode