Subagent Driven Development

Subagent-Driven Development

Overview

Execute implementation plans by dispatching fresh subagents per task with a systematic two-stage review pipeline: spec compliance first, code quality second. Each task gets its own implementer subagent with clean context, followed by two independent reviewer subagents that gate progression.

Core principle: Fresh subagent per task + two-stage review (spec then quality) = high quality, fast iteration, no context pollution.

When to Use

Use this skill when any of the following are true:

• You have an implementation plan (from the plan skill, a plan file, or user requirements broken into tasks)
• Tasks are mostly independent and can be executed sequentially or in parallel
• Quality and spec compliance are important — you need gates, not just "it ran"
• You want automated review between tasks to catch issues before they compound

Do NOT use when:

• You have no plan or task breakdown — create one with the plan skill first
• Tasks are deeply interdependent and cannot be reviewed in isolation
• The work is a single trivial change (just do it directly)

vs. manual execution in a single session:

• Fresh context per task — no confusion from accumulated state across tasks
• Automated review process catches issues early, before they propagate
• Consistent quality checks across all tasks
• Subagents can ask clarifying questions before starting work

Prerequisites

1. An implementation plan — either a plan file (e.g., docs/plans/feature-plan.md) or a structured task list. If you don't have one, use the plan skill first.
2. Task granularity of 2–5 minutes each. If a task is larger, split it before starting.
3. Project context available — language, framework, test runner, existing file structure, dependencies.
4. delegate_task capability — the controller agent must be able to dispatch subagents with specified toolsets.

Procedure

Step 1 — Read and Parse the Plan (Once)

Read the plan file once in the controller session. Extract ALL tasks with their full text and context upfront. Do not make subagents re-read the plan file — you will provide full task text directly in each subagent's context.

Create a todo list capturing every task:

# Read the plan
read_file("docs/plans/feature-plan.md")

# Create todo list with all tasks
todo([
    {"id": "task-1", "content": "Create User model with email field", "status": "pending"},
    {"id": "task-2", "content": "Add password hashing utility", "status": "pending"},
    {"id": "task-3", "content": "Create login endpoint", "status": "pending"},
])

Key rule: Read the plan ONCE. Extract everything. Each subagent receives the full task text inline — never instruct a subagent to read the plan file itself.

Step 2 — Per-Task Workflow (Repeat for Every Task)

For EACH task in the plan, execute the four-phase loop below. Do not advance to the next task until the current task passes both reviews.

Phase 2a — Dispatch Implementer Subagent

Use delegate_task with complete context including: the task spec, TDD instructions, and project context (scene-setting).

delegate_task(
    goal="Implement Task 1: Create User model with email and password_hash fields",
    context="""
    TASK FROM PLAN:
    - Create: src/models/user.py
    - Add User class with email (str) and password_hash (str) fields
    - Use bcrypt for password hashing
    - Include __repr__ for debugging

    FOLLOW TDD:
    1. Write failing test in tests/models/test_user.py
    2. Run: pytest tests/models/test_user.py -v (verify FAIL)
    3. Write minimal implementation
    4. Run: pytest tests/models/test_user.py -v (verify PASS)
    5. Run: pytest tests/ -q (verify no regressions)
    6. Commit: git add -A && git commit -m "feat: add User model with password hashing"

    PROJECT CONTEXT:
    - Python 3.11, Flask app in src/app.py
    - Existing models in src/models/
    - Tests use pytest, run from project root
    - bcrypt already in requirements.txt
    """,
    toolsets=['terminal', 'file']
)

If the subagent asks questions: Answer clearly and completely before letting it proceed. Provide additional context if needed. Do not rush it into implementation.

Phase 2b — Dispatch Spec Compliance Reviewer

After the implementer completes, verify the implementation against the original spec — not against what the implementer thought they were asked to do.

delegate_task(
    goal="Review if implementation matches the spec from the plan",
    context="""
    ORIGINAL TASK SPEC:
    - Create src/models/user.py with User class
    - Fields: email (str), password_hash (str)
    - Use bcrypt for password hashing
    - Include __repr__

    CHECK:
    - [ ] All requirements from spec implemented?
    - [ ] File paths match spec?
    - [ ] Function signatures match spec?
    - [ ] Behavior matches expected?
    - [ ] Nothing extra added (no scope creep)?

    OUTPUT: PASS or list of specific spec gaps to fix.
    """,
    toolsets=['file']
)

If spec issues found: Dispatch a fix subagent (or re-dispatch the implementer) with specific instructions about the gaps. Then re-run the spec review. Continue only when the reviewer outputs PASS.

HARD RULE: Never start code quality review before spec compliance returns PASS. The order is non-negotiable: spec first, quality second.

Phase 2c — Dispatch Code Quality Reviewer

Only after spec compliance passes:

delegate_task(
    goal="Review code quality for Task 1 implementation",
    context="""
    FILES TO REVIEW:
    - src/models/user.py
    - tests/models/test_user.py

    CHECK:
    - [ ] Follows project conventions and style?
    - [ ] Proper error handling?
    - [ ] Clear variable/function names?
    - [ ] Adequate test coverage?
    - [ ] No obvious bugs or missed edge cases?
    - [ ] No security issues?

    OUTPUT FORMAT:
    - Critical Issues: [must fix before proceeding]
    - Important Issues: [should fix]
    - Minor Issues: [optional]
    - Verdict: APPROVED or REQUEST_CHANGES
    """,
    toolsets=['file']
)

If quality issues found: Fix critical and important issues (minor are optional), then re-review. Continue only when the reviewer returns APPROVED.

HARD RULE: Never let the implementer self-review replace either of these review stages. Both spec and quality reviews must be independent subagents.

Phase 2d — Mark Task Complete

todo([{"id": "task-1", "content": "Create User model with email field", "status": "completed"}], merge=True)

Step 3 — Final Integration Review

After ALL tasks are complete, dispatch a final integration reviewer to check cross-task consistency:

delegate_task(
    goal="Review the entire implementation for consistency and integration issues",
    context="""
    All tasks from the plan are complete. Review the full implementation:
    - Do all components work together?
    - Any inconsistencies between tasks?
    - All tests passing?
    - Ready for merge?
    """,
    toolsets=['terminal', 'file']
)

For the final integration review, use the review dimensions from the requesting-code-review skill.

Step 4 — Verify and Commit

Run the full test suite and review all changes before the final commit:

Windows (PowerShell):

# Run full test suite
pytest tests/ -q

# Review all changes
git diff --stat

# Final commit if needed
git add -A; git commit -m "feat: complete [feature name] implementation"

Linux/macOS:

# Run full test suite
pytest tests/ -q

# Review all changes
git diff --stat

# Final commit if needed
git add -A && git commit -m "feat: complete [feature name] implementation"

Task Granularity

Each task = 2–5 minutes of focused work.

If a task feels larger than 5 minutes, split it into sub-tasks before dispatching.

Handling Issues

If a Subagent Asks Questions

• Answer clearly and completely
• Provide additional context if needed
• Do not rush them into implementation — unanswered questions lead to wrong implementations

If a Reviewer Finds Issues

• Dispatch a fix subagent (or re-dispatch the implementer) with specific instructions about what went wrong
• Re-run the same review stage after the fix
• Repeat until that stage passes
• Never skip the re-review — a fix without re-review is an unverified fix

If a Subagent Fails a Task

• Dispatch a new fix subagent with specific instructions about what went wrong
• Do not try to fix manually in the controller session — this causes context pollution and defeats the purpose of fresh subagents

Pitfalls — Never Do These

1. Start implementation without a plan. Always have a task breakdown first.
2. Skip reviews — neither spec compliance nor code quality may be skipped.
3. Proceed with unfixed critical or important issues. Fix and re-review first.
4. Dispatch multiple implementation subagents for tasks that touch the same files in parallel — this causes conflicts. Run them sequentially.
5. Make subagents read the plan file. Provide full task text directly in context instead.
6. Skip scene-setting context. Subagents need to understand where the task fits in the project.
7. Ignore subagent questions. Answer before letting them proceed.
8. Accept "close enough" on spec compliance. Either it matches the spec or it doesn't.
9. Skip review loops. Reviewer found issues → implementer fixes → review again. Always.
10. Let implementer self-review replace actual review. Both spec and quality reviews must be independent subagents.
11. Start code quality review before spec compliance is PASS. Wrong order — spec first, quality second.
12. Move to the next task while either review has open issues. Both must be cleared.

Verification

After the full workflow completes, verify the outcome:

# All tests pass
pytest tests/ -q
# Expected: all tests pass, 0 failures

# No uncommitted changes left behind (or intentional final commit)
git status
# Expected: clean working tree (or only expected untracked files)

# Review the full diff
git diff --stat
# Expected: only files from the plan, no unexpected changes

# Verify each task has a commit
git log --oneline -10
# Expected: commits matching each task's commit message

Checklist before declaring done:

• Every task in the todo list is marked completed
• Every task passed spec compliance review (PASS)
• Every task passed code quality review (APPROVED)
• Final integration review completed with no open issues
• Full test suite passes
• All changes committed

Efficiency Notes

Why fresh subagent per task:

• Prevents context pollution from accumulated state
• Each subagent gets clean, focused context
• No confusion from prior tasks' code or reasoning

Why two-stage review (spec then quality):

• Spec review catches under-building (missing requirements) and over-building (scope creep) early
• Quality review ensures the implementation is well-built
• Catches issues before they compound across tasks

Cost trade-off:

• More subagent invocations (implementer + 2 reviewers per task)
• But catches issues early — far cheaper than debugging compounded problems later

Integration with Other Skills

With plan

This skill executes plans created by the plan skill:

1. User requirements → plan → implementation plan
2. Implementation plan → subagent-driven-development → working code

With test-driven-development

Implementer subagents should follow TDD. Include TDD instructions in every implementer context:

1. Write failing test first
2. Implement minimal code
3. Verify test passes
4. Commit

With requesting-code-review

The two-stage review process IS the code review for each task. For the final integration review, use the requesting-code-review skill's review dimensions.

With systematic-debugging

If a subagent encounters bugs during implementation:

1. Follow the systematic-debugging process
2. Find root cause before fixing
3. Write a regression test
4. Resume implementation

Example Workflow

[Read plan: docs/plans/auth-feature.md]
[Create todo list with 5 tasks]

--- Task 1: Create User model ---
[Dispatch implementer subagent]
  Implementer: "Should email be unique?"
  You: "Yes, email must be unique"
  Implementer: Implemented, 3/3 tests passing, committed.

[Dispatch spec reviewer]
  Spec reviewer: ✅ PASS — all requirements met

[Dispatch quality reviewer]
  Quality reviewer: ✅ APPROVED — clean code, good tests

[Mark Task 1 complete]

--- Task 2: Password hashing ---
[Dispatch implementer subagent]
  Implementer: No questions, implemented, 5/5 tests passing.

[Dispatch spec reviewer]
  Spec reviewer: ❌ Missing: password strength validation (spec says "min 8 chars")

[Dispatch fix subagent]
  Fix subagent: Added validation, 7/7 tests passing.

[Dispatch spec reviewer again]
  Spec reviewer: ✅ PASS

[Dispatch quality reviewer]
  Quality reviewer: Important — magic number 8, extract to constant
  Fix subagent: Extracted MIN_PASSWORD_LENGTH constant
  Quality reviewer: ✅ APPROVED

[Mark Task 2 complete]

... (continue for all tasks)

[After all tasks: dispatch final integration reviewer]
[Run full test suite: all passing]
[Done!]

References (Load When Relevant)

When orchestration involves significant context usage, long review loops, or complex validation checkpoints, load these reference files for the specific discipline:

• references/context-budget-discipline.md — Four-tier context degradation model (PEAK / GOOD / DEGRADING / POOR), read-depth rules that scale with context window size, and early warning signs of silent degradation. Load when a run will clearly consume significant context (multi-phase plans, many subagents, large artifacts).

• references/gates-taxonomy.md — The four canonical gate types (Pre-flight, Revision, Escalation, Abort) with behavior, recovery, and examples. Load when designing or reviewing any workflow that has validation checkpoints — use the vocabulary explicitly so each gate has defined entry, failure behavior, and resumption rules.

Both references adapted from gsd-build/get-shit-done (MIT © 2025 Lex Christopherson).

Remember

Fresh subagent per task
Two-stage review every time
Spec compliance FIRST
Code quality SECOND
Never skip reviews
Catch issues early

Quality is not an accident. It's the result of systematic process.