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skills/software-development/subagent-driven-development/references/gates-taxonomy.md

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+# Gates Taxonomy
+
+Canonical gate types for validation checkpoints across any workflow that spawns subagents, runs review loops, or has human-approval pauses. Every validation checkpoint maps to one of these four types — naming them explicitly makes the workflow legible and prevents "what happens when this check fails?" confusion.
+
+Adapted from the GSD (Get Shit Done) project's gates reference — MIT © 2025 Lex Christopherson ([gsd-build/get-shit-done](https://github.com/gsd-build/get-shit-done)).
+
+## The four gate types
+
+### 1. Pre-flight gate
+
+**Purpose:** Validates preconditions before starting an operation.
+
+**Behavior:** Blocks entry if conditions unmet. No partial work created — bail before anything changes.
+
+**Recovery:** Fix the missing precondition, then retry.
+
+**Examples:**
+- Implementation phase checks that the plan file exists before it starts writing code.
+- Delegated subagent checks that required env vars are set before making API calls.
+- Commit checks that tests passed before pushing.
+
+### 2. Revision gate
+
+**Purpose:** Evaluates output quality and routes to revision if insufficient.
+
+**Behavior:** Loops back to the producer with specific feedback. Bounded by an iteration cap (typically 3).
+
+**Recovery:** Producer addresses feedback; checker re-evaluates. The loop escalates early if issue count does not decrease between consecutive iterations (stall detection). After max iterations, escalates to the user unconditionally — never loop forever.
+
+**Examples:**
+- Plan reviewer reads a draft plan, returns specific issues, planner revises, reviewer re-reads (max 3 cycles).
+- Code reviewer checks subagent-produced code against must-haves; dispatches fixes back to the implementer if any must-have failed.
+- Test coverage checker validates new tests exercise the new paths; if not, sends back to author.
+
+### 3. Escalation gate
+
+**Purpose:** Surfaces unresolvable issues to the human for a decision.
+
+**Behavior:** Pauses workflow, presents options, waits for human input. Never guesses, never picks a default.
+
+**Recovery:** Human chooses action; workflow resumes on the selected path.
+
+**Examples:**
+- Revision loop exhausted after 3 iterations.
+- Merge conflict during automated worktree cleanup.
+- Ambiguous requirement — two reasonable interpretations and the choice changes the approach.
+- Subagent reports "the plan says X but the codebase actually does Y" — human decides which is right.
+
+### 4. Abort gate
+
+**Purpose:** Terminates the operation to prevent damage or waste.
+
+**Behavior:** Stops immediately, preserves state (checkpoint current progress), reports the specific reason.
+
+**Recovery:** Human investigates root cause, fixes, restarts from checkpoint.
+
+**Examples:**
+- Context window critically low during execution (POOR tier, >70%) — abort cleanly rather than produce truncated output.
+- Critical dependency unavailable mid-run (network down, API key revoked).
+- Unrecoverable filesystem state (disk full, permissions lost).
+- Safety invariant violated (agent attempted an irreversible destructive action outside approved scope).
+
+## How to use this in a skill
+
+When you write an orchestration skill that has validation checkpoints, **name each checkpoint by its gate type explicitly** and answer three questions:
+
+1. **What condition triggers this gate?** (e.g., "plan file missing", "issue count didn't decrease", "context >70%")
+2. **What happens when it fails?** (block / loop back / ask human / abort)
+3. **Who resumes, and from where?** (fix precondition + retry, revise + re-check, human decision, restart from checkpoint)
+
+Answering these three up front means your skill never hits "what do we do now?" at runtime.
+
+## Example — a review loop with all four gate types
+
+```
+[Pre-flight] plan.md exists and is non-empty?   → no: bail, ask user to write a plan first
+                ↓ yes
+[Execute]  subagent implements task
+                ↓
+[Revision] reviewer checks against must-haves  → fail: loop back to subagent (max 3)
+                ↓ pass
+[Pre-flight] tests pass?                       → no: bail, report failing tests
+                ↓ yes
+[Commit]
+                ↓
+(on revision loop exhaustion)
+[Escalation] "3 review cycles failed to converge on issue X — pick: force-merge, rewrite task, abandon"
+                ↓ user picks
+(on any tier-POOR context pressure during loop)
+[Abort] "context at 73%, checkpointing and stopping"
+```
+
+The vocabulary is small on purpose. Every gate in every workflow should fit one of these four. If you find yourself inventing a fifth, it's probably a revision gate with extra branching, or an escalation gate in disguise.