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skills/condition-based-waiting/SKILL.md

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+---
+name: condition-based-waiting
+description: Use when tests have race conditions, timing dependencies, or inconsistent pass/fail behavior - replaces arbitrary timeouts with condition polling to wait for actual state changes, eliminating flaky tests from timing guesses
+---
+
+# Condition-Based Waiting
+
+## Overview
+
+Flaky tests often guess at timing with arbitrary delays. This creates race conditions where tests pass on fast machines but fail under load or in CI.
+
+**Core principle:** Wait for the actual condition you care about, not a guess about how long it takes.
+
+## When to Use
+
+```dot
+digraph when_to_use {
+    "Test uses setTimeout/sleep?" [shape=diamond];
+    "Testing timing behavior?" [shape=diamond];
+    "Document WHY timeout needed" [shape=box];
+    "Use condition-based waiting" [shape=box];
+
+    "Test uses setTimeout/sleep?" -> "Testing timing behavior?" [label="yes"];
+    "Testing timing behavior?" -> "Document WHY timeout needed" [label="yes"];
+    "Testing timing behavior?" -> "Use condition-based waiting" [label="no"];
+}
+```
+
+**Use when:**
+- Tests have arbitrary delays (`setTimeout`, `sleep`, `time.sleep()`)
+- Tests are flaky (pass sometimes, fail under load)
+- Tests timeout when run in parallel
+- Waiting for async operations to complete
+
+**Don't use when:**
+- Testing actual timing behavior (debounce, throttle intervals)
+- Always document WHY if using arbitrary timeout
+
+## Core Pattern
+
+```typescript
+// ❌ BEFORE: Guessing at timing
+await new Promise(r => setTimeout(r, 50));
+const result = getResult();
+expect(result).toBeDefined();
+
+// ✅ AFTER: Waiting for condition
+await waitFor(() => getResult() !== undefined);
+const result = getResult();
+expect(result).toBeDefined();
+```
+
+## Quick Patterns
+
+| Scenario | Pattern |
+|----------|---------|
+| Wait for event | `waitFor(() => events.find(e => e.type === 'DONE'))` |
+| Wait for state | `waitFor(() => machine.state === 'ready')` |
+| Wait for count | `waitFor(() => items.length >= 5)` |
+| Wait for file | `waitFor(() => fs.existsSync(path))` |
+| Complex condition | `waitFor(() => obj.ready && obj.value > 10)` |
+
+## Implementation
+
+Generic polling function:
+```typescript
+async function waitFor<T>(
+  condition: () => T | undefined | null | false,
+  description: string,
+  timeoutMs = 5000
+): Promise<T> {
+  const startTime = Date.now();
+
+  while (true) {
+    const result = condition();
+    if (result) return result;
+
+    if (Date.now() - startTime > timeoutMs) {
+      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
+    }
+
+    await new Promise(r => setTimeout(r, 10)); // Poll every 10ms
+  }
+}
+```
+
+See @example.ts for complete implementation with domain-specific helpers (`waitForEvent`, `waitForEventCount`, `waitForEventMatch`) from actual debugging session.
+
+## Common Mistakes
+
+**❌ Polling too fast:** `setTimeout(check, 1)` - wastes CPU
+**✅ Fix:** Poll every 10ms
+
+**❌ No timeout:** Loop forever if condition never met
+**✅ Fix:** Always include timeout with clear error
+
+**❌ Stale data:** Cache state before loop
+**✅ Fix:** Call getter inside loop for fresh data
+
+## When Arbitrary Timeout IS Correct
+
+```typescript
+// Tool ticks every 100ms - need 2 ticks to verify partial output
+await waitForEvent(manager, 'TOOL_STARTED'); // First: wait for condition
+await new Promise(r => setTimeout(r, 200));   // Then: wait for timed behavior
+// 200ms = 2 ticks at 100ms intervals - documented and justified
+```
+
+**Requirements:**
+1. First wait for triggering condition
+2. Based on known timing (not guessing)
+3. Comment explaining WHY
+
+## Real-World Impact
+
+From debugging session (2025-10-03):
+- Fixed 15 flaky tests across 3 files
+- Pass rate: 60% → 100%
+- Execution time: 40% faster
+- No more race conditions