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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

skills/condition-based-waiting/example.ts

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+// Complete implementation of condition-based waiting utilities
+// From: Lace test infrastructure improvements (2025-10-03)
+// Context: Fixed 15 flaky tests by replacing arbitrary timeouts
+
+import type { ThreadManager } from '~/threads/thread-manager';
+import type { LaceEvent, LaceEventType } from '~/threads/types';
+
+/**
+ * Wait for a specific event type to appear in thread
+ *
+ * @param threadManager - The thread manager to query
+ * @param threadId - Thread to check for events
+ * @param eventType - Type of event to wait for
+ * @param timeoutMs - Maximum time to wait (default 5000ms)
+ * @returns Promise resolving to the first matching event
+ *
+ * Example:
+ *   await waitForEvent(threadManager, agentThreadId, 'TOOL_RESULT');
+ */
+export function waitForEvent(
+  threadManager: ThreadManager,
+  threadId: string,
+  eventType: LaceEventType,
+  timeoutMs = 5000
+): Promise<LaceEvent> {
+  return new Promise((resolve, reject) => {
+    const startTime = Date.now();
+
+    const check = () => {
+      const events = threadManager.getEvents(threadId);
+      const event = events.find((e) => e.type === eventType);
+
+      if (event) {
+        resolve(event);
+      } else if (Date.now() - startTime > timeoutMs) {
+        reject(new Error(`Timeout waiting for ${eventType} event after ${timeoutMs}ms`));
+      } else {
+        setTimeout(check, 10); // Poll every 10ms for efficiency
+      }
+    };
+
+    check();
+  });
+}
+
+/**
+ * Wait for a specific number of events of a given type
+ *
+ * @param threadManager - The thread manager to query
+ * @param threadId - Thread to check for events
+ * @param eventType - Type of event to wait for
+ * @param count - Number of events to wait for
+ * @param timeoutMs - Maximum time to wait (default 5000ms)
+ * @returns Promise resolving to all matching events once count is reached
+ *
+ * Example:
+ *   // Wait for 2 AGENT_MESSAGE events (initial response + continuation)
+ *   await waitForEventCount(threadManager, agentThreadId, 'AGENT_MESSAGE', 2);
+ */
+export function waitForEventCount(
+  threadManager: ThreadManager,
+  threadId: string,
+  eventType: LaceEventType,
+  count: number,
+  timeoutMs = 5000
+): Promise<LaceEvent[]> {
+  return new Promise((resolve, reject) => {
+    const startTime = Date.now();
+
+    const check = () => {
+      const events = threadManager.getEvents(threadId);
+      const matchingEvents = events.filter((e) => e.type === eventType);
+
+      if (matchingEvents.length >= count) {
+        resolve(matchingEvents);
+      } else if (Date.now() - startTime > timeoutMs) {
+        reject(
+          new Error(
+            `Timeout waiting for ${count} ${eventType} events after ${timeoutMs}ms (got ${matchingEvents.length})`
+          )
+        );
+      } else {
+        setTimeout(check, 10);
+      }
+    };
+
+    check();
+  });
+}
+
+/**
+ * Wait for an event matching a custom predicate
+ * Useful when you need to check event data, not just type
+ *
+ * @param threadManager - The thread manager to query
+ * @param threadId - Thread to check for events
+ * @param predicate - Function that returns true when event matches
+ * @param description - Human-readable description for error messages
+ * @param timeoutMs - Maximum time to wait (default 5000ms)
+ * @returns Promise resolving to the first matching event
+ *
+ * Example:
+ *   // Wait for TOOL_RESULT with specific ID
+ *   await waitForEventMatch(
+ *     threadManager,
+ *     agentThreadId,
+ *     (e) => e.type === 'TOOL_RESULT' && e.data.id === 'call_123',
+ *     'TOOL_RESULT with id=call_123'
+ *   );
+ */
+export function waitForEventMatch(
+  threadManager: ThreadManager,
+  threadId: string,
+  predicate: (event: LaceEvent) => boolean,
+  description: string,
+  timeoutMs = 5000
+): Promise<LaceEvent> {
+  return new Promise((resolve, reject) => {
+    const startTime = Date.now();
+
+    const check = () => {
+      const events = threadManager.getEvents(threadId);
+      const event = events.find(predicate);
+
+      if (event) {
+        resolve(event);
+      } else if (Date.now() - startTime > timeoutMs) {
+        reject(new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`));
+      } else {
+        setTimeout(check, 10);
+      }
+    };
+
+    check();
+  });
+}
+
+// Usage example from actual debugging session:
+//
+// BEFORE (flaky):
+// ---------------
+// const messagePromise = agent.sendMessage('Execute tools');
+// await new Promise(r => setTimeout(r, 300)); // Hope tools start in 300ms
+// agent.abort();
+// await messagePromise;
+// await new Promise(r => setTimeout(r, 50));  // Hope results arrive in 50ms
+// expect(toolResults.length).toBe(2);         // Fails randomly
+//
+// AFTER (reliable):
+// ----------------
+// const messagePromise = agent.sendMessage('Execute tools');
+// await waitForEventCount(threadManager, threadId, 'TOOL_CALL', 2); // Wait for tools to start
+// agent.abort();
+// await messagePromise;
+// await waitForEventCount(threadManager, threadId, 'TOOL_RESULT', 2); // Wait for results
+// expect(toolResults.length).toBe(2); // Always succeeds
+//
+// Result: 60% pass rate → 100%, 40% faster execution