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diff --git a/packages/host-bin/src/mem-telemetry.ts b/packages/host-bin/src/mem-telemetry.ts
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+/**
+ * Periodic memory telemetry — leak-localization edge effect.
+ *
+ * Owns the timers (setInterval) that periodically log process.memoryUsage()
+ * so RSS growth can be correlated with active conversations/turns and the
+ * leaking subsystem pinpointed. This is the composition-root edge effect
+ * (AGENTS.md: timers are edge effects owned by host-bin, NOT the kernel).
+ *
+ * All effects are injected (sampler, GC, clock, logger, active-conversation
+ * count) so the timer logic is fully testable — tests pass fakes and assert
+ * the emitted log calls, never waiting on a real wall clock. No ambient
+ * state (P3): the timers are owned explicitly and returned as a `stop()`
+ * handle that the composition root clears on shutdown.
+ *
+ * The per-turn before/after sampling lives in session-orchestrator (it wraps
+ * the stream boundary); this module owns the PERIODIC baseline + GC logging.
+ */
+
+import type { Logger } from "@dispatch/kernel";
+import {
+  type MemorySample,
+  memoryDelta,
+  memorySampleAttributes,
+} from "@dispatch/session-orchestrator";
+
+/** Default periodic sample interval: every 15s. */
+export const DEFAULT_MEMORY_SAMPLE_INTERVAL_MS = 15_000;
+
+/** Default GC interval: every 5 min (longer than the sample interval). */
+export const DEFAULT_GC_INTERVAL_MS = 5 * 60_000;
+
+/**
+ * Deps injected into {@link startMemoryTelemetry}. Every effect is explicit so
+ * the timer logic is reproducible from its inputs (no ambient state, P3) and
+ * testable without a real clock or process.
+ */
+export interface MemoryTelemetryDeps {
+  /** Logger (auto-scoped to host-bin by the composition root). */
+  readonly logger: Logger;
+  /** Edge effect: capture a {@link MemorySample} now (process.memoryUsage()). */
+  readonly sampleMemory: () => MemorySample;
+  /**
+   * Edge effect: run a full GC cycle. In production this is `() =>
+   * Bun.gc(true)`; tests pass a no-op or a counting fake. Used on the longer
+   * GC interval to distinguish live retained objects from GC fragmentation.
+   */
+  readonly gc: () => void;
+  /**
+   * The number of conversations currently driving a turn (from the
+   * session-orchestrator's activeConversations set). Tags each periodic
+   * sample so growth can be attributed to the streaming/turn path vs an idle
+   * baseline.
+   */
+  readonly getActiveConversationCount: () => number;
+  /** Periodic sample interval (ms). Defaults to 15s. */
+  readonly sampleIntervalMs?: number;
+  /** GC interval (ms). Defaults to 5 min. */
+  readonly gcIntervalMs?: number;
+  /**
+   * Injected timer scheduler (defaults to global setInterval). Tests pass a
+   * fake to drive ticks deterministically without real wall-clock waits. The
+   * handle type is opaque (the same type {@link clearInterval} accepts).
+   */
+  readonly setInterval?: (fn: () => void, ms: number) => MemoryTimerHandle;
+  /** Injected timer clearer (defaults to global clearInterval). */
+  readonly clearInterval?: (handle: MemoryTimerHandle | undefined) => void;
+}
+
+/** Opaque timer handle shared by {@link MemoryTelemetryDeps.setInterval} / clearInterval. */
+export type MemoryTimerHandle = ReturnType<typeof globalThis.setInterval>;
+
+/** Handle returned by {@link startMemoryTelemetry} to stop the timers. */
+export interface MemoryTelemetryHandle {
+  /** Stop both timers. Idempotent. Called by the composition root on shutdown. */
+  readonly stop: () => void;
+}
+
+/**
+ * Start periodic memory telemetry. Logs process.memoryUsage() every
+ * `sampleIntervalMs` (default 15s) tagged with the active-conversation count,
+ * and every `gcIntervalMs` (default 5 min) runs `gc()` and logs RSS
+ * before/after to distinguish live retained objects from GC fragmentation.
+ *
+ * Returns a `stop()` handle that clears both timers. The composition root
+ * (host-bin main) owns this handle and calls `stop()` on shutdown so the
+ * timers never leak across a restart.
+ *
+ * Pure decision logic: {@link buildPeriodicAttributes} /
+ * {@link buildGcAttributes} are exported separately for unit testing.
+ */
+export function startMemoryTelemetry(deps: MemoryTelemetryDeps): MemoryTelemetryHandle {
+  const sampleIntervalMs = deps.sampleIntervalMs ?? DEFAULT_MEMORY_SAMPLE_INTERVAL_MS;
+  const gcIntervalMs = deps.gcIntervalMs ?? DEFAULT_GC_INTERVAL_MS;
+  const setIntervalFn = deps.setInterval ?? globalThis.setInterval;
+  const clearIntervalFn = deps.clearInterval ?? globalThis.clearInterval;
+
+  let gcHandle: MemoryTimerHandle | undefined;
+
+  // Periodic sample: log rss/heap/external/arrayBuffers + active-conversation
+  // count every 15s. Correlates RSS growth with active turns so the leak can
+  // be attributed to the streaming path vs an idle baseline.
+  const sampleHandle: MemoryTimerHandle | undefined = setIntervalFn(() => {
+    const sample = deps.sampleMemory();
+    const activeConversations = deps.getActiveConversationCount();
+    deps.logger.info("memory:periodic", buildPeriodicAttributes(sample, activeConversations));
+  }, sampleIntervalMs);
+
+  // GC log: on a longer interval, force a full GC and log RSS before/after.
+  // A small/no drop after gc means the memory is LIVE (retained objects — the
+  // leak); a large drop means it was GC fragmentation (reclaimable). This
+  // distinguishes the two failure modes the crash investigation flagged.
+  gcHandle = setIntervalFn(() => {
+    const before = deps.sampleMemory();
+    deps.gc();
+    const after = deps.sampleMemory();
+    deps.logger.info("memory:gc", buildGcAttributes(before, after));
+  }, gcIntervalMs);
+
+  let stopped = false;
+  return {
+    stop() {
+      if (stopped) return; // idempotent — safe to call on every shutdown path
+      stopped = true;
+      clearIntervalFn(sampleHandle);
+      clearIntervalFn(gcHandle);
+    },
+  };
+}
+
+/**
+ * Pure: build the logger attributes for a periodic sample. Exported for unit
+ * testing (no I/O, no clock). The `activeConversations` count tags the sample
+ * so growth can be attributed to the streaming/turn path vs idle baseline.
+ */
+export function buildPeriodicAttributes(
+  sample: MemorySample,
+  activeConversations: number,
+): ReturnType<typeof memorySampleAttributes> & { activeConversations: number } {
+  return { ...memorySampleAttributes(sample), activeConversations };
+}
+
+/**
+ * Pure: build the logger attributes for a GC sample. Exported for unit testing.
+ * Carries the absolute after-sample plus the `reclaimed` delta
+ * (`before - after`): a POSITIVE `reclaimedRssMB` means GC freed memory
+ * (fragmentation, reclaimable); near-zero/negative means the memory is LIVE
+ * (retained objects — the leak). This distinguishes the two failure modes the
+ * crash investigation flagged.
+ */
+export function buildGcAttributes(
+  before: MemorySample,
+  after: MemorySample,
+): ReturnType<typeof memorySampleAttributes> {
+  // reclaimed = before - after (how much GC freed). memoryDelta(a, b) = b - a,
+  // so pass (after, before) to get before - after.
+  return {
+    ...memorySampleAttributes(after),
+    ...memorySampleAttributes(memoryDelta(after, before), "reclaimed"),
+  };
+}