diff --git a/reference-implementation/runtime/scheduler/run-executor.ts b/reference-implementation/runtime/scheduler/run-executor.ts
index b030e6f20..88c0ef808 100644
--- a/reference-implementation/runtime/scheduler/run-executor.ts
+++ b/reference-implementation/runtime/scheduler/run-executor.ts
@@ -383,6 +383,15 @@ function isBoundedWallClock(maxRunWallClockMs: number): boolean {
   return Number.isFinite(maxRunWallClockMs) && maxRunWallClockMs > 0;
 }
 
+// `setTimeout` cannot delay past 2^31 - 1 ms: Node fires it almost at once.
+// `budget * 4` can also overflow to Infinity, which reads as "unbounded", so a
+// huge finite budget would silently lose its ceiling.
+const MAX_TIMER_DELAY_MS = 2 ** 31 - 1;
+
+function clampTimerDelay(ms: number): number {
+  return Math.min(ms, MAX_TIMER_DELAY_MS);
+}
+
 export function createAttemptWatchdog(maxRunWallClockMs: number): AttemptWatchdog {
   const cancellation = new AbortController();
   let timedOut = false;
@@ -391,7 +400,9 @@ export function createAttemptWatchdog(maxRunWallClockMs: number): AttemptWatchdo
   // watchdog. Keep the exemption bounded: four primary budgets is long enough
   // for the existing local materialization path while still guaranteeing that
   // a connector that stops making progress cannot survive until restart.
-  const localOnlyPhaseMaxRunWallClockMs = maxRunWallClockMs * 4;
+  const localOnlyPhaseMaxRunWallClockMs = isBoundedWallClock(maxRunWallClockMs)
+    ? clampTimerDelay(maxRunWallClockMs * 4)
+    : maxRunWallClockMs;
   // Set once a connector reports `phase_boundary: "local_only_phase_started"`
   // and never cleared for the rest of this attempt. `maxRunWallClockMs` is
   // sized for provider-rate-limited walks (external API pagination); once a
@@ -414,10 +425,16 @@ export function createAttemptWatchdog(maxRunWallClockMs: number): AttemptWatchdo
     timer = setTimeout(() => {
       timedOut = true;
       cancellation.abort("run_timed_out");
-    }, maxRunWallClockMs);
+    }, clampTimerDelay(maxRunWallClockMs));
     timer.unref?.();
   };
 
+  // Suppression is only real while a primary timer could still fire: with an
+  // opt-out budget, after the watchdog fired, or after an owner cancel there is
+  // nothing to suppress, and the caller must not record that it did.
+  const canSuppressPrimaryBudget = () =>
+    isBoundedWallClock(maxRunWallClockMs) && !timedOut && !cancellation.signal.aborted;
+
   const armLocalOnlyPhaseCeiling = () => {
     if (
       !isBoundedWallClock(localOnlyPhaseMaxRunWallClockMs) ||
@@ -432,7 +449,7 @@ export function createAttemptWatchdog(maxRunWallClockMs: number): AttemptWatchdo
     timer = setTimeout(() => {
       timedOut = true;
       cancellation.abort("run_timed_out");
-    }, localOnlyPhaseMaxRunWallClockMs);
+    }, clampTimerDelay(localOnlyPhaseMaxRunWallClockMs));
     timer.unref?.();
   };
 
@@ -453,7 +470,7 @@ export function createAttemptWatchdog(maxRunWallClockMs: number): AttemptWatchdo
       }
     },
     markProgress(extra) {
-      if (extra?.phase_boundary === "local_only_phase_started" && !localOnlyPhase) {
+      if (extra?.phase_boundary === "local_only_phase_started" && !localOnlyPhase && canSuppressPrimaryBudget()) {
         localOnlyPhase = true;
         if (timer) {
           clearTimeout(timer);
diff --git a/reference-implementation/test/run-executor-watchdog-limits.test.ts b/reference-implementation/test/run-executor-watchdog-limits.test.ts
new file mode 100644
index 000000000..e3eabf7fc
--- /dev/null
+++ b/reference-implementation/test/run-executor-watchdog-limits.test.ts
@@ -0,0 +1,90 @@
+// Copyright The PDP-Connect Contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import assert from "node:assert/strict";
+import { mock, test } from "node:test";
+import { createAttemptWatchdog } from "../runtime/scheduler/run-executor.ts";
+
+const PHASE_BOUNDARY = { phase_boundary: "local_only_phase_started" };
+const MAX_TIMER_DELAY_MS = 2 ** 31 - 1;
+const SHORT_BUDGET_MS = 5;
+
+function waitForAbort(signal: AbortSignal): Promise<void> {
+  return new Promise((resolve) => {
+    signal.addEventListener("abort", () => resolve(), { once: true });
+  });
+}
+
+// `setTimeout` cannot delay past 2^31 - 1 ms (Node fires it almost at once), and
+// `budget * 4` can overflow to Infinity, which reads as "unbounded". A huge but
+// finite budget must keep a real, finite ceiling.
+for (const hugeBudget of [2 ** 30, 1e308]) {
+  test(`a finite budget of ${String(hugeBudget)} ms arms timers no longer than the timer limit`, () => {
+    const realSetTimeout = globalThis.setTimeout;
+    const delays: number[] = [];
+    const spy = mock.method(globalThis, "setTimeout", ((callback: () => void, ms?: number) => {
+      delays.push(Number(ms));
+      // Never let the test arm a real long timer.
+      return realSetTimeout(callback, 60_000);
+    }) as typeof setTimeout);
+    const watchdog = createAttemptWatchdog(hugeBudget);
+    try {
+      watchdog.markProgress(PHASE_BOUNDARY);
+      assert.ok(delays.length >= 2, "the primary timer and the local-only ceiling are both armed");
+      for (const delay of delays) {
+        assert.ok(
+          Number.isFinite(delay) && delay <= MAX_TIMER_DELAY_MS,
+          `armed delay ${delay} exceeds the timer limit`
+        );
+      }
+      assert.equal(delays.at(-1), MAX_TIMER_DELAY_MS, "the local-only ceiling stays bounded at the limit");
+    } finally {
+      watchdog.clear();
+      spy.mock.restore();
+    }
+  });
+}
+
+// A "watchdog suppressed" record is written when markProgress reports true.
+// That must be true only when a timer was armed and could still have fired.
+for (const optOut of [0, Number.POSITIVE_INFINITY]) {
+  test(`a phase boundary with the opt-out budget ${String(optOut)} suppresses nothing`, () => {
+    const watchdog = createAttemptWatchdog(optOut);
+    try {
+      assert.equal(watchdog.markProgress(PHASE_BOUNDARY), false);
+    } finally {
+      watchdog.clear();
+    }
+  });
+}
+
+test("a phase boundary arriving after the watchdog fired suppresses nothing", async () => {
+  const watchdog = createAttemptWatchdog(SHORT_BUDGET_MS);
+  try {
+    await waitForAbort(watchdog.signal);
+    assert.equal(watchdog.timedOut(), true);
+    assert.equal(watchdog.markProgress(PHASE_BOUNDARY), false);
+  } finally {
+    watchdog.clear();
+  }
+});
+
+test("a phase boundary arriving after an owner cancel suppresses nothing", () => {
+  const watchdog = createAttemptWatchdog(60_000);
+  try {
+    watchdog.cancel();
+    assert.equal(watchdog.markProgress(PHASE_BOUNDARY), false);
+  } finally {
+    watchdog.clear();
+  }
+});
+
+test("a phase boundary on an armed watchdog latches exactly once", () => {
+  const watchdog = createAttemptWatchdog(60_000);
+  try {
+    assert.equal(watchdog.markProgress(PHASE_BOUNDARY), true);
+    assert.equal(watchdog.markProgress(PHASE_BOUNDARY), false);
+  } finally {
+    watchdog.clear();
+  }
+});