diff --git a/src/logging/diagnostic-memory.test.ts b/src/logging/diagnostic-memory.test.ts
index dce15fead67b..ff87998e9c73 100644
--- a/src/logging/diagnostic-memory.test.ts
+++ b/src/logging/diagnostic-memory.test.ts
@@ -156,6 +156,75 @@ describe("diagnostic memory", () => {
     expect(events.map((event) => event.type)).toEqual(["diagnostic.memory.pressure"]);
   });
 
+  it("scales default heap pressure thresholds with enlarged V8 limits", () => {
+    const events: DiagnosticEventPayload[] = [];
+    const stop = onDiagnosticEvent((event) => events.push(event));
+    const gb = 1024 ** 3;
+
+    emitDiagnosticMemorySample({
+      now: 1000,
+      heapSizeLimitBytes: 8 * gb,
+      memoryUsage: memoryUsage({ heapUsed: 2.1 * gb }),
+    });
+    expect(events.filter((event) => event.type === "diagnostic.memory.pressure")).toEqual([]);
+
+    emitDiagnosticMemorySample({
+      now: 2000,
+      heapSizeLimitBytes: 8 * gb,
+      memoryUsage: memoryUsage({ heapUsed: 4.1 * gb }),
+    });
+    emitDiagnosticMemorySample({
+      now: 3000,
+      heapSizeLimitBytes: 8 * gb,
+      memoryUsage: memoryUsage({ heapUsed: 6.1 * gb }),
+    });
+    stop();
+
+    expect(
+      events
+        .filter((event) => event.type === "diagnostic.memory.pressure")
+        .map((event) => ({
+          level: event.level,
+          reason: event.reason,
+          threshold: event.thresholdBytes,
+        })),
+    ).toEqual([
+      { level: "warning", reason: "heap_threshold", threshold: 4 * gb },
+      { level: "critical", reason: "heap_threshold", threshold: 6 * gb },
+    ]);
+  });
+
+  it("scales default heap pressure thresholds down for constrained V8 limits", () => {
+    const events: DiagnosticEventPayload[] = [];
+    const stop = onDiagnosticEvent((event) => events.push(event));
+    const mb = 1024 ** 2;
+
+    emitDiagnosticMemorySample({
+      now: 1000,
+      heapSizeLimitBytes: 1024 * mb,
+      memoryUsage: memoryUsage({ heapUsed: 600 * mb }),
+    });
+    emitDiagnosticMemorySample({
+      now: 2000,
+      heapSizeLimitBytes: 1024 * mb,
+      memoryUsage: memoryUsage({ heapUsed: 800 * mb }),
+    });
+    stop();
+
+    expect(
+      events
+        .filter((event) => event.type === "diagnostic.memory.pressure")
+        .map((event) => ({
+          level: event.level,
+          reason: event.reason,
+          threshold: event.thresholdBytes,
+        })),
+    ).toEqual([
+      { level: "warning", reason: "heap_threshold", threshold: 512 * mb },
+      { level: "critical", reason: "heap_threshold", threshold: 768 * mb },
+    ]);
+  });
+
   it("emits pressure when RSS grows quickly", () => {
     const events: DiagnosticEventPayload[] = [];
     const stop = onDiagnosticEvent((event) => events.push(event));
diff --git a/src/logging/diagnostic-memory.ts b/src/logging/diagnostic-memory.ts
index 26fc94eeb759..d34fad3c5388 100644
--- a/src/logging/diagnostic-memory.ts
+++ b/src/logging/diagnostic-memory.ts
@@ -1,4 +1,5 @@
 // Diagnostic memory helpers capture process memory facts for support diagnostics.
+import { getHeapStatistics } from "node:v8";
 import {
   emitInternalDiagnosticEvent as emitDiagnosticEvent,
   type DiagnosticMemoryPressureEvent,
@@ -9,10 +10,15 @@ import { createSubsystemLogger } from "./subsystem.js";
 
 // Diagnostic memory sampler with threshold/growth pressure detection and repeat suppression.
 const MB = 1024 * 1024;
+const GB = 1024 * MB;
 const DEFAULT_RSS_WARNING_BYTES = 1536 * MB;
 const DEFAULT_RSS_CRITICAL_BYTES = 3072 * MB;
 const DEFAULT_HEAP_WARNING_BYTES = 1024 * MB;
 const DEFAULT_HEAP_CRITICAL_BYTES = 2048 * MB;
+const DEFAULT_HEAP_WARNING_RATIO = 0.5;
+const DEFAULT_HEAP_CRITICAL_RATIO = 0.75;
+const DEFAULT_HEAP_WARNING_MAX_BYTES = 4 * GB;
+const DEFAULT_HEAP_CRITICAL_MAX_BYTES = 6 * GB;
 const DEFAULT_RSS_GROWTH_WARNING_BYTES = 512 * MB;
 const DEFAULT_RSS_GROWTH_CRITICAL_BYTES = 1024 * MB;
 const DEFAULT_GROWTH_WINDOW_MS = 10 * 60 * 1000;
@@ -60,12 +66,31 @@ function normalizeMemoryUsage(memory: NodeJS.MemoryUsage): DiagnosticMemoryUsage
 
 function resolveThresholds(
   thresholds?: DiagnosticMemoryThresholds,
+  heapSizeLimitBytes?: number,
 ): Required<DiagnosticMemoryThresholds> {
+  const hasHeapLimit =
+    typeof heapSizeLimitBytes === "number" &&
+    Number.isFinite(heapSizeLimitBytes) &&
+    heapSizeLimitBytes > 0;
+  // Scale both directions with V8's effective limit, but keep a warning/critical
+  // ceiling so very large heaps still surface actionable pressure diagnostics.
+  const heapWarningBytes = hasHeapLimit
+    ? Math.min(
+        Math.floor(heapSizeLimitBytes * DEFAULT_HEAP_WARNING_RATIO),
+        DEFAULT_HEAP_WARNING_MAX_BYTES,
+      )
+    : DEFAULT_HEAP_WARNING_BYTES;
+  const heapCriticalBytes = hasHeapLimit
+    ? Math.min(
+        Math.floor(heapSizeLimitBytes * DEFAULT_HEAP_CRITICAL_RATIO),
+        DEFAULT_HEAP_CRITICAL_MAX_BYTES,
+      )
+    : DEFAULT_HEAP_CRITICAL_BYTES;
   return {
     rssWarningBytes: thresholds?.rssWarningBytes ?? DEFAULT_RSS_WARNING_BYTES,
     rssCriticalBytes: thresholds?.rssCriticalBytes ?? DEFAULT_RSS_CRITICAL_BYTES,
-    heapUsedWarningBytes: thresholds?.heapUsedWarningBytes ?? DEFAULT_HEAP_WARNING_BYTES,
-    heapUsedCriticalBytes: thresholds?.heapUsedCriticalBytes ?? DEFAULT_HEAP_CRITICAL_BYTES,
+    heapUsedWarningBytes: thresholds?.heapUsedWarningBytes ?? heapWarningBytes,
+    heapUsedCriticalBytes: thresholds?.heapUsedCriticalBytes ?? heapCriticalBytes,
     rssGrowthWarningBytes: thresholds?.rssGrowthWarningBytes ?? DEFAULT_RSS_GROWTH_WARNING_BYTES,
     rssGrowthCriticalBytes: thresholds?.rssGrowthCriticalBytes ?? DEFAULT_RSS_GROWTH_CRITICAL_BYTES,
     growthWindowMs: thresholds?.growthWindowMs ?? DEFAULT_GROWTH_WINDOW_MS,
@@ -263,6 +288,7 @@ function logMemoryPressure(params: {
 export function emitDiagnosticMemorySample(options?: {
   now?: number;
   memoryUsage?: NodeJS.MemoryUsage;
+  heapSizeLimitBytes?: number;
   uptimeMs?: number;
   thresholds?: DiagnosticMemoryThresholds;
   emitSample?: boolean;
@@ -274,7 +300,10 @@ export function emitDiagnosticMemorySample(options?: {
   const now = options?.now ?? Date.now();
   const memory = normalizeMemoryUsage(options?.memoryUsage ?? process.memoryUsage());
   const current = { ts: now, memory };
-  const thresholds = resolveThresholds(options?.thresholds);
+  const thresholds = resolveThresholds(
+    options?.thresholds,
+    options?.heapSizeLimitBytes ?? getHeapStatistics().heap_size_limit,
+  );
   const shouldEmitSample = options?.emitSample !== false;
 
   if (shouldEmitSample) {