diff --git a/.env.example b/.env.example
index 9de3c87966f..9a79c763526 100644
--- a/.env.example
+++ b/.env.example
@@ -949,13 +949,24 @@ APP_LOG_TO_FILE=true
 
 # Node.js V8 heap limit in MB, passed to the server via --max-old-space-size.
 # Used by the standalone launcher (Docker CMD) and `omniroute serve`.
-# Default: 512. Clamped to [64, 16384]. Raise it (e.g. 1024) if you see random
-# OOM crashes under load or with a large SQLite DB (#2939).
+# Clamped to [64, 16384]. Default: 512 (safe for a 1 GB / 1 core VPS). Size it to
+# roughly half the box's RAM, leaving the rest for native memory (better-sqlite3,
+# buffers — ~300 MB) and the OS:
+#   1 GB RAM  → 512   (default)
+#   2 GB RAM  → 1024
+#   4 GB RAM  → 2048
+# In a memory-capped container, set this EXPLICITLY: Node reads the HOST's RAM,
+# not the cgroup limit, so leaving it to a RAM heuristic can oversize the heap and
+# get the container OOM-killed. (#2939)
 # OMNIROUTE_MEMORY_MB=512
 
-# Heap pressure threshold (MB) — when V8 heap exceeds this, chatCore returns 503.
-# Used by: open-sse/handlers/chatCore.ts. Default: 200.
-# HEAP_PRESSURE_THRESHOLD_MB=200
+# Heap-pressure shed threshold (MB) — chatCore returns 503 when V8 heapUsed exceeds
+# it, to avoid hard OOM under concurrent large-context load.
+# LEAVE UNSET: it now AUTO-CALIBRATES to 85% of the actual V8 heap ceiling, so it
+# tracks OMNIROUTE_MEMORY_MB above and never sits below the ~260 MB runtime baseline
+# (a fixed 200 here used to reject every request). Used by: open-sse/utils/heapPressure.ts.
+# Override only to hand-tune for a known workload.
+# HEAP_PRESSURE_THRESHOLD_MB=
 
 # ── CLI helpers (bin/cli/) ──
 # Override UI language for CLI output. Accepts BCP-47 locale (e.g. en, pt-BR).
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 6a9eb700e2a..96d25e4cf68 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -1,4 +1,5 @@
 import { CORS_HEADERS } from "../utils/cors.ts";
+import { HEAP_PRESSURE_THRESHOLD_MB } from "../utils/heapPressure.ts";
 import { normalizeHeaders } from "../utils/headers.ts";
 import { detectFormatFromEndpoint, getTargetFormat } from "../services/provider.ts";
 import { injectSystemPrompt } from "../services/systemPrompt.ts";
@@ -233,8 +234,10 @@ const MEMORY_EXTRACTION_TEXT_LIMIT = 64 * 1024;
 
 // ── Global memory pressure guard ────────────────────────────────────────
 // Prevents OOM by rejecting new requests when V8 heap exceeds threshold.
-// Self-healing: no counters to leak, no cleanup needed.
-const HEAP_PRESSURE_THRESHOLD_MB = parseInt(process.env.HEAP_PRESSURE_THRESHOLD_MB || "200", 10);
+// Self-healing: no counters to leak, no cleanup needed. The threshold
+// auto-calibrates to 85% of the actual V8 heap ceiling (see heapPressure.ts) so
+// it tracks --max-old-space-size across 1GB/2GB/large VPS instead of a fixed
+// 200MB that sat below the app's own ~260MB baseline and rejected every request.
 
 function capMemoryExtractionText(value: string): string {
   if (value.length <= MEMORY_EXTRACTION_TEXT_LIMIT) return value;
diff --git a/open-sse/utils/heapPressure.ts b/open-sse/utils/heapPressure.ts
new file mode 100644
index 00000000000..85f6050d2c5
--- /dev/null
+++ b/open-sse/utils/heapPressure.ts
@@ -0,0 +1,44 @@
+import v8 from "node:v8";
+
+/**
+ * Compute the V8 heap-pressure shed threshold (MB).
+ *
+ * The chat pipeline rejects new requests with a 503 once `heapUsed` exceeds this
+ * value, to avoid hard "JavaScript heap out of memory" crashes under concurrent
+ * large-context load. The threshold is derived from the process's *actual* V8
+ * heap ceiling (`heap_size_limit`, which reflects `--max-old-space-size` when set,
+ * otherwise Node's RAM-derived default) so it auto-adapts across 1 GB / 2 GB /
+ * large VPS instead of using a fixed number.
+ *
+ * A fixed default was the bug: 200 MB sat *below* the app's ~260 MB working set,
+ * so the guard rejected every request once the heap warmed up (the v3.8.8
+ * "resource pressure" outage). We shed at 85% of the ceiling — leaving headroom
+ * for in-flight requests + GC — with a floor that always clears the runtime
+ * baseline so a small/undersized heap never rejects all traffic.
+ *
+ * @param heapSizeLimitMb  `v8.getHeapStatistics().heap_size_limit` expressed in MB
+ * @param override         `HEAP_PRESSURE_THRESHOLD_MB` env value — wins when it
+ *                         parses to a positive number; invalid/unset → auto-calibrate
+ */
+export function computeHeapPressureThresholdMb(
+  heapSizeLimitMb: number,
+  override?: string | number | null
+): number {
+  const explicit = Number(override);
+  if (Number.isFinite(explicit) && explicit > 0) return Math.floor(explicit);
+  // Shed at 85% of the heap ceiling, but never below a floor that clears the
+  // runtime's own ~260 MB baseline (+ margin) so an undersized heap degrades to
+  // "guard never fires" rather than "guard rejects everything".
+  const SHED_RATIO = 0.85;
+  const FLOOR_MB = 400;
+  return Math.max(Math.round(heapSizeLimitMb * SHED_RATIO), FLOOR_MB);
+}
+
+/**
+ * Heap-pressure threshold (MB) resolved once at module load from the live V8 heap
+ * ceiling. Read by `open-sse/handlers/chatCore.ts`'s memory-pressure guard.
+ */
+export const HEAP_PRESSURE_THRESHOLD_MB = computeHeapPressureThresholdMb(
+  v8.getHeapStatistics().heap_size_limit / (1024 * 1024),
+  process.env.HEAP_PRESSURE_THRESHOLD_MB
+);
diff --git a/tests/unit/chatcore-memory-pressure.test.ts b/tests/unit/chatcore-memory-pressure.test.ts
index 79cc0295070..dad17e4d89b 100644
--- a/tests/unit/chatcore-memory-pressure.test.ts
+++ b/tests/unit/chatcore-memory-pressure.test.ts
@@ -37,11 +37,31 @@ test("estimateSizeFast handles circular references", async () => {
   assert.ok(size < 1000, `Simple circular ref should be small, got ${size}`);
 });
 
-// ── HEAP_PRESSURE_THRESHOLD_MB default ─────────────────────────────────
+// ── HEAP_PRESSURE_THRESHOLD_MB auto-calibration ────────────────────────
+// Regression: the old fixed 200MB default sat below the app's ~260MB working set,
+// so the chatCore heap guard 503'd every request ("resource pressure" outage).
+// The live constant must now auto-calibrate from the real V8 heap ceiling.
 
-test("HEAP_PRESSURE_THRESHOLD_MB defaults to 200 when env is unset", () => {
-  const val = parseInt(process.env.HEAP_PRESSURE_THRESHOLD_MB || "200", 10);
-  assert.equal(val, 200);
+test("HEAP_PRESSURE_THRESHOLD_MB auto-calibrates from the live V8 heap ceiling (no fixed 200)", async () => {
+  const { getHeapStatistics } = await import("node:v8");
+  const { HEAP_PRESSURE_THRESHOLD_MB, computeHeapPressureThresholdMb } = await import(
+    "../../open-sse/utils/heapPressure.ts"
+  );
+  const limitMb = getHeapStatistics().heap_size_limit / (1024 * 1024);
+  // The live constant must equal the pure helper applied to this process's
+  // actual ceiling — no drift between the resolved value and the formula.
+  assert.equal(
+    HEAP_PRESSURE_THRESHOLD_MB,
+    computeHeapPressureThresholdMb(limitMb, process.env.HEAP_PRESSURE_THRESHOLD_MB)
+  );
+  // With no operator override it must clear the ~260MB baseline, otherwise the
+  // guard would reject every request at idle (the bug we are fixing).
+  if (!process.env.HEAP_PRESSURE_THRESHOLD_MB) {
+    assert.ok(
+      HEAP_PRESSURE_THRESHOLD_MB >= 400,
+      `live threshold ${HEAP_PRESSURE_THRESHOLD_MB}MB must clear the ~260MB app baseline`
+    );
+  }
 });
 
 // ── estimateSizeFast vs MAX_LOG_BODY_CHARS threshold ───────────────────
diff --git a/tests/unit/heap-pressure.test.ts b/tests/unit/heap-pressure.test.ts
new file mode 100644
index 00000000000..621b1ea7123
--- /dev/null
+++ b/tests/unit/heap-pressure.test.ts
@@ -0,0 +1,48 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { computeHeapPressureThresholdMb } from "../../open-sse/utils/heapPressure.ts";
+
+// Regression guard for the v3.8.8 "Service temporarily unavailable due to resource
+// pressure" outage: a fixed 200MB threshold sat below the app's ~260MB working set,
+// so the chatCore heap guard rejected every request. The threshold must now track
+// the real V8 heap ceiling so it stays above the baseline on every VPS tier.
+describe("computeHeapPressureThresholdMb", () => {
+  it("sheds at 85% of a no-cap heap ceiling (2240MB → 1904)", () => {
+    assert.equal(computeHeapPressureThresholdMb(2240), 1904);
+  });
+
+  it("tracks the default 512MB cap (heap_limit ~704 → 598), clearing the ~260MB baseline", () => {
+    const t = computeHeapPressureThresholdMb(704);
+    assert.equal(t, 598);
+    assert.ok(t > 260, "threshold must stay above the ~260MB app baseline");
+  });
+
+  it("tracks a 1 GB-box 640MB cap (heap_limit 832 → 707)", () => {
+    assert.equal(computeHeapPressureThresholdMb(832), 707);
+  });
+
+  it("tracks a 2 GB-box 1536MB cap (heap_limit 1728 → 1469)", () => {
+    assert.equal(computeHeapPressureThresholdMb(1728), 1469);
+  });
+
+  it("floors at 400MB so a tiny/undersized heap never rejects all traffic", () => {
+    // 85% of 300 = 255, which would sit below the baseline — the floor wins.
+    assert.equal(computeHeapPressureThresholdMb(300), 400);
+  });
+
+  it("honors a positive explicit override (string or number)", () => {
+    assert.equal(computeHeapPressureThresholdMb(2240, "1024"), 1024);
+    assert.equal(computeHeapPressureThresholdMb(2240, 800), 800);
+    assert.equal(computeHeapPressureThresholdMb(2240, "1500.9"), 1500);
+  });
+
+  it("ignores invalid/zero/negative overrides and auto-calibrates", () => {
+    for (const bad of ["", "0", "-5", "abc", null, undefined]) {
+      assert.equal(
+        computeHeapPressureThresholdMb(2240, bad as string | number | null | undefined),
+        1904,
+        `override ${JSON.stringify(bad)} must fall back to auto-calibration`
+      );
+    }
+  });
+});