From 6d043674c2dae3b09c0c6ad58ef9aa4306ad9025 Mon Sep 17 00:00:00 2001
From: Markus Hartung <mail@hartmark.se>
Date: Thu, 20 Aug 2026 20:34:19 -0300
Subject: [PATCH] fix: skip expensive RTK compression stats computation on
 no-op runs (#10765)

---
 .../10765-rtk-unconditional-stats-cpu.md      |  1 +
 .../services/compression/engines/rtk/index.ts | 12 +++++++
 tests/unit/compression/rtk-engine.test.ts     |  3 +-
 tests/unit/probe-10765-rtk-noop-stats.test.ts | 32 +++++++++++++++++++
 4 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/fixes/10765-rtk-unconditional-stats-cpu.md
 create mode 100644 tests/unit/probe-10765-rtk-noop-stats.test.ts

diff --git a/changelog.d/fixes/10765-rtk-unconditional-stats-cpu.md b/changelog.d/fixes/10765-rtk-unconditional-stats-cpu.md
new file mode 100644
index 0000000000..ff36462d47
--- /dev/null
+++ b/changelog.d/fixes/10765-rtk-unconditional-stats-cpu.md
@@ -0,0 +1 @@
+- fix(compression): skip the expensive `createCompressionStats()` pass in RTK when no message was actually compressed, matching every sibling stacked engine (#10765)
diff --git a/open-sse/services/compression/engines/rtk/index.ts b/open-sse/services/compression/engines/rtk/index.ts
index 33b6c90b59..b3791bfc54 100644
--- a/open-sse/services/compression/engines/rtk/index.ts
+++ b/open-sse/services/compression/engines/rtk/index.ts
@@ -656,6 +656,18 @@ export function applyRtkCompression(
     };
   });
 
+  // Mirror the sibling stacked engines (headroom, session-dedup, ccr, relevance,
+  // ionizer, readLifecycle): skip the expensive createCompressionStats() pass
+  // (full JSON.stringify + tokenizer over the whole body, twice) when nothing
+  // actually changed. Untouched messages keep their original reference above,
+  // so a reference-identity scan is enough to detect the no-op case (#10765).
+  const anyMessageChanged = compressedMessages.some(
+    (message, index) => message !== messages[index]
+  );
+  if (!anyMessageChanged) {
+    return { body, compressed: false, stats: null };
+  }
+
   const compressedBody = { ...adapter.body, messages: compressedMessages };
   const stats = createCompressionStats(
     adapter.body,
diff --git a/tests/unit/compression/rtk-engine.test.ts b/tests/unit/compression/rtk-engine.test.ts
index 7406f53634..4741ddb6e6 100644
--- a/tests/unit/compression/rtk-engine.test.ts
+++ b/tests/unit/compression/rtk-engine.test.ts
@@ -70,7 +70,8 @@ describe("RTK compression engine", () => {
     assert.equal(rtkEngine.validateConfig({ intensity: "invalid" }).valid, false);
     assert.equal(rtkEngine.validateConfig({ rawOutputRetention: "always" }).valid, true);
 
-    const body = { messages: [{ role: "tool", content: "same\nsame\nsame\nsame" }] };
+    const repeated = Array.from({ length: 20 }, () => "same").join("\n");
+    const body = { messages: [{ role: "tool", content: repeated }] };
     assert.equal(
       rtkEngine.apply(body, { config: { rtkConfig: { enabled: true } } }).stats?.engine,
       "rtk"
diff --git a/tests/unit/probe-10765-rtk-noop-stats.test.ts b/tests/unit/probe-10765-rtk-noop-stats.test.ts
new file mode 100644
index 0000000000..2826af2a71
--- /dev/null
+++ b/tests/unit/probe-10765-rtk-noop-stats.test.ts
@@ -0,0 +1,32 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { applyRtkCompression } from "../../open-sse/services/compression/engines/rtk/index.ts";
+
+// Issue #10765: enabling RTK causes ~100% CPU even when the engine finds nothing to
+// compress ("It also occurs when the engine does not modify the request and reports
+// no token savings."). Root cause: applyRtkCompression() unconditionally calls
+// createCompressionStats() at the end of the function — which does a full
+// JSON.stringify() (+ tiktoken tokenize for Codex bodies) of the ENTIRE request body,
+// TWICE (original + compressed) — even when zero messages were touched.
+//
+// Every sibling stacked engine (headroom, session-dedup, ccr, relevance, ionizer,
+// readLifecycle) returns `stats: null` early when nothing changed, skipping this
+// expensive computation entirely. RTK is the outlier: it always pays the cost.
+test("RTK no-op run should skip the expensive stats computation (like sibling engines)", () => {
+  const body = {
+    model: "codex/gpt-5",
+    provider: "codex",
+    messages: [
+      { role: "user", content: "hello, this is a simple message with nothing to compress" },
+    ],
+  };
+
+  const result = applyRtkCompression(body, { config: { enabled: true } });
+
+  assert.equal(result.compressed, false, "RTK made no changes");
+  assert.equal(
+    result.stats,
+    null,
+    "RTK should return stats: null on a no-op run, like every sibling stacked engine"
+  );
+});