diff --git a/changelog.d/fixes/11977-antigravity-stale-compaction-log.md b/changelog.d/fixes/11977-antigravity-stale-compaction-log.md
new file mode 100644
index 0000000000..decd6ea604
--- /dev/null
+++ b/changelog.d/fixes/11977-antigravity-stale-compaction-log.md
@@ -0,0 +1 @@
+- fix(routing): stop the reactive-compaction debug log from lying when compression is globally disabled (#11977)
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index f622da986c..d81248a472 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -1958,7 +1958,9 @@ export async function handleChatCore({
     if (!promptCompressionEnabled) {
       log?.debug?.(
         "CONTEXT",
-        "Prompt Compression engines disabled; reactive context compaction still applies when over threshold"
+        reactiveContextCompactionEnabled
+          ? "Prompt Compression engines disabled; reactive context compaction still applies when over threshold"
+          : "Prompt Compression engines disabled; reactive context compaction is ALSO disabled — large histories will NOT be trimmed before reaching the upstream provider"
       );
     }
     if (isCombo && comboName) {
diff --git a/tests/unit/chatcore-stale-compaction-log.test.mjs b/tests/unit/chatcore-stale-compaction-log.test.mjs
new file mode 100644
index 0000000000..8cb70ff2cc
--- /dev/null
+++ b/tests/unit/chatcore-stale-compaction-log.test.mjs
@@ -0,0 +1,71 @@
+// Regression guard for #11977: the diagnostic log fired unconditionally whenever
+// promptCompressionEnabled was false, claiming "reactive context compaction still
+// applies when over threshold" even on a default install where
+// reactiveContextCompactionEnabled is ALSO false (DEFAULT_COMPRESSION_CONFIG.enabled
+// === false, per #9200's gating). That left operators with zero diagnostic signal for
+// the real failure mode: large histories reaching the upstream provider untrimmed
+// (e.g. Antigravity's 400 once session history grows past its real request-size
+// ceiling). The fix branches the log on reactiveContextCompactionEnabled so the
+// message reflects which safety net, if any, is actually still active.
+import assert from "node:assert/strict";
+import test from "node:test";
+import { readFileSync } from "node:fs";
+
+const source = readFileSync(
+  new URL("../../open-sse/handlers/chatCore.ts", import.meta.url),
+  "utf8"
+);
+
+test("gating expressions exist as documented (sanity check, tracks real source)", () => {
+  assert.match(
+    source,
+    /let promptCompressionEnabled =\s*\n\s*compressionSettingsResult\.enabled && !compressionExcluded && apiKeyCompressionEnabled;/
+  );
+  assert.match(
+    source,
+    /reactiveContextCompactionEnabled = compressionSettingsResult\.enabled && !compressionExcluded;/
+  );
+});
+
+test("on default install, reactiveContextCompactionEnabled is provably false whenever the log fires", () => {
+  const compressionSettingsResultEnabled = false; // DEFAULT_COMPRESSION_CONFIG.enabled
+  const compressionExcluded = false;
+  const apiKeyCompressionEnabled = true;
+
+  const promptCompressionEnabled =
+    compressionSettingsResultEnabled && !compressionExcluded && apiKeyCompressionEnabled;
+  const reactiveContextCompactionEnabled = compressionSettingsResultEnabled && !compressionExcluded;
+
+  const logFires = !promptCompressionEnabled;
+
+  assert.equal(logFires, true, "the log fires on every default-config request");
+  assert.equal(
+    reactiveContextCompactionEnabled,
+    false,
+    "reactive compaction is ALSO disabled here — the old unconditional message was false for this branch"
+  );
+});
+
+test("the log statement branches on reactiveContextCompactionEnabled so it stays accurate in both cases", () => {
+  const blockMatch = source.match(
+    /if \(!promptCompressionEnabled\) \{[\s\S]{0,400}\}/
+  );
+  assert.ok(blockMatch, "expected to find the promptCompressionEnabled debug-log block");
+  const block = blockMatch[0];
+
+  assert.match(
+    block,
+    /reactiveContextCompactionEnabled/,
+    "expected the debug-log block to branch on reactiveContextCompactionEnabled"
+  );
+  assert.match(
+    block,
+    /still applies when over threshold/,
+    "expected the true-branch message (reactive compaction still active) to be preserved"
+  );
+  assert.match(
+    block,
+    /reactive context compaction is ALSO disabled/,
+    "expected a distinct false-branch message for the fully-disabled default-install case"
+  );
+});