diff --git a/lib/entry-points.js b/lib/entry-points.js
index 904592609..18a338702 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -149918,7 +149918,7 @@ function runnerHasSufficientDiskSpace(diskUsage, logger, minimumDiskSpaceMb) {
     return false;
   }
   logger.debug(
-    `Disk space available for CodeQL analysis is ${diskSpaceMb} MB, which is above the minimum of ${minimumDiskSpaceMb} MB.`
+    `Disk space available for CodeQL analysis is ${diskSpaceMb} MB, which is at or above the minimum of ${minimumDiskSpaceMb} MB.`
   );
   return true;
 }
diff --git a/src/config-utils.test.ts b/src/config-utils.test.ts
index f4a664c79..bedf219ef 100644
--- a/src/config-utils.test.ts
+++ b/src/config-utils.test.ts
@@ -1333,26 +1333,57 @@ checkOverlayEnablementMacro.serial(
   },
 );
 
-checkOverlayEnablementMacro.serial(
-  "Overlay-base database on default branch if runner disk space is above the limit lowered by a feature flag",
-  {
-    languages: [BuiltInLanguage.javascript],
-    features: [
-      Feature.OverlayAnalysis,
-      Feature.OverlayAnalysisCodeScanningJavascript,
-      Feature.OverlayAnalysisMinDisk10Gb,
-    ],
-    isDefaultBranch: true,
-    diskUsage: {
-      numAvailableBytes: 11_000_000_000,
-      numTotalBytes: 100_000_000_000,
+// Check that each feature flag lowers the limit to the threshold that its name
+// declares. Both sides of the boundary are needed to pin the threshold down: a
+// mapping to a lower value would still pass the case at the limit, and one to a
+// higher value would still fail the case below it.
+for (const [feature, thresholdGb] of [
+  [Feature.OverlayAnalysisMinDisk8Gb, 8],
+  [Feature.OverlayAnalysisMinDisk9Gb, 9],
+  [Feature.OverlayAnalysisMinDisk10Gb, 10],
+  [Feature.OverlayAnalysisMinDisk11Gb, 11],
+  [Feature.OverlayAnalysisMinDisk12Gb, 12],
+  [Feature.OverlayAnalysisMinDisk13Gb, 13],
+] as Array<[Feature, number]>) {
+  const features = [
+    Feature.OverlayAnalysis,
+    Feature.OverlayAnalysisCodeScanningJavascript,
+    feature,
+  ];
+
+  checkOverlayEnablementMacro.serial(
+    `Overlay-base database on default branch if ${feature} is enabled and runner disk space is at its limit`,
+    {
+      languages: [BuiltInLanguage.javascript],
+      features,
+      isDefaultBranch: true,
+      diskUsage: {
+        numAvailableBytes: thresholdGb * 1_000_000_000,
+        numTotalBytes: 100_000_000_000,
+      },
     },
-  },
-  {
-    overlayDatabaseMode: OverlayDatabaseMode.OverlayBase,
-    useOverlayDatabaseCaching: true,
-  },
-);
+    {
+      overlayDatabaseMode: OverlayDatabaseMode.OverlayBase,
+      useOverlayDatabaseCaching: true,
+    },
+  );
+
+  checkOverlayEnablementMacro.serial(
+    `No overlay-base database on default branch if ${feature} is enabled and runner disk space is below its limit`,
+    {
+      languages: [BuiltInLanguage.javascript],
+      features,
+      isDefaultBranch: true,
+      diskUsage: {
+        numAvailableBytes: thresholdGb * 1_000_000_000 - 1_000_000,
+        numTotalBytes: 100_000_000_000,
+      },
+    },
+    {
+      disabledReason: OverlayDisabledReason.InsufficientDiskSpace,
+    },
+  );
+}
 
 checkOverlayEnablementMacro.serial(
   "Overlay-base database on default branch if runner disk space is exactly at the lowest limit enabled by a feature flag",
diff --git a/src/config-utils.ts b/src/config-utils.ts
index dac83aa2c..a0880d14a 100644
--- a/src/config-utils.ts
+++ b/src/config-utils.ts
@@ -630,8 +630,8 @@ function runnerHasSufficientDiskSpace(
   }
 
   logger.debug(
-    `Disk space available for CodeQL analysis is ${diskSpaceMb} MB, which is above the minimum ` +
-      `of ${minimumDiskSpaceMb} MB.`,
+    `Disk space available for CodeQL analysis is ${diskSpaceMb} MB, which is at or above the ` +
+      `minimum of ${minimumDiskSpaceMb} MB.`,
   );
   return true;
 }