diff --git a/lib/init-action.js b/lib/init-action.js
index 86b8fb3a16..fda7608f0b 100644
--- a/lib/init-action.js
+++ b/lib/init-action.js
@@ -105442,6 +105442,17 @@ var CODEQL_OVERLAY_MINIMUM_VERSION_PYTHON = "2.23.9";
 var CODEQL_OVERLAY_MINIMUM_VERSION_RUBY = "2.23.9";
 var OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_MB = 7500;
 var OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_BYTES = OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_MB * 1e6;
+function revertOverlayModeIfDiffInformedUnavailable(config, diffInformedAnalysisExpected, logger) {
+  if (config.overlayDatabaseMode !== "overlay" /* Overlay */) {
+    return;
+  }
+  if (diffInformedAnalysisExpected && !fs4.existsSync(getDiffRangesJsonFilePath())) {
+    logger.warning(
+      `Diff-informed analysis is not available for this pull request. Reverting overlay database mode to ${"none" /* None */}.`
+    );
+    config.overlayDatabaseMode = "none" /* None */;
+  }
+}
 async function writeBaseDatabaseOidsFile(config, sourceRoot) {
   const gitFileOids = await getFileOidsUnderPath(sourceRoot);
   const gitFileOidsJson = JSON.stringify(gitFileOids);
@@ -110338,6 +110349,24 @@ async function run(startedAt) {
     }
     return;
   }
+  let diffInformedAnalysisExpected = false;
+  try {
+    diffInformedAnalysisExpected = await shouldPerformDiffInformedAnalysis(
+      codeql,
+      features,
+      logger
+    );
+  } catch (e) {
+    logger.warning(
+      `Failed to determine diff-informed analysis availability: ${getErrorMessage(e)}`
+    );
+    diffInformedAnalysisExpected = true;
+  }
+  revertOverlayModeIfDiffInformedUnavailable(
+    config,
+    diffInformedAnalysisExpected,
+    logger
+  );
   let overlayBaseDatabaseStats;
   let dependencyCachingStatus;
   try {
diff --git a/src/init-action.ts b/src/init-action.ts
index 70d5d79ce5..263008113d 100644
--- a/src/init-action.ts
+++ b/src/init-action.ts
@@ -40,6 +40,7 @@ import {
 import {
   getDiffInformedAnalysisBranches,
   getPullRequestEditedDiffRanges,
+  shouldPerformDiffInformedAnalysis,
   writeDiffRangesJsonFile,
 } from "./diff-informed-analysis-utils";
 import { EnvVar } from "./environment";
@@ -64,6 +65,7 @@ import {
   downloadOverlayBaseDatabaseFromCache,
   OverlayBaseDatabaseDownloadStats,
   OverlayDatabaseMode,
+  revertOverlayModeIfDiffInformedUnavailable,
 } from "./overlay";
 import { getRepositoryNwo, RepositoryNwo } from "./repository";
 import { ToolsSource } from "./setup-codeql";
@@ -438,6 +440,27 @@ async function run(startedAt: Date) {
     return;
   }
 
+  let diffInformedAnalysisExpected = false;
+  try {
+    diffInformedAnalysisExpected = await shouldPerformDiffInformedAnalysis(
+      codeql,
+      features,
+      logger,
+    );
+  } catch (e) {
+    logger.warning(
+      `Failed to determine diff-informed analysis availability: ${getErrorMessage(e)}`,
+    );
+    // Treat errors conservatively: assume diff-informed was expected so that
+    // the overlay fallback triggers if the diff ranges file is missing.
+    diffInformedAnalysisExpected = true;
+  }
+  revertOverlayModeIfDiffInformedUnavailable(
+    config,
+    diffInformedAnalysisExpected,
+    logger,
+  );
+
   let overlayBaseDatabaseStats: OverlayBaseDatabaseDownloadStats | undefined;
   let dependencyCachingStatus: DependencyCacheRestoreStatusReport | undefined;
   try {
diff --git a/src/overlay/index.test.ts b/src/overlay/index.test.ts
index 2d0b4d3fcb..114909c0a2 100644
--- a/src/overlay/index.test.ts
+++ b/src/overlay/index.test.ts
@@ -24,6 +24,7 @@ import {
   getCacheRestoreKeyPrefix,
   getCacheSaveKey,
   OverlayDatabaseMode,
+  revertOverlayModeIfDiffInformedUnavailable,
   writeBaseDatabaseOidsFile,
   writeOverlayChangesFile,
 } from ".";
@@ -602,3 +603,77 @@ test.serial("overlay-base database cache keys remain stable", async (t) => {
     `Expected save key "${saveKey}" to start with restore key prefix "${restoreKeyPrefix}"`,
   );
 });
+
+test.serial(
+  "revertOverlayModeIfDiffInformedUnavailable: no-op when mode is not Overlay",
+  (t) => {
+    const config = createTestConfig({});
+    config.overlayDatabaseMode = OverlayDatabaseMode.None;
+    const logger = getRunnerLogger(true);
+
+    revertOverlayModeIfDiffInformedUnavailable(config, true, logger);
+
+    t.is(config.overlayDatabaseMode, OverlayDatabaseMode.None);
+  },
+);
+
+test.serial(
+  "revertOverlayModeIfDiffInformedUnavailable: no-op when diff ranges file exists",
+  async (t) => {
+    await withTmpDir(async (tmpDir) => {
+      const config = createTestConfig({});
+      config.overlayDatabaseMode = OverlayDatabaseMode.Overlay;
+      const logger = getRunnerLogger(true);
+
+      // Create the diff ranges file so it exists
+      const diffRangesPath = path.join(tmpDir, "pr-diff-range.json");
+      await fs.promises.writeFile(diffRangesPath, "[]");
+      const stub = sinon
+        .stub(actionsUtil, "getDiffRangesJsonFilePath")
+        .returns(diffRangesPath);
+
+      try {
+        revertOverlayModeIfDiffInformedUnavailable(config, true, logger);
+        t.is(config.overlayDatabaseMode, OverlayDatabaseMode.Overlay);
+      } finally {
+        stub.restore();
+      }
+    });
+  },
+);
+
+test.serial(
+  "revertOverlayModeIfDiffInformedUnavailable: reverts to None when diff ranges file is missing",
+  (t) => {
+    const config = createTestConfig({});
+    config.overlayDatabaseMode =
+      OverlayDatabaseMode.Overlay as OverlayDatabaseMode;
+    const logger = getRunnerLogger(true);
+
+    const stub = sinon
+      .stub(actionsUtil, "getDiffRangesJsonFilePath")
+      .returns("/nonexistent/path/pr-diff-range.json");
+
+    try {
+      revertOverlayModeIfDiffInformedUnavailable(config, true, logger);
+      t.is(
+        config.overlayDatabaseMode,
+        OverlayDatabaseMode.None as OverlayDatabaseMode,
+      );
+    } finally {
+      stub.restore();
+    }
+  },
+);
+
+test.serial(
+  "revertOverlayModeIfDiffInformedUnavailable: no-op when diff-informed analysis is not expected",
+  (t) => {
+    const config = createTestConfig({});
+    config.overlayDatabaseMode = OverlayDatabaseMode.Overlay;
+    const logger = getRunnerLogger(true);
+
+    revertOverlayModeIfDiffInformedUnavailable(config, false, logger);
+    t.is(config.overlayDatabaseMode, OverlayDatabaseMode.Overlay);
+  },
+);
diff --git a/src/overlay/index.ts b/src/overlay/index.ts
index 4905b254f7..bafa74f6be 100644
--- a/src/overlay/index.ts
+++ b/src/overlay/index.ts
@@ -62,6 +62,39 @@ const OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_MB = 7500;
 const OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_BYTES =
   OVERLAY_BASE_DATABASE_MAX_UPLOAD_SIZE_MB * 1_000_000;
 
+/**
+ * If overlay analysis is enabled but diff-informed analysis failed to produce
+ * its output file, reverts the overlay database mode to `None`. Overlay
+ * without diff-informed is an untested combination that can produce inaccurate
+ * results, so we fall back to a full non-overlay analysis instead.
+ *
+ * @param config The configuration object whose `overlayDatabaseMode` may be mutated.
+ * @param diffInformedAnalysisExpected Whether diff-informed analysis was expected
+ *   to run for this workflow. When true and the diff ranges file is missing, the
+ *   overlay mode is reverted.
+ * @param logger The logger instance.
+ */
+export function revertOverlayModeIfDiffInformedUnavailable(
+  config: Config,
+  diffInformedAnalysisExpected: boolean,
+  logger: Logger,
+): void {
+  if (config.overlayDatabaseMode !== OverlayDatabaseMode.Overlay) {
+    return;
+  }
+
+  if (
+    diffInformedAnalysisExpected &&
+    !fs.existsSync(actionsUtil.getDiffRangesJsonFilePath())
+  ) {
+    logger.warning(
+      "Diff-informed analysis is not available for this pull request. " +
+        `Reverting overlay database mode to ${OverlayDatabaseMode.None}.`,
+    );
+    config.overlayDatabaseMode = OverlayDatabaseMode.None;
+  }
+}
+
 /**
  * Writes a JSON file containing Git OIDs for all tracked files (represented
  * by path relative to the source root) under the source root. The file is