diff --git a/.github/workflows/codescanning-config-cli.yml b/.github/workflows/codescanning-config-cli.yml
index 0c4829339f..38b5b6ecce 100644
--- a/.github/workflows/codescanning-config-cli.yml
+++ b/.github/workflows/codescanning-config-cli.yml
@@ -6,13 +6,6 @@ env:
   # Diff informed queries add an additional query filter which is not yet
   # taken into account by these tests.
   CODEQL_ACTION_DIFF_INFORMED_QUERIES: false
-  # Specify overlay enablement manually to ensure stability around the exclude-from-incremental
-  # query filter. Here we only enable for the default code scanning suite.
-  CODEQL_ACTION_OVERLAY_ANALYSIS: true
-  CODEQL_ACTION_OVERLAY_ANALYSIS_JAVASCRIPT: false
-  CODEQL_ACTION_OVERLAY_ANALYSIS_CODE_SCANNING_JAVASCRIPT: true
-  CODEQL_ACTION_OVERLAY_ANALYSIS_STATUS_CHECK: false
-  CODEQL_ACTION_OVERLAY_ANALYSIS_SKIP_RESOURCE_CHECKS: true
 
 on:
   push:
@@ -79,33 +72,13 @@ jobs:
       with:
         version: ${{ matrix.version }}
 
-    # On PRs, overlay analysis may change the config that is passed to the CLI.
-    # Therefore, we have two variants of the following test, one for PRs and one for other events.
-    - name: Empty file (non-PR)
-      if: github.event_name != 'pull_request'
+    - name: Empty file
       uses: ./../action/.github/actions/check-codescanning-config
       with:
         expected-config-file-contents: "{}"
         languages: javascript
         tools: ${{ steps.prepare-test.outputs.tools-url }}
 
-    - name: Empty file (PR)
-      if: github.event_name == 'pull_request'
-      uses: ./../action/.github/actions/check-codescanning-config
-      with:
-        expected-config-file-contents: |
-          {
-            "query-filters": [
-              {
-                "exclude": {
-                  "tags": "exclude-from-incremental"
-                }
-              }
-            ]
-          }
-        languages: javascript
-        tools: ${{ steps.prepare-test.outputs.tools-url }}
-
     - name: Packs from input
       if: success() || failure()
       uses: ./../action/.github/actions/check-codescanning-config
diff --git a/lib/init-action.js b/lib/init-action.js
index 6acd2a5670..c707def1fd 100644
--- a/lib/init-action.js
+++ b/lib/init-action.js
@@ -106359,9 +106359,6 @@ function initFeatures(gitHubVersion, repositoryNwo, tempDir, logger) {
 }
 
 // src/diff-informed-analysis-utils.ts
-async function shouldPerformDiffInformedAnalysis(codeql, features, logger) {
-  return await getDiffInformedAnalysisBranches(codeql, features, logger) !== void 0;
-}
 async function getDiffInformedAnalysisBranches(codeql, features, logger) {
   if (!await features.getValue("diff_informed_queries" /* DiffInformedQueries */, codeql)) {
     return void 0;
@@ -106378,6 +106375,30 @@ async function getDiffInformedAnalysisBranches(codeql, features, logger) {
   }
   return branches;
 }
+async function prepareDiffInformedAnalysis(codeql, features, logger) {
+  let branches;
+  try {
+    branches = await getDiffInformedAnalysisBranches(codeql, features, logger);
+  } catch (e) {
+    logger.warning(
+      `Failed to determine branch information for diff-informed analysis: ${getErrorMessage(e)}`
+    );
+    return false;
+  }
+  if (!branches) {
+    return false;
+  }
+  return await withGroupAsync("Computing PR diff ranges", async () => {
+    try {
+      return await computeAndPersistDiffRanges(branches, logger);
+    } catch (e) {
+      logger.warning(
+        `Failed to compute diff-informed analysis ranges: ${getErrorMessage(e)}`
+      );
+      return false;
+    }
+  });
+}
 function writeDiffRangesJsonFile(logger, ranges) {
   const jsonContents = JSON.stringify(ranges, null, 2);
   const jsonFilePath = getDiffRangesJsonFilePath();
@@ -106408,6 +106429,18 @@ async function getPullRequestEditedDiffRanges(branches, logger) {
   }
   return results;
 }
+async function computeAndPersistDiffRanges(branches, logger) {
+  const ranges = await getPullRequestEditedDiffRanges(branches, logger);
+  if (ranges === void 0) {
+    return false;
+  }
+  writeDiffRangesJsonFile(logger, ranges);
+  const distinctFiles = new Set(ranges.map((r) => r.path)).size;
+  logger.info(
+    `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s).`
+  );
+  return true;
+}
 async function getFileDiffsWithBasehead(branches, logger) {
   const repositoryNwo = getRepositoryNwoFromEnv(
     "CODE_SCANNING_REPOSITORY",
@@ -107251,6 +107284,25 @@ function userConfigFromActionPath(tempDir) {
 function hasQueryCustomisation(userConfig) {
   return isDefined2(userConfig["disable-default-queries"]) || isDefined2(userConfig.queries) || isDefined2(userConfig["query-filters"]);
 }
+async function applyIncrementalAnalysisSettings(config, hasDiffRanges, codeql, logger) {
+  if (config.overlayDatabaseMode === "overlay" /* Overlay */ && !hasDiffRanges) {
+    logger.info(
+      `Reverting overlay database mode to ${"none" /* None */} because the PR diff ranges could not be computed.`
+    );
+    config.overlayDatabaseMode = "none" /* None */;
+    config.useOverlayDatabaseCaching = false;
+    await addOverlayDisablementDiagnostics(
+      config,
+      codeql,
+      "pr-diff-ranges-not-computed" /* PrDiffRangesNotComputed */
+    );
+  }
+  if (hasDiffRanges) {
+    config.extraQueryExclusions.push({
+      exclude: { tags: "exclude-from-incremental" }
+    });
+  }
+}
 async function initConfig(features, inputs) {
   const { logger, tempDir } = inputs;
   if (inputs.configInput) {
@@ -107360,15 +107412,17 @@ async function initConfig(features, inputs) {
       overlayDisabledReason
     );
   }
-  if (config.overlayDatabaseMode === "overlay" /* Overlay */ || await shouldPerformDiffInformedAnalysis(
+  const hasDiffRanges = await prepareDiffInformedAnalysis(
     inputs.codeql,
     inputs.features,
     logger
-  )) {
-    config.extraQueryExclusions.push({
-      exclude: { tags: "exclude-from-incremental" }
-    });
-  }
+  );
+  await applyIncrementalAnalysisSettings(
+    config,
+    hasDiffRanges,
+    inputs.codeql,
+    logger
+  );
   if (await isTrapCachingEnabled(features, config.overlayDatabaseMode)) {
     const { trapCaches, trapCacheDownloadTime } = await downloadCacheWithTime(
       inputs.codeql,
@@ -110636,7 +110690,6 @@ async function run(startedAt) {
       logFileCoverageOnPrsDeprecationWarning(logger);
     }
     await checkInstallPython311(config.languages, codeql);
-    await computeAndPersistDiffRanges(codeql, features, logger);
   } catch (unwrappedError) {
     const error3 = wrapError(unwrappedError);
     core15.setFailed(error3.message);
@@ -110909,33 +110962,6 @@ async function loadRepositoryProperties(repositoryNwo, logger) {
     return new Failure(error3);
   }
 }
-async function computeAndPersistDiffRanges(codeql, features, logger) {
-  await withGroupAsync("Computing PR diff ranges", async () => {
-    try {
-      const branches = await getDiffInformedAnalysisBranches(
-        codeql,
-        features,
-        logger
-      );
-      if (!branches) {
-        return;
-      }
-      const ranges = await getPullRequestEditedDiffRanges(branches, logger);
-      if (ranges === void 0) {
-        return;
-      }
-      writeDiffRangesJsonFile(logger, ranges);
-      const distinctFiles = new Set(ranges.map((r) => r.path)).size;
-      logger.info(
-        `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s).`
-      );
-    } catch (e) {
-      logger.warning(
-        `Failed to compute and persist PR diff ranges: ${getErrorMessage(e)}`
-      );
-    }
-  });
-}
 async function recordZstdAvailability(config, zstdAvailability) {
   addNoLanguageDiagnostic(
     config,
diff --git a/src/config-utils.test.ts b/src/config-utils.test.ts
index 25aa414334..417b61c753 100644
--- a/src/config-utils.test.ts
+++ b/src/config-utils.test.ts
@@ -2200,3 +2200,79 @@ test.serial(
     });
   },
 );
+
+test("applyIncrementalAnalysisSettings: no-op when mode is not Overlay and diff ranges are unavailable", async (t) => {
+  const config = createTestConfig({});
+  config.overlayDatabaseMode = OverlayDatabaseMode.None;
+  const codeql = createStubCodeQL({});
+  const logger = getRunnerLogger(true);
+
+  await configUtils.applyIncrementalAnalysisSettings(
+    config,
+    false,
+    codeql,
+    logger,
+  );
+
+  t.is(config.overlayDatabaseMode, OverlayDatabaseMode.None);
+  t.deepEqual(config.extraQueryExclusions, []);
+});
+
+test("applyIncrementalAnalysisSettings: keeps overlay mode and adds exclusions when diff ranges are available", async (t) => {
+  const config = createTestConfig({
+    overlayDatabaseMode: OverlayDatabaseMode.Overlay,
+  });
+  const codeql = createStubCodeQL({});
+  const logger = getRunnerLogger(true);
+
+  await configUtils.applyIncrementalAnalysisSettings(
+    config,
+    true,
+    codeql,
+    logger,
+  );
+
+  t.is(config.overlayDatabaseMode, OverlayDatabaseMode.Overlay);
+  t.deepEqual(config.extraQueryExclusions, [
+    { exclude: { tags: "exclude-from-incremental" } },
+  ]);
+});
+
+test("applyIncrementalAnalysisSettings: disables overlay analysis when diff ranges are unavailable", async (t) => {
+  const config = createTestConfig({
+    overlayDatabaseMode: OverlayDatabaseMode.Overlay,
+  });
+  config.useOverlayDatabaseCaching = true;
+  const codeql = createStubCodeQL({});
+  const logger = getRunnerLogger(true);
+
+  await configUtils.applyIncrementalAnalysisSettings(
+    config,
+    false,
+    codeql,
+    logger,
+  );
+
+  t.is(config.overlayDatabaseMode, OverlayDatabaseMode.None);
+  t.is(config.useOverlayDatabaseCaching, false);
+  t.deepEqual(config.extraQueryExclusions, []);
+});
+
+test("applyIncrementalAnalysisSettings: adds exclusions for diff-informed-only runs", async (t) => {
+  const config = createTestConfig({});
+  config.overlayDatabaseMode = OverlayDatabaseMode.None;
+  const codeql = createStubCodeQL({});
+  const logger = getRunnerLogger(true);
+
+  await configUtils.applyIncrementalAnalysisSettings(
+    config,
+    true,
+    codeql,
+    logger,
+  );
+
+  t.is(config.overlayDatabaseMode, OverlayDatabaseMode.None);
+  t.deepEqual(config.extraQueryExclusions, [
+    { exclude: { tags: "exclude-from-incremental" } },
+  ]);
+});
diff --git a/src/config-utils.ts b/src/config-utils.ts
index 0b07010070..563cd31c42 100644
--- a/src/config-utils.ts
+++ b/src/config-utils.ts
@@ -31,7 +31,7 @@ import {
   addNoLanguageDiagnostic,
   makeTelemetryDiagnostic,
 } from "./diagnostics";
-import { shouldPerformDiffInformedAnalysis } from "./diff-informed-analysis-utils";
+import { prepareDiffInformedAnalysis } from "./diff-informed-analysis-utils";
 import { EnvVar } from "./environment";
 import * as errorMessages from "./error-messages";
 import { Feature, FeatureEnablement } from "./feature-flags";
@@ -1076,6 +1076,48 @@ function hasQueryCustomisation(userConfig: UserConfig): boolean {
   );
 }
 
+/**
+ * Finalize the incremental-analysis configuration for this run.
+ *
+ * Overlay analysis has only been validated in combination with diff-informed
+ * analysis, so if `Overlay` mode was selected for a pull request but the diff
+ * ranges could not be computed, fall back to a full non-overlay analysis.
+ *
+ * Query exclusions for incremental-only queries are then applied whenever the
+ * diff ranges are available — which, after the fallback above, is exactly the
+ * set of runs where any kind of incremental analysis (overlay or
+ * diff-informed) is in effect.
+ */
+export async function applyIncrementalAnalysisSettings(
+  config: Config,
+  hasDiffRanges: boolean,
+  codeql: CodeQL,
+  logger: Logger,
+): Promise<void> {
+  if (
+    config.overlayDatabaseMode === OverlayDatabaseMode.Overlay &&
+    !hasDiffRanges
+  ) {
+    logger.info(
+      `Reverting overlay database mode to ${OverlayDatabaseMode.None} ` +
+        "because the PR diff ranges could not be computed.",
+    );
+    config.overlayDatabaseMode = OverlayDatabaseMode.None;
+    config.useOverlayDatabaseCaching = false;
+    await addOverlayDisablementDiagnostics(
+      config,
+      codeql,
+      OverlayDisabledReason.PrDiffRangesNotComputed,
+    );
+  }
+
+  if (hasDiffRanges) {
+    config.extraQueryExclusions.push({
+      exclude: { tags: "exclude-from-incremental" },
+    });
+  }
+}
+
 /**
  * Load and return the config.
  *
@@ -1230,18 +1272,18 @@ export async function initConfig(
     );
   }
 
-  if (
-    config.overlayDatabaseMode === OverlayDatabaseMode.Overlay ||
-    (await shouldPerformDiffInformedAnalysis(
-      inputs.codeql,
-      inputs.features,
-      logger,
-    ))
-  ) {
-    config.extraQueryExclusions.push({
-      exclude: { tags: "exclude-from-incremental" },
-    });
-  }
+  const hasDiffRanges = await prepareDiffInformedAnalysis(
+    inputs.codeql,
+    inputs.features,
+    logger,
+  );
+
+  await applyIncrementalAnalysisSettings(
+    config,
+    hasDiffRanges,
+    inputs.codeql,
+    logger,
+  );
 
   if (await isTrapCachingEnabled(features, config.overlayDatabaseMode)) {
     const { trapCaches, trapCacheDownloadTime } = await downloadCacheWithTime(
diff --git a/src/diff-informed-analysis-utils.test.ts b/src/diff-informed-analysis-utils.test.ts
index bec0c2c0ab..96fce23eba 100644
--- a/src/diff-informed-analysis-utils.test.ts
+++ b/src/diff-informed-analysis-utils.test.ts
@@ -5,14 +5,16 @@ import * as actionsUtil from "./actions-util";
 import type { PullRequestBranches } from "./actions-util";
 import * as apiClient from "./api-client";
 import {
-  shouldPerformDiffInformedAnalysis,
+  getDiffInformedAnalysisBranches,
+  prepareDiffInformedAnalysis,
   exportedForTesting,
 } from "./diff-informed-analysis-utils";
-import { Feature, initFeatures } from "./feature-flags";
+import { Feature, FeatureEnablement, initFeatures } from "./feature-flags";
 import { getRunnerLogger } from "./logging";
 import { parseRepositoryNwo } from "./repository";
 import {
   setupTests,
+  createFeatures,
   mockCodeQLVersion,
   mockFeatureFlagApiEndpoint,
   setupActionsVars,
@@ -80,13 +82,13 @@ const testShouldPerformDiffInformedAnalysis = test.macro({
         .stub(actionsUtil, "getPullRequestBranches")
         .returns(testCase.pullRequestBranches);
 
-      const result = await shouldPerformDiffInformedAnalysis(
+      const branches = await getDiffInformedAnalysisBranches(
         codeql,
         features,
         logger,
       );
 
-      t.is(result, expectedResult);
+      t.is(branches !== undefined, expectedResult);
 
       delete process.env.CODEQL_ACTION_DIFF_INFORMED_QUERIES;
 
@@ -94,7 +96,7 @@ const testShouldPerformDiffInformedAnalysis = test.macro({
       getPullRequestBranchesStub.restore();
     });
   },
-  title: (_, title) => `shouldPerformDiffInformedAnalysis: ${title}`,
+  title: (_, title) => `getDiffInformedAnalysisBranches: ${title}`,
 });
 
 test.serial(
@@ -187,6 +189,135 @@ test.serial(
   false,
 );
 
+test.serial(
+  "prepareDiffInformedAnalysis: returns false when not a pull request",
+  async (t) => {
+    await withTmpDir(async (tmpDir) => {
+      setupActionsVars(tmpDir, tmpDir);
+      const logger = getRunnerLogger(true);
+      const codeql = mockCodeQLVersion("2.21.0");
+      const features = createFeatures([Feature.DiffInformedQueries]);
+
+      sinon.stub(actionsUtil, "getPullRequestBranches").returns(undefined);
+      sinon
+        .stub(apiClient, "getGitHubVersion")
+        .resolves({ type: GitHubVariant.DOTCOM });
+
+      const result = await prepareDiffInformedAnalysis(
+        codeql,
+        features,
+        logger,
+      );
+
+      t.false(result);
+    });
+  },
+);
+
+test.serial(
+  "prepareDiffInformedAnalysis: returns false when applicability check throws",
+  async (t) => {
+    await withTmpDir(async (tmpDir) => {
+      setupActionsVars(tmpDir, tmpDir);
+      const logger = getRunnerLogger(true);
+      const codeql = mockCodeQLVersion("2.21.0");
+      // A features implementation whose getValue rejects, simulating an
+      // unexpected failure when determining whether diff-informed analysis
+      // should run.
+      const features: FeatureEnablement = {
+        getDefaultCliVersion: async () => {
+          throw new Error("not implemented");
+        },
+        getValue: async () => {
+          throw new Error("feature flag lookup failed");
+        },
+      };
+
+      const result = await prepareDiffInformedAnalysis(
+        codeql,
+        features,
+        logger,
+      );
+
+      t.false(result);
+    });
+  },
+);
+
+test.serial(
+  "prepareDiffInformedAnalysis: returns true when the diff is fetched successfully",
+  async (t) => {
+    await withTmpDir(async (tmpDir) => {
+      setupActionsVars(tmpDir, tmpDir);
+      const logger = getRunnerLogger(true);
+      const codeql = mockCodeQLVersion("2.21.0");
+      const features = createFeatures([Feature.DiffInformedQueries]);
+
+      sinon
+        .stub(actionsUtil, "getPullRequestBranches")
+        .returns({ base: "main", head: "feature" });
+      sinon
+        .stub(apiClient, "getGitHubVersion")
+        .resolves({ type: GitHubVariant.DOTCOM });
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-argument
+      sinon.stub(apiClient, "getApiClient").returns({
+        rest: {
+          repos: {
+            compareCommitsWithBasehead: sinon
+              .stub()
+              .resolves({ data: { files: [] } }),
+          },
+        },
+      } as any);
+
+      const result = await prepareDiffInformedAnalysis(
+        codeql,
+        features,
+        logger,
+      );
+
+      t.true(result);
+    });
+  },
+);
+
+test.serial(
+  "prepareDiffInformedAnalysis: returns false when the diff API call fails",
+  async (t) => {
+    await withTmpDir(async (tmpDir) => {
+      setupActionsVars(tmpDir, tmpDir);
+      const logger = getRunnerLogger(true);
+      const codeql = mockCodeQLVersion("2.21.0");
+      const features = createFeatures([Feature.DiffInformedQueries]);
+
+      sinon
+        .stub(actionsUtil, "getPullRequestBranches")
+        .returns({ base: "main", head: "feature" });
+      sinon
+        .stub(apiClient, "getGitHubVersion")
+        .resolves({ type: GitHubVariant.DOTCOM });
+      const notFoundError: any = new Error("Not Found");
+      notFoundError.status = 404;
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-argument
+      sinon.stub(apiClient, "getApiClient").returns({
+        rest: {
+          repos: {
+            compareCommitsWithBasehead: sinon.stub().rejects(notFoundError),
+          },
+        },
+      } as any);
+
+      const result = await prepareDiffInformedAnalysis(
+        codeql,
+        features,
+        logger,
+      );
+
+      t.false(result);
+    });
+  },
+);
+
 function runGetDiffRanges(changes: number, patch: string[] | undefined): any {
   return exportedForTesting.getDiffRanges(
     {
diff --git a/src/diff-informed-analysis-utils.ts b/src/diff-informed-analysis-utils.ts
index 1c98d4caca..a48c6dcfdf 100644
--- a/src/diff-informed-analysis-utils.ts
+++ b/src/diff-informed-analysis-utils.ts
@@ -5,9 +5,9 @@ import type { PullRequestBranches } from "./actions-util";
 import { getApiClient, getGitHubVersion } from "./api-client";
 import type { CodeQL } from "./codeql";
 import { Feature, FeatureEnablement } from "./feature-flags";
-import { Logger } from "./logging";
+import { Logger, withGroupAsync } from "./logging";
 import { getRepositoryNwoFromEnv } from "./repository";
-import { GitHubVariant, satisfiesGHESVersion } from "./util";
+import { getErrorMessage, GitHubVariant, satisfiesGHESVersion } from "./util";
 
 /**
  * This interface is an abbreviated version of the file diff object returned by
@@ -21,20 +21,6 @@ interface FileDiff {
   patch?: string | undefined;
 }
 
-/**
- * Check if the action should perform diff-informed analysis.
- */
-export async function shouldPerformDiffInformedAnalysis(
-  codeql: CodeQL,
-  features: FeatureEnablement,
-  logger: Logger,
-): Promise<boolean> {
-  return (
-    (await getDiffInformedAnalysisBranches(codeql, features, logger)) !==
-    undefined
-  );
-}
-
 /**
  * Get the branches to use for diff-informed analysis.
  *
@@ -69,6 +55,46 @@ export async function getDiffInformedAnalysisBranches(
   return branches;
 }
 
+/**
+ * Prepares the diff ranges needed for diff-informed analysis for the current
+ * run.
+ *
+ * @returns `true` if the diff ranges were successfully computed and persisted
+ *   and are therefore available for use, `false` otherwise.
+ */
+export async function prepareDiffInformedAnalysis(
+  codeql: CodeQL,
+  features: FeatureEnablement,
+  logger: Logger,
+): Promise<boolean> {
+  let branches: PullRequestBranches | undefined;
+  try {
+    branches = await getDiffInformedAnalysisBranches(codeql, features, logger);
+  } catch (e) {
+    // If we cannot determine whether diff-informed analysis applies (for
+    // example, because a feature-flag lookup failed), treat it as not
+    // applicable rather than triggering the overlay fallback.
+    logger.warning(
+      `Failed to determine branch information for diff-informed analysis: ${getErrorMessage(e)}`,
+    );
+    return false;
+  }
+  if (!branches) {
+    return false;
+  }
+
+  return await withGroupAsync("Computing PR diff ranges", async () => {
+    try {
+      return await computeAndPersistDiffRanges(branches, logger);
+    } catch (e) {
+      logger.warning(
+        `Failed to compute diff-informed analysis ranges: ${getErrorMessage(e)}`,
+      );
+      return false;
+    }
+  });
+}
+
 export interface DiffThunkRange {
   /** Relative path from the repository root, using forward slashes as separators. */
   path: string;
@@ -151,6 +177,33 @@ export async function getPullRequestEditedDiffRanges(
   return results;
 }
 
+/**
+ * Compute and persist the diff ranges for a pull request. This fetches the
+ * diff from the GitHub API and writes it to the diff ranges JSON file so that
+ * CodeQL can use it for diff-informed analysis.
+ *
+ * @param branches The base and head branches of the pull request, as returned
+ *   by `getDiffInformedAnalysisBranches`.
+ * @param logger
+ * @returns `true` if the diff ranges were successfully computed and persisted,
+ *   otherwise `false`.
+ */
+export async function computeAndPersistDiffRanges(
+  branches: PullRequestBranches,
+  logger: Logger,
+): Promise<boolean> {
+  const ranges = await getPullRequestEditedDiffRanges(branches, logger);
+  if (ranges === undefined) {
+    return false;
+  }
+  writeDiffRangesJsonFile(logger, ranges);
+  const distinctFiles = new Set(ranges.map((r) => r.path)).size;
+  logger.info(
+    `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s).`,
+  );
+  return true;
+}
+
 async function getFileDiffsWithBasehead(
   branches: PullRequestBranches,
   logger: Logger,
diff --git a/src/init-action.ts b/src/init-action.ts
index 859dcefa2c..50a66f9b01 100644
--- a/src/init-action.ts
+++ b/src/init-action.ts
@@ -37,11 +37,6 @@ import {
   makeDiagnostic,
   makeTelemetryDiagnostic,
 } from "./diagnostics";
-import {
-  getDiffInformedAnalysisBranches,
-  getPullRequestEditedDiffRanges,
-  writeDiffRangesJsonFile,
-} from "./diff-informed-analysis-utils";
 import { EnvVar } from "./environment";
 import { Feature, FeatureEnablement, initFeatures } from "./feature-flags";
 import {
@@ -427,7 +422,6 @@ async function run(startedAt: Date) {
     }
 
     await checkInstallPython311(config.languages, codeql);
-    await computeAndPersistDiffRanges(codeql, features, logger);
   } catch (unwrappedError) {
     const error = wrapError(unwrappedError);
     core.setFailed(error.message);
@@ -823,42 +817,6 @@ async function loadRepositoryProperties(
   }
 }
 
-/**
- * Compute and persist diff ranges when diff-informed analysis is enabled
- * (feature flag + PR context). This writes the standard pr-diff-range.json
- * file for later reuse in the analyze step. Failures are logged but non-fatal.
- */
-async function computeAndPersistDiffRanges(
-  codeql: CodeQL,
-  features: FeatureEnablement,
-  logger: Logger,
-): Promise<void> {
-  await withGroupAsync("Computing PR diff ranges", async () => {
-    try {
-      const branches = await getDiffInformedAnalysisBranches(
-        codeql,
-        features,
-        logger,
-      );
-      if (!branches) {
-        return;
-      }
-      const ranges = await getPullRequestEditedDiffRanges(branches, logger);
-      if (ranges === undefined) {
-        return;
-      }
-      writeDiffRangesJsonFile(logger, ranges);
-      const distinctFiles = new Set(ranges.map((r) => r.path)).size;
-      logger.info(
-        `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s).`,
-      );
-    } catch (e) {
-      logger.warning(
-        `Failed to compute and persist PR diff ranges: ${getErrorMessage(e)}`,
-      );
-    }
-  });
-}
 async function recordZstdAvailability(
   config: configUtils.Config,
   zstdAvailability: ZstdAvailability,
diff --git a/src/overlay/diagnostics.ts b/src/overlay/diagnostics.ts
index 6bc11a73ff..d350e38f3e 100644
--- a/src/overlay/diagnostics.ts
+++ b/src/overlay/diagnostics.ts
@@ -39,6 +39,13 @@ export enum OverlayDisabledReason {
   NotPullRequestOrDefaultBranch = "not-pull-request-or-default-branch",
   /** The top-level overlay analysis feature flag is not enabled. */
   OverallFeatureNotEnabled = "overall-feature-not-enabled",
+  /**
+   * Overlay analysis was selected for a pull request, but the PR diff ranges
+   * needed for diff-informed analysis could not be computed. Overlay analysis
+   * has only been validated in combination with diff-informed analysis, so we
+   * fall back to a non-overlay analysis in this case.
+   */
+  PrDiffRangesNotComputed = "pr-diff-ranges-not-computed",
   /** Overlay analysis was skipped because it previously failed with similar hardware resources. */
   SkippedDueToCachedStatus = "skipped-due-to-cached-status",
   /** Disk usage could not be determined during the overlay status check. */