diff --git a/lib/entry-points.js b/lib/entry-points.js
index 674e6425f..629ee5b6b 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -177425,7 +177425,7 @@ async function getCodeQLForCmd(logger2, cmd, checkVersion) {
       }
       await new toolrunner3.ToolRunner(cmd, args).exec();
     },
-    async diagnosticsExport(sarifFile, automationDetailsId, config) {
+    async diagnosticsExport(sarifFile, automationDetailsId, config, diagnosticDir) {
       const args = [
         "diagnostics",
         "export",
@@ -177434,6 +177434,7 @@ async function getCodeQLForCmd(logger2, cmd, checkVersion) {
         `--sarif-codescanning-config=${getGeneratedCodeScanningConfigPath(
           config
         )}`,
+        ...diagnosticDir !== void 0 ? [`--diagnostic-dir=${diagnosticDir}`] : [],
         ...getExtraOptionsFromEnv(["diagnostics", "export"])
       ];
       if (automationDetailsId !== void 0) {
@@ -187021,7 +187022,13 @@ async function generateFailedSarif(logger2, features, config, category, checkout
     sarifFile = "../codeql-failed-run.sarif";
   }
   if (databasePath === void 0 || !await features.getValue("export_diagnostics_enabled" /* ExportDiagnosticsEnabled */, codeql)) {
-    await codeql.diagnosticsExport(sarifFile, category, config);
+    let diagnosticDir = getTempDiagnosticPath(
+      config.tempDir
+    );
+    if (!fs31.existsSync(diagnosticDir)) {
+      diagnosticDir = void 0;
+    }
+    await codeql.diagnosticsExport(sarifFile, category, config, diagnosticDir);
   } else {
     await codeql.databaseExportDiagnostics(databasePath, sarifFile, category);
   }
diff --git a/src/codeql.ts b/src/codeql.ts
index a76e29727..5981929c1 100644
--- a/src/codeql.ts
+++ b/src/codeql.ts
@@ -199,6 +199,7 @@ export interface CodeQL {
     sarifFile: string,
     automationDetailsId: string | undefined,
     config: ActionConfig<"tempDir">,
+    diagnosticDir: string | undefined,
   ): Promise<void>;
   /** Get the location of an extractor for the specified language. */
   resolveExtractor(language: Language): Promise<string>;
@@ -915,6 +916,7 @@ async function getCodeQLForCmd(
       sarifFile: string,
       automationDetailsId: string | undefined,
       config: ActionConfig<"tempDir">,
+      diagnosticDir: string | undefined,
     ): Promise<void> {
       const args = [
         "diagnostics",
@@ -924,6 +926,9 @@ async function getCodeQLForCmd(
         `--sarif-codescanning-config=${getGeneratedCodeScanningConfigPath(
           config,
         )}`,
+        ...(diagnosticDir !== undefined
+          ? [`--diagnostic-dir=${diagnosticDir}`]
+          : []),
         ...getExtraOptionsFromEnv(["diagnostics", "export"]),
       ];
       if (automationDetailsId !== undefined) {
diff --git a/src/init-action-post-helper.test.ts b/src/init-action-post-helper.test.ts
index 554add71f..b90ad3047 100644
--- a/src/init-action-post-helper.test.ts
+++ b/src/init-action-post-helper.test.ts
@@ -1,3 +1,6 @@
+import * as fs from "fs";
+import * as path from "path";
+
 import * as core from "@actions/core";
 import test, { ExecutionContext } from "ava";
 import * as sinon from "sinon";
@@ -8,6 +11,7 @@ import * as apiClient from "./api-client";
 import * as codeql from "./codeql";
 import * as configUtils from "./config-utils";
 import * as debugArtifacts from "./debug-artifacts";
+import { getTempDiagnosticPath } from "./diagnostics";
 import { EnvVar } from "./environment";
 import { Feature } from "./feature-flags";
 import * as initActionPostHelper from "./init-action-post-helper";
@@ -35,6 +39,47 @@ const NUM_BYTES_PER_GIB = 1024 * 1024 * 1024;
 
 setupTests(test);
 
+test.serial("generateFailedSarif picks up unwritten diagnostics", async (t) =>
+  util.withTmpDir(async (tmpDir) => {
+    const logger = new RecordingLogger();
+
+    const tempDiagDir = getTempDiagnosticPath(tmpDir);
+    fs.mkdirSync(tempDiagDir);
+
+    const codeqlObject = await codeql.getCodeQLForTesting();
+    const diagnosticsExport = sinon
+      .stub(codeqlObject, "diagnosticsExport")
+      .resolves();
+
+    sinon.stub(codeql, "getCodeQL").resolves(codeqlObject);
+
+    const sarifFile = path.join(tmpDir, "failed.sarif");
+    const result = await initActionPostHelper.generateFailedSarif(
+      logger,
+      createFeatures([]),
+      createTestConfig({
+        tempDir: tmpDir,
+        // Explicitly undefined to ensure we get the "no database" path.
+        dbLocation: undefined,
+      }),
+      "category",
+      "",
+      sarifFile,
+    );
+
+    t.truthy(result);
+    t.is(diagnosticsExport.callCount, 1);
+    t.true(
+      diagnosticsExport.calledOnceWith(
+        sarifFile,
+        "category",
+        sinon.match.any,
+        tempDiagDir,
+      ),
+    );
+  }),
+);
+
 test.serial("init-post action with debug mode off", async (t) => {
   return await util.withTmpDir(async (tmpDir) => {
     setupActionsVars(tmpDir, tmpDir);
diff --git a/src/init-action-post-helper.ts b/src/init-action-post-helper.ts
index cfaab312f..83c2aec9f 100644
--- a/src/init-action-post-helper.ts
+++ b/src/init-action-post-helper.ts
@@ -24,6 +24,7 @@ import {
   type UploadDebugArtifactsConfig,
 } from "./debug-artifacts";
 import * as dependencyCaching from "./dependency-caching";
+import { getTempDiagnosticPath } from "./diagnostics";
 import { EnvVar, ReadOnlyEnv } from "./environment";
 import { Feature, FeatureEnablement } from "./feature-flags";
 import { Logger } from "./logging";
@@ -177,7 +178,16 @@ async function prepareFailedSarif(
   }
 }
 
-async function generateFailedSarif(
+/**
+ * Generates a SARIF file for a failed run, i.e. an analysis where no SARIF was produced as a result of the analysis.
+ *
+ * Not intended to be called directly, other than by `prepareFailedSarif`.
+ *
+ * @param category The analysis category to include in the SARIF.
+ * @param checkoutPath The path where the repository was checked out at.
+ * @param sarifFile The SARIF file to produce.
+ */
+export async function generateFailedSarif(
   logger: Logger,
   features: FeatureEnablement,
   config: ActionConfig<"codeQLCmd" | "tempDir">,
@@ -198,7 +208,19 @@ async function generateFailedSarif(
     databasePath === undefined ||
     !(await features.getValue(Feature.ExportDiagnosticsEnabled, codeql))
   ) {
-    await codeql.diagnosticsExport(sarifFile, category, config);
+    // If we don't have a database path, then we may have diagnostics that were written to a
+    // temporary location. Get the path that they would have been written to and check whether
+    // it exists. If so, pass it on to the CLI. The CLI will not complain if we give it a
+    // path that doesn't exist, so we are just being extra defensive here.
+    let diagnosticDir: string | undefined = getTempDiagnosticPath(
+      config.tempDir,
+    );
+
+    if (!fs.existsSync(diagnosticDir)) {
+      diagnosticDir = undefined;
+    }
+
+    await codeql.diagnosticsExport(sarifFile, category, config, diagnosticDir);
   } else {
     // We call 'database export-diagnostics' to find any per-database diagnostics.
     await codeql.databaseExportDiagnostics(databasePath, sarifFile, category);