@@ -668,6 +668,7 @@ const injectedConfigMacro = makeMacro({
         new RecordingLogger(),
         thisStubConfig.originalUserInput,
         augmentationProperties,
+        thisStubConfig.languages,
       );
 
       await codeqlObject.databaseInitCluster(
@@ -737,6 +738,54 @@ injectedConfigMacro.serial(
   },
 );
 
+const additivePackConfigurations: Array<Record<string, string[]>> = [
+  {
+    javascript: ["acme/javascript-queries"],
+    python: ["acme/python-queries"],
+  },
+  {
+    python: ["acme/python-queries"],
+    javascript: ["acme/javascript-queries"],
+  },
+  { javascript: ["acme/javascript-queries"] },
+  { javascript: ["acme/javascript-queries"], python: [] },
+  {},
+];
+for (const packs of additivePackConfigurations) {
+  injectedConfigMacro.serial(
+    `additive packs target the analyzed language in ${JSON.stringify(packs)}`,
+    {
+      ...defaultAugmentationProperties,
+      packsInputCombines: true,
+      packsInput: ["acme/python-extra"],
+    },
+    {
+      languages: [BuiltInLanguage.python],
+      originalUserInput: { packs },
+    },
+    {
+      packs: {
+        ...packs,
+        python: [...(packs.python || []), "acme/python-extra"],
+      },
+    },
+  );
+}
+
+injectedConfigMacro.serial(
+  "additive packs preserve an existing array",
+  {
+    ...defaultAugmentationProperties,
+    packsInputCombines: true,
+    packsInput: ["acme/python-extra"],
+  },
+  {
+    languages: [BuiltInLanguage.python],
+    originalUserInput: { packs: ["acme/python-queries"] },
+  },
+  { packs: ["acme/python-queries", "acme/python-extra"] },
+);
+
 injectedConfigMacro.serial(
   "injected packs from input with existing packs overrides",
   {

@@ -622,6 +622,47 @@ test.serial(
   },
 );
 
+test.serial(
+  "additive packs use the job language with a shared config file",
+  async (t) => {
+    await withTmpDir(async (tempDir) => {
+      setupActionsVars(tempDir, tempDir);
+      const userConfig: UserConfig = {
+        packs: {
+          javascript: ["acme/javascript-queries"],
+          python: ["acme/python-queries"],
+        },
+      };
+      const codeql = createStubCodeQL({
+        async resolveLanguages() {
+          return {
+            extractors: { python: [{ extractor_root: "" }] },
+          };
+        },
+      });
+      const config = await configUtils.initConfig(
+        initAllState(),
+        createTestInitConfigInputs({
+          languagesInput: "python",
+          packsInput: "+acme/python-extra",
+          configFile: createConfigFile(yaml.dump(userConfig), tempDir),
+          tempDir,
+          codeql,
+          workspacePath: tempDir,
+        }),
+      );
+
+      t.deepEqual(config.computedConfig, {
+        packs: {
+          javascript: ["acme/javascript-queries"],
+          python: ["acme/python-queries", "acme/python-extra"],
+        },
+      });
+      t.deepEqual(config.originalUserInput, userConfig);
+    });
+  },
+);
+
 test.serial("API client used when reading remote config", async (t) => {
   return await withTmpDir(async (tempDir) => {
     const codeql = createStubCodeQL({

@@ -437,6 +437,7 @@ export async function initActionState(
     logger,
     userConfig,
     augmentationProperties,
+    languages,
   );
 
   return {

@@ -588,6 +588,7 @@ export function generateCodeScanningConfig(
   logger: Logger,
   originalUserInput: UserConfig,
   augmentationProperties: AugmentationProperties,
+  languages: Language[],
 ): UserConfig {
   // make a copy so we can modify it
   const augmentedConfig = cloneObject(originalUserInput);
@@ -616,12 +617,10 @@ export function generateCodeScanningConfig(
       } else if (!augmentedConfig.packs) {
         augmentedConfig.packs = augmentationProperties.packsInput;
       } else {
-        // At this point, we know there is only one language.
-        // If there were more than one language, an error would already have been thrown.
-        const language = Object.keys(augmentedConfig.packs)[0];
-        augmentedConfig.packs[language] = augmentedConfig.packs[
-          language
-        ].concat(augmentationProperties.packsInput);
+        const language = languages[0];
+        augmentedConfig.packs[language] = (
+          augmentedConfig.packs[language] ?? []
+        ).concat(augmentationProperties.packsInput);
       }
     } else {
       augmentedConfig.packs = augmentationProperties.packsInput;

@@ -2,6 +2,8 @@ name: Pack testing in the CodeQL Action
 
 disable-default-queries: true
 packs:
+  ruby:
+    - codeql/ruby-queries
   javascript:
     - codeql-testing/codeql-pack2
     - codeql-testing/codeql-pack3:other-query.ql

@@ -0,0 +1 @@
+- Fixed additive `packs` inputs being applied to the wrong language when a shared configuration file lists packs for multiple languages.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		- Fixed additive `packs` inputs being applied to the wrong language when a shared configuration file lists packs for multiple languages.