reproduction: customProvider loses video models from v3 fallback providers

.ai-sdk-factory/reproduction-replay.json

@@ -0,0 +1,13 @@
+{
+  "version": 2,
+  "issueId": 21110,
+  "expectationRevision": 1,
+  "command": "pnpm -C examples/ai-functions exec tsx src/reproduction/custom-provider-v3-video-fallback.ts",
+  "artifactPaths": [
+    "examples/ai-functions/src/reproduction/custom-provider-v3-video-fallback.ts"
+  ],
+  "expectedFailure": {
+    "exitCode": 1,
+    "signal": "ISSUE #21110: customProvider fallback did not return the v3 video model adapted to v4; received AI_NoSuchModelError"
+  }
+}

examples/ai-functions/src/reproduction/custom-provider-v3-video-fallback.ts

@@ -0,0 +1,89 @@
+import assert from 'node:assert/strict';
+import {
+  NoSuchModelError,
+  type Experimental_VideoModelV3,
+  type ProviderV3,
+} from '@ai-sdk/provider';
+import { createProviderRegistry, customProvider } from 'ai';
+
+const modelId = 'fal-ai/kling-video/v2.1/standard/text-to-video';
+const failureSignal =
+  'ISSUE #21110: customProvider fallback did not return the v3 video model adapted to v4';
+
+type ProviderV3WithVideo = ProviderV3 & {
+  videoModel(modelId: string): Experimental_VideoModelV3;
+};
+
+function noSuchModel(
+  modelId: string,
+  modelType: NoSuchModelError['modelType'],
+): never {
+  throw new NoSuchModelError({ modelId, modelType });
+}
+
+const falV3LikeProvider: ProviderV3WithVideo = {
+  specificationVersion: 'v3',
+  languageModel: id => noSuchModel(id, 'languageModel'),
+  embeddingModel: id => noSuchModel(id, 'embeddingModel'),
+  imageModel: id => noSuchModel(id, 'imageModel'),
+  videoModel(id) {
+    assert.equal(id, modelId);
+
+    return {
+      specificationVersion: 'v3',
+      provider: 'fal.video',
+      modelId: id,
+      maxVideosPerCall: 1,
+      async doGenerate() {
+        return {
+          videos: [],
+          warnings: [],
+          response: {
+            timestamp: new Date(0),
+            modelId: id,
+            headers: undefined,
+          },
+        };
+      },
+    };
+  },
+};
+
+async function main() {
+  const directModel = falV3LikeProvider.videoModel(modelId);
+  assert.equal(directModel.specificationVersion, 'v3');
+
+  const aliasModel = customProvider({
+    videoModels: { clip: directModel },
+  }).videoModel('clip');
+  assert.equal(aliasModel.specificationVersion, 'v4');
+
+  const registryModel = createProviderRegistry({
+    fal: falV3LikeProvider,
+  }).videoModel(`fal:${modelId}`);
+  assert.equal(registryModel.specificationVersion, 'v4');
+
+  let fallbackModel;
+  try {
+    fallbackModel = customProvider({
+      fallbackProvider: falV3LikeProvider,
+    }).videoModel(modelId);
+  } catch (error) {
+    if (
+      NoSuchModelError.isInstance(error) &&
+      error.modelId === modelId &&
+      error.modelType === 'videoModel'
+    ) {
+      throw new Error(`${failureSignal}; received AI_NoSuchModelError`);
+    }
+
+    throw error;
+  }
+
+  assert.equal(fallbackModel.specificationVersion, 'v4', failureSignal);
+}
+
+main().catch(error => {
+  console.error(error);
+  process.exitCode = 1;
+});