diff --git a/js/testapps/anthropic/README.md b/js/testapps/anthropic/README.md
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+# Anthropic Plugin Sample
+
+This test app demonstrates usage of the Genkit Anthropic plugin against both the stable and beta runners, organized by feature.
+
+## Directory Structure
+
+```
+src/
+  stable/
+    basic.ts          - Basic stable API examples (hello, streaming)
+    text-plain.ts     - Text/plain error handling demonstration
+    webp.ts           - WEBP image handling demonstration
+    pdf.ts            - PDF document processing examples
+    attention-first-page.pdf - Sample PDF file for testing
+  beta/
+    basic.ts          - Basic beta API examples
+```
+
+## Setup
+
+1. From the repo root run `pnpm install` followed by `pnpm run setup` to link workspace dependencies.
+2. In this directory, optionally run `pnpm install` if you want a local `node_modules/`.
+3. Export an Anthropic API key (or add it to a `.env` file) before running any samples:
+
+   ```bash
+   export ANTHROPIC_API_KEY=your-key
+   ```
+
+## Available scripts
+
+### Basic Examples
+- `pnpm run build` – Compile the TypeScript sources into `lib/`.
+- `pnpm run start:stable` – Run the compiled stable basic sample.
+- `pnpm run start:beta` – Run the compiled beta basic sample.
+- `pnpm run dev:stable` – Start the Genkit Dev UI over `src/stable/basic.ts` with live reload.
+- `pnpm run dev:beta` – Start the Genkit Dev UI over `src/beta/basic.ts` with live reload.
+
+### Feature-Specific Examples
+- `pnpm run dev:stable:text-plain` – Start Dev UI for text/plain error handling demo.
+- `pnpm run dev:stable:webp` – Start Dev UI for WEBP image handling demo.
+- `pnpm run dev:stable:pdf` – Start Dev UI for PDF document processing demo.
+
+## Flows
+
+Each source file defines flows that can be invoked from the Dev UI or the Genkit CLI:
+
+### Basic Examples
+- `anthropic-stable-hello` – Simple greeting using stable API
+- `anthropic-stable-stream` – Streaming response example
+- `anthropic-beta-hello` – Simple greeting using beta API
+- `anthropic-beta-stream` – Streaming response with beta API
+- `anthropic-beta-opus41` – Test Opus 4.1 model with beta API
+
+### Text/Plain Handling
+- `stable-text-plain-error` – Demonstrates the helpful error when using text/plain as media
+- `stable-text-plain-correct` – Shows the correct way to send text content
+
+### WEBP Image Handling
+- `stable-webp-matching` – WEBP image with matching contentType
+- `stable-webp-mismatched` – WEBP image with mismatched contentType (demonstrates the fix)
+
+### PDF Document Processing
+- `stable-pdf-base64` – Process a PDF from a local file using base64 encoding
+- `stable-pdf-url` – Process a PDF from a publicly accessible URL
+- `stable-pdf-analysis` – Analyze a PDF document for key topics, concepts, and visual elements
+
+Example: `genkit flow:run anthropic-stable-hello`
diff --git a/js/testapps/anthropic/package.json b/js/testapps/anthropic/package.json
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+{
+  "name": "anthropic-testapp",
+  "version": "0.0.1",
+  "description": "Sample Genkit app showcasing Anthropic plugin stable and beta usage.",
+  "main": "lib/stable/basic.js",
+  "scripts": {
+    "build": "tsc",
+    "build:watch": "tsc --watch",
+    "start:stable": "node lib/stable/basic.js",
+    "start:beta": "node lib/beta/basic.js",
+    "dev:stable": "genkit start -- npx tsx --watch src/stable/basic.ts",
+    "dev:beta": "genkit start -- npx tsx --watch src/beta/basic.ts",
+    "dev:stable:text-plain": "genkit start -- npx tsx --watch src/stable/text-plain.ts",
+    "dev:stable:webp": "genkit start -- npx tsx --watch src/stable/webp.ts",
+    "dev:stable:pdf": "genkit start -- npx tsx --watch src/stable/pdf.ts",
+    "genkit:dev": "cross-env GENKIT_ENV=dev npm run dev:stable",
+    "genkit:start": "cross-env GENKIT_ENV=dev genkit start -- tsx --watch src/stable/basic.ts",
+    "dev": "export GENKIT_RUNTIME_ID=$(openssl rand -hex 8) && node lib/stable/basic.js 2>&1"
+  },
+  "keywords": [
+    "genkit",
+    "anthropic",
+    "sample"
+  ],
+  "author": "",
+  "license": "Apache-2.0",
+  "dependencies": {
+    "genkit": "workspace:*",
+    "@genkit-ai/anthropic": "workspace:*"
+  },
+  "devDependencies": {
+    "cross-env": "^10.1.0",
+    "tsx": "^4.19.2",
+    "typescript": "^5.6.2"
+  }
+}
diff --git a/js/testapps/anthropic/src/beta/basic.ts b/js/testapps/anthropic/src/beta/basic.ts
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+/**
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { anthropic } from '@genkit-ai/anthropic';
+import { genkit } from 'genkit';
+
+const ai = genkit({
+  plugins: [
+    // Default all flows in this sample to the beta surface
+    anthropic({ apiVersion: 'beta', cacheSystemPrompt: true }),
+  ],
+});
+
+const betaHaiku = anthropic.model('claude-3-5-haiku', { apiVersion: 'beta' });
+const betaSonnet = anthropic.model('claude-sonnet-4-5', { apiVersion: 'beta' });
+const betaOpus41 = anthropic.model('claude-opus-4-1', { apiVersion: 'beta' });
+
+ai.defineFlow('anthropic-beta-hello', async () => {
+  const { text } = await ai.generate({
+    model: betaHaiku,
+    prompt:
+      'You are Claude on the beta API. Provide a concise greeting that mentions that you are using the beta API.',
+    config: { temperature: 0.6 },
+  });
+
+  return text;
+});
+
+ai.defineFlow('anthropic-beta-stream', async (_, { sendChunk }) => {
+  const { stream } = ai.generateStream({
+    model: betaSonnet,
+    prompt: [
+      {
+        text: 'Outline two experimental capabilities unlocked by the Anthropic beta API.',
+      },
+    ],
+    config: {
+      apiVersion: 'beta',
+      temperature: 0.4,
+    },
+  });
+
+  const collected: string[] = [];
+  for await (const chunk of stream) {
+    if (chunk.text) {
+      collected.push(chunk.text);
+      sendChunk(chunk.text);
+    }
+  }
+
+  return collected.join('');
+});
+
+ai.defineFlow('anthropic-beta-opus41', async () => {
+  const { text } = await ai.generate({
+    model: betaOpus41,
+    prompt:
+      'You are Claude Opus 4.1 on the beta API. Provide a brief greeting that confirms you are using the beta API.',
+    config: { temperature: 0.6 },
+  });
+
+  return text;
+});
diff --git a/js/testapps/anthropic/src/stable/attention-first-page.pdf b/js/testapps/anthropic/src/stable/attention-first-page.pdf
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Binary files /dev/null and b/js/testapps/anthropic/src/stable/attention-first-page.pdf differ
diff --git a/js/testapps/anthropic/src/stable/basic.ts b/js/testapps/anthropic/src/stable/basic.ts
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+/**
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { anthropic } from '@genkit-ai/anthropic';
+import { genkit } from 'genkit';
+
+const ai = genkit({
+  plugins: [
+    // Configure the plugin with environment-driven API key
+    anthropic(),
+  ],
+});
+
+ai.defineFlow('anthropic-stable-hello', async () => {
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    prompt: 'You are a friendly Claude assistant. Greet the user briefly.',
+  });
+
+  return text;
+});
+
+ai.defineFlow('anthropic-stable-stream', async (_, { sendChunk }) => {
+  const { stream } = ai.generateStream({
+    model: anthropic.model('claude-sonnet-4-5'),
+    prompt: 'Compose a short limerick about using Genkit with Anthropic.',
+  });
+
+  let response = '';
+  for await (const chunk of stream) {
+    response += chunk.text ?? '';
+    if (chunk.text) {
+      sendChunk(chunk.text);
+    }
+  }
+
+  return response;
+});
diff --git a/js/testapps/anthropic/src/stable/pdf.ts b/js/testapps/anthropic/src/stable/pdf.ts
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+++ b/js/testapps/anthropic/src/stable/pdf.ts
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+/**
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { anthropic } from '@genkit-ai/anthropic';
+import * as fs from 'fs';
+import { genkit } from 'genkit';
+import * as path from 'path';
+
+const ai = genkit({
+  plugins: [anthropic()],
+});
+
+/**
+ * This flow demonstrates PDF document processing with Claude using base64 encoding.
+ * The PDF is read from the source directory and sent as a base64 data URL.
+ */
+ai.defineFlow('stable-pdf-base64', async () => {
+  // Read PDF file from the same directory as this source file
+  const pdfPath = path.join(__dirname, 'attention-first-page.pdf');
+  const pdfBuffer = fs.readFileSync(pdfPath);
+  const pdfBase64 = pdfBuffer.toString('base64');
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          {
+            text: 'What are the key findings or main points in this document?',
+          },
+          {
+            media: {
+              url: `data:application/pdf;base64,${pdfBase64}`,
+              contentType: 'application/pdf',
+            },
+          },
+        ],
+      },
+    ],
+  });
+
+  return text;
+});
+
+/**
+ * This flow demonstrates PDF document processing with a URL reference.
+ * Note: This requires the PDF to be hosted at a publicly accessible URL.
+ */
+ai.defineFlow('stable-pdf-url', async () => {
+  // Example: Using a publicly hosted PDF URL
+  // In a real application, you would use your own hosted PDF
+  const pdfUrl =
+    'https://assets.anthropic.com/m/1cd9d098ac3e6467/original/Claude-3-Model-Card-October-Addendum.pdf';
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          {
+            text: 'Summarize the key points from this document.',
+          },
+          {
+            media: {
+              url: pdfUrl,
+              contentType: 'application/pdf',
+            },
+          },
+        ],
+      },
+    ],
+  });
+
+  return text;
+});
+
+/**
+ * This flow demonstrates analyzing specific aspects of a PDF document.
+ * Claude can understand both text and visual elements (charts, tables, images) in PDFs.
+ */
+ai.defineFlow('stable-pdf-analysis', async () => {
+  const pdfPath = path.join(__dirname, 'attention-first-page.pdf');
+  const pdfBuffer = fs.readFileSync(pdfPath);
+  const pdfBase64 = pdfBuffer.toString('base64');
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          {
+            text: 'Analyze this document and provide:\n1. The main topic or subject\n2. Any key technical concepts mentioned\n3. Any visual elements (charts, tables, diagrams) if present',
+          },
+          {
+            media: {
+              url: `data:application/pdf;base64,${pdfBase64}`,
+              contentType: 'application/pdf',
+            },
+          },
+        ],
+      },
+    ],
+  });
+
+  return text;
+});
diff --git a/js/testapps/anthropic/src/stable/text-plain.ts b/js/testapps/anthropic/src/stable/text-plain.ts
new file mode 100644
index 0000000000..0b290d53e6
--- /dev/null
+++ b/js/testapps/anthropic/src/stable/text-plain.ts
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+/**
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { anthropic } from '@genkit-ai/anthropic';
+import { genkit } from 'genkit';
+
+const ai = genkit({
+  plugins: [anthropic()],
+});
+
+/**
+ * This flow demonstrates the error that occurs when trying to use text/plain
+ * files as media. The plugin will throw a helpful error message guiding users
+ * to use text content instead.
+ *
+ * Error message: "Unsupported media type: text/plain. Text files should be sent
+ * as text content in the message, not as media. For example, use { text: '...' }
+ * instead of { media: { url: '...', contentType: 'text/plain' } }"
+ */
+ai.defineFlow('stable-text-plain-error', async () => {
+  try {
+    await ai.generate({
+      model: anthropic.model('claude-sonnet-4-5'),
+      messages: [
+        {
+          role: 'user',
+          content: [
+            {
+              media: {
+                url: 'data:text/plain;base64,SGVsbG8gV29ybGQ=',
+                contentType: 'text/plain',
+              },
+            },
+          ],
+        },
+      ],
+    });
+    return 'Unexpected: Should have thrown an error';
+  } catch (error: any) {
+    return {
+      error: error.message,
+      note: 'This demonstrates the helpful error message for text/plain files',
+    };
+  }
+});
+
+/**
+ * This flow demonstrates the correct way to send text content.
+ * Instead of using media with text/plain, use the text field directly.
+ */
+ai.defineFlow('stable-text-plain-correct', async () => {
+  // Read the text content (in a real app, you'd read from a file)
+  const textContent = 'Hello World\n\nThis is a text file content.';
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          {
+            text: `Please summarize this text file content:\n\n${textContent}`,
+          },
+        ],
+      },
+    ],
+  });
+
+  return text;
+});
diff --git a/js/testapps/anthropic/src/stable/webp.ts b/js/testapps/anthropic/src/stable/webp.ts
new file mode 100644
index 0000000000..f8a861024b
--- /dev/null
+++ b/js/testapps/anthropic/src/stable/webp.ts
@@ -0,0 +1,95 @@
+/**
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { anthropic } from '@genkit-ai/anthropic';
+import { genkit } from 'genkit';
+
+const ai = genkit({
+  plugins: [anthropic()],
+});
+
+/**
+ * This flow demonstrates WEBP image handling with matching contentType.
+ * Both the data URL and the contentType field specify image/webp.
+ */
+ai.defineFlow('stable-webp-matching', async () => {
+  // Minimal valid WEBP image (1x1 pixel, transparent)
+  // In a real app, you'd load an actual WEBP image file
+  const webpImageData =
+    'data:image/webp;base64,UklGRiQAAABXRUJQVlA4IBgAAAAwAQCdASoBAAEAAwA0JaQAA3AA/vuUAAA=';
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          { text: 'Describe this image:' },
+          {
+            media: {
+              url: webpImageData,
+              contentType: 'image/webp',
+            },
+          },
+        ],
+      },
+    ],
+  });
+
+  return text;
+});
+
+/**
+ * This flow demonstrates the fix for WEBP images with mismatched contentType.
+ * Even if contentType says 'image/png', the plugin will use 'image/webp' from
+ * the data URL, preventing API validation errors.
+ *
+ * This fix ensures that the media_type sent to Anthropic matches the actual
+ * image data, which is critical for WEBP images that were previously causing
+ * "Image does not match the provided media type" errors.
+ */
+ai.defineFlow('stable-webp-mismatched', async () => {
+  // Minimal valid WEBP image (1x1 pixel, transparent)
+  const webpImageData =
+    'data:image/webp;base64,UklGRiQAAABXRUJQVlA4IBgAAAAwAQCdASoBAAEAAwA0JaQAA3AA/vuUAAA=';
+
+  const { text } = await ai.generate({
+    model: anthropic.model('claude-sonnet-4-5'),
+    messages: [
+      {
+        role: 'user',
+        content: [
+          {
+            text: 'Describe this image (note: contentType is wrong but data URL is correct):',
+          },
+          {
+            media: {
+              // Data URL says WEBP, but contentType says PNG
+              // The plugin will use WEBP from the data URL (the fix)
+              url: webpImageData,
+              contentType: 'image/png', // This mismatch is handled correctly
+            },
+          },
+        ],
+      },
+    ],
+  });
+
+  return {
+    result: text,
+    note: 'The plugin correctly used image/webp from the data URL, not image/png from contentType',
+  };
+});
diff --git a/js/testapps/anthropic/tsconfig.json b/js/testapps/anthropic/tsconfig.json
new file mode 100644
index 0000000000..efbb566bf7
--- /dev/null
+++ b/js/testapps/anthropic/tsconfig.json
@@ -0,0 +1,14 @@
+{
+  "compileOnSave": true,
+  "include": ["src"],
+  "compilerOptions": {
+    "module": "commonjs",
+    "noImplicitReturns": true,
+    "outDir": "lib",
+    "sourceMap": true,
+    "strict": true,
+    "target": "es2017",
+    "skipLibCheck": true,
+    "esModuleInterop": true
+  }
+}