fix(chat): scaffold a Markdown renderer so chat renders answers

deno.json

@@ -41,6 +41,7 @@
     "./react",
     "./scripts/codemods",
     "./extensions/ext-llm-anthropic",
+    "./extensions/ext-markdown-react",
     "./extensions/ext-llm-google",
     "./extensions/ext-llm-openai",
     "./extensions/ext-auth-jwt",
@@ -155,6 +156,7 @@
     "./extensions/distributed/rate-limit-support": "./src/extensions/distributed/rate-limit-support.ts",
     "./extensions/distributed/routing-invalidation-support": "./src/extensions/distributed/routing-invalidation-support.ts",
     "./extensions/dev-ui": "./src/extensions/dev-ui/index.ts",
+    "./extensions/markdown": "./src/extensions/markdown/index.ts",
     "./extensions/dev-ui/protocol": "./src/extensions/dev-ui/protocol.ts",
     "./extensions/rendering": "./src/extensions/rendering/index.ts",
     "./extensions/contracts": "./src/extensions/contracts.ts",

docs/guides/chat-ui.md

@@ -304,12 +304,38 @@ different requests would share the same store.
 Use chat context providers only when nested components need direct state access.
 Prefer preset props or composition components first.
 
+## Render Markdown in chat
+
+`Chat` renders assistant answers and reasoning as semantic Markdown out of the
+box. The built-in `@veryfront/ext-markdown-react` extension supplies the
+renderer, so CommonMark plus GFM tables, task lists, strikethrough, and
+autolinks work with no project setup. Fenced code renders through the shared
+code block, with a language label and a copy button.
+
+To use a different renderer, install one for the subtree. An application
+renderer takes precedence over the built-in extension everywhere, including
+inside chat:
+
+```tsx
+import { Chat } from "veryfront/chat";
+import { MarkdownRendererProvider } from "veryfront/markdown";
+import { ProjectMarkdownRenderer } from "./project-markdown-renderer.tsx";
+
+export default function ChatPage() {
+  return (
+    <MarkdownRendererProvider renderer={ProjectMarkdownRenderer}>
+      <Chat agentId="assistant" />
+    </MarkdownRendererProvider>
+  );
+}
+```
+
 ## Present Markdown source safely
 
-`veryfront/markdown` is the dependency-free Markdown boundary used by chat
-surfaces. Without an installed rich renderer, it preserves the exact source in
-an escaped `<pre><code>` element. This is useful when source visibility matters
-more than semantic formatting:
+`veryfront/markdown` is the dependency-free Markdown boundary. Used on its own,
+without an installed rich renderer, it preserves the exact source in an escaped
+`<pre><code>` element. This is useful when source visibility matters more than
+semantic formatting:
 
 ````tsx
 import { Markdown } from "veryfront/markdown";
@@ -337,10 +363,26 @@ escaped source is present in server HTML.
 
 ### Install a semantic renderer
 
-Semantic Markdown is an explicit extension capability. Select a trusted
-extension or application adapter that implements `MarkdownRendererProps`, then
-install its component for the relevant subtree. In this example,
-`ProjectMarkdownRenderer` comes from that adapter:
+Semantic Markdown is an extension capability. Chat installs
+`@veryfront/ext-markdown-react` for you; outside chat, install a renderer for
+the relevant subtree yourself. Use the built-in extension:
+
+```tsx
+import { Markdown, MarkdownRendererProvider } from "veryfront/markdown";
+import { MarkdownRenderer } from "@veryfront/ext-markdown-react/renderer";
+
+export default function Result() {
+  return (
+    <MarkdownRendererProvider renderer={MarkdownRenderer}>
+      <Markdown>{answer}</Markdown>
+    </MarkdownRendererProvider>
+  );
+}
+```
+
+Or select another trusted extension or application adapter that implements
+`MarkdownRendererProps`. In this example, `ProjectMarkdownRenderer` comes from
+that adapter:
 
 ```tsx
 import { Markdown, MarkdownRendererProvider } from "veryfront/markdown";

extensions/README.md

@@ -52,6 +52,7 @@ Extension availability is separate from contract requirement:
 | Package                                           | Contract                      | Description                                           |
 | ------------------------------------------------- | ----------------------------- | ----------------------------------------------------- |
 | [`@veryfront/ext-content-mdx`](./ext-content-mdx) | `ContentProcessor`            | MDX and Markdown processing via unified/remark/rehype |
+| [`@veryfront/ext-markdown-react`](./ext-markdown-react) | `MarkdownRendererProvider` | Rich Markdown rendering for React chat surfaces via react-markdown and remark-gfm |
 | [`@veryfront/ext-yaml`](./ext-yaml)               | `SkillDocumentParserProvider` | YAML parsing for skill and agent documents            |
 
 ### Development and rendering
@@ -128,6 +129,7 @@ baseline subset used by ordinary apps and local development.
 | `@veryfront/ext-parser-babel`                | `CodeParser`                  | Deferred; standard npm baseline           |
 | `@veryfront/ext-yaml`                        | `SkillDocumentParserProvider` | Deferred; standard npm baseline           |
 | `@veryfront/ext-content-mdx`                 | `ContentProcessor`            | Deferred; standard npm baseline           |
+| `@veryfront/ext-markdown-react`              | `MarkdownRendererProvider`    | Deferred; standard npm baseline           |
 | `@veryfront/ext-css-tailwind`                | `CSSProcessor`                | Deferred; standard npm baseline           |
 | `@veryfront/ext-node-websocket-ws`           | `NodeWebSocketServerProvider` | Deferred; standard npm baseline           |
 | `@veryfront/ext-dev-ui-react`                | `DevUiAssetProvider`          | Deferred; standard npm baseline           |
@@ -175,7 +177,7 @@ raw transitive dependencies such as `bash-tool`, `just-bash`, `jose`,
 
 | Runtime or service role                      | Install these extension packages                                                                                                                                                                                       |
 | -------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| CLI, build image, or project server runtime  | `@veryfront/ext-bundler-esbuild`, `@veryfront/ext-content-mdx`, `@veryfront/ext-css-tailwind`, `@veryfront/ext-dev-ui-react`, `@veryfront/ext-node-websocket-ws`, `@veryfront/ext-parser-babel`, `@veryfront/ext-yaml` |
+| CLI, build image, or project server runtime  | `@veryfront/ext-bundler-esbuild`, `@veryfront/ext-content-mdx`, `@veryfront/ext-css-tailwind`, `@veryfront/ext-dev-ui-react`, `@veryfront/ext-markdown-react`, `@veryfront/ext-node-websocket-ws`, `@veryfront/ext-parser-babel`, `@veryfront/ext-yaml` |
 | Build with CSS optimization                  | `@veryfront/ext-css-lightning` (register explicitly)                                                                                                                                                                   |
 | Build with CSS purging or critical CSS       | `@veryfront/ext-css-purgecss` (register explicitly)                                                                                                                                                                    |
 | Build with image optimization                | `@veryfront/ext-image-sharp` (register explicitly)                                                                                                                                                                     |

extensions/ext-markdown-react/README.md

@@ -0,0 +1,58 @@
+# @veryfront/ext-markdown-react
+
+Rich Markdown rendering for React surfaces, built on
+[react-markdown](https://github.com/remarkjs/react-markdown) and
+[remark-gfm](https://github.com/remarkjs/remark-gfm).
+
+Provides the `MarkdownRendererProvider` contract. Chat installs this renderer by
+default, so assistant answers render as semantic HTML without any project setup.
+
+## What it renders
+
+- CommonMark: headings, paragraphs, lists, blockquotes, links, images, emphasis,
+  inline code, and fenced code.
+- GFM: tables, task lists, strikethrough, and autolinks.
+- Fenced code through the shared `CodeBlock` primitive (language label, copy
+  button, and an optional syntax-highlight renderer).
+
+## Safety
+
+- Raw HTML in Markdown source is never injected. `rehype-raw` is not installed,
+  so `<script>` in source reaches the DOM as escaped text.
+- react-markdown's default URL transform drops unsafe link protocols such as
+  `javascript:`.
+- Links open in a new tab with `rel="noopener noreferrer"`.
+
+## Use it directly
+
+Chat wires this up for you. Install it explicitly when you render
+`veryfront/markdown` outside chat:
+
+```tsx
+import { Markdown, MarkdownRendererProvider } from "veryfront/markdown";
+import { MarkdownRenderer } from "@veryfront/ext-markdown-react/renderer";
+
+export default function Answer({ source }: { source: string }) {
+  return (
+    <MarkdownRendererProvider renderer={MarkdownRenderer}>
+      <Markdown>{source}</Markdown>
+    </MarkdownRendererProvider>
+  );
+}
+```
+
+Import from `@veryfront/ext-markdown-react/renderer` in client code. The package
+root additionally exports the extension factory, which pulls in the extension
+registry.
+
+## Replace it
+
+Pass your own renderer to `MarkdownRendererProvider` to override this one
+everywhere, including inside chat. Pass `renderer={null}` on a single `Markdown`
+instance to show plain escaped source instead.
+
+## Test
+
+```bash
+deno task test
+```

extensions/ext-markdown-react/deno.json

@@ -0,0 +1,45 @@
+{
+  "name": "@veryfront/ext-markdown-react",
+  "version": "0.1.0",
+  "exports": {
+    ".": "./src/index.ts",
+    "./renderer": "./src/renderer.tsx"
+  },
+  "veryfront": {
+    "extension": true,
+    "activation": "auto",
+    "contracts": {
+      "provides": [
+        "MarkdownRendererProvider"
+      ]
+    },
+    "capabilities": [],
+    "npm": {
+      "runtimeVersionFromManifest": true,
+      "peerDependencies": {
+        "react": "^19.2.4",
+        "react-dom": "^19.2.4"
+      }
+    }
+  },
+  "imports": {
+    "react": "../../react/react.ts",
+    "react/jsx-runtime": "../../react/jsx-runtime.ts",
+    "react-dom/server": "../../react/react-dom-server.ts",
+    "react-markdown": "npm:react-markdown@9.0.3",
+    "remark-gfm": "npm:remark-gfm@4.0.1",
+    "@std/assert": "jsr:@std/assert@1.0.19",
+    "@std/testing/bdd": "jsr:@std/testing@1.0.17/bdd",
+    "veryfront/extensions": "../../src/extensions/types.ts",
+    "veryfront/extensions/markdown": "../../src/extensions/markdown/index.ts",
+    "veryfront/markdown": "../../src/markdown/index.ts"
+  },
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "jsxImportSource": "react"
+  },
+  "tasks": {
+    "test": "deno test --no-check --allow-all src/",
+    "check": "deno check src/index.ts src/renderer.tsx"
+  }
+}

extensions/ext-markdown-react/src/index.ts

@@ -0,0 +1,37 @@
+/**
+ * react-markdown implementation of the `MarkdownRendererProvider` extension
+ * contract.
+ *
+ * @module extensions/ext-markdown-react
+ */
+
+import type { ExtensionFactory } from "veryfront/extensions";
+import {
+  createMarkdownRendererProvider,
+  MarkdownRendererProviderName,
+} from "veryfront/extensions/markdown";
+import extensionPackage from "../deno.json" with { type: "json" };
+import { MarkdownRenderer } from "./renderer.tsx";
+
+const extMarkdownReact: ExtensionFactory = () => {
+  const provider = createMarkdownRendererProvider("ext-markdown-react", MarkdownRenderer);
+
+  return {
+    name: "ext-markdown-react",
+    version: extensionPackage.version,
+    contracts: {
+      provides: [MarkdownRendererProviderName],
+    },
+    capabilities: [],
+    setup(ctx) {
+      ctx.provide(MarkdownRendererProviderName, provider);
+      ctx.logger.debug("[ext-markdown-react] MarkdownRendererProvider registered");
+    },
+    teardown() {
+      // No resources to release.
+    },
+  };
+};
+
+export default extMarkdownReact;
+export { MarkdownRenderer } from "./renderer.tsx";