feat(slack): use native markdown_text for outgoing messages

.changeset/slack-native-markdown.md

@@ -0,0 +1,22 @@
+---
+"@chat-adapter/slack": minor
+---
+
+use Slack's native `markdown_text` field for outgoing markdown messages
+
+Slack now natively renders markdown via the `markdown_text` parameter on
+`chat.postMessage`, `chat.postEphemeral`, `chat.update`, and
+`chat.scheduleMessage`. The adapter passes markdown through directly instead
+of converting to mrkdwn, so tables, headings, fenced code blocks, blockquotes,
+and other rich formatting now render natively in Slack.
+
+- Tables are rendered by Slack natively (no more ASCII-table fallback or
+  Block Kit `table` block fabrication).
+- Plain `string` and `{ raw }` messages still go to the `text` field so
+  literal `*` / `_` characters are preserved.
+- `markdown_text` has a 12,000 character limit (vs. ~40,000 for `text`).
+- The deprecated `SlackMarkdownConverter` alias has been removed; use
+  `SlackFormatConverter` instead.
+- `renderFormatted(ast)` now returns standard markdown instead of mrkdwn.
+- Incoming `message` events are unchanged — they still arrive as mrkdwn
+  and are parsed as before.

apps/docs/content/docs/api/markdown.mdx

@@ -241,7 +241,7 @@ const value = getNodeValue(node);       // string | undefined
 
 ### tableToAscii
 
-Render an mdast `Table` node as a padded ASCII table string. Used by adapters that lack native table support (Slack, Google Chat, Discord, Telegram).
+Render an mdast `Table` node as a padded ASCII table string. Used by adapters that lack native table support (Google Chat, Discord, Telegram).
 
 ```typescript
 import { parseMarkdown, tableToAscii, isTableNode } from "chat";
@@ -280,17 +280,21 @@ The SDK uses mdast as the canonical format and each adapter converts it to the p
 
 | Feature | Slack | Teams | Google Chat |
 |---------|-------|-------|-------------|
-| Bold | `*text*` | `**text**` | `*text*` |
+| Bold | `**text**` | `**text**` | `*text*` |
 | Italic | `_text_` | `_text_` | `_text_` |
-| Strikethrough | `~text~` | `~~text~~` | `~text~` |
+| Strikethrough | `~~text~~` | `~~text~~` | `~text~` |
 | Code | `` `code` `` | `` `code` `` | `` `code` `` |
 | Code blocks | ```` ``` ```` | ```` ``` ```` | ```` ``` ```` |
-| Links | `<url\|text>` | `[text](url)` | `[text](url)` |
+| Links | `[text](url)` | `[text](url)` | `[text](url)` |
 | Lists | Supported | Supported | Supported |
 | Blockquotes | `>` | `>` | Simulated with `>` prefix |
-| Tables | ASCII fallback | Native GFM | ASCII fallback |
+| Tables | Native (markdown_text) | Native GFM | ASCII fallback |
 | Mentions | `<@USER>` | `<at>name</at>` | `<users/{id}>` |
 
+<Callout type="info">
+Slack accepts standard markdown via the `markdown_text` field on `chat.postMessage` and friends, so the SDK passes markdown through directly. Incoming Slack messages still arrive as legacy mrkdwn (`*bold*`, `<url|text>`) and are parsed transparently. If you need to send mrkdwn yourself, use `{ raw: "..." }`.
+</Callout>
+
 <Callout type="info">
 You don't need to worry about these differences when using the SDK — the AST builders and `parseMarkdown` handle conversion automatically. This table is useful if you're working with `raw` platform payloads or debugging formatting issues.
 </Callout>

apps/docs/content/docs/contributing/building.mdx

@@ -535,7 +535,7 @@ export class MatrixFormatConverter extends BaseFormatConverter {
 }
 ```
 
-For platforms with non-standard formatting (e.g., Slack's `mrkdwn`), implement custom parsing in `toAst()` and rendering in `fromAst()`. See the [Discord adapter](https://github.com/vercel/chat/blob/main/packages/adapter-discord/src/markdown.ts) for an example of handling platform-specific mention syntax.
+For platforms with non-standard formatting, implement custom parsing in `toAst()` and rendering in `fromAst()`. See the [Discord adapter](https://github.com/vercel/chat/blob/main/packages/adapter-discord/src/markdown.ts) for an example of handling platform-specific mention syntax.
 
 ## Optional methods
 

apps/docs/content/docs/posting-messages.mdx

@@ -24,15 +24,15 @@ This sends the string directly without any formatting conversion.
 
 ## Markdown
 
-Pass a `{ markdown }` object to have the SDK convert standard markdown to each platform's native format — mrkdwn for Slack, HTML for Teams, and so on.
+Pass a `{ markdown }` object to have the SDK render standard markdown on each platform — passed through to Slack's native `markdown_text` field, converted to HTML for Teams, and so on.
 
 ```typescript title="lib/bot.ts" lineNumbers
 await thread.post({
   markdown: "**Bold**, _italic_, and `code`",
 });
 ```
 
-Under the hood, the SDK parses the markdown into an mdast AST, then each adapter converts it to the platform's format.
+Under the hood, the SDK parses the markdown into an mdast AST, then each adapter handles it natively or converts it to the platform's format.
 
 ## AST builders
 

packages/adapter-slack/src/index.test.ts

@@ -1326,7 +1326,7 @@ describe("renderFormatted", () => {
     logger: mockLogger,
   });
 
-  it("renders AST to Slack mrkdwn format", () => {
+  it("renders AST to standard markdown (Slack now accepts markdown natively)", () => {
     const ast = {
       type: "root" as const,
       children: [
@@ -1343,7 +1343,7 @@ describe("renderFormatted", () => {
     };
 
     const result = adapter.renderFormatted(ast);
-    expect(result).toBe("*bold*");
+    expect(result.trim()).toBe("**bold**");
   });
 });
 
@@ -5303,16 +5303,24 @@ describe("editMessage via response_url", () => {
       .spyOn(global, "fetch")
       .mockResolvedValue(new Response("ok", { status: 200 }));
 
-    await adapter.editMessage(
-      "slack:C123:1234567890.000000",
-      ephemeralId,
-      "Updated text"
-    );
+    await adapter.editMessage("slack:C123:1234567890.000000", ephemeralId, {
+      markdown:
+        "**Updated** [text](https://example.com)\n\n| A | B |\n|---|---|\n| 1 | 2 |",
+    });
 
     expect(fetchSpy).toHaveBeenCalledWith(
       "https://hooks.slack.com/respond",
       expect.objectContaining({ method: "POST" })
     );
+    const callBody = JSON.parse(fetchSpy.mock.calls[0][1]?.body as string);
+    expect(callBody).toEqual(
+      expect.objectContaining({
+        replace_original: true,
+        text: expect.stringContaining("*Updated* <https://example.com|text>"),
+      })
+    );
+    expect(callBody.text).toContain("```");
+    expect(callBody).not.toHaveProperty("markdown_text");
     fetchSpy.mockRestore();
   });
 });