feat(types): add BlockChunk type to chat.{start,append,stop}Stream methods (#2580)

Author: srtaalej

.changeset/add-blocks-chunk.md

@@ -0,0 +1,5 @@
+---
+'@slack/types': patch
+---
+
+Add `BlocksChunk` type for passing Block Kit blocks within streaming messages

packages/types/src/chunk.ts

@@ -1,4 +1,6 @@
 import type { URLSourceElement } from './block-kit/block-elements';
+import type { AnyBlock } from './block-kit/blocks';
+
 /**
  * Base interface for streaming message chunks.
  * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
@@ -7,6 +9,16 @@ export interface Chunk {
   type: string;
 }
 
+/**
+ * Used for passing an array of blocks within a streaming message.
+ * https://docs.slack.dev/changelog/2026/04/16/block-kit-new-blocks/
+ */
+export interface BlocksChunk extends Chunk {
+  type: 'blocks';
+  /** @description An array of {@link AnyBlock} objects. Maximum of 50 blocks. */
+  blocks: AnyBlock[];
+}
+
 /**
  * Used for streaming text content with markdown formatting support.
  * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
@@ -42,4 +54,4 @@ export interface TaskUpdateChunk extends Chunk {
 /**
  * Union type of all possible chunk types
  */
-export type AnyChunk = MarkdownTextChunk | PlanUpdateChunk | TaskUpdateChunk;
+export type AnyChunk = BlocksChunk | MarkdownTextChunk | PlanUpdateChunk | TaskUpdateChunk;

packages/types/test/chunk.test-d.ts

@@ -0,0 +1,32 @@
+import { expectAssignable, expectError } from 'tsd';
+import type { AnyChunk, BlocksChunk } from '../src/index';
+
+// BlocksChunk
+// -- sad path
+expectError<BlocksChunk>({});
+expectError<BlocksChunk>({ type: 'blocks' });
+expectError<BlocksChunk>({ blocks: [] });
+
+// -- happy path
+expectAssignable<BlocksChunk>({ type: 'blocks', blocks: [] });
+expectAssignable<BlocksChunk>({
+  type: 'blocks',
+  blocks: [
+    {
+      type: 'section',
+      text: { type: 'plain_text', text: "Sandra's plan outline" },
+    },
+  ],
+});
+expectAssignable<BlocksChunk>({
+  type: 'blocks',
+  blocks: [{ type: 'divider' }, { type: 'header', text: { type: 'plain_text', text: 'Hello' } }],
+});
+// Unknown block types accepted via generic Block interface
+expectAssignable<BlocksChunk>({
+  type: 'blocks',
+  blocks: [{ type: 'future_block_type' }],
+});
+
+// BlocksChunk is assignable to AnyChunk
+expectAssignable<AnyChunk>({ type: 'blocks', blocks: [] });

packages/web-api/test/types/methods/chat.test-d.ts

@@ -32,6 +32,10 @@ expectAssignable<Parameters<typeof web.chat.appendStream>>([
     channel: 'C1234',
     ts: '1234.56',
     chunks: [
+      {
+        type: 'blocks',
+        blocks: [{ type: 'section', text: { type: 'mrkdwn', text: 'Hello' } }],
+      },
       {
         type: 'markdown_text',
         text: 'Hello world',
@@ -789,6 +793,10 @@ expectAssignable<Parameters<typeof web.chat.startStream>>([
     channel: 'C1234',
     thread_ts: '1234.56',
     chunks: [
+      {
+        type: 'blocks',
+        blocks: [{ type: 'section', text: { type: 'mrkdwn', text: 'Hello' } }],
+      },
       {
         type: 'markdown_text',
         text: 'Hello world',
@@ -913,6 +921,10 @@ expectAssignable<Parameters<typeof web.chat.stopStream>>([
     channel: 'C1234',
     ts: '1234.56',
     chunks: [
+      {
+        type: 'blocks',
+        blocks: [{ type: 'section', text: { type: 'mrkdwn', text: 'Hello' } }],
+      },
       {
         type: 'markdown_text',
         text: 'Hello world',