From e3d7b9b534899aeb7cac0e367c38fa6cc679874a Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Sun, 11 Jan 2026 16:27:26 -0500
Subject: [PATCH 01/13] feat: accept chunks as arguments to
 chat.{start,append,stop}Stream methods

---
 packages/types/src/chunk.ts                   | 89 +++++++++++++++++++
 packages/types/src/index.ts                   |  1 +
 packages/web-api/src/types/request/chat.ts    | 18 +++-
 .../web-api/test/types/methods/chat.test-d.ts | 74 +++++++++++++++
 4 files changed, 181 insertions(+), 1 deletion(-)
 create mode 100644 packages/types/src/chunk.ts

diff --git a/packages/types/src/chunk.ts b/packages/types/src/chunk.ts
new file mode 100644
index 000000000..acc7cdc23
--- /dev/null
+++ b/packages/types/src/chunk.ts
@@ -0,0 +1,89 @@
+/**
+ * Base interface for streaming message chunks.
+ * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
+ */
+export interface Chunk {
+  type: string;
+}
+
+/**
+ * Used for streaming text content with markdown formatting support.
+ * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
+ */
+export interface MarkdownTextChunk extends Chunk {
+  type: 'markdown_text';
+  text: string;
+}
+
+/**
+ * URL source for task update chunks.
+ */
+export interface URLSource {
+  type: 'url';
+  url: string;
+  text: string;
+  icon_url?: string;
+}
+
+/**
+ * Used for displaying tool execution progress in a timeline-style UI.
+ * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
+ */
+export interface TaskUpdateChunk extends Chunk {
+  type: 'task_update';
+  id: string;
+  title: string;
+  status: 'pending' | 'in_progress' | 'complete' | 'error';
+  details?: string;
+  output?: string;
+  sources?: URLSource[];
+}
+
+/**
+ * Union type of all possible chunk types
+ */
+export type AnyChunk = MarkdownTextChunk | TaskUpdateChunk;
+
+/**
+ * Parse a chunk object and return the appropriate typed chunk.
+ * Returns null if the chunk is invalid or unknown.
+ */
+export function parseChunk(chunk: unknown): AnyChunk | null {
+  if (!chunk || typeof chunk !== 'object') {
+    return null;
+  }
+
+  const chunkObj = chunk as Record<string, unknown>;
+
+  if (!('type' in chunkObj) || typeof chunkObj.type !== 'string') {
+    console.warn('Unknown chunk detected and skipped (missing type)', chunk);
+    return null;
+  }
+
+  const { type } = chunkObj;
+
+  if (type === 'markdown_text') {
+    if (typeof chunkObj.text === 'string') {
+      return chunkObj as unknown as MarkdownTextChunk;
+    }
+    console.warn('Invalid MarkdownTextChunk (missing text property)', chunk);
+    return null;
+  }
+
+  if (type === 'task_update') {
+    const taskChunk = chunkObj as Partial<TaskUpdateChunk>;
+    if (
+      typeof taskChunk.id === 'string' &&
+      typeof taskChunk.title === 'string' &&
+      typeof taskChunk.status === 'string' &&
+      ['pending', 'in_progress', 'complete', 'error'].includes(taskChunk.status)
+    ) {
+      return chunkObj as unknown as TaskUpdateChunk;
+    }
+    console.warn('Invalid TaskUpdateChunk (missing required properties)', chunk);
+    return null;
+  }
+
+  console.warn(`Unknown chunk type detected and skipped: ${type}`, chunk);
+  return null;
+}
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index 9992536de..2740a2290 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -3,6 +3,7 @@ export * from './block-kit/blocks';
 export * from './block-kit/composition-objects';
 export * from './block-kit/extensions';
 export * from './calls';
+export * from './chunk';
 export * from './dialog';
 export * from './events';
 export * from './message-attachments';
diff --git a/packages/web-api/src/types/request/chat.ts b/packages/web-api/src/types/request/chat.ts
index 7734d7a7f..bc43a9fb8 100644
--- a/packages/web-api/src/types/request/chat.ts
+++ b/packages/web-api/src/types/request/chat.ts
@@ -1,4 +1,5 @@
 import type {
+  AnyChunk,
   Block, // TODO: these will be combined into one in a new types release
   EntityMetadata,
   KnownBlock,
@@ -168,7 +169,13 @@ export interface Unfurls {
   unfurl_media?: boolean;
 }
 
-export interface ChatAppendStreamArguments extends TokenOverridable, ChannelAndTS, MarkdownText {}
+export interface ChatAppendStreamArguments extends TokenOverridable, ChannelAndTS, MarkdownText {
+  /**
+   * @description An array of {@link https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming chunk objects} to append to the stream.
+   * Either `markdown_text` or `chunks` is required.
+   */
+  chunks?: AnyChunk[];
+}
 
 // https://docs.slack.dev/reference/methods/chat.delete
 export interface ChatDeleteArguments extends ChannelAndTS, AsUser, TokenOverridable {}
@@ -233,6 +240,11 @@ export type ChatScheduledMessagesListArguments = OptionalArgument<
 >;
 
 export interface ChatStartStreamArguments extends TokenOverridable, Channel, Partial<MarkdownText>, ThreadTS {
+  /**
+   * @description An array of {@link https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming chunk objects} to start the stream with.
+   * Either `markdown_text` or `chunks` is required.
+   */
+  chunks?: AnyChunk[];
   /**
    * @description The ID of the team that is associated with `recipient_user_id`.
    * This is required when starting a streaming conversation outside of a DM.
@@ -249,6 +261,10 @@ export type ChatStopStreamArguments = TokenOverridable &
   ChannelAndTS &
   Partial<MarkdownText> &
   Partial<Metadata> & {
+    /**
+     * @description An array of {@link https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming chunk objects} to finalize the stream with.
+     */
+    chunks?: AnyChunk[];
     /**
      * Block formatted elements will be appended to the end of the message.
      */
diff --git a/packages/web-api/test/types/methods/chat.test-d.ts b/packages/web-api/test/types/methods/chat.test-d.ts
index f1ec5f025..09916b0bb 100644
--- a/packages/web-api/test/types/methods/chat.test-d.ts
+++ b/packages/web-api/test/types/methods/chat.test-d.ts
@@ -32,6 +32,26 @@ expectAssignable<Parameters<typeof web.chat.appendStream>>([
     markdown_text: 'hello',
   },
 ]);
+expectAssignable<Parameters<typeof web.chat.appendStream>>([
+  {
+    channel: 'C1234',
+    ts: '1234.56',
+    markdown_text: 'hello',
+    chunks: [
+      {
+        type: 'markdown_text',
+        text: 'Hello world',
+      },
+      {
+        type: 'task_update',
+        id: 'task-1',
+        title: 'Processing request',
+        status: 'in_progress',
+        details: 'Working on it...',
+      },
+    ],
+  },
+]);
 
 // chat.delete
 // -- sad path
@@ -636,6 +656,39 @@ expectAssignable<Parameters<typeof web.chat.startStream>>([
     channel: 'C1234',
     thread_ts: '1234.56',
     markdown_text: 'hello',
+    chunks: [
+      {
+        type: 'markdown_text',
+        text: 'Hello world',
+      },
+      {
+        type: 'task_update',
+        id: 'task-1',
+        title: 'Processing request',
+        status: 'in_progress',
+        details: 'Working on it...',
+      },
+    ],
+  },
+]);
+expectAssignable<Parameters<typeof web.chat.startStream>>([
+  {
+    channel: 'C1234',
+    thread_ts: '1234.56',
+    markdown_text: 'hello',
+    chunks: [
+      {
+        type: 'markdown_text',
+        text: 'Hello world',
+      },
+      {
+        type: 'task_update',
+        id: 'task-1',
+        title: 'Processing request',
+        status: 'in_progress',
+        details: 'Working on it...',
+      },
+    ],
     recipient_team_id: 'T1234',
     recipient_user_id: 'U1234',
   },
@@ -670,6 +723,27 @@ expectAssignable<Parameters<typeof web.chat.stopStream>>([
     blocks: [],
   },
 ]);
+expectAssignable<Parameters<typeof web.chat.stopStream>>([
+  {
+    channel: 'C1234',
+    ts: '1234.56',
+    markdown_text: 'hello',
+    chunks: [
+      {
+        type: 'markdown_text',
+        text: 'Hello world',
+      },
+      {
+        type: 'task_update',
+        id: 'task-1',
+        title: 'Processing request',
+        status: 'in_progress',
+        details: 'Working on it...',
+      },
+    ],
+    blocks: [],
+  },
+]);
 
 // chat.unfurl
 // -- sad path

From 260119954e2df1757d6c1e0d28add29efc27b888 Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Sun, 11 Jan 2026 17:17:17 -0500
Subject: [PATCH 02/13] build: update slack/types version in package json

---
 packages/web-api/package.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/web-api/package.json b/packages/web-api/package.json
index 36ffc23bf..c41da4903 100644
--- a/packages/web-api/package.json
+++ b/packages/web-api/package.json
@@ -49,7 +49,7 @@
   },
   "dependencies": {
     "@slack/logger": "^4.0.0",
-    "@slack/types": "^2.18.0",
+    "@slack/types": "^2.19.0",
     "@types/node": ">=18.0.0",
     "@types/retry": "0.12.0",
     "axios": "^1.11.0",

From 160cfb7aee50dce5759a2968267cbb0da7055562 Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Thu, 15 Jan 2026 17:43:38 -0500
Subject: [PATCH 03/13] fix: make markdown text optional and add missing chunk
 type

---
 packages/types/src/chunk.ts                | 19 ++++++++++++++++++-
 packages/web-api/src/types/request/chat.ts |  2 +-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/packages/types/src/chunk.ts b/packages/types/src/chunk.ts
index acc7cdc23..6acddb06b 100644
--- a/packages/types/src/chunk.ts
+++ b/packages/types/src/chunk.ts
@@ -25,6 +25,15 @@ export interface URLSource {
   icon_url?: string;
 }
 
+/**
+ * An updated title of plans for task and tool calls.
+ * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
+ */
+export interface PlanUpdateChunk extends Chunk {
+  type: 'plan_update';
+  title: string;
+}
+
 /**
  * Used for displaying tool execution progress in a timeline-style UI.
  * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
@@ -42,7 +51,7 @@ export interface TaskUpdateChunk extends Chunk {
 /**
  * Union type of all possible chunk types
  */
-export type AnyChunk = MarkdownTextChunk | TaskUpdateChunk;
+export type AnyChunk = MarkdownTextChunk | PlanUpdateChunk | TaskUpdateChunk;
 
 /**
  * Parse a chunk object and return the appropriate typed chunk.
@@ -70,6 +79,14 @@ export function parseChunk(chunk: unknown): AnyChunk | null {
     return null;
   }
 
+  if(type === 'plan_update') {
+    if (typeof chunkObj.title === 'string') {
+      return chunkObj as unknown as PlanUpdateChunk;
+    }
+    console.warn('Invalid PlanUpdateChunk (missing title property)', chunk);
+    return null;
+  }
+
   if (type === 'task_update') {
     const taskChunk = chunkObj as Partial<TaskUpdateChunk>;
     if (
diff --git a/packages/web-api/src/types/request/chat.ts b/packages/web-api/src/types/request/chat.ts
index bc43a9fb8..9c5c2292c 100644
--- a/packages/web-api/src/types/request/chat.ts
+++ b/packages/web-api/src/types/request/chat.ts
@@ -169,7 +169,7 @@ export interface Unfurls {
   unfurl_media?: boolean;
 }
 
-export interface ChatAppendStreamArguments extends TokenOverridable, ChannelAndTS, MarkdownText {
+export interface ChatAppendStreamArguments extends TokenOverridable, ChannelAndTS, Partial<MarkdownText> {
   /**
    * @description An array of {@link https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming chunk objects} to append to the stream.
    * Either `markdown_text` or `chunks` is required.

From 81140ccaea3eb4afa9db09f14a2aa39734d5411b Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Fri, 16 Jan 2026 15:20:46 -0500
Subject: [PATCH 04/13] tests: add missing chunk type to tests

---
 .../web-api/test/types/methods/chat.test-d.ts  | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/packages/web-api/test/types/methods/chat.test-d.ts b/packages/web-api/test/types/methods/chat.test-d.ts
index 09916b0bb..97ac7f59e 100644
--- a/packages/web-api/test/types/methods/chat.test-d.ts
+++ b/packages/web-api/test/types/methods/chat.test-d.ts
@@ -42,6 +42,10 @@ expectAssignable<Parameters<typeof web.chat.appendStream>>([
         type: 'markdown_text',
         text: 'Hello world',
       },
+      {
+        type: 'plan_update',
+        title: 'Analyzing request',
+      },
       {
         type: 'task_update',
         id: 'task-1',
@@ -655,12 +659,15 @@ expectAssignable<Parameters<typeof web.chat.startStream>>([
   {
     channel: 'C1234',
     thread_ts: '1234.56',
-    markdown_text: 'hello',
     chunks: [
       {
         type: 'markdown_text',
         text: 'Hello world',
       },
+      {
+        type: 'plan_update',
+        title: 'Analyzing request',
+      },
       {
         type: 'task_update',
         id: 'task-1',
@@ -681,6 +688,10 @@ expectAssignable<Parameters<typeof web.chat.startStream>>([
         type: 'markdown_text',
         text: 'Hello world',
       },
+      {
+        type: 'plan_update',
+        title: 'Analyzing request',
+      },
       {
         type: 'task_update',
         id: 'task-1',
@@ -727,12 +738,15 @@ expectAssignable<Parameters<typeof web.chat.stopStream>>([
   {
     channel: 'C1234',
     ts: '1234.56',
-    markdown_text: 'hello',
     chunks: [
       {
         type: 'markdown_text',
         text: 'Hello world',
       },
+      {
+        type: 'plan_update',
+        title: 'Analyzing request',
+      },
       {
         type: 'task_update',
         id: 'task-1',

From dbca9c953d18709f1067238d2ab2f6cf24ceb86b Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Thu, 22 Jan 2026 13:27:39 -0500
Subject: [PATCH 05/13] build: use @slack/logger for debugging parsechunks

---
 packages/types/package.json |  3 +++
 packages/types/src/chunk.ts | 17 +++++++++++------
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/packages/types/package.json b/packages/types/package.json
index 59af1faee..e5210345a 100644
--- a/packages/types/package.json
+++ b/packages/types/package.json
@@ -27,6 +27,9 @@
   "bugs": {
     "url": "https://github.com/slackapi/node-slack-sdk/issues"
   },
+  "dependencies": {
+    "@slack/logger": "^4.0.0"
+  },
   "scripts": {
     "prepare": "npm run build",
     "build": "npm run build:clean && tsc",
diff --git a/packages/types/src/chunk.ts b/packages/types/src/chunk.ts
index 6acddb06b..6b3986cd1 100644
--- a/packages/types/src/chunk.ts
+++ b/packages/types/src/chunk.ts
@@ -1,3 +1,8 @@
+import { ConsoleLogger, LogLevel } from '@slack/logger';
+
+const logger = new ConsoleLogger();
+logger.setLevel(LogLevel.DEBUG);
+
 /**
  * Base interface for streaming message chunks.
  * https://docs.slack.dev/messaging/sending-and-scheduling-messages#text-streaming
@@ -65,7 +70,7 @@ export function parseChunk(chunk: unknown): AnyChunk | null {
   const chunkObj = chunk as Record<string, unknown>;
 
   if (!('type' in chunkObj) || typeof chunkObj.type !== 'string') {
-    console.warn('Unknown chunk detected and skipped (missing type)', chunk);
+    logger.debug('Unknown chunk detected and skipped (missing type)', chunk);
     return null;
   }
 
@@ -75,15 +80,15 @@ export function parseChunk(chunk: unknown): AnyChunk | null {
     if (typeof chunkObj.text === 'string') {
       return chunkObj as unknown as MarkdownTextChunk;
     }
-    console.warn('Invalid MarkdownTextChunk (missing text property)', chunk);
+    logger.debug('Invalid MarkdownTextChunk (missing text property)', chunk);
     return null;
   }
 
-  if(type === 'plan_update') {
+  if (type === 'plan_update') {
     if (typeof chunkObj.title === 'string') {
       return chunkObj as unknown as PlanUpdateChunk;
     }
-    console.warn('Invalid PlanUpdateChunk (missing title property)', chunk);
+    logger.debug('Invalid PlanUpdateChunk (missing title property)', chunk);
     return null;
   }
 
@@ -97,10 +102,10 @@ export function parseChunk(chunk: unknown): AnyChunk | null {
     ) {
       return chunkObj as unknown as TaskUpdateChunk;
     }
-    console.warn('Invalid TaskUpdateChunk (missing required properties)', chunk);
+    logger.debug('Invalid TaskUpdateChunk (missing required properties)', chunk);
     return null;
   }
 
-  console.warn(`Unknown chunk type detected and skipped: ${type}`, chunk);
+  logger.debug(`Unknown chunk type detected and skipped: ${type}`, chunk);
   return null;
 }

From 1d480f6312e056d1f1c5d0561069ab5e2702f690 Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Thu, 22 Jan 2026 15:39:51 -0500
Subject: [PATCH 06/13] type narrowing for parseChunks

---
 packages/types/src/chunk.ts | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/packages/types/src/chunk.ts b/packages/types/src/chunk.ts
index 6b3986cd1..892c31d21 100644
--- a/packages/types/src/chunk.ts
+++ b/packages/types/src/chunk.ts
@@ -67,40 +67,38 @@ export function parseChunk(chunk: unknown): AnyChunk | null {
     return null;
   }
 
-  const chunkObj = chunk as Record<string, unknown>;
-
-  if (!('type' in chunkObj) || typeof chunkObj.type !== 'string') {
+  if (!('type' in chunk) || typeof chunk.type !== 'string') {
     logger.debug('Unknown chunk detected and skipped (missing type)', chunk);
     return null;
   }
 
-  const { type } = chunkObj;
+  const type = chunk.type;
 
   if (type === 'markdown_text') {
-    if (typeof chunkObj.text === 'string') {
-      return chunkObj as unknown as MarkdownTextChunk;
+    if ('text' in chunk && typeof chunk.text === 'string') {
+      return chunk as MarkdownTextChunk;
     }
     logger.debug('Invalid MarkdownTextChunk (missing text property)', chunk);
     return null;
   }
 
   if (type === 'plan_update') {
-    if (typeof chunkObj.title === 'string') {
-      return chunkObj as unknown as PlanUpdateChunk;
+    if ('title' in chunk && typeof chunk.title === 'string') {
+      return chunk as PlanUpdateChunk;
     }
     logger.debug('Invalid PlanUpdateChunk (missing title property)', chunk);
     return null;
   }
 
   if (type === 'task_update') {
-    const taskChunk = chunkObj as Partial<TaskUpdateChunk>;
+    const taskChunk = chunk as Partial<TaskUpdateChunk>;
     if (
       typeof taskChunk.id === 'string' &&
       typeof taskChunk.title === 'string' &&
       typeof taskChunk.status === 'string' &&
       ['pending', 'in_progress', 'complete', 'error'].includes(taskChunk.status)
     ) {
-      return chunkObj as unknown as TaskUpdateChunk;
+      return chunk as TaskUpdateChunk;
     }
     logger.debug('Invalid TaskUpdateChunk (missing required properties)', chunk);
     return null;

From 14c9f36bcfd6422a022ef1dc8ed7d79d16ae8476 Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Fri, 16 Jan 2026 14:49:03 -0500
Subject: [PATCH 07/13] feat: chunk processing in chat stream helper

---
 packages/web-api/src/chat-stream.ts | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index 695100b14..4db44b6a6 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -89,7 +89,14 @@ export class ChatStreamer {
     if (args.token) {
       this.token = args.token;
     }
-    this.buffer += args.markdown_text;
+  
+    if (args.chunks) {
+      return await this.flushBuffer(args);
+    }
+  
+    if (args.markdown_text) {
+      this.buffer += args.markdown_text;
+    }
     if (this.buffer.length >= this.options.buffer_size) {
       return await this.flushBuffer(args);
     }
@@ -144,12 +151,13 @@ export class ChatStreamer {
       this.streamTs = response.ts;
       this.state = 'in_progress';
     }
+    const finalArgs = args?.chunks ? args : { ...args, markdown_text: this.buffer };
+
     const response = await this.client.chat.stopStream({
       token: this.token,
       channel: this.streamArgs.channel,
       ts: this.streamTs,
-      ...args,
-      markdown_text: this.buffer,
+      ...finalArgs,
     });
     this.state = 'completed';
     return response;
@@ -158,12 +166,14 @@ export class ChatStreamer {
   private async flushBuffer(
     args: Omit<ChatStartStreamArguments | ChatAppendStreamArguments, 'channel' | 'ts'>,
   ): Promise<ChatStartStreamResponse | ChatAppendStreamResponse> {
+
+    const finalArgs = args.chunks ? args : { ...args, markdown_text: this.buffer };
+
     if (!this.streamTs) {
       const response = await this.client.chat.startStream({
         ...this.streamArgs,
         token: this.token,
-        ...args,
-        markdown_text: this.buffer,
+        ...finalArgs,
       });
       this.buffer = '';
       this.streamTs = response.ts;
@@ -174,8 +184,7 @@ export class ChatStreamer {
       token: this.token,
       channel: this.streamArgs.channel,
       ts: this.streamTs,
-      ...args,
-      markdown_text: this.buffer,
+      ...finalArgs,
     });
     this.buffer = '';
     return response;

From 35cceedec3bc2902275e701a8fe759a16e51348f Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Tue, 20 Jan 2026 11:58:42 -0500
Subject: [PATCH 08/13] fix: buffer logic for chunks and markdown

---
 packages/web-api/src/chat-stream.ts | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index 4db44b6a6..2808a5282 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -151,8 +151,9 @@ export class ChatStreamer {
       this.streamTs = response.ts;
       this.state = 'in_progress';
     }
-    const finalArgs = args?.chunks ? args : { ...args, markdown_text: this.buffer };
-
+    const finalArgs = this.buffer.length > 0 && !args?.chunks
+      ? { ...args, markdown_text: this.buffer }
+      : args;
     const response = await this.client.chat.stopStream({
       token: this.token,
       channel: this.streamArgs.channel,
@@ -167,7 +168,9 @@ export class ChatStreamer {
     args: Omit<ChatStartStreamArguments | ChatAppendStreamArguments, 'channel' | 'ts'>,
   ): Promise<ChatStartStreamResponse | ChatAppendStreamResponse> {
 
-    const finalArgs = args.chunks ? args : { ...args, markdown_text: this.buffer };
+    const finalArgs = this.buffer.length > 0 && !args.chunks
+      ? { ...args, markdown_text: this.buffer }
+      : args;
 
     if (!this.streamTs) {
       const response = await this.client.chat.startStream({

From 6f5f8f14517070d70c01121f5f86115bd79dbcb5 Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Wed, 21 Jan 2026 16:14:52 -0500
Subject: [PATCH 09/13] bug: handle undefined args in ChatStream append

---
 packages/web-api/src/chat-stream.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index 2808a5282..8bca267fe 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -90,11 +90,11 @@ export class ChatStreamer {
       this.token = args.token;
     }
   
-    if (args.chunks) {
+    if (args?.chunks) {
       return await this.flushBuffer(args);
     }
   
-    if (args.markdown_text) {
+    if (args?.markdown_text) {
       this.buffer += args.markdown_text;
     }
     if (this.buffer.length >= this.options.buffer_size) {

From 5e86b29928d03c4f4f471b61f6857602493c5c2e Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Thu, 22 Jan 2026 16:09:32 -0500
Subject: [PATCH 10/13] linter errors

---
 packages/web-api/src/chat-stream.ts | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index 8bca267fe..0ff177240 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -89,14 +89,15 @@ export class ChatStreamer {
     if (args.token) {
       this.token = args.token;
     }
-  
+
     if (args?.chunks) {
       return await this.flushBuffer(args);
     }
-  
+
     if (args?.markdown_text) {
       this.buffer += args.markdown_text;
     }
+
     if (this.buffer.length >= this.options.buffer_size) {
       return await this.flushBuffer(args);
     }
@@ -151,9 +152,7 @@ export class ChatStreamer {
       this.streamTs = response.ts;
       this.state = 'in_progress';
     }
-    const finalArgs = this.buffer.length > 0 && !args?.chunks
-      ? { ...args, markdown_text: this.buffer }
-      : args;
+    const finalArgs = this.buffer.length > 0 && !args?.chunks ? { ...args, markdown_text: this.buffer } : args;
     const response = await this.client.chat.stopStream({
       token: this.token,
       channel: this.streamArgs.channel,
@@ -167,10 +166,7 @@ export class ChatStreamer {
   private async flushBuffer(
     args: Omit<ChatStartStreamArguments | ChatAppendStreamArguments, 'channel' | 'ts'>,
   ): Promise<ChatStartStreamResponse | ChatAppendStreamResponse> {
-
-    const finalArgs = this.buffer.length > 0 && !args.chunks
-      ? { ...args, markdown_text: this.buffer }
-      : args;
+    const finalArgs = this.buffer.length > 0 && !args.chunks ? { ...args, markdown_text: this.buffer } : args;
 
     if (!this.streamTs) {
       const response = await this.client.chat.startStream({

From 076d3a58c749f6f38bdce96df66b18fc2638ab2b Mon Sep 17 00:00:00 2001
From: Michael Brooks <mbrooks@slack-corp.com>
Date: Fri, 23 Jan 2026 14:16:59 -0800
Subject: [PATCH 11/13] Revert "build: use @slack/logger for debugging
 parsechunks"

This reverts commit dbca9c953d18709f1067238d2ab2f6cf24ceb86b.
---
 packages/types/package.json | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/packages/types/package.json b/packages/types/package.json
index e5210345a..59af1faee 100644
--- a/packages/types/package.json
+++ b/packages/types/package.json
@@ -27,9 +27,6 @@
   "bugs": {
     "url": "https://github.com/slackapi/node-slack-sdk/issues"
   },
-  "dependencies": {
-    "@slack/logger": "^4.0.0"
-  },
   "scripts": {
     "prepare": "npm run build",
     "build": "npm run build:clean && tsc",

From 0e553a7d08704bfe8981dff9aa5cdfc7130ed51d Mon Sep 17 00:00:00 2001
From: Ale Mercado <maria.mercado@slack-corp.com>
Date: Fri, 23 Jan 2026 18:30:14 -0500
Subject: [PATCH 12/13] fix: flush logic for chunks

---
 packages/web-api/src/chat-stream.ts           | 41 +++++++++++++------
 .../web-api/test/types/methods/chat.test-d.ts |  1 -
 2 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index 0ff177240..e7b72badd 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -1,4 +1,5 @@
 import type { Logger } from '@slack/logger';
+import type { AnyChunk } from '@slack/types';
 import type { ChatAppendStreamArguments, ChatStartStreamArguments, ChatStopStreamArguments } from './types/request';
 import type { ChatAppendStreamResponse, ChatStartStreamResponse, ChatStopStreamResponse } from './types/response';
 import type WebClient from './WebClient';
@@ -89,16 +90,10 @@ export class ChatStreamer {
     if (args.token) {
       this.token = args.token;
     }
-
-    if (args?.chunks) {
-      return await this.flushBuffer(args);
-    }
-
     if (args?.markdown_text) {
       this.buffer += args.markdown_text;
     }
-
-    if (this.buffer.length >= this.options.buffer_size) {
+    if (this.buffer.length >= this.options.buffer_size || args?.chunks) {
       return await this.flushBuffer(args);
     }
     const details = {
@@ -152,12 +147,24 @@ export class ChatStreamer {
       this.streamTs = response.ts;
       this.state = 'in_progress';
     }
-    const finalArgs = this.buffer.length > 0 && !args?.chunks ? { ...args, markdown_text: this.buffer } : args;
+
+    const flushings: AnyChunk[] = [];
+
+    if (this.buffer.length > 0) {
+      flushings.push({
+        type: 'markdown_text',
+        text: this.buffer,
+      });
+    }
+    if (args?.chunks) {
+      flushings.push(...args.chunks);
+    }
+
     const response = await this.client.chat.stopStream({
       token: this.token,
       channel: this.streamArgs.channel,
       ts: this.streamTs,
-      ...finalArgs,
+      chunks: flushings,
     });
     this.state = 'completed';
     return response;
@@ -166,13 +173,23 @@ export class ChatStreamer {
   private async flushBuffer(
     args: Omit<ChatStartStreamArguments | ChatAppendStreamArguments, 'channel' | 'ts'>,
   ): Promise<ChatStartStreamResponse | ChatAppendStreamResponse> {
-    const finalArgs = this.buffer.length > 0 && !args.chunks ? { ...args, markdown_text: this.buffer } : args;
+    const flushings: AnyChunk[] = [];
+
+    if (this.buffer.length > 0) {
+      flushings.push({
+        type: 'markdown_text',
+        text: this.buffer,
+      });
+    }
+    if (args?.chunks) {
+      flushings.push(...args.chunks);
+    }
 
     if (!this.streamTs) {
       const response = await this.client.chat.startStream({
         ...this.streamArgs,
         token: this.token,
-        ...finalArgs,
+        chunks: flushings,
       });
       this.buffer = '';
       this.streamTs = response.ts;
@@ -183,7 +200,7 @@ export class ChatStreamer {
       token: this.token,
       channel: this.streamArgs.channel,
       ts: this.streamTs,
-      ...finalArgs,
+      chunks: flushings,
     });
     this.buffer = '';
     return response;
diff --git a/packages/web-api/test/types/methods/chat.test-d.ts b/packages/web-api/test/types/methods/chat.test-d.ts
index 88c5a4b5f..a73e40249 100644
--- a/packages/web-api/test/types/methods/chat.test-d.ts
+++ b/packages/web-api/test/types/methods/chat.test-d.ts
@@ -676,7 +676,6 @@ expectAssignable<Parameters<typeof web.chat.startStream>>([
   {
     channel: 'C1234',
     thread_ts: '1234.56',
-    markdown_text: 'hello',
     chunks: [
       {
         type: 'markdown_text',

From fc5ed1ab6bd948daf2b258e87bfaad82ec0d4490 Mon Sep 17 00:00:00 2001
From: Michael Brooks <mbrooks@slack-corp.com>
Date: Fri, 23 Jan 2026 17:23:39 -0800
Subject: [PATCH 13/13] test: update charStream tests to support chunks

---
 packages/web-api/src/WebClient.spec.ts | 18 +++++++++++++-----
 packages/web-api/src/chat-stream.ts    |  5 +++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/packages/web-api/src/WebClient.spec.ts b/packages/web-api/src/WebClient.spec.ts
index 15dcb3fbe..c0c326297 100644
--- a/packages/web-api/src/WebClient.spec.ts
+++ b/packages/web-api/src/WebClient.spec.ts
@@ -1228,7 +1228,11 @@ describe('WebClient', () => {
           ok: true,
           ts: '123.123',
         })
-        .post('/api/chat.stopStream', { channel: 'C0123456789', ts: '123.123', markdown_text: 'nice!' })
+        .post('/api/chat.stopStream', {
+          channel: 'C0123456789',
+          ts: '123.123',
+          chunks: JSON.stringify([{ type: 'markdown_text', text: 'nice!' }]),
+        })
         .reply(200, {
           ok: true,
         });
@@ -1279,7 +1283,7 @@ describe('WebClient', () => {
       const scope = nock('https://slack.com')
         .post('/api/chat.startStream', {
           channel: 'C0123456789',
-          markdown_text: '**this messag',
+          chunks: JSON.stringify([{ type: 'markdown_text', text: '**this messag' }]),
           recipient_team_id: 'T0123456789',
           recipient_user_id: 'U0123456789',
           thread_ts: '123.000',
@@ -1290,7 +1294,7 @@ describe('WebClient', () => {
         })
         .post('/api/chat.appendStream', {
           channel: 'C0123456789',
-          markdown_text: 'e is bold!',
+          chunks: JSON.stringify([{ type: 'markdown_text', text: 'e is bold!' }]),
           token: 'xoxb-updated-1',
           ts: '123.123',
         })
@@ -1300,7 +1304,7 @@ describe('WebClient', () => {
         .post('/api/chat.stopStream', {
           blocks: JSON.stringify([contextActionsBlock]),
           channel: 'C0123456789',
-          markdown_text: '**',
+          chunks: JSON.stringify([{ type: 'markdown_text', text: '**' }]),
           token: 'xoxb-updated-2',
           ts: '123.123',
         })
@@ -1370,7 +1374,11 @@ describe('WebClient', () => {
           ok: true,
           ts: '123.123',
         })
-        .post('/api/chat.stopStream', { channel: 'C0123456789', ts: '123.123', markdown_text: 'nice!' })
+        .post('/api/chat.stopStream', {
+          channel: 'C0123456789',
+          ts: '123.123',
+          chunks: JSON.stringify([{ type: 'markdown_text', text: 'nice!' }]),
+        })
         .reply(200, {
           ok: true,
         });
diff --git a/packages/web-api/src/chat-stream.ts b/packages/web-api/src/chat-stream.ts
index e7b72badd..779aee80d 100644
--- a/packages/web-api/src/chat-stream.ts
+++ b/packages/web-api/src/chat-stream.ts
@@ -92,6 +92,7 @@ export class ChatStreamer {
     }
     if (args?.markdown_text) {
       this.buffer += args.markdown_text;
+      args.markdown_text = undefined;
     }
     if (this.buffer.length >= this.options.buffer_size || args?.chunks) {
       return await this.flushBuffer(args);
@@ -135,6 +136,7 @@ export class ChatStreamer {
     }
     if (args?.markdown_text) {
       this.buffer += args.markdown_text;
+      args.markdown_text = undefined;
     }
     if (!this.streamTs) {
       const response = await this.client.chat.startStream({
@@ -165,6 +167,7 @@ export class ChatStreamer {
       channel: this.streamArgs.channel,
       ts: this.streamTs,
       chunks: flushings,
+      ...args,
     });
     this.state = 'completed';
     return response;
@@ -190,6 +193,7 @@ export class ChatStreamer {
         ...this.streamArgs,
         token: this.token,
         chunks: flushings,
+        ...args,
       });
       this.buffer = '';
       this.streamTs = response.ts;
@@ -201,6 +205,7 @@ export class ChatStreamer {
       channel: this.streamArgs.channel,
       ts: this.streamTs,
       chunks: flushings,
+      ...args,
     });
     this.buffer = '';
     return response;