microsoft · OEvgeny · Apr 9, 2026 · Apr 3, 2026 · Apr 3, 2026 · Apr 3, 2026
@@ -161,6 +161,7 @@ Breaking changes in this release:
 - Added pull-based capabilities system for dynamically discovering adapter capabilities at runtime, in PR [#5679](https://github.com/microsoft/BotFramework-WebChat/pull/5679), by [@pranavjoshi001](https://github.com/pranavjoshi001)
 - Added Speech-to-Speech (S2S) support for real-time voice conversations, in PR [#5654](https://github.com/microsoft/BotFramework-WebChat/pull/5654), by [@pranavjoshi](https://github.com/pranavjoshi001)
 - Added core mute/unmute functionality for speech-to-speech via `useRecorder` hook (silent chunks keep server connection alive), in PR [#5688](https://github.com/microsoft/BotFramework-WebChat/pull/5688), by [@pranavjoshi](https://github.com/pranavjoshi001)
+- 🧪 Added incremental streaming Markdown renderer for livestreaming, in PR [#5799](https://github.com/microsoft/BotFramework-WebChat/pull/5799), by [@OEvgeny](https://github.com/OEvgeny)
 
 ### Changed
 

@@ -0,0 +1,236 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <title>Event Stream Adapter Custom Element</title>
+</head>
+<body>
+  <script type="module">
+    import { customElement } from '/assets/custom-element/custom-element.js';
+    import { getActivityLivestreamingMetadata } from 'botframework-webchat-core';
+    import { EventSourceParserStream } from 'eventsource-parser/stream';
+    import createStreamCoalescer from '/assets/esm/adapter/createStreamCoalescer.js';
+    import { forIterator } from '/assets/esm/adapter/demuxChainOfThought.js';
+
+    function createObservable(subscribe) {
+      return Object.freeze({ subscribe });
+    }
+
+    customElement('event-stream-adapter', currentDocument =>
+      class EventStreamAdapterElement extends HTMLElement {
+        static get observedAttributes() { return ['compat', 'href']; }
+
+        #abortController = null;
+        #activities = [];
+        #activityBuffer = [];
+        #activityObservers = [];
+        #activityWaiters = new Set();
+        #directLine;
+
+        constructor() {
+          super();
+
+          const self = this;
+
+          this.#directLine = Object.freeze({
+            activity$: createObservable(observerOrNext => {
+              const observer = typeof observerOrNext === 'function'
+                ? { next: observerOrNext }
+                : observerOrNext;
+
+              for (const activity of self.#activityBuffer) {
+                observer.next(activity);
+              }
+
+              self.#activityObservers.push(observer);
+
+              return Object.freeze({
+                unsubscribe() {
+                  const i = self.#activityObservers.indexOf(observer);
+                  i !== -1 && self.#activityObservers.splice(i, 1);
+                }
+              });
+            }),
+
+            connectionStatus$: createObservable(observerOrNext => {
+              const observer = typeof observerOrNext === 'function'
+                ? { next: observerOrNext }
+                : observerOrNext;
+
+              observer.next(0);
+              observer.next(1);
+              observer.next(2);
+
+              return Object.freeze({ unsubscribe() {} });
+            }),
+
+            end() {},
+
+            postActivity() {
+              return createObservable(observerOrNext => {
+                const observer = typeof observerOrNext === 'function'
+                  ? { next: observerOrNext }
+                  : observerOrNext;
+
+                observer.next(crypto.randomUUID());
+                observer.complete?.();
+
+                return Object.freeze({ unsubscribe() {} });
+              });
+            }
+          });
+        }
+
+        get compat() {
+          return this.getAttribute('compat') || 'webchat';
+        }
+
+        get directLine() {
+          return this.#directLine;
+        }
+
+        connectedCallback() {
+          this.#load();
+        }
+
+        disconnectedCallback() {
+          this.#abortController?.abort();
+          this.#abortController = null;
+        }
+
+        #emitToAdapter(activity) {
+          this.#activityBuffer.push(activity);
+
+          for (const observer of this.#activityObservers) {
+            observer.next(activity);
+          }
+        }
+
+        #emitEvent(activity) {
+          this.#activities.push(activity);
+
+          for (const resolve of this.#activityWaiters) {
+            resolve();
+          }
+
+          this.#activityWaiters.clear();
+
+          const meta = getActivityLivestreamingMetadata(activity);
+          const element = this;
+          const detail = { activity };
+
+          meta && Object.defineProperty(detail, 'activities', {
+            enumerable: true,
+            get: () => element.#iterateStreamActivities(meta.sessionId)
+          });
+
+          this.dispatchEvent(new CustomEvent('activity', {
+            bubbles: true,
+            detail: Object.freeze(detail)
+          }));
+        }
+
+        async *#iterateStreamActivities(sessionId) {
+          let cursor = 0;
+
+          for (;;) {
+            while (cursor < this.#activities.length) {
+              const activity = this.#activities[cursor++];
+              const activityMeta = getActivityLivestreamingMetadata(activity);
+
+              if (!activityMeta || activityMeta.sessionId !== sessionId) {
+                continue;
+              }
+
+              yield activity;
+
+              if (activityMeta.type === 'final activity') {
+                return;
+              }
+            }
+
+            await new Promise(resolve => this.#activityWaiters.add(resolve));
+          }
+        }
+
+        async #load() {
+          const href = this.getAttribute('href');
+
+          if (!href) {
+            return;
+          }
+
+          this.#abortController?.abort();
+          this.#abortController = new AbortController();
+
+          const { signal } = this.#abortController;
+          const compat = this.compat;
+          const useAdapter = compat === 'webchat' || compat === 'both';
+          const useEvents = compat === 'events' || compat === 'both';
+
+          try {
+            for await (const activity of forIterator({}, this.#fetchStreamed(href, signal))) {
+              if (signal.aborted) {
+                break;
+              }
+
+              useAdapter && this.#emitToAdapter(activity);
+              useEvents && this.#emitEvent(activity.raw);
+            }
+          } catch (error) {
+            signal.aborted || console.error('event-stream-adapter:', error);
+          }
+        }
+
+        async *#fetchStreamed(href, signal) {
+          const typingMap = new Map();
+          const res = await fetch(new URL(href, location.href), { signal });
+
+          yield* res.body
+            .pipeThrough(new TextDecoderStream())
+            .pipeThrough(new EventSourceParserStream())
+            .pipeThrough(
+              new TransformStream({
+                transform({ data, event }, controller) {
+                  if (event === 'end') {
+                    controller.terminate();
+                    return;
+                  }
+
+                  if (event !== 'activity') {
+                    return;
+                  }
+
+                  const activity = JSON.parse(data);
+
+                  activity.raw = { ...activity };
+
+                  if (
+                    activity.type === 'typing' &&
+                    activity.text &&
+                    activity.channelData?.streamType === 'streaming'
+                  ) {
+                    const streamId = activity.channelData?.streamId || activity.id;
+                    let accumulated = typingMap.get(streamId) || '';
+
+                    if (activity.channelData?.chunkType === 'delta') {
+                      accumulated += activity.text;
+                      activity.text = accumulated;
+                    } else {
+                      accumulated = activity.text;
+                    }
+
+                    typingMap.set(streamId, accumulated);
+                  }
+
+                  controller.enqueue(activity);
+                }
+              })
+            )
+            .pipeThrough(createStreamCoalescer());
+        }
+      }
+    );
+  </script>
+</body>
+</html>
@@ -0,0 +1,54 @@
+/* eslint-disable security/detect-object-injection */
+/*!
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ */
+
+const NextLivestreamSequence = Symbol();
+const PreviousActivitySymbol = Symbol();
+const SessionIdSymbol = Symbol();
+
+export default class LivestreamSession {
+  constructor(sessionId) {
+    this[NextLivestreamSequence] = 1;
+    this[PreviousActivitySymbol] = undefined;
+    this[SessionIdSymbol] = sessionId;
+  }
+
+  /**
+   * Last string, useful for decompressing delta-compressed chunks.
+   */
+  get previousActivity() {
+    return this[PreviousActivitySymbol];
+  }
+
+  set previousActivity(value) {
+    this[PreviousActivitySymbol] = value;
+  }
+
+  /**
+   * Activity ID of the session (and the first activity.)
+   *
+   * @type {string}
+   */
+  get sessionId() {
+    return this[SessionIdSymbol];
+  }
+
+  get isConcluded() {
+    return this[NextLivestreamSequence] === Infinity;
+  }
+
+  /** @return {number} */
+  getNextLivestreamSequence(
+    /** @type {boolean | undefined} */
+    isFinal = false
+  ) {
+    if (isFinal) {
+      this.previousActivity = undefined;
+
+      return (this[NextLivestreamSequence] = Infinity);
+    }
+
+    return this[NextLivestreamSequence]++;
+  }
+}
@@ -0,0 +1,80 @@
+/* eslint-disable security/detect-object-injection */
+
+/*!
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ */
+
+import LivestreamSession from './LivestreamSession.js';
+
+const ActiveLivestreamSymbol = Symbol();
+
+export default class LivestreamSessionManager {
+  constructor() {
+    this[ActiveLivestreamSymbol] = new Map();
+  }
+
+  *concludeAll() {
+    for (const [sessionId, session] of this[ActiveLivestreamSymbol]) {
+      if (!session.isConcluded) {
+        const { previousActivity } = session;
+        const entitiesWithoutStreamInfo = (previousActivity?.entities ?? []).filter(
+          ({ type }) => type !== 'streaminfo'
+        );
+
+        yield Object.freeze({
+          ...previousActivity,
+          channelData: Object.freeze({
+            ...previousActivity?.channelData,
+            chunkType: undefined,
+            streamId: sessionId,
+            streamSequence: undefined,
+            streamType: 'final'
+          }),
+          entities: Object.freeze([...entitiesWithoutStreamInfo]),
+          id: `${sessionId}/final`,
+          text: previousActivity?.text,
+          type: 'message'
+        });
+      }
+    }
+  }
+
+  has(livestreamSessionId) {
+    return this[ActiveLivestreamSymbol].has(livestreamSessionId);
+  }
+
+  *sequence(livestreamSessionId, activity, isFinal = false) {
+    let livestreamSession = this[ActiveLivestreamSymbol].get(livestreamSessionId);
+
+    if (!livestreamSession) {
+      livestreamSession = new LivestreamSession(livestreamSessionId);
+
+      this[ActiveLivestreamSymbol].set(livestreamSessionId, livestreamSession);
+    }
+
+    if (livestreamSession.isConcluded) {
+      return;
+    }
+
+    const streamSequence = livestreamSession.getNextLivestreamSequence(isFinal);
+    const entitiesWithoutStreamInfo = (activity.entities ?? []).filter(({ type }) => type !== 'streaminfo');
+
+    // We assume the chat adapter will do delta decompression.
+    livestreamSession.previousActivity = activity;
+
+    yield Object.freeze({
+      ...activity,
+      channelData: Object.freeze({
+        ...activity.channelData,
+        chunkType: undefined,
+        streamId: streamSequence === 1 ? undefined : livestreamSessionId,
+        streamSequence: streamSequence === Infinity ? undefined : streamSequence,
+        streamType: streamSequence === Infinity ? 'final' : 'streaming'
+      }),
+      entities: Object.freeze([...entitiesWithoutStreamInfo]),
+      id: streamSequence === 1 ? livestreamSessionId : activity.id,
+      text: livestreamSession.previousActivity.text,
+      type: streamSequence === Infinity ? 'message' : 'typing'
+    });
+  }
+}