diff --git a/extensions/msteams/src/graph-thread.test.ts b/extensions/msteams/src/graph-thread.test.ts
new file mode 100644
index 00000000000..8e1266f44ec
--- /dev/null
+++ b/extensions/msteams/src/graph-thread.test.ts
@@ -0,0 +1,246 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  _teamGroupIdCacheForTest,
+  fetchChannelMessage,
+  fetchThreadReplies,
+  formatThreadContext,
+  resolveTeamGroupId,
+  stripHtmlFromTeamsMessage,
+} from "./graph-thread.js";
+import { fetchGraphJson } from "./graph.js";
+
+vi.mock("./graph.js", () => ({
+  fetchGraphJson: vi.fn(),
+}));
+
+describe("stripHtmlFromTeamsMessage", () => {
+  it("preserves @mention display names from <at> tags", () => {
+    expect(stripHtmlFromTeamsMessage("<at>Alice</at> hello")).toBe("@Alice hello");
+  });
+
+  it("strips other HTML tags", () => {
+    expect(stripHtmlFromTeamsMessage("<p>Hello <b>world</b></p>")).toBe("Hello world");
+  });
+
+  it("decodes common HTML entities", () => {
+    expect(stripHtmlFromTeamsMessage("&amp; &lt;b&gt; &quot;x&quot; &#39;y&#39; &nbsp;z")).toBe(
+      "& <b> \"x\" 'y' z",
+    );
+  });
+
+  it("normalizes multiple whitespace to single space", () => {
+    expect(stripHtmlFromTeamsMessage("hello   world")).toBe("hello world");
+  });
+
+  it("handles <at> tags with attributes", () => {
+    expect(stripHtmlFromTeamsMessage('<at id="123">Bob</at> please review')).toBe(
+      "@Bob please review",
+    );
+  });
+
+  it("returns empty string for empty input", () => {
+    expect(stripHtmlFromTeamsMessage("")).toBe("");
+  });
+});
+
+describe("resolveTeamGroupId", () => {
+  beforeEach(() => {
+    vi.mocked(fetchGraphJson).mockReset();
+    _teamGroupIdCacheForTest.clear();
+  });
+
+  it("fetches team id from Graph and caches it", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({ id: "group-guid-1" } as never);
+
+    const result = await resolveTeamGroupId("tok", "team-123");
+    expect(result).toBe("group-guid-1");
+    expect(fetchGraphJson).toHaveBeenCalledWith({
+      token: "tok",
+      path: "/teams/team-123?$select=id",
+    });
+  });
+
+  it("returns cached value without calling Graph again", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({ id: "group-guid-2" } as never);
+
+    await resolveTeamGroupId("tok", "team-456");
+    await resolveTeamGroupId("tok", "team-456");
+
+    expect(fetchGraphJson).toHaveBeenCalledTimes(1);
+  });
+
+  it("falls back to conversationTeamId when Graph returns no id", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({} as never);
+
+    const result = await resolveTeamGroupId("tok", "team-fallback");
+    expect(result).toBe("team-fallback");
+  });
+});
+
+describe("fetchChannelMessage", () => {
+  beforeEach(() => {
+    vi.mocked(fetchGraphJson).mockReset();
+  });
+
+  it("fetches the parent message with correct path", async () => {
+    const mockMsg = { id: "msg-1", body: { content: "hello", contentType: "text" } };
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce(mockMsg as never);
+
+    const result = await fetchChannelMessage("tok", "group-1", "channel-1", "msg-1");
+
+    expect(result).toEqual(mockMsg);
+    expect(fetchGraphJson).toHaveBeenCalledWith({
+      token: "tok",
+      path: "/teams/group-1/channels/channel-1/messages/msg-1?$select=id,from,body,createdDateTime",
+    });
+  });
+
+  it("returns undefined on fetch error", async () => {
+    vi.mocked(fetchGraphJson).mockRejectedValueOnce(new Error("forbidden") as never);
+
+    const result = await fetchChannelMessage("tok", "group-1", "channel-1", "msg-1");
+    expect(result).toBeUndefined();
+  });
+
+  it("URL-encodes group, channel, and message IDs", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({} as never);
+
+    await fetchChannelMessage("tok", "g/1", "c/2", "m/3");
+
+    expect(fetchGraphJson).toHaveBeenCalledWith({
+      token: "tok",
+      path: "/teams/g%2F1/channels/c%2F2/messages/m%2F3?$select=id,from,body,createdDateTime",
+    });
+  });
+});
+
+describe("fetchThreadReplies", () => {
+  beforeEach(() => {
+    vi.mocked(fetchGraphJson).mockReset();
+  });
+
+  it("fetches replies with correct path and default limit", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({
+      value: [{ id: "reply-1" }, { id: "reply-2" }],
+    } as never);
+
+    const result = await fetchThreadReplies("tok", "group-1", "channel-1", "msg-1");
+
+    expect(result).toHaveLength(2);
+    expect(fetchGraphJson).toHaveBeenCalledWith({
+      token: "tok",
+      path: "/teams/group-1/channels/channel-1/messages/msg-1/replies?$top=50&$select=id,from,body,createdDateTime",
+    });
+  });
+
+  it("clamps limit to 50 maximum", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({ value: [] } as never);
+
+    await fetchThreadReplies("tok", "g", "c", "m", 200);
+
+    const path = vi.mocked(fetchGraphJson).mock.calls[0]?.[0]?.path ?? "";
+    expect(path).toContain("$top=50");
+  });
+
+  it("clamps limit to 1 minimum", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({ value: [] } as never);
+
+    await fetchThreadReplies("tok", "g", "c", "m", 0);
+
+    const path = vi.mocked(fetchGraphJson).mock.calls[0]?.[0]?.path ?? "";
+    expect(path).toContain("$top=1");
+  });
+
+  it("returns empty array when value is missing", async () => {
+    vi.mocked(fetchGraphJson).mockResolvedValueOnce({} as never);
+
+    const result = await fetchThreadReplies("tok", "g", "c", "m");
+    expect(result).toEqual([]);
+  });
+});
+
+describe("formatThreadContext", () => {
+  it("formats messages as sender: content lines", () => {
+    const messages = [
+      {
+        id: "m1",
+        from: { user: { displayName: "Alice" } },
+        body: { content: "Hello!", contentType: "text" },
+      },
+      {
+        id: "m2",
+        from: { user: { displayName: "Bob" } },
+        body: { content: "World!", contentType: "text" },
+      },
+    ];
+    expect(formatThreadContext(messages)).toBe("Alice: Hello!\nBob: World!");
+  });
+
+  it("skips the current message by id", () => {
+    const messages = [
+      {
+        id: "m1",
+        from: { user: { displayName: "Alice" } },
+        body: { content: "Hello!", contentType: "text" },
+      },
+      {
+        id: "m2",
+        from: { user: { displayName: "Bob" } },
+        body: { content: "Current", contentType: "text" },
+      },
+    ];
+    expect(formatThreadContext(messages, "m2")).toBe("Alice: Hello!");
+  });
+
+  it("strips HTML from html contentType messages", () => {
+    const messages = [
+      {
+        id: "m1",
+        from: { user: { displayName: "Carol" } },
+        body: { content: "<p>Hello <b>world</b></p>", contentType: "html" },
+      },
+    ];
+    expect(formatThreadContext(messages)).toBe("Carol: Hello world");
+  });
+
+  it("uses application displayName when user is absent", () => {
+    const messages = [
+      {
+        id: "m1",
+        from: { application: { displayName: "BotApp" } },
+        body: { content: "automated msg", contentType: "text" },
+      },
+    ];
+    expect(formatThreadContext(messages)).toBe("BotApp: automated msg");
+  });
+
+  it("skips messages with empty content", () => {
+    const messages = [
+      {
+        id: "m1",
+        from: { user: { displayName: "Alice" } },
+        body: { content: "", contentType: "text" },
+      },
+      {
+        id: "m2",
+        from: { user: { displayName: "Bob" } },
+        body: { content: "actual content", contentType: "text" },
+      },
+    ];
+    expect(formatThreadContext(messages)).toBe("Bob: actual content");
+  });
+
+  it("falls back to 'unknown' sender when from is missing", () => {
+    const messages = [
+      {
+        id: "m1",
+        body: { content: "orphan msg", contentType: "text" },
+      },
+    ];
+    expect(formatThreadContext(messages)).toBe("unknown: orphan msg");
+  });
+
+  it("returns empty string for empty messages array", () => {
+    expect(formatThreadContext([])).toBe("");
+  });
+});
diff --git a/extensions/msteams/src/graph-thread.ts b/extensions/msteams/src/graph-thread.ts
new file mode 100644
index 00000000000..258f37cebde
--- /dev/null
+++ b/extensions/msteams/src/graph-thread.ts
@@ -0,0 +1,142 @@
+import { fetchGraphJson, type GraphResponse } from "./graph.js";
+
+export type GraphThreadMessage = {
+  id?: string;
+  from?: {
+    user?: { displayName?: string; id?: string };
+    application?: { displayName?: string; id?: string };
+  };
+  body?: { content?: string; contentType?: string };
+  createdDateTime?: string;
+};
+
+// TTL cache for team ID -> group GUID mapping.
+const teamGroupIdCache = new Map<string, { groupId: string; expiresAt: number }>();
+const CACHE_TTL_MS = 10 * 60 * 1000; // 10 minutes
+
+/**
+ * Strip HTML tags from Teams message content, preserving @mention display names.
+ * Teams wraps mentions in <at>Name</at> tags.
+ */
+export function stripHtmlFromTeamsMessage(html: string): string {
+  // Preserve mention display names by replacing <at>Name</at> with @Name.
+  let text = html.replace(/<at[^>]*>(.*?)<\/at>/gi, "@$1");
+  // Strip remaining HTML tags.
+  text = text.replace(/<[^>]*>/g, " ");
+  // Decode common HTML entities.
+  text = text
+    .replace(/&amp;/g, "&")
+    .replace(/&lt;/g, "<")
+    .replace(/&gt;/g, ">")
+    .replace(/&quot;/g, '"')
+    .replace(/&#39;/g, "'")
+    .replace(/&nbsp;/g, " ");
+  // Normalize whitespace.
+  return text.replace(/\s+/g, " ").trim();
+}
+
+/**
+ * Resolve the Azure AD group GUID for a Teams conversation team ID.
+ * Results are cached with a TTL to avoid repeated Graph API calls.
+ */
+export async function resolveTeamGroupId(
+  token: string,
+  conversationTeamId: string,
+): Promise<string> {
+  const cached = teamGroupIdCache.get(conversationTeamId);
+  if (cached && cached.expiresAt > Date.now()) {
+    return cached.groupId;
+  }
+
+  // The team ID in channelData is typically the group ID itself for standard teams.
+  // Validate by fetching /teams/{id} and returning the confirmed id.
+  // Requires Team.ReadBasic.All permission; fall back to raw ID if missing.
+  try {
+    const path = `/teams/${encodeURIComponent(conversationTeamId)}?$select=id`;
+    const team = await fetchGraphJson<{ id?: string }>({ token, path });
+    const groupId = team.id ?? conversationTeamId;
+
+    // Only cache when the Graph lookup succeeds — caching a fallback raw ID
+    // can cause silent failures for the entire TTL if the ID is not a valid
+    // Graph team GUID (e.g. Bot Framework conversation key).
+    teamGroupIdCache.set(conversationTeamId, {
+      groupId,
+      expiresAt: Date.now() + CACHE_TTL_MS,
+    });
+
+    return groupId;
+  } catch {
+    // Fallback to raw team ID without caching so subsequent calls retry the
+    // Graph lookup instead of using a potentially invalid cached value.
+    return conversationTeamId;
+  }
+}
+
+/**
+ * Fetch a single channel message (the parent/root of a thread).
+ * Returns undefined on error so callers can degrade gracefully.
+ */
+export async function fetchChannelMessage(
+  token: string,
+  groupId: string,
+  channelId: string,
+  messageId: string,
+): Promise<GraphThreadMessage | undefined> {
+  const path = `/teams/${encodeURIComponent(groupId)}/channels/${encodeURIComponent(channelId)}/messages/${encodeURIComponent(messageId)}?$select=id,from,body,createdDateTime`;
+  try {
+    return await fetchGraphJson<GraphThreadMessage>({ token, path });
+  } catch {
+    return undefined;
+  }
+}
+
+/**
+ * Fetch thread replies for a channel message, ordered chronologically.
+ *
+ * **Limitation:** The Graph API replies endpoint (`/messages/{id}/replies`) does not
+ * support `$orderby`, so results are always returned in ascending (oldest-first) order.
+ * Combined with the `$top` cap of 50, this means only the **oldest 50 replies** are
+ * returned for long threads — newer replies are silently omitted. There is currently no
+ * Graph API workaround for this; pagination via `@odata.nextLink` can retrieve more
+ * replies but still in ascending order only.
+ */
+export async function fetchThreadReplies(
+  token: string,
+  groupId: string,
+  channelId: string,
+  messageId: string,
+  limit = 50,
+): Promise<GraphThreadMessage[]> {
+  const top = Math.min(Math.max(limit, 1), 50);
+  // NOTE: Graph replies endpoint returns oldest-first and does not support $orderby.
+  // For threads with >50 replies, only the oldest 50 are returned. The most recent
+  // replies (often the most relevant context) may be truncated.
+  const path = `/teams/${encodeURIComponent(groupId)}/channels/${encodeURIComponent(channelId)}/messages/${encodeURIComponent(messageId)}/replies?$top=${top}&$select=id,from,body,createdDateTime`;
+  const res = await fetchGraphJson<GraphResponse<GraphThreadMessage>>({ token, path });
+  return res.value ?? [];
+}
+
+/**
+ * Format thread messages into a context string for the agent.
+ * Skips the current message (by id) and blank messages.
+ */
+export function formatThreadContext(
+  messages: GraphThreadMessage[],
+  currentMessageId?: string,
+): string {
+  const lines: string[] = [];
+  for (const msg of messages) {
+    if (msg.id && msg.id === currentMessageId) continue; // Skip the triggering message.
+    const sender = msg.from?.user?.displayName ?? msg.from?.application?.displayName ?? "unknown";
+    const contentType = msg.body?.contentType ?? "text";
+    const rawContent = msg.body?.content ?? "";
+    const content =
+      contentType === "html" ? stripHtmlFromTeamsMessage(rawContent) : rawContent.trim();
+    if (!content) continue;
+    lines.push(`${sender}: ${content}`);
+  }
+  return lines.join("\n");
+}
+
+// Exported for testing only.
+export { teamGroupIdCache as _teamGroupIdCacheForTest };
diff --git a/extensions/msteams/src/monitor-handler/message-handler.ts b/extensions/msteams/src/monitor-handler/message-handler.ts
index 0ad5053f91e..badac50c118 100644
--- a/extensions/msteams/src/monitor-handler/message-handler.ts
+++ b/extensions/msteams/src/monitor-handler/message-handler.ts
@@ -28,6 +28,12 @@ import {
 } from "../attachments.js";
 import type { StoredConversationReference } from "../conversation-store.js";
 import { formatUnknownError } from "../errors.js";
+import {
+  fetchChannelMessage,
+  fetchThreadReplies,
+  formatThreadContext,
+  resolveTeamGroupId,
+} from "../graph-thread.js";
 import {
   extractMSTeamsConversationMessageId,
   extractMSTeamsQuoteInfo,
@@ -474,6 +480,29 @@ export function createMSTeamsMessageHandler(deps: MSTeamsMessageHandlerDeps) {
     });
 
     const mediaPayload = buildMSTeamsMediaPayload(mediaList);
+
+    // Fetch thread history when the message is a reply inside a Teams channel thread.
+    // This is a best-effort enhancement; errors are logged and do not block the reply.
+    let threadContext: string | undefined;
+    if (activity.replyToId && isChannel && teamId) {
+      try {
+        const graphToken = await tokenProvider.getAccessToken("https://graph.microsoft.com");
+        const groupId = await resolveTeamGroupId(graphToken, teamId);
+        const [parentMsg, replies] = await Promise.all([
+          fetchChannelMessage(graphToken, groupId, conversationId, activity.replyToId),
+          fetchThreadReplies(graphToken, groupId, conversationId, activity.replyToId),
+        ]);
+        const allMessages = parentMsg ? [parentMsg, ...replies] : replies;
+        const formatted = formatThreadContext(allMessages, activity.id);
+        if (formatted) {
+          threadContext = formatted;
+        }
+      } catch (err) {
+        log.debug?.("failed to fetch thread history", { error: String(err) });
+        // Graceful degradation: thread history is an optional enhancement.
+      }
+    }
+
     const envelopeFrom = isDirectMessage ? senderName : conversationType;
     const { storePath, envelopeOptions, previousTimestamp } = resolveInboundSessionEnvelopeContext({
       cfg,
@@ -518,9 +547,14 @@ export function createMSTeamsMessageHandler(deps: MSTeamsMessageHandlerDeps) {
         : undefined;
     const commandBody = text.trim();
 
+    // Prepend thread history to the agent body so the agent has full thread context.
+    const bodyForAgent = threadContext
+      ? `[Thread history]\n${threadContext}\n[/Thread history]\n\n${rawBody}`
+      : rawBody;
+
     const ctxPayload = core.channel.reply.finalizeInboundContext({
       Body: combinedBody,
-      BodyForAgent: rawBody,
+      BodyForAgent: bodyForAgent,
       InboundHistory: inboundHistory,
       RawBody: rawBody,
       CommandBody: commandBody,