diff --git a/extensions/msteams/src/welcome-card.test.ts b/extensions/msteams/src/welcome-card.test.ts
index b9e068be029c..3a7da92c7ec7 100644
--- a/extensions/msteams/src/welcome-card.test.ts
+++ b/extensions/msteams/src/welcome-card.test.ts
@@ -85,6 +85,15 @@ describe("buildWelcomeCard", () => {
     expect(actions[0]?.title).toBe("What can you do?");
   });
 
+  it("styles the heading with valid PascalCase Adaptive Card enum values", () => {
+    // Lowercase weight/size fall back to Default in the Teams renderer, so the heading must use the
+    // schema's PascalCase enums to render bold/medium.
+    const card = buildWelcomeCard();
+    const heading = (card.body as Array<{ weight?: string; size?: string }>)[0];
+    expect(heading?.weight).toBe("Bolder");
+    expect(heading?.size).toBe("Medium");
+  });
+
   it("uses custom bot name", () => {
     const card = buildWelcomeCard({ botName: "TestBot" });
     const body = card.body as Array<{ text: string }>;
diff --git a/extensions/msteams/src/welcome-card.ts b/extensions/msteams/src/welcome-card.ts
index dcc29106139b..f9c934805889 100644
--- a/extensions/msteams/src/welcome-card.ts
+++ b/extensions/msteams/src/welcome-card.ts
@@ -31,8 +31,10 @@ export function buildWelcomeCard(options?: WelcomeCardOptions): Record<string, u
       {
         type: "TextBlock",
         text: `Hi! I'm ${botName}.`,
-        weight: "bolder",
-        size: "medium",
+        // Adaptive Card TextWeight/TextSize enums are PascalCase ("Bolder"/"Medium"); lowercase
+        // values fall back to Default, so the greeting rendered unstyled (matches polls/presentation).
+        weight: "Bolder",
+        size: "Medium",
       },
       {
         type: "TextBlock",