diff --git a/packages/agent/CHANGELOG.md b/packages/agent/CHANGELOG.md
index 04ad9fb1dd..abe731f4f0 100644
--- a/packages/agent/CHANGELOG.md
+++ b/packages/agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Removed unnecessary `as any` casts when accessing `errorMessage` on `AssistantMessage` in `Agent` turn-end handling — the field is already on the type.
+
 ## [16.1.16] - 2026-06-23
 
 ### Added
diff --git a/packages/agent/src/agent.ts b/packages/agent/src/agent.ts
index b5be52d785..ee23c84017 100644
--- a/packages/agent/src/agent.ts
+++ b/packages/agent/src/agent.ts
@@ -1202,8 +1202,8 @@ export class Agent {
 						break;
 
 					case "turn_end":
-						if (event.message.role === "assistant" && (event.message as any).errorMessage) {
-							this.#state.error = (event.message as any).errorMessage;
+						if (event.message.role === "assistant" && event.message.errorMessage) {
+							this.#state.error = event.message.errorMessage;
 						}
 						break;
 
diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 34c31852ea..29610c19e6 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- Removed redundant `undefined as any` casts in `EventStream.end()` and `endWaiting()`. The waiter type is now explicitly `IteratorResult<T, undefined>` (instead of the default `IteratorResult<T, any>`), so the `undefined` value is type-checked without relying on the implicit `any` for `TReturn`.
+
 ## [16.1.16] - 2026-06-23
 
 ### Fixed
diff --git a/packages/ai/src/utils/event-stream.ts b/packages/ai/src/utils/event-stream.ts
index f4819d98f4..7184b96999 100644
--- a/packages/ai/src/utils/event-stream.ts
+++ b/packages/ai/src/utils/event-stream.ts
@@ -3,7 +3,7 @@ import type { AssistantMessage, AssistantMessageEvent } from "../types";
 // Generic event stream class for async iteration
 export class EventStream<T, R = T> implements AsyncIterable<T> {
 	queue: T[] = [];
-	waiting: Array<{ resolve: (value: IteratorResult<T>) => void; reject: (err: unknown) => void }> = [];
+	waiting: Array<{ resolve: (value: IteratorResult<T, undefined>) => void; reject: (err: unknown) => void }> = [];
 	done = false;
 	/** True once finalResultPromise has been resolved or rejected. */
 	resultSettled = false;
@@ -68,14 +68,14 @@ export class EventStream<T, R = T> implements AsyncIterable<T> {
 		// Notify all waiting consumers that we're done
 		while (this.waiting.length > 0) {
 			const waiter = this.waiting.shift()!;
-			waiter.resolve({ value: undefined as any, done: true });
+			waiter.resolve({ value: undefined, done: true });
 		}
 	}
 
 	endWaiting(): void {
 		while (this.waiting.length > 0) {
 			const waiter = this.waiting.shift()!;
-			waiter.resolve({ value: undefined as any, done: true });
+			waiter.resolve({ value: undefined, done: true });
 		}
 	}
 
@@ -101,7 +101,7 @@ export class EventStream<T, R = T> implements AsyncIterable<T> {
 			} else if (this.done) {
 				return;
 			} else {
-				const result = await new Promise<IteratorResult<T>>((resolve, reject) =>
+				const result = await new Promise<IteratorResult<T, undefined>>((resolve, reject) =>
 					this.waiting.push({ resolve, reject }),
 				);
 				if (result.done) return;