fix(agent): restore streamFn extension compatibility

Keep streamFn required for typed callers while preserving the legacy runtime fallback for extensions that omit it.\n\nfixes #6915

packages/agent/test/agent-loop.test.ts

@@ -9,6 +9,7 @@ import {
 import { Type } from "typebox";
 import { describe, expect, it } from "vitest";
 import { agentLoop, agentLoopContinue } from "../src/agent-loop.ts";
+import { setDefaultStreamFn } from "../src/index.ts";
 import type { AgentContext, AgentEvent, AgentLoopConfig, AgentMessage, AgentTool } from "../src/types.ts";
 
 // Mock stream for testing - mimics MockAssistantStream
@@ -80,6 +81,40 @@ function identityConverter(messages: AgentMessage[]): Message[] {
 	return messages.filter((m) => m.role === "user" || m.role === "assistant" || m.role === "toolResult") as Message[];
 }
 
+describe("default stream function compatibility", () => {
+	it("uses the configured default when a legacy caller omits streamFn", async () => {
+		let calls = 0;
+		setDefaultStreamFn(() => {
+			calls++;
+			const stream = new MockAssistantStream();
+			queueMicrotask(() => {
+				stream.push({
+					type: "done",
+					reason: "stop",
+					message: createAssistantMessage([{ type: "text", text: "fallback" }]),
+				});
+			});
+			return stream;
+		});
+
+		try {
+			const context: AgentContext = { systemPrompt: "", messages: [], tools: [] };
+			const config: AgentLoopConfig = { model: createModel(), convertToLlm: identityConverter };
+			const stream = Reflect.apply(agentLoop, undefined, [
+				[createUserMessage("Hello")],
+				context,
+				config,
+				undefined,
+			]) as ReturnType<typeof agentLoop>;
+
+			await stream.result();
+			expect(calls).toBe(1);
+		} finally {
+			setDefaultStreamFn(undefined);
+		}
+	});
+});
+
 describe("agentLoop with AgentMessage", () => {
 	it("should emit events with AgentMessage types", async () => {
 		const context: AgentContext = {