fix(chat): apply agent-level options in chat.toStreamTextOptions

chat.agent({ system, registry, cacheControl, systemProviderOptions })
reached only the bound streamText; the documented spread form ran without
them. The options are published for the run and toStreamTextOptions
defaults from them, caller options winning.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AxuSksX18bj1yhnLpkcQ6a

Author: ericallam

packages/trigger-sdk/src/v3/ai.ts

@@ -4811,6 +4811,15 @@ export type ChatStreamText = AiStreamTextFn;
  * version, and dropping one silently is the failure this seam exists to
  * prevent. Injected instructions append to whichever one is in play.
  */
+/**
+ * The agent-level managed options (`registry`, `system`, `cacheControl`,
+ * `systemProviderOptions`), published for the run so that
+ * `chat.toStreamTextOptions()` applies them too. Without this only the bound
+ * `streamText` saw them, and the documented spread form silently ran without
+ * the agent's system prompt or model.
+ */
+const chatAgentManagedConfigKey = locals.create<ManagedStreamTextConfig>("chat.agentManagedConfig");
+
 type ManagedStreamTextConfig = {
   registry?: ToStreamTextOptionsOptions["registry"];
   system?: ToStreamTextOptionsOptions["system"];
@@ -4928,6 +4937,16 @@ function createBoundStreamText(
 }
 
 function toStreamTextOptions(options?: ToStreamTextOptionsOptions): Record<string, unknown> {
+  const agentDefaults = locals.get(chatAgentManagedConfigKey);
+  if (agentDefaults) {
+    options = {
+      registry: agentDefaults.registry,
+      system: agentDefaults.system,
+      cacheControl: agentDefaults.cacheControl,
+      systemProviderOptions: agentDefaults.systemProviderOptions,
+      ...options,
+    };
+  }
   const prompt = locals.get(chatPromptKey);
   const skills = locals.get(chatSkillsKey);
   const result: Record<string, unknown> = {};
@@ -7493,6 +7512,13 @@ function chatAgent<
           }
         }
 
+        locals.set(chatAgentManagedConfigKey, {
+          registry: promptRegistry,
+          system: agentSystem,
+          cacheControl: agentCacheControl,
+          systemProviderOptions: agentSystemProviderOptions,
+        });
+
         // Make the seeded UI accumulator visible to `chat.history.*`
         // before any hook (`onChatStart`, `onTurnStart`, etc.) fires.
         locals.set(chatCurrentUIMessagesKey, accumulatedUIMessages);

packages/trigger-sdk/test/spread-form-agent-options.test.ts

@@ -0,0 +1,50 @@
+import { mockChatAgent } from "../src/v3/test/index.js";
+import type { LanguageModelV3StreamPart } from "@ai-sdk/provider";
+import { simulateReadableStream, streamText } from "ai";
+import { MockLanguageModelV3 } from "ai/test";
+import { describe, expect, it } from "vitest";
+import { chat } from "../src/v3/ai.js";
+
+/**
+ * The docs call `streamText({ ...chat.toStreamTextOptions() })` equivalent to
+ * the `streamText` handed to `run()`. That holds only if the agent-level
+ * options (`system`, `registry`, `cacheControl`, `systemProviderOptions`)
+ * reach the helper too, not just the bound function.
+ */
+const USAGE = {
+  inputTokens: { total: 1, noCache: 1, cacheRead: undefined, cacheWrite: undefined },
+  outputTokens: { total: 1, text: 1, reasoning: undefined },
+};
+const textChunks = (text: string): LanguageModelV3StreamPart[] => [
+  { type: "text-start", id: "t1" },
+  { type: "text-delta", id: "t1", delta: text },
+  { type: "text-end", id: "t1" },
+  { type: "finish", finishReason: { unified: "stop", raw: "stop" }, usage: USAGE },
+];
+
+describe("chat.toStreamTextOptions with agent-level options", () => {
+  it("carries chat.agent({ system }) in the spread form", async () => {
+    const prompts: string[] = [];
+    const model = new MockLanguageModelV3({
+      doStream: async ({ prompt }) => {
+        prompts.push(JSON.stringify(prompt));
+        return {
+          stream: simulateReadableStream({ chunks: textChunks("ok"), initialDelayInMs: 5 }),
+        };
+      },
+    });
+    const agent = chat.agent({
+      id: "spread-form-agent-system",
+      system: "AGENT-SYSTEM-VIA-SPREAD",
+      run: async ({ messages, signal }) =>
+        streamText({ model, messages, abortSignal: signal, ...chat.toStreamTextOptions() }),
+    });
+    const harness = mockChatAgent(agent, { chatId: "spread-form-agent-system" });
+    try {
+      await harness.sendMessage({ id: "u1", role: "user", parts: [{ type: "text", text: "hi" }] });
+      expect(prompts.at(-1)!).toContain("AGENT-SYSTEM-VIA-SPREAD");
+    } finally {
+      await harness.close();
+    }
+  });
+});