feat(provider): systemPrompt modes (rules|message|omit) in transcript mapping

Author: justin-carper

src/provider/message-map.ts

@@ -1,6 +1,16 @@
 import type { LanguageModelV3Prompt } from "@ai-sdk/provider";
 import type { SDKImage, SDKUserMessage } from "@cursor/sdk";
 
+/**
+ * How opencode's system prompt reaches the Cursor agent.
+ *  - "rules" (default): delivered out-of-band as a Cursor project rule
+ *    (see system-rule.ts) and therefore omitted from this flattened transcript.
+ *  - "message": legacy — inlined as a `# System` block in the transcript.
+ *    Injection-hardened Cursor models may reject this; kept for back-compat.
+ *  - "omit": dropped entirely (no rule, no inline).
+ */
+export type SystemPromptMode = "rules" | "message" | "omit";
+
 /**
  * Caps on inlined tool payloads in the flattened transcript. Tool outputs are
  * INCLUDED (truncated) rather than dropped so a fresh/diverged agent still sees
@@ -37,6 +47,7 @@ function truncate(text: string, cap: number): string {
  */
 export function promptToCursorMessage(
 	prompt: LanguageModelV3Prompt,
+	systemPrompt: SystemPromptMode = "rules",
 ): SDKUserMessage {
 	const lines: string[] = [];
 	const images: SDKImage[] = [];
@@ -45,7 +56,12 @@ export function promptToCursorMessage(
 		const isLast = index === prompt.length - 1;
 		switch (message.role) {
 			case "system":
-				lines.push(`# System\n${message.content}`);
+				// Only inline in legacy "message" mode. In "rules" mode the system
+				// prompt is delivered via .cursor/rules (authoritative, not flagged);
+				// in "omit" mode it is dropped.
+				if (systemPrompt === "message") {
+					lines.push(`# System\n${message.content}`);
+				}
 				break;
 			case "user": {
 				const text: string[] = [];

test/message-map.test.ts

@@ -6,21 +6,42 @@ import {
 } from "../src/provider/message-map.js";
 
 describe("promptToCursorMessage", () => {
-	it("flattens a multi-role conversation into a transcript", () => {
+	it("omits the system prompt from the transcript by default (rules mode)", () => {
 		const prompt: LanguageModelV3Prompt = [
 			{ role: "system", content: "Be concise." },
 			{ role: "user", content: [{ type: "text", text: "Hello" }] },
 			{ role: "assistant", content: [{ type: "text", text: "Hi there" }] },
 			{ role: "user", content: [{ type: "text", text: "What is 2+2?" }] },
 		];
 		const msg = promptToCursorMessage(prompt);
-		expect(msg.text).toContain("# System\nBe concise.");
+		// System prompt is delivered via the Cursor rules channel, not inline.
+		expect(msg.text).not.toContain("# System");
+		expect(msg.text).not.toContain("Be concise.");
 		expect(msg.text).toContain("# User\nHello");
 		expect(msg.text).toContain("# Assistant\nHi there");
 		expect(msg.text).toContain("# User\nWhat is 2+2?");
 		expect(msg.images).toBeUndefined();
 	});
 
+	it("includes the system prompt inline in 'message' (legacy) mode", () => {
+		const prompt: LanguageModelV3Prompt = [
+			{ role: "system", content: "Be concise." },
+			{ role: "user", content: [{ type: "text", text: "Hi" }] },
+		];
+		const msg = promptToCursorMessage(prompt, "message");
+		expect(msg.text).toContain("# System\nBe concise.");
+	});
+
+	it("omits the system prompt in 'omit' mode", () => {
+		const prompt: LanguageModelV3Prompt = [
+			{ role: "system", content: "Be concise." },
+			{ role: "user", content: [{ type: "text", text: "Hi" }] },
+		];
+		const msg = promptToCursorMessage(prompt, "omit");
+		expect(msg.text).not.toContain("Be concise.");
+		expect(msg.text).toBe("# User\nHi");
+	});
+
 	it("attaches images from the final user turn as base64 data", () => {
 		const bytes = new Uint8Array([1, 2, 3, 4]);
 		const prompt: LanguageModelV3Prompt = [