feat(provider): deliver system prompt via Cursor rules in rules mode

Author: justin-carper

src/provider/language-model.ts

@@ -14,7 +14,12 @@ import type {
 	AgentModeOption,
 } from "@cursor/sdk";
 import { resolveCursorApiKey } from "../api-key.js";
-import { latestUserMessage, promptToCursorMessage } from "./message-map.js";
+import {
+	latestUserMessage,
+	promptToCursorMessage,
+	type SystemPromptMode,
+} from "./message-map.js";
+import { extractSystemText, writeSystemRule } from "./system-rule.js";
 import { streamAgentTurn, type CursorEvent } from "./agent-events.js";
 import {
 	cursorEventsToContent,
@@ -64,6 +69,11 @@ export interface CursorModelConfig {
 	 * Defaults to `"blocks"`.
 	 */
 	toolDisplay?: ToolDisplay;
+	/**
+	 * How opencode's system prompt reaches the Cursor agent (see
+	 * {@link SystemPromptMode}). Defaults to "rules".
+	 */
+	systemPrompt?: SystemPromptMode;
 }
 
 /**
@@ -184,14 +194,25 @@ export class CursorLanguageModel implements LanguageModelV3 {
 			}
 		}
 
+		// In "rules" mode (default), deliver opencode's system prompt through
+		// Cursor's authoritative rules channel instead of the user transcript.
+		const systemMode: SystemPromptMode = this.config.systemPrompt ?? "rules";
+		let settingSources = this.config.settingSources;
+		if (systemMode === "rules") {
+			const systemText = extractSystemText(options.prompt);
+			if (writeSystemRule(this.config.cwd, systemText)) {
+				settingSources = settingSources?.includes("project")
+					? settingSources
+					: [...(settingSources ?? []), "project"];
+			}
+		}
+
 		const acquired = await acquireAgent({
 			apiKey: this.requireApiKey(),
 			modelSelection,
 			mode,
 			cwd: this.config.cwd,
-			...(this.config.settingSources
-				? { settingSources: this.config.settingSources }
-				: {}),
+			...(settingSources ? { settingSources } : {}),
 			...(this.config.sandbox !== undefined
 				? { sandbox: this.config.sandbox }
 				: {}),
@@ -207,8 +228,8 @@ export class CursorLanguageModel implements LanguageModelV3 {
 		// new turn; otherwise send the full transcript.
 		const message = acquired.resumed
 			? (latestUserMessage(options.prompt) ??
-				promptToCursorMessage(options.prompt))
-			: promptToCursorMessage(options.prompt);
+				promptToCursorMessage(options.prompt, systemMode))
+			: promptToCursorMessage(options.prompt, systemMode);
 
 		try {
 			yield* streamAgentTurn(acquired.agent, message, {