diff --git a/apps/pi-extension/server/network.test.ts b/apps/pi-extension/server/network.test.ts
index 8c8182802..1efc34ebe 100644
--- a/apps/pi-extension/server/network.test.ts
+++ b/apps/pi-extension/server/network.test.ts
@@ -285,6 +285,42 @@ describe("pi browser no-op sentinels", () => {
 	});
 });
 
+describe("pi PLANNOTATOR_BROWSER script path", () => {
+	// #1391: on darwin, `open -a <script>` fails with LaunchServices -10811,
+	// so a slash-containing non-.app value must be executed directly.
+	test.if(process.platform !== "win32")(
+		"executes a script path directly with the URL as argument",
+		async () => {
+			clearEnv();
+			const { mkdtempSync, writeFileSync, readFileSync, existsSync, rmSync } =
+				await import("node:fs");
+			const { join } = await import("node:path");
+			const { tmpdir } = await import("node:os");
+			const dir = mkdtempSync(join(tmpdir(), "pn-browser-"));
+			try {
+				const marker = join(dir, "opened.txt");
+				const script = join(dir, "handler.sh");
+				writeFileSync(script, `#!/bin/sh\nprintf '%s' \"$1\" > \"${marker}\"\n`, {
+					mode: 0o755,
+				});
+				process.env.PLANNOTATOR_BROWSER = script;
+
+				const result = await openBrowser("http://127.0.0.1:19432");
+				expect(result.opened).toBe(true);
+
+				// spawn is detached; poll briefly for the marker
+				const deadline = Date.now() + 2000;
+				while (!existsSync(marker) && Date.now() < deadline) {
+					await new Promise((r) => setTimeout(r, 25));
+				}
+				expect(readFileSync(marker, "utf-8")).toBe("http://127.0.0.1:19432");
+			} finally {
+				rmSync(dir, { recursive: true, force: true });
+			}
+		},
+	);
+});
+
 describe("pi buildAdvertisedUrl", () => {
 	test("defaults to localhost", () => {
 		clearEnv();
diff --git a/apps/pi-extension/server/network.ts b/apps/pi-extension/server/network.ts
index d97ad2977..ec563778c 100644
--- a/apps/pi-extension/server/network.ts
+++ b/apps/pi-extension/server/network.ts
@@ -333,8 +333,17 @@ export async function openBrowser(url: string): Promise<{
 
 		if (browser) {
 			if (plannotatorBrowser && platform === "darwin") {
-				cmd = "open";
-				args = ["-a", plannotatorBrowser, url];
+				if (
+					plannotatorBrowser.includes("/") &&
+					!plannotatorBrowser.endsWith(".app")
+				) {
+					// Script/executable path — run directly (open -a fails with -10811)
+					cmd = plannotatorBrowser;
+					args = [url];
+				} else {
+					cmd = "open";
+					args = ["-a", plannotatorBrowser, url];
+				}
 			} else if ((platform === "win32" || wsl) && plannotatorBrowser) {
 				cmd = "cmd.exe";
 				args = ["/c", "start", "", plannotatorBrowser, url];