fix(subagents): run bundled JS launcher via runtime on Windows (#975)

Co-authored-by: Letta <noreply@letta.com>

src/tests/agent/subagent-model-resolution.test.ts

@@ -1,5 +1,122 @@
 import { describe, expect, test } from "bun:test";
-import { resolveSubagentModel } from "../../agent/subagents/manager";
+import {
+  resolveSubagentLauncher,
+  resolveSubagentModel,
+} from "../../agent/subagents/manager";
+
+describe("resolveSubagentLauncher", () => {
+  test("explicit launcher takes precedence over .ts script autodetection", () => {
+    const launcher = resolveSubagentLauncher(["-p", "hi"], {
+      env: {
+        LETTA_CODE_BIN: "custom-bun",
+        LETTA_CODE_BIN_ARGS_JSON: JSON.stringify(["run", "src/index.ts"]),
+      } as NodeJS.ProcessEnv,
+      argv: ["bun", "/tmp/dev-entry.ts"],
+      execPath: "/opt/homebrew/bin/bun",
+      platform: "darwin",
+    });
+
+    expect(launcher).toEqual({
+      command: "custom-bun",
+      args: ["run", "src/index.ts", "-p", "hi"],
+    });
+  });
+
+  test("explicit launcher takes precedence over .js script autodetection", () => {
+    const launcher = resolveSubagentLauncher(["-p", "hi"], {
+      env: {
+        LETTA_CODE_BIN: "custom-node",
+      } as NodeJS.ProcessEnv,
+      argv: ["node", "/tmp/letta.js"],
+      execPath: "/usr/local/bin/node",
+      platform: "win32",
+    });
+
+    expect(launcher).toEqual({
+      command: "custom-node",
+      args: ["-p", "hi"],
+    });
+  });
+
+  test("preserves existing .ts dev behavior for any ts entrypoint", () => {
+    const launcher = resolveSubagentLauncher(
+      ["--output-format", "stream-json"],
+      {
+        env: {} as NodeJS.ProcessEnv,
+        argv: ["bun", "/tmp/custom-runner.ts"],
+        execPath: "/opt/homebrew/bin/bun",
+        platform: "darwin",
+      },
+    );
+
+    expect(launcher).toEqual({
+      command: "/opt/homebrew/bin/bun",
+      args: ["/tmp/custom-runner.ts", "--output-format", "stream-json"],
+    });
+  });
+
+  test("uses node runtime for bundled js on win32", () => {
+    const launcher = resolveSubagentLauncher(["-p", "prompt"], {
+      env: {} as NodeJS.ProcessEnv,
+      argv: ["node", "C:\\Program Files\\Letta\\letta.js"],
+      execPath: "C:\\Program Files\\nodejs\\node.exe",
+      platform: "win32",
+    });
+
+    expect(launcher).toEqual({
+      command: "C:\\Program Files\\nodejs\\node.exe",
+      args: ["C:\\Program Files\\Letta\\letta.js", "-p", "prompt"],
+    });
+  });
+
+  test("keeps direct js spawn behavior on non-win32", () => {
+    const launcher = resolveSubagentLauncher(["-p", "prompt"], {
+      env: {} as NodeJS.ProcessEnv,
+      argv: ["node", "/usr/local/lib/letta.js"],
+      execPath: "/usr/local/bin/node",
+      platform: "linux",
+    });
+
+    expect(launcher).toEqual({
+      command: "/usr/local/lib/letta.js",
+      args: ["-p", "prompt"],
+    });
+  });
+
+  test("falls back to global letta when no launcher hints available", () => {
+    const launcher = resolveSubagentLauncher(["-p", "prompt"], {
+      env: {} as NodeJS.ProcessEnv,
+      argv: ["node", ""],
+      execPath: "/usr/local/bin/node",
+      platform: "linux",
+    });
+
+    expect(launcher).toEqual({
+      command: "letta",
+      args: ["-p", "prompt"],
+    });
+  });
+
+  test("keeps explicit launcher with spaces as a single command token", () => {
+    const launcher = resolveSubagentLauncher(
+      ["--output-format", "stream-json"],
+      {
+        env: {
+          LETTA_CODE_BIN:
+            '"C:\\Users\\Example User\\AppData\\Roaming\\npm\\letta.cmd"',
+        } as NodeJS.ProcessEnv,
+        argv: ["node", "C:\\Program Files\\Letta\\letta.js"],
+        execPath: "C:\\Program Files\\nodejs\\node.exe",
+        platform: "win32",
+      },
+    );
+
+    expect(launcher).toEqual({
+      command: "C:\\Users\\Example User\\AppData\\Roaming\\npm\\letta.cmd",
+      args: ["--output-format", "stream-json"],
+    });
+  });
+});
 
 describe("resolveSubagentModel", () => {
   test("prefers BYOK-swapped handle when available", async () => {