fix(mcp): tolerate synthetic entrypoint paths during import

Treat an unresolvable argv entry path as an imported module instead of throwing during entrypoint detection. Keep canonical direct-entry comparison private.

Validation: real child imports compiled runtime with a nonexistent argv path and no CLI side effects; all six runtime-error tests pass.

Based on #3162.

Co-authored-by: qorvr <330186599+qorvr@users.noreply.github.com>

src/main.ts

@@ -109,10 +109,20 @@ export const runEntrypoint = async (
   }
 };
 
+/** Return false when the runtime's argv path is synthetic or no longer exists. */
+const isDirectEntrypoint = (
+  entryPath: string | undefined,
+  moduleUrl: string,
+): boolean => {
+  if (entryPath === undefined) return false;
+  try {
+    return pathToFileURL(realpathSync(entryPath)).href === moduleUrl;
+  } catch {
+    return false;
+  }
+};
+
 const entryPath = process.argv[1];
-if (
-  entryPath !== undefined &&
-  pathToFileURL(realpathSync(entryPath)).href === import.meta.url
-) {
+if (isDirectEntrypoint(entryPath, import.meta.url)) {
   void runEntrypoint();
 }

tests/boundary/mcp/mainErrors.test.ts

@@ -1,4 +1,7 @@
 import type { ServeStdioOptions } from "@modelcontextprotocol/server/stdio";
+import { spawnSync } from "node:child_process";
+import { resolve } from "node:path";
+import { pathToFileURL } from "node:url";
 import { describe, expect, it } from "vitest";
 
 import { run, runEntrypoint } from "../../../src/main.js";
@@ -35,6 +38,25 @@ const dependencies = ({
 });
 
 describe("MCP runtime errors", () => {
+  it("imports the compiled runtime when argv[1] is a missing synthetic path", () => {
+    const runtimeUrl = pathToFileURL(resolve("dist/main.js")).href;
+    const script = [
+      'process.argv[1] = "/rea-test-missing-entrypoint/main.js";',
+      `await import(${JSON.stringify(runtimeUrl)});`,
+      'console.log("IMPORT_OK");',
+    ].join("\n");
+    const child = spawnSync(
+      process.execPath,
+      ["--input-type=module", "--eval", script],
+      { cwd: process.cwd(), encoding: "utf8", timeout: 5_000 },
+    );
+
+    expect(child.error).toBeUndefined();
+    expect(child.status).toBe(0);
+    expect(child.stdout).toBe("IMPORT_OK\n");
+    expect(child.stderr).toBe("");
+  });
+
   it("reports connection loss without the transport cause", async () => {
     const output: string[] = [];
     const shutdown: Array<() => void> = [];