fix(session): select the registered entrypoint as the running runtime does

Node resolves its script entrypoint lexically, so a registered spelling that
crosses a symlink before `..` names the running file lexically, while Bun
follows native traversal (#8844). Select the expected server path with the
shared runtime-aware entrypoint normalizer before comparing it with the
running entrypoint, and fall back to file identity only when they differ.

Co-Authored-By: gensokyo-dev <342580529+gensokyo-dev@users.noreply.github.com>

src/serverIdentity.ts

@@ -2,6 +2,12 @@ import { statSync } from "node:fs";
 import { dirname, basename, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 
+import {
+  javaScriptRuntime,
+  type JavaScriptRuntime,
+} from "./domain/runtimeVersion.js";
+import { normalizeEntrypointPath } from "./process/CurrentEntrypointPath.js";
+
 import type { createCatalogIdentity } from "./catalogIdentity.js";
 import { PRODUCT_IDENTITY, SDK_IDENTITY } from "./identity.js";
 
@@ -28,12 +34,15 @@ const packageRoot = (): string => {
 const compareServerPaths = (
   expected: string,
   observed: string,
+  runtime: JavaScriptRuntime["id"],
 ): "aligned" | "mismatch" | "unknown" => {
-  if (expected === observed) return "aligned";
-  if (process.platform === "win32")
-    return resolve(expected) === observed ? "aligned" : "mismatch";
+  // Select the registered script the way this runtime does: Node resolves it
+  // lexically, while Bun follows native traversal through a symlink before `..`.
+  const script = normalizeEntrypointPath(expected, runtime) ?? expected;
+  if (script === observed) return "aligned";
+  if (process.platform === "win32") return "mismatch";
   try {
-    const selected = statSync(expected, { bigint: true });
+    const selected = statSync(script, { bigint: true });
     const running = statSync(observed, { bigint: true });
     return selected.dev === running.dev && selected.ino === running.ino
       ? "aligned"
@@ -51,13 +60,19 @@ export const createServerIdentity = (input: {
   readonly client?: ConnectedClientIdentity;
   readonly protocolVersion?: string | undefined;
   readonly catalog: ReturnType<typeof createCatalogIdentity>;
+  /** The runtime that selected `commandPath`; defaults to the current one. */
+  readonly runtime?: JavaScriptRuntime["id"];
 }) => {
   const catalog = input.catalog;
   const serverPath = input.commandPath || undefined;
   const pathComparison =
     input.expected?.server_path === undefined || serverPath === undefined
       ? "unknown"
-      : compareServerPaths(input.expected.server_path, serverPath);
+      : compareServerPaths(
+          input.expected.server_path,
+          serverPath,
+          input.runtime ?? javaScriptRuntime(process.versions).id,
+        );
   const reasons: string[] = [];
   if (
     input.expected?.package_version !== undefined &&

tests/boundary/mcp/serverIdentity.test.ts

@@ -259,12 +259,14 @@ it.skipIf(process.platform === "win32")(
     await writeFile(other, "other entrypoint");
     await symlink(scripts, join(scratch, "link"));
     const selected = `${scratch}/link/../scripts/rea.mjs`;
+    // Bun selects a script through native traversal (#8844).
     const identity = (server_path: string) =>
       createServerIdentity({
         catalog: DEFAULT_CATALOG_IDENTITY,
         startedAt: "2026-07-13T00:00:00.000Z",
         commandPath: canonical,
         expected: { server_path },
+        runtime: "bun",
       });
 
     expect(identity(selected).alignment).toMatchObject({
@@ -279,6 +281,25 @@ it.skipIf(process.platform === "win32")(
       state: "unknown",
       reasons: [],
     });
+    // Node resolves its entrypoint lexically, so this spelling runs
+    // scratch/rea.mjs even though traversal through the link leaves scratch.
+    // Bun traverses the link and runs a different file.
+    const lexical = join(scratch, "rea.mjs");
+    await writeFile(lexical, "lexical entrypoint");
+    await writeFile(join(root, "package", "rea.mjs"), "traversed entrypoint");
+    const crossing = (runtime: "node" | "bun") =>
+      createServerIdentity({
+        catalog: DEFAULT_CATALOG_IDENTITY,
+        startedAt: "2026-07-13T00:00:00.000Z",
+        commandPath: lexical,
+        expected: { server_path: `${scratch}/link/../rea.mjs` },
+        runtime,
+      }).alignment;
+    expect(crossing("node")).toMatchObject({ state: "aligned", reasons: [] });
+    expect(crossing("bun")).toMatchObject({
+      state: "mcp_server_restart_required",
+      reasons: ["registration_path_mismatch"],
+    });
   },
 );