test(mcp-server): add regression tests for importLocalModule candidate resolution

Extracts _buildImportCandidates() as a testable export and adds 5 tests
verifying src/<->dist/ path swapping and .ts extension fallback logic.

Refs #3954

packages/mcp-server/src/import-candidates.test.ts

@@ -0,0 +1,48 @@
+// GSD-2 — Regression tests for importLocalModule candidate resolution (#3954)
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+
+import { _buildImportCandidates } from "./workflow-tools.js";
+
+describe("_buildImportCandidates", () => {
+  it("includes dist/ fallback for src/ paths", () => {
+    const candidates = _buildImportCandidates("../../../src/resources/extensions/gsd/db-writer.js");
+    assert.ok(
+      candidates.some((c) => c.includes("/dist/resources/extensions/gsd/db-writer.js")),
+      "should include dist/ swapped candidate",
+    );
+  });
+
+  it("includes src/ fallback for dist/ paths", () => {
+    const candidates = _buildImportCandidates("../../../dist/resources/extensions/gsd/db-writer.js");
+    assert.ok(
+      candidates.some((c) => c.includes("/src/resources/extensions/gsd/db-writer.js")),
+      "should include src/ swapped candidate",
+    );
+  });
+
+  it("includes .ts variants for .js paths", () => {
+    const candidates = _buildImportCandidates("../../../src/resources/extensions/gsd/db-writer.js");
+    assert.ok(
+      candidates.some((c) => c.endsWith("db-writer.ts") && c.includes("/src/")),
+      "should include .ts variant for original src/ path",
+    );
+    assert.ok(
+      candidates.some((c) => c.endsWith("db-writer.ts") && c.includes("/dist/")),
+      "should include .ts variant for swapped dist/ path",
+    );
+  });
+
+  it("returns original path first", () => {
+    const input = "../../../src/resources/extensions/gsd/db-writer.js";
+    const candidates = _buildImportCandidates(input);
+    assert.equal(candidates[0], input, "first candidate should be the original path");
+  });
+
+  it("handles paths without src/ or dist/ gracefully", () => {
+    const candidates = _buildImportCandidates("./local-module.js");
+    assert.equal(candidates.length, 2, "should have original + .ts variant only");
+    assert.equal(candidates[0], "./local-module.js");
+    assert.equal(candidates[1], "./local-module.ts");
+  });
+});

packages/mcp-server/src/workflow-tools.ts

@@ -337,24 +337,29 @@ function toFileUrl(modulePath: string): string {
   return pathToFileURL(resolve(modulePath)).href;
 }
 
-async function importLocalModule<T>(relativePath: string): Promise<T> {
+/** @internal — exported for testing only */
+export function _buildImportCandidates(relativePath: string): string[] {
   // Build candidate paths: try the given path first, then swap src/<->dist/
   // and try .ts extension. This handles both dev (tsx from src/) and prod
   // (compiled from dist/) execution contexts.
-  const candidates: string[] = [
-    new URL(relativePath, import.meta.url).href,
-  ];
+  const candidates: string[] = [relativePath];
   const swapped = relativePath.includes("/src/")
     ? relativePath.replace("/src/", "/dist/")
     : relativePath.includes("/dist/")
       ? relativePath.replace("/dist/", "/src/")
       : null;
-  if (swapped) candidates.push(new URL(swapped, import.meta.url).href);
+  if (swapped) candidates.push(swapped);
   // Also try .ts variants for dev-mode tsx execution
   if (relativePath.endsWith(".js")) {
-    candidates.push(new URL(relativePath.replace(/\.js$/, ".ts"), import.meta.url).href);
-    if (swapped) candidates.push(new URL(swapped.replace(/\.js$/, ".ts"), import.meta.url).href);
+    candidates.push(relativePath.replace(/\.js$/, ".ts"));
+    if (swapped) candidates.push(swapped.replace(/\.js$/, ".ts"));
   }
+  return candidates;
+}
+
+async function importLocalModule<T>(relativePath: string): Promise<T> {
+  const candidates = _buildImportCandidates(relativePath)
+    .map((p) => new URL(p, import.meta.url).href);
 
   let lastErr: unknown;
   for (const candidate of candidates) {