fix(compiler): resolve entrypoint from exports["."]["typespec"] when tspMain is absent

.chronus/changes/copilot-add-support-for-exports-2026-6-28-13-56-41.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+`tsp compile .` now resolves the entrypoint from `exports["."]["typespec"]` in package.json, taking precedence over the legacy `tspMain` field

packages/compiler/src/core/entrypoint-resolution.ts

@@ -1,4 +1,7 @@
 import { loadTypeSpecConfigForPath } from "../config/config-loader.js";
+import { resolvePackageExports } from "../module-resolver/esm/resolve-package-exports.js";
+import { NoMatchingConditionsError } from "../module-resolver/esm/utils.js";
+import { fileURLToPath, pathToFileURL } from "../module-resolver/utils.js";
 import { doIO, loadFile } from "../utils/io.js";
 import { resolveTspMain } from "../utils/misc.js";
 import { DiagnosticHandler } from "./diagnostics.js";
@@ -39,12 +42,40 @@ export async function resolveTypeSpecEntrypointForDir(
     return resolvePath(dir, config.entrypoint);
   }
 
-  // Otherwise fall back to the `tspMain` declared in package.json. A `tspconfig.yaml`
-  // without an explicit `entrypoint` must not override `tspMain`.
   const pkgJsonPath = resolvePath(dir, "package.json");
   const [pkg] = await loadFile(host, pkgJsonPath, JSON.parse, reportDiagnostic, {
     allowFileNotFound: true,
   });
+
+  // Try exports["."]["typespec"] first using the existing ESM package exports resolver.
+  if (pkg?.exports) {
+    try {
+      const match = await resolvePackageExports(
+        {
+          packageUrl: pathToFileURL(dir),
+          specifier: ".",
+          moduleDirs: ["node_modules"],
+          conditions: ["typespec"],
+          ignoreDefaultCondition: true,
+          resolveId: () => {
+            throw new Error("not supported");
+          },
+        },
+        ".",
+        pkg.exports,
+      );
+      if (match) {
+        return fileURLToPath(match);
+      }
+    } catch (e) {
+      if (!(e instanceof NoMatchingConditionsError)) {
+        throw e;
+      }
+      // No matching typespec condition — fall through to tspMain / main.tsp
+    }
+  }
+
+  // Fall back to the legacy `tspMain` declared in package.json.
   const tspMain = resolveTspMain(pkg);
   if (tspMain !== undefined) {
     return resolvePath(dir, tspMain);

packages/compiler/src/utils/misc.ts

@@ -219,9 +219,8 @@ export function omitUndefined<T extends Record<string, unknown>>(data: T): T {
 }
 
 /**
- * Extract package.json's tspMain entry point in a given path.
- * @param path Path that contains package.json
- * @param reportDiagnostic optional diagnostic handler.
+ * Extract package.json's tspMain entry point.
+ * @param packageJson Parsed package.json object.
  */
 export function resolveTspMain(packageJson: any): string | undefined {
   if (packageJson?.tspMain !== undefined) {

packages/compiler/test/core/entrypoint-resolution.test.ts

@@ -43,3 +43,41 @@ it("defaults to main.tsp when neither tspconfig entrypoint nor package.json tspM
   const entrypoint = await resolveTypeSpecEntrypointForDir(host, dir, () => {});
   expect(entrypoint).toBe(resolveVirtualPath("my-lib/main.tsp"));
 });
+
+it("uses package.json exports['.']['typespec'] when there is no tspMain", async () => {
+  fs.add(
+    "my-lib/package.json",
+    JSON.stringify({
+      name: "my-lib",
+      exports: { ".": { typespec: "./lib/main.tsp", default: "./dist/index.js" } },
+    }),
+  );
+  const entrypoint = await resolveTypeSpecEntrypointForDir(host, dir, () => {});
+  expect(entrypoint).toBe(resolveVirtualPath("my-lib/lib/main.tsp"));
+});
+
+it("exports['.']['typespec'] takes precedence over tspMain", async () => {
+  fs.add(
+    "my-lib/package.json",
+    JSON.stringify({
+      name: "my-lib",
+      tspMain: "lib/main.tsp",
+      exports: { ".": { typespec: "./lib/other.tsp", default: "./dist/index.js" } },
+    }),
+  );
+  const entrypoint = await resolveTypeSpecEntrypointForDir(host, dir, () => {});
+  expect(entrypoint).toBe(resolveVirtualPath("my-lib/lib/other.tsp"));
+});
+
+it("an explicit entrypoint in tspconfig.yaml takes precedence over exports['.']['typespec']", async () => {
+  fs.add(
+    "my-lib/package.json",
+    JSON.stringify({
+      name: "my-lib",
+      exports: { ".": { typespec: "./lib/main.tsp", default: "./dist/index.js" } },
+    }),
+  );
+  fs.add("my-lib/tspconfig.yaml", `kind: project\nentrypoint: other.tsp`);
+  const entrypoint = await resolveTypeSpecEntrypointForDir(host, dir, () => {});
+  expect(entrypoint).toBe(resolveVirtualPath("my-lib/other.tsp"));
+});