[DO NOT MERGE]feat(compiler): prefer client.tsp over main.tsp when no entrypoint configured

.chronus/changes/prefer-client-tsp-entrypoint-2026-6-10-15-15-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Default entrypoint resolution now prefers `client.tsp` over `main.tsp` when a project's `tspconfig.yaml` does not explicitly set `entrypoint` and a sibling `client.tsp` exists. This matches the convention used by `tsp-client` and allows augment decorators in `client.tsp` (e.g. `@@clientName`) to participate in compilation and linting without needing to add `imports: - ./client.tsp` to `tspconfig.yaml`.

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

@@ -36,7 +36,7 @@ export async function resolveTypeSpecEntrypointForDir(
   // Check for project tspconfig first
   const config = await loadTypeSpecConfigForPath(host, dir, false, false);
   if (config.kind === "project") {
-    const entrypoint = config.entrypoint ?? "main.tsp";
+    const entrypoint = config.entrypoint ?? (await resolveDefaultEntrypoint(host, dir));
     return resolvePath(dir, entrypoint);
   }
 
@@ -49,5 +49,11 @@ export async function resolveTypeSpecEntrypointForDir(
     return resolvePath(dir, tspMain);
   }
 
-  return resolvePath(dir, "main.tsp");
+  return resolvePath(dir, await resolveDefaultEntrypoint(host, dir));
+}
+
+async function resolveDefaultEntrypoint(host: CompilerHost, dir: string): Promise<string> {
+  const clientTspPath = resolvePath(dir, "client.tsp");
+  const stat = await doIO(host.stat, clientTspPath, () => {}, { allowFileNotFound: true });
+  return stat?.isFile() ? "client.tsp" : "main.tsp";
 }

packages/compiler/src/server/entrypoint-resolver.ts

@@ -24,14 +24,15 @@ export async function resolveEntrypointFile(
   let dir = isFilePath ? getDirectoryPath(path) : path;
 
   if (!entrypoints) {
-    entrypoints = ["main.tsp"];
+    entrypoints = ["client.tsp", "main.tsp"];
   }
 
   while (true) {
     // Check for project tspconfig first (highest priority)
     const config = await loadTypeSpecConfigForPath(host, dir, false, false);
     if (config.kind === "project") {
-      const entrypoint = config.entrypoint ?? "main.tsp";
+      const entrypoint =
+        config.entrypoint ?? ((await existingFile(dir, "client.tsp")) ? "client.tsp" : "main.tsp");
       const candidate = await existingFile(dir, entrypoint);
       logDebug({
         level: "debug",

packages/compiler/test/server/entrypoint-resolver.test.ts

@@ -70,6 +70,13 @@ describe("entrypoint resolution", () => {
     const resultForUndefined = await resolveEntrypoint(files, "project/src/file.tsp", undefined);
     expect(resultForUndefined).toBe(resolveVirtualPath("project/main.tsp"));
   });
+
+  it("prefers client.tsp over main.tsp in default fallback when both exist", async () => {
+    const files = { "project/main.tsp": "", "project/client.tsp": "" };
+
+    const result = await resolveEntrypoint(files, "project/src/file.tsp");
+    expect(result).toBe(resolveVirtualPath("project/client.tsp"));
+  });
 });
 
 describe("project tspconfig entrypoint resolution", () => {
@@ -106,6 +113,18 @@ describe("project tspconfig entrypoint resolution", () => {
     expect(result).toBe(resolveVirtualPath("project/main.tsp"));
   });
 
+  it("defaults to client.tsp when kind is project, no entrypoint specified, and client.tsp exists", async () => {
+    const result = await resolveEntrypoint(
+      {
+        "project/tspconfig.yaml": "kind: project\n",
+        "project/main.tsp": "",
+        "project/client.tsp": "",
+      },
+      "project/src/doc.tsp",
+    );
+    expect(result).toBe(resolveVirtualPath("project/client.tsp"));
+  });
+
   it("project tspconfig stops the walk even if entrypoint file doesn't exist", async () => {
     const result = await resolveEntrypoint(
       {