fix: include stdout diagnostics when stderr is empty on swift package failure

SPM writes compiler diagnostics to stdout, not stderr. The ?? operator
does not treat empty string as falsy, so when stderr is "" the diagnostics
in stdout were never included in the error response. Switching to ||
fixes the fallback for all three affected tools.

Fixes #243

Author: doovers

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed `swift_package_build`, `swift_package_test`, and `swift_package_clean` swallowing compiler diagnostics on failure by treating empty stderr as falsy, so stdout diagnostics are included in the error response ([#243](https://github.com/getsentry/XcodeBuildMCP/issues/243)).
+
 ## [2.1.0]
 
 ### Added

src/mcp/tools/swift-package/__tests__/swift_package_build.test.ts

@@ -228,6 +228,32 @@ describe('swift_package_build plugin', () => {
       });
     });
 
+    it('should include stdout diagnostics when stderr is empty on build failure', async () => {
+      const executor = createMockExecutor({
+        success: false,
+        error: '',
+        output:
+          "main.swift:10:25: error: cannot find type 'DOESNOTEXIST' in scope\nlet broken: DOESNOTEXIST = 42",
+      });
+
+      const result = await swift_package_buildLogic(
+        {
+          packagePath: '/test/package',
+        },
+        executor,
+      );
+
+      expect(result).toEqual({
+        content: [
+          {
+            type: 'text',
+            text: "Error: Swift package build failed\nDetails: main.swift:10:25: error: cannot find type 'DOESNOTEXIST' in scope\nlet broken: DOESNOTEXIST = 42",
+          },
+        ],
+        isError: true,
+      });
+    });
+
     it('should handle spawn error', async () => {
       const executor = async () => {
         throw new Error('spawn ENOENT');

src/mcp/tools/swift-package/__tests__/swift_package_test.test.ts

@@ -218,6 +218,32 @@ describe('swift_package_test plugin', () => {
       });
     });
 
+    it('should include stdout diagnostics when stderr is empty on test failure', async () => {
+      const mockExecutor = createMockExecutor({
+        success: false,
+        error: '',
+        output:
+          "main.swift:10:25: error: cannot find type 'DOESNOTEXIST' in scope\nlet broken: DOESNOTEXIST = 42",
+      });
+
+      const result = await swift_package_testLogic(
+        {
+          packagePath: '/test/package',
+        },
+        mockExecutor,
+      );
+
+      expect(result).toEqual({
+        content: [
+          {
+            type: 'text',
+            text: "Error: Swift package tests failed\nDetails: main.swift:10:25: error: cannot find type 'DOESNOTEXIST' in scope\nlet broken: DOESNOTEXIST = 42",
+          },
+        ],
+        isError: true,
+      });
+    });
+
     it('should handle spawn error', async () => {
       const mockExecutor = async () => {
         throw new Error('spawn ENOENT');

src/mcp/tools/swift-package/swift_package_build.ts

@@ -57,7 +57,7 @@ export async function swift_package_buildLogic(
   try {
     const result = await executor(['swift', ...swiftArgs], 'Swift Package Build', false, undefined);
     if (!result.success) {
-      const errorMessage = result.error ?? result.output ?? 'Unknown error';
+      const errorMessage = result.error || result.output || 'Unknown error';
       return createErrorResponse('Swift package build failed', errorMessage);
     }
 

src/mcp/tools/swift-package/swift_package_clean.ts

@@ -26,7 +26,7 @@ export async function swift_package_cleanLogic(
   try {
     const result = await executor(['swift', ...swiftArgs], 'Swift Package Clean', false, undefined);
     if (!result.success) {
-      const errorMessage = result.error ?? result.output ?? 'Unknown error';
+      const errorMessage = result.error || result.output || 'Unknown error';
       return createErrorResponse('Swift package clean failed', errorMessage);
     }
 

src/mcp/tools/swift-package/swift_package_test.ts

@@ -67,7 +67,7 @@ export async function swift_package_testLogic(
   try {
     const result = await executor(['swift', ...swiftArgs], 'Swift Package Test', false, undefined);
     if (!result.success) {
-      const errorMessage = result.error ?? result.output ?? 'Unknown error';
+      const errorMessage = result.error || result.output || 'Unknown error';
       return createErrorResponse('Swift package tests failed', errorMessage);
     }