diff --git a/src/index.runtime.test.ts b/src/index.runtime.test.ts
index d4e245c..40de2b0 100644
--- a/src/index.runtime.test.ts
+++ b/src/index.runtime.test.ts
@@ -331,6 +331,81 @@ describe('OpenCodeRulesPlugin', () => {
     expect(result.system).toContain('Rule Content');
   });
 
+  it('should consolidate array system messages into single string', async () => {
+    const { testDir, globalRulesDir } = getTestDirs();
+    writeFileSync(path.join(globalRulesDir, 'rule.md'), '# My Rule');
+    process.env.XDG_CONFIG_HOME = path.join(testDir, '.config');
+
+    const {
+      default: { server: plugin },
+    } = await import('./index.js');
+    const mockInput = createMockPluginInput({ testDir });
+
+    const hooks = await plugin(
+      mockInput as unknown as Parameters<typeof plugin>[0]
+    );
+    const systemTransform = hooks['experimental.chat.system.transform'] as (
+      input: unknown,
+      output: { system: string[] }
+    ) => Promise<{ system: string }>;
+    const result = await systemTransform(
+      {},
+      { system: ['First message.', 'Second message.'] }
+    );
+
+    expect(typeof result.system).toBe('string');
+    expect(result.system).toContain('First message.');
+    expect(result.system).toContain('Second message.');
+    expect(result.system).toContain('My Rule');
+  });
+
+  it('should handle empty array system messages', async () => {
+    const { testDir, globalRulesDir } = getTestDirs();
+    writeFileSync(path.join(globalRulesDir, 'rule.md'), '# My Rule');
+    process.env.XDG_CONFIG_HOME = path.join(testDir, '.config');
+
+    const {
+      default: { server: plugin },
+    } = await import('./index.js');
+    const mockInput = createMockPluginInput({ testDir });
+
+    const hooks = await plugin(
+      mockInput as unknown as Parameters<typeof plugin>[0]
+    );
+    const systemTransform = hooks['experimental.chat.system.transform'] as (
+      input: unknown,
+      output: { system: string[] }
+    ) => Promise<{ system: string }>;
+    const result = await systemTransform({}, { system: [] });
+
+    expect(typeof result.system).toBe('string');
+    expect(result.system).toContain('My Rule');
+  });
+
+  it('should handle single-element array system message', async () => {
+    const { testDir, globalRulesDir } = getTestDirs();
+    writeFileSync(path.join(globalRulesDir, 'rule.md'), '# My Rule');
+    process.env.XDG_CONFIG_HOME = path.join(testDir, '.config');
+
+    const {
+      default: { server: plugin },
+    } = await import('./index.js');
+    const mockInput = createMockPluginInput({ testDir });
+
+    const hooks = await plugin(
+      mockInput as unknown as Parameters<typeof plugin>[0]
+    );
+    const systemTransform = hooks['experimental.chat.system.transform'] as (
+      input: unknown,
+      output: { system: string[] }
+    ) => Promise<{ system: string }>;
+    const result = await systemTransform({}, { system: ['Only message.'] });
+
+    expect(typeof result.system).toBe('string');
+    expect(result.system).toContain('Only message.');
+    expect(result.system).toContain('My Rule');
+  });
+
   it('should not modify messages in messages.transform hook', async () => {
     const { testDir, globalRulesDir } = getTestDirs();
     writeFileSync(path.join(globalRulesDir, 'rule.md'), '# Rule');
diff --git a/src/runtime.ts b/src/runtime.ts
index 27dab6c..0582d9e 100644
--- a/src/runtime.ts
+++ b/src/runtime.ts
@@ -37,7 +37,7 @@ interface SystemTransformInput {
 }
 
 interface SystemTransformOutput {
-  system?: string | string[];
+  system?: string;
 }
 
 interface OpenCodeRulesRuntimeOptions {
@@ -324,7 +324,10 @@ export class OpenCodeRulesRuntime {
     }
 
     if (Array.isArray(output.system)) {
-      output.system.push(combinedSystem);
+      output.system =
+        output.system.join('\n\n') +
+        (output.system.length > 0 ? '\n\n' : '') +
+        combinedSystem;
     } else {
       output.system = output.system
         ? `${output.system}\n\n${combinedSystem}`