diff --git a/package-lock.json b/package-lock.json
index 7068f9e7a..d5c031163 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1727,7 +1727,6 @@
       "integrity": "sha512-qzQZRBqkFsYyaSWXuEHc2WR9c0a0CXwiE5FWUvn7ZM+vdy1uZLfCunD38UzhuB7YN/J11ndbDBcTmOdxJo9Q7A==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "undici-types": "~7.16.0"
       }
@@ -3575,7 +3574,6 @@
       "resolved": "https://registry.npmjs.org/jiti/-/jiti-2.6.1.tgz",
       "integrity": "sha512-ekilCSN1jwRvIbgeg/57YFh8qQDNbwDb9xT/qu2DAHbFFZUicIl4ygVaAvzveMhMVr3LnpSKTNnwt8PoOfmKhQ==",
       "license": "MIT",
-      "peer": true,
       "bin": {
         "jiti": "lib/jiti-cli.mjs"
       }
@@ -5465,7 +5463,6 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -5529,7 +5526,6 @@
       "integrity": "sha512-ytQKuwgmrrkDTFP4LjR0ToE2nqgy886GpvRSpU0JAnrdBYppuY5rLkRUYPU1yCryb24SsKBTL/hlDQAEFVwtZg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "esbuild": "~0.25.0",
         "get-tsconfig": "^4.7.5"
@@ -5672,7 +5668,6 @@
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       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "esbuild": "^0.25.0",
         "fdir": "^6.5.0",
@@ -5789,7 +5784,6 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -5803,7 +5797,6 @@
       "integrity": "sha512-LUCP5ev3GURDysTWiP47wRRUpLKMOfPh+yKTx3kVIEiu5KOMeqzpnYNsKyOoVrULivR8tLcks4+lga33Whn90A==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@types/chai": "^5.2.2",
         "@vitest/expect": "3.2.4",
@@ -6059,7 +6052,6 @@
       "resolved": "https://registry.npmjs.org/zod/-/zod-4.1.12.tgz",
       "integrity": "sha512-JInaHOamG8pt5+Ey8kGmdcAcg3OL9reK8ltczgHTAwNhMys/6ThXHityHxVV2p3fkw/c+MAvBHFVYHFZDmjMCQ==",
       "license": "MIT",
-      "peer": true,
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
       }
diff --git a/src/config/configLoad.ts b/src/config/configLoad.ts
index 1f6066ae1..2fa0d36b7 100644
--- a/src/config/configLoad.ts
+++ b/src/config/configLoad.ts
@@ -12,6 +12,7 @@ import {
   type RepomixConfigCli,
   type RepomixConfigFile,
   type RepomixConfigMerged,
+  type RepomixOutputStyle,
   repomixConfigFileSchema,
   repomixConfigMergedSchema,
 } from './configSchema.js';
@@ -167,6 +168,14 @@ const loadAndValidateConfig = async (
   }
 };
 
+// Mapping of output styles to their file extensions
+const styleToExtensionMap: Record<RepomixOutputStyle, string> = {
+  xml: '.xml',
+  markdown: '.md',
+  plain: '.txt',
+  json: '.json',
+} as const;
+
 export const mergeConfigs = (
   cwd: string,
   fileConfig: RepomixConfigFile,
@@ -205,6 +214,15 @@ export const mergeConfigs = (
           mergedOutput.filePath = desiredPath;
           logger.trace('Adjusted output file path to match style:', mergedOutput.filePath);
         }
+      } else {
+        // If filePath is explicitly set, check if it has an extension
+        const currentExtension = path.extname(mergedOutput.filePath);
+        if (!currentExtension) {
+          // No extension found, add the appropriate extension based on style
+          const extensionToAdd = styleToExtensionMap[style];
+          mergedOutput.filePath = `${mergedOutput.filePath}${extensionToAdd}`;
+          logger.trace('Added file extension to output path:', mergedOutput.filePath);
+        }
       }
 
       return mergedOutput;
diff --git a/tests/config/configLoad.test.ts b/tests/config/configLoad.test.ts
index d864ab096..14f04af6e 100644
--- a/tests/config/configLoad.test.ts
+++ b/tests/config/configLoad.test.ts
@@ -329,5 +329,65 @@ describe('configLoad', () => {
       expect(merged.output.filePath).toBe('repomix-output.txt');
       expect(merged.output.style).toBe('plain');
     });
+
+    test('should add extension when CLI filePath has no extension (default style)', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'myoutput' } });
+      expect(merged.output.filePath).toBe('myoutput.xml');
+      expect(merged.output.style).toBe('xml');
+    });
+
+    test('should add extension when CLI filePath has no extension (markdown style)', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'myoutput', style: 'markdown' } });
+      expect(merged.output.filePath).toBe('myoutput.md');
+      expect(merged.output.style).toBe('markdown');
+    });
+
+    test('should add extension when CLI filePath has no extension (plain style)', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'myoutput', style: 'plain' } });
+      expect(merged.output.filePath).toBe('myoutput.txt');
+      expect(merged.output.style).toBe('plain');
+    });
+
+    test('should add extension when CLI filePath has no extension (json style)', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'myoutput', style: 'json' } });
+      expect(merged.output.filePath).toBe('myoutput.json');
+      expect(merged.output.style).toBe('json');
+    });
+
+    test('should keep extension when CLI filePath already has extension', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'myoutput.txt', style: 'markdown' } });
+      expect(merged.output.filePath).toBe('myoutput.txt');
+      expect(merged.output.style).toBe('markdown');
+    });
+
+    test('should add extension when file config filePath has no extension', () => {
+      const merged = mergeConfigs(process.cwd(), { output: { filePath: 'myoutput', style: 'markdown' } }, {});
+      expect(merged.output.filePath).toBe('myoutput.md');
+      expect(merged.output.style).toBe('markdown');
+    });
+
+    test('should keep extension when file config filePath already has extension', () => {
+      const merged = mergeConfigs(process.cwd(), { output: { filePath: 'myoutput.custom', style: 'markdown' } }, {});
+      expect(merged.output.filePath).toBe('myoutput.custom');
+      expect(merged.output.style).toBe('markdown');
+    });
+
+    test('should add extension for paths with directories when no extension', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'output/myfile', style: 'json' } });
+      expect(merged.output.filePath).toBe('output/myfile.json');
+      expect(merged.output.style).toBe('json');
+    });
+
+    test('should keep extension for paths with directories when extension exists', () => {
+      const merged = mergeConfigs(process.cwd(), {}, { output: { filePath: 'output/myfile.txt', style: 'json' } });
+      expect(merged.output.filePath).toBe('output/myfile.txt');
+      expect(merged.output.style).toBe('json');
+    });
+
+    test('should add extension when style is changed via CLI but filePath has no extension', () => {
+      const merged = mergeConfigs(process.cwd(), { output: { filePath: 'myfile' } }, { output: { style: 'markdown' } });
+      expect(merged.output.filePath).toBe('myfile.md');
+      expect(merged.output.style).toBe('markdown');
+    });
   });
 });