diff --git a/CHANGELOG.md b/CHANGELOG.md
index 706edfa..15a100e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 ## Unreleased
 
+### New Features
+
+- **Style ID validation for `style_comparison_tool`**: `before` and `after` inputs are now validated to contain only alphanumeric characters, hyphens, and underscores (after stripping the optional `mapbox://styles/` prefix). Validation is enforced at both the Zod schema layer and inside `processStyleId()`. Malformed style IDs are rejected with a descriptive error before any URL is constructed.
+
 ## 0.8.1 - 2026-06-11
 
 ### Changed
diff --git a/src/tools/style-comparison-tool/StyleComparisonTool.schema.ts b/src/tools/style-comparison-tool/StyleComparisonTool.schema.ts
index ec25e25..1265c11 100644
--- a/src/tools/style-comparison-tool/StyleComparisonTool.schema.ts
+++ b/src/tools/style-comparison-tool/StyleComparisonTool.schema.ts
@@ -6,11 +6,19 @@ import { z } from 'zod';
 export const StyleComparisonSchema = z.object({
   before: z
     .string()
+    .regex(
+      /^(?:mapbox:\/\/styles\/)?[a-zA-Z0-9_-]+(\/[a-zA-Z0-9_-]+)?$/,
+      'Invalid style format. Use mapbox://styles/username/styleId, username/styleId, or a styleId containing only letters, numbers, hyphens, and underscores.'
+    )
     .describe(
       'Mapbox style for the "before" side. Accepts: full style URL (mapbox://styles/username/styleId), username/styleId format, or just styleId if using your own styles'
     ),
   after: z
     .string()
+    .regex(
+      /^(?:mapbox:\/\/styles\/)?[a-zA-Z0-9_-]+(\/[a-zA-Z0-9_-]+)?$/,
+      'Invalid style format. Use mapbox://styles/username/styleId, username/styleId, or a styleId containing only letters, numbers, hyphens, and underscores.'
+    )
     .describe(
       'Mapbox style for the "after" side. Accepts: full style URL (mapbox://styles/username/styleId), username/styleId format, or just styleId if using your own styles'
     ),
diff --git a/src/tools/style-comparison-tool/StyleComparisonTool.ts b/src/tools/style-comparison-tool/StyleComparisonTool.ts
index 82f7638..e916b5a 100644
--- a/src/tools/style-comparison-tool/StyleComparisonTool.ts
+++ b/src/tools/style-comparison-tool/StyleComparisonTool.ts
@@ -40,33 +40,49 @@ export class StyleComparisonTool extends BaseTool<
     super({ inputSchema: StyleComparisonSchema });
   }
 
+  /**
+   * Validates that a resolved username/styleId contains only safe characters.
+   * Style IDs must be alphanumeric with hyphens and underscores only.
+   */
+  private validateStyleId(resolved: string): void {
+    if (!/^[a-zA-Z0-9_-]+\/[a-zA-Z0-9_-]+$/.test(resolved)) {
+      throw new Error(
+        `Invalid style ID format: "${resolved}". ` +
+          `Style IDs must be in username/styleId format using only letters, numbers, hyphens, and underscores.`
+      );
+    }
+  }
+
   /**
    * Processes style input to extract username/styleId format
    */
   private processStyleId(style: string, accessToken: string): string {
+    let resolved: string;
+
     // If it's a full URL, extract the username/styleId part
     if (style.startsWith('mapbox://styles/')) {
-      return style.replace('mapbox://styles/', '');
-    }
-
-    // If it contains a slash, assume it's already username/styleId format
-    if (style.includes('/')) {
-      return style;
+      resolved = style.replace('mapbox://styles/', '');
+    } else if (style.includes('/')) {
+      // If it contains a slash, assume it's already username/styleId format
+      resolved = style;
+    } else {
+      // If it's just a style ID, try to get username from the token
+      try {
+        const username = getUserNameFromToken(accessToken);
+        resolved = `${username}/${style}`;
+      } catch (error) {
+        throw new Error(
+          `Could not determine username for style ID "${style}". ${error instanceof Error ? error.message : ''}\n` +
+            `Please provide either:\n` +
+            `1. Full style URL: mapbox://styles/username/${style}\n` +
+            `2. Username/styleId format: username/${style}\n` +
+            `3. Just the style ID with a valid Mapbox token that contains username information`
+        );
+      }
     }
 
-    // If it's just a style ID, try to get username from the token
-    try {
-      const username = getUserNameFromToken(accessToken);
-      return `${username}/${style}`;
-    } catch (error) {
-      throw new Error(
-        `Could not determine username for style ID "${style}". ${error instanceof Error ? error.message : ''}\n` +
-          `Please provide either:\n` +
-          `1. Full style URL: mapbox://styles/username/${style}\n` +
-          `2. Username/styleId format: username/${style}\n` +
-          `3. Just the style ID with a valid Mapbox token that contains username information`
-      );
-    }
+    this.validateStyleId(resolved);
+    return resolved;
   }
 
   protected async execute(
diff --git a/test/tools/style-comparison-tool/StyleComparisonTool.test.ts b/test/tools/style-comparison-tool/StyleComparisonTool.test.ts
index e715011..5024af7 100644
--- a/test/tools/style-comparison-tool/StyleComparisonTool.test.ts
+++ b/test/tools/style-comparison-tool/StyleComparisonTool.test.ts
@@ -130,6 +130,36 @@ describe('StyleComparisonTool', () => {
       ).toContain('Invalid token type');
     });
 
+    it('should reject style IDs with invalid characters', async () => {
+      const input = {
+        before: 'mapbox/streets-v12',
+        after: 'bad</code><img onerror=alert(1)>',
+        accessToken: 'pk.test.token'
+      };
+
+      const result = await tool.run(input);
+
+      expect(result.isError).toBe(true);
+      expect(
+        (result.content[0] as { type: 'text'; text: string }).text
+      ).toContain('Invalid style format');
+    });
+
+    it('should reject style URLs with invalid characters after stripping scheme', async () => {
+      const input = {
+        before: 'mapbox://styles/mapbox/streets-v12',
+        after: 'mapbox://styles/bad<user>/evil"style',
+        accessToken: 'pk.test.token'
+      };
+
+      const result = await tool.run(input);
+
+      expect(result.isError).toBe(true);
+      expect(
+        (result.content[0] as { type: 'text'; text: string }).text
+      ).toContain('Invalid style format');
+    });
+
     it('should return error for style ID without valid username in token', async () => {
       // Mock getUserNameFromToken to throw an error
       vi.spyOn(jwtUtils, 'getUserNameFromToken').mockImplementation(() => {