diff --git a/src/lib/types/fileTypes.ts b/src/lib/types/fileTypes.ts
index fd8491354..287656f55 100644
--- a/src/lib/types/fileTypes.ts
+++ b/src/lib/types/fileTypes.ts
@@ -57,6 +57,7 @@ export type FileProcessingResult = {
     confidence: number;
     size?: number;
     filename?: string;
+    extension?: string | null; // Original file extension (e.g., 'csv', 'tsv', 'txt')
     // CSV-specific metadata
     rowCount?: number;
     columnCount?: number;
@@ -99,6 +100,7 @@ export type CSVProcessorOptions = {
   formatStyle?: "raw" | "markdown" | "json";
   includeHeaders?: boolean;
   sampleDataFormat?: SampleDataFormat;
+  extension?: string | null;
 };
 
 /**
diff --git a/src/lib/utils/csvProcessor.ts b/src/lib/utils/csvProcessor.ts
index 7a5e30dc3..72fa35eda 100644
--- a/src/lib/utils/csvProcessor.ts
+++ b/src/lib/utils/csvProcessor.ts
@@ -84,6 +84,7 @@ export class CSVProcessor {
       formatStyle = "raw",
       includeHeaders = true,
       sampleDataFormat = "json",
+      extension = null,
     } = options || {};
 
     const maxRows = Math.max(1, Math.min(10000, rawMaxRows));
@@ -153,6 +154,7 @@ export class CSVProcessor {
           size: content.length,
           rowCount,
           columnCount: (limitedLines[0] || "").split(",").length,
+          extension,
         },
       };
     }
@@ -219,6 +221,7 @@ export class CSVProcessor {
         columnNames,
         sampleData,
         hasEmptyColumns,
+        extension,
       },
     };
   }
diff --git a/src/lib/utils/fileDetector.ts b/src/lib/utils/fileDetector.ts
index c0d93af0f..faf183f74 100644
--- a/src/lib/utils/fileDetector.ts
+++ b/src/lib/utils/fileDetector.ts
@@ -457,7 +457,12 @@ export class FileDetector {
   ): Promise<FileProcessingResult> {
     switch (detection.type) {
       case "csv":
-        return await CSVProcessor.process(content, options);
+        // Pass original extension through to CSV processor; if detection has none,
+        // fall back to any extension provided in csvOptions.
+        return await CSVProcessor.process(content, {
+          ...options,
+          extension: detection.extension ?? options?.extension,
+        });
       case "image":
         return await ImageProcessor.process(content);
       case "pdf":
diff --git a/test/fixtures/sample.tsv b/test/fixtures/sample.tsv
new file mode 100644
index 000000000..203d31142
--- /dev/null
+++ b/test/fixtures/sample.tsv
@@ -0,0 +1,5 @@
+name	age	city
+Alice	30	New York
+Bob	25	London
+Charlie	35	Tokyo
+
diff --git a/test/unit/csv-extension-metadata.test.ts b/test/unit/csv-extension-metadata.test.ts
new file mode 100644
index 000000000..0766048d3
--- /dev/null
+++ b/test/unit/csv-extension-metadata.test.ts
@@ -0,0 +1,199 @@
+/**
+ * Tests for CSV-023: File Extension in Metadata
+ * Validates that CSV metadata includes original file extension
+ */
+
+import { describe, it, expect } from "vitest";
+import { FileDetector } from "../../src/lib/utils/fileDetector.js";
+import { CSVProcessor } from "../../src/lib/utils/csvProcessor.js";
+import { readFile } from "fs/promises";
+import { join } from "path";
+
+describe("CSV Extension Metadata (CSV-023)", () => {
+  const fixturesDir = join(process.cwd(), "test", "fixtures");
+
+  describe("FileDetector with CSV files", () => {
+    it("should include extension metadata for .csv files", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const result = await FileDetector.detectAndProcess(csvPath);
+
+      expect(result.type).toBe("csv");
+      expect(result.metadata.extension).toBe("csv");
+    });
+
+    it("should include extension metadata for .tsv files", async () => {
+      const tsvPath = join(fixturesDir, "sample.tsv");
+      const result = await FileDetector.detectAndProcess(tsvPath);
+
+      expect(result.type).toBe("csv"); // Type is normalized to 'csv'
+      expect(result.metadata.extension).toBe("tsv"); // Extension preserves original
+    });
+
+    it("should handle extension when processing from buffer", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      // When processing a buffer without csvOptions, detection can't infer an extension,
+      // so we expect metadata.extension to be present and null.
+      const result = await FileDetector.detectAndProcess(buffer);
+
+      expect(result.type).toBe("csv");
+      expect(result.metadata).toHaveProperty("extension");
+      expect(result.metadata.extension).toBeNull();
+    });
+
+    it("should use csvOptions.extension as fallback when detection has no extension", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      // When processing buffer with csvOptions.extension, it should be used as fallback
+      const result = await FileDetector.detectAndProcess(buffer, {
+        csvOptions: { extension: "csv" },
+      });
+
+      expect(result.type).toBe("csv");
+      expect(result.metadata.extension).toBe("csv");
+    });
+  });
+
+  describe("CSVProcessor direct usage", () => {
+    it("should include extension when provided in options", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: "csv",
+        formatStyle: "raw",
+      });
+
+      expect(result.metadata.extension).toBe("csv");
+    });
+
+    it("should preserve tsv extension in metadata", async () => {
+      const tsvPath = join(fixturesDir, "sample.tsv");
+      const buffer = await readFile(tsvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: "tsv",
+        formatStyle: "raw",
+      });
+
+      expect(result.metadata.extension).toBe("tsv");
+    });
+
+    it("should handle null extension gracefully", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: null,
+        formatStyle: "raw",
+      });
+
+      expect(result.metadata.extension).toBeNull();
+    });
+
+    it("should work without extension option (backward compatibility)", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        formatStyle: "raw",
+      });
+
+      // Should not error, extension should be null by default
+      expect(result.metadata.extension).toBeNull();
+    });
+  });
+
+  describe("Extension metadata for different formats", () => {
+    it("should include extension for raw format", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: "csv",
+        formatStyle: "raw",
+      });
+
+      expect(result.metadata.extension).toBe("csv");
+      expect(typeof result.content).toBe("string");
+    });
+
+    it("should include extension for json format", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: "csv",
+        formatStyle: "json",
+      });
+
+      expect(result.metadata.extension).toBe("csv");
+      expect(result.metadata.columnNames).toBeDefined();
+    });
+
+    it("should include extension for markdown format", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const buffer = await readFile(csvPath);
+
+      const result = await CSVProcessor.process(buffer, {
+        extension: "csv",
+        formatStyle: "markdown",
+      });
+
+      expect(result.metadata.extension).toBe("csv");
+      expect(typeof result.content).toBe("string");
+    });
+  });
+
+  describe("Extension detection for various CSV-like files", () => {
+    it("should distinguish CSV from TSV based on extension", async () => {
+      const csvPath = join(fixturesDir, "basic.csv");
+      const tsvPath = join(fixturesDir, "sample.tsv");
+
+      const csvResult = await FileDetector.detectAndProcess(csvPath);
+      const tsvResult = await FileDetector.detectAndProcess(tsvPath);
+
+      // Both are type 'csv'
+      expect(csvResult.type).toBe("csv");
+      expect(tsvResult.type).toBe("csv");
+
+      // But extensions differ
+      expect(csvResult.metadata.extension).toBe("csv");
+      expect(tsvResult.metadata.extension).toBe("tsv");
+    });
+
+    it("should preserve extension through the full processing pipeline", async () => {
+      const tsvPath = join(fixturesDir, "sample.tsv");
+
+      const result = await FileDetector.detectAndProcess(tsvPath, {
+        csvOptions: {
+          formatStyle: "json",
+          maxRows: 100,
+        },
+      });
+
+      expect(result.type).toBe("csv");
+      expect(result.metadata.extension).toBe("tsv");
+      expect(result.metadata.rowCount).toBeGreaterThan(0);
+    });
+
+    it("should prioritize detected extension over csvOptions.extension", async () => {
+      const tsvPath = join(fixturesDir, "sample.tsv");
+
+      // When both detection.extension (tsv) and csvOptions.extension (csv) are present,
+      // detection should win to preserve the actual file extension
+      const result = await FileDetector.detectAndProcess(tsvPath, {
+        csvOptions: {
+          extension: "csv", // Try to override with different extension
+          formatStyle: "raw",
+        },
+      });
+
+      expect(result.type).toBe("csv");
+      // Detection should win: tsv from file path, not csv from options
+      expect(result.metadata.extension).toBe("tsv");
+    });
+  });
+});