diff --git a/open-sse/services/autoCombo/autoPrefix.ts b/open-sse/services/autoCombo/autoPrefix.ts
new file mode 100644
index 0000000000..9501f9ed19
--- /dev/null
+++ b/open-sse/services/autoCombo/autoPrefix.ts
@@ -0,0 +1,50 @@
+export type AutoVariant = "coding" | "fast" | "cheap" | "offline" | "smart" | "lkgp";
+
+export interface AutoPrefixParseResult {
+  valid: boolean;
+  variant?: AutoVariant;
+  error?: string;
+}
+
+const VALID_VARIANTS: AutoVariant[] = ["coding", "fast", "cheap", "offline", "smart", "lkgp"];
+
+/**
+ * Parses a model name to determine if it's an auto-prefixed model and extracts the variant.
+ *
+ * Examples:
+ * - "auto"         -> { valid: true, variant: undefined } (default)
+ * - "auto/coding"  -> { valid: true, variant: "coding" }
+ * - "auto/lkgp"    -> { valid: true, variant: "lkgp" }
+ * - "auto/"        -> { valid: true, variant: undefined } (default)
+ * - "autocoding"   -> { valid: false, error: "Invalid auto prefix format" }
+ * - "otherModel"   -> { valid: false, error: "Not an auto-prefixed model" }
+ */
+export function parseAutoPrefix(model: string): AutoPrefixParseResult {
+  if (!model.startsWith("auto")) {
+    return { valid: false, error: "Not an auto-prefixed model" };
+  }
+
+  const parts = model.split("/");
+
+  if (parts.length === 1) {
+    if (parts[0] === "auto") {
+      return { valid: true, variant: undefined }; // Default auto
+    } else {
+      return { valid: false, error: "Invalid auto prefix format" };
+    }
+  }
+
+  if (parts.length === 2) {
+    if (parts[0] !== "auto") {
+      return { valid: false, error: "Invalid auto prefix format" };
+    }
+    const variant = parts[1] as AutoVariant;
+    if (variant === "" || VALID_VARIANTS.includes(variant)) {
+      return { valid: true, variant: variant === "" ? undefined : variant };
+    } else {
+      return { valid: false, error: `Invalid auto variant: ${variant}` };
+    }
+  }
+
+  return { valid: false, error: "Invalid auto prefix format" };
+}
diff --git a/tests/unit/autoPrefix.test.ts b/tests/unit/autoPrefix.test.ts
new file mode 100644
index 0000000000..cb0588dc61
--- /dev/null
+++ b/tests/unit/autoPrefix.test.ts
@@ -0,0 +1,89 @@
+import { parseAutoPrefix } from "../../open-sse/services/autoCombo/autoPrefix.ts";
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+describe("parseAutoPrefix", () => {
+  it('should return valid for "auto" with no variant', () => {
+    const result = parseAutoPrefix("auto");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, undefined);
+  });
+
+  it('should return valid for "auto/" with no variant', () => {
+    const result = parseAutoPrefix("auto/");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, undefined);
+  });
+
+  it('should return valid for "auto/coding" with coding variant', () => {
+    const result = parseAutoPrefix("auto/coding");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "coding");
+  });
+
+  it('should return valid for "auto/fast" with fast variant', () => {
+    const result = parseAutoPrefix("auto/fast");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "fast");
+  });
+
+  it('should return valid for "auto/cheap" with cheap variant', () => {
+    const result = parseAutoPrefix("auto/cheap");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "cheap");
+  });
+
+  it('should return valid for "auto/offline" with offline variant', () => {
+    const result = parseAutoPrefix("auto/offline");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "offline");
+  });
+
+  it('should return valid for "auto/smart" with smart variant', () => {
+    const result = parseAutoPrefix("auto/smart");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "smart");
+  });
+
+  it('should return valid for "auto/lkgp" with lkgp variant', () => {
+    const result = parseAutoPrefix("auto/lkgp");
+    assert.strictEqual(result.valid, true);
+    assert.strictEqual(result.variant, "lkgp");
+  });
+
+  it('should return invalid for "autocoding" (invalid format)', () => {
+    const result = parseAutoPrefix("autocoding");
+    assert.strictEqual(result.valid, false);
+    assert.match(result.error || "", /Invalid auto prefix format/);
+  });
+
+  it('should return invalid for "auto/unknown" (invalid variant)', () => {
+    const result = parseAutoPrefix("auto/unknown");
+    assert.strictEqual(result.valid, false);
+    assert.match(result.error || "", /Invalid auto variant: unknown/);
+  });
+
+  it("should return invalid for a non-auto prefixed model", () => {
+    const result = parseAutoPrefix("otherModel");
+    assert.strictEqual(result.valid, false);
+    assert.match(result.error || "", /Not an auto-prefixed model/);
+  });
+
+  it("should return invalid for empty string", () => {
+    const result = parseAutoPrefix("");
+    assert.strictEqual(result.valid, false);
+    assert.match(result.error || "", /Not an auto-prefixed model/);
+  });
+
+  it("should return invalid for null/undefined input (handled by TS but for robustness)", () => {
+    // @ts-ignore testing invalid input that TS normally prevents
+    const result = parseAutoPrefix(null);
+    assert.strictEqual(result.valid, false);
+    assert.match(result.error || "", /Not an auto-prefixed model/);
+
+    // @ts-ignore
+    const result2 = parseAutoPrefix(undefined);
+    assert.strictEqual(result2.valid, false);
+    assert.match(result2.error || "", /Not an auto-prefixed model/);
+  });
+});