diff --git a/.changeset/shiny-ghosts-argue.md b/.changeset/shiny-ghosts-argue.md
new file mode 100644
index 0000000..d9194a3
--- /dev/null
+++ b/.changeset/shiny-ghosts-argue.md
@@ -0,0 +1,5 @@
+---
+"bako-ui": patch
+---
+
+rhf-combobox can work only with lowercase value
diff --git a/packages/ui/src/components/RhfCombobox/rhf-combobox.test.tsx b/packages/ui/src/components/RhfCombobox/rhf-combobox.test.tsx
index 8cf7dc4..ec0523e 100644
--- a/packages/ui/src/components/RhfCombobox/rhf-combobox.test.tsx
+++ b/packages/ui/src/components/RhfCombobox/rhf-combobox.test.tsx
@@ -253,4 +253,87 @@ describe('RhfCombobox', () => {
     ) as HTMLInputElement;
     expect(hiddenInput?.value).toBe('banana');
   });
+
+  it('converts input to lowercase when onlyLowercase is true', async () => {
+    const onSubmit = vi.fn();
+    const { user } = renderSingleCombobox(
+      { onlyLowercase: true },
+      { onSubmit }
+    );
+
+    const input = screen.getByRole('combobox');
+    await user.click(input);
+    await user.type(input, 'BaNaNa');
+
+    // Wait for the value to be normalized to lowercase letters
+    await waitFor(() => {
+      const hiddenInput = document.querySelector(
+        'input[type="hidden"]'
+      ) as HTMLInputElement;
+      expect(hiddenInput?.value).toBe('banana');
+    });
+
+    await user.click(screen.getByText('Submit'));
+
+    await waitFor(() => {
+      expect(onSubmit).toHaveBeenCalledWith(
+        expect.objectContaining({ single: 'banana' }),
+        expect.anything()
+      );
+    });
+  });
+
+  it('converts selected option to lowercase when onlyLowercase is true', async () => {
+    const onSubmit = vi.fn();
+    const { user } = renderSingleCombobox(
+      {
+        onlyLowercase: true,
+        options: [
+          { label: 'Apple', value: 'APPLE' },
+          { label: 'Banana', value: 'BANANA' },
+          { label: 'Cherry', value: 'CHERRY' },
+        ],
+      },
+      { onSubmit }
+    );
+
+    await user.click(screen.getByRole('combobox'));
+    await user.click(await screen.findByText('Banana'));
+
+    await waitFor(() => {
+      const hiddenInput = document.querySelector(
+        'input[type="hidden"]'
+      ) as HTMLInputElement;
+      expect(hiddenInput?.value).toBe('banana');
+    });
+
+    await user.click(screen.getByText('Submit'));
+
+    await waitFor(() => {
+      expect(onSubmit).toHaveBeenCalledWith(
+        expect.objectContaining({ single: 'banana' }),
+        expect.anything()
+      );
+    });
+  });
+
+  it('converts defaultValue to lowercase when onlyLowercase is true', async () => {
+    renderSingleCombobox({
+      defaultValue: 'APPLE',
+      onlyLowercase: true,
+    });
+
+    const input = screen.getByRole('combobox');
+
+    // The input should display “apple” (converted to lowercase)
+    await waitFor(() => {
+      expect(input).toHaveValue('apple');
+    });
+
+    // The hidden input must also have a lowercase value
+    const hiddenInput = document.querySelector(
+      'input[type="hidden"]'
+    ) as HTMLInputElement;
+    expect(hiddenInput?.value).toBe('apple');
+  });
 });
diff --git a/packages/ui/src/components/RhfCombobox/rhf-combobox.tsx b/packages/ui/src/components/RhfCombobox/rhf-combobox.tsx
index 070d4f7..3fbf542 100644
--- a/packages/ui/src/components/RhfCombobox/rhf-combobox.tsx
+++ b/packages/ui/src/components/RhfCombobox/rhf-combobox.tsx
@@ -69,6 +69,7 @@ export function RhfCombobox<
   clearTriggerIcon,
   showTrigger = false,
   allowCustomValue = true,
+  onlyLowercase = false,
   onInputValueChange,
 }: RhfComboboxProps<TFieldValues, TName>) {
   const {
@@ -80,46 +81,57 @@ export function RhfCombobox<
     filter: contains,
   });
 
-  const [inputValue, setInputValue] = useState(value || '');
+  const normalizeValue = useCallback(
+    (val: string) => {
+      if (!val) return val;
+      return onlyLowercase ? val.toLowerCase() : val;
+    },
+    [onlyLowercase]
+  );
+
+  const [inputValue, setInputValue] = useState(normalizeValue(value || ''));
   const [isTyping, setIsTyping] = useState(false);
 
   useEffect(() => {
     if (isTyping) return;
 
-    if (value !== inputValue) {
-      setInputValue(value || '');
+    const normalizedValue = normalizeValue(value || '');
+    if (normalizedValue !== inputValue) {
+      setInputValue(normalizedValue || '');
     }
-  }, [value, inputValue, isTyping]);
+  }, [value, inputValue, isTyping, normalizeValue]);
 
   const handleValueChange = useCallback(
     (details: ComboboxValueChangeDetails<RhfComboboxOptions>) => {
       const newValue = details.value[0] || '';
+      const normalizedValue = normalizeValue(newValue);
       setIsTyping(false);
-      onChange(newValue);
-      setInputValue(newValue);
-      onInputValueChange?.(newValue);
+      onChange(normalizedValue);
+      setInputValue(normalizedValue);
+      onInputValueChange?.(normalizedValue);
     },
-    [onChange, onInputValueChange]
+    [onChange, onInputValueChange, normalizeValue]
   );
 
   const handleInputValueChange = useCallback(
     (details: ComboboxInputValueChangeDetails) => {
       setIsTyping(true);
 
-      setInputValue(details.inputValue);
+      const normalizedInputValue = normalizeValue(details.inputValue);
+      setInputValue(normalizedInputValue);
 
       flushSync(() => {
-        filter(details.inputValue);
+        filter(normalizedInputValue);
       });
 
-      onInputValueChange?.(details.inputValue);
+      onInputValueChange?.(normalizedInputValue);
       if (allowCustomValue) {
-        onChange(details.inputValue);
+        onChange(normalizedInputValue);
       }
 
       setTimeout(() => setIsTyping(false), 100);
     },
-    [filter, allowCustomValue, onChange, onInputValueChange]
+    [filter, allowCustomValue, onChange, onInputValueChange, normalizeValue]
   );
 
   const handleOpenChange = useCallback(
@@ -176,7 +188,7 @@ export function RhfCombobox<
         width="full"
         variant={variant}
         inputValue={inputValue}
-        value={value ? [value] : []}
+        value={value ? [normalizeValue(value)] : []}
         borderRadius="lg"
         onValueChange={handleValueChange}
         onOpenChange={handleOpenChange}
@@ -187,7 +199,7 @@ export function RhfCombobox<
         invalid={!!error}
         allowCustomValue={allowCustomValue}
         selectionBehavior="preserve"
-        defaultValue={[defaultValue || '']}
+        defaultValue={[normalizeValue(defaultValue || '')]}
         placeholder={placeholder}
         {...slotProps?.root}
       >
@@ -210,7 +222,10 @@ export function RhfCombobox<
           </Combobox.IndicatorGroup>
         </Combobox.Control>
 
-        <ComboboxHiddenInput name={rest.name} value={value ?? ''} />
+        <ComboboxHiddenInput
+          name={rest.name}
+          value={normalizeValue(value ?? '')}
+        />
 
         {helperText && <Field.HelperText>{helperText}</Field.HelperText>}
         {error?.message && <Field.ErrorText>{error.message}</Field.ErrorText>}
diff --git a/packages/ui/src/components/RhfCombobox/rhf-combobox.types.tsx b/packages/ui/src/components/RhfCombobox/rhf-combobox.types.tsx
index a0251d4..098e1b2 100644
--- a/packages/ui/src/components/RhfCombobox/rhf-combobox.types.tsx
+++ b/packages/ui/src/components/RhfCombobox/rhf-combobox.types.tsx
@@ -35,4 +35,5 @@ export type RhfComboboxProps<
     label?: FieldLabelProps;
     input?: Omit<InputProps, 'value' | 'onChange' | 'disabled' | 'type'>;
   };
+  onlyLowercase?: boolean;
 };