diff --git a/README.md b/README.md
index 9f44062..5a0b526 100644
--- a/README.md
+++ b/README.md
@@ -423,6 +423,90 @@ function FavoriteSkill() {
 }
 ```
 
+#### Server-only Validations
+
+While this library tries to lean-into shared validations between client and server, there are also good reasons not to share validations entirely. Most of the time, this comes down to keeping client-bundles small and/or needing direct server or DB access for certain validations.
+
+One approach is to just perform server-only validations manually _after_ calling `validateServerFormInfo`:
+
+```js
+import { validateServerFormData } from "remix-validity-state";
+
+export async function action({ request }: ActionArgs) {
+  const formData = await request.formData();
+  const serverFormInfo = await validateServerFormData(
+    formData,
+    formDefinitions
+  );
+  if (!serverFormInfo.valid) {
+    return json({ serverFormInfo });
+  }
+
+  // Now that we know our shared validations passed, we can perform more complex validations
+  let isEmailUnique = await checkEmail(
+    serverFormInfo.submittedValues.emailAddress
+  );
+  if (!isEmailUnique) {
+    return json({
+      serverFormInfo,
+      errors: { email: "Email address is not unique " },
+    });
+  }
+  // ...
+}
+```
+
+This may be sufficient in some cases, but also now requires you to support a new error messaging UI separate from the one already handled via `<Input>` and/or provided by `useValidatedInput().info.errorMessages`.
+
+To support this common use-case, you can pass a set of `customValidations` server-only implementations to `validateServerFormData`, which will be used instead of the validations you define in the shared `formDefinition`. Usually, you'll just put a stub `() => true` in your shared validations so the client is aware of the validation.
+
+```ts
+import type { ServerOnlyCustomValidations } from 'remix-validity-state'
+import { validateServerFormData } from 'remix-validity-state'
+
+let formDefinition: FormSchema = {
+  inputs: {
+    emailAddress: {
+      validationAttrs: {
+        type: "email",
+        required: true,
+      },
+      customValidations: {
+        // always "valid" in shared validations
+        uniqueEmail: () => true,
+      },
+      errorMessages: {
+        uniqueEmail: () => 'Email address already in use!'',
+      },
+    },
+  }
+};
+
+const serverCustomValidations: ServerOnlyCustomValidations<FormSchema> = {
+  emailAddress: {
+    async uniqueEmail(value) {
+      let isUnique = await checkEmail(value);
+      return isUnique;
+    },
+  },
+};
+
+export async function action({ request }: ActionArgs) {
+  const formData = await request.formData();
+  const serverFormInfo = await validateServerFormData(
+    formData,
+    formDefinitions,
+    serverCustomValidations
+  );
+  // serverFormInfo.valid here will be reflective of the custom server-only
+  // validation and will leverage your shared `errorMessages`
+  if (!serverFormInfo.valid) {
+    return json({ serverFormInfo });
+  }
+  // ...
+}
+```
+
 #### Error Messages
 
 Basic error messaging is handled out of the box by `<Input>` for built-in HTML validations. If you are using custom validations, or if you want to override the built-in messaging, you can provide custom error messages in our `formDefinition`. Custom error messages can either be a static string, or a function that receives the attribute value (built-in validations only), the input name, and the input value:
diff --git a/src/index.tsx b/src/index.tsx
index 828fd84..52cdd89 100644
--- a/src/index.tsx
+++ b/src/index.tsx
@@ -176,6 +176,10 @@ export interface InputInfo {
   errorMessages?: Record<string, string>;
 }
 
+export type ServerOnlyCustomValidations<T extends FormDefinition> = Partial<{
+  [key in KeyOf<T["inputs"]>]: CustomValidations;
+}>;
+
 // Server-side only (currently) - validate all specified inputs in the formData
 export type ServerFormInfo<T extends FormDefinition> = {
   submittedValues: Record<KeyOf<T["inputs"]>, string | string[] | null>;
@@ -365,7 +369,8 @@ function IsInputDefinition(
 // Called in a useEffect client side and from validateServerFormIno server-side
 async function validateInput(
   inputName: string,
-  inputDef: ControlDefinition,
+  validationAttrs: ControlDefinition["validationAttrs"],
+  customValidations: ControlDefinition["customValidations"],
   value: string,
   inputEl?: SupportedHTMLElements | SupportedHTMLElements[], // CSR
   formData?: FormData // SSR
@@ -381,8 +386,8 @@ async function validateInput(
     formData = new FormData(formEl);
   }
 
-  if (inputDef.validationAttrs) {
-    for (let _attr of Object.keys(inputDef.validationAttrs)) {
+  if (validationAttrs) {
+    for (let _attr of Object.keys(validationAttrs)) {
       let attr = _attr as KeyOf<BuiltInValidationAttrs>;
       // Ignoring this "error" since the type narrowing to accomplish this
       // would be nasty due to the differences in attribute values per input
@@ -390,7 +395,7 @@ async function validateInput(
       // users are specifying valid attributes up front in their schemas and
       // just yolo this lookup
       // @ts-expect-error
-      let _attrValue = inputDef.validationAttrs[attr] || null;
+      let _attrValue = validationAttrs[attr] || null;
       let attrValue = calculateValidationAttr(_attrValue, formData);
       // Undefined attr values means the attribute doesn't exist and there's
       // nothing to validate
@@ -413,9 +418,9 @@ async function validateInput(
     }
   }
 
-  if (inputDef.customValidations) {
-    for (let name of Object.keys(inputDef.customValidations)) {
-      let validate = inputDef.customValidations[name];
+  if (customValidations) {
+    for (let name of Object.keys(customValidations)) {
+      let validate = customValidations[name];
       let isInvalid = !(await validate(value, formData));
       validity[name] = isInvalid;
       validity.valid = validity.valid && !isInvalid;
@@ -428,7 +433,8 @@ async function validateInput(
 // Perform all validations for a submitted form on the server
 export async function validateServerFormData<T extends FormDefinition>(
   formData: FormData,
-  formDefinition: T
+  formDefinition: T,
+  serverCustomValidations?: ServerOnlyCustomValidations<T>
 ): Promise<ServerFormInfo<T>> {
   // Unsure if there's a better way to do this type of object mapping while
   // keeping the keys strongly typed - but this currently complains since we
@@ -454,7 +460,11 @@ export async function validateServerFormData<T extends FormDefinition>(
               state: "done",
               validity: await validateInput(
                 inputName,
-                inputDef,
+                inputDef.validationAttrs,
+                {
+                  ...inputDef.customValidations,
+                  ...serverCustomValidations?.[inputName],
+                },
                 value,
                 undefined,
                 formData
@@ -482,7 +492,8 @@ export async function validateServerFormData<T extends FormDefinition>(
           state: "done",
           validity: await validateInput(
             inputName,
-            inputDef,
+            inputDef.validationAttrs,
+            inputDef.customValidations,
             "",
             undefined,
             formData
@@ -882,7 +893,8 @@ function useValidatedControl<
       if (inputType === "radio" || inputType === "checkbox") {
         validity = await validateInput(
           name,
-          inputDef,
+          inputDef.validationAttrs,
+          inputDef.customValidations,
           value,
           Array.from(
             inputRef.current?.form?.querySelectorAll(
@@ -893,7 +905,8 @@ function useValidatedControl<
       } else {
         validity = await validateInput(
           name,
-          inputDef,
+          inputDef.validationAttrs,
+          inputDef.customValidations,
           value,
           inputRef.current || undefined
         );