diff --git a/.chronus/changes/decorator-post-validator-2025-10-26-18-27-31.md b/.chronus/changes/decorator-post-validator-2025-10-26-18-27-31.md
new file mode 100644
index 00000000000..90099e0719c
--- /dev/null
+++ b/.chronus/changes/decorator-post-validator-2025-10-26-18-27-31.md
@@ -0,0 +1,21 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: internal
+packages:
+  - "@typespec/events"
+  - "@typespec/http-client"
+  - "@typespec/http"
+  - "@typespec/json-schema"
+  - "@typespec/openapi"
+  - "@typespec/openapi3"
+  - "@typespec/protobuf"
+  - "@typespec/rest"
+  - "@typespec/spector"
+  - "@typespec/sse"
+  - "@typespec/streams"
+  - "@typespec/tspd"
+  - "@typespec/versioning"
+  - "@typespec/xml"
+---
+
+Regenerate signature
diff --git a/packages/compiler/generated-defs/TypeSpec.Prototypes.ts b/packages/compiler/generated-defs/TypeSpec.Prototypes.ts
index 0819a6017d2..97a473e2868 100644
--- a/packages/compiler/generated-defs/TypeSpec.Prototypes.ts
+++ b/packages/compiler/generated-defs/TypeSpec.Prototypes.ts
@@ -1,6 +1,9 @@
-import type { DecoratorContext, Type } from "../src/index.js";
+import type { DecoratorContext, DecoratorValidatorCallback, Type } from "../src/index.js";
 
-export type GetterDecorator = (context: DecoratorContext, target: Type) => void;
+export type GetterDecorator = (
+  context: DecoratorContext,
+  target: Type,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecPrototypesDecorators = {
   getter: GetterDecorator;
diff --git a/packages/compiler/generated-defs/TypeSpec.ts b/packages/compiler/generated-defs/TypeSpec.ts
index 3325db0772a..3804f8e730a 100644
--- a/packages/compiler/generated-defs/TypeSpec.ts
+++ b/packages/compiler/generated-defs/TypeSpec.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Enum,
   EnumValue,
   Interface,
@@ -70,7 +71,7 @@ export type MediaTypeHintDecorator = (
   context: DecoratorContext,
   target: Model | Scalar | Enum | Union,
   mediaType: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify how to encode the target type.
@@ -102,7 +103,7 @@ export type EncodeDecorator = (
   target: Scalar | ModelProperty,
   encodingOrEncodeAs: Scalar | string | EnumValue,
   encodedAs?: Scalar,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Attach a documentation string. Content support CommonMark markdown formatting.
@@ -120,17 +121,23 @@ export type DocDecorator = (
   target: Type,
   doc: string,
   formatArgs?: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the model with required properties removed.
  */
-export type WithOptionalPropertiesDecorator = (context: DecoratorContext, target: Model) => void;
+export type WithOptionalPropertiesDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the model with non-updateable properties removed.
  */
-export type WithUpdateablePropertiesDecorator = (context: DecoratorContext, target: Model) => void;
+export type WithUpdateablePropertiesDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the model with the given properties omitted.
@@ -141,7 +148,7 @@ export type WithoutOmittedPropertiesDecorator = (
   context: DecoratorContext,
   target: Model,
   omit: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the model with only the given properties included.
@@ -152,12 +159,15 @@ export type WithPickedPropertiesDecorator = (
   context: DecoratorContext,
   target: Model,
   pick: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the model with any default values removed.
  */
-export type WithoutDefaultValuesDecorator = (context: DecoratorContext, target: Model) => void;
+export type WithoutDefaultValuesDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Set the visibility of key properties in a model if not already set.
@@ -178,7 +188,7 @@ export type WithDefaultKeyVisibilityDecorator = (
   context: DecoratorContext,
   target: Model,
   visibility: EnumValue,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Typically a short, single-line description.
@@ -190,7 +200,11 @@ export type WithDefaultKeyVisibilityDecorator = (
  * model Pet {}
  * ```
  */
-export type SummaryDecorator = (context: DecoratorContext, target: Type, summary: string) => void;
+export type SummaryDecorator = (
+  context: DecoratorContext,
+  target: Type,
+  summary: string,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Attach a documentation string to describe the successful return types of an operation.
@@ -207,7 +221,7 @@ export type ReturnsDocDecorator = (
   context: DecoratorContext,
   target: Operation,
   doc: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Attach a documentation string to describe the error return types of an operation.
@@ -224,7 +238,7 @@ export type ErrorsDocDecorator = (
   context: DecoratorContext,
   target: Operation,
   doc: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark this namespace as describing a service and configure service properties.
@@ -245,7 +259,7 @@ export type ServiceDecorator = (
   context: DecoratorContext,
   target: Namespace,
   options?: ServiceOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this model is an error type. Operations return error types when the operation has failed.
@@ -259,7 +273,10 @@ export type ServiceDecorator = (
  * }
  * ```
  */
-export type ErrorDecorator = (context: DecoratorContext, target: Model) => void;
+export type ErrorDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify a known data format hint for this string type. For example `uuid`, `uri`, etc.
@@ -277,7 +294,7 @@ export type FormatDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   format: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the the pattern this string should respect using simple regular expression syntax.
@@ -302,7 +319,7 @@ export type PatternDecorator = (
   target: Scalar | ModelProperty,
   pattern: string,
   validationMessage?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the minimum length this string type should be.
@@ -318,7 +335,7 @@ export type MinLengthDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the maximum length this string type should be.
@@ -334,7 +351,7 @@ export type MaxLengthDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the minimum number of items this array should have.
@@ -350,7 +367,7 @@ export type MinItemsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: Numeric,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the maximum number of items this array should have.
@@ -366,7 +383,7 @@ export type MaxItemsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: Numeric,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the minimum value this numeric type should be.
@@ -382,7 +399,7 @@ export type MinValueDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric | ScalarValue | ScalarValue | ScalarValue | ScalarValue | ScalarValue,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the maximum value this numeric type should be.
@@ -398,7 +415,7 @@ export type MaxValueDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric | ScalarValue | ScalarValue | ScalarValue | ScalarValue | ScalarValue,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the minimum value this numeric type should be, exclusive of the given
@@ -415,7 +432,7 @@ export type MinValueExclusiveDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric | ScalarValue | ScalarValue | ScalarValue | ScalarValue | ScalarValue,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the maximum value this numeric type should be, exclusive of the given
@@ -432,7 +449,7 @@ export type MaxValueExclusiveDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric | ScalarValue | ScalarValue | ScalarValue | ScalarValue | ScalarValue,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark this value as a secret value that should be treated carefully to avoid exposure
@@ -446,7 +463,7 @@ export type MaxValueExclusiveDecorator = (
 export type SecretDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty | Model | Union | Enum,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Attaches a tag to an operation, interface, or namespace. Multiple `@tag` decorators can be specified to attach multiple tags to a TypeSpec element.
@@ -457,7 +474,7 @@ export type TagDecorator = (
   context: DecoratorContext,
   target: Namespace | Interface | Operation,
   tag: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specifies how a templated type should name their instances.
@@ -478,7 +495,7 @@ export type FriendlyNameDecorator = (
   target: Type,
   name: string,
   formatArgs?: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark a model property as the key to identify instances of that type
@@ -495,7 +512,7 @@ export type KeyDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   altName?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this operation is an overload of the given operation.
@@ -514,7 +531,7 @@ export type OverloadDecorator = (
   context: DecoratorContext,
   target: Operation,
   overloadbase: Operation,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Provide an alternative name for this type when serialized to the given mime type.
@@ -541,7 +558,7 @@ export type EncodedNameDecorator = (
   target: Type,
   mimeType: string,
   name: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this union is discriminated.
@@ -603,7 +620,7 @@ export type DiscriminatedDecorator = (
   context: DecoratorContext,
   target: Union,
   options?: DiscriminatedOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the property to be used to discriminate this type.
@@ -622,7 +639,7 @@ export type DiscriminatorDecorator = (
   context: DecoratorContext,
   target: Model,
   propertyName: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Provide an example value for a data type.
@@ -643,7 +660,7 @@ export type ExampleDecorator = (
   target: Model | Enum | Scalar | Union | ModelProperty | UnionVariant,
   example: unknown,
   options?: ExampleOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Provide example values for an operation's parameters and corresponding return type.
@@ -661,12 +678,15 @@ export type OpExampleDecorator = (
   target: Operation,
   example: OperationExample,
   options?: ExampleOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark this operation as a `list` operation that returns a paginated list of items.
  */
-export type ListDecorator = (context: DecoratorContext, target: Operation) => void;
+export type ListDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining the number of items to skip.
@@ -679,7 +699,10 @@ export type ListDecorator = (context: DecoratorContext, target: Operation) => vo
  * @list op listPets(@offset skip: int32, @pageSize pageSize: int8): Page<Pet>;
  * ```
  */
-export type OffsetDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type OffsetDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining the page index.
@@ -692,7 +715,10 @@ export type OffsetDecorator = (context: DecoratorContext, target: ModelProperty)
  * @list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;
  * ```
  */
-export type PageIndexDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type PageIndexDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the pagination parameter that controls the maximum number of items to include in a page.
@@ -705,7 +731,10 @@ export type PageIndexDecorator = (context: DecoratorContext, target: ModelProper
  * @list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;
  * ```
  */
-export type PageSizeDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type PageSizeDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the the property that contains the array of page items.
@@ -718,7 +747,10 @@ export type PageSizeDecorator = (context: DecoratorContext, target: ModelPropert
  * @list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;
  * ```
  */
-export type PageItemsDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type PageItemsDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining the token to get to the next page.
@@ -733,7 +765,10 @@ export type PageItemsDecorator = (context: DecoratorContext, target: ModelProper
  * @list op listPets(@continuationToken continuationToken: string): Page<Pet>;
  * ```
  */
-export type ContinuationTokenDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type ContinuationTokenDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining a link to the next page.
@@ -752,7 +787,10 @@ export type ContinuationTokenDecorator = (context: DecoratorContext, target: Mod
  * @list op listPets(): Page<Pet>;
  * ```
  */
-export type NextLinkDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type NextLinkDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining a link to the previous page.
@@ -771,7 +809,10 @@ export type NextLinkDecorator = (context: DecoratorContext, target: ModelPropert
  * @list op listPets(): Page<Pet>;
  * ```
  */
-export type PrevLinkDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type PrevLinkDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining a link to the first page.
@@ -790,7 +831,10 @@ export type PrevLinkDecorator = (context: DecoratorContext, target: ModelPropert
  * @list op listPets(): Page<Pet>;
  * ```
  */
-export type FirstLinkDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type FirstLinkDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Pagination property defining a link to the last page.
@@ -809,14 +853,21 @@ export type FirstLinkDecorator = (context: DecoratorContext, target: ModelProper
  * @list op listPets(): Page<Pet>;
  * ```
  */
-export type LastLinkDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type LastLinkDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * A debugging decorator used to inspect a type.
  *
  * @param text Custom text to log
  */
-export type InspectTypeDecorator = (context: DecoratorContext, target: Type, text: string) => void;
+export type InspectTypeDecorator = (
+  context: DecoratorContext,
+  target: Type,
+  text: string,
+) => DecoratorValidatorCallback | void;
 
 /**
  * A debugging decorator used to inspect a type name.
@@ -827,7 +878,7 @@ export type InspectTypeNameDecorator = (
   context: DecoratorContext,
   target: Type,
   text: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Sets the visibility modifiers that are active on a property, indicating that it is only considered to be present
@@ -879,7 +930,7 @@ export type VisibilityDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Indicates that a property is not visible in the given visibility class.
@@ -901,7 +952,7 @@ export type InvisibleDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   visibilityClass: Enum,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Removes visibility modifiers from a property.
@@ -926,7 +977,7 @@ export type RemoveVisibilityDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Removes properties that do not have at least one of the given visibility modifiers
@@ -975,7 +1026,7 @@ export type WithVisibilityDecorator = (
   context: DecoratorContext,
   target: Model,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Declares the visibility constraint of the parameters of a given operation.
@@ -991,7 +1042,7 @@ export type ParameterVisibilityDecorator = (
   context: DecoratorContext,
   target: Operation,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Declares the visibility constraint of the return type of a given operation.
@@ -1007,7 +1058,7 @@ export type ReturnTypeVisibilityDecorator = (
   context: DecoratorContext,
   target: Operation,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Declares the default visibility modifiers for a visibility class.
@@ -1023,7 +1074,7 @@ export type DefaultVisibilityDecorator = (
   context: DecoratorContext,
   target: Enum,
   ...visibilities: EnumValue[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Applies the given visibility filter to the properties of the target model.
@@ -1058,7 +1109,7 @@ export type WithVisibilityFilterDecorator = (
   target: Model,
   filter: VisibilityFilter,
   nameTemplate?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Transforms the `target` model to include only properties that are visible during the
@@ -1096,7 +1147,7 @@ export type WithLifecycleUpdateDecorator = (
   context: DecoratorContext,
   target: Model,
   nameTemplate?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecDecorators = {
   mediaTypeHint: MediaTypeHintDecorator;
diff --git a/packages/compiler/src/core/checker.ts b/packages/compiler/src/core/checker.ts
index b39e55cd85f..de5359e3daa 100644
--- a/packages/compiler/src/core/checker.ts
+++ b/packages/compiler/src/core/checker.ts
@@ -56,6 +56,7 @@ import {
   DecoratorContext,
   DecoratorDeclarationStatementNode,
   DecoratorExpressionNode,
+  DecoratorValidatorCallback,
   Diagnostic,
   DiagnosticTarget,
   DocContent,
@@ -368,6 +369,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
    * Key is the SymId of a node. It can be retrieved with getNodeSymId(node)
    */
   const pendingResolutions = new PendingResolutions();
+  const postCheckValidators: DecoratorValidatorCallback[] = [];
 
   const typespecNamespaceBinding = resolver.symbols.global.exports!.get("TypeSpec");
   if (typespecNamespaceBinding) {
@@ -3485,6 +3487,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
 
     internalDecoratorValidation();
     assertNoPendingResolutions();
+    runPostValidators(postCheckValidators);
   }
 
   function assertNoPendingResolutions() {
@@ -5956,9 +5959,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
 
     if (!options.skipDecorators) {
       if ("decorators" in typeDef) {
-        for (const decApp of typeDef.decorators) {
-          applyDecoratorToType(program, decApp, typeDef);
-        }
+        applyDecoratorsToType(typeDef);
       }
       typeDef.isFinished = true;
       Object.setPrototypeOf(typeDef, typePrototype);
@@ -5968,6 +5969,31 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
     return typeDef;
   }
 
+  function applyDecoratorsToType(typeDef: Type & { decorators: DecoratorApplication[] }) {
+    const postSelfValidators: DecoratorValidatorCallback[] = [];
+    for (const decApp of typeDef.decorators) {
+      const validator = applyDecoratorToType(program, decApp, typeDef);
+      if (validator) {
+        switch (validator.when) {
+          case "onFinish":
+            postSelfValidators.push(validator);
+            break;
+          case "onGraphFinish":
+            postCheckValidators.push(validator);
+            break;
+        }
+      }
+    }
+    runPostValidators(postSelfValidators);
+  }
+
+  /** Run a list of post validator */
+  function runPostValidators(validators: DecoratorValidatorCallback[]) {
+    for (const validator of validators) {
+      program.reportDiagnostics(validator.validator());
+    }
+  }
+
   function markAsChecked<T extends Type>(type: T) {
     if (!type.creating) return;
     delete type.creating;
@@ -6669,7 +6695,11 @@ function reportDeprecation(
   }
 }
 
-function applyDecoratorToType(program: Program, decApp: DecoratorApplication, target: Type) {
+function applyDecoratorToType(
+  program: Program,
+  decApp: DecoratorApplication,
+  target: Type,
+): DecoratorValidatorCallback | void {
   compilerAssert("decorators" in target, "Cannot apply decorator to non-decoratable type", target);
 
   for (const arg of decApp.args) {
@@ -6697,7 +6727,7 @@ function applyDecoratorToType(program: Program, decApp: DecoratorApplication, ta
     const args = decApp.args.map((x) => x.jsValue);
     const fn = decApp.decorator;
     const context = createDecoratorContext(program, decApp);
-    fn(context, target, ...args);
+    return fn(context, target, ...args);
   } catch (error: any) {
     // do not fail the language server for exceptions in decorators
     if (program.compilerOptions.designTimeBuild) {
diff --git a/packages/compiler/src/core/types.ts b/packages/compiler/src/core/types.ts
index 7dd56a806a2..6fa70b4ee07 100644
--- a/packages/compiler/src/core/types.ts
+++ b/packages/compiler/src/core/types.ts
@@ -48,11 +48,27 @@ export interface DecoratorApplication {
   node?: DecoratorExpressionNode | AugmentDecoratorStatementNode;
 }
 
+/**
+ * Signature for a decorator JS implementation function.
+ * Use `@typespec/tspd` to generate an accurate signature from the `extern dec`
+ */
 export interface DecoratorFunction {
-  (program: DecoratorContext, target: any, ...customArgs: any[]): void;
+  (program: DecoratorContext, target: any, ...customArgs: any[]): DecoratorValidatorCallback | void;
   namespace?: string;
 }
 
+export interface DecoratorValidatorCallback {
+  /**
+   * When should this validator run.
+   * "onFinish": After all decorators are run on the same type. Useful if trying to validate this decorator is compatible with other decorators without relying on the order they are applied.
+   * "onGraphFinish": After everything is checked in the type graph. Useful when trying to get an overall view of the program.
+   */
+  readonly when: "onFinish" | "onGraphFinish";
+
+  /** Validator implementation. This function will be run according to the kind defined above. */
+  readonly validator: () => readonly Diagnostic[];
+}
+
 export interface BaseType {
   readonly entityKind: "Type";
   kind: string;
diff --git a/packages/compiler/src/index.ts b/packages/compiler/src/index.ts
index 55ae15c153d..61e9a06012e 100644
--- a/packages/compiler/src/index.ts
+++ b/packages/compiler/src/index.ts
@@ -322,6 +322,7 @@ export type {
   DecoratorContext,
   DecoratorFunction,
   DecoratorImplementations,
+  DecoratorValidatorCallback,
   DeprecatedDirective,
   Diagnostic,
   DiagnosticCreator,
diff --git a/packages/compiler/test/checker/decorators.test.ts b/packages/compiler/test/checker/decorators.test.ts
index 0841d5ec379..be266e24917 100644
--- a/packages/compiler/test/checker/decorators.test.ts
+++ b/packages/compiler/test/checker/decorators.test.ts
@@ -3,7 +3,9 @@ import { beforeEach, describe, it } from "vitest";
 import { numericRanges } from "../../src/core/numeric-ranges.js";
 import { Numeric } from "../../src/core/numeric.js";
 import {
+  DecoratorContext,
   DecoratorFunction,
+  Model,
   Namespace,
   PackageFlags,
   setTypeSpecNamespace,
@@ -14,7 +16,9 @@ import {
   createTestHost,
   createTestWrapper,
   expectDiagnostics,
+  mockFile,
 } from "../../src/testing/index.js";
+import { Tester } from "../tester.js";
 
 describe("compiler: checker: decorators", () => {
   let testHost: TestHost;
@@ -546,3 +550,88 @@ describe("compiler: checker: decorators", () => {
     ok(result, "expected Foo to be blue in isBlue decorator");
   });
 });
+
+describe("validators", () => {
+  async function testerForDecorator(fn: DecoratorFunction) {
+    return await Tester.files({
+      "dec.tsp": `
+        import "./dec.js";
+        namespace MyLibrary;
+        extern dec myDecorator(target: unknown);
+      `,
+      "dec.js": mockFile.js({
+        $decorators: {
+          MyLibrary: {
+            myDecorator: fn,
+          },
+        },
+      }),
+    })
+      .import("./dec.tsp")
+      .using("MyLibrary");
+  }
+
+  it("postSelf apply validator after checking the type", async () => {
+    const order: string[] = [];
+    const tester = await testerForDecorator((_: DecoratorContext, target: Model) => {
+      order.push(`apply(${target.name})`);
+      return {
+        when: "onFinish",
+        validator: () => {
+          order.push(`validate(${target.name})`);
+          return [];
+        },
+      };
+    });
+    await tester.compile(`
+      @myDecorator
+      @myDecorator
+      model A {}  
+      @myDecorator
+      @myDecorator
+      model B {}  
+    `);
+    deepStrictEqual(order, [
+      `apply(A)`,
+      `apply(A)`,
+      `validate(A)`,
+      `validate(A)`,
+      `apply(B)`,
+      `apply(B)`,
+      `validate(B)`,
+      `validate(B)`,
+    ]);
+  });
+
+  it("post apply validator after checking every type", async () => {
+    const order: string[] = [];
+    const tester = await testerForDecorator((_: DecoratorContext, target: Model) => {
+      order.push(`apply(${target.name})`);
+      return {
+        when: "onGraphFinish",
+        validator: () => {
+          order.push(`validate(${target.name})`);
+          return [];
+        },
+      };
+    });
+    await tester.compile(`
+      @myDecorator
+      @myDecorator
+      model A {}  
+      @myDecorator
+      @myDecorator
+      model B {}  
+    `);
+    deepStrictEqual(order, [
+      `apply(A)`,
+      `apply(A)`,
+      `apply(B)`,
+      `apply(B)`,
+      `validate(A)`,
+      `validate(A)`,
+      `validate(B)`,
+      `validate(B)`,
+    ]);
+  });
+});
diff --git a/packages/events/generated-defs/TypeSpec.Events.ts b/packages/events/generated-defs/TypeSpec.Events.ts
index 00f7e0c222c..1765080ca03 100644
--- a/packages/events/generated-defs/TypeSpec.Events.ts
+++ b/packages/events/generated-defs/TypeSpec.Events.ts
@@ -1,4 +1,10 @@
-import type { DecoratorContext, ModelProperty, Union, UnionVariant } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  ModelProperty,
+  Union,
+  UnionVariant,
+} from "@typespec/compiler";
 
 /**
  * Specify that this union describes a set of events.
@@ -13,7 +19,10 @@ import type { DecoratorContext, ModelProperty, Union, UnionVariant } from "@type
  * }
  * ```
  */
-export type EventsDecorator = (context: DecoratorContext, target: Union) => void;
+export type EventsDecorator = (
+  context: DecoratorContext,
+  target: Union,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specifies the content type of the event envelope, event body, or event payload.
@@ -41,7 +50,7 @@ export type ContentTypeDecorator = (
   context: DecoratorContext,
   target: UnionVariant | ModelProperty,
   contentType: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies the payload of an event.
@@ -54,7 +63,10 @@ export type ContentTypeDecorator = (
  * }
  * ```
  */
-export type DataDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type DataDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecEventsDecorators = {
   events: EventsDecorator;
diff --git a/packages/http-client/generated-defs/TypeSpec.HttpClient.ts b/packages/http-client/generated-defs/TypeSpec.HttpClient.ts
index d6001ffcbe3..76e97d71714 100644
--- a/packages/http-client/generated-defs/TypeSpec.HttpClient.ts
+++ b/packages/http-client/generated-defs/TypeSpec.HttpClient.ts
@@ -1,4 +1,4 @@
-import type { DecoratorContext, Type } from "@typespec/compiler";
+import type { DecoratorContext, DecoratorValidatorCallback, Type } from "@typespec/compiler";
 
 export interface FeatureLifecycleOptions {
   readonly emitterScope?: string;
@@ -8,7 +8,7 @@ export type ExperimentalDecorator = (
   context: DecoratorContext,
   target: Type,
   options?: FeatureLifecycleOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecHttpClientDecorators = {
   experimental: ExperimentalDecorator;
diff --git a/packages/http/generated-defs/TypeSpec.Http.Private.ts b/packages/http/generated-defs/TypeSpec.Http.Private.ts
index ec331e57121..6ba1d811f91 100644
--- a/packages/http/generated-defs/TypeSpec.Http.Private.ts
+++ b/packages/http/generated-defs/TypeSpec.Http.Private.ts
@@ -1,4 +1,10 @@
-import type { DecoratorContext, Model, ModelProperty, Type } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  Model,
+  ModelProperty,
+  Type,
+} from "@typespec/compiler";
 
 export interface HttpPartOptions {
   readonly name?: string;
@@ -8,16 +14,22 @@ export interface ApplyMergePatchOptions {
   readonly visibilityMode: unknown;
 }
 
-export type PlainDataDecorator = (context: DecoratorContext, target: Model) => void;
+export type PlainDataDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
-export type HttpFileDecorator = (context: DecoratorContext, target: Model) => void;
+export type HttpFileDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 export type HttpPartDecorator = (
   context: DecoratorContext,
   target: Model,
   type: Type,
   options: HttpPartOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Performs the canonical merge-patch transformation on the given model and injects its
@@ -29,7 +41,7 @@ export type ApplyMergePatchDecorator = (
   source: Model,
   nameTemplate: string,
   options: ApplyMergePatchOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify if inapplicable metadata should be included in the payload for the given entity.
@@ -40,7 +52,7 @@ export type IncludeInapplicableMetadataInPayloadDecorator = (
   context: DecoratorContext,
   target: Type,
   value: boolean,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Marks a model that was generated by applying the MergePatch
@@ -50,7 +62,7 @@ export type MergePatchModelDecorator = (
   context: DecoratorContext,
   target: Model,
   source: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Links a modelProperty mutated as part of a mergePatch transform to
@@ -60,7 +72,7 @@ export type MergePatchPropertyDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   source: ModelProperty,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecHttpPrivateDecorators = {
   plainData: PlainDataDecorator;
diff --git a/packages/http/generated-defs/TypeSpec.Http.ts b/packages/http/generated-defs/TypeSpec.Http.ts
index 7e0a318f61d..a78efa32fc5 100644
--- a/packages/http/generated-defs/TypeSpec.Http.ts
+++ b/packages/http/generated-defs/TypeSpec.Http.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Interface,
   ModelProperty,
   Namespace,
@@ -46,7 +47,10 @@ export interface PatchOptions {
  * };
  * ```
  */
-export type StatusCodeDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type StatusCodeDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Explicitly specify that this property type will be exactly the HTTP body.
@@ -60,7 +64,10 @@ export type StatusCodeDecorator = (context: DecoratorContext, target: ModelPrope
  * op download(): {@body image: bytes};
  * ```
  */
-export type BodyDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type BodyDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this property is to be sent or received as an HTTP header.
@@ -83,7 +90,7 @@ export type HeaderDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   headerNameOrOptions?: string | HeaderOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this property is to be sent or received in the cookie.
@@ -106,7 +113,7 @@ export type CookieDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   cookieNameOrOptions?: string | CookieOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this property is to be sent as a query parameter.
@@ -122,7 +129,7 @@ export type QueryDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   queryNameOrOptions?: string | QueryOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Explicitly specify that this property is to be interpolated as a path parameter.
@@ -138,7 +145,7 @@ export type PathDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   paramNameOrOptions?: string | PathOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the body resolution should be resolved from that property.
@@ -151,7 +158,10 @@ export type PathDecorator = (
  * op download(): {@bodyRoot user: {name: string, @header id: string}};
  * ```
  */
-export type BodyRootDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type BodyRootDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this property shouldn't be included in the HTTP body.
@@ -162,7 +172,10 @@ export type BodyRootDecorator = (context: DecoratorContext, target: ModelPropert
  * op upload(name: string, @bodyIgnore headers: {@header id: string}): void;
  * ```
  */
-export type BodyIgnoreDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type BodyIgnoreDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  *
@@ -179,7 +192,10 @@ export type BodyIgnoreDecorator = (context: DecoratorContext, target: ModelPrope
  * ): void;
  * ```
  */
-export type MultipartBodyDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type MultipartBodyDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `GET`.
@@ -189,7 +205,10 @@ export type MultipartBodyDecorator = (context: DecoratorContext, target: ModelPr
  * @get op read(): string
  * ```
  */
-export type GetDecorator = (context: DecoratorContext, target: Operation) => void;
+export type GetDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `PUT`.
@@ -199,7 +218,10 @@ export type GetDecorator = (context: DecoratorContext, target: Operation) => voi
  * @put op set(pet: Pet): void
  * ```
  */
-export type PutDecorator = (context: DecoratorContext, target: Operation) => void;
+export type PutDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `POST`.
@@ -209,7 +231,10 @@ export type PutDecorator = (context: DecoratorContext, target: Operation) => voi
  * @post op create(pet: Pet): void
  * ```
  */
-export type PostDecorator = (context: DecoratorContext, target: Operation) => void;
+export type PostDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `PATCH`.
@@ -231,7 +256,7 @@ export type PatchDecorator = (
   context: DecoratorContext,
   target: Operation,
   options?: PatchOptions,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `DELETE`.
@@ -241,7 +266,10 @@ export type PatchDecorator = (
  * @delete op set(petId: string): void
  * ```
  */
-export type DeleteDecorator = (context: DecoratorContext, target: Operation) => void;
+export type DeleteDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the HTTP verb for the target operation to be `HEAD`.
@@ -251,7 +279,10 @@ export type DeleteDecorator = (context: DecoratorContext, target: Operation) =>
  * @head op ping(petId: string): void
  * ```
  */
-export type HeadDecorator = (context: DecoratorContext, target: Operation) => void;
+export type HeadDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify an endpoint for this service. Multiple `@server` decorators can be used to specify multiple endpoints.
@@ -296,7 +327,7 @@ export type ServerDecorator = (
   url: string,
   description?: string,
   parameters?: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify authentication for a whole service or specific methods. See the [documentation in the Http library](https://typespec.io/docs/libraries/http/authentication) for full details.
@@ -313,7 +344,7 @@ export type UseAuthDecorator = (
   context: DecoratorContext,
   target: Namespace | Interface | Operation,
   auth: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Defines the relative route URI template for the target operation as defined by [RFC 6570](https://datatracker.ietf.org/doc/html/rfc6570#section-3.2.3)
@@ -340,7 +371,7 @@ export type RouteDecorator = (
   context: DecoratorContext,
   target: Namespace | Interface | Operation,
   path: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * `@sharedRoute` marks the operation as sharing a route path with other operations.
@@ -356,7 +387,10 @@ export type RouteDecorator = (
  * op getWidget(@path id: string): Widget;
  * ```
  */
-export type SharedRouteDecorator = (context: DecoratorContext, target: Operation) => void;
+export type SharedRouteDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecHttpDecorators = {
   statusCode: StatusCodeDecorator;
diff --git a/packages/http/src/private.decorators.ts b/packages/http/src/private.decorators.ts
index aa0bb3aa9ed..f6d26823e42 100644
--- a/packages/http/src/private.decorators.ts
+++ b/packages/http/src/private.decorators.ts
@@ -20,6 +20,16 @@ import {
   HttpPartOptions,
   PlainDataDecorator,
 } from "../generated-defs/TypeSpec.Http.Private.js";
+import {
+  getCookieParamOptions,
+  getHeaderFieldOptions,
+  getPathOptions,
+  getQueryOptions,
+  isBody,
+  isBodyRoot,
+  isMultipartBodyProperty,
+  isStatusCode,
+} from "./decorators.js";
 import { HttpStateKeys, reportDiagnostic } from "./lib.js";
 import { HttpOperationFileBody } from "./types.js";
 
@@ -151,8 +161,73 @@ export const $httpFile: HttpFileDecorator = (context: DecoratorContext, target:
 
     return undefined;
   }
+
+  return {
+    when: "onGraphFinish",
+    validator: () => {
+      validateHttpFileModel(context.program, target);
+      return [];
+    },
+  };
 };
 
+function validateHttpFileModel(program: Program, model: Model) {
+  for (const prop of model.properties.values()) {
+    switch (prop.name) {
+      case "contentType":
+      case "contents": {
+        // Check if these properties have any HTTP metadata and if so, report an error
+        const annotations = {
+          header: getHeaderFieldOptions(program, prop),
+          cookie: getCookieParamOptions(program, prop),
+          query: getQueryOptions(program, prop),
+          path: getPathOptions(program, prop),
+          body: isBody(program, prop),
+          bodyRoot: isBodyRoot(program, prop),
+          multipartBody: isMultipartBodyProperty(program, prop),
+          statusCode: isStatusCode(program, prop),
+        };
+
+        reportDisallowed(prop, annotations);
+        break;
+      }
+      case "filename": {
+        const annotations = {
+          body: isBody(program, prop),
+          bodyRoot: isBodyRoot(program, prop),
+          multipartBody: isMultipartBodyProperty(program, prop),
+          statusCode: isStatusCode(program, prop),
+          cookie: getCookieParamOptions(program, prop),
+        };
+
+        reportDisallowed(prop, annotations);
+        break;
+      }
+      default:
+        reportDiagnostic(program, {
+          code: "http-file-extra-property",
+          format: { propName: prop.name },
+          target: prop,
+        });
+    }
+  }
+  for (const child of model.derivedModels) {
+    validateHttpFileModel(program, child);
+  }
+
+  function reportDisallowed(target: ModelProperty, annotations: Record<string, unknown>) {
+    const metadataEntries = Object.entries(annotations).filter((e) => !!e[1]);
+
+    for (const [metadataType] of metadataEntries) {
+      reportDiagnostic(program, {
+        code: "http-file-disallowed-metadata",
+        format: { propName: target.name, metadataType },
+        target: target,
+      });
+    }
+  }
+}
+
 /**
  * Check if the given type is an `HttpFile`
  */
diff --git a/packages/http/src/validate.ts b/packages/http/src/validate.ts
index 340356a2594..ea9d2c76500 100644
--- a/packages/http/src/validate.ts
+++ b/packages/http/src/validate.ts
@@ -1,16 +1,6 @@
-import type { Model, ModelProperty, Program } from "@typespec/compiler";
-import {
-  getCookieParamOptions,
-  getHeaderFieldOptions,
-  getPathOptions,
-  getQueryOptions,
-  isBody,
-  isBodyRoot,
-  isMultipartBodyProperty,
-  isStatusCode,
-} from "./decorators.js";
+import type { Program } from "@typespec/compiler";
 import { isSharedRoute } from "./decorators/shared-route.js";
-import { HttpStateKeys, reportDiagnostic } from "./lib.js";
+import { reportDiagnostic } from "./lib.js";
 import { getAllHttpServices } from "./operations.js";
 import { HttpOperation, HttpService } from "./types.js";
 
@@ -21,74 +11,6 @@ export function $onValidate(program: Program) {
     program.reportDiagnostics(diagnostics);
   }
   validateSharedRouteConsistency(program, services);
-  validateHttpFiles(program);
-}
-
-function validateHttpFiles(program: Program) {
-  const httpFiles = [...program.stateSet(HttpStateKeys.file)];
-
-  for (const model of httpFiles) {
-    if (model.kind === "Model") {
-      validateHttpFileModel(program, model);
-    }
-  }
-}
-
-function validateHttpFileModel(program: Program, model: Model) {
-  for (const prop of model.properties.values()) {
-    switch (prop.name) {
-      case "contentType":
-      case "contents": {
-        // Check if these properties have any HTTP metadata and if so, report an error
-        const annotations = {
-          header: getHeaderFieldOptions(program, prop),
-          cookie: getCookieParamOptions(program, prop),
-          query: getQueryOptions(program, prop),
-          path: getPathOptions(program, prop),
-          body: isBody(program, prop),
-          bodyRoot: isBodyRoot(program, prop),
-          multipartBody: isMultipartBodyProperty(program, prop),
-          statusCode: isStatusCode(program, prop),
-        };
-
-        reportDisallowed(prop, annotations);
-        break;
-      }
-      case "filename": {
-        const annotations = {
-          body: isBody(program, prop),
-          bodyRoot: isBodyRoot(program, prop),
-          multipartBody: isMultipartBodyProperty(program, prop),
-          statusCode: isStatusCode(program, prop),
-          cookie: getCookieParamOptions(program, prop),
-        };
-
-        reportDisallowed(prop, annotations);
-        break;
-      }
-      default:
-        reportDiagnostic(program, {
-          code: "http-file-extra-property",
-          format: { propName: prop.name },
-          target: prop,
-        });
-    }
-  }
-  for (const child of model.derivedModels) {
-    validateHttpFileModel(program, child);
-  }
-
-  function reportDisallowed(target: ModelProperty, annotations: Record<string, unknown>) {
-    const metadataEntries = Object.entries(annotations).filter((e) => !!e[1]);
-
-    for (const [metadataType] of metadataEntries) {
-      reportDiagnostic(program, {
-        code: "http-file-disallowed-metadata",
-        format: { propName: target.name, metadataType },
-        target: target,
-      });
-    }
-  }
 }
 
 function groupHttpOperations(
diff --git a/packages/json-schema/generated-defs/TypeSpec.JsonSchema.Private.ts b/packages/json-schema/generated-defs/TypeSpec.JsonSchema.Private.ts
index 2c4f69bbc62..a0613346f47 100644
--- a/packages/json-schema/generated-defs/TypeSpec.JsonSchema.Private.ts
+++ b/packages/json-schema/generated-defs/TypeSpec.JsonSchema.Private.ts
@@ -1,10 +1,10 @@
-import type { DecoratorContext, Model, Type } from "@typespec/compiler";
+import type { DecoratorContext, DecoratorValidatorCallback, Model, Type } from "@typespec/compiler";
 
 export type ValidatesRawJsonDecorator = (
   context: DecoratorContext,
   target: Model,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecJsonSchemaPrivateDecorators = {
   validatesRawJson: ValidatesRawJsonDecorator;
diff --git a/packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts b/packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts
index ee71c6f53a6..8b9b4530a40 100644
--- a/packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts
+++ b/packages/json-schema/generated-defs/TypeSpec.JsonSchema.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   ModelProperty,
   Namespace,
   Numeric,
@@ -21,7 +22,7 @@ export type JsonSchemaDecorator = (
   context: DecoratorContext,
   target: Type,
   baseUri?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Set the base URI for any schemas emitted from types within this namespace.
@@ -32,7 +33,7 @@ export type BaseUriDecorator = (
   context: DecoratorContext,
   target: Namespace,
   baseUri: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the JSON Schema id. If this model or a parent namespace has a base URI,
@@ -42,12 +43,19 @@ export type BaseUriDecorator = (
  *
  * @param id The id of the JSON schema for this declaration.
  */
-export type IdDecorator = (context: DecoratorContext, target: Type, id: string) => void;
+export type IdDecorator = (
+  context: DecoratorContext,
+  target: Type,
+  id: string,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that `oneOf` should be used instead of `anyOf` for that union.
  */
-export type OneOfDecorator = (context: DecoratorContext, target: Union | ModelProperty) => void;
+export type OneOfDecorator = (
+  context: DecoratorContext,
+  target: Union | ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the numeric type must be a multiple of some numeric value.
@@ -58,7 +66,7 @@ export type MultipleOfDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Numeric,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the array must contain at least one instance of the provided type.
@@ -70,7 +78,7 @@ export type ContainsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Used in conjunction with the `@contains` decorator,
@@ -82,7 +90,7 @@ export type MinContainsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: number,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Used in conjunction with the `@contains` decorator,
@@ -94,7 +102,7 @@ export type MaxContainsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: number,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that every item in the array must be unique.
@@ -102,7 +110,7 @@ export type MaxContainsDecorator = (
 export type UniqueItemsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the minimum number of properties this object can have.
@@ -113,7 +121,7 @@ export type MinPropertiesDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: number,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the maximum number of properties this object can have.
@@ -124,7 +132,7 @@ export type MaxPropertiesDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: number,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the encoding used for the contents of a string.
@@ -137,7 +145,7 @@ export type ContentEncodingDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the target array must begin with the provided types.
@@ -148,7 +156,7 @@ export type PrefixItemsDecorator = (
   context: DecoratorContext,
   target: Type | ModelProperty,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the content type of content stored in a string.
@@ -159,7 +167,7 @@ export type ContentMediaTypeDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the schema for the contents of a string when interpreted according to the content's
@@ -171,7 +179,7 @@ export type ContentSchemaDecorator = (
   context: DecoratorContext,
   target: Scalar | ModelProperty,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify a custom property to add to the emitted schema. This is useful for adding custom keywords
@@ -197,7 +205,7 @@ export type ExtensionDecorator = (
   target: Type,
   key: string,
   value: Type | unknown,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecJsonSchemaDecorators = {
   jsonSchema: JsonSchemaDecorator;
diff --git a/packages/openapi/generated-defs/TypeSpec.OpenAPI.ts b/packages/openapi/generated-defs/TypeSpec.OpenAPI.ts
index b032c2f6889..9d82d0a0a42 100644
--- a/packages/openapi/generated-defs/TypeSpec.OpenAPI.ts
+++ b/packages/openapi/generated-defs/TypeSpec.OpenAPI.ts
@@ -1,4 +1,11 @@
-import type { DecoratorContext, Model, Namespace, Operation, Type } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  Model,
+  Namespace,
+  Operation,
+  Type,
+} from "@typespec/compiler";
 
 export interface AdditionalInfo {
   readonly [key: string]: unknown;
@@ -49,7 +56,7 @@ export type OperationIdDecorator = (
   context: DecoratorContext,
   target: Operation,
   operationId: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Attach some custom data to the OpenAPI element generated from this type.
@@ -68,7 +75,7 @@ export type ExtensionDecorator = (
   target: Type,
   key: string,
   value: unknown,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this model is to be treated as the OpenAPI `default` response.
@@ -82,7 +89,10 @@ export type ExtensionDecorator = (
  * op listPets(): Pet[] | PetStoreResponse;
  * ```
  */
-export type DefaultResponseDecorator = (context: DecoratorContext, target: Model) => void;
+export type DefaultResponseDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the OpenAPI `externalDocs` property for this type.
@@ -100,7 +110,7 @@ export type ExternalDocsDecorator = (
   target: Type,
   url: string,
   description?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify OpenAPI additional information.
@@ -112,7 +122,7 @@ export type InfoDecorator = (
   context: DecoratorContext,
   target: Namespace,
   additionalInfo: AdditionalInfo,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify OpenAPI additional information.
@@ -131,7 +141,7 @@ export type TagMetadataDecorator = (
   target: Namespace,
   name: string,
   tagMetadata: TagMetadata,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecOpenAPIDecorators = {
   operationId: OperationIdDecorator;
diff --git a/packages/openapi3/generated-defs/TypeSpec.OpenAPI.ts b/packages/openapi3/generated-defs/TypeSpec.OpenAPI.ts
index 1a9d7dc6755..385683874ef 100644
--- a/packages/openapi3/generated-defs/TypeSpec.OpenAPI.ts
+++ b/packages/openapi3/generated-defs/TypeSpec.OpenAPI.ts
@@ -1,9 +1,18 @@
-import type { DecoratorContext, Model, ModelProperty, Union } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  Model,
+  ModelProperty,
+  Union,
+} from "@typespec/compiler";
 
 /**
  * Specify that `oneOf` should be used instead of `anyOf` for that union.
  */
-export type OneOfDecorator = (context: DecoratorContext, target: Union | ModelProperty) => void;
+export type OneOfDecorator = (
+  context: DecoratorContext,
+  target: Union | ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify an external reference that should be used inside of emitting this type.
@@ -14,7 +23,7 @@ export type UseRefDecorator = (
   context: DecoratorContext,
   target: Model | ModelProperty,
   ref: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecOpenAPIDecorators = {
   oneOf: OneOfDecorator;
diff --git a/packages/protobuf/generated-defs/TypeSpec.Protobuf.Private.ts b/packages/protobuf/generated-defs/TypeSpec.Protobuf.Private.ts
index 9e0312b7668..7da472fe47c 100644
--- a/packages/protobuf/generated-defs/TypeSpec.Protobuf.Private.ts
+++ b/packages/protobuf/generated-defs/TypeSpec.Protobuf.Private.ts
@@ -1,13 +1,16 @@
-import type { DecoratorContext, Model, Type } from "@typespec/compiler";
+import type { DecoratorContext, DecoratorValidatorCallback, Model, Type } from "@typespec/compiler";
 
 export type ExternRefDecorator = (
   context: DecoratorContext,
   target: Model,
   path: Type,
   name: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
-export type _mapDecorator = (context: DecoratorContext, target: Model) => void;
+export type _mapDecorator = (
+  context: DecoratorContext,
+  target: Model,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecProtobufPrivateDecorators = {
   externRef: ExternRefDecorator;
diff --git a/packages/protobuf/generated-defs/TypeSpec.Protobuf.ts b/packages/protobuf/generated-defs/TypeSpec.Protobuf.ts
index 0e59e7977b2..adfa8411457 100644
--- a/packages/protobuf/generated-defs/TypeSpec.Protobuf.ts
+++ b/packages/protobuf/generated-defs/TypeSpec.Protobuf.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Interface,
   ModelProperty,
   Namespace,
@@ -17,7 +18,10 @@ import type {
  *
  * This decorator will force the emitter to check and emit a model.
  */
-export type MessageDecorator = (context: DecoratorContext, target: Type) => void;
+export type MessageDecorator = (
+  context: DecoratorContext,
+  target: Type,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Defines the field index of a model property for conversion to a Protobuf
@@ -52,7 +56,7 @@ export type FieldDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   index: number,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Reserve a field index, range, or name. If a field definition collides with a reservation, the emitter will produce
@@ -90,13 +94,16 @@ export type ReserveDecorator = (
   context: DecoratorContext,
   target: Type,
   ...reservations: (string | unknown | number)[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Declares that a TypeSpec interface constitutes a Protobuf service. The contents of the interface will be converted to
  * a `service` declaration in the resulting Protobuf file.
  */
-export type ServiceDecorator = (context: DecoratorContext, target: Interface) => void;
+export type ServiceDecorator = (
+  context: DecoratorContext,
+  target: Interface,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Declares that a TypeSpec namespace constitutes a Protobuf package. The contents of the namespace will be emitted to a
@@ -108,7 +115,7 @@ export type PackageDecorator = (
   context: DecoratorContext,
   target: Namespace,
   details?: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Set the streaming mode of an operation. See [StreamMode](./data-types#TypeSpec.Protobuf.StreamMode) for more information.
@@ -125,7 +132,11 @@ export type PackageDecorator = (
  * op connectToMessageService(...Message): Message;
  * ```
  */
-export type StreamDecorator = (context: DecoratorContext, target: Operation, mode: Type) => void;
+export type StreamDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+  mode: Type,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecProtobufDecorators = {
   message: MessageDecorator;
diff --git a/packages/rest/generated-defs/TypeSpec.Rest.Private.ts b/packages/rest/generated-defs/TypeSpec.Rest.Private.ts
index 0c6a8327b19..5e4185d2b27 100644
--- a/packages/rest/generated-defs/TypeSpec.Rest.Private.ts
+++ b/packages/rest/generated-defs/TypeSpec.Rest.Private.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Model,
   ModelProperty,
   Operation,
@@ -11,31 +12,31 @@ export type ResourceLocationDecorator = (
   context: DecoratorContext,
   target: Scalar,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type ValidateHasKeyDecorator = (
   context: DecoratorContext,
   target: Type,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type ValidateIsErrorDecorator = (
   context: DecoratorContext,
   target: Type,
   value: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type ActionSegmentDecorator = (
   context: DecoratorContext,
   target: Operation,
   value: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type ResourceTypeForKeyParamDecorator = (
   context: DecoratorContext,
   entity: ModelProperty,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecRestPrivateDecorators = {
   resourceLocation: ResourceLocationDecorator;
diff --git a/packages/rest/generated-defs/TypeSpec.Rest.ts b/packages/rest/generated-defs/TypeSpec.Rest.ts
index 90f68ad386e..2d2d987bad7 100644
--- a/packages/rest/generated-defs/TypeSpec.Rest.ts
+++ b/packages/rest/generated-defs/TypeSpec.Rest.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Interface,
   Model,
   ModelProperty,
@@ -17,7 +18,10 @@ import type {
  * }
  * ```
  */
-export type AutoRouteDecorator = (context: DecoratorContext, target: Interface | Operation) => void;
+export type AutoRouteDecorator = (
+  context: DecoratorContext,
+  target: Interface | Operation,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Defines the preceding path segment for a
@@ -38,7 +42,7 @@ export type SegmentDecorator = (
   context: DecoratorContext,
   target: Model | ModelProperty | Operation,
   name: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Returns the URL segment of a given model if it has `@segment` and `@key` decorator.
@@ -49,7 +53,7 @@ export type SegmentOfDecorator = (
   context: DecoratorContext,
   target: Operation,
   type: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Defines the separator string that is inserted before the action name in auto-generated routes for actions.
@@ -60,7 +64,7 @@ export type ActionSeparatorDecorator = (
   context: DecoratorContext,
   target: Model | ModelProperty | Operation,
   seperator: "/" | ":" | "/:",
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark this model as a resource type with a name.
@@ -71,7 +75,7 @@ export type ResourceDecorator = (
   context: DecoratorContext,
   target: Model,
   collectionName: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark model as a child of the given parent resource.
@@ -82,7 +86,7 @@ export type ParentResourceDecorator = (
   context: DecoratorContext,
   target: Model,
   parent: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a Read operation for a given resource.
@@ -96,7 +100,7 @@ export type ReadsResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a Create operation for a given resource.
@@ -110,7 +114,7 @@ export type CreatesResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a CreateOrReplace operation for a given resource.
@@ -124,7 +128,7 @@ export type CreatesOrReplacesResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a CreatesOrUpdate operation for a given resource.
@@ -138,7 +142,7 @@ export type CreatesOrUpdatesResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a Update operation for a given resource.
@@ -152,7 +156,7 @@ export type UpdatesResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a Delete operation for a given resource.
@@ -166,7 +170,7 @@ export type DeletesResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that this is a List operation for a given resource.
@@ -180,14 +184,18 @@ export type ListsResourceDecorator = (
   context: DecoratorContext,
   target: Operation,
   resourceType: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this operation is an action. (Scoped to a resource item /pets/{petId}/my-action)
  *
  * @param name Name of the action. If not specified, the name of the operation will be used.
  */
-export type ActionDecorator = (context: DecoratorContext, target: Operation, name?: string) => void;
+export type ActionDecorator = (
+  context: DecoratorContext,
+  target: Operation,
+  name?: string,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify this operation is a collection action. (Scopped to a resource, /pets/my-action)
@@ -203,7 +211,7 @@ export type CollectionActionDecorator = (
   target: Operation,
   resourceType: Model,
   name?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Copy the resource key parameters on the model
@@ -214,7 +222,7 @@ export type CopyResourceKeyParametersDecorator = (
   context: DecoratorContext,
   target: Model,
   filter?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecRestDecorators = {
   autoRoute: AutoRouteDecorator;
diff --git a/packages/spector/generated-defs/TypeSpec.Spector.ts b/packages/spector/generated-defs/TypeSpec.Spector.ts
index 6003f5b7864..44a7f4fbf96 100644
--- a/packages/spector/generated-defs/TypeSpec.Spector.ts
+++ b/packages/spector/generated-defs/TypeSpec.Spector.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Interface,
   Model,
   Namespace,
@@ -15,7 +16,7 @@ export type ScenarioServiceDecorator = (
   target: Namespace,
   route: string,
   options?: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark an operation, interface or namespace as a scenario. All containing operations will be part of the same scenario.
@@ -24,7 +25,7 @@ export type ScenarioDecorator = (
   context: DecoratorContext,
   target: Namespace | Interface | Operation,
   name?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify documentation on how to implement this scenario.
@@ -37,7 +38,7 @@ export type ScenarioDocDecorator = (
   target: Namespace | Interface | Operation,
   doc: string,
   formatArgs?: Model,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecSpectorDecorators = {
   scenarioService: ScenarioServiceDecorator;
diff --git a/packages/sse/generated-defs/TypeSpec.SSE.ts b/packages/sse/generated-defs/TypeSpec.SSE.ts
index 387c44b7bda..f6ed4ad4a3d 100644
--- a/packages/sse/generated-defs/TypeSpec.SSE.ts
+++ b/packages/sse/generated-defs/TypeSpec.SSE.ts
@@ -1,10 +1,17 @@
-import type { DecoratorContext, UnionVariant } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  UnionVariant,
+} from "@typespec/compiler";
 
 /**
  * Indicates that the presence of this event is a terminal event,
  * and the client should disconnect from the server.
  */
-export type TerminalEventDecorator = (context: DecoratorContext, target: UnionVariant) => void;
+export type TerminalEventDecorator = (
+  context: DecoratorContext,
+  target: UnionVariant,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecSSEDecorators = {
   terminalEvent: TerminalEventDecorator;
diff --git a/packages/streams/generated-defs/TypeSpec.Streams.ts b/packages/streams/generated-defs/TypeSpec.Streams.ts
index 92e5538c695..717dce87e1e 100644
--- a/packages/streams/generated-defs/TypeSpec.Streams.ts
+++ b/packages/streams/generated-defs/TypeSpec.Streams.ts
@@ -1,4 +1,4 @@
-import type { DecoratorContext, Model, Type } from "@typespec/compiler";
+import type { DecoratorContext, DecoratorValidatorCallback, Model, Type } from "@typespec/compiler";
 
 /**
  * Specify that a model represents a stream protocol type whose data is described
@@ -18,7 +18,11 @@ import type { DecoratorContext, Model, Type } from "@typespec/compiler";
  * }
  * ```
  */
-export type StreamOfDecorator = (context: DecoratorContext, target: Model, type: Type) => void;
+export type StreamOfDecorator = (
+  context: DecoratorContext,
+  target: Model,
+  type: Type,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecStreamsDecorators = {
   streamOf: StreamOfDecorator;
diff --git a/packages/tspd/src/gen-extern-signatures/components/decorator-signature-type.tsx b/packages/tspd/src/gen-extern-signatures/components/decorator-signature-type.tsx
index 9a9211dbf80..3ca06304371 100644
--- a/packages/tspd/src/gen-extern-signatures/components/decorator-signature-type.tsx
+++ b/packages/tspd/src/gen-extern-signatures/components/decorator-signature-type.tsx
@@ -1,4 +1,4 @@
-import { For, join, List, Refkey, refkey } from "@alloy-js/core";
+import { code, For, join, List, Refkey, refkey } from "@alloy-js/core";
 import * as ts from "@alloy-js/typescript";
 import {
   getSourceLocation,
@@ -58,7 +58,10 @@ export function DecoratorSignatureType(props: Readonly<DecoratorSignatureProps>)
       name={props.signature.typeName}
       doc={getDocComment(props.signature.decorator)}
     >
-      <ts.FunctionType parameters={parameters} />
+      <ts.FunctionType
+        parameters={parameters}
+        returnType={code`${typespecCompiler.DecoratorValidatorCallback} | void`}
+      />
     </ts.TypeDeclaration>
   );
 }
diff --git a/packages/tspd/src/gen-extern-signatures/external-packages/compiler.ts b/packages/tspd/src/gen-extern-signatures/external-packages/compiler.ts
index b734fb02f17..bba2cd1ca25 100644
--- a/packages/tspd/src/gen-extern-signatures/external-packages/compiler.ts
+++ b/packages/tspd/src/gen-extern-signatures/external-packages/compiler.ts
@@ -21,6 +21,7 @@ export const typespecCompiler = createPackage({
         "EnumValue",
         "Numeric",
         "ScalarValue",
+        "DecoratorValidatorCallback",
       ],
     },
   },
diff --git a/packages/tspd/test/gen-extern-signature/decorators-signatures.test.ts b/packages/tspd/test/gen-extern-signature/decorators-signatures.test.ts
index 4f0be902852..42a32053438 100644
--- a/packages/tspd/test/gen-extern-signature/decorators-signatures.test.ts
+++ b/packages/tspd/test/gen-extern-signature/decorators-signatures.test.ts
@@ -44,7 +44,7 @@ it("generate simple decorator with no parameters", async () => {
     expected: `
 ${importLine(["Type"])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -72,7 +72,7 @@ describe("generate target type", () => {
         expected: `
 ${importLine([expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -92,7 +92,7 @@ export type Decorators = {
         expected: `
 ${importLine([...expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: ${expected.join(" | ")}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: ${expected.join(" | ")}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -113,7 +113,7 @@ export type Decorators = {
         expected: `
 ${importLine([expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -129,7 +129,7 @@ export type Decorators = {
       expected: `
 ${importLine(["Model", "Scalar"])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Scalar | Model) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Scalar | Model) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -158,7 +158,7 @@ describe("generate parameter type", () => {
         expected: `
 ${importLine(["Type", expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -178,7 +178,7 @@ export type Decorators = {
         expected: `
 ${importLine(["Type", ...expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected.join(" | ")}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected.join(" | ")}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -210,7 +210,7 @@ export type Decorators = {
         expected: `
 ${importLine(["Type", ...(expected === "Numeric" ? ["Numeric"] : [])])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -232,7 +232,7 @@ export type SimpleDecorator = (
     readonly [key: string]: number;
     readonly other: string;
   },
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -254,7 +254,7 @@ export type SimpleDecorator = (
     readonly name: string;
     readonly age?: number;
   },
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -269,7 +269,7 @@ export type Decorators = {
         expected: `
 ${importLine(["ScalarValue", "Type"])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: number | string | ScalarValue) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: number | string | ScalarValue) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -291,7 +291,7 @@ export interface Info {
   readonly age?: number;
 }
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: Info) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: Info) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -311,7 +311,7 @@ export type Decorators = {
         expected: `
 ${importLine(["Type", expected])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -334,7 +334,7 @@ ${importLine(["Type", "Model"])}
 /**
  * Some doc comment
  */
-export type SimpleDecorator = (context: DecoratorContext, target: Type, ...args: Model[]) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, ...args: Model[]) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -362,7 +362,7 @@ export type Decorators = {
         expected: `
 ${importLine(["Type", ...(expected === "Numeric[]" ? ["Numeric"] : [])])}
 
-export type SimpleDecorator = (context: DecoratorContext, target: Type, ...args: ${expected}) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, ...args: ${expected}) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -385,7 +385,7 @@ ${importLine(["Type"])}
 /**
  * Some doc comment
  */
-export type SimpleDecorator = (context: DecoratorContext, target: Type) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -413,7 +413,7 @@ ${importLine(["Type"])}
  * @param arg1 This is the first argument
  * @param arg2 This is the second argument
  */
-export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: Type, arg2: Type) => void;
+export type SimpleDecorator = (context: DecoratorContext, target: Type, arg1: Type, arg2: Type) => DecoratorValidatorCallback | void;
 
 export type Decorators = {
   simple: SimpleDecorator;
@@ -424,6 +424,6 @@ export type Decorators = {
 });
 
 function importLine(imports: string[]) {
-  const all = new Set(["DecoratorContext", ...imports]);
+  const all = new Set(["DecoratorContext", "DecoratorValidatorCallback", ...imports]);
   return `import type { ${[...all].sort().join(", ")} } from "@typespec/compiler";`;
 }
diff --git a/packages/versioning/generated-defs/TypeSpec.Versioning.ts b/packages/versioning/generated-defs/TypeSpec.Versioning.ts
index 03e4b22cf1f..e74c13d1e62 100644
--- a/packages/versioning/generated-defs/TypeSpec.Versioning.ts
+++ b/packages/versioning/generated-defs/TypeSpec.Versioning.ts
@@ -1,5 +1,6 @@
 import type {
   DecoratorContext,
+  DecoratorValidatorCallback,
   Enum,
   EnumMember,
   Interface,
@@ -32,7 +33,7 @@ export type VersionedDecorator = (
   context: DecoratorContext,
   target: Namespace,
   versions: Enum,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies that a namespace or a given versioning enum member relies upon a versioned package.
@@ -63,7 +64,7 @@ export type UseDependencyDecorator = (
   context: DecoratorContext,
   target: EnumMember | Namespace,
   ...versionRecords: EnumMember[]
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when the target was added.
@@ -98,7 +99,7 @@ export type AddedDecorator = (
     | Scalar
     | Interface,
   version: EnumMember,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when the target was removed.
@@ -133,7 +134,7 @@ export type RemovedDecorator = (
     | Scalar
     | Interface,
   version: EnumMember,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when the target has been renamed.
@@ -160,7 +161,7 @@ export type RenamedFromDecorator = (
     | Interface,
   version: EnumMember,
   oldName: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when a target was made optional.
@@ -179,7 +180,7 @@ export type MadeOptionalDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   version: EnumMember,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when a target was made required.
@@ -198,7 +199,7 @@ export type MadeRequiredDecorator = (
   context: DecoratorContext,
   target: ModelProperty,
   version: EnumMember,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when the target type changed.
@@ -211,7 +212,7 @@ export type TypeChangedFromDecorator = (
   target: ModelProperty,
   version: EnumMember,
   oldType: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Identifies when the target type changed.
@@ -224,7 +225,7 @@ export type ReturnTypeChangedFromDecorator = (
   target: Operation,
   version: EnumMember,
   oldType: Type,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecVersioningDecorators = {
   versioned: VersionedDecorator;
diff --git a/packages/xml/generated-defs/TypeSpec.Xml.ts b/packages/xml/generated-defs/TypeSpec.Xml.ts
index 99ebfb9c9e5..4c2a5d5167d 100644
--- a/packages/xml/generated-defs/TypeSpec.Xml.ts
+++ b/packages/xml/generated-defs/TypeSpec.Xml.ts
@@ -1,4 +1,10 @@
-import type { DecoratorContext, Enum, ModelProperty, Type } from "@typespec/compiler";
+import type {
+  DecoratorContext,
+  DecoratorValidatorCallback,
+  Enum,
+  ModelProperty,
+  Type,
+} from "@typespec/compiler";
 
 /**
  * Provide the name of the XML element or attribute. This means the same thing as
@@ -23,7 +29,11 @@ import type { DecoratorContext, Enum, ModelProperty, Type } from "@typespec/comp
  * </XmlBook>
  * ```
  */
-export type NameDecorator = (context: DecoratorContext, target: Type, name: string) => void;
+export type NameDecorator = (
+  context: DecoratorContext,
+  target: Type,
+  name: string,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the target property should be encoded as an XML attribute instead of node.
@@ -54,7 +64,10 @@ export type NameDecorator = (context: DecoratorContext, target: Type, name: stri
  * </Blob>
  * ```
  */
-export type AttributeDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type AttributeDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify that the target property shouldn't create a wrapper node. This can be used to flatten list nodes into the model node or to include raw text in the model node.
@@ -121,7 +134,10 @@ export type AttributeDecorator = (context: DecoratorContext, target: ModelProper
  * </BlobName>
  * ```
  */
-export type UnwrappedDecorator = (context: DecoratorContext, target: ModelProperty) => void;
+export type UnwrappedDecorator = (
+  context: DecoratorContext,
+  target: ModelProperty,
+) => DecoratorValidatorCallback | void;
 
 /**
  * Specify the XML namespace for this element. It can be used in 2 different ways:
@@ -164,12 +180,15 @@ export type NsDecorator = (
   target: Type,
   ns: Type,
   prefix?: string,
-) => void;
+) => DecoratorValidatorCallback | void;
 
 /**
  * Mark an enum as declaring XML namespaces. See `@ns`
  */
-export type NsDeclarationsDecorator = (context: DecoratorContext, target: Enum) => void;
+export type NsDeclarationsDecorator = (
+  context: DecoratorContext,
+  target: Enum,
+) => DecoratorValidatorCallback | void;
 
 export type TypeSpecXmlDecorators = {
   name: NameDecorator;