diff --git a/.chronus/changes/joheredi-is-builtin-2025-4-20-22-7-54.md b/.chronus/changes/joheredi-is-builtin-2025-4-20-22-7-54.md
new file mode 100644
index 00000000000..aa2844b0720
--- /dev/null
+++ b/.chronus/changes/joheredi-is-builtin-2025-4-20-22-7-54.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Introduce builtin.is(type: Type): boolean, which returns true for any type defined in the global TypeSpec namespace (i.e. built-in/standard library types).
\ No newline at end of file
diff --git a/packages/compiler/src/typekit/kits/builtin.ts b/packages/compiler/src/typekit/kits/builtin.ts
index 7fb43448045..457dfc1dde0 100644
--- a/packages/compiler/src/typekit/kits/builtin.ts
+++ b/packages/compiler/src/typekit/kits/builtin.ts
@@ -1,4 +1,4 @@
-import type { Scalar } from "../../core/types.js";
+import type { Namespace, Scalar, Type } from "../../core/types.js";
 import { defineKit } from "../define-kit.js";
 
 /**
@@ -6,6 +6,12 @@ import { defineKit } from "../define-kit.js";
  * @typekit builtin
  */
 export interface BuiltinKit {
+  /**
+   * Checks if the given type is a built-in type.
+   * A type is considered built-in if it is defined in the TypeSpec namespace.
+   */
+  is(type: Type): boolean;
+
   /**
    * Accessor for the string builtin type.
    */
@@ -142,6 +148,36 @@ declare module "../define-kit.js" {
 
 defineKit<TypekitExtension>({
   builtin: {
+    is(type: Type): boolean {
+      // Model property does not have a namespace,
+      // so we will use the Model where it is defined.
+      if (type.kind === "ModelProperty" && type.model) {
+        type = type.model;
+      }
+
+      // Consider the type not built-in if it has no namespace
+      // TypeKit might create types without a namespace
+      const ns = (type as any).namespace as Namespace | undefined;
+      if (!ns) {
+        return false;
+      }
+
+      const globalNs = this.program.getGlobalNamespaceType();
+      // If it’s already in the global root, it's not in global.TypeSpec
+      if (ns === globalNs) {
+        return false;
+      }
+
+      // Find the immediate child of `globalNs` in this namespace's ancestry.
+      const topLevel = getImmediateChildOfGlobal(ns, globalNs);
+      if (!topLevel) {
+        return false;
+      }
+
+      // Finally, compare by identity
+      const typeSpecNs = globalNs.namespaces.get("TypeSpec");
+      return topLevel === typeSpecNs;
+    },
     get string(): Scalar {
       return this.program.checker.getStdType("string");
     },
@@ -219,3 +255,16 @@ defineKit<TypekitExtension>({
     },
   },
 });
+
+/**
+ * Walks from `ns` up to `root`, returning the first namespace
+ * whose parent is `root`.  Returns undefined if `root` is not
+ * actually an ancestor.
+ */
+function getImmediateChildOfGlobal(ns: Namespace, root: Namespace): Namespace | undefined {
+  let current: Namespace | undefined = ns;
+  while (current.namespace && current.namespace !== root) {
+    current = current.namespace;
+  }
+  return current?.namespace === root ? current : undefined;
+}
diff --git a/packages/compiler/test/typekit/builtin.test.ts b/packages/compiler/test/typekit/builtin.test.ts
index e0337c9e3c2..4a99dabd451 100644
--- a/packages/compiler/test/typekit/builtin.test.ts
+++ b/packages/compiler/test/typekit/builtin.test.ts
@@ -1,5 +1,11 @@
-import { beforeAll, expect, it } from "vitest";
-import { Program } from "../../src/index.js";
+import { beforeAll, beforeEach, describe, expect, it } from "vitest";
+import {
+  unsafe_mutateSubgraphWithNamespace,
+  unsafe_MutatorWithNamespace,
+} from "../../src/experimental/index.js";
+import { Model, ModelProperty, Namespace, Program, Scalar, Union } from "../../src/index.js";
+import { createTestRunner } from "../../src/testing/test-host.js";
+import { BasicTestRunner } from "../../src/testing/types.js";
 import { $ } from "../../src/typekit/index.js";
 import { createContextMock } from "./utils.js";
 
@@ -159,3 +165,270 @@ it("can get the builtin utcDateTime type", async () => {
   expect(utcDateTimeType).toBeDefined();
   expect(utcDateTimeType.name).toBe("utcDateTime");
 });
+
+describe("builtin.is() tests", () => {
+  let runner: BasicTestRunner;
+
+  beforeEach(async () => {
+    runner = await createTestRunner();
+  });
+
+  it("simple model with a string property", async () => {
+    // ------------------------------------------------------------
+    //   - Foo (Model)                     → false
+    //   - Foo.bar (ModelProperty)         → false
+    //   - type of Foo.bar (intrinsic)     → true
+    // ------------------------------------------------------------
+    const { Foo, bar } = (await runner.compile(
+      `
+      @test model Foo {
+        @test bar: string;
+      }
+    `,
+    )) as { Foo: Model; bar: ModelProperty };
+
+    program = runner.program;
+    const _$ = $(program);
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    expect(_$.builtin.is(bar)).toBe(false);
+    expect(_$.builtin.is(bar.type)).toBe(true);
+  });
+
+  it("Model Property reference", async () => {
+    // ------------------------------------------------------------
+    //   - Foo (Model)                     → false
+    //   - Foo.bar (ModelProperty)         → false
+    //   - type of Foo.bar (ModelProperty)     → false
+    //   - type of Foo.bar.type (Intrinsic) → true
+    // ------------------------------------------------------------
+    const { Foo, bar } = (await runner.compile(
+      `
+      @test model Bar {
+         prop: string
+      }
+         
+      @test model Foo {
+        @test bar: Bar.prop;
+      }
+    `,
+    )) as { Foo: Model; bar: ModelProperty };
+
+    program = runner.program;
+    const _$ = $(program);
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    expect(_$.builtin.is(bar)).toBe(false);
+    expect(_$.builtin.is(bar.type)).toBe(false);
+    expect(_$.builtin.is((bar.type as ModelProperty).type)).toBe(true);
+  });
+
+  it("model property is a union of string | int32", async () => {
+    // ------------------------------------------------------------
+    //   - Foo (Model)                             → false
+    //   - Foo.bar (ModelProperty)                 → false
+    //   - Foo.bar.type (Union)                    → false
+    //   - each variant (UnionVariant)             → false
+    //   - variant.type (intrinsic)                → true
+    // ------------------------------------------------------------
+    const { Foo, bar } = (await runner.compile(
+      `
+      @test model Foo {
+        @test bar: string | int32;
+      }
+    `,
+    )) as { Foo: Model; bar: ModelProperty };
+
+    program = runner.program;
+    const _$ = $(program);
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    expect(_$.builtin.is(bar)).toBe(false);
+    expect(_$.builtin.is(bar.type)).toBe(false);
+    const [variant1, variant2] = [...(bar.type as Union).variants.values()];
+    expect(_$.builtin.is(variant1)).toBe(false);
+    expect(_$.builtin.is(variant2)).toBe(false);
+    expect(_$.builtin.is(variant1.type)).toBe(true);
+    expect(_$.builtin.is(variant2.type)).toBe(true);
+  });
+
+  it("works with enums and models", async () => {
+    // -------------------------------------------------------------------
+    //   - FooEnum (Enum variants)               → false
+    //   - Foo.bar (ModelProperty of FooEnum)    → false
+    //   - Foo.bar.type (Enum)                   → false
+    //   - Baz.baz (ModelProperty of FooEnum.one)→ false
+    //   - Baz.baz.type (EnumMember)             → false
+    // -------------------------------------------------------------------
+    const { Foo, bar, Baz } = (await runner.compile(
+      `
+      @test enum FooEnum {
+        one: "1";
+        two: "2";
+      };
+
+      @test model Foo {
+        @test bar: FooEnum;
+      }
+      @test model Baz {
+        @test baz: FooEnum.one;
+      }
+    `,
+    )) as { Foo: Model; bar: ModelProperty; Baz: Model };
+
+    program = runner.program;
+    const _$ = $(program);
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    expect(_$.builtin.is(bar)).toBe(false);
+    expect(_$.builtin.is(bar.type)).toBe(false);
+    expect(_$.builtin.is(Baz)).toBe(false);
+    expect(_$.builtin.is(Baz.properties.get("baz")!)).toBe(false);
+    expect(_$.builtin.is(Baz.properties.get("baz")!.type)).toBe(false);
+  });
+
+  it("scalar extends an intrinsic (string)", async () => {
+    // ------------------------------------------------------------
+    //   - NotBuiltin (Scalar)             → false
+    //   - NotBuiltin.baseScalar (string)  → true
+    // ------------------------------------------------------------
+    const { NotBuiltin } = (await runner.compile(
+      `
+      @test scalar NotBuiltin extends string; 
+      `,
+    )) as { NotBuiltin: Scalar };
+
+    program = runner.program;
+    const _$ = $(program);
+    expect(_$.builtin.is(NotBuiltin)).toBe(false);
+    expect(_$.builtin.is(NotBuiltin.baseScalar!)).toBe(true);
+  });
+
+  it("alias an intrinsic then extend it", async () => {
+    // ------------------------------------------------------------
+    //   - NotBuiltin (Scalar extends alias)  → false
+    //   - NotBuiltin.baseScalar (int builtin)→ true
+    // ------------------------------------------------------------
+    const { NotBuiltin } = (await runner.compile(
+      `
+      alias UnixTimeStamp32 = unixTimestamp32;
+      @test scalar NotBuiltin extends UnixTimeStamp32;
+      `,
+    )) as { NotBuiltin: Scalar };
+
+    program = runner.program;
+    const _$ = $(program);
+    expect(_$.builtin.is(NotBuiltin)).toBe(false);
+    expect(_$.builtin.is(NotBuiltin.baseScalar!)).toBe(true);
+  });
+
+  it("should recognize a model in global.TypeSpec", async () => {
+    // ------------------------------------------------------------
+    // A non-typespec model references a ModelProperty defined in TypeSpec
+    // ------------------------------------------------------------
+    const { InTypeSpec, Foo } = (await runner.compile(`
+      @test namespace TypeSpec {
+        @test model InTypeSpec {
+          @test foo: string;
+        }
+      }
+
+      @test model Foo {
+        @test bar: TypeSpec.InTypeSpec.foo;
+      } 
+    `)) as { InTypeSpec: Model; Foo: Model };
+
+    const program = runner.program;
+    const _$ = $(program);
+
+    expect(_$.builtin.is(InTypeSpec)).toBe(true);
+
+    const prop = InTypeSpec.properties.get("foo")!;
+    expect(_$.builtin.is(prop)).toBe(true);
+
+    expect(_$.builtin.is(prop.type)).toBe(true);
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    const bar = Foo.properties.get("bar") as ModelProperty;
+    expect(_$.builtin.is(bar)).toBe(false); // bar is defined outside TypeSpec
+    expect(_$.builtin.is(bar.type)).toBe(true); // TypeSpec.InTypeSpec.foo is deifined inside TypeSpec
+  });
+
+  it("should NOT recognize a nested TypeSpec namespace", async () => {
+    // ----------------------------------------------------------------
+    // A namespace called "TypeSpec" but nested inside another namespace
+    // ----------------------------------------------------------------
+    const { InNested } = (await runner.compile(`
+      @test namespace Outer {
+        @test namespace TypeSpec {
+          @test model InNested {
+            @test bar: int32;
+          }
+        }
+      }
+    `)) as { InNested: Model };
+
+    const program = runner.program;
+    const _$ = $(program);
+
+    // Although the local namespace is called "TypeSpec",
+    // it's not direct child of the global namespace → false
+    expect(_$.builtin.is(InNested)).toBe(false);
+  });
+
+  it("should always return true for pure intrinsics", async () => {
+    const _$ = $(program);
+    const stringType = _$.builtin.string;
+
+    expect(_$.builtin.is(stringType)).toBe(true);
+
+    const int32Type = _$.builtin.int32;
+    expect(_$.builtin.is(int32Type)).toBe(true);
+  });
+
+  it("should work with mutators", async () => {
+    // ------------------------------------------------------------
+    // A non-typespec model references a ModelProperty defined in TypeSpec
+    // ------------------------------------------------------------
+    const mutator: unsafe_MutatorWithNamespace = {
+      name: "test",
+      Namespace: {
+        mutate: (_ns, clone) => {
+          clone.models.delete("Bar");
+        },
+      },
+    };
+
+    (await runner.compile(`
+      namespace TypeSpec {
+        model InTypeSpec {
+          foo: string;
+        }
+      }
+
+      model Foo {
+        bar: TypeSpec.InTypeSpec.foo;
+      } 
+
+      model Bar {}
+    `)) as { InTypeSpec: Model; Foo: Model };
+
+    const program = runner.program;
+    const _$ = $(program);
+
+    const globalNs = program.getGlobalNamespaceType();
+    const mutatedGlobalNs = unsafe_mutateSubgraphWithNamespace(program, [mutator], globalNs)
+      .type as Namespace;
+
+    const Foo = mutatedGlobalNs.models.get("Foo")!;
+    const Bar = mutatedGlobalNs.models.get("Bar");
+
+    expect(Bar).toBeUndefined();
+    expect(Foo).toBeDefined();
+
+    expect(_$.builtin.is(Foo)).toBe(false);
+    expect(_$.builtin.is(Foo.properties.get("bar")!)).toBe(false);
+    expect(_$.builtin.is(Foo.properties.get("bar")!.type)).toBe(true);
+    expect(_$.builtin.is((Foo.properties.get("bar")!.type as ModelProperty).type)).toBe(true);
+  });
+});