diff --git a/common/changes/@cadl-lang/compiler/fix-proj-plus-GET_2022-08-06-01-58.json b/common/changes/@cadl-lang/compiler/fix-proj-plus-GET_2022-08-06-01-58.json
new file mode 100644
index 0000000000..28dad57fd4
--- /dev/null
+++ b/common/changes/@cadl-lang/compiler/fix-proj-plus-GET_2022-08-06-01-58.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@cadl-lang/compiler",
+      "comment": "Run projections on types returned from getEffectiveType",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@cadl-lang/compiler"
+}
\ No newline at end of file
diff --git a/common/changes/@cadl-lang/openapi3/fix-proj-plus-GET_2022-08-06-01-58.json b/common/changes/@cadl-lang/openapi3/fix-proj-plus-GET_2022-08-06-01-58.json
new file mode 100644
index 0000000000..e2ee484c99
--- /dev/null
+++ b/common/changes/@cadl-lang/openapi3/fix-proj-plus-GET_2022-08-06-01-58.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@cadl-lang/openapi3",
+      "comment": "Run projections on types returned from getEffectiveType",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@cadl-lang/openapi3"
+}
\ No newline at end of file
diff --git a/packages/compiler/core/checker.ts b/packages/compiler/core/checker.ts
index 0cc0f67100..c4fc004c8a 100644
--- a/packages/compiler/core/checker.ts
+++ b/packages/compiler/core/checker.ts
@@ -4110,6 +4110,19 @@ export function createChecker(program: Program): Checker {
     return false;
   }
 
+  function getProjectedEffectiveModelType(type: ModelType): ModelType {
+    if (!program.currentProjector) {
+      return type;
+    }
+
+    const projectedType = program.currentProjector.projectType(type);
+    if (projectedType.kind !== "Model") {
+      compilerAssert(false, "Fail");
+    }
+
+    return projectedType;
+  }
+
   function getEffectiveModelType(
     model: ModelType,
     filter?: (property: ModelTypeProperty) => boolean
@@ -4120,7 +4133,7 @@ export function createChecker(program: Program): Checker {
 
     if (model.name) {
       // named model
-      return model;
+      return getProjectedEffectiveModelType(model);
     }
 
     // We would need to change the algorithm if this doesn't hold. We
@@ -4188,7 +4201,7 @@ export function createChecker(program: Program): Checker {
       }
     }
 
-    return match ?? model;
+    return match ? getProjectedEffectiveModelType(match) : model;
   }
 
   function filterModelProperties(
diff --git a/packages/compiler/lib/decorators.ts b/packages/compiler/lib/decorators.ts
index a87419fadf..a6f1b96234 100644
--- a/packages/compiler/lib/decorators.ts
+++ b/packages/compiler/lib/decorators.ts
@@ -449,7 +449,7 @@ export function $withOptionalProperties(context: DecoratorContext, target: Type)
   target.properties.forEach((p) => (p.optional = true));
 }
 
-// -- @withUpdatableProperties decorator ----------------------
+// -- @withUpdateableProperties decorator ----------------------
 
 export function $withUpdateableProperties(context: DecoratorContext, target: Type) {
   if (!validateDecoratorTarget(context, target, "@withUpdateableProperties", "Model")) {
diff --git a/packages/openapi3/test/test-host.ts b/packages/openapi3/test/test-host.ts
index 591e8735cb..ef89b111f3 100644
--- a/packages/openapi3/test/test-host.ts
+++ b/packages/openapi3/test/test-host.ts
@@ -15,14 +15,22 @@ export async function createOpenAPITestHost() {
   });
 }
 
-export async function createOpenAPITestRunner() {
+export async function createOpenAPITestRunner({
+  withVersioning,
+}: { withVersioning?: boolean } = {}) {
   const host = await createOpenAPITestHost();
-  return createTestWrapper(
-    host,
-    (code) =>
-      `import "@cadl-lang/rest"; import "@cadl-lang/openapi"; import "@cadl-lang/openapi3"; using Cadl.Rest; using Cadl.Http; using OpenAPI; ${code}`,
-    { emitters: { "@cadl-lang/openapi3": {} } }
-  );
+  const importAndUsings = `
+  import "@cadl-lang/rest"; import "@cadl-lang/openapi";
+  import "@cadl-lang/openapi3"; 
+  ${withVersioning ? `import "@cadl-lang/versioning"` : ""};
+  using Cadl.Rest;
+  using Cadl.Http;
+  using OpenAPI;
+  ${withVersioning ? "using Cadl.Versioning;" : ""}
+`;
+  return createTestWrapper(host, (code) => `${importAndUsings} ${code}`, {
+    emitters: { "@cadl-lang/openapi3": {} },
+  });
 }
 
 function versionedOutput(path: string, version: string) {
diff --git a/packages/openapi3/test/versioning.test.ts b/packages/openapi3/test/versioning.test.ts
index 725a701012..f5567038a7 100644
--- a/packages/openapi3/test/versioning.test.ts
+++ b/packages/openapi3/test/versioning.test.ts
@@ -1,7 +1,7 @@
 import { DecoratorContext, NamespaceType } from "@cadl-lang/compiler";
 import { createTestWrapper } from "@cadl-lang/compiler/testing";
 import { deepStrictEqual, strictEqual } from "assert";
-import { createOpenAPITestHost, openApiFor } from "./test-host.js";
+import { createOpenAPITestHost, createOpenAPITestRunner, openApiFor } from "./test-host.js";
 
 describe("openapi3: versioning", () => {
   it("works with models", async () => {
@@ -118,19 +118,16 @@ describe("openapi3: versioning", () => {
         `import "@cadl-lang/rest"; import "@cadl-lang/openapi";
           import "@cadl-lang/openapi3"; import "@cadl-lang/versioning";
           import "./test.js";
-
           using Cadl.Rest; using Cadl.Http; using OpenAPI; using Cadl.Versioning; ${code}`,
       { emitters: { "@cadl-lang/openapi3": {} } }
     );
 
     await runner.compile(`
-
     @versioned(Contoso.Library.Versions)
     namespace Contoso.Library {
       namespace Blah { }
       enum Versions { v1 };
     }
-
     @armNamespace
     @serviceTitle("Widgets 'r' Us")
     @versionedDependency(Contoso.Library.Versions.v1)
@@ -140,7 +137,6 @@ describe("openapi3: versioning", () => {
         @segment("widgets")
         id: string;
       }
-
       interface Operations {
         @test
         op get(id: string): Widget;
@@ -150,4 +146,33 @@ describe("openapi3: versioning", () => {
 
     strictEqual(storedNamespace, "Contoso.WidgetService");
   });
+
+  // Test for https://github.com/microsoft/cadl/issues/812
+  it("doesn't throw errors when using UpdateableProperties", async () => {
+    // if this test throws a duplicate name diagnostic, check that getEffectiveType
+    // is returning the projected type.
+    const runner = await createOpenAPITestRunner({ withVersioning: true });
+    await runner.compile(`
+      @versioned(Library.Versions)
+      namespace Library {
+        enum Versions {
+          v1,
+          v2,
+        }
+      }
+      
+      @serviceTitle("Service")
+      @versionedDependency(Library.Versions.v1)
+      namespace Service {
+        model Widget {
+          details?: WidgetDetails;
+        }
+      
+        model WidgetDetails {}
+        interface Projects {
+          oops(...UpdateableProperties<Widget>): Widget;
+        }
+      }
+    `);
+  });
 });