[tcgc] linter raise for encoding mfd bytes

.changeset/chilly-ladybugs-jam.md

@@ -0,0 +1,5 @@
+---
+"@azure-tools/typespec-client-generator-core": minor
+---
+
+error out if user tries to encode bytes in multipart input

packages/typespec-client-generator-core/src/lib.ts

@@ -92,6 +92,13 @@ export const $lib = createTypeSpecLibrary({
         default: "@client or @operationGroup should decorate namespace or interface in client.tsp",
       },
     },
+    "encoding-multipart-bytes": {
+      severity: "error",
+      messages: {
+        default:
+          "Encoding should not be applied to bytes content in a multipart request. This is semi-incompatible with how multipart works in HTTP.",
+      },
+    },
   },
 });
 

packages/typespec-client-generator-core/src/types.ts

@@ -804,6 +804,12 @@ function getSdkBodyModelPropertyType(
   const isBytesInput =
     base.type.kind === "bytes" ||
     (base.type.kind === "array" && base.type.valueType.kind === "bytes");
+  if (isBytesInput && getEncode(context.program, type)) {
+    reportDiagnostic(context.program, {
+      code: "encoding-multipart-bytes",
+      target: type,
+    });
+  }
   return {
     ...base,
     kind: "property",

packages/typespec-client-generator-core/test/types.test.ts

@@ -2101,6 +2101,24 @@ describe("typespec-client-generator-core: types", () => {
       strictEqual(pictures.kind, "property");
       strictEqual(pictures.isMultipartFileInput, true);
     });
+
+    it("multipart with encoding bytes raises error", async function () {
+      const diagnostics = await runner.diagnose(
+        `
+        @service({title: "Test Service"}) namespace TestService;
+        model EncodedBytesMFD {
+          @encode("base64")
+          pictures: bytes;
+        }
+
+        @put op multipartOp(@header contentType: "multipart/form-data", @body body: EncodedBytesMFD): void;
+        `
+      );
+      getAllModels(runner.context);
+      expectDiagnostics(diagnostics, {
+        code: "@azure-tools/typespec-client-generator-core/encoding-multipart-bytes",
+      });
+    });
   });
   describe("SdkTupleType", () => {
     it("model with tupled properties", async function () {