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+This doc is to design TCGC API of multipart for language emitters.
+
+# multipart payload
+
+Here is classic payload of multipart:
+```
+POST /upload HTTP/1.1
+Content-Length: 428
+Content-Type: multipart/form-data; boundary=abcde12345
+--abcde12345
+Content-Disposition: form-data; name="id"
+Content-Type: text/plain
+
+123e4567-e89b-12d3-a456-426655440000
+--abcde12345
+Content-Disposition: form-data; name="profileImage"; filename="image1.png"
+Content-Type: image/png
+
+{…file content…}
+--abcde12345--
+```
+According to https://datatracker.ietf.org/doc/html/rfc7578, multipart request payload contains multi independent parts which could be divided into two kinds: one is non-file part which contains `required "name"/optional "Content-Type" / content`; the other one is file part which contains one more `optional "filename"`.
+
+# Current TCGC API for multipart
+Currently TCGC only has boolean flag [isMultipartFileInput](https://github.com/Azure/typespec-azure/blob/ab7a066d4ac0ae23a40f9ff8f4b6037559bda34c/packages/typespec-client-generator-core/src/interfaces.ts#L368) to distinguish file and non-file. After https://github.com/microsoft/typespec/issues/3046 complete, Typespec permit users to define more info explicitly(e.g. content-type) for each part so boolean flag is not enough.
+
+# Multipart in Typespec
+Typespec support two kinds of definition for multipart:
+
+1. Model format
+
+```
+op upload(
+  @header `content-type`: "multipart/form-data",
+  @multipartBody body: {
+    fullName: HttpPart<string>,
+    headShots: HttpPart<Image>[]
+  }
+): void;
+```
+
+2. Tuple format
+
+ ```
+op upload(
+  @header `content-type`: "multipart/form-data",
+  @multipartBody body: [
+    HttpPart<string, #{ name: "fullName" }>,
+    HttpPart<Image, #{ name: "headShots" }>[]
+  ]
+): void;
+```
+
+# Proposal about new TCGC API for multipart
+Since all language emitters emit multipart body as model format, TCGC will uniform the multipart body type as model, and each model property equals to property of model format or part of tuple format in Typespec.
+
+```typescript
+export interface multipartOptionsType {
+  isNameDefined: boolean; // whether name is defined in Typespec. For multipart/mixed, name may not be defined for some parts
+  isFilePart: boolean; // whether this part is for file
+  isMulti: boolean; // whether this part is multi in request payload
+  headers: HeaderProperty[]; // relates to custom header
+  filename?: SdkModelPropertyTypeBase;
+  contentType?: SdkModelPropertyTypeBase;
+}
+
+export interface SdkBodyModelPropertyType extends SdkModelPropertyTypeBase {
+  kind: "property";
+  discriminator: boolean;
+  serializedName: string;
+  isMultipartFileInput: boolean;  // deprecated
+  multipartOptions?: multipartOptionsType; // new options for multipart
+  visibility?: Visibility[];
+  flatten: boolean;
+}
+```
+
+notes:
+- `isNameDefined`: Whether name is defined in Typespec. For multipart/mixed, name may not be defined for some parts.
+- `isFilePart`: Same with `isMultipartFileInput` before
+- `isMulti`: Mainly for explicity of `Type[]`. In old design for `Model[]`, Typespec can't declare it clearly that SDK shall
+ (a) serialize array of model as single part or (b) serialize model as single part then send it multi times. With new design, if
+ `HttpPart<Model[]>`, isMulti is false and SDK shall follow (a); if `HttpPart<Model>[]`, isMulti is true and follow (b)
+- `headers`: Equals to custom headers in swagger https://swagger.io/docs/specification/describing-request-body/multipart-requests/  
+- `filename`: When Typespec author use `httpFile` change requiredness for optional metadata properties "filename", this property has value; otherwise it is "undefined".
+- `contentType`: When Typespec author use `httpFile` change requiredness for optional metadata properties "contentType", this property has value; otherwise it is "undefined".
+
+# Examples
+
+## Define "multipart/form-data" with "@body" (compatible for old scenario)
+Typespec example:
+
+```
+model JSON {
+  name: string;
+}
+
+op upload(
+  @header `content-type`: "multipart/form-data",
+  @body body: {
+    basic: string,
+    singleJson: JSON,
+    singleBytes: bytes,
+    multiJsonMultiParts: JSON[],
+    multiBytes: bytes[]
+  }
+): void;
+```
+
+
+TCGC API example:
+
+```json
+
+{
+  "properties": [
+    {
+      "name": "basic",
+      "type": { "kind": "string" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "singleJson",
+      "type": { "kind": "model" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "singleBytes",
+      "type": { "kind": "bytes" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "multiJsonMultiParts",
+      "type": { "kind": "model" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": true,
+        "headers": []
+      }
+    },
+    {
+      "name": "multiBytes",
+      "type": { "kind": "bytes" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": true,
+        "headers": []
+      }
+    }
+  ]
+}
+
+```
+
+## Define "multipart/form-data" with "@multipartBody"
+Typespec example:
+
+```
+// "File" is predefined model for user to declare requiredness of metadata properties in part of multipart payload: https://github.com/microsoft/typespec/blob/602fe73692dddf7897363b2ac5d98f5871954612/packages/http/lib/http.tsp#L109-L114
+
+model RequiredFileMetadata is File {
+  contentType: string;
+  filename: string;
+}
+
+op upload(
+  @header `content-type`: "multipart/form-data",
+  @multipartBody body: [
+    // single
+    HttpPart<string, #{ name: "basic" }>,
+    HttpPart<JSON, #{ name: "singleJson" }>,
+    HttpPart<bytes, #{ name: "singleBytes" }>,
+    HttpPart<File, #{ name: "singleFileWithOptionalMetadata" }>,
+    HttpPart<RequiredFileMetadata, #{ name: "singleFileWithRequiredMetadata" }>,
+
+    // multi
+    HttpPart<JSON[], #{ name: "multiJsonOnePart" }>,
+    HttpPart<bytes, #{ name: "multiBytes" }>[],
+    HttpPart<JSON, #{ name: "multiJsonMultiParts" }>[],
+
+    // custom header
+    HttpPart<{ @header `x-ms-header`: "x-custom", @body image: File }>
+  ]
+): void;
+```
+
+TCGC API example:
+
+```json
+
+{
+  "properties": [
+    {
+      "name": "basic",
+      "type": { "kind": "string" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "singleJson",
+      "type": { "kind": "model" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "singleBytes",
+      "type": { "kind": "bytes" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "singleFileWithOptionalMetadata",
+      "type": { "kind": "model" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": false,
+        "headers": [],
+        "fileName": { "optional": true },
+        "contentType": { "optional": true }
+      }
+    },
+    {
+      "name": "singleFileWithRequiredMetadata",
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": false,
+        "headers": [],
+        "fileName": { "optional": false },
+        "contentType": { "optional": false }
+      }
+    },
+    {
+      "name": "multiJsonOnePart",
+      "type": { "kind": "array" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    },
+    {
+      "name": "multiBytes",
+      "type": { "kind": "bytes" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": true,
+        "headers": []
+      }
+    },
+    {
+      "name": "multiJsonMultiParts",
+      "type": { "kind": "model" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": true,
+        "headers": []
+      }
+    },
+    {
+      "name": "image",
+      "type": { "kind": "bytes" },
+      "multipartOptions": {
+        "isNameDefined": true,
+        "isFilePart": true,
+        "isMulti": false,
+        "headers": [{"kind": "header", "name": "x-ms-header"}]
+      }
+    }
+  ]
+}
+
+```
+
+## Define "multipart/mixed" with "@multipartBody"
+Typespec example:
+
+```
+op upload(
+  @header `content-type`: "multipart/mixed",
+  @multipartBody body: [HttpPart<string>]): void;
+```
+
+TCGC API example:
+
+```json
+{
+  "properties": [
+    {
+      "name": "",
+      "type": { "kind": "string" },
+      "multipartOptions": {
+        "isNameDefined": false,
+        "isFilePart": false,
+        "isMulti": false,
+        "headers": []
+      }
+    }
+  ]
+}
+```