RFC: Client / Server Content capabilities

docs/docs/concepts/tools.mdx

@@ -333,6 +333,7 @@ The MCP specification defines the following annotations for tools:
 | `destructiveHint` | boolean | true | If true, the tool may perform destructive updates (only meaningful when `readOnlyHint` is false) |
 | `idempotentHint` | boolean | false | If true, calling the tool repeatedly with the same arguments has no additional effect (only meaningful when `readOnlyHint` is false) |
 | `openWorldHint` | boolean | true | If true, the tool may interact with an "open world" of external entities |
+| `generatesHint` | string[] | None | If present, provides non-exhaustive list of MIME types this Tool may generate in a `CallToolResult`. |
 
 ### Example usage
 

docs/specification/2025-03-26/basic/lifecycle.mdx

@@ -144,16 +144,17 @@ available during the session.
 
 Key capabilities include:
 
-| Category | Capability     | Description                                                                |
-| -------- | -------------- | -------------------------------------------------------------------------- |
+| Category | Capability     | Description                                                                         |
+| -------- | -------------- | ----------------------------------------------------------------------------------- |
 | Client   | `roots`        | Ability to provide filesystem [roots](/specification/2025-03-26/client/roots)       |
 | Client   | `sampling`     | Support for LLM [sampling](/specification/2025-03-26/client/sampling) requests      |
-| Client   | `experimental` | Describes support for non-standard experimental features                   |
+| Client   | `contentTypes` | Describes content types the Host can show the User, or tokenize for the LLM         |
+| Client   | `experimental` | Describes support for non-standard experimental features                            |
 | Server   | `prompts`      | Offers [prompt templates](/specification/2025-03-26/server/prompts)                 |
 | Server   | `resources`    | Provides readable [resources](/specification/2025-03-26/server/resources)           |
 | Server   | `tools`        | Exposes callable [tools](/specification/2025-03-26/server/tools)                    |
 | Server   | `logging`      | Emits structured [log messages](/specification/2025-03-26/server/utilities/logging) |
-| Server   | `experimental` | Describes support for non-standard experimental features                   |
+| Server   | `experimental` | Describes support for non-standard experimental features                            |
 
 Capability objects can describe sub-capabilities like:
 

schema/2025-03-26/schema.json

@@ -170,6 +170,26 @@
         "ClientCapabilities": {
             "description": "Capabilities a client may support. Known capabilities are defined here, in this schema, but this is not a closed set: any client can define its own, additional capabilities.",
             "properties": {
+                "contentTypes": {
+                    "description": "Present if the client advertises content types it can handle.",
+                    "properties": {
+                        "renders": {
+                            "description": "Optional non-exclusive List of MIME types that the client can render to Users.",
+                            "items": {
+                                "type": "string"
+                            },
+                            "type": "array"
+                        },
+                        "tokenizes": {
+                            "description": "Optional non-exclusive list of MIME types that the client can tokenize for the LLM.",
+                            "items": {
+                                "type": "string"
+                            },
+                            "type": "array"
+                        }
+                    },
+                    "type": "object"
+                },
                 "experimental": {
                     "additionalProperties": {
                         "additionalProperties": true,
@@ -2064,6 +2084,13 @@
                     "description": "If true, the tool may perform destructive updates to its environment.\nIf false, the tool performs only additive updates.\n\n(This property is meaningful only when `readOnlyHint == false`)\n\nDefault: true",
                     "type": "boolean"
                 },
+                "generatesHint": {
+                    "description": "Optional list of MIME types that this Tool may produce in a CallToolResult.",
+                    "items": {
+                        "type": "string"
+                    },
+                    "type": "array"
+                },
                 "idempotentHint": {
                     "description": "If true, calling the tool repeatedly with the same arguments\nwill have no additional effect on the its environment.\n\n(This property is meaningful only when `readOnlyHint == false`)\n\nDefault: false",
                     "type": "boolean"

schema/2025-03-26/schema.ts

@@ -220,6 +220,19 @@ export interface ClientCapabilities {
    * Present if the client supports sampling from an LLM.
    */
   sampling?: object;
+  /**
+   * Present if the client advertises content types it can handle.
+   */
+  contentTypes?: {
+    /**
+     * Optional non-exclusive List of MIME types that the client can render to Users.
+     */
+    renders?: string[];
+    /**
+     * Optional non-exclusive list of MIME types that the client can tokenize for the LLM.
+     */
+    tokenizes?: string[];
+  };
 }
 
 /**
@@ -269,6 +282,7 @@ export interface ServerCapabilities {
      */
     listChanged?: boolean;
   };
+
 }
 
 /**
@@ -776,6 +790,12 @@ export interface ToolAnnotations {
    * Default: true
    */
   openWorldHint?: boolean;
+
+
+  /**
+   * Optional list of MIME types that this Tool may produce in a CallToolResult.
+   */
+  generatesHint?: string[];
 }
 
 /**