modelcontextprotocol · mcp-commander · May 18, 2026 · Jan 7, 2026 · Jan 7, 2026 · Jan 7, 2026
@@ -452,6 +452,7 @@
           {
             "group": "Draft",
             "pages": [
+              "seps/2106-json-schema-2020-12",
               "seps/2484-conformance-tests-required-for-final-seps"
             ]
           }

@@ -473,7 +473,7 @@ Embedded resources support the same [Resource annotations](/specification/draft/
 
 #### Structured Content
 
-**Structured** content is returned as a JSON object in the `structuredContent` field of a result.
+**Structured** content is returned as a JSON value in the `structuredContent` field of a result. This can be any JSON value (object, array, string, number, boolean, or null) that conforms to the tool's `outputSchema` if one is defined.
 
 For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.
 
@@ -545,6 +545,53 @@ Example valid response for this tool:
 }
 ```
 
+Example tool with array output schema:
+
+```json
+{
+  "name": "list_users",
+  "title": "User List",
+  "description": "Returns a list of all users",
+  "inputSchema": {
+    "type": "object",
+    "properties": {}
+  },
+  "outputSchema": {
+    "type": "array",
+    "items": {
+      "type": "object",
+      "properties": {
+        "id": { "type": "string" },
+        "name": { "type": "string" },
+        "email": { "type": "string" }
+      },
+      "required": ["id", "name", "email"]
+    }
+  }
+}
+```
+
+Example valid response for a tool with array output:
+
+```json
+{
+  "jsonrpc": "2.0",
+  "id": 6,
+  "result": {
+    "content": [
+      {
+        "type": "text",
+        "text": "Found 2 users: Alice ([email protected]) and Bob ([email protected])."
+      }
+    ],
+    "structuredContent": [
+      { "id": "1", "name": "Alice", "email": "[email protected]" },
+      { "id": "2", "name": "Bob", "email": "[email protected]" }
+    ]
+  }
+}
+```
+
 Providing an output schema helps clients and LLMs understand and properly handle structured tool outputs by:
 
 - Enabling strict schema validation of responses

@@ -0,0 +1,13 @@
+{
+  "resultType": "complete",
+  "content": [
+    {
+      "type": "text",
+      "text": "Found 2 users: Alice ([email protected]) and Bob ([email protected])."
+    }
+  ],
+  "structuredContent": [
+    { "id": "1", "name": "Alice", "email": "[email protected]" },
+    { "id": "2", "name": "Bob", "email": "[email protected]" }
+  ]
+}
@@ -0,0 +1,30 @@
+{
+  "name": "list_users",
+  "title": "User List",
+  "description": "Returns a list of all users",
+  "inputSchema": {
+    "type": "object",
+    "properties": {}
+  },
+  "outputSchema": {
+    "type": "array",
+    "items": {
+      "type": "object",
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "User ID"
+        },
+        "name": {
+          "type": "string",
+          "description": "User name"
+        },
+        "email": {
+          "type": "string",
+          "description": "User email"
+        }
+      },
+      "required": ["id", "name", "email"]
+    }
+  }
+}
@@ -0,0 +1,28 @@
+{
+  "name": "find_resource",
+  "title": "Resource Finder",
+  "description": "Find a resource by ID or name",
+  "inputSchema": {
+    "type": "object",
+    "oneOf": [
+      {
+        "properties": {
+          "id": {
+            "type": "string",
+            "description": "Resource ID"
+          }
+        },
+        "required": ["id"]
+      },
+      {
+        "properties": {
+          "name": {
+            "type": "string",
+            "description": "Resource name"
+          }
+        },
+        "required": ["name"]
+      }
+    ]
+  }
+}
@@ -1641,9 +1641,12 @@ export interface CallToolResult extends Result {
   content: ContentBlock[];
 
   /**
-   * An optional JSON object that represents the structured result of the tool call.
+   * An optional JSON value that represents the structured result of the tool call.
+   *
+   * This can be any JSON value (object, array, string, number, boolean, or null)
+   * that conforms to the tool's outputSchema if one is defined.
    */
-  structuredContent?: { [key: string]: unknown };
+  structuredContent?: unknown;
 
   /**
    * Whether the tool call ended in an error.
@@ -1805,27 +1808,24 @@ export interface Tool extends BaseMetadata, Icons {
 
   /**
    * A JSON Schema object defining the expected parameters for the tool.
+   *
+   * Tool arguments are always JSON objects, so `type: "object"` is required at the root.
+   * Beyond that, any JSON Schema 2020-12 keyword may appear alongside `type` — including
+   * composition keywords (`oneOf`, `anyOf`, `allOf`, `not`), conditional keywords
+   * (`if`/`then`/`else`), reference keywords (`$ref`, `$defs`, `$anchor`), and any other
+   * standard validation or annotation keywords.
+   *
+   * Defaults to JSON Schema 2020-12 when no explicit `$schema` is provided.
    */
-  inputSchema: {
-    $schema?: string;
-    type: "object";
-    properties?: { [key: string]: JSONValue };
-    required?: string[];
-  };
+  inputSchema: { $schema?: string; type: "object"; [key: string]: unknown };
 
   /**
    * An optional JSON Schema object defining the structure of the tool's output returned in
-   * the structuredContent field of a {@link CallToolResult}.
+   * the structuredContent field of a {@link CallToolResult}. This can be any valid JSON Schema 2020-12.
    *
    * Defaults to JSON Schema 2020-12 when no explicit `$schema` is provided.
-   * Currently restricted to `type: "object"` at the root level.
    */
-  outputSchema?: {
-    $schema?: string;
-    type: "object";
-    properties?: { [key: string]: JSONValue };
-    required?: string[];
-  };
+  outputSchema?: { $schema?: string; [key: string]: unknown };
 
   /**
    * Optional additional tool information.
@@ -2240,11 +2240,12 @@ export interface ToolResultContent {
   content: ContentBlock[];
 
   /**
-   * An optional structured result object.
+   * An optional structured result value.
    *
+   * This can be any JSON value (object, array, string, number, boolean, or null).
    * If the tool defined an {@link Tool.outputSchema}, this SHOULD conform to that schema.
    */
-  structuredContent?: { [key: string]: unknown };
+  structuredContent?: unknown;
 
   /**
    * Whether the tool use resulted in an error.