Conform Tool Schemas to JSON Schema 2020-12 per SEP-2106

README.md

@@ -698,8 +698,28 @@ class WeatherTool < MCP::Tool
 end
 ```
 
-Please note: in this case, you must provide `type: "array"`. The default type
-for output schemas is `object`.
+Please note: in this case, you must provide `type: "array"`. The default type for output schemas is `object`,
+applied only when the schema declares no root keyword (`type`, `$ref`, `oneOf`, `anyOf`, `allOf`, `not`, `if`, `const`, `enum`).
+
+Per SEP-2106, an output schema may be any valid JSON Schema 2020-12 document, including a primitive root
+(`{ type: "string" }`) or a root-level composition:
+
+```ruby
+class FlexibleTool < MCP::Tool
+  output_schema(
+    oneOf: [
+      { type: "string" },
+      { type: "array", items: { type: "number" } }
+    ]
+  )
+end
+```
+
+Input schemas keep `type: "object"` at the root but accept the full 2020-12 vocabulary below it
+(`$defs`/`$ref`, `oneOf`/`anyOf`/`allOf`/`not`, `if`/`then`/`else`). Two resource bounds apply to
+all tool schemas: only same-document `$ref`s (starting with `#`) are accepted, and documents are
+capped at `MCP::Tool::Schema::MAX_SCHEMA_DEPTH` nesting levels and `MCP::Tool::Schema::MAX_SUBSCHEMA_COUNT` subschema objects;
+violations raise `ArgumentError` at construction time.
 
 MCP spec for the [Output Schema](https://modelcontextprotocol.io/specification/latest/server/tools#output-schema) specifies that:
 
@@ -729,6 +749,10 @@ Tools can return structured data alongside text content using the `structured_co
 
 The structured content will be included in the JSON-RPC response as the `structuredContent` field.
 
+Per SEP-2106, `structured_content` may be any JSON value, not only an object. When a tool returns a non-object value (e.g. an array)
+without providing any content blocks, the server automatically mirrors it into `content` as serialized JSON text so older clients
+that only read `content` still receive the data.
+
 ```ruby
 class WeatherTool < MCP::Tool
   description "Get current weather and return structured data"

lib/mcp/server.rb

@@ -636,7 +636,7 @@ def call_tool(request, session: nil, related_request_id: nil, cancellation: nil)
         tool, arguments, server_context_with_meta(request), progress_token: progress_token, session: session, related_request_id: related_request_id, cancellation: cancellation
       )
       validate_tool_call_result!(tool, result)
-      result
+      serialize_structured_content_fallback(result)
     rescue RequestHandlerError, CancelledError
       # CancelledError is intentionally not wrapped so `handle_request` can turn it into
       # `JsonRpcHandler::NO_RESPONSE` per the MCP cancellation spec.
@@ -801,6 +801,18 @@ def validate_tool_call_result!(tool, result)
       tool.output_schema.validate_result(result[:structuredContent])
     end
 
+    # Per SEP-2106, `structuredContent` may be any JSON value, not only an object.
+    # Clients on older protocol versions may only read `content`,
+    # so when a tool returns non-object structured content without providing
+    # any content blocks, mirror the value into `content` as serialized JSON text.
+    def serialize_structured_content_fallback(result)
+      structured = result[:structuredContent]
+      return result if structured.nil? || structured.is_a?(Hash)
+      return result unless result[:content].nil? || result[:content].empty?
+
+      result.merge(content: [{ type: "text", text: JSON.generate(structured) }])
+    end
+
     # Whether a tool/prompt handler opts in to receiving an `MCP::ServerContext`.
     # Recognizes `:keyrest` (`**kwargs`) because tools are invoked without a positional argument
     # (`tool.call(**args, server_context:)`), soa `**kwargs`-only signature safely captures `server_context:`.

lib/mcp/tool/output_schema.rb

@@ -7,12 +7,29 @@ class Tool
     class OutputSchema < Schema
       class ValidationError < StandardError; end
 
+      # Root-level keywords whose presence means the user already chose a root schema shape,
+      # so no `type: "object"` default should be merged in.
+      ROOT_SCHEMA_KEYWORDS = [:type, :"$ref", :oneOf, :anyOf, :allOf, :not, :if, :const, :enum].freeze
+
       def validate_result(result)
         errors = fully_validate(result)
         if errors.any?
           raise ValidationError, "Invalid result: #{errors.join(", ")}"
         end
       end
+
+      private
+
+      # Per SEP-2106, an output schema may be ANY valid JSON Schema 2020-12 document: object, array, primitive,
+      # or a root-level composition.
+      # Default the root to an object only when no root schema keyword is present, which preserves the wire output
+      # of the common `properties`-only shape while leaving e.g. `{ type: "array" }` or `{ oneOf: [...] }` untouched
+      # (the old unconditional default merged `type: "object"` into root combinators, producing a wrong schema).
+      def apply_default_root_type!
+        return if ROOT_SCHEMA_KEYWORDS.any? { |keyword| @schema.key?(keyword) }
+
+        super
+      end
     end
   end
 end

lib/mcp/tool/schema.rb

@@ -36,16 +36,29 @@ def clear
       end
       VALIDATION_CACHE = ValidationCache.new
 
-      # JSON Schema 2020-12 is the default dialect for MCP schema definitions
-      # per MCP 2025-11-25 (SEP-1613). Note: emission only — runtime validation
-      # is still performed against the JSON Schema draft-04 metaschema.
+      # JSON Schema 2020-12 is the default dialect for MCP schema definitions per MCP 2025-11-25 (SEP-1613),
+      # and SEP-2106 requires tool schemas to conform to the full 2020-12 vocabulary. Both emission and
+      # runtime validation use this dialect. Because MCP mandates 2020-12, the SDK validates against it
+      # regardless of any `$schema` a document embeds; for compliant schemas this is the same dialect
+      # the Python SDK's `jsonschema.validate` resolves to.
       JSON_SCHEMA_2020_12_URI = "https://json-schema.org/draft/2020-12/schema"
 
-      DRAFT4_META_SCHEMA_URI = "http://json-schema.org/draft-04/schema#"
+      # Resource bounds for schema compilation, mirroring the TypeScript SDK's schema bounds (SEP-2106):
+      # schemas may use the full JSON Schema 2020-12 vocabulary including composition keywords and `$ref`,
+      # so adversarial documents must be rejected before they can cause excessive validation cost.
+      # Only same-document references (starting with `#`) are accepted, so schema handling can never trigger network
+      # or file access.
+      MAX_SCHEMA_DEPTH = 64
+      MAX_SUBSCHEMA_COUNT = 10_000
+
+      # Reference keywords whose targets the SDK refuses to dereference. Both `$ref` and `$dynamicRef` may carry
+      # an absolute URI under JSON Schema 2020-12, so a non-same-document value is an external reference.
+      REFERENCE_KEYWORDS = [:"$ref", :"$dynamicRef"].freeze
 
       def initialize(schema = {})
         @schema = JSON.parse(JSON.dump(schema), symbolize_names: true)
-        @schema[:type] ||= "object"
+        apply_default_root_type!
+        validate_schema_bounds!
         validate_schema!
       end
 
@@ -61,6 +74,48 @@ def to_h
 
       private
 
+      # Root-type defaulting hook. The base class preserves the historical behavior of defaulting the root
+      # to an object schema; `OutputSchema` overrides this because SEP-2106 allows any root schema there.
+      def apply_default_root_type!
+        @schema[:type] ||= "object"
+      end
+
+      # Enforces `MAX_SCHEMA_DEPTH` / `MAX_SUBSCHEMA_COUNT` and the same-document reference rule over
+      # the whole schema document.
+      def validate_schema_bounds!
+        subschema_count = 0
+        stack = [[@schema, 1]]
+
+        until stack.empty?
+          node, depth = stack.pop
+          if depth > MAX_SCHEMA_DEPTH
+            raise ArgumentError,
+              "Invalid JSON Schema: nesting exceeds the maximum depth of #{MAX_SCHEMA_DEPTH}."
+          end
+
+          case node
+          when Hash
+            subschema_count += 1
+            if subschema_count > MAX_SUBSCHEMA_COUNT
+              raise ArgumentError,
+                "Invalid JSON Schema: document exceeds the maximum of #{MAX_SUBSCHEMA_COUNT} subschema objects."
+            end
+
+            REFERENCE_KEYWORDS.each do |keyword|
+              ref = node[keyword]
+              next unless ref.is_a?(String) && !ref.start_with?("#")
+
+              raise ArgumentError,
+                "Invalid JSON Schema: only same-document #{keyword} (starting with '#') is supported, got #{ref.inspect}."
+            end
+
+            node.each_value { |child| stack << [child, depth + 1] }
+          when Array
+            node.each { |child| stack << [child, depth + 1] }
+          end
+        end
+      end
+
       def stringify(obj)
         case obj
         when Hash
@@ -78,13 +133,16 @@ def stringify(obj)
       # Memoized per Schema instance because schema content is fixed at construction,
       # so the compiled schemer is reusable across many `fully_validate` calls.
       #
+      # Validated against the JSON Schema 2020-12 metaschema per SEP-2106, so `$defs`/`$ref` and
+      # the rest of the 2020-12 vocabulary resolve natively.
+      #
       # `format: false` preserves the legacy behavior of the previous `json-schema` based implementation,
       # which did not enforce `format` keywords. `RegexpError` from a malformed `pattern` is re-raised as
       # `ArgumentError` so callers see the same exception class they used to.
       def schemer
         @schemer ||= JSONSchemer.schema(
           stringify(schema_for_validation),
-          meta_schema: DRAFT4_META_SCHEMA_URI,
+          meta_schema: JSON_SCHEMA_2020_12_URI,
           format: false,
         )
       rescue RegexpError => e
@@ -112,8 +170,8 @@ def validate_schema!
         VALIDATION_CACHE.store(key)
       end
 
-      # `json_schemer` is pinned to the draft-04 metaschema, so strip top-level `$schema` before validation:
-      # this preserves the legacy behavior of ignoring the advertised dialect URI when the SDK validates schemas.
+      # Strip the top-level `$schema` before validation so the SDK always validates against
+      # the 2020-12 metaschema (SEP-2106) regardless of any dialect URI a caller embedded in the document.
       def schema_for_validation
         return @schema unless @schema.key?(:"$schema")
 

test/mcp/server_test.rb

@@ -2884,6 +2884,58 @@ def server_context
       end
     end
 
+    test "#handle tools/call mirrors non-object structuredContent into serialized text content" do
+      # Per SEP-2106, `structuredContent` may be any JSON value. Older clients may only read `content`,
+      # so the server adds a serialized fallback when the tool provided no content blocks.
+      server = Server.new(name: "structured_test", tools: [])
+      server.define_tool(name: "array_tool") do
+        Tool::Response.new(nil, structured_content: [1, 2])
+      end
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        method: "tools/call",
+        id: 1,
+        params: { name: "array_tool", arguments: {} },
+      })
+
+      assert_equal [1, 2], response.dig(:result, :structuredContent)
+      assert_equal [{ type: "text", text: "[1,2]" }], response.dig(:result, :content)
+    end
+
+    test "#handle tools/call does not overwrite explicit content when structuredContent is non-object" do
+      server = Server.new(name: "structured_test", tools: [])
+      server.define_tool(name: "array_tool") do
+        Tool::Response.new([{ type: "text", text: "two items" }], structured_content: [1, 2])
+      end
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        method: "tools/call",
+        id: 1,
+        params: { name: "array_tool", arguments: {} },
+      })
+
+      assert_equal [{ type: "text", text: "two items" }], response.dig(:result, :content)
+    end
+
+    test "#handle tools/call leaves object structuredContent without a text fallback" do
+      server = Server.new(name: "structured_test", tools: [])
+      server.define_tool(name: "object_tool") do
+        Tool::Response.new(nil, structured_content: { answer: 42 })
+      end
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        method: "tools/call",
+        id: 1,
+        params: { name: "object_tool", arguments: {} },
+      })
+
+      assert_equal({ answer: 42 }, response.dig(:result, :structuredContent))
+      assert_empty response.dig(:result, :content)
+    end
+
     test "#handle tools/list returns paginated results when page_size is set" do
       tool_a = Tool.define(name: "tool_a", title: "Tool A", description: "Tool A")
       tool_b = Tool.define(name: "tool_b", title: "Tool B", description: "Tool B")

test/mcp/tool/input_schema_test.rb

@@ -101,6 +101,22 @@ class InputSchemaTest < ActiveSupport::TestCase
         end
       end
 
+      test "rejects a draft-04-only boolean exclusiveMinimum under the 2020-12 dialect" do
+        # SEP-2106 validates tool schemas against the JSON Schema 2020-12 metaschema,
+        # where `exclusiveMinimum` must be a number. The draft-04 boolean form (deprecated since draft-06)
+        # is rejected at construction, matching the Python SDK's `jsonschema` validator selection.
+        error = assert_raises(ArgumentError) do
+          InputSchema.new(properties: { age: { type: "integer", minimum: 0, exclusiveMinimum: true } })
+        end
+        assert_includes error.message, "Invalid JSON Schema"
+      end
+
+      test "accepts the 2020-12 numeric exclusiveMinimum form" do
+        assert_nothing_raised do
+          InputSchema.new(properties: { age: { type: "integer", exclusiveMinimum: 0 } })
+        end
+      end
+
       test "schema without required arguments is valid" do
         assert_nothing_raised do
           InputSchema.new(properties: { foo: { type: "string" } })
@@ -183,6 +199,26 @@ class InputSchemaTest < ActiveSupport::TestCase
         assert_equal "#/definitions/bar", schema.to_h[:properties][:foo][:$ref]
       end
 
+      test "keeps the object root type while accepting 2020-12 composition keywords" do
+        # Per SEP-2106, an input schema root must stay `type: "object"` but may use
+        # the full 2020-12 vocabulary below the root.
+        schema = InputSchema.new(
+          "$defs": { name: { type: "string", minLength: 1 } },
+          properties: {
+            name: { "$ref": "#/$defs/name" },
+            value: { oneOf: [{ type: "string" }, { type: "integer" }] },
+          },
+          if: { properties: { value: { type: "integer" } } },
+          then: { required: ["name"] },
+          allOf: [{ properties: { extra: { type: "boolean" } } }],
+        )
+
+        assert_equal "object", schema.to_h[:type]
+        assert schema.to_h.key?(:"$defs")
+        assert schema.to_h.key?(:if)
+        assert schema.to_h.key?(:allOf)
+      end
+
       test "== compares two input schemas with the same properties, required fields" do
         schema1 = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])
         schema2 = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])

test/mcp/tool/output_schema_test.rb

@@ -215,6 +215,51 @@ class OutputSchemaTest < ActiveSupport::TestCase
         end
       end
 
+      test "does not inject a root type into a root-level oneOf schema" do
+        # Per SEP-2106, an output schema may be any JSON Schema 2020-12 document.
+        # Merging `type: "object"` into a root combinator would produce a wrong schema.
+        schema = OutputSchema.new(oneOf: [{ type: "string" }, { type: "integer" }])
+
+        refute schema.to_h.key?(:type)
+        assert_equal [{ type: "string" }, { type: "integer" }], schema.to_h[:oneOf]
+        assert_nothing_raised { schema.validate_result("text") }
+        assert_nothing_raised { schema.validate_result(42) }
+        assert_raises(OutputSchema::ValidationError) { schema.validate_result(1.5) }
+      end
+
+      test "does not inject a root type into a root-level $ref schema" do
+        schema = OutputSchema.new(
+          "$ref": "#/$defs/result",
+          "$defs": { result: { type: "string" } },
+        )
+
+        refute schema.to_h.key?(:type)
+        assert_nothing_raised { schema.validate_result("text") }
+        assert_raises(OutputSchema::ValidationError) { schema.validate_result(42) }
+      end
+
+      test "allows primitive root schemas" do
+        schema = OutputSchema.new(type: "string")
+
+        assert_nothing_raised { schema.validate_result("text") }
+        assert_raises(OutputSchema::ValidationError) { schema.validate_result(42) }
+      end
+
+      test "does not inject a root type into a root-level enum schema" do
+        schema = OutputSchema.new(enum: ["red", "green", "blue"])
+
+        refute schema.to_h.key?(:type)
+        assert_nothing_raised { schema.validate_result("red") }
+        assert_raises(OutputSchema::ValidationError) { schema.validate_result("yellow") }
+      end
+
+      test "defaults a properties-only schema to a root object" do
+        # Wire-format regression: the common shorthand keeps serializing with the injected `type: "object"`.
+        schema = OutputSchema.new(properties: { result: { type: "string" } })
+
+        assert_equal "object", schema.to_h[:type]
+      end
+
       test "allow to declare array schemas" do
         schema = OutputSchema.new({
           type: "array",