Expose the user-defined server_context in instrumentation data

APM integrations need to tag a span with the subject of the request, but the
user-defined `server_context` hash passed to `Server.new` never reaches
`around_request` or `instrumentation_callback`. The exception reporter's
`server_context` argument is a different value — it describes where a failure
occurred (`{ request: ... }`, `{ notification: ... }`), not who made the call —
so there is currently no way to read the request subject from a hook.

Add `Configuration#instrument_server_context`, off by default. When enabled,
`instrument_call` puts the host's `server_context` into the instrumentation data
before invoking `around_request`, so it is readable both before and after
`request_handler.call`.

The flag is opt-in rather than unconditional because the hash is
application-supplied and may hold values a tracing backend should not receive.
Hosts that do not expose a `server_context` reader are unaffected.

`instrument_call` reads it through `self.server_context`: the existing
`server_context:` keyword shadows the reader, and the value wanted here is the
user-defined hash, not the reporter context.

Follows the same shape as the earlier additions of `tool_arguments` (#218) and
`client` (#221) to this hash.

README.md

@@ -346,7 +346,8 @@ server = MCP::Server.new(
 ```
 
 This hash is then passed as the `server_context` keyword argument to tool and prompt calls.
-Note that exception and instrumentation callbacks do not receive this user-defined hash.
+Note that the exception reporter does not receive this user-defined hash, and instrumentation
+callbacks omit it unless you opt in with `instrument_server_context`.
 See the relevant sections below for the arguments they receive.
 
 #### Request-specific `_meta` Parameter
@@ -460,10 +461,31 @@ around_request = ->(data, &request_handler) { request_handler.call }
 
 **`data` availability by timing:**
 
-- Before `request_handler.call`: `method`
+- Before `request_handler.call`: `method`, and `server_context` when `instrument_server_context` is enabled
 - After `request_handler.call`: `tool_name`, `tool_arguments`, `prompt_name`, `resource_uri`, `error`, `client`
 - Not available inside `around_request`: `duration` (added after `around_request` returns)
 
+**Exposing the user-defined `server_context` (opt in):**
+
+`data` omits the user-defined `server_context` by default, because that hash is
+application-supplied and may hold values a tracing backend should not receive.
+Enable it when you need to tag spans with the request's subject:
+
+```ruby
+MCP.configure do |config|
+  config.instrument_server_context = true
+
+  config.around_request = ->(data, &request_handler) {
+    Sentry.set_user(id: data.dig(:server_context, :user_id))
+    request_handler.call
+  }
+end
+```
+
+`data[:server_context]` is the hash passed to `Server.new` — `nil` when the host
+set none. It is not the exception reporter's context argument, which describes
+where a failure occurred rather than who made the request.
+
 > [!NOTE]
 > `tool_name`, `prompt_name` and `resource_uri` may only be populated for the corresponding request methods
 > (`tools/call`, `prompts/get`, `resources/read`), and may not be set depending on how the request is handled
@@ -515,6 +537,8 @@ It receives a hash with the following possible keys:
 - `error`: (String, optional) Error code if a lookup failed
 - `duration`: (Float) Duration of the call in seconds
 - `client`: (Hash, optional) Client information with `name` and `version` keys, from the initialize request
+- `server_context`: (Any, optional) The user-defined hash passed to `Server.new`, present only when
+  `instrument_server_context` is enabled
 
 **Signature:**
 

lib/mcp/configuration.rb

@@ -33,7 +33,7 @@ module MCP
     attr_writer :instrumentation_callback
 
     def initialize(exception_reporter: nil, around_request: nil, instrumentation_callback: nil, protocol_version: nil,
-      validate_tool_call_arguments: true, validate_tool_call_results: false)
+      validate_tool_call_arguments: true, validate_tool_call_results: false, instrument_server_context: false)
       @exception_reporter = exception_reporter
       @around_request = around_request
       @instrumentation_callback = instrumentation_callback
@@ -43,9 +43,11 @@ module MCP
       end
       validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
       validate_value_of_validate_tool_call_results!(validate_tool_call_results)
+      validate_value_of_instrument_server_context!(instrument_server_context)
 
       @validate_tool_call_arguments = validate_tool_call_arguments
       @validate_tool_call_results = validate_tool_call_results
+      @instrument_server_context = instrument_server_context
     end
 
     def protocol_version=(protocol_version)
@@ -60,6 +62,16 @@ module MCP
       @validate_tool_call_arguments = validate_tool_call_arguments
     end
 
+    # Opt in to exposing the user-defined `server_context` in the
+    # `around_request` / `instrumentation_callback` data hash. Off by default:
+    # the hash is application-supplied and may hold values a tracing backend
+    # should not receive, so surfacing it has to be a deliberate choice.
+    def instrument_server_context=(instrument_server_context)
+      validate_value_of_instrument_server_context!(instrument_server_context)
+
+      @instrument_server_context = instrument_server_context
+    end
+
     def validate_tool_call_results=(validate_tool_call_results)
       validate_value_of_validate_tool_call_results!(validate_tool_call_results)
 
@@ -107,6 +119,10 @@ module MCP
     attr_reader :validate_tool_call_arguments
     attr_reader :validate_tool_call_results
 
+    def instrument_server_context?
+      !!@instrument_server_context
+    end
+
     def validate_tool_call_arguments?
       !!@validate_tool_call_arguments
     end
@@ -152,6 +168,7 @@ module MCP
         protocol_version: protocol_version,
         validate_tool_call_arguments: validate_tool_call_arguments,
         validate_tool_call_results: validate_tool_call_results,
+        instrument_server_context: other.instrument_server_context?,
       )
     end
 
@@ -176,6 +193,12 @@ module MCP
       end
     end
 
+    def validate_value_of_instrument_server_context!(instrument_server_context)
+      unless instrument_server_context.is_a?(TrueClass) || instrument_server_context.is_a?(FalseClass)
+        raise ArgumentError, "instrument_server_context must be a boolean"
+      end
+    end
+
     def default_exception_reporter
       @default_exception_reporter ||= ->(exception, server_context) {}
     end

lib/mcp/instrumentation.rb

@@ -7,6 +7,12 @@ module MCP
       begin
         @instrumentation_data = {}
         add_instrumentation_data(method: method)
+        # `self.` is required: the `server_context:` keyword above shadows the
+        # reader, and the value we want here is the user-defined hash passed to
+        # `Server.new`, not the per-call reporter context.
+        if configuration.instrument_server_context? && respond_to?(:server_context)
+          add_instrumentation_data(server_context: self.server_context)
+        end
 
         result = configuration.around_request.call(@instrumentation_data, &block)
 

test/mcp/configuration_test.rb

@@ -145,6 +145,35 @@ module MCP
       refute merged.validate_tool_call_arguments
     end
 
+    test "defaults instrument_server_context to false" do
+      config = Configuration.new
+
+      refute_predicate config, :instrument_server_context?
+    end
+
+    test "accepts instrument_server_context in the constructor" do
+      config = Configuration.new(instrument_server_context: true)
+
+      assert_predicate config, :instrument_server_context?
+    end
+
+    test "raises ArgumentError when instrument_server_context is not a boolean value" do
+      config = Configuration.new
+
+      exception = assert_raises(ArgumentError) do
+        config.instrument_server_context = "true"
+      end
+      assert_equal("instrument_server_context must be a boolean", exception.message)
+    end
+
+    test "merge carries instrument_server_context from the other configuration" do
+      base = Configuration.new(instrument_server_context: true)
+      other = Configuration.new(instrument_server_context: false)
+
+      refute_predicate base.merge(other), :instrument_server_context?
+      assert_predicate other.merge(base), :instrument_server_context?
+    end
+
     test "defaults validate_tool_call_results to false" do
       config = Configuration.new
       refute config.validate_tool_call_results

test/mcp/instrumentation_test.rb

@@ -40,6 +40,79 @@ module MCP
       end
     end
 
+    # Mirrors `MCP::Server`, which exposes the user-defined hash through
+    # `attr_accessor :server_context`.
+    class SubjectWithServerContext < Subject
+      attr_accessor :server_context
+
+      def initialize(server_context)
+        super()
+        @server_context = server_context
+      end
+    end
+
+    test "#instrument_call omits the user-defined server_context by default" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+
+      subject.instrumented_method
+
+      refute_includes subject.instrumentation_data_received.keys, :server_context
+    end
+
+    test "#instrument_call exposes the user-defined server_context when instrument_server_context is enabled" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      assert_equal({ user_id: 42 }, subject.instrumentation_data_received[:server_context])
+    end
+
+    test "#instrument_call exposes the user-defined server_context to around_request" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+      seen = nil
+      subject.configuration.around_request = ->(data, &handler) {
+        seen = data[:server_context]
+        handler.call
+      }
+
+      subject.instrumented_method
+
+      assert_equal({ user_id: 42 }, seen)
+    end
+
+    test "#instrument_call reports the user-defined server_context as nil when the host does not set one" do
+      subject = SubjectWithServerContext.new(nil)
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      assert_includes subject.instrumentation_data_received.keys, :server_context
+      assert_nil subject.instrumentation_data_received[:server_context]
+    end
+
+    test "#instrument_call does not confuse the reporter context with the user-defined server_context" do
+      # `instrument_call(server_context:)` is the exception-reporter context, a
+      # different value from the user-defined hash. Enabling the flag must
+      # surface the latter, not the former.
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method_with_server_context({ request: "reporter-context" })
+
+      assert_equal({ user_id: 42 }, subject.instrumentation_data_received[:server_context])
+    end
+
+    test "#instrument_call skips server_context for hosts without the reader even when enabled" do
+      subject = Subject.new
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      refute_includes subject.instrumentation_data_received.keys, :server_context
+    end
+
     test "#instrument_call adds the method name to the instrumentation data" do
       subject = Subject.new