[dotnet-port-api] Port message source attribution helpers

agent/context.go

@@ -149,13 +149,7 @@ func (p *defaultContextProvider) Invoking(ctx context.Context, invoking Invoking
 		copy(outMessages, invoking.Messages)
 		source := message.Source{Type: SourceTypeContextProvider, ID: p.config.SourceID}
 		for _, msg := range providedMessages {
-			if msg == nil || msg.Source == source {
-				outMessages = append(outMessages, msg)
-				continue
-			}
-			marked := msg.Clone()
-			marked.Source = source
-			outMessages = append(outMessages, marked)
+			outMessages = append(outMessages, msg.WithSource(source))
 		}
 	}
 

agent/history.go

@@ -118,13 +118,7 @@ func (p *defaultHistoryProvider) Invoking(ctx context.Context, invoking Invoking
 	outMessages := make([]*message.Message, 0, len(providedMessages)+len(invoking.Messages))
 	source := message.Source{Type: SourceTypeHistoryProvider, ID: p.config.SourceID}
 	for _, msg := range providedMessages {
-		if msg == nil || msg.Source == source {
-			outMessages = append(outMessages, msg)
-			continue
-		}
-		marked := msg.Clone()
-		marked.Source = source
-		outMessages = append(outMessages, marked)
+		outMessages = append(outMessages, msg.WithSource(source))
 	}
 	outMessages = append(outMessages, invoking.Messages...)
 

agent/middleware.go

@@ -71,13 +71,11 @@ func (mr middlewareRunner) Run(ctx context.Context, messages []*message.Message,
 			if msg == nil || msg.Source != (message.Source{}) {
 				continue
 			}
-			marked := msg.Clone()
-			marked.Source = message.Source{Type: SourceTypeMiddleware}
 			if !outMessagesCloned {
 				outMessages = slices.Clone(outMessages)
 				outMessagesCloned = true
 			}
-			outMessages[i] = marked
+			outMessages[i] = msg.WithSource(message.Source{Type: SourceTypeMiddleware})
 		}
 		return mr.next(ctx, outMessages, opts...)
 	}

docs/dotnet-go-sdk-feature-comparison.md

@@ -34,7 +34,7 @@ Intentional contract choices in this parity pass:
 | Feature area | .NET SDK | Go SDK | Status | Misalignment |
 | --- | --- | --- | --- | --- |
 | Core agent abstraction | `AIAgent`, `DelegatingAIAgent`, `AgentRunOptions`, `AgentResponse`, `AgentResponseUpdate`, current run context, metadata, typed structured responses. | `agent.Agent`, `agent.ProviderConfig`, `agent.Config`, `agent.Option`, `Response`, `ResponseUpdate`, `Run`, `RunText`, `RunMessage`, `ResponseStream`; response aggregation preserves provider `RawRepresentation`. | Aligned | .NET exposes extension-method adapters around `Microsoft.Extensions.AI`; Go uses a provider `RunFunc` contract and package-level option wrappers. |
-| Agent identity and metadata | `AIAgentMetadata`, agent ID/name/description, source attribution extensions. | Agent ID/name/description, provider name, response author stamping. | Partial | Go does not expose the same request source attribution helpers as .NET. |
+| Agent identity and metadata | `AIAgentMetadata`, agent ID/name/description, source attribution extensions. | Agent ID/name/description, provider name, response author stamping, `message.Message` source attribution helpers. | Aligned | API shape differs: .NET exposes extension methods around `Microsoft.Extensions.AI`; Go exposes an explicit `message.Source` field plus helper methods. |
 | Sessions | `AgentSession`, `AgentSessionStateBag`, session serialization helpers, provider session state. | `agent.Session`, marshal/unmarshal hooks, provider session hooks, local/service ID support. | Aligned | .NET has a richer typed state bag and extension helpers; Go stores provider/session values through its own session abstraction. |
 | Chat history | `ChatHistoryProvider`, `InMemoryChatHistoryProvider`, per-service-call persistence, reducer triggers. | `HistoryProvider`, default in-memory history for local sessions, third-party storage example. | Partial | Go has the core lifecycle but fewer built-in storage providers and no first-class reducer trigger options on the history provider. |
 | Context providers and memory injection | `AIContextProvider`, `MessageAIContextProvider`, provider invoking/invoked lifecycle. | `agent.ContextProvider`, `(*agent.ContextProvider).Middleware`, before/after lifecycle. | Aligned | .NET context providers are integrated with `Microsoft.Extensions.AI`; Go providers directly transform `message.Message` slices and options. |

message/message.go

@@ -75,6 +75,37 @@ func (m *Message) Usage() UsageDetails {
 	return m.Contents.Usage()
 }
 
+// SourceType returns the message source type.
+//
+// When no explicit source type is set, [SourceTypeExternal] is returned.
+func (m *Message) SourceType() SourceType {
+	if m == nil {
+		return SourceTypeExternal
+	}
+	return m.Source.Type
+}
+
+// SourceID returns the message source identifier.
+func (m *Message) SourceID() string {
+	if m == nil {
+		return ""
+	}
+	return m.Source.ID
+}
+
+// WithSource returns the message tagged with the provided source.
+//
+// If the message already has the requested source, the original message is
+// returned. Otherwise, a cloned message is returned with the updated source.
+func (m *Message) WithSource(source Source) *Message {
+	if m == nil || m.Source == source {
+		return m
+	}
+	v := m.Clone()
+	v.Source = source
+	return v
+}
+
 // Clone creates a shallow copy of the message, cloning its top-level map and
 // slice containers while sharing their values and content objects.
 func (m *Message) Clone() *Message {

message/message_test.go

@@ -27,6 +27,58 @@ func TestMessage_Clone_ClonesAdditionalProperties(t *testing.T) {
 	}
 }
 
+func TestMessage_SourceType_DefaultsToExternal(t *testing.T) {
+	msg := message.NewText("hello")
+
+	if got := msg.SourceType(); got != message.SourceTypeExternal {
+		t.Fatalf("SourceType() = %q, want %q", got, message.SourceTypeExternal)
+	}
+}
+
+func TestMessage_SourceTypeAndSourceID_ReturnExplicitSource(t *testing.T) {
+	msg := message.NewText("hello")
+	msg.Source = message.Source{Type: message.SourceType("context-provider"), ID: "ctx"}
+
+	if got := msg.SourceType(); got != message.SourceType("context-provider") {
+		t.Fatalf("SourceType() = %q, want %q", got, message.SourceType("context-provider"))
+	}
+	if got := msg.SourceID(); got != "ctx" {
+		t.Fatalf("SourceID() = %q, want %q", got, "ctx")
+	}
+}
+
+func TestMessage_WithSource_ClonesWhenSourceChanges(t *testing.T) {
+	original := message.NewText("hello")
+	original.AdditionalProperties = map[string]any{"k": "v"}
+
+	got := original.WithSource(message.Source{Type: message.SourceType("history-provider"), ID: "history"})
+	if got == nil {
+		t.Fatal("expected sourced message")
+	}
+	if got == original {
+		t.Fatal("expected WithSource to clone when source changes")
+	}
+	if got.Source != (message.Source{Type: message.SourceType("history-provider"), ID: "history"}) {
+		t.Fatalf("WithSource source = %#v", got.Source)
+	}
+	if got.AdditionalProperties["k"] != "v" {
+		t.Fatalf("expected cloned additional properties, got %v", got.AdditionalProperties["k"])
+	}
+	if original.Source != (message.Source{}) {
+		t.Fatalf("expected original source to remain unchanged, got %#v", original.Source)
+	}
+}
+
+func TestMessage_WithSource_ReturnsOriginalWhenUnchanged(t *testing.T) {
+	original := message.NewText("hello")
+	original.Source = message.Source{Type: message.SourceType("context-provider"), ID: "ctx"}
+
+	got := original.WithSource(message.Source{Type: message.SourceType("context-provider"), ID: "ctx"})
+	if got != original {
+		t.Fatal("expected WithSource to return original message when source is unchanged")
+	}
+}
+
 func TestMessage_Clone_ClonesContentsSlice(t *testing.T) {
 	content := &message.TextContent{Text: "original"}
 	original := message.New(content)