diff --git a/agent/compaction/index.go b/agent/compaction/index.go
index 279018a1..cf063fae 100644
--- a/agent/compaction/index.go
+++ b/agent/compaction/index.go
@@ -399,6 +399,26 @@ func computeContentByteCount(content message.Content) int {
 		return stringByteCount(typed.Message) + stringByteCount(typed.ErrorCode) + stringByteCount(typed.Details)
 	case *message.HostedFileContent:
 		return stringByteCount(typed.FileID) + stringByteCount(typed.MediaType) + stringByteCount(typed.Name)
+	case *message.MCPServerToolCallContent:
+		return stringByteCount(typed.CallID) + stringByteCount(typed.Name) + stringByteCount(typed.ServerName) + stringByteCount(typed.Arguments)
+	case *message.MCPServerToolResultContent:
+		total := stringByteCount(typed.CallID) + stringByteCount(typed.Name) + stringByteCount(typed.ServerName) + stringByteCount(typed.Error)
+		for _, output := range typed.Outputs {
+			total += computeContentByteCount(output)
+		}
+		return total
+	case *message.CodeInterpreterToolCallContent:
+		total := stringByteCount(typed.CallID)
+		for _, input := range typed.Inputs {
+			total += computeContentByteCount(input)
+		}
+		return total
+	case *message.CodeInterpreterToolResultContent:
+		total := stringByteCount(typed.CallID)
+		for _, output := range typed.Outputs {
+			total += computeContentByteCount(output)
+		}
+		return total
 	default:
 		return 0
 	}
diff --git a/agent/compaction/index_test.go b/agent/compaction/index_test.go
index c0c557ea..4b96bedd 100644
--- a/agent/compaction/index_test.go
+++ b/agent/compaction/index_test.go
@@ -5,6 +5,7 @@ package compaction_test
 import (
 	"context"
 	"slices"
+	"strings"
 	"testing"
 
 	"github.com/microsoft/agent-framework-go/agent/compaction"
@@ -370,3 +371,31 @@ func TestMessageIndex_Update_RepeatedContentDoesNotDropMessages(t *testing.T) {
 		t.Fatalf("after turn 2 (repeated \"continue\"): got %v, want %v — appended turns were dropped", got, want)
 	}
 }
+
+func TestMessageIndex_CountsHostedToolResultBytes(t *testing.T) {
+	big := strings.Repeat("x", 10000)
+	// A hosted tool result carrying 10k bytes of Outputs must be byte-accounted
+	// like the equivalent FunctionResultContent, so token/byte-based compaction
+	// triggers see the payload that most needs compacting.
+	fnMsg := &message.Message{Role: message.RoleTool, Contents: message.Contents{
+		&message.FunctionResultContent{CallID: "c1", Result: big},
+	}}
+	mcpMsg := &message.Message{Role: message.RoleTool, Contents: message.Contents{
+		&message.MCPServerToolResultContent{CallID: "c1", Outputs: message.Contents{&message.TextContent{Text: big}}},
+	}}
+	ciMsg := &message.Message{Role: message.RoleTool, Contents: message.Contents{
+		&message.CodeInterpreterToolResultContent{CallID: "c1", Outputs: message.Contents{&message.TextContent{Text: big}}},
+	}}
+	prefix := textMessage(message.RoleUser, "hi")
+
+	fnBytes := compaction.CreateMessageIndex([]*message.Message{prefix, fnMsg}, nil).TotalByteCount()
+	mcpBytes := compaction.CreateMessageIndex([]*message.Message{prefix, mcpMsg}, nil).TotalByteCount()
+	ciBytes := compaction.CreateMessageIndex([]*message.Message{prefix, ciMsg}, nil).TotalByteCount()
+
+	if mcpBytes < fnBytes {
+		t.Errorf("MCP tool-result bytes = %d, want >= FunctionResult bytes %d (Outputs undercounted)", mcpBytes, fnBytes)
+	}
+	if ciBytes < fnBytes {
+		t.Errorf("code-interpreter result bytes = %d, want >= FunctionResult bytes %d (Outputs undercounted)", ciBytes, fnBytes)
+	}
+}