fix(mcp): wrap slice structuredContent in object per MCP spec

The MCP specification requires structuredContent to marshal to a JSON
object, but NewStructuredResult could receive slice/array values. Add
ensureStructuredObject to automatically wrap slices in {"items": [...]}
so that the result always complies with the spec.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Marc Nuri <marc@marcnuri.com>

Author: manusa

pkg/mcp/mcp.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/http"
 	"os"
+	"reflect"
 	"slices"
 	"sync"
 	"time"
@@ -399,6 +400,10 @@ func NewTextResult(content string, err error) *mcp.CallToolResult {
 // The Content field contains the JSON-serialized form of structuredContent
 // for backward compatibility with MCP clients that don't support structuredContent.
 //
+// Per the MCP specification, structuredContent must marshal to a JSON object.
+// If structuredContent is a slice/array, it is automatically wrapped in
+// {"items": [...]} to satisfy this requirement.
+//
 // Per the MCP specification:
 // "For backwards compatibility, a tool that returns structured content SHOULD
 // also return the serialized JSON in a TextContent block."
@@ -425,7 +430,19 @@ func NewStructuredResult(content string, structuredContent any, err error) *mcp.
 		},
 	}
 	if structuredContent != nil {
-		result.StructuredContent = structuredContent
+		result.StructuredContent = ensureStructuredObject(structuredContent)
 	}
 	return result
 }
+
+// ensureStructuredObject wraps slice/array values in a {"items": ...} object
+// because the MCP specification requires structuredContent to be a JSON object.
+// Note: this checks the top-level reflect.Kind, so a pointer-to-slice (*[]T)
+// would not be wrapped. All current callers pass value types.
+func ensureStructuredObject(v any) any {
+	rv := reflect.ValueOf(v)
+	if rv.Kind() == reflect.Slice || rv.Kind() == reflect.Array {
+		return map[string]any{"items": v}
+	}
+	return v
+}

pkg/mcp/text_result_test.go

@@ -48,6 +48,19 @@ func (s *TextResultSuite) TestNewStructuredResult() {
 		s.Equal(`{"pods":["pod-1","pod-2"]}`, tc.Text)
 		s.Equal(structured, result.StructuredContent)
 	})
+	s.Run("wraps slice in object for MCP spec compliance", func() {
+		items := []map[string]any{{"name": "ns-1"}, {"name": "ns-2"}}
+		result := NewStructuredResult("text", items, nil)
+		s.False(result.IsError)
+		wrapped, ok := result.StructuredContent.(map[string]any)
+		s.Require().True(ok, "expected map[string]any wrapper")
+		s.Equal(items, wrapped["items"])
+	})
+	s.Run("does not wrap map structured content", func() {
+		structured := map[string]any{"key": "value"}
+		result := NewStructuredResult("text", structured, nil)
+		s.Equal(structured, result.StructuredContent)
+	})
 	s.Run("omits structured content when nil", func() {
 		result := NewStructuredResult("text output", nil, nil)
 		s.False(result.IsError)