fix(inventory): normalize owned metadata

Canonicalize JSON metadata at builder boundaries, report invalid values from Build, and preserve wire semantics for typed mutable values. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1e4a1ca6-53f7-4158-af22-35d2448d0b13
SamMorrowDrums · Sep 3, 2026 · Aug 27, 2026 · Aug 27, 2026 · Sep 1, 2026 · Sep 1, 2026
commit e070a779d16b0749692054a591dde2f1f4cdcd61
@@ -24,8 +24,8 @@ const mcpAppsFeatureFlag FeatureFlag = "remote_mcp_ui_apps"
 type ToolFilter func(ctx context.Context, tool *ServerTool) (bool, error)
 
 // Builder builds a Registry with the specified configuration. SetTools,
-// SetResources, and SetPrompts copy the feature and metadata state retained by
-// the inventory.
+// SetResources, and SetPrompts normalize retained metadata through JSON so the
+// inventory owns it; Build reports metadata that cannot be represented as JSON.
 // Use NewBuilder to create a builder, chain configuration methods,
 // then call Build() to create the final inventory.
 //
@@ -45,6 +45,9 @@ type Builder struct {
 	tools             []ServerTool
 	resourceTemplates []ServerResourceTemplate
 	prompts           []ServerPrompt
+	toolsErr          error
+	resourcesErr      error
+	promptsErr        error
 	deprecatedAliases map[string]string
 
 	// Configuration options (processed at Build time)
@@ -68,30 +71,66 @@ func NewBuilder() *Builder {
 // SetTools sets the tools for the inventory. Returns self for chaining.
 func (b *Builder) SetTools(tools []ServerTool) *Builder {
 	b.tools = slices.Clone(tools)
+	b.toolsErr = nil
 	for i := range b.tools {
-		b.tools[i] = cloneServerTool(b.tools[i])
+		tool, err := ownServerTool(b.tools[i])
+		if err != nil {
+			b.toolsErr = errors.Join(b.toolsErr, fmt.Errorf("tool %q metadata: %w", b.tools[i].Tool.Name, err))
+		}
+		b.tools[i] = tool
 	}
 	return b
 }
 
 // SetResources sets the resource templates for the inventory. Returns self for chaining.
 func (b *Builder) SetResources(resources []ServerResourceTemplate) *Builder {
 	b.resourceTemplates = slices.Clone(resources)
+	b.resourcesErr = nil
 	for i := range b.resourceTemplates {
-		b.resourceTemplates[i] = cloneResourceTemplate(b.resourceTemplates[i])
+		resource, err := ownResourceTemplate(b.resourceTemplates[i])
+		if err != nil {
+			b.resourcesErr = errors.Join(b.resourcesErr, fmt.Errorf("resource template %q metadata: %w", b.resourceTemplates[i].Template.Name, err))
+		}
+		b.resourceTemplates[i] = resource
 	}
 	return b
 }
 
 // SetPrompts sets the prompts for the inventory. Returns self for chaining.
 func (b *Builder) SetPrompts(prompts []ServerPrompt) *Builder {
 	b.prompts = slices.Clone(prompts)
+	b.promptsErr = nil
 	for i := range b.prompts {
-		b.prompts[i] = clonePrompt(b.prompts[i])
+		prompt, err := ownPrompt(b.prompts[i])
+		if err != nil {
+			b.promptsErr = errors.Join(b.promptsErr, fmt.Errorf("prompt %q metadata: %w", b.prompts[i].Prompt.Name, err))
+		}
+		b.prompts[i] = prompt
 	}
 	return b
 }
 
+func ownServerTool(tool ServerTool) (ServerTool, error) {
+	meta, err := normalizeMeta(tool.Tool.Meta)
+	tool.Tool.Meta = meta
+	tool.FeatureRule = tool.FeatureRule.clone()
+	return tool, err
+}
+
+func ownResourceTemplate(resource ServerResourceTemplate) (ServerResourceTemplate, error) {
+	meta, err := normalizeMeta(resource.Template.Meta)
+	resource.Template.Meta = meta
+	resource.FeatureRule = resource.FeatureRule.clone()
+	return resource, err
+}
+
+func ownPrompt(prompt ServerPrompt) (ServerPrompt, error) {
+	meta, err := normalizeMeta(prompt.Prompt.Meta)
+	prompt.Prompt.Meta = meta
+	prompt.FeatureRule = prompt.FeatureRule.clone()
+	return prompt, err
+}
+
 func cloneServerTool(tool ServerTool) ServerTool {
 	tool.Tool.Meta = cloneMeta(tool.Tool.Meta)
 	tool.FeatureRule = tool.FeatureRule.clone()
@@ -234,6 +273,10 @@ func cleanTools(tools []string) []string {
 // (i.e., they don't exist in the tool set and are not deprecated aliases).
 // This ensures invalid tool configurations fail fast at build time.
 func (b *Builder) Build() (*Inventory, error) {
+	if err := errors.Join(b.toolsErr, b.resourcesErr, b.promptsErr); err != nil {
+		return nil, fmt.Errorf("invalid inventory metadata: %w", err)
+	}
+
 	tools := b.tools
 
 	filters := b.filters

diff --git a/pkg/inventory/metadata.go b/pkg/inventory/metadata.go
@@ -1,11 +1,27 @@
 package inventory
 
 import (
-	"slices"
+	"encoding/json"
+	"fmt"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
+func normalizeMeta(meta mcp.Meta) (mcp.Meta, error) {
+	if meta == nil {
+		return nil, nil
+	}
+	data, err := json.Marshal(meta)
+	if err != nil {
+		return nil, fmt.Errorf("marshal metadata: %w", err)
+	}
+	var normalized mcp.Meta
+	if err := json.Unmarshal(data, &normalized); err != nil {
+		return nil, fmt.Errorf("unmarshal metadata: %w", err)
+	}
+	return normalized, nil
+}
+
 func cloneMeta(meta mcp.Meta) mcp.Meta {
 	if meta == nil {
 		return nil
@@ -33,9 +49,9 @@ func cloneMetaValue(value any) any {
 			clone[i] = cloneMetaValue(item)
 		}
 		return clone
-	case []string:
-		return slices.Clone(value)
-	default:
+	case nil, bool, float64, string:
 		return value
+	default:
+		panic(fmt.Sprintf("metadata value %T is not normalized", value))
 	}
 }
@@ -2,6 +2,7 @@ package inventory
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"testing"
@@ -1309,6 +1310,9 @@ func TestBuilderOwnsFeatureMetadataInputs(t *testing.T) {
 	prompts := []ServerPrompt{mockPrompt("prompt", "toolset1")}
 	prompts[0].FeatureRule = promptRule
 	prompts[0].Prompt.Meta = nestedTestMeta("prompt", "prompt")
+	expectedToolMeta := mustMarshalJSON(t, tools[0].Tool.Meta)
+	expectedResourceMeta := mustMarshalJSON(t, resources[0].Template.Meta)
+	expectedPromptMeta := mustMarshalJSON(t, prompts[0].Prompt.Meta)
 
 	checker := func(_ context.Context, flag FeatureFlag) (bool, error) {
 		return flag == "tool" || flag == "resource" || flag == "prompt" || flag == mcpAppsFeatureFlag, nil
@@ -1320,16 +1324,16 @@ func TestBuilderOwnsFeatureMetadataInputs(t *testing.T) {
 		WithToolsets([]string{"all"}).
 		WithFeatureChecker(checker)
 
-	setNestedTestMeta(tools[0].Tool.Meta, "ui", "after-set")
-	setNestedTestMeta(resources[0].Template.Meta, "resource", "after-set")
-	setNestedTestMeta(prompts[0].Prompt.Meta, "prompt", "after-set")
+	mutateSourceTestMeta(tools[0].Tool.Meta, "ui", "after-set", 's')
+	mutateSourceTestMeta(resources[0].Template.Meta, "resource", "after-set", 's')
+	mutateSourceTestMeta(prompts[0].Prompt.Meta, "prompt", "after-set", 's')
 	inv := mustBuild(t, builder)
 	tools[0].FeatureRule.features[0] = "changed"
-	setNestedTestMeta(tools[0].Tool.Meta, "ui", "after-build")
+	mutateSourceTestMeta(tools[0].Tool.Meta, "ui", "after-build", 'b')
 	resources[0].FeatureRule.features[0] = "changed"
-	setNestedTestMeta(resources[0].Template.Meta, "resource", "after-build")
+	mutateSourceTestMeta(resources[0].Template.Meta, "resource", "after-build", 'b')
 	prompts[0].FeatureRule.features[0] = "changed"
-	setNestedTestMeta(prompts[0].Prompt.Meta, "prompt", "after-build")
+	mutateSourceTestMeta(prompts[0].Prompt.Meta, "prompt", "after-build", 'b')
 
 	require.Equal(t, []FeatureFlag{"prompt", "remote_mcp_ui_apps", "resource", "tool"}, inv.RequiredFeatures())
 	availableTools := inv.AvailableTools(context.Background())
@@ -1342,18 +1346,18 @@ func TestBuilderOwnsFeatureMetadataInputs(t *testing.T) {
 	requireNestedTestMeta(t, availableResources[0].Template.Meta, "resource", "resource")
 	requireNestedTestMeta(t, availablePrompts[0].Prompt.Meta, "prompt", "prompt")
 
-	setNestedTestMeta(availableTools[0].Tool.Meta, "ui", "after-available")
+	mutateNormalizedTestMeta(availableTools[0].Tool.Meta, "ui", "after-available")
 	availableTools[0].FeatureRule.features[0] = "changed"
-	setNestedTestMeta(availableResources[0].Template.Meta, "resource", "after-available")
+	mutateNormalizedTestMeta(availableResources[0].Template.Meta, "resource", "after-available")
 	availableResources[0].FeatureRule.features[0] = "changed"
-	setNestedTestMeta(availablePrompts[0].Prompt.Meta, "prompt", "after-available")
+	mutateNormalizedTestMeta(availablePrompts[0].Prompt.Meta, "prompt", "after-available")
 	availablePrompts[0].FeatureRule.features[0] = "changed"
 
 	allTools := inv.AllTools()
-	setNestedTestMeta(allTools[0].Tool.Meta, "ui", "after-all")
+	mutateNormalizedTestMeta(allTools[0].Tool.Meta, "ui", "after-all")
 	foundTool, _, err := inv.FindToolByName("tool")
 	require.NoError(t, err)
-	setNestedTestMeta(foundTool.Tool.Meta, "ui", "after-find")
+	mutateNormalizedTestMeta(foundTool.Tool.Meta, "ui", "after-find")
 
 	server := mcp.NewServer(&mcp.Implementation{Name: "test-server", Version: "v0.0.1"}, nil)
 	inv.RegisterAll(context.Background(), server, nil)
@@ -1369,40 +1373,110 @@ func TestBuilderOwnsFeatureMetadataInputs(t *testing.T) {
 	registeredTools, err := clientSession.ListTools(context.Background(), nil)
 	require.NoError(t, err)
 	requireNestedTestMeta(t, registeredTools.Tools[0].Meta, "ui", "tool")
+	require.JSONEq(t, string(expectedToolMeta), string(mustMarshalJSON(t, registeredTools.Tools[0].Meta)))
 	registeredResources, err := clientSession.ListResourceTemplates(context.Background(), nil)
 	require.NoError(t, err)
 	requireNestedTestMeta(t, registeredResources.ResourceTemplates[0].Meta, "resource", "resource")
+	require.JSONEq(t, string(expectedResourceMeta), string(mustMarshalJSON(t, registeredResources.ResourceTemplates[0].Meta)))
 	registeredPrompts, err := clientSession.ListPrompts(context.Background(), nil)
 	require.NoError(t, err)
 	requireNestedTestMeta(t, registeredPrompts.Prompts[0].Meta, "prompt", "prompt")
+	require.JSONEq(t, string(expectedPromptMeta), string(mustMarshalJSON(t, registeredPrompts.Prompts[0].Meta)))
+}
+
+type testMetaMap map[string]string
+type testMetaSlice []string
+type testMetaPointer struct {
+	Value string `json:"value"`
 }
 
 func nestedTestMeta(key, value string) mcp.Meta {
 	return mcp.Meta{
 		key: map[string]any{
-			"objects": []any{map[string]any{"value": value}},
-			"strings": []string{value},
+			"objects":     []any{map[string]any{"value": value}},
+			"strings":     []string{value},
+			"typed_map":   testMetaMap{"value": value},
+			"typed_slice": testMetaSlice{value},
+			"bytes":       []byte(value),
+			"pointer":     &testMetaPointer{Value: value},
 		},
 	}
 }
 
-func setNestedTestMeta(meta mcp.Meta, key, value string) {
+func mutateSourceTestMeta(meta mcp.Meta, key, value string, marker byte) {
 	nested := meta[key].(map[string]any)
 	nested["objects"].([]any)[0].(map[string]any)["value"] = value
 	nested["strings"].([]string)[0] = value
+	nested["typed_map"].(testMetaMap)["value"] = value
+	nested["typed_slice"].(testMetaSlice)[0] = value
+	nested["bytes"].([]byte)[0] = marker
+	nested["pointer"].(*testMetaPointer).Value = value
+}
+
+func mutateNormalizedTestMeta(meta mcp.Meta, key, value string) {
+	nested := meta[key].(map[string]any)
+	nested["objects"].([]any)[0].(map[string]any)["value"] = value
+	nested["strings"].([]any)[0] = value
+	nested["typed_map"].(map[string]any)["value"] = value
+	nested["typed_slice"].([]any)[0] = value
+	nested["bytes"] = value
+	nested["pointer"].(map[string]any)["value"] = value
 }
 
 func requireNestedTestMeta(t *testing.T, meta mcp.Meta, key, value string) {
 	t.Helper()
 	nested := meta[key].(map[string]any)
 	require.Equal(t, value, nested["objects"].([]any)[0].(map[string]any)["value"])
-	switch strings := nested["strings"].(type) {
-	case []string:
-		require.Equal(t, value, strings[0])
-	case []any:
-		require.Equal(t, value, strings[0])
-	default:
-		require.Failf(t, "unexpected strings metadata", "type %T", strings)
+	require.Equal(t, value, nested["strings"].([]any)[0])
+	require.Equal(t, value, nested["typed_map"].(map[string]any)["value"])
+	require.Equal(t, value, nested["typed_slice"].([]any)[0])
+	require.Equal(t, base64.StdEncoding.EncodeToString([]byte(value)), nested["bytes"])
+	require.Equal(t, value, nested["pointer"].(map[string]any)["value"])
+}
+
+func mustMarshalJSON(t *testing.T, value any) []byte {
+	t.Helper()
+	data, err := json.Marshal(value)
+	require.NoError(t, err)
+	return data
+}
+
+func TestBuildRejectsInvalidMetadata(t *testing.T) {
+	tests := []struct {
+		name      string
+		builder   *Builder
+		errorText string
+	}{
+		{
+			name: "tool",
+			builder: NewBuilder().SetTools([]ServerTool{{
+				Tool: mcp.Tool{Name: "tool", Meta: mcp.Meta{"invalid": make(chan int)}},
+			}}),
+			errorText: `tool "tool" metadata`,
+		},
+		{
+			name: "resource",
+			builder: NewBuilder().SetResources([]ServerResourceTemplate{{
+				Template: mcp.ResourceTemplate{Name: "resource", Meta: mcp.Meta{"invalid": make(chan int)}},
+			}}),
+			errorText: `resource template "resource" metadata`,
+		},
+		{
+			name: "prompt",
+			builder: NewBuilder().SetPrompts([]ServerPrompt{{
+				Prompt: mcp.Prompt{Name: "prompt", Meta: mcp.Meta{"invalid": make(chan int)}},
+			}}),
+			errorText: `prompt "prompt" metadata`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := tt.builder.Build()
+			require.ErrorContains(t, err, "invalid inventory metadata")
+			require.ErrorContains(t, err, tt.errorText)
+			require.ErrorContains(t, err, "unsupported type: chan int")
+		})
 	}
 }