diff --git a/marketplace/indexer.go b/marketplace/indexer.go
new file mode 100644
index 0000000..2af7328
--- /dev/null
+++ b/marketplace/indexer.go
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: EUPL-1.2
+
+package marketplace
+
+import (
+	"sort"
+
+	"dappco.re/go/core/io"
+	"dappco.re/go/core/scm/manifest"
+)
+
+// IndexOptions controls how BuildIndex populates marketplace metadata.
+// Usage: IndexOptions{...}
+type IndexOptions struct {
+	// Org is the default organisation used when constructing repo URLs.
+	Org string
+
+	// ForgeURL is the base URL used when constructing repo URLs.
+	ForgeURL string
+
+	// CategoryFn assigns a category to a module code.
+	CategoryFn func(code string) string
+}
+
+// BuildIndex reads .core/manifest.yaml from each repository root and produces
+// a marketplace index. Repositories without a manifest are skipped silently.
+// Categories are deduplicated and sorted.
+// Usage: BuildIndex(...)
+func BuildIndex(medium io.Medium, repoPaths []string, opts IndexOptions) (*Index, error) {
+	idx := &Index{
+		Version: IndexVersion,
+	}
+
+	seen := make(map[string]bool)
+	categories := make(map[string]bool)
+
+	for _, repoPath := range repoPaths {
+		m, err := manifest.Load(medium, repoPath)
+		if err != nil {
+			continue
+		}
+		if m == nil || m.Code == "" {
+			continue
+		}
+		if seen[m.Code] {
+			continue
+		}
+		seen[m.Code] = true
+
+		module := Module{
+			Code: m.Code,
+			Name: m.Name,
+		}
+		if opts.ForgeURL != "" && opts.Org != "" {
+			module.Repo = opts.ForgeURL + "/" + opts.Org + "/" + m.Code
+		}
+		if opts.CategoryFn != nil {
+			module.Category = opts.CategoryFn(m.Code)
+		}
+		if module.Category != "" {
+			categories[module.Category] = true
+		}
+
+		idx.Modules = append(idx.Modules, module)
+	}
+
+	sort.Slice(idx.Modules, func(i, j int) bool {
+		return idx.Modules[i].Code < idx.Modules[j].Code
+	})
+
+	for category := range categories {
+		idx.Categories = append(idx.Categories, category)
+	}
+	sort.Strings(idx.Categories)
+
+	return idx, nil
+}
diff --git a/marketplace/indexer_test.go b/marketplace/indexer_test.go
new file mode 100644
index 0000000..830c71b
--- /dev/null
+++ b/marketplace/indexer_test.go
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: EUPL-1.2
+
+package marketplace
+
+import (
+	"testing"
+
+	"dappco.re/go/core/io"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildIndex_Good_CategoriesAndRepoURLs_Good(t *testing.T) {
+	medium := io.NewMockMedium()
+
+	require.NoError(t, medium.Write("/repos/a/.core/manifest.yaml", `
+code: a
+name: Alpha
+version: 1.0.0
+`))
+	require.NoError(t, medium.Write("/repos/b/.core/manifest.yaml", `
+code: b
+name: Beta
+version: 1.0.0
+`))
+	require.NoError(t, medium.Write("/repos/c/.core/manifest.yaml", `
+code: c
+name: Gamma
+version: 1.0.0
+`))
+
+	idx, err := BuildIndex(medium, []string{"/repos/a", "/repos/b", "/repos/c"}, IndexOptions{
+		ForgeURL: "https://forge.example.com",
+		Org:      "core",
+		CategoryFn: func(code string) string {
+			switch code {
+			case "a", "b":
+				return "tools"
+			default:
+				return "products"
+			}
+		},
+	})
+	require.NoError(t, err)
+
+	require.Len(t, idx.Modules, 3)
+	assert.Equal(t, "a", idx.Modules[0].Code)
+	assert.Equal(t, "https://forge.example.com/core/a", idx.Modules[0].Repo)
+	assert.Equal(t, "tools", idx.Modules[0].Category)
+	assert.Equal(t, []string{"products", "tools"}, idx.Categories)
+}
+
+func TestBuildIndex_Good_SkipsMissingManifest_Good(t *testing.T) {
+	medium := io.NewMockMedium()
+
+	require.NoError(t, medium.Write("/repos/one/.core/manifest.yaml", `
+code: one
+name: One
+version: 1.0.0
+`))
+
+	idx, err := BuildIndex(medium, []string{"/repos/one", "/repos/missing"}, IndexOptions{})
+	require.NoError(t, err)
+
+	require.Len(t, idx.Modules, 1)
+	assert.Equal(t, "one", idx.Modules[0].Code)
+	assert.Empty(t, idx.Categories)
+}