diff --git a/pkg/marketplace/marketplace.go b/pkg/marketplace/marketplace.go
new file mode 100644
index 0000000..52b4a8f
--- /dev/null
+++ b/pkg/marketplace/marketplace.go
@@ -0,0 +1,67 @@
+package marketplace
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Module is a marketplace entry pointing to a module's Git repo.
+type Module struct {
+	Code     string `json:"code"`
+	Name     string `json:"name"`
+	Repo     string `json:"repo"`
+	SignKey  string `json:"sign_key"`
+	Category string `json:"category"`
+}
+
+// Index is the root marketplace catalog.
+type Index struct {
+	Version    int      `json:"version"`
+	Modules    []Module `json:"modules"`
+	Categories []string `json:"categories"`
+}
+
+// ParseIndex decodes a marketplace index.json.
+func ParseIndex(data []byte) (*Index, error) {
+	var idx Index
+	if err := json.Unmarshal(data, &idx); err != nil {
+		return nil, fmt.Errorf("marketplace.ParseIndex: %w", err)
+	}
+	return &idx, nil
+}
+
+// Search returns modules matching the query in code, name, or category.
+func (idx *Index) Search(query string) []Module {
+	q := strings.ToLower(query)
+	var results []Module
+	for _, m := range idx.Modules {
+		if strings.Contains(strings.ToLower(m.Code), q) ||
+			strings.Contains(strings.ToLower(m.Name), q) ||
+			strings.Contains(strings.ToLower(m.Category), q) {
+			results = append(results, m)
+		}
+	}
+	return results
+}
+
+// ByCategory returns all modules in the given category.
+func (idx *Index) ByCategory(category string) []Module {
+	var results []Module
+	for _, m := range idx.Modules {
+		if m.Category == category {
+			results = append(results, m)
+		}
+	}
+	return results
+}
+
+// Find returns the module with the given code, or false if not found.
+func (idx *Index) Find(code string) (Module, bool) {
+	for _, m := range idx.Modules {
+		if m.Code == code {
+			return m, true
+		}
+	}
+	return Module{}, false
+}
diff --git a/pkg/marketplace/marketplace_test.go b/pkg/marketplace/marketplace_test.go
new file mode 100644
index 0000000..c51d0ee
--- /dev/null
+++ b/pkg/marketplace/marketplace_test.go
@@ -0,0 +1,65 @@
+package marketplace
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestParseIndex_Good(t *testing.T) {
+	raw := `{
+		"version": 1,
+		"modules": [
+			{"code": "mining-xmrig", "name": "XMRig Miner", "repo": "https://forge.lthn.io/host-uk/mod-xmrig.git", "sign_key": "abc123", "category": "miner"},
+			{"code": "utils-cyberchef", "name": "CyberChef", "repo": "https://forge.lthn.io/host-uk/mod-cyberchef.git", "sign_key": "def456", "category": "utils"}
+		],
+		"categories": ["miner", "utils"]
+	}`
+	idx, err := ParseIndex([]byte(raw))
+	require.NoError(t, err)
+	assert.Equal(t, 1, idx.Version)
+	assert.Len(t, idx.Modules, 2)
+	assert.Equal(t, "mining-xmrig", idx.Modules[0].Code)
+}
+
+func TestSearch_Good(t *testing.T) {
+	idx := &Index{
+		Modules: []Module{
+			{Code: "mining-xmrig", Name: "XMRig Miner", Category: "miner"},
+			{Code: "utils-cyberchef", Name: "CyberChef", Category: "utils"},
+		},
+	}
+	results := idx.Search("miner")
+	assert.Len(t, results, 1)
+	assert.Equal(t, "mining-xmrig", results[0].Code)
+}
+
+func TestByCategory_Good(t *testing.T) {
+	idx := &Index{
+		Modules: []Module{
+			{Code: "a", Category: "miner"},
+			{Code: "b", Category: "utils"},
+			{Code: "c", Category: "miner"},
+		},
+	}
+	miners := idx.ByCategory("miner")
+	assert.Len(t, miners, 2)
+}
+
+func TestFind_Good(t *testing.T) {
+	idx := &Index{
+		Modules: []Module{
+			{Code: "mining-xmrig", Name: "XMRig"},
+		},
+	}
+	m, ok := idx.Find("mining-xmrig")
+	assert.True(t, ok)
+	assert.Equal(t, "XMRig", m.Name)
+}
+
+func TestFind_Bad_NotFound(t *testing.T) {
+	idx := &Index{}
+	_, ok := idx.Find("nope")
+	assert.False(t, ok)
+}