diff --git a/Makefile b/Makefile
index 1cd508e..758a093 100644
--- a/Makefile
+++ b/Makefile
@@ -8,17 +8,18 @@ MAPPER_GEN_FILE := ./api/eiaapi_funcmap.gen.go
 GO_FORMATTER := gofumpt
 
 IS_GNU_SED := $(shell sed --version >/dev/null 2>&1 && echo true || echo false)
-SED_INLINE := $(if $(IS_GNU_SED),-i '',-i)
+SED_INLINE := $(if $(IS_GNU_SED),-i,-i '')
 
-.PHONY: all schema generate build install clean
+.PHONY: all schema generate test build install clean
 
 # Default target
-all: schema generate build install
+all: schema generate test build install
 
 # Retrieve and prepare schema
 schema:
 	curl https://www.eia.gov/opendata/$(SCHEMA_ZIP) -o $(SCHEMA_DIR)/$(SCHEMA_ZIP)
 	unzip -o $(SCHEMA_DIR)/$(SCHEMA_ZIP) -d $(SCHEMA_DIR)
+	@echo "Using GNU sed: $(IS_GNU_SED)"
 	sed -E $(SED_INLINE) 's/responses\/data/schemas\/DataResponseContainer/g' $(SCHEMA_DIR)/$(SCHEMA_YML)
 	sed -E $(SED_INLINE) 's/responses\/route/schemas\/RouteResponse/g' $(SCHEMA_DIR)/$(SCHEMA_YML)
 	sed -E $(SED_INLINE) 's/responses\/facets/schemas\/FacetOptionListContainer/g' $(SCHEMA_DIR)/$(SCHEMA_YML)
@@ -30,6 +31,7 @@ schema:
 # Generate code
 generate:
 	go generate ./...
+	@echo "Using GNU sed: $(IS_GNU_SED)"
 	sed -E $(SED_INLINE) 's/Total[[:space:]]+\*int/Total *string/g' $(CLIENT_GEN_FILE)
 	sed -E $(SED_INLINE) 's/Command[[:space:]]+\*\[\]string/Command *string/g' $(CLIENT_GEN_FILE)
 	sed -E $(SED_INLINE) 's/Routes[[:space:]]+\*\[\]string/Routes *[]Routes/g' $(CLIENT_GEN_FILE)
@@ -37,8 +39,12 @@ generate:
 	$(GO_FORMATTER) -w $(CLIENT_GEN_FILE)
 	$(GO_FORMATTER) -w $(MAPPER_GEN_FILE)
 
+test:
+	# Test EIA API client package
+	go test -v ./pkg/eia
+
 # Build the client command binary
-build: generate
+build: generate test
 	go build -o bin/eia-client $(CLIENT_PKG)
 
 # Install the client command binary
diff --git a/main.go b/main.go
index 16f245c..36090e7 100644
--- a/main.go
+++ b/main.go
@@ -2,3 +2,5 @@ package main
 
 //go:generate go run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen --config=api/genclient.yaml schema/eia-api-swagger.yaml
 //go:generate go run api/funcmapper/funcmapper.go
+
+func main() {}
diff --git a/pkg/eia/eia_test.go b/pkg/eia/eia_test.go
new file mode 100644
index 0000000..403496f
--- /dev/null
+++ b/pkg/eia/eia_test.go
@@ -0,0 +1,67 @@
+package eia
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNewFacets(t *testing.T) {
+	type args struct {
+		facets []*Facet
+	}
+	tests := []struct {
+		name string
+		args args
+		want map[string]interface{}
+	}{
+		{
+			name: "Empty facets",
+			args: args{facets: []*Facet{}},
+			want: map[string]interface{}{},
+		},
+		{
+			name: "Single facet",
+			args: args{
+				facets: []*Facet{
+					{Name: "seriesId", Data: "Region1"},
+				},
+			},
+			want: map[string]interface{}{
+				"facets[seriesId][0]": "Region1",
+			},
+		},
+		{
+			name: "Multiple facets with unique names",
+			args: args{
+				facets: []*Facet{
+					{Name: "seriesId", Data: "Region1"},
+					{Name: "scenario", Data: "low-gdp"},
+				},
+			},
+			want: map[string]interface{}{
+				"facets[seriesId][0]": "Region1",
+				"facets[scenario][0]": "low-gdp",
+			},
+		},
+		{
+			name: "Multiple facets with duplicate names",
+			args: args{
+				facets: []*Facet{
+					{Name: "scenario", Data: "low-gdp"},
+					{Name: "scenario", Data: "high-gdp"},
+				},
+			},
+			want: map[string]interface{}{
+				"facets[scenario][0]": "low-gdp",
+				"facets[scenario][1]": "high-gdp",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := NewFacets(tt.args.facets...); !reflect.DeepEqual(*got, tt.want) {
+				t.Errorf("NewFacets() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}