diff --git a/internal/graphql/content_type_test.go b/internal/graphql/content_type_test.go
new file mode 100644
index 0000000..2eefab2
--- /dev/null
+++ b/internal/graphql/content_type_test.go
@@ -0,0 +1,34 @@
+package graphql
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestExecuteGraphQLDoesNotDuplicateContentType(t *testing.T) {
+	for _, tc := range []struct {
+		headers []string
+		want    string
+	}{
+		{nil, "application/json"},
+		{[]string{"content-type: application/json; charset=utf-8"}, "application/json; charset=utf-8"},
+	} {
+		received := make(chan []string, 1)
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			received <- r.Header.Values("Content-Type")
+			_, _ = w.Write([]byte(`{"data":{}}`))
+		}))
+		result, err := ExecuteGraphQL(Options{URL: server.URL, Query: "{ hello }", Headers: tc.headers})
+		if err != nil {
+			server.Close()
+			t.Fatal(err)
+		}
+		result.Response.Body.Close()
+		server.Close()
+		values := <-received
+		if len(values) != 1 || values[0] != tc.want {
+			t.Errorf("got Content-Type %v, want only %q", values, tc.want)
+		}
+	}
+}
diff --git a/internal/graphql/graphql.go b/internal/graphql/graphql.go
index 6ab4f88..fdc18ec 100644
--- a/internal/graphql/graphql.go
+++ b/internal/graphql/graphql.go
@@ -1,10 +1,12 @@
 package graphql
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"net/http"
 	"os"
+	"strings"
 
 	"github.com/kavix/kurl/client"
 )
@@ -63,7 +65,12 @@ func BuildPayload(queryStr, variablesStr string, introspect bool) ([]byte, error
 
 	if variablesStr != "" {
 		var vars map[string]interface{}
-		if err := json.Unmarshal([]byte(variablesStr), &vars); err != nil {
+		if !json.Valid([]byte(variablesStr)) {
+			return nil, fmt.Errorf("invalid GraphQL variables JSON")
+		}
+		decoder := json.NewDecoder(bytes.NewBufferString(variablesStr))
+		decoder.UseNumber()
+		if err := decoder.Decode(&vars); err != nil {
 			return nil, fmt.Errorf("invalid GraphQL variables JSON: %w", err)
 		}
 		payload.Variables = vars
@@ -92,7 +99,18 @@ func ExecuteGraphQL(opts Options) (*client.Result, error) {
 		return nil, err
 	}
 
-	headers := append([]string{"Content-Type: application/json"}, opts.Headers...)
+	headers := append([]string(nil), opts.Headers...)
+	hasContentType := false
+	for _, header := range headers {
+		name, _, ok := strings.Cut(header, ":")
+		if ok && strings.EqualFold(strings.TrimSpace(name), "Content-Type") {
+			hasContentType = true
+			break
+		}
+	}
+	if !hasContentType {
+		headers = append([]string{"Content-Type: application/json"}, headers...)
+	}
 
 	fetchOpts := client.Options{
 		Method:  http.MethodPost,
diff --git a/internal/graphql/graphql_test.go b/internal/graphql/graphql_test.go
index e381ce6..0aa31e4 100644
--- a/internal/graphql/graphql_test.go
+++ b/internal/graphql/graphql_test.go
@@ -50,3 +50,34 @@ func TestGenerateQueryForType(t *testing.T) {
 		t.Errorf("expected %q, got %q", expected, query)
 	}
 }
+
+func TestBuildPayloadPreservesNumericVariables(t *testing.T) {
+	payload, err := BuildPayload(`query ($id: ID!) { user(id: $id) { name } }`, `{"id":9007199254740993,"nested":{"ratio":0.1234567890123456789}}`, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var result struct {
+		Variables json.RawMessage `json:"variables"`
+	}
+	if err := json.Unmarshal(payload, &result); err != nil {
+		t.Fatal(err)
+	}
+	var variables map[string]json.RawMessage
+	if err := json.Unmarshal(result.Variables, &variables); err != nil {
+		t.Fatal(err)
+	}
+	if string(variables["id"]) != `9007199254740993` {
+		t.Fatalf("numeric ID changed: %s", variables["id"])
+	}
+	if string(variables["nested"]) != `{"ratio":0.1234567890123456789}` {
+		t.Fatalf("decimal changed: %s", variables["nested"])
+	}
+}
+
+func TestBuildPayloadRejectsInvalidVariableObjects(t *testing.T) {
+	for _, variables := range []string{`{"x":1} {}`, `[]`, `1`, `{`} {
+		if _, err := BuildPayload(`{ user { name } }`, variables, false); err == nil {
+			t.Errorf("accepted invalid variables %s", variables)
+		}
+	}
+}