apitypes: rename apiVersions to api_versions

apitypes/capabilities.go

@@ -12,5 +12,5 @@ const APIVersionV1 = "v1"
 // so they can decide whether, and at which version, to use the HTTP API instead of falling back to the gRPC API.
 type CapabilitiesResponse struct {
 	// APIVersions lists the HTTP API versions the Coordinator supports, e.g. ["v1"].
-	APIVersions []string `json:"apiVersions"`
+	APIVersions []string `json:"api_versions"`
 }

apitypes/capabilities_test.go

@@ -0,0 +1,28 @@
+// Copyright 2026 Edgeless Systems GmbH
+// SPDX-License-Identifier: BUSL-1.1
+
+package apitypes
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestCapabilitiesResponseFieldName pins the wire field name, which is part of the API contract and can't be changed once clients rely on it.
+func TestCapabilitiesResponseFieldName(t *testing.T) {
+	data, err := json.Marshal(CapabilitiesResponse{APIVersions: []string{APIVersionV1}})
+	require.NoError(t, err)
+
+	assert.JSONEq(t, `{"api_versions":["v1"]}`, string(data))
+}
+
+// TestCapabilitiesResponseIgnoresUnknownFields pins the rule that lets a Coordinator extend this response without breaking older clients.
+func TestCapabilitiesResponseIgnoresUnknownFields(t *testing.T) {
+	var resp CapabilitiesResponse
+	require.NoError(t, json.Unmarshal([]byte(`{"api_versions":["v1"],"something_new":42}`), &resp))
+
+	assert.Equal(t, []string{APIVersionV1}, resp.APIVersions)
+}