diff --git a/relay/channel/vertex/adaptor.go b/relay/channel/vertex/adaptor.go
index c60d75d29f21..33749a34ecbc 100644
--- a/relay/channel/vertex/adaptor.go
+++ b/relay/channel/vertex/adaptor.go
@@ -94,6 +94,21 @@ func removeFunctionResponseID(request *dto.GeminiChatRequest) {
 }
 
 func (a *Adaptor) ConvertClaudeRequest(c *gin.Context, info *relaycommon.RelayInfo, request *dto.ClaudeRequest) (any, error) {
+	if a.RequestMode == RequestModeGemini {
+		// Gemini upstreams reject the Anthropic Messages body (anthropic_version,
+		// messages, max_tokens). The /v1/messages request must be converted to
+		// Gemini generateContent, same as the /v1/chat/completions path.
+		result, err := service.ConvertRequest(c, info, types.RelayFormatGemini, request)
+		if err != nil {
+			return nil, err
+		}
+		geminiRequest, ok := result.Value.(*dto.GeminiChatRequest)
+		if !ok {
+			return nil, fmt.Errorf("expected Gemini generateContent request, got %T", result.Value)
+		}
+		c.Set("request_model", request.Model)
+		return geminiRequest, nil
+	}
 	if v, ok := claudeModelMap[info.UpstreamModelName]; ok {
 		c.Set("request_model", v)
 	} else {
diff --git a/relay/channel/vertex/adaptor_test.go b/relay/channel/vertex/adaptor_test.go
new file mode 100644
index 000000000000..9f7b894eec8d
--- /dev/null
+++ b/relay/channel/vertex/adaptor_test.go
@@ -0,0 +1,99 @@
+package vertex
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/QuantumNous/new-api/common"
+	relaycommon "github.com/QuantumNous/new-api/relay/common"
+	"github.com/QuantumNous/new-api/relaykit/dto"
+	"github.com/QuantumNous/new-api/relaykit/types"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newTestClaudeRequest() *dto.ClaudeRequest {
+	maxTokens := uint(16)
+	return &dto.ClaudeRequest{
+		Model:     "gemini-2.5-pro",
+		MaxTokens: &maxTokens,
+		Messages: []dto.ClaudeMessage{
+			{Role: "user", Content: "hi"},
+		},
+	}
+}
+
+// A Vertex channel whose upstream is Gemini must convert the incoming Anthropic
+// Messages body to Gemini generateContent. Forwarding the raw Anthropic body to
+// the generateContent URL makes Vertex reject anthropic_version/messages/max_tokens
+// with HTTP 400 (issue #6715). The channel test dispatches through this same
+// ConvertClaudeRequest for the Anthropic endpoint, so this also fixes the test.
+func TestConvertClaudeRequestByRequestMode(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	cases := []struct {
+		name        string
+		requestMode int
+		wantKeys    []string
+		absentKeys  []string
+	}{
+		{
+			name:        "gemini upstream converts to generateContent",
+			requestMode: RequestModeGemini,
+			wantKeys:    []string{"contents", "generationConfig"},
+			absentKeys:  []string{"anthropic_version", "messages", "max_tokens"},
+		},
+		{
+			name:        "claude upstream keeps anthropic messages body",
+			requestMode: RequestModeClaude,
+			wantKeys:    []string{"anthropic_version", "messages", "max_tokens"},
+			absentKeys:  []string{"contents", "generationConfig"},
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			c, _ := gin.CreateTestContext(httptest.NewRecorder())
+			info := &relaycommon.RelayInfo{
+				RelayFormat: types.RelayFormatClaude,
+				ChannelMeta: &relaycommon.ChannelMeta{
+					UpstreamModelName: "gemini-2.5-pro",
+				},
+			}
+			adaptor := &Adaptor{RequestMode: tc.requestMode}
+
+			converted, err := adaptor.ConvertClaudeRequest(c, info, newTestClaudeRequest())
+			require.NoError(t, err)
+			require.NotNil(t, converted)
+
+			raw, err := common.Marshal(converted)
+			require.NoError(t, err)
+
+			var body map[string]any
+			require.NoError(t, common.Unmarshal(raw, &body))
+			for _, key := range tc.wantKeys {
+				assert.Contains(t, body, key, "expected key %q in converted body", key)
+			}
+			for _, key := range tc.absentKeys {
+				assert.NotContains(t, body, key, "unexpected key %q in converted body", key)
+			}
+
+			switch tc.requestMode {
+			case RequestModeGemini:
+				geminiReq, ok := converted.(*dto.GeminiChatRequest)
+				require.True(t, ok, "expected *dto.GeminiChatRequest, got %T", converted)
+				require.Len(t, geminiReq.Contents, 1)
+				require.Len(t, geminiReq.Contents[0].Parts, 1)
+				assert.Equal(t, "hi", geminiReq.Contents[0].Parts[0].Text)
+				require.NotNil(t, geminiReq.GenerationConfig.MaxOutputTokens)
+				assert.Equal(t, uint(16), *geminiReq.GenerationConfig.MaxOutputTokens)
+			case RequestModeClaude:
+				claudeReq, ok := converted.(*VertexAIClaudeRequest)
+				require.True(t, ok, "expected *VertexAIClaudeRequest, got %T", converted)
+				assert.Equal(t, anthropicVersion, claudeReq.AnthropicVersion)
+			}
+		})
+	}
+}