Add Gemini-to-Gemini request normalization and passthrough support

- Introduced a `ConvertGeminiRequestToGemini` function to normalize Gemini v1beta requests by ensuring valid or default roles.
- Added passthrough response handlers for both streamed and non-streamed Gemini responses.
- Registered translators for Gemini-to-Gemini traffic in the initialization process.
- Updated `gemini-cli` request normalization to align with the new Gemini translator logic.

internal/translator/gemini-cli/gemini/gemini-cli_gemini_request.go

@@ -49,6 +49,33 @@ func ConvertGeminiRequestToGeminiCLI(_ string, rawJSON []byte, _ bool) []byte {
 	}
 	rawJSON = []byte(template)
 
+	// Normalize roles in request.contents: default to valid values if missing/invalid
+	contents := gjson.GetBytes(rawJSON, "request.contents")
+	if contents.Exists() {
+		prevRole := ""
+		idx := 0
+		contents.ForEach(func(_ gjson.Result, value gjson.Result) bool {
+			role := value.Get("role").String()
+			valid := role == "user" || role == "model"
+			if role == "" || !valid {
+				var newRole string
+				if prevRole == "" {
+					newRole = "user"
+				} else if prevRole == "user" {
+					newRole = "model"
+				} else {
+					newRole = "user"
+				}
+				path := fmt.Sprintf("request.contents.%d.role", idx)
+				rawJSON, _ = sjson.SetBytes(rawJSON, path, newRole)
+				role = newRole
+			}
+			prevRole = role
+			idx++
+			return true
+		})
+	}
+
 	return rawJSON
 }
 

internal/translator/gemini/gemini/gemini_gemini_request.go

@@ -0,0 +1,54 @@
+// Package gemini provides in-provider request normalization for Gemini API.
+// It ensures incoming v1beta requests meet minimal schema requirements
+// expected by Google's Generative Language API.
+package gemini
+
+import (
+	"fmt"
+
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+// ConvertGeminiRequestToGemini normalizes Gemini v1beta requests.
+//   - Adds a default role for each content if missing or invalid.
+//     The first message defaults to "user", then alternates user/model when needed.
+//
+// It keeps the payload otherwise unchanged.
+func ConvertGeminiRequestToGemini(_ string, rawJSON []byte, _ bool) []byte {
+	// Fast path: if no contents field, return as-is
+	contents := gjson.GetBytes(rawJSON, "contents")
+	if !contents.Exists() {
+		return rawJSON
+	}
+
+	// Walk contents and fix roles
+	out := rawJSON
+	prevRole := ""
+	idx := 0
+	contents.ForEach(func(_ gjson.Result, value gjson.Result) bool {
+		role := value.Get("role").String()
+
+		// Only user/model are valid for Gemini v1beta requests
+		valid := role == "user" || role == "model"
+		if role == "" || !valid {
+			var newRole string
+			if prevRole == "" {
+				newRole = "user"
+			} else if prevRole == "user" {
+				newRole = "model"
+			} else {
+				newRole = "user"
+			}
+			path := fmt.Sprintf("contents.%d.role", idx)
+			out, _ = sjson.SetBytes(out, path, newRole)
+			role = newRole
+		}
+
+		prevRole = role
+		idx++
+		return true
+	})
+
+	return out
+}

internal/translator/gemini/gemini/gemini_gemini_response.go

@@ -0,0 +1,15 @@
+package gemini
+
+import (
+	"context"
+)
+
+// PassthroughGeminiResponseStream forwards Gemini responses unchanged.
+func PassthroughGeminiResponseStream(_ context.Context, _ string, rawJSON []byte, _ *any) []string {
+	return []string{string(rawJSON)}
+}
+
+// PassthroughGeminiResponseNonStream forwards Gemini responses unchanged.
+func PassthroughGeminiResponseNonStream(_ context.Context, _ string, rawJSON []byte, _ *any) string {
+	return string(rawJSON)
+}

internal/translator/gemini/gemini/init.go

@@ -0,0 +1,21 @@
+package gemini
+
+import (
+	. "github.com/luispater/CLIProxyAPI/internal/constant"
+	"github.com/luispater/CLIProxyAPI/internal/interfaces"
+	"github.com/luispater/CLIProxyAPI/internal/translator/translator"
+)
+
+// Register a no-op response translator and a request normalizer for Gemini→Gemini.
+// The request converter ensures missing or invalid roles are normalized to valid values.
+func init() {
+	translator.Register(
+		GEMINI,
+		GEMINI,
+		ConvertGeminiRequestToGemini,
+		interfaces.TranslateResponse{
+			Stream:    PassthroughGeminiResponseStream,
+			NonStream: PassthroughGeminiResponseNonStream,
+		},
+	)
+}

internal/translator/init.go

@@ -12,6 +12,7 @@ import (
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini-cli/gemini"
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini-cli/openai"
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini/claude"
+	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini/gemini"
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini/gemini-cli"
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/gemini/openai"
 	_ "github.com/luispater/CLIProxyAPI/internal/translator/openai/claude"