From 791a42ad8ba355366b842803dfee2445d7039eea Mon Sep 17 00:00:00 2001
From: Alex Castillo <alex.castillog33@gmail.com>
Date: Mon, 26 Oct 2026 20:19:42 -0400
Subject: [PATCH] fix: honor Claude tool_choice when converting requests to
 Gemini

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 internal/transformer/convert/claude_gemini.go | 34 +++++++++++--
 .../convert/claude_gemini_tool_choice_test.go | 49 +++++++++++++++++++
 2 files changed, 80 insertions(+), 3 deletions(-)
 create mode 100644 internal/transformer/convert/claude_gemini_tool_choice_test.go

diff --git a/internal/transformer/convert/claude_gemini.go b/internal/transformer/convert/claude_gemini.go
index a7fc3c4..25f2924 100644
--- a/internal/transformer/convert/claude_gemini.go
+++ b/internal/transformer/convert/claude_gemini.go
@@ -73,15 +73,43 @@ func ClaudeReqToGemini(claudeReq []byte, model string) ([]byte, error) {
 		geminiReq["tools"] = []map[string]interface{}{{"functionDeclarations": funcDecls}}
 		// Add toolConfig to enable function calling
 		geminiReq["toolConfig"] = map[string]interface{}{
-			"functionCallingConfig": map[string]interface{}{
-				"mode": "AUTO",
-			},
+			"functionCallingConfig": claudeToolChoiceToGeminiConfig(req.ToolChoice),
 		}
 	}
 
 	return json.Marshal(geminiReq)
 }
 
+// claudeToolChoiceToGeminiConfig maps a Claude tool_choice to a Gemini
+// functionCallingConfig. A missing or unknown choice stays AUTO.
+func claudeToolChoiceToGeminiConfig(toolChoice interface{}) map[string]interface{} {
+	config := map[string]interface{}{"mode": "AUTO"}
+
+	var choiceType string
+	var name string
+	switch tc := toolChoice.(type) {
+	case map[string]interface{}:
+		choiceType, _ = tc["type"].(string)
+		name, _ = tc["name"].(string)
+	case string:
+		choiceType = tc
+	}
+
+	switch choiceType {
+	case "any", "required":
+		config["mode"] = "ANY"
+	case "tool":
+		config["mode"] = "ANY"
+		if name != "" {
+			config["allowedFunctionNames"] = []string{name}
+		}
+	case "none":
+		config["mode"] = "NONE"
+	}
+
+	return config
+}
+
 // GeminiReqToClaude converts Gemini request to Claude request
 func GeminiReqToClaude(geminiReq []byte, model string) ([]byte, error) {
 	var req transformer.GeminiRequest
diff --git a/internal/transformer/convert/claude_gemini_tool_choice_test.go b/internal/transformer/convert/claude_gemini_tool_choice_test.go
new file mode 100644
index 0000000..d7c367d
--- /dev/null
+++ b/internal/transformer/convert/claude_gemini_tool_choice_test.go
@@ -0,0 +1,49 @@
+package convert
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestClaudeReqToGeminiToolChoice(t *testing.T) {
+	tests := []struct {
+		name       string
+		toolChoice string
+		wantMode   string
+		wantNames  []interface{}
+	}{
+		{"omitted", ``, "AUTO", nil},
+		{"auto", `,"tool_choice":{"type":"auto"}`, "AUTO", nil},
+		{"any", `,"tool_choice":{"type":"any"}`, "ANY", nil},
+		{"tool", `,"tool_choice":{"type":"tool","name":"get_weather"}`, "ANY", []interface{}{"get_weather"}},
+		{"none", `,"tool_choice":{"type":"none"}`, "NONE", nil},
+		{"string any", `,"tool_choice":"any"`, "ANY", nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			claudeReq := `{"model":"claude","max_tokens":100,"messages":[{"role":"user","content":"hi"}],
+				"tools":[{"name":"get_weather","description":"d","input_schema":{"type":"object","properties":{}}}]` + tt.toolChoice + `}`
+
+			out, err := ClaudeReqToGemini([]byte(claudeReq), "gemini-2.5-pro")
+			if err != nil {
+				t.Fatalf("ClaudeReqToGemini failed: %v", err)
+			}
+
+			var got map[string]interface{}
+			if err := json.Unmarshal(out, &got); err != nil {
+				t.Fatalf("invalid output: %v", err)
+			}
+			toolConfig, _ := got["toolConfig"].(map[string]interface{})
+			fcc, _ := toolConfig["functionCallingConfig"].(map[string]interface{})
+			if fcc["mode"] != tt.wantMode {
+				t.Errorf("mode = %v, want %v", fcc["mode"], tt.wantMode)
+			}
+			gotNames, _ := fcc["allowedFunctionNames"].([]interface{})
+			if !reflect.DeepEqual(gotNames, tt.wantNames) {
+				t.Errorf("allowedFunctionNames = %v, want %v", gotNames, tt.wantNames)
+			}
+		})
+	}
+}