diff --git a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
index d1f3bccc00..46cfda43b3 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
@@ -88,10 +88,11 @@ func ChatCompletionsToResponses(req *ChatCompletionsRequest) (*ResponsesRequest,
 		out.Tools = convertChatToolsToResponses(req.Tools, req.Functions)
 	}
 
-	// tool_choice: already compatible format — pass through directly.
+	// tool_choice: strings and Responses-shaped objects pass through; a named
+	// Chat choice nests the name under "function" and must be flattened.
 	// Legacy function_call needs mapping.
 	if len(req.ToolChoice) > 0 {
-		out.ToolChoice = req.ToolChoice
+		out.ToolChoice = convertChatToolChoiceToResponses(req.ToolChoice)
 	} else if len(req.FunctionCall) > 0 {
 		tc, err := convertChatFunctionCallToToolChoice(req.FunctionCall)
 		if err != nil {
@@ -490,6 +491,34 @@ func defaultStrictFalse(src *bool) *bool {
 	return src
 }
 
+// convertChatToolChoiceToResponses maps a Chat Completions tool_choice to the
+// Responses API shape.
+//
+//	{"type":"function","function":{"name":"X"}} → {"type":"function","name":"X"}
+//
+// Strings ("auto", "none", "required") and objects that already use the
+// Responses shape are returned unchanged.
+func convertChatToolChoiceToResponses(raw json.RawMessage) json.RawMessage {
+	var choice struct {
+		Type     string `json:"type"`
+		Function *struct {
+			Name string `json:"name"`
+		} `json:"function"`
+	}
+	if err := json.Unmarshal(raw, &choice); err != nil || choice.Type != "function" || choice.Function == nil {
+		return raw
+	}
+	name := strings.TrimSpace(choice.Function.Name)
+	if name == "" {
+		return raw
+	}
+	flat, err := json.Marshal(map[string]string{"type": "function", "name": name})
+	if err != nil {
+		return raw
+	}
+	return flat
+}
+
 // convertChatFunctionCallToToolChoice maps the legacy function_call field to a
 // Responses API tool_choice value.
 //
diff --git a/backend/internal/pkg/apicompat/chatcompletions_to_responses_tool_choice_test.go b/backend/internal/pkg/apicompat/chatcompletions_to_responses_tool_choice_test.go
new file mode 100644
index 0000000000..736a0b5acf
--- /dev/null
+++ b/backend/internal/pkg/apicompat/chatcompletions_to_responses_tool_choice_test.go
@@ -0,0 +1,43 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// Chat Completions nests a named choice under "function"; the Responses API
+// expects the name at the top level and rejects the nested shape.
+func TestChatCompletionsToResponses_ToolChoiceShape(t *testing.T) {
+	tests := []struct {
+		name   string
+		choice string
+		want   string
+	}{
+		{name: "named chat function is flattened", choice: `{"type":"function","function":{"name":"get_weather"}}`, want: `{"type":"function","name":"get_weather"}`},
+		{name: "flat responses function is kept", choice: `{"type":"function","name":"get_weather"}`, want: `{"type":"function","name":"get_weather"}`},
+		{name: "auto string is kept", choice: `"auto"`, want: `"auto"`},
+		{name: "required string is kept", choice: `"required"`, want: `"required"`},
+		{name: "none string is kept", choice: `"none"`, want: `"none"`},
+		{name: "allowed_tools policy is kept", choice: `{"type":"allowed_tools","mode":"auto","tools":[{"type":"function","name":"get_weather"}]}`, want: `{"type":"allowed_tools","mode":"auto","tools":[{"type":"function","name":"get_weather"}]}`},
+		{name: "function without name is kept", choice: `{"type":"function","function":{}}`, want: `{"type":"function","function":{}}`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := &ChatCompletionsRequest{
+				Model:    "gpt-4o",
+				Messages: []ChatMessage{{Role: "user", Content: json.RawMessage(`"weather?"`)}},
+				Tools: []ChatTool{{
+					Type:     "function",
+					Function: &ChatFunction{Name: "get_weather", Parameters: json.RawMessage(`{"type":"object"}`)},
+				}},
+				ToolChoice: json.RawMessage(tt.choice),
+			}
+
+			resp, err := ChatCompletionsToResponses(req)
+			require.NoError(t, err)
+			require.JSONEq(t, tt.want, string(resp.ToolChoice))
+		})
+	}
+}