diff --git a/backend/internal/pkg/apicompat/anthropic_responses_test.go b/backend/internal/pkg/apicompat/anthropic_responses_test.go
index facfe572..e8b25c2b 100644
--- a/backend/internal/pkg/apicompat/anthropic_responses_test.go
+++ b/backend/internal/pkg/apicompat/anthropic_responses_test.go
@@ -991,9 +991,40 @@ func TestAnthropicToResponses_ToolChoiceSpecific(t *testing.T) {
 	var tc map[string]any
 	require.NoError(t, json.Unmarshal(resp.ToolChoice, &tc))
 	assert.Equal(t, "function", tc["type"])
-	fn, ok := tc["function"].(map[string]any)
-	require.True(t, ok)
-	assert.Equal(t, "get_weather", fn["name"])
+	assert.Equal(t, "get_weather", tc["name"])
+	assert.NotContains(t, tc, "function")
+}
+
+func TestResponsesToAnthropicRequest_ToolChoiceFunctionName(t *testing.T) {
+	req := &ResponsesRequest{
+		Model:      "gpt-5.2",
+		Input:      json.RawMessage(`[{"role":"user","content":"Hello"}]`),
+		ToolChoice: json.RawMessage(`{"type":"function","name":"get_weather"}`),
+	}
+
+	resp, err := ResponsesToAnthropicRequest(req)
+	require.NoError(t, err)
+
+	var tc map[string]string
+	require.NoError(t, json.Unmarshal(resp.ToolChoice, &tc))
+	assert.Equal(t, "tool", tc["type"])
+	assert.Equal(t, "get_weather", tc["name"])
+}
+
+func TestResponsesToAnthropicRequest_ToolChoiceLegacyFunctionName(t *testing.T) {
+	req := &ResponsesRequest{
+		Model:      "gpt-5.2",
+		Input:      json.RawMessage(`[{"role":"user","content":"Hello"}]`),
+		ToolChoice: json.RawMessage(`{"type":"function","function":{"name":"get_weather"}}`),
+	}
+
+	resp, err := ResponsesToAnthropicRequest(req)
+	require.NoError(t, err)
+
+	var tc map[string]string
+	require.NoError(t, json.Unmarshal(resp.ToolChoice, &tc))
+	assert.Equal(t, "tool", tc["type"])
+	assert.Equal(t, "get_weather", tc["name"])
 }
 
 // ---------------------------------------------------------------------------
diff --git a/backend/internal/pkg/apicompat/anthropic_to_responses.go b/backend/internal/pkg/apicompat/anthropic_to_responses.go
index 485262e8..268f9f22 100644
--- a/backend/internal/pkg/apicompat/anthropic_to_responses.go
+++ b/backend/internal/pkg/apicompat/anthropic_to_responses.go
@@ -75,7 +75,7 @@ func AnthropicToResponses(req *AnthropicRequest) (*ResponsesRequest, error) {
 //	{"type":"auto"}            → "auto"
 //	{"type":"any"}             → "required"
 //	{"type":"none"}            → "none"
-//	{"type":"tool","name":"X"} → {"type":"function","function":{"name":"X"}}
+//	{"type":"tool","name":"X"} → {"type":"function","name":"X"}
 func convertAnthropicToolChoiceToResponses(raw json.RawMessage) (json.RawMessage, error) {
 	var tc struct {
 		Type string `json:"type"`
@@ -94,8 +94,8 @@ func convertAnthropicToolChoiceToResponses(raw json.RawMessage) (json.RawMessage
 		return json.Marshal("none")
 	case "tool":
 		return json.Marshal(map[string]any{
-			"type":     "function",
-			"function": map[string]string{"name": tc.Name},
+			"type": "function",
+			"name": tc.Name,
 		})
 	default:
 		// Pass through unknown types as-is
diff --git a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
index c140449a..35d42999 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
@@ -281,6 +281,8 @@ func TestChatCompletionsToResponses_LegacyFunctions(t *testing.T) {
 	var tc map[string]any
 	require.NoError(t, json.Unmarshal(resp.ToolChoice, &tc))
 	assert.Equal(t, "function", tc["type"])
+	assert.Equal(t, "get_weather", tc["name"])
+	assert.NotContains(t, tc, "function")
 }
 
 func TestChatCompletionsToResponses_ServiceTier(t *testing.T) {
diff --git a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
index c2725406..64ef5781 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
@@ -420,7 +420,7 @@ func convertChatToolsToResponses(tools []ChatTool, functions []ChatFunction) []R
 //
 //	"auto" → "auto"
 //	"none" → "none"
-//	{"name":"X"} → {"type":"function","function":{"name":"X"}}
+//	{"name":"X"} → {"type":"function","name":"X"}
 func convertChatFunctionCallToToolChoice(raw json.RawMessage) (json.RawMessage, error) {
 	// Try string first ("auto", "none", etc.) — pass through as-is.
 	var s string
@@ -436,7 +436,7 @@ func convertChatFunctionCallToToolChoice(raw json.RawMessage) (json.RawMessage,
 		return nil, err
 	}
 	return json.Marshal(map[string]any{
-		"type":     "function",
-		"function": map[string]string{"name": obj.Name},
+		"type": "function",
+		"name": obj.Name,
 	})
 }
diff --git a/backend/internal/pkg/apicompat/responses_to_anthropic_request.go b/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
index 49426b88..8fa652f2 100644
--- a/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
+++ b/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
@@ -428,7 +428,8 @@ func normalizeAnthropicInputSchema(schema json.RawMessage) json.RawMessage {
 //	"auto"                                     → {"type":"auto"}
 //	"required"                                 → {"type":"any"}
 //	"none"                                     → {"type":"none"}
-//	{"type":"function","function":{"name":"X"}} → {"type":"tool","name":"X"}
+//	{"type":"function","name":"X"}                 → {"type":"tool","name":"X"}
+//	{"type":"function","function":{"name":"X"}}     → {"type":"tool","name":"X"} // legacy
 func convertResponsesToAnthropicToolChoice(raw json.RawMessage) (json.RawMessage, error) {
 	// Try as string first
 	var s string
@@ -448,14 +449,22 @@ func convertResponsesToAnthropicToolChoice(raw json.RawMessage) (json.RawMessage
 	// Try as object with type=function
 	var tc struct {
 		Type     string `json:"type"`
+		Name     string `json:"name"`
 		Function struct {
 			Name string `json:"name"`
 		} `json:"function"`
 	}
-	if err := json.Unmarshal(raw, &tc); err == nil && tc.Type == "function" && tc.Function.Name != "" {
+	if err := json.Unmarshal(raw, &tc); err == nil && tc.Type == "function" {
+		name := strings.TrimSpace(tc.Name)
+		if name == "" {
+			name = strings.TrimSpace(tc.Function.Name)
+		}
+		if name == "" {
+			return raw, nil
+		}
 		return json.Marshal(map[string]string{
 			"type": "tool",
-			"name": tc.Function.Name,
+			"name": name,
 		})
 	}
 
diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index e765d7e9..0fda16b0 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -141,9 +141,7 @@ func applyCodexOAuthTransform(reqBody map[string]any, isCodexCLI bool, isCompact
 			if name, ok := fcObj["name"].(string); ok && strings.TrimSpace(name) != "" {
 				reqBody["tool_choice"] = map[string]any{
 					"type": "function",
-					"function": map[string]any{
-						"name": name,
-					},
+					"name": name,
 				}
 			}
 		}
@@ -219,9 +217,38 @@ func normalizeCodexToolChoice(reqBody map[string]any) bool {
 		return false
 	}
 	choiceType := strings.TrimSpace(firstNonEmptyString(choiceMap["type"]))
-	if choiceType == "" || codexToolsContainType(reqBody["tools"], choiceType) {
+	if choiceType == "" {
 		return false
 	}
+	modified := false
+	if choiceType == "function" {
+		name := strings.TrimSpace(firstNonEmptyString(choiceMap["name"]))
+		if name == "" {
+			if function, ok := choiceMap["function"].(map[string]any); ok {
+				name = strings.TrimSpace(firstNonEmptyString(function["name"]))
+			}
+		}
+		if name == "" {
+			reqBody["tool_choice"] = "auto"
+			return true
+		}
+		if strings.TrimSpace(firstNonEmptyString(choiceMap["name"])) != name {
+			choiceMap["name"] = name
+			modified = true
+		}
+		if _, ok := choiceMap["function"]; ok {
+			delete(choiceMap, "function")
+			modified = true
+		}
+		if !codexToolsContainFunctionName(reqBody["tools"], name) {
+			reqBody["tool_choice"] = "auto"
+			return true
+		}
+		return modified
+	}
+	if codexToolsContainType(reqBody["tools"], choiceType) {
+		return modified
+	}
 	reqBody["tool_choice"] = "auto"
 	return true
 }
@@ -243,6 +270,33 @@ func codexToolsContainType(rawTools any, toolType string) bool {
 	return false
 }
 
+func codexToolsContainFunctionName(rawTools any, name string) bool {
+	tools, ok := rawTools.([]any)
+	if !ok || strings.TrimSpace(name) == "" {
+		return false
+	}
+	normalizedName := strings.TrimSpace(name)
+	for _, rawTool := range tools {
+		tool, ok := rawTool.(map[string]any)
+		if !ok {
+			continue
+		}
+		if strings.TrimSpace(firstNonEmptyString(tool["type"])) != "function" {
+			continue
+		}
+		toolName := strings.TrimSpace(firstNonEmptyString(tool["name"]))
+		if toolName == "" {
+			if function, ok := tool["function"].(map[string]any); ok {
+				toolName = strings.TrimSpace(firstNonEmptyString(function["name"]))
+			}
+		}
+		if toolName == normalizedName {
+			return true
+		}
+	}
+	return false
+}
+
 func normalizeCodexToolRoleMessages(input []any) ([]any, bool) {
 	if len(input) == 0 {
 		return input, false
diff --git a/backend/internal/service/openai_codex_transform_test.go b/backend/internal/service/openai_codex_transform_test.go
index 75f5c55c..8d9f8574 100644
--- a/backend/internal/service/openai_codex_transform_test.go
+++ b/backend/internal/service/openai_codex_transform_test.go
@@ -249,6 +249,44 @@ func TestApplyCodexOAuthTransform_PreservesKnownToolChoice(t *testing.T) {
 	require.Equal(t, "custom", choice["type"])
 }
 
+func TestApplyCodexOAuthTransform_NormalizesLegacyFunctionToolChoice(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "gpt-5.4",
+		"tools": []any{
+			map[string]any{"type": "function", "name": "shell"},
+		},
+		"tool_choice": map[string]any{
+			"type":     "function",
+			"function": map[string]any{"name": "shell"},
+		},
+	}
+
+	applyCodexOAuthTransform(reqBody, true, false)
+
+	choice, ok := reqBody["tool_choice"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "function", choice["type"])
+	require.Equal(t, "shell", choice["name"])
+	require.NotContains(t, choice, "function")
+}
+
+func TestApplyCodexOAuthTransform_DowngradesMissingFunctionToolChoice(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "gpt-5.4",
+		"tools": []any{
+			map[string]any{"type": "function", "name": "shell"},
+		},
+		"tool_choice": map[string]any{
+			"type":     "function",
+			"function": map[string]any{"name": "missing"},
+		},
+	}
+
+	applyCodexOAuthTransform(reqBody, true, false)
+
+	require.Equal(t, "auto", reqBody["tool_choice"])
+}
+
 func TestApplyCodexOAuthTransform_AddsFallbackNameForFunctionCallInput(t *testing.T) {
 	reqBody := map[string]any{
 		"model": "gpt-5.4",