diff --git a/pkg/providers/codex_provider.go b/pkg/providers/codex_provider.go
index 9e36217ae..7617bf716 100644
--- a/pkg/providers/codex_provider.go
+++ b/pkg/providers/codex_provider.go
@@ -217,12 +217,18 @@ func buildCodexParams(messages []Message, tools []ToolDefinition, model string,
 					})
 				}
 				for _, tc := range msg.ToolCalls {
-					argsJSON, _ := json.Marshal(tc.Arguments)
+					name, args, ok := resolveCodexToolCall(tc)
+					if !ok {
+						logger.WarnCF("provider.codex", "Skipping invalid tool call in history", map[string]interface{}{
+							"call_id": tc.ID,
+						})
+						continue
+					}
 					inputItems = append(inputItems, responses.ResponseInputItemUnionParam{
 						OfFunctionCall: &responses.ResponseFunctionToolCallParam{
 							CallID:    tc.ID,
-							Name:      tc.Name,
-							Arguments: string(argsJSON),
+							Name:      name,
+							Arguments: args,
 						},
 					})
 				}
@@ -267,6 +273,30 @@ func buildCodexParams(messages []Message, tools []ToolDefinition, model string,
 	return params
 }
 
+func resolveCodexToolCall(tc ToolCall) (name string, arguments string, ok bool) {
+	name = tc.Name
+	if name == "" && tc.Function != nil {
+		name = tc.Function.Name
+	}
+	if name == "" {
+		return "", "", false
+	}
+
+	if len(tc.Arguments) > 0 {
+		argsJSON, err := json.Marshal(tc.Arguments)
+		if err != nil {
+			return "", "", false
+		}
+		return name, string(argsJSON), true
+	}
+
+	if tc.Function != nil && tc.Function.Arguments != "" {
+		return name, tc.Function.Arguments, true
+	}
+
+	return name, "{}", true
+}
+
 func translateToolsForCodex(tools []ToolDefinition) []responses.ToolUnionParam {
 	result := make([]responses.ToolUnionParam, 0, len(tools))
 	for _, t := range tools {
diff --git a/pkg/providers/codex_provider_test.go b/pkg/providers/codex_provider_test.go
index c34593e7b..8406760c4 100644
--- a/pkg/providers/codex_provider_test.go
+++ b/pkg/providers/codex_provider_test.go
@@ -68,6 +68,45 @@ func TestBuildCodexParams_ToolCallConversation(t *testing.T) {
 	}
 }
 
+func TestBuildCodexParams_ToolCallFunctionFallback(t *testing.T) {
+	messages := []Message{
+		{Role: "user", Content: "Read a file"},
+		{
+			Role: "assistant",
+			ToolCalls: []ToolCall{
+				{
+					ID:   "call_1",
+					Type: "function",
+					Function: &FunctionCall{
+						Name:      "read_file",
+						Arguments: `{"path":"README.md"}`,
+					},
+				},
+			},
+		},
+		{Role: "tool", Content: "ok", ToolCallID: "call_1"},
+	}
+
+	params := buildCodexParams(messages, nil, "gpt-4o", map[string]interface{}{})
+	if params.Input.OfInputItemList == nil {
+		t.Fatal("Input.OfInputItemList should not be nil")
+	}
+	if len(params.Input.OfInputItemList) != 3 {
+		t.Fatalf("len(Input items) = %d, want 3", len(params.Input.OfInputItemList))
+	}
+
+	fc := params.Input.OfInputItemList[1].OfFunctionCall
+	if fc == nil {
+		t.Fatal("assistant tool call should be converted to function_call input item")
+	}
+	if fc.Name != "read_file" {
+		t.Errorf("Function call name = %q, want %q", fc.Name, "read_file")
+	}
+	if fc.Arguments != `{"path":"README.md"}` {
+		t.Errorf("Function call arguments = %q, want %q", fc.Arguments, `{"path":"README.md"}`)
+	}
+}
+
 func TestBuildCodexParams_WithTools(t *testing.T) {
 	tools := []ToolDefinition{
 		{