diff --git a/model/renderers/qwen3coder.go b/model/renderers/qwen3coder.go
index df3b3a45..32611791 100644
--- a/model/renderers/qwen3coder.go
+++ b/model/renderers/qwen3coder.go
@@ -99,9 +99,7 @@ func Qwen3CoderRenderer(messages []api.Message, tools []api.Tool, _ *api.ThinkVa
 					sb.WriteString("\n<name>" + name + "</name>")
 
 					if len(prop.Type) > 0 {
-						// TODO(!!!)(drifkin): we should match the reference implementation for
-						// more complex types here instead of using this format
-						sb.WriteString("\n<type>" + prop.ToTypeScriptType() + "</type>")
+						sb.WriteString("\n<type>" + formatToolDefinitionType(prop.Type) + "</type>")
 					}
 
 					if prop.Description != "" {
@@ -215,3 +213,24 @@ func formatToolCallArgument(value any) string {
 
 	return fmt.Sprintf("%v", value)
 }
+
+func formatToolDefinitionType(tp api.PropertyType) string {
+	if len(tp) == 0 {
+		return "[]"
+	}
+
+	if len(tp) == 1 {
+		return tp[0]
+	}
+
+	// TODO(drifkin): it would be nice to format the JSON here similarly to
+	// python's default json.dumps behavior (spaces after commas and colons).
+	// This would let us be byte-for-byte compatible with the reference
+	// implementation for most common inputs
+	jsonBytes, err := json.Marshal(tp)
+	if err != nil {
+		return "[]"
+	}
+
+	return string(jsonBytes)
+}
diff --git a/model/renderers/qwen3coder_test.go b/model/renderers/qwen3coder_test.go
index 4aaa066d..6a9e5ecc 100644
--- a/model/renderers/qwen3coder_test.go
+++ b/model/renderers/qwen3coder_test.go
@@ -336,3 +336,35 @@ func TestFormatToolCallArgument(t *testing.T) {
 		})
 	}
 }
+
+func TestQwen3ToolDefinitionTypes(t *testing.T) {
+	tests := []struct {
+		name         string
+		propertyType api.PropertyType
+		expected     string
+	}{
+		{
+			name:         "simple",
+			propertyType: api.PropertyType{"string"},
+			expected:     "string",
+		},
+		{
+			name:         "multiple",
+			propertyType: api.PropertyType{"string", "number"},
+			expected:     "[\"string\",\"number\"]",
+		},
+		{
+			name:         "empty",
+			propertyType: api.PropertyType{},
+			expected:     "[]",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := formatToolDefinitionType(tt.propertyType)
+			if got != tt.expected {
+				t.Errorf("formatToolDefinitionType() = %v, want %v", got, tt.expected)
+			}
+		})
+	}
+}