diff --git a/hilti/lib/hilti.hlt b/hilti/lib/hilti.hlt
index 018e5e6fd..6bb65f30a 100644
--- a/hilti/lib/hilti.hlt
+++ b/hilti/lib/hilti.hlt
@@ -24,8 +24,8 @@ declare public void debug(string dbg_stream, any obj) &cxxname="hilti::rt::debug
 declare public void debugIndent(string dbg_stream) &cxxname="hilti::rt::debug::indent" &have_prototype;
 declare public void debugDedent(string dbg_stream) &cxxname="hilti::rt::debug::dedent" &have_prototype;
 
-declare public time current_time() &cxxname="hilti::rt::time::current_time" &have_prototype;
-declare public time mktime(uint<64> y, uint<64> m, uint<64> d, uint<64> H, uint<64> M, uint<64> S) &cxxname="hilti::rt::time::mktime" &have_prototype;
+declare public time current_time() &cxxname="hilti::rt::time::current_time" &have_prototype &pure;
+declare public time mktime(uint<64> y, uint<64> m, uint<64> d, uint<64> H, uint<64> M, uint<64> S) &cxxname="hilti::rt::time::mktime" &have_prototype &pure;
 
 declare public void abort() &cxxname="hilti::rt::abort_with_backtrace" &have_prototype;
 
@@ -49,5 +49,5 @@ public type RecoverableFailure = exception &cxxname="hilti::rt::RecoverableFailu
 public type MissingData = exception &cxxname="hilti::rt::MissingData";
 
 # Returns the message associated with an exception.
-declare public string exception_what(SystemException excpt) &cxxname="hilti::rt::exception::what" &have_prototype;
+declare public string exception_what(SystemException excpt) &cxxname="hilti::rt::exception::what" &have_prototype &pure;
 }
diff --git a/spicy/lib/spicy_rt.hlt b/spicy/lib/spicy_rt.hlt
index 5741bddb2..7a67d712e 100644
--- a/spicy/lib/spicy_rt.hlt
+++ b/spicy/lib/spicy_rt.hlt
@@ -29,7 +29,7 @@ public type Sink = struct {
     method void set_auto_trim(bool enable);
     method void set_initial_sequence_number(uint<64> seq);
     method void set_policy(any policy);
-    method uint<64> size();
+    method uint<64> size() &pure;
     method void skip(uint<64> seq);
     method void trim(uint<64> seq);
     method void write(bytes data, optional<uint<64>> seq = Null, optional<uint<64>> len = Null);