diff --git a/lib/completions/dialects/dialect.rb b/lib/completions/dialects/dialect.rb
index d36c8f1d..7d49e396 100644
--- a/lib/completions/dialects/dialect.rb
+++ b/lib/completions/dialects/dialect.rb
@@ -95,7 +95,17 @@ module DiscourseAi
 
           range = (0..-1)
           if messages.dig(0, :type) == :system
+            max_system_tokens = prompt_limit * 0.6
             system_message = messages[0]
+            system_size = calculate_message_token(system_message)
+
+            if system_size > max_system_tokens
+              system_message[:content] = tokenizer.truncate(
+                system_message[:content],
+                max_system_tokens,
+              )
+            end
+
             trimmed_messages << system_message
             current_token_count += calculate_message_token(system_message)
             range = (1..-1)
diff --git a/spec/lib/completions/dialects/dialect_spec.rb b/spec/lib/completions/dialects/dialect_spec.rb
index e5511bbf..a7667a7c 100644
--- a/spec/lib/completions/dialects/dialect_spec.rb
+++ b/spec/lib/completions/dialects/dialect_spec.rb
@@ -8,22 +8,20 @@ class TestDialect < DiscourseAi::Completions::Dialects::Dialect
   end
 
   def tokenizer
-    Class.new do
-      def self.size(str)
-        str.length
-      end
-    end
+    DiscourseAi::Tokenizer::OpenAiTokenizer
   end
 end
 
 RSpec.describe DiscourseAi::Completions::Dialects::Dialect do
   describe "#trim_messages" do
+    let(:five_token_msg) { "This represents five tokens." }
+
     it "should trim tool messages if tool_calls are trimmed" do
-      prompt = DiscourseAi::Completions::Prompt.new("12345")
-      prompt.push(type: :user, content: "12345")
-      prompt.push(type: :tool_call, content: "12345", id: 1)
-      prompt.push(type: :tool, content: "12345", id: 1)
-      prompt.push(type: :user, content: "12345")
+      prompt = DiscourseAi::Completions::Prompt.new(five_token_msg)
+      prompt.push(type: :user, content: five_token_msg)
+      prompt.push(type: :tool_call, content: five_token_msg, id: 1)
+      prompt.push(type: :tool, content: five_token_msg, id: 1)
+      prompt.push(type: :user, content: five_token_msg)
 
       dialect = TestDialect.new(prompt, "test")
       dialect.max_prompt_tokens = 15 # fits the user messages and the tool_call message
@@ -31,7 +29,24 @@ RSpec.describe DiscourseAi::Completions::Dialects::Dialect do
       trimmed = dialect.trim(prompt.messages)
 
       expect(trimmed).to eq(
-        [{ type: :system, content: "12345" }, { type: :user, content: "12345" }],
+        [{ type: :system, content: five_token_msg }, { type: :user, content: five_token_msg }],
+      )
+    end
+
+    it "limits the system message to 60% of available tokens" do
+      prompt = DiscourseAi::Completions::Prompt.new("I'm a system message consisting of 10 tokens")
+      prompt.push(type: :user, content: five_token_msg)
+
+      dialect = TestDialect.new(prompt, "test")
+      dialect.max_prompt_tokens = 15
+
+      trimmed = dialect.trim(prompt.messages)
+
+      expect(trimmed).to eq(
+        [
+          { type: :system, content: "I'm a system message consisting of 10" },
+          { type: :user, content: five_token_msg },
+        ],
       )
     end
   end