FIX: Limit system message size to 60% of available tokens. (#714)

Using RAG fragments can lead to considerably big system messages, which becomes problematic when models have a smaller context window.

Before this change, we only look at the rest of the conversation to make sure we don't surpass the limit, which could lead to two unwanted scenarios when having large system messages:

All other messages are excluded due to size.
The system message already exceeds the limit.

As a result, I'm putting a hard-limit of 60% of available tokens. We don't want to aggresively truncate because if rag fragments are included, the system message contains a lot of context to improve the model response, but we also want to make room for the recent messages in the conversation.

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)

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