fix(adapters): #528 Gemini adapter fails if system prompt is removed

lua/codecompanion/adapters/gemini.lua

@@ -44,20 +44,25 @@ return {
     form_messages = function(self, messages)
       -- Format system prompts
       local system = utils.pluck_messages(vim.deepcopy(messages), "system")
-      for _, msg in ipairs(system) do
-        msg.text = msg.content
-
-        -- Remove unnecessary fields
-        msg.tag = nil
-        msg.content = nil
-        msg.role = nil
-        msg.id = nil
-        msg.opts = nil
+      local system_instruction
+
+      -- Only create system_instruction if there are system messages
+      if #system > 0 then
+        for _, msg in ipairs(system) do
+          msg.text = msg.content
+
+          -- Remove unnecessary fields
+          msg.tag = nil
+          msg.content = nil
+          msg.role = nil
+          msg.id = nil
+          msg.opts = nil
+        end
+        system_instruction = {
+          role = self.roles.user,
+          parts = system,
+        }
       end
-      local sys_prompts = {
-        role = self.roles.user,
-        parts = system,
-      }
 
       -- Format messages (remove all system prompts)
       local output = {}
@@ -71,10 +76,15 @@ return {
         })
       end
 
-      return {
-        system_instruction = sys_prompts,
+      -- Only include system_instruction if it exists
+      local result = {
         contents = output,
       }
+      if system_instruction then
+        result.system_instruction = system_instruction
+      end
+
+      return result
     end,
 
     ---Returns the number of tokens generated from the LLM

tests/adapters/test_gemini.lua

@@ -65,6 +65,92 @@ describe("Gemini adapter", function()
     h.eq(output, adapter.handlers.form_messages(adapter, messages))
   end)
 
+  it("can form messages with system prompt", function()
+    adapter = require("codecompanion.adapters").extend("gemini")
+    local messages_with_system = {
+      {
+        content = "You are a helpful assistant",
+        role = "system",
+        id = 1,
+        opts = { visible = false },
+      },
+      {
+        content = "hello",
+        id = 2,
+        opts = { visible = true },
+        role = "user",
+      },
+      {
+        content = "Hi, how can I help?",
+        id = 3,
+        opts = { visible = true },
+        role = "llm",
+      },
+    }
+
+    local output = {
+      system_instruction = {
+        role = "user",
+        parts = {
+          { text = "You are a helpful assistant" },
+        },
+      },
+      contents = {
+        {
+          role = "user",
+          parts = {
+            { text = "hello" },
+          },
+        },
+        {
+          role = "user",
+          parts = {
+            { text = "Hi, how can I help?" },
+          },
+        },
+      },
+    }
+
+    h.eq(output, adapter.handlers.form_messages(adapter, messages_with_system))
+  end)
+
+  it("can form messages without system prompt", function()
+    adapter = require("codecompanion.adapters").extend("gemini")
+    local messages_without_system = {
+      {
+        content = "hello",
+        id = 1,
+        opts = { visible = true },
+        role = "user",
+      },
+      {
+        content = "Hi, how can I help?",
+        id = 2,
+        opts = { visible = true },
+        role = "llm",
+      },
+    }
+
+    local output = {
+      contents = {
+        {
+          role = "user",
+          parts = {
+            { text = "hello" },
+          },
+        },
+        {
+          role = "user",
+          parts = {
+            { text = "Hi, how can I help?" },
+          },
+        },
+      },
+    }
+
+    h.eq(output, adapter.handlers.form_messages(adapter, messages_without_system))
+  end)
+
   it("can output streamed data into a format for the chat buffer", function()
     h.eq(stream_response[#stream_response].output, adapter_helpers.chat_buffer_output(stream_response, adapter))
   end)