FIX: gemini 0801 tool calls (#748)

Gemini experimental model requires tool_config.

Previously defaults would apply.

This corrects prompts containing multiple tools on gemini.

lib/completions/endpoints/gemini.rb

@@ -65,7 +65,10 @@ module DiscourseAi
             role: "system",
             parts: [{ text: prompt[:system_instruction].to_s }],
           } if prompt[:system_instruction].present?
-          payload[:tools] = tools if tools.present?
+          if tools.present?
+            payload[:tools] = tools
+            payload[:tool_config] = { function_calling_config: { mode: "AUTO" } }
+          end
           payload[:generationConfig].merge!(model_params) if model_params.present?
           payload
         end

spec/lib/completions/endpoints/gemini_spec.rb

@@ -145,6 +145,43 @@ RSpec.describe DiscourseAi::Completions::Endpoints::Gemini do
     EndpointsCompliance.new(self, endpoint, DiscourseAi::Completions::Dialects::Gemini, user)
   end
 
+  let(:echo_tool) do
+    {
+      name: "echo",
+      description: "echo something",
+      parameters: [{ name: "text", type: "string", description: "text to echo", required: true }],
+    }
+  end
+
+  # by default gemini is meant to use AUTO mode, however new experimental models
+  # appear to require this to be explicitly set
+  it "Explicitly specifies tool config" do
+    prompt = DiscourseAi::Completions::Prompt.new("Hello", tools: [echo_tool])
+
+    response = gemini_mock.response("World").to_json
+
+    req_body = nil
+
+    llm = DiscourseAi::Completions::Llm.proxy("custom:#{model.id}")
+    url = "#{model.url}:generateContent?key=123"
+
+    stub_request(:post, url).with(
+      body:
+        proc do |_req_body|
+          req_body = _req_body
+          true
+        end,
+    ).to_return(status: 200, body: response)
+
+    response = llm.generate(prompt, user: user)
+
+    expect(response).to eq("World")
+
+    parsed = JSON.parse(req_body, symbolize_names: true)
+
+    expect(parsed[:tool_config]).to eq({ function_calling_config: { mode: "AUTO" } })
+  end
+
   it "Supports Vision API" do
     prompt =
       DiscourseAi::Completions::Prompt.new(