Switch default Gemini model to 3.1 Flash-Lite preview (#6625)

3.1 Flash-Lite is faster (throughput + TTFT), meaningfully smarter, and
still among the cheapest Gemini options, making it a strict upgrade
over 2.5 Flash-Lite for narrative text generation.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docs/LLM_MODEL_COMPARISON.md

@@ -42,16 +42,17 @@ Time-to-first-token (TTFT) was measured from request initiation to the first tex
 
 ### For Narrative Text Generation (Default)
 
-**Gemini 2.5 Flash-Lite** is recommended as the default:
-- Fastest TTFT (~0.6s) - nearly 3x faster than alternatives
-- Cheapest pricing ($0.10/$0.40 per 1M tokens)
-- Quality is acceptable for short narrative snippets
+**Gemini 3.1 Flash-Lite** is recommended as the default:
+- Priced at $0.25/$1.50 per 1M input/output tokens
+- Significantly faster than 2.5 Flash-Lite on throughput and TTFT
+- Meaningfully smarter than 2.5 Flash-Lite, while remaining one of the cheapest options
+- Quality is more than sufficient for short narrative snippets
 
 ### Alternative Options
 
 | Priority | Model | When to Use |
 |----------|-------|-------------|
-| Speed + Cost | Gemini 2.5 Flash-Lite | Default for most use cases |
+| Speed + Cost | Gemini 3.1 Flash-Lite | Default for most use cases |
 | Speed + Quality | gpt-4.1-mini | When you need OpenAI quality with good speed |
 | Instruction Following | claude-3-5-haiku | Complex multi-step prompts, consistent tone |
 | Maximum Quality | claude-sonnet-4 or gpt-5.2 | When output quality is paramount |
@@ -70,7 +71,7 @@ LLM settings can be changed at runtime via the admin console:
 1. Navigate to Admin Console → Settings
 2. Change `LlmProvider` to select vendor (gemini, openai, claude)
 3. Change the corresponding model name setting:
-   - `GeminiModelName` (default: gemini-2.5-flash-lite)
+   - `GeminiModelName` (default: gemini-3.1-flash-lite-preview)
    - `OpenAiModelName` (default: gpt-4.1-mini)
    - `ClaudeModelName` (default: claude-3-5-haiku-20241022)
 

src/main/go/net/eagle0/admin_server/admin_server.go

@@ -1566,7 +1566,7 @@ type SettingInfo struct {
 var llmProviderOptions = []string{"gemini", "openai", "claude"}
 var openAiModelOptions = []string{"gpt-4.1-mini", "gpt-5-mini", "gpt-5.1", "gpt-5.2", "gpt-4.1", "gpt-4o", "gpt-4o-mini", "o1", "o1-mini", "o3-mini"}
 var claudeModelOptions = []string{"claude-3-5-haiku-20241022", "claude-sonnet-4-20250514", "claude-opus-4-20250514", "claude-3-7-sonnet-20250219", "claude-3-5-sonnet-20241022"}
-var geminiModelOptions = []string{"gemini-2.5-flash-lite", "gemini-2.5-flash", "gemini-3-flash-preview", "gemini-2.5-pro", "gemini-3-pro-preview"}
+var geminiModelOptions = []string{"gemini-3.1-flash-lite-preview", "gemini-2.5-flash-lite", "gemini-2.5-flash", "gemini-3-flash-preview", "gemini-2.5-pro", "gemini-3-pro-preview"}
 
 type SettingsPageData struct {
 	Title     string

src/main/scala/net/eagle0/common/llm_integration/GeminiServiceImpl.scala

@@ -12,7 +12,7 @@ import org.json4s.jvalue2monadic
 import org.json4s.native.{Json, Serialization}
 
 object GeminiServiceImpl {
-  val defaultModel: String = "gemini-2.5-flash-lite"
+  val defaultModel: String = "gemini-3.1-flash-lite-preview"
 
   private val apiKey  = ApiKeys.gemini
   private val baseURL = "https://generativelanguage.googleapis.com/v1beta/models"

src/main/scala/net/eagle0/eagle/library/settings/BUILD.bazel

@@ -3607,7 +3607,7 @@ scala_setting_library(
     name = "gemini_model_name",
     setting_name = "GeminiModelName",
     setting_type = "String",
-    setting_value = "gemini-2.5-flash-lite",
+    setting_value = "gemini-3.1-flash-lite-preview",
     visibility = [
         "//src/main/scala/net/eagle0/eagle:__subpackages__",
         "//src/main/scala/net/eagle0/util:__subpackages__",

src/main/scala/net/eagle0/eagle/service/GameAdminServiceImpl.scala

@@ -370,7 +370,7 @@ class GameAdminServiceImpl(
         case "gemini" =>
           settingsMap.getOrElse(
             "GeminiModelName",
-            currentSettings.getOrElse("GeminiModelName", "gemini-2.5-flash-lite")
+            currentSettings.getOrElse("GeminiModelName", "gemini-3.1-flash-lite-preview")
           )
         case _        => ""
       }