feat: add fallback to Sonnet 3.5 before trying Gemini Flash 2.0

This commit adds an intermediate fallback step to Claude Sonnet 3.5 when Sonnet 3.7
fails, before attempting to use Gemini Flash 2.0. It also updates the default LLM
to Sonnet 3.7.

Author: agentfarmx[bot]

sidecar/src/agentic/tool/session/tool_use_agent.rs

@@ -1421,8 +1421,36 @@ You accomplish a given task iteratively, breaking it down into clear steps and w
             }
         }
 
-        // If original LLM (Sonnet) failed, try with gemini-flash-2.0
-        if llm_properties.llm() == &LLMType::ClaudeSonnet {
+        // If original LLM (Sonnet3.7) failed, try with sonnet3.5 first
+        if llm_properties.llm() == &LLMType::ClaudeSonnet37 {
+            println!("sonnet37_failed::failing_back_to_sonnet35");
+            let sonnet35_properties = llm_properties.clone().set_llm(LLMType::ClaudeSonnet);
+            if let Ok(Some(result)) = self
+                .try_with_llm(
+                    sonnet35_properties,
+                    cancellation_token.clone(),
+                    root_request_id.clone(),
+                    ui_sender.clone(),
+                    exchange_id,
+                    final_messages.to_vec(),
+                    None,
+                )
+                .await
+            {
+                if matches!(
+                    result,
+                    ToolUseAgentOutput {
+                        r#type: ToolUseAgentOutputType::Success(_),
+                        usage_statistics: _,
+                    }
+                ) {
+                    return Ok(result);
+                }
+            }
+        }
+
+        // If sonnet3.5 failed or if using a different model, try with gemini-flash-2.0
+        if llm_properties.llm() == &LLMType::ClaudeSonnet37 || llm_properties.llm() == &LLMType::ClaudeSonnet {
             println!("sonnet_failed::failing_back_to_gemini-2.0-flash");
             let gemini_pro_properties = llm_properties.clone().set_llm(LLMType::Gemini2_0Flash);
             if let Ok(Some(result)) = self
@@ -2870,4 +2898,4 @@ To get the current time in Timbuktu, Mali, I'll need to use the mcp::time::get_c
         let tool_use_possible = tool_use_generator.tool_input_partial;
         assert!(tool_use_possible.is_some());
     }
-}
+}

sidecar/src/bin/agent_bin.rs

@@ -141,7 +141,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let llm_model = if let Some(model_name) = args.model_name {
         LLMType::Custom(model_name)
     } else {
-        LLMType::ClaudeSonnet
+        LLMType::ClaudeSonnet37
     };
 
     let llm_provider = LLMProperties::new(