fix: prevent MALFORMED_FUNCTION_CALL infinite retry loop

Root cause: after stripping LS tool definitions, two things remained:
1. toolConfig with mode=VALIDATED (forces function calling even with
   empty tools array)
2. Model's training/identity context causing it to attempt function
   calls in text

Fix:
- Remove empty tools array and toolConfig when no custom tools injected
- Strip functionCall/functionResponse parts from conversation history
- Append explicit 'no tools available' instruction to system prompt
- Remove debug dump code

src/mitm/modify.rs

@@ -48,7 +48,16 @@ pub fn modify_request(body: &[u8], tool_ctx: Option<&ToolContext>) -> Option<Vec
         let identity = extract_xml_section(&sys, "identity");
 
         if let Some(identity_text) = identity {
-            let new_sys = format!("<identity>\n{}\n</identity>", identity_text.trim());
+            let mut new_sys = format!("<identity>\n{}\n</identity>", identity_text.trim());
+
+            // When no tools are available, explicitly tell the model not to attempt
+            // function calls. Without this, the model's training causes it to try
+            // calling tools from its identity context, resulting in MALFORMED_FUNCTION_CALL.
+            let has_tools = tool_ctx.as_ref().map_or(false, |ctx| ctx.tools.is_some());
+            if !has_tools {
+                new_sys.push_str("\n\nIMPORTANT: You have NO tools available. Do not attempt to call any functions or tools. Respond with text only.");
+            }
+
             let stripped = original_len - new_sys.len();
             if stripped > 0 {
                 changes.push(format!(
@@ -181,11 +190,24 @@ pub fn modify_request(body: &[u8], tool_ctx: Option<&ToolContext>) -> Option<Vec
         }
     }
 
-    // ── 3a. Strip functionCall/functionResponse from history when no tools ──
+    // ── 3a. When no tools remain, clean up all tool-related config ────────
-    // When LS tools are stripped and no custom tools are injected, the model
+    // The LS sets toolConfig.functionCallingConfig.mode = "VALIDATED" which
-    // may try to reference LS tools from conversation history, causing Google
+    // forces Google to attempt function calls even with an empty tools array,
-    // to return MALFORMED_FUNCTION_CALL in an infinite retry loop.
+    // causing MALFORMED_FUNCTION_CALL in an infinite retry loop.
     if STRIP_ALL_TOOLS && !has_custom_tools {
+        if let Some(req) = json.get_mut("request").and_then(|v| v.as_object_mut()) {
+            // Remove the empty tools array entirely
+            if req.get("tools").and_then(|v| v.as_array()).map_or(false, |a| a.is_empty()) {
+                req.remove("tools");
+                changes.push("remove empty tools array".to_string());
+            }
+            // Remove toolConfig (VALIDATED mode with no tools = MALFORMED_FUNCTION_CALL)
+            if req.remove("toolConfig").is_some() {
+                changes.push("remove toolConfig (no tools)".to_string());
+            }
+        }
+
+        // Also strip functionCall/functionResponse from conversation history
         if let Some(contents) = json
             .pointer_mut("/request/contents")
             .and_then(|v| v.as_array_mut())