diff --git a/src/providers/gemini.rs b/src/providers/gemini.rs
index 611b3c75..5db27d8c 100644
--- a/src/providers/gemini.rs
+++ b/src/providers/gemini.rs
@@ -309,8 +309,10 @@ impl GeminiProvider {
                         let args = serde_json::from_str::<Value>(&tc.function.arguments)
                             .unwrap_or_else(|_| serde_json::json!({}));
                         
-                        // RESTORE: Use tc.id as thought_signature if it was originally one
-                        let thought_signature = if tc.id.starts_with("sig_") || !tc.id.contains('-') {
+                        // RESTORE: Use tc.id as thought_signature if it was originally one.
+                        // We skip our own generated IDs (which start with 'call_') 
+                        // because they are not valid base64-encoded Gemini signatures.
+                        let thought_signature = if !tc.id.starts_with("call_") {
                             Some(tc.id.clone())
                         } else {
                             None