fix: preserve provider schema drift safely

crates/aether-ai-formats/src/formats/registry.rs

@@ -2785,6 +2785,59 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn same_format_canonical_roundtrip_preserves_future_root_fields() {
+        let cases = [
+            (
+                "openai:chat",
+                json!({
+                    "model": "gpt-source",
+                    "messages": [{"role": "user", "content": "hello"}],
+                    "future_field": {"enabled": true}
+                }),
+                "future_field",
+            ),
+            (
+                "openai:responses",
+                json!({
+                    "model": "gpt-source",
+                    "input": [{"role": "user", "content": "hello"}],
+                    "future_field": {"enabled": true}
+                }),
+                "future_field",
+            ),
+            (
+                "claude:messages",
+                json!({
+                    "model": "claude-source",
+                    "max_tokens": 1024,
+                    "messages": [{"role": "user", "content": "hello"}],
+                    "future_field": {"enabled": true}
+                }),
+                "future_field",
+            ),
+            (
+                "gemini:generate_content",
+                json!({
+                    "model": "gemini-source",
+                    "contents": [{"role": "user", "parts": [{"text": "hello"}]}],
+                    "futureField": {"enabled": true}
+                }),
+                "futureField",
+            ),
+        ];
+
+        for (format, body, field) in cases {
+            let converted = convert_request_pure(format, format, &body)
+                .unwrap_or_else(|err| panic!("{format} same-format roundtrip failed: {err}"));
+            assert_eq!(
+                converted.value.get(field),
+                body.get(field),
+                "{format} must preserve unknown provider root fields in canonical roundtrip"
+            );
+        }
+    }
+
     #[test]
     fn pure_openai_chat_to_claude_blocks_target_unsupported_generation_field() {
         let body = json!({

crates/aether-ai-formats/src/protocol/canonical.rs

@@ -4582,9 +4582,29 @@ pub(crate) fn apply_gemini_request_extensions(
     if let Some(raw_tool_config) = gemini.get("raw_tool_config").cloned() {
         output_object.insert("toolConfig".to_string(), raw_tool_config);
     }
+    for (key, value) in gemini {
+        if GEMINI_REQUEST_EXTENSION_INTERNAL_KEYS.contains(&key.as_str()) {
+            continue;
+        }
+        output_object
+            .entry(key.clone())
+            .or_insert_with(|| value.clone());
+    }
     Some(())
 }
 
+const GEMINI_REQUEST_EXTENSION_INTERNAL_KEYS: &[&str] = &[
+    "builtin_tools",
+    "cached_content",
+    "generation_config_extra",
+    "grounding",
+    "raw_tool_config",
+    "raw_tools",
+    "response_modalities",
+    "safety_settings",
+    "thinking_config",
+];
+
 fn should_reuse_raw_gemini_tools(gemini: &Map<String, Value>) -> bool {
     let Some(google_search) = gemini
         .get("grounding")