fix(ai-formats): preserve OpenAI tool call item ids

crates/aether-ai-formats/src/formats/openai/responses/request.rs

@@ -1431,6 +1431,7 @@ mod tests {
 
         assert_eq!(body["input"].as_array().expect("input").len(), 2);
         assert_eq!(body["input"][0]["type"], "function_call");
+        assert_eq!(body["input"][0]["id"], "fc_call_auto_0");
         assert_eq!(body["input"][0]["call_id"], "call_auto_0");
         assert_eq!(body["input"][0]["name"], "unknown");
         assert_eq!(body["input"][0]["arguments"], "{\"q\":\"rust\"}");

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

@@ -2580,6 +2580,7 @@ mod tests {
             .value;
 
         assert_eq!(converted["input"][0]["type"], "function_call");
+        assert_eq!(converted["input"][0]["id"], "fc_call_lookup_1");
         assert_eq!(converted["input"][0]["call_id"], "call_lookup_1");
         assert_eq!(converted["input"][1]["type"], "function_call_output");
         assert_eq!(converted["input"][1]["call_id"], "call_lookup_1");

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

@@ -795,15 +795,27 @@ pub(crate) fn canonical_tool_use_to_openai_responses_item(
             "input": openai_custom_tool_input_text(input),
         });
     }
+    let item_id = openai_responses_function_call_item_id(id);
     json!({
         "type": "function_call",
-        "id": id,
+        "id": item_id,
         "call_id": id,
         "name": name,
         "arguments": canonicalize_tool_arguments(input),
     })
 }
 
+fn openai_responses_function_call_item_id(call_id: &str) -> String {
+    let trimmed = call_id.trim();
+    if trimmed.starts_with("fc") {
+        trimmed.to_string()
+    } else if trimmed.is_empty() {
+        "fc_auto".to_string()
+    } else {
+        format!("fc_{trimmed}")
+    }
+}
+
 fn openai_responses_hosted_tool_call_item_type(
     extensions: &BTreeMap<String, Value>,
 ) -> Option<&str> {