diff --git a/crates/agent2/src/thread.rs b/crates/agent2/src/thread.rs
index 97ea1caf1d766be0314a16cc0f518ad701564569..8ff5b845066c8af90eb713aef2a0c87e6d114a85 100644
--- a/crates/agent2/src/thread.rs
+++ b/crates/agent2/src/thread.rs
@@ -484,11 +484,15 @@ impl AgentMessage {
         };
 
         for tool_result in self.tool_results.values() {
+            let mut tool_result = tool_result.clone();
+            // Surprisingly, the API fails if we return an empty string here.
+            // It thinks we are sending a tool use without a tool result.
+            if tool_result.content.is_empty() {
+                tool_result.content = "<Tool returned an empty string>".into();
+            }
             user_message
                 .content
-                .push(language_model::MessageContent::ToolResult(
-                    tool_result.clone(),
-                ));
+                .push(language_model::MessageContent::ToolResult(tool_result));
         }
 
         let mut messages = Vec::new();