fix: empty tool results can't cause errors in bedrock, claude, openai, or vertexai clients anymore

src/client/bedrock.rs

@@ -457,10 +457,15 @@ fn build_chat_completions_body(
                                 }
                             }));
                         }
-                        vec![
-                            json!({ "role": "assistant", "content": assistant_parts }),
-                            json!({ "role": "user", "content": user_parts }),
-                        ]
+                        // Empty tool_results (reachable via deserialized sessions) must not emit an empty pair.
+                        if assistant_parts.is_empty() {
+                            vec![]
+                        } else {
+                            vec![
+                                json!({ "role": "assistant", "content": assistant_parts }),
+                                json!({ "role": "user", "content": user_parts }),
+                            ]
+                        }
                     } else {
                         // One pair per round: Claude can reuse tool_use IDs across API calls.
                         // A round boundary is detected by the presence of round text, but
@@ -872,7 +877,7 @@ mod tests {
         }
     }
 
-    fn build_body(tool_results: Vec<ToolResult>) -> Value {
+    fn build_body_with(tool_results: Vec<ToolResult>, text: &str, sequence: bool) -> Value {
         let data = ChatCompletionsData {
             messages: vec![
                 Message::new(MessageRole::User, MessageContent::Text("hello".to_string())),
@@ -880,8 +885,8 @@ mod tests {
                     MessageRole::Assistant,
                     MessageContent::ToolCalls(MessageContentToolCalls {
                         tool_results,
-                        text: String::new(),
-                        sequence: true,
+                        text: text.to_string(),
+                        sequence,
                     }),
                 ),
             ],
@@ -894,6 +899,10 @@ mod tests {
         build_chat_completions_body(data, &Model::new("bedrock", "test"), false).unwrap()
     }
 
+    fn build_body(tool_results: Vec<ToolResult>) -> Value {
+        build_body_with(tool_results, "", true)
+    }
+
     fn tool_result_content_block(output: Value) -> Value {
         let body = build_body(vec![tool_result(output)]);
         body["messages"][2]["content"][0]["toolResult"]["content"][0].clone()
@@ -1063,6 +1072,44 @@ mod tests {
         assert_unique_tool_use_ids_per_message(&body);
     }
 
+    #[test]
+    fn non_sequence_emits_assistant_user_pair() {
+        let body = build_body_with(
+            vec![
+                sequence_tool_result("toolu_A", None),
+                sequence_tool_result("toolu_B", None),
+            ],
+            "",
+            false,
+        );
+
+        let messages = body["messages"].as_array().unwrap();
+
+        assert_eq!(messages.len(), 3, "body: {body}");
+        assert_eq!(messages[1]["role"], "assistant");
+        assert_eq!(messages[1]["content"].as_array().unwrap().len(), 2);
+        assert_eq!(messages[2]["role"], "user");
+        assert_eq!(messages[2]["content"].as_array().unwrap().len(), 2);
+    }
+
+    #[test]
+    fn non_sequence_empty_tool_results_emits_no_messages() {
+        let body = build_body_with(vec![], "leftover text", false);
+
+        let messages = body["messages"].as_array().unwrap();
+
+        assert_eq!(messages.len(), 1, "body: {body}");
+    }
+
+    #[test]
+    fn sequence_empty_tool_results_emits_no_messages() {
+        let body = build_body_with(vec![], "leftover text", true);
+
+        let messages = body["messages"].as_array().unwrap();
+
+        assert_eq!(messages.len(), 1, "body: {body}");
+    }
+
     #[test]
     fn test_bedrock_tool_result_object_uses_json_block() {
         assert_eq!(