Fix streaming + custom tool calls

crates/lingua/src/processing/transform.rs

@@ -22,10 +22,10 @@ use crate::providers::openai::model_needs_transforms;
 use crate::serde_json;
 use crate::serde_json::Value;
 use crate::universal::{
-    AssistantContent, AssistantContentPart, Message, TextContentPart, UniversalReasoningDelta,
-    UniversalRequest, UniversalResponse, UniversalStreamChoice, UniversalStreamChunk,
-    UniversalStreamDelta, UniversalToolCallDelta, UniversalToolFunctionDelta, UserContent,
-    UserContentPart,
+    AssistantContent, AssistantContentPart, Message, TextContentPart, ToolCallArguments,
+    UniversalReasoningDelta, UniversalRequest, UniversalResponse, UniversalStreamChoice,
+    UniversalStreamChunk, UniversalStreamDelta, UniversalToolCallDelta, UniversalToolFunctionDelta,
+    UserContent, UserContentPart,
 };
 use serde::de::DeserializeOwned;
 use thiserror::Error;
@@ -694,6 +694,8 @@ fn assistant_content_to_stream_delta(content: &AssistantContent) -> UniversalStr
                             index: Some(tool_call_index),
                             id: Some(tool_call_id.clone()),
                             call_type: Some("function".to_string()),
+                            custom_tool_call: matches!(arguments, ToolCallArguments::Custom(_))
+                                .then_some(true),
                             function: Some(UniversalToolFunctionDelta {
                                 name: Some(tool_name.clone()),
                                 arguments: Some(arguments.to_string()),
@@ -2071,6 +2073,42 @@ mod tests {
         );
     }
 
+    #[test]
+    #[cfg(feature = "openai")]
+    fn test_response_to_stream_chunk_preserves_responses_custom_tool_call() {
+        let response = UniversalResponse {
+            id: None,
+            id_format: None,
+            model: Some("gpt-5.6-terra".to_string()),
+            messages: vec![Message::Assistant {
+                id: None,
+                content: AssistantContent::Array(vec![AssistantContentPart::ToolCall {
+                    tool_call_id: "call_custom".to_string(),
+                    tool_name: "exec".to_string(),
+                    arguments: ToolCallArguments::Custom("await tools.exec();".to_string()),
+                    status: None,
+                    caller: None,
+                    encrypted_content: None,
+                    provider_options: None,
+                    provider_executed: None,
+                }]),
+            }],
+            usage: None,
+            finish_reason: None,
+        };
+
+        let chunk = response_to_stream_chunk(response);
+        let output = crate::providers::openai::responses_adapter::ResponsesAdapter
+            .stream_from_universal(&chunk)
+            .unwrap();
+
+        assert_eq!(output["type"], json!("response.output_item.added"));
+        assert_eq!(output["item"]["type"], json!("custom_tool_call"));
+        assert_eq!(output["item"]["input"], json!(""));
+        assert_eq!(output["item"]["call_id"], json!("call_custom"));
+        assert_eq!(output["item"]["name"], json!("exec"));
+    }
+
     #[test]
     #[cfg(feature = "openai")]
     fn test_transform_response_passthrough() {

crates/lingua/src/providers/anthropic/adapter.rs

@@ -1046,6 +1046,7 @@ impl ProviderAdapter for AnthropicAdapter {
                                 index: Some(0),
                                 id: part.id,
                                 call_type: Some("function".to_string()),
+                                custom_tool_call: None,
                                 function: Some(UniversalToolFunctionDelta {
                                     name: part.name,
                                     arguments: Some(arguments),
@@ -1109,6 +1110,7 @@ impl ProviderAdapter for AnthropicAdapter {
                                     index: Some(block_index),
                                     id: Some(id.to_string()),
                                     call_type: Some("function".to_string()),
+                                    custom_tool_call: None,
                                     function: Some(UniversalToolFunctionDelta {
                                         name: Some(name.to_string()),
                                         arguments: Some(String::new()),

crates/lingua/src/providers/bedrock/adapter.rs

@@ -424,6 +424,7 @@ impl ProviderAdapter for BedrockAdapter {
                                     index: Some(content_block_start.content_block_index),
                                     id: Some(tool_use.tool_use_id),
                                     call_type: Some("function".to_string()),
+                                    custom_tool_call: None,
                                     function: Some(UniversalToolFunctionDelta {
                                         name: Some(tool_use.name),
                                         arguments: Some(String::new()),

crates/lingua/src/providers/google/adapter.rs

@@ -642,6 +642,7 @@ impl ProviderAdapter for GoogleAdapter {
                                 })
                             }),
                             call_type: Some("function".to_string()),
+                            custom_tool_call: None,
                             function: Some(UniversalToolFunctionDelta {
                                 name: function_call.name.clone(),
                                 arguments: function_call

crates/lingua/src/providers/openai/adapter.rs

@@ -733,7 +733,10 @@ impl ProviderAdapter for OpenAIAdapter {
                 choice_map.insert("index".into(), serde_json::json!(c.index));
                 choice_map.insert(
                     "delta".into(),
-                    c.delta.clone().unwrap_or(Value::Object(Map::new())),
+                    c.delta
+                        .clone()
+                        .map(chat_stream_delta_from_universal)
+                        .unwrap_or(Value::Object(Map::new())),
                 );
                 let finish_reason_val = match &c.finish_reason {
                     Some(reason) => Value::String(reason.clone()),
@@ -771,6 +774,22 @@ impl ProviderAdapter for OpenAIAdapter {
     }
 }
 
+fn chat_stream_delta_from_universal(mut delta: Value) -> Value {
+    if let Some(tool_calls) = delta
+        .as_object_mut()
+        .and_then(|delta| delta.get_mut("tool_calls"))
+        .and_then(Value::as_array_mut)
+    {
+        for tool_call in tool_calls {
+            if let Some(tool_call) = tool_call.as_object_mut() {
+                tool_call.remove("custom_tool_call");
+            }
+        }
+    }
+
+    delta
+}
+
 // =============================================================================
 // Helper Functions
 // =============================================================================
@@ -918,6 +937,61 @@ mod tests {
         assert_eq!(value["prompt_cache_key"], json!("cache-key-1"));
     }
 
+    #[test]
+    fn test_openai_stream_from_universal_omits_custom_tool_call_marker() {
+        let adapter = OpenAIAdapter;
+        let chunk = UniversalStreamChunk::new(
+            None,
+            None,
+            vec![UniversalStreamChoice {
+                index: 0,
+                delta: Some(json!({
+                    "role": "assistant",
+                    "content": null,
+                    "tool_calls": [{
+                        "index": 0,
+                        "id": "call_1",
+                        "type": "function",
+                        "custom_tool_call": true,
+                        "function": {
+                            "name": "exec",
+                            "arguments": ""
+                        }
+                    }]
+                })),
+                finish_reason: None,
+            }],
+            None,
+            None,
+        );
+
+        let value = adapter.stream_from_universal(&chunk).unwrap();
+
+        assert_eq!(
+            value,
+            json!({
+                "object": "chat.completion.chunk",
+                "choices": [{
+                    "index": 0,
+                    "delta": {
+                        "role": "assistant",
+                        "content": null,
+                        "tool_calls": [{
+                            "index": 0,
+                            "id": "call_1",
+                            "type": "function",
+                            "function": {
+                                "name": "exec",
+                                "arguments": ""
+                            }
+                        }]
+                    },
+                    "finish_reason": null
+                }]
+            })
+        );
+    }
+
     #[test]
     fn test_openai_prompt_cache_key_canonical_value_overrides_stale_extra() {
         let adapter = OpenAIAdapter;