diff --git a/server/src/provider/openai_chat.rs b/server/src/provider/openai_chat.rs
index acf9614..996fa8e 100644
--- a/server/src/provider/openai_chat.rs
+++ b/server/src/provider/openai_chat.rs
@@ -95,12 +95,14 @@ impl Provider for OpenAiChatProvider {
             let mut body = json!({
                 "model": request.model.model_id,
                 "messages": messages,
-                "tools": request.prompt.tools.iter().map(|tool| json!({"type":"function","function":{
-                    "name": tool.name, "description": tool.description, "parameters": tool.parameters
-                }})).collect::<Vec<_>>(),
                 "stream": true,
                 "stream_options": {"include_usage": true}
             });
+            if !request.prompt.tools.is_empty() {
+                body["tools"] = json!(request.prompt.tools.iter().map(|tool| json!({"type":"function","function":{
+                    "name": tool.name, "description": tool.description, "parameters": tool.parameters
+                }})).collect::<Vec<_>>());
+            }
             apply_model(&mut body, &request.model, config.max_output_tokens)?;
             merge_extra_params(&mut body, &request.model.extra_params)?;
             apply_openai_prompt_cache_key(&mut body, &request.model.model_id)?;
@@ -164,42 +166,27 @@ impl Provider for OpenAiChatProvider {
             let mut loop_iteration: u64 = 0;
             loop {
                 loop_iteration += 1;
-                dbg_log!("openai_chat.rs:stream:poll", "Polling SSE event", serde_json::json!({
-                    "iteration": loop_iteration,
-                    "saw_done": saw_done_marker,
-                    "has_finish": finish.is_some(),
-                    "tool_count": tools.len(),
-                    "text_open": text_open,
-                    "thinking_open": thinking_open,
-                    "reasoning_len": reasoning.len()
-                }));
                 let event = tokio::select! {
                     _ = cancellation.cancelled() => {
                         dbg_log!("openai_chat.rs:stream", "Stream cancelled by token", serde_json::json!({
                             "iteration": loop_iteration,
                             "saw_done_marker": saw_done_marker,
-                            "tool_count": tools.len(),
-                            "text_open": text_open
+                            "tool_count": tools.len()
                         }));
                         return;
                     }
                     event = source.next() => event,
                 };
                 let Some(event) = event else {
-                    dbg_log!("openai_chat.rs:stream:poll", "SSE stream ended (None from source)", serde_json::json!({
+                    dbg_log!("openai_chat.rs:stream", "SSE stream ended (None)", serde_json::json!({
                         "iteration": loop_iteration,
-                        "saw_done_marker": saw_done_marker,
-                        "has_finish": finish.is_some(),
-                        "tool_count": tools.len(),
-                        "text_open": text_open,
-                        "thinking_open": thinking_open,
-                        "reasoning_len": reasoning.len()
+                        "saw_done_marker": saw_done_marker
                     }));
                     break;
                 };
                 let event = event.map_err(|error| {
                     let err_msg = error.to_string();
-                    dbg_log!("openai_chat.rs:stream:poll", "SSE event error", serde_json::json!({
+                    dbg_log!("openai_chat.rs:stream", "SSE event error", serde_json::json!({
                         "iteration": loop_iteration,
                         "error": err_msg.clone()
                     }));
@@ -212,7 +199,7 @@ impl Provider for OpenAiChatProvider {
                 }
                 let Some(choice) = value.get("choices").and_then(Value::as_array).and_then(|values| values.first()) else { continue; };
                 let delta = choice.get("delta").unwrap_or(&Value::Null);
-                if let Some(reasoning_delta) = delta.get("reasoning_content").and_then(Value::as_str).filter(|text| !text.is_empty()) {
+                if let Some(reasoning_delta) = delta.get("reasoning_content").or_else(|| delta.get("reasoning")).and_then(Value::as_str).filter(|text| !text.is_empty()) {
                     if !thinking_open { thinking_open = true; yield ModelEvent::ThinkingStart; }
                     reasoning.push_str(reasoning_delta);
                     yield ModelEvent::ThinkingDelta(reasoning_delta.into());
@@ -236,15 +223,6 @@ impl Provider for OpenAiChatProvider {
                     finish = Some(map_finish(reason, !tools.is_empty()));
                 }
             }
-            dbg_log!("openai_chat.rs:stream", "SSE loop exited", serde_json::json!({
-                "total_iterations": loop_iteration,
-                "saw_done_marker": saw_done_marker,
-                "finish_reason": finish.as_ref().map(|f| format!("{:?}", f)),
-                "reasoning_len": reasoning.len(),
-                "tool_count": tools.len(),
-                "text_open": text_open,
-                "thinking_open": thinking_open
-            }));
             if thinking_open { yield ModelEvent::ThinkingEnd; }
             if text_open { yield ModelEvent::TextEnd; }
             for (index, tool) in &mut tools {
@@ -319,12 +297,27 @@ fn openai_chat_messages(instructions: &str, messages: &[ProjectedMessage]) -> Re
                 calls,
                 ..
             } => {
-                value.insert("content".into(), Value::String(text.clone()));
                 let replay_reasoning = replay_state
                     .as_ref()
                     .filter(|state| state.provider_kind == "openai_chat")
                     .and_then(|state| state.value.get("reasoning_content"))
-                    .and_then(Value::as_str);
+                    .and_then(Value::as_str)
+                    .filter(|reasoning| !reasoning.is_empty());
+
+                // Chat Completions rejects an empty assistant content string. Tool-call
+                // assistant messages use null content, while an assistant with no visible
+                // content at all does not need to be sent.
+                if text.is_empty() && calls.is_empty() && replay_reasoning.is_none() {
+                    continue;
+                }
+                value.insert(
+                    "content".into(),
+                    if text.is_empty() {
+                        Value::Null
+                    } else {
+                        Value::String(text.clone())
+                    },
+                );
                 if let Some(reasoning) = replay_reasoning {
                     value.insert("reasoning_content".into(), Value::String(reasoning.into()));
                 }
@@ -487,7 +480,7 @@ mod tests {
         model::{ContentPart, ProjectedContent, ProjectedMessage, ToolResultContent},
         model::{ProviderReplayState, Role, ToolCallContent},
     };
-    use serde_json::json;
+    use serde_json::{json, Value};
 
     #[test]
     fn chat_replay_state_is_encoded_as_reasoning_content() {
@@ -519,6 +512,52 @@ mod tests {
         assert_eq!(messages[0]["tool_calls"][0]["id"], "call-1");
     }
 
+    #[test]
+    fn chat_tool_call_assistant_uses_null_content() {
+        let messages = openai_chat_messages(
+            "",
+            &[ProjectedMessage {
+                message_id: "test".into(),
+                role: Role::Assistant,
+                content: ProjectedContent::Assistant {
+                    text: String::new(),
+                    thinking: String::new(),
+                    replay_state: None,
+                    calls: vec![ToolCallContent {
+                        index: 0,
+                        call_id: "call-1".into(),
+                        name: "Read".into(),
+                        arguments: json!({"path": "README.md"}),
+                    }],
+                },
+            }],
+        )
+        .unwrap();
+
+        assert_eq!(messages[0]["content"], Value::Null);
+        assert!(messages[0]["tool_calls"].is_array());
+    }
+
+    #[test]
+    fn chat_contentless_assistant_is_omitted() {
+        let messages = openai_chat_messages(
+            "",
+            &[ProjectedMessage {
+                message_id: "test".into(),
+                role: Role::Assistant,
+                content: ProjectedContent::Assistant {
+                    text: String::new(),
+                    thinking: String::new(),
+                    replay_state: None,
+                    calls: vec![],
+                },
+            }],
+        )
+        .unwrap();
+
+        assert!(messages.is_empty());
+    }
+
     #[test]
     fn another_provider_replay_does_not_invent_chat_reasoning_content() {
         let messages = openai_chat_messages(