diff --git a/test/twin/openai/src/openai/models.rs b/test/twin/openai/src/openai/models.rs
index abd2e47e7..c6b33bc31 100644
--- a/test/twin/openai/src/openai/models.rs
+++ b/test/twin/openai/src/openai/models.rs
@@ -625,15 +625,34 @@ impl ChatCompletionsRequest {
 #[derive(Clone, Debug, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct ChatMessage {
-    pub role:    String,
-    pub content: Value,
+    pub role:              String,
+    pub content:           Option<Value>,
+    pub reasoning_content: Option<String>,
+    pub tool_call_id:      Option<String>,
+    pub tool_calls:        Option<Vec<ChatMessageToolCall>>,
+}
+
+#[derive(Clone, Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ChatMessageToolCall {
+    pub id:       String,
+    #[serde(rename = "type")]
+    pub kind:     String,
+    pub function: ChatMessageToolCallFunction,
+}
+
+#[derive(Clone, Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ChatMessageToolCallFunction {
+    pub name:      String,
+    pub arguments: String,
 }
 
 impl ChatMessage {
     fn extract_texts(&self) -> Vec<String> {
-        match &self.content {
-            Value::String(text) => vec![normalize_whitespace(text)],
-            Value::Array(parts) => parts
+        match self.content.as_ref() {
+            Some(Value::String(text)) => vec![normalize_whitespace(text)],
+            Some(Value::Array(parts)) => parts
                 .iter()
                 .filter_map(|part| {
                     part.get("text")
@@ -647,13 +666,21 @@ impl ChatMessage {
 
     fn contains_reasoning_content(&self) -> bool {
         self.role == "assistant"
-            && self.content.as_array().is_some_and(|parts| {
-                parts.iter().any(|part| {
-                    part.get("type")
-                        .and_then(Value::as_str)
-                        .is_some_and(|kind| kind == "reasoning")
-                })
-            })
+            && (self
+                .reasoning_content
+                .as_deref()
+                .is_some_and(|reasoning| !reasoning.trim().is_empty())
+                || self
+                    .content
+                    .as_ref()
+                    .and_then(Value::as_array)
+                    .is_some_and(|parts| {
+                        parts.iter().any(|part| {
+                            part.get("type")
+                                .and_then(Value::as_str)
+                                .is_some_and(|kind| kind == "reasoning")
+                        })
+                    }))
     }
 }
 
@@ -680,14 +707,24 @@ fn validate_chat_message(message: &ChatMessage) -> Result<(), OpenAiError> {
         ));
     }
 
-    match &message.content {
-        Value::String(_) => Ok(()),
-        Value::Array(parts) if !parts.is_empty() => {
+    validate_chat_tool_fields(message)?;
+
+    match message.content.as_ref() {
+        Some(Value::String(_)) => Ok(()),
+        Some(Value::Array(parts)) if !parts.is_empty() => {
             for part in parts {
                 validate_chat_message_part(part, &message.role)?;
             }
             Ok(())
         }
+        None if message.role == "assistant"
+            && message
+                .tool_calls
+                .as_ref()
+                .is_some_and(|tool_calls| !tool_calls.is_empty()) =>
+        {
+            Ok(())
+        }
         _ => Err(OpenAiError::invalid_request(
             "messages",
             "unsupported message content shape",
@@ -695,6 +732,56 @@ fn validate_chat_message(message: &ChatMessage) -> Result<(), OpenAiError> {
     }
 }
 
+fn validate_chat_tool_fields(message: &ChatMessage) -> Result<(), OpenAiError> {
+    if message.role == "tool" {
+        if message
+            .tool_call_id
+            .as_deref()
+            .is_none_or(|tool_call_id| tool_call_id.trim().is_empty())
+        {
+            return Err(OpenAiError::invalid_request(
+                "messages",
+                "tool messages require a tool_call_id",
+            ));
+        }
+    } else if message.tool_call_id.is_some() {
+        return Err(OpenAiError::invalid_request(
+            "messages",
+            "tool_call_id is only supported on tool messages",
+        ));
+    }
+
+    let Some(tool_calls) = &message.tool_calls else {
+        return Ok(());
+    };
+    if message.role != "assistant" {
+        return Err(OpenAiError::invalid_request(
+            "messages",
+            "tool_calls are only supported on assistant messages",
+        ));
+    }
+    if tool_calls.is_empty() {
+        return Err(OpenAiError::invalid_request(
+            "messages",
+            "tool_calls must not be empty",
+        ));
+    }
+    for tool_call in tool_calls {
+        if tool_call.id.trim().is_empty()
+            || tool_call.kind != "function"
+            || tool_call.function.name.trim().is_empty()
+            || tool_call.function.arguments.trim().is_empty()
+        {
+            return Err(OpenAiError::invalid_request(
+                "messages",
+                "invalid assistant tool call",
+            ));
+        }
+    }
+
+    Ok(())
+}
+
 fn validate_chat_message_part(part: &Value, role: &str) -> Result<(), OpenAiError> {
     let Some(object) = part.as_object() else {
         return Err(OpenAiError::invalid_request(
diff --git a/test/twin/openai/tests/chat_completions_contract.rs b/test/twin/openai/tests/chat_completions_contract.rs
index db1ace170..6fe4c27fe 100644
--- a/test/twin/openai/tests/chat_completions_contract.rs
+++ b/test/twin/openai/tests/chat_completions_contract.rs
@@ -90,6 +90,44 @@ async fn chat_completions_accepts_supported_openai_compatible_fields() {
     );
 }
 
+#[tokio::test]
+async fn chat_completions_accepts_tool_call_history() {
+    let server = common::spawn_server().await.expect("server should start");
+
+    let response = server
+        .post_chat(json!({
+            "model": "gpt-test",
+            "messages": [
+                { "role": "user", "content": "replace old with new" },
+                {
+                    "role": "assistant",
+                    "tool_calls": [{
+                        "id": "call_edit",
+                        "type": "function",
+                        "function": {
+                            "name": "edit_file",
+                            "arguments": "{\"old\":\"old\",\"new\":\"new\"}"
+                        }
+                    }]
+                },
+                {
+                    "role": "tool",
+                    "content": "Updated data.txt",
+                    "tool_call_id": "call_edit"
+                }
+            ],
+            "stream": false
+        }))
+        .await;
+
+    assert_eq!(response.status(), 200);
+    let body = response.json::<serde_json::Value>().await.expect("json");
+    assert_eq!(
+        body["choices"][0]["message"]["content"],
+        "deterministic: replace old with new"
+    );
+}
+
 #[tokio::test]
 async fn chat_completions_supports_scripted_tool_call_and_json_schema() {
     let server = common::spawn_server().await.expect("server should start");