Validate message content on chat completions endpoint (#383)

* Add content validation

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Add no_detectors tests for chat completions

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Add empty content message test for chat completions

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Add comments about changing validation

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Update chat completions passthrough tests for empty message

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Add further validation for empty content

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Test case: passthrough with last message as an array

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* Prevent content array on last message when input detector is present

Signed-off-by: Mateus Devino <mdevino@ibm.com>

* nit: combine if clauses

Signed-off-by: Mateus Devino <mdevino@ibm.com>

---------

Signed-off-by: Mateus Devino <mdevino@ibm.com>

Author: mdevino

src/clients/openai.rs

@@ -259,6 +259,26 @@ impl ChatCompletionsRequest {
                 "`messages` must not be empty".into(),
             ));
         }
+
+        if !self.detectors.input.is_empty() {
+            // Content of type Array is not supported yet
+            // Adding this validation separately as we do plan to support arrays of string in the future
+            if let Some(Content::Array(_)) = self.messages.last().unwrap().content {
+                return Err(ValidationError::Invalid(
+                    "Detection on array is not supported".into(),
+                ));
+            }
+
+            // As text_content detections only run on last message at the moment, only the last
+            // message is being validated.
+            if self.messages.last().unwrap().is_text_content_empty() {
+                return Err(ValidationError::Invalid(
+                    "if input detectors are provided, `content` must not be empty on last message"
+                        .into(),
+                ));
+            }
+        }
+
         Ok(())
     }
 }
@@ -414,6 +434,32 @@ pub struct Message {
     pub tool_call_id: Option<String>,
 }
 
+impl Message {
+    /// Checks if text content of a message is empty.
+    ///
+    /// The following messages are considered empty:
+    /// 1. [`Message::content`] is None.
+    /// 2. [`Message::content`] is an empty string.
+    /// 3. [`Message::content`] is an empty array.
+    /// 4. [`Message::content`] is an array of empty strings and ContentType is Text.
+    pub fn is_text_content_empty(&self) -> bool {
+        match &self.content {
+            Some(content) => match content {
+                Content::Text(string) => string.is_empty(),
+                Content::Array(content_parts) => {
+                    content_parts.is_empty()
+                        || content_parts.iter().all(|content_part| {
+                            content_part.text.is_none()
+                                || (content_part.r#type == ContentType::Text
+                                    && content_part.text.as_ref().unwrap().is_empty())
+                        })
+                }
+            },
+            None => true,
+        }
+    }
+}
+
 /// Content.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(untagged)]

src/orchestrator/handlers/chat_completions_detection/unary.rs

@@ -108,6 +108,8 @@ async fn handle_input_detection(
     let model_id = task.request.model.clone();
 
     // Input detectors are only applied to the last message
+    // If this changes, the empty content validation in [`ChatCompletionsRequest::validate`]
+    // should also change.
     // Get the last message
     let messages = task.request.messages();
     let message = if let Some(message) = messages.last() {