Expose typed message sources in Rust send APIs

Add opt-in user/system source builders for MessageOptions and SendRequest while preserving default payloads and existing send semantics.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: jmoseley

rust/README.md

@@ -818,6 +818,40 @@ let client = Client::start(opts).await?;
 
 The SDK injects the appropriate environment variables (`COPILOT_OTEL_EXPORTER_TYPE`, `OTEL_EXPORTER_OTLP_ENDPOINT`, `OTEL_EXPORTER_OTLP_PROTOCOL`, ...) into the spawned CLI process. The SDK takes no OpenTelemetry dependency; the CLI itself owns the exporter pipeline. Caller-supplied `ClientOptions::env` entries override telemetry-injected values.
 
+### Message Source
+
+Use `MessageSource::System` for automated messages sent by your application. Ordinary human sends leave `source` unset, so the field is omitted from the request. Use `MessageSource::User` when you need to set it explicitly.
+
+```rust,no_run
+use github_copilot_sdk::{MessageOptions, MessageSource, session::Session};
+
+# async fn example(session: &Session) -> Result<(), github_copilot_sdk::Error> {
+session
+    .send(MessageOptions::new("Context updated").with_source(MessageSource::System))
+    .await?;
+# Ok(())
+# }
+```
+
+The raw RPC path supports the same builder, including requests with JSON attachments:
+
+```rust,no_run
+use github_copilot_sdk::{MessageSource, rpc::SendRequest, session::Session};
+
+# async fn example(session: &Session) -> Result<(), github_copilot_sdk::Error> {
+let mut request = SendRequest::default().with_source(MessageSource::System);
+request.prompt = "Context updated".into();
+request.attachments = Some(vec![serde_json::json!({
+    "type": "github_url",
+    "url": "https://github.com/github/copilot-sdk"
+})]);
+session.rpc().send(request).await?;
+# Ok(())
+# }
+```
+
+Both paths use ordinary `session.send`. Source does not select a delivery mode or set billing flags; the runtime applies its existing source behavior. `send_and_wait` still completes on `session.idle` and may return `Ok(None)` when no assistant message was emitted. Genuine errors still propagate.
+
 ### Progress Reporting (`send_and_wait`)
 
 For fire-and-forget messaging where you need to block until the agent finishes:

rust/src/rpc.rs

@@ -10,3 +10,19 @@
 
 pub use crate::generated::api_types::*;
 pub use crate::generated::rpc::*;
+
+impl SendRequest {
+    /// Set the message provenance without changing other request options.
+    ///
+    /// When this is not called, the source field is omitted by default.
+    pub fn with_source(mut self, source: crate::MessageSource) -> Self {
+        self.source = Some(
+            match source {
+                crate::MessageSource::User => "user",
+                crate::MessageSource::System => "system",
+            }
+            .to_string(),
+        );
+        self
+    }
+}

rust/src/session.rs

@@ -522,6 +522,9 @@ impl Session {
             "sessionId": self.id,
             "prompt": opts.prompt,
         });
+        if let Some(source) = opts.source {
+            params["source"] = serde_json::to_value(source)?;
+        }
         if let Some(m) = opts.mode {
             params["mode"] = serde_json::to_value(m)?;
         }

rust/src/types.rs

@@ -5315,6 +5315,20 @@ pub fn ensure_attachment_display_names(attachments: &mut [Attachment]) {
     }
 }
 
+/// Provenance of a message sent through `session.send`.
+///
+/// Source is independent of delivery mode. Leaving [`MessageOptions::source`]
+/// unset omits the field and preserves the runtime's default for user messages.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+#[non_exhaustive]
+pub enum MessageSource {
+    /// A message from a human user.
+    User,
+    /// An automated message from the integrating application.
+    System,
+}
+
 /// Message delivery mode for [`MessageOptions::mode`].
 ///
 /// Controls how a prompt is delivered relative to in-flight session work.
@@ -5380,6 +5394,9 @@ pub enum AgentMode {
 pub struct MessageOptions {
     /// The user prompt to send.
     pub prompt: String,
+    /// Optional message provenance. When `None`, the field is omitted,
+    /// preserving the runtime's default for user messages.
+    pub source: Option<MessageSource>,
     /// Optional message delivery mode for this turn.
     ///
     /// Controls whether the prompt is queued behind in-flight work
@@ -5419,6 +5436,7 @@ impl MessageOptions {
     pub fn new(prompt: impl Into<String>) -> Self {
         Self {
             prompt: prompt.into(),
+            source: None,
             mode: None,
             agent_mode: None,
             attachments: None,
@@ -5430,6 +5448,12 @@ impl MessageOptions {
         }
     }
 
+    /// Set the message provenance without changing its delivery mode.
+    pub fn with_source(mut self, source: MessageSource) -> Self {
+        self.source = Some(source);
+        self
+    }
+
     /// Set the message delivery mode for this turn.
     ///
     /// Pass [`DeliveryMode::Immediate`] to interrupt the session and run