feat: steering messages with ACP

crates/goose-providers/src/conversation/message.rs

@@ -667,6 +667,11 @@ pub struct MessageMetadata {
     pub agent_visible: bool,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub inference: Option<InferenceMetadata>,
+    /// Whether this message is a steer injected into an active run. UI-only:
+    /// surfaced as `_meta.goose.steer` so clients can mark the steer boundary
+    /// without matching user-visible text. Never sent to providers.
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub steer: bool,
 }
 
 impl Default for MessageMetadata {
@@ -675,6 +680,7 @@ impl Default for MessageMetadata {
             user_visible: true,
             agent_visible: true,
             inference: None,
+            steer: false,
         }
     }
 }
@@ -743,6 +749,11 @@ impl MessageMetadata {
         self.inference = Some(inference);
         self
     }
+
+    pub fn with_steer(mut self) -> Self {
+        self.steer = true;
+        self
+    }
 }
 
 #[derive(ToSchema, Clone, PartialEq, Serialize, Deserialize, Debug)]
@@ -1028,6 +1039,11 @@ impl Message {
         self
     }
 
+    pub fn with_steer(mut self) -> Self {
+        self.metadata.steer = true;
+        self
+    }
+
     pub fn with_inference_if_assistant(self, inference: InferenceMetadata) -> Self {
         if self.role == Role::Assistant && self.metadata.inference.is_none() {
             self.with_inference(inference)

crates/goose-sdk-types/src/custom_requests.rs

@@ -1,4 +1,4 @@
-use agent_client_protocol::schema::McpServer;
+use agent_client_protocol::schema::{ContentBlock, McpServer};
 use agent_client_protocol::{JsonRpcRequest, JsonRpcResponse};
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
@@ -140,6 +140,30 @@ pub struct SetSessionSystemPromptRequest {
     pub text: String,
 }
 
+/// Add user input to the currently active prompt without starting a new prompt.
+#[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema, JsonRpcRequest)]
+#[request(
+    method = "_goose/unstable/session/steer",
+    response = SteerSessionResponse
+)]
+#[serde(rename_all = "camelCase")]
+pub struct SteerSessionRequest {
+    pub session_id: String,
+    #[serde(default)]
+    pub prompt: Vec<ContentBlock>,
+    pub expected_run_id: String,
+}
+
+#[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema, JsonRpcResponse)]
+#[serde(rename_all = "camelCase")]
+pub struct SteerSessionResponse {
+    pub run_id: String,
+    /// Stable id of the queued steer message. The same id later appears as
+    /// `messageId` on the streamed `UserMessageChunk` (with `_meta.goose.steer`),
+    /// letting clients correlate a queued steer with its pickup.
+    pub message_id: String,
+}
+
 /// Delete a session.
 #[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema, JsonRpcRequest)]
 #[request(method = "session/delete", response = EmptyResponse)]

crates/goose/acp-meta.json

@@ -35,6 +35,11 @@
       "requestType": "SetSessionSystemPromptRequest_unstable",
       "responseType": "EmptyResponse"
     },
+    {
+      "method": "_goose/unstable/session/steer",
+      "requestType": "SteerSessionRequest_unstable",
+      "responseType": "SteerSessionResponse_unstable"
+    },
     {
       "method": "session/delete",
       "requestType": "DeleteSessionRequest",