Add ACP session system prompt setter (#9478)

Signed-off-by: Bradley Axen <baxen@squareup.com>

crates/goose-sdk/src/custom_requests.rs

@@ -108,6 +108,37 @@ pub struct UpdateWorkingDirRequest {
     pub working_dir: String,
 }
 
+/// How a session system prompt update should be applied.
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum SessionSystemPromptMode {
+    /// Replace Goose's base system prompt with the provided text.
+    Set,
+    /// Append the provided text under Goose's "Additional Instructions" section.
+    #[default]
+    Append,
+}
+
+/// Set, append, or clear system prompt text for a session.
+///
+/// `mode: "set"` replaces Goose's base system prompt. `mode: "append"` adds an
+/// instruction under "Additional Instructions". Reusing a key replaces the
+/// previous value for that mode/key; sending empty text clears it.
+#[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema, JsonRpcRequest)]
+#[request(
+    method = "_goose/unstable/session/system-prompt/set",
+    response = EmptyResponse
+)]
+#[serde(rename_all = "camelCase")]
+pub struct SetSessionSystemPromptRequest {
+    pub session_id: String,
+    #[serde(default)]
+    pub mode: SessionSystemPromptMode,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub key: Option<String>,
+    pub text: String,
+}
+
 /// Delete a session.
 #[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema, JsonRpcRequest)]
 #[request(method = "session/delete", response = EmptyResponse)]

crates/goose/acp-meta.json

@@ -30,6 +30,11 @@
       "requestType": "UpdateWorkingDirRequest_unstable",
       "responseType": "EmptyResponse"
     },
+    {
+      "method": "_goose/unstable/session/system-prompt/set",
+      "requestType": "SetSessionSystemPromptRequest_unstable",
+      "responseType": "EmptyResponse"
+    },
     {
       "method": "session/delete",
       "requestType": "DeleteSessionRequest",

crates/goose/acp-schema.json

@@ -167,6 +167,49 @@
       "x-side": "agent",
       "x-method": "_goose/unstable/session/working-dir/update"
     },
+    "SetSessionSystemPromptRequest_unstable": {
+      "type": "object",
+      "properties": {
+        "sessionId": {
+          "type": "string"
+        },
+        "mode": {
+          "$ref": "#/$defs/SessionSystemPromptMode",
+          "default": "append"
+        },
+        "key": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "text": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "sessionId",
+        "text"
+      ],
+      "description": "Set, append, or clear system prompt text for a session.\n\n`mode: \"set\"` replaces Goose's base system prompt. `mode: \"append\"` adds an\ninstruction under \"Additional Instructions\". Reusing a key replaces the\nprevious value for that mode/key; sending empty text clears it.",
+      "x-side": "agent",
+      "x-method": "_goose/unstable/session/system-prompt/set"
+    },
+    "SessionSystemPromptMode": {
+      "oneOf": [
+        {
+          "type": "string",
+          "const": "set",
+          "description": "Replace Goose's base system prompt with the provided text."
+        },
+        {
+          "type": "string",
+          "const": "append",
+          "description": "Append the provided text under Goose's \"Additional Instructions\" section."
+        }
+      ],
+      "description": "How a session system prompt update should be applied."
+    },
     "DeleteSessionRequest": {
       "type": "object",
       "properties": {
@@ -2640,6 +2683,15 @@
                   "description": "Params for _goose/unstable/session/working-dir/update",
                   "title": "UpdateWorkingDirRequest_unstable"
                 },
+                {
+                  "allOf": [
+                    {
+                      "$ref": "#/$defs/SetSessionSystemPromptRequest_unstable"
+                    }
+                  ],
+                  "description": "Params for _goose/unstable/session/system-prompt/set",
+                  "title": "SetSessionSystemPromptRequest_unstable"
+                },
                 {
                   "allOf": [
                     {

crates/goose/src/acp/server/custom_dispatch.rs

@@ -59,6 +59,14 @@ impl GooseAcpAgent {
         self.on_update_working_dir(req).await
     }
 
+    #[custom_method(SetSessionSystemPromptRequest)]
+    async fn dispatch_set_session_system_prompt(
+        &self,
+        req: SetSessionSystemPromptRequest,
+    ) -> Result<EmptyResponse, agent_client_protocol::Error> {
+        self.on_set_session_system_prompt(req).await
+    }
+
     #[custom_method(DeleteSessionRequest)]
     async fn dispatch_delete_session(
         &self,

crates/goose/src/acp/server/sessions.rs

@@ -34,6 +34,47 @@ impl GooseAcpAgent {
         Ok(EmptyResponse {})
     }
 
+    pub(super) async fn on_set_session_system_prompt(
+        &self,
+        req: SetSessionSystemPromptRequest,
+    ) -> Result<EmptyResponse, agent_client_protocol::Error> {
+        let session_id = req.session_id.trim();
+        if session_id.is_empty() {
+            return Err(
+                agent_client_protocol::Error::invalid_params().data("sessionId cannot be empty")
+            );
+        }
+
+        let agent = self.get_session_agent_provider_ready(session_id).await?;
+        match req.mode {
+            SessionSystemPromptMode::Set => {
+                if req.text.trim().is_empty() {
+                    agent.clear_system_prompt_override().await;
+                } else {
+                    agent.override_system_prompt(req.text).await;
+                }
+            }
+            SessionSystemPromptMode::Append => {
+                let key = req
+                    .key
+                    .as_deref()
+                    .map(str::trim)
+                    .filter(|key| !key.is_empty())
+                    .ok_or_else(|| {
+                        agent_client_protocol::Error::invalid_params()
+                            .data("key cannot be empty for append mode")
+                    })?;
+                if req.text.trim().is_empty() {
+                    agent.remove_system_prompt_extra(key).await;
+                } else {
+                    agent.extend_system_prompt(key.to_string(), req.text).await;
+                }
+            }
+        }
+
+        Ok(EmptyResponse {})
+    }
+
     pub(super) async fn on_delete_session(
         &self,
         req: DeleteSessionRequest,

crates/goose/src/agents/agent.rs

@@ -66,8 +66,7 @@ use tracing::{debug, error, info, instrument, warn};
 
 const DEFAULT_MAX_TURNS: u32 = 1000;
 const COMPACTION_THINKING_TEXT: &str = "goose is compacting the conversation...";
-const DEFAULT_FRONTEND_INSTRUCTIONS: &str =
-    "The following tools are provided directly by the frontend and will be executed by the frontend when called.";
+const DEFAULT_FRONTEND_INSTRUCTIONS: &str = "The following tools are provided directly by the frontend and will be executed by the frontend when called.";
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum ToolCategory {
@@ -2293,6 +2292,11 @@ impl Agent {
         prompt_manager.add_system_prompt_extra(key, instruction);
     }
 
+    pub async fn remove_system_prompt_extra(&self, key: &str) {
+        let mut prompt_manager = self.prompt_manager.lock().await;
+        prompt_manager.remove_system_prompt_extra(key);
+    }
+
     pub async fn set_goal(&self, goal: Option<String>) {
         *self.goal.lock().await = goal;
     }
@@ -2463,6 +2467,11 @@ impl Agent {
         prompt_manager.set_system_prompt_override(template);
     }
 
+    pub async fn clear_system_prompt_override(&self) {
+        let mut prompt_manager = self.prompt_manager.lock().await;
+        prompt_manager.clear_system_prompt_override();
+    }
+
     pub async fn list_extension_prompts(&self, session_id: &str) -> HashMap<String, Vec<Prompt>> {
         self.extension_manager
             .list_prompts(session_id, CancellationToken::default())

crates/goose/src/agents/prompt_manager.rs

@@ -224,6 +224,10 @@ impl PromptManager {
         self.system_prompt_extras.insert(key, instruction);
     }
 
+    pub fn remove_system_prompt_extra(&mut self, key: &str) {
+        self.system_prompt_extras.shift_remove(key);
+    }
+
     pub fn record_tool_arguments(
         &mut self,
         arguments: &Option<serde_json::Map<String, serde_json::Value>>,
@@ -247,6 +251,10 @@ impl PromptManager {
         self.system_prompt_override = Some(template);
     }
 
+    pub fn clear_system_prompt_override(&mut self) {
+        self.system_prompt_override = None;
+    }
+
     pub fn builder<'a>(&'a self) -> SystemPromptBuilder<'a, Self> {
         SystemPromptBuilder {
             manager: self,
@@ -326,6 +334,29 @@ mod tests {
         assert!(result.contains("Thirdinstruction"));
     }
 
+    #[test]
+    fn test_remove_system_prompt_extra() {
+        let mut manager = PromptManager::new();
+        manager.add_system_prompt_extra("agent".to_string(), "Agent instruction".to_string());
+        manager.add_system_prompt_extra("project".to_string(), "Project instruction".to_string());
+
+        manager.remove_system_prompt_extra("agent");
+        let result = manager.builder().build();
+
+        assert!(!result.contains("Agent instruction"));
+        assert!(result.contains("Project instruction"));
+    }
+
+    #[test]
+    fn test_clear_system_prompt_override() {
+        let mut manager = PromptManager::new();
+        manager.set_system_prompt_override("Replacement prompt".to_string());
+        assert!(manager.builder().build().contains("Replacement prompt"));
+
+        manager.clear_system_prompt_override();
+        assert!(!manager.builder().build().contains("Replacement prompt"));
+    }
+
     #[test]
     fn test_build_system_prompt_preserves_legitimate_unicode_in_extras() {
         let mut manager = PromptManager::new();