acp: Advertise model picker support to Cursor (#59694)

Fixes an issue where users couldn't see when fast mode was enabled

Release Notes:

- acp: Fix for Cursor agent only being able to use Fast mode with
Composer 2.5

crates/agent_servers/src/acp.rs

@@ -41,9 +41,10 @@ use acp_thread::{AcpThread, AuthRequired, LoadError, TerminalProviderEvent};
 use terminal::TerminalBuilder;
 use terminal::terminal_settings::{AlternateScroll, CursorShape};
 
-use crate::GEMINI_ID;
+use crate::{CURSOR_ID, GEMINI_ID};
 
 pub const GEMINI_TERMINAL_AUTH_METHOD_ID: &str = "spawn-gemini-cli";
+const PARAMETERIZED_MODEL_PICKER_META_KEY: &str = "parameterizedModelPicker";
 const MAX_DEBUG_BACKLOG_MESSAGES: usize = 2000;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -735,6 +736,25 @@ fn connect_client_future(
         )
 }
 
+fn client_capabilities_for_agent(agent_id: &AgentId) -> acp::ClientCapabilities {
+    let mut meta = acp::Meta::from_iter([
+        ("terminal_output".into(), true.into()),
+        ("terminal-auth".into(), true.into()),
+    ]);
+
+    if agent_id.as_ref() == CURSOR_ID {
+        meta.insert(PARAMETERIZED_MODEL_PICKER_META_KEY.into(), true.into());
+    }
+
+    acp::ClientCapabilities::new()
+        .fs(acp::FileSystemCapabilities::new()
+            .read_text_file(true)
+            .write_text_file(true))
+        .terminal(true)
+        .auth(acp::AuthCapabilities::new().terminal(true))
+        .meta(meta)
+}
+
 impl AcpConnection {
     pub fn subscribe_debug_messages(
         &self,
@@ -922,18 +942,7 @@ impl AcpConnection {
         let initialize_response = connection
             .send_request(
                 acp::InitializeRequest::new(ProtocolVersion::V1)
-                    .client_capabilities(
-                        acp::ClientCapabilities::new()
-                            .fs(acp::FileSystemCapabilities::new()
-                                .read_text_file(true)
-                                .write_text_file(true))
-                            .terminal(true)
-                            .auth(acp::AuthCapabilities::new().terminal(true))
-                            .meta(acp::Meta::from_iter([
-                                ("terminal_output".into(), true.into()),
-                                ("terminal-auth".into(), true.into()),
-                            ])),
-                    )
+                    .client_capabilities(client_capabilities_for_agent(&agent_id))
                     .client_info(
                         acp::Implementation::new("zed", version)
                             .title(release_channel.map(ToOwned::to_owned)),
@@ -2457,6 +2466,31 @@ mod tests {
     use super::*;
     use settings::Settings as _;
 
+    #[test]
+    fn cursor_client_capabilities_include_parameterized_model_picker_meta() {
+        let capabilities = client_capabilities_for_agent(&AgentId::new(CURSOR_ID));
+        let meta = capabilities
+            .meta
+            .expect("expected client capabilities meta");
+
+        assert_eq!(
+            meta.get(PARAMETERIZED_MODEL_PICKER_META_KEY),
+            Some(&serde_json::json!(true))
+        );
+        assert_eq!(meta.get("terminal_output"), Some(&serde_json::json!(true)));
+        assert_eq!(meta.get("terminal-auth"), Some(&serde_json::json!(true)));
+    }
+
+    #[test]
+    fn non_cursor_client_capabilities_do_not_include_parameterized_model_picker_meta() {
+        let capabilities = client_capabilities_for_agent(&AgentId::new("codex-acp"));
+        let meta = capabilities
+            .meta
+            .expect("expected client capabilities meta");
+
+        assert!(!meta.contains_key(PARAMETERIZED_MODEL_PICKER_META_KEY));
+    }
+
     #[test]
     fn terminal_auth_task_builds_spawn_from_prebuilt_command() {
         let command = AgentServerCommand {

crates/agent_servers/src/custom.rs

@@ -17,6 +17,7 @@ use ui::IconName;
 pub const GEMINI_ID: &str = "gemini";
 pub const CLAUDE_AGENT_ID: &str = "claude-acp";
 pub const CODEX_ID: &str = "codex-acp";
+pub const CURSOR_ID: &str = "cursor";
 
 /// A generic agent server implementation for custom user-defined agents
 pub struct CustomAgentServer {