VSCode extension uses symposium-acp-agent registry commands

Centralize registry access through the Rust binary instead of
fetching directly from GitHub in TypeScript:

Rust changes:
- Add extensions field to RegistryJson struct
- Add list_extensions() function and ExtensionListEntry type
- Add 'registry list-extensions' subcommand

TypeScript changes:
- agentRegistry.ts: fetchRegistry() calls 'registry list'
- agentRegistry.ts: resolveDistribution() calls 'registry resolve'
- agentRegistry.ts: Remove binary download/cache code (now in Rust)
- agentRegistry.ts: Export runRegistryCommand for extensionRegistry
- extensionRegistry.ts: fetchRegistryExtensions() calls 'registry list-extensions'
- Remove isSymposiumBuiltin from ResolvedCommand (binary returns full path)
- Simplify spawn logic in acpAgentActor.ts and languageModelProvider.ts

Co-authored-by: Claude <claude@anthropic.com>

Author: nikomatsakis

src/symposium-acp-agent/src/main.rs

@@ -116,6 +116,9 @@ enum RegistryCommand {
     /// List all available agents (built-ins + registry)
     List,
 
+    /// List all available extensions from the registry
+    ListExtensions,
+
     /// Resolve an agent ID to an executable McpServer configuration.
     /// Downloads binaries if needed.
     Resolve {
@@ -289,6 +292,10 @@ async fn main() -> Result<()> {
                 let agents = registry::list_agents().await?;
                 println!("{}", serde_json::to_string(&agents)?);
             }
+            RegistryCommand::ListExtensions => {
+                let extensions = registry::list_extensions().await?;
+                println!("{}", serde_json::to_string(&extensions)?);
+            }
             RegistryCommand::Resolve { agent_id } => {
                 let server = registry::resolve_agent(&agent_id).await?;
                 println!("{}", serde_json::to_string(&server)?);

src/symposium-acp-agent/src/registry.rs

@@ -25,6 +25,8 @@ const REGISTRY_URL: &str =
 pub struct RegistryJson {
     pub version: String,
     pub agents: Vec<RegistryEntry>,
+    #[serde(default)]
+    pub extensions: Vec<RegistryEntry>,
 }
 
 /// A single entry in the registry (agent or extension)
@@ -197,6 +199,39 @@ pub async fn list_agents() -> Result<Vec<AgentListEntry>> {
     Ok(agents)
 }
 
+/// Extension listing entry - what `registry list-extensions` outputs
+#[derive(Debug, Clone, Serialize)]
+pub struct ExtensionListEntry {
+    pub id: String,
+    pub name: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub version: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub description: Option<String>,
+}
+
+/// List all available extensions from the registry
+pub async fn list_extensions() -> Result<Vec<ExtensionListEntry>> {
+    let registry = fetch_registry().await?;
+
+    let extensions: Vec<ExtensionListEntry> = registry
+        .extensions
+        .into_iter()
+        .map(|e| ExtensionListEntry {
+            id: e.id,
+            name: e.name,
+            version: if e.version.is_empty() {
+                None
+            } else {
+                Some(e.version)
+            },
+            description: e.description,
+        })
+        .collect();
+
+    Ok(extensions)
+}
+
 // ============================================================================
 // Distribution Resolution
 // ============================================================================

vscode-extension/src/acpAgentActor.ts

@@ -262,18 +262,8 @@ export class AcpAgentActor {
       spawnArgs.push(arg);
     }
 
-    if (resolved.isSymposiumBuiltin) {
-      // Symposium builtin (e.g., eliza) - run with the same binary
-      spawnArgs.push(
-        "--",
-        conductorCommand,
-        resolved.command,
-        ...resolved.args,
-      );
-    } else {
-      // External agent - wrap with conductor
-      spawnArgs.push("--", resolved.command, ...resolved.args);
-    }
+    // Add the downstream agent command after "--"
+    spawnArgs.push("--", resolved.command, ...resolved.args);
 
     logger.important("agent", "Spawning ACP agent", {
       command: conductorCommand,