feat(acp): add BUZZ_ACP_MCP_ARGS for MCP servers that need arguments

.env.example

@@ -140,6 +140,11 @@ RUST_LOG=buzz_relay=debug,buzz_db=debug,buzz_auth=debug,buzz_pubsub=debug,tower_
 # Binary for an optional MCP server sidecar (e.g. buzz-dev-mcp for buzz-agent).
 # BUZZ_ACP_MCP_COMMAND=
 
+# Comma-separated arguments passed to the MCP server binary. Needed by servers
+# that are launched through a runner rather than as a bare binary, for example
+# "npx" with args "-y,some-mcp-server".
+# BUZZ_ACP_MCP_ARGS=
+
 # Number of parallel agent subprocesses (1–32).
 # BUZZ_ACP_AGENTS=1
 

crates/buzz-acp/README.md

@@ -102,6 +102,7 @@ All configuration is via environment variables (or CLI flags — every env var h
 | `BUZZ_ACP_AGENT_COMMAND` | no | `goose` | Agent binary to spawn. |
 | `BUZZ_ACP_AGENT_ARGS` | no | `acp` | Agent arguments (comma-separated). |
 | `BUZZ_ACP_MCP_COMMAND` | no | `""` (empty) | Path to an optional MCP server binary to provide to the agent subprocess. |
+| `BUZZ_ACP_MCP_ARGS` | no | `""` (empty) | MCP server arguments (comma-separated). Lets a server be launched through a runner, e.g. command `npx` with args `-y,some-mcp-server`. |
 | `BUZZ_ACP_IDLE_TIMEOUT` | no | `620` | Idle timeout: max seconds of silence before cancelling a turn. Resets on any agent stdout activity. |
 | `BUZZ_ACP_MAX_TURN_DURATION` | no | `7200` | Absolute wall-clock cap per turn (safety valve). |
 | `BUZZ_API_TOKEN` | no | — | API token (required if relay enforces token auth). |

crates/buzz-acp/src/config.rs

@@ -261,6 +261,15 @@ pub struct CliArgs {
     #[arg(long, env = "BUZZ_ACP_MCP_COMMAND", default_value = "")]
     pub mcp_command: String,
 
+    /// Arguments passed to the MCP server binary.
+    #[arg(
+        long,
+        env = "BUZZ_ACP_MCP_ARGS",
+        default_value = "",
+        value_delimiter = ','
+    )]
+    pub mcp_args: Vec<String>,
+
     /// Idle timeout: max seconds of silence before killing a turn.
     /// Resets on any agent stdout activity.
     #[arg(long, env = "BUZZ_ACP_IDLE_TIMEOUT")]
@@ -489,6 +498,7 @@ pub struct Config {
     pub agent_command: String,
     pub agent_args: Vec<String>,
     pub mcp_command: String,
+    pub mcp_args: Vec<String>,
     pub idle_timeout_secs: u64,
     pub max_turn_duration_secs: u64,
     pub agents: u32,
@@ -964,6 +974,7 @@ impl Config {
             agent_command,
             agent_args,
             mcp_command: args.mcp_command,
+            mcp_args: args.mcp_args,
             idle_timeout_secs,
             max_turn_duration_secs,
             agents: args.agents,
@@ -1024,12 +1035,13 @@ impl Config {
             format!(" allowed_respond_to=[{}]", modes.join(","))
         };
         format!(
-            "relay={} pubkey={} agent_cmd={} {} mcp_cmd={} idle_timeout={}s max_turn={}s agents={} heartbeat={}s subscribe={:?} dedup={:?} meh={:?} ignore_self={} context_limit={} max_turns_per_session={} presence={} typing={} memory={} model={} permission_mode={} {}{}",
+            "relay={} pubkey={} agent_cmd={} {} mcp_cmd={} {} idle_timeout={}s max_turn={}s agents={} heartbeat={}s subscribe={:?} dedup={:?} meh={:?} ignore_self={} context_limit={} max_turns_per_session={} presence={} typing={} memory={} model={} permission_mode={} {}{}",
             self.relay_url,
             self.keys.public_key().to_hex(),
             self.agent_command,
             self.agent_args.join(" "),
             self.mcp_command,
+            self.mcp_args.join(" "),
             self.idle_timeout_secs,
             self.max_turn_duration_secs,
             self.agents,
@@ -1338,6 +1350,7 @@ mod tests {
             agent_command: "goose".into(),
             agent_args: vec!["acp".into()],
             mcp_command: "".into(),
+            mcp_args: vec![],
             idle_timeout_secs: DEFAULT_IDLE_TIMEOUT_SECS,
             max_turn_duration_secs: DEFAULT_MAX_TURN_DURATION_SECS,
             agents: 1,

crates/buzz-acp/src/lib.rs

@@ -4105,7 +4105,7 @@ fn build_mcp_servers(config: &Config) -> Vec<McpServer> {
             .unwrap_or("mcp")
             .to_string(),
         command: config.mcp_command.clone(),
-        args: vec![],
+        args: config.mcp_args.clone(),
         env: {
             let mut env = vec![
                 EnvVar {
@@ -4906,6 +4906,7 @@ mod build_mcp_servers_tests {
             agent_command: "goose".into(),
             agent_args: vec!["acp".into()],
             mcp_command: "test-mcp-server".into(),
+            mcp_args: vec![],
             idle_timeout_secs: config::DEFAULT_IDLE_TIMEOUT_SECS,
             max_turn_duration_secs: config::DEFAULT_MAX_TURN_DURATION_SECS,
             agents: 1,
@@ -5035,6 +5036,40 @@ mod build_mcp_servers_tests {
             "Path::new(\".\").file_stem() is None — should fall back to \"mcp\""
         );
     }
+
+    #[test]
+    fn mcp_args_default_to_empty() {
+        let mut config = test_config();
+        config.mcp_command = "/opt/bin/my-mcp-server".into();
+        config.mcp_args = vec![];
+        let servers = build_mcp_servers(&config);
+        assert_eq!(servers.len(), 1);
+        assert!(
+            servers[0].args.is_empty(),
+            "an MCP command with no args should spawn with no args"
+        );
+    }
+
+    #[test]
+    fn mcp_args_are_passed_to_the_server() {
+        let mut config = test_config();
+        config.mcp_command = "npx".into();
+        config.mcp_args = vec!["-y".into(), "some-mcp-server".into()];
+        let servers = build_mcp_servers(&config);
+        assert_eq!(servers.len(), 1);
+        assert_eq!(servers[0].command, "npx");
+        assert_eq!(servers[0].args, vec!["-y", "some-mcp-server"]);
+    }
+
+    #[test]
+    fn mcp_args_do_not_affect_the_derived_server_name() {
+        // The name still comes from the command's file stem, not the args.
+        let mut config = test_config();
+        config.mcp_command = "/opt/bin/my-mcp-server".into();
+        config.mcp_args = vec!["--port".into(), "8080".into()];
+        let servers = build_mcp_servers(&config);
+        assert_eq!(servers[0].name, "my-mcp-server");
+    }
 }
 
 #[cfg(test)]
@@ -5072,6 +5107,7 @@ mod error_outcome_emission_tests {
             agent_command: "true".into(),
             agent_args: vec![],
             mcp_command: "test-mcp-server".into(),
+            mcp_args: vec![],
             idle_timeout_secs: config::DEFAULT_IDLE_TIMEOUT_SECS,
             max_turn_duration_secs: config::DEFAULT_MAX_TURN_DURATION_SECS,
             agents: 1,