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feat(acp): allow multiple MCP servers per agent #2900

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7 changes: 5 additions & 2 deletions .env.example
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Original file line number Diff line number Diff line change
Expand Up @@ -137,8 +137,11 @@ RUST_LOG=buzz_relay=debug,buzz_db=debug,buzz_auth=debug,buzz_pubsub=debug,tower_
# Goose default: "acp". Codex/Claude default: "" (empty).
# BUZZ_ACP_AGENT_ARGS=acp

# Binary for an optional MCP server sidecar (e.g. buzz-dev-mcp for buzz-agent).
# BUZZ_ACP_MCP_COMMAND=
# MCP server binaries to expose to the agent (e.g. buzz-dev-mcp for buzz-agent).
# Comma-separated for more than one; --mcp-command may also be repeated.
# Each server is spawned with BUZZ_RELAY_URL / BUZZ_PRIVATE_KEY / BUZZ_AUTH_TAG
# injected. Servers are named after each binary's file stem.
# BUZZ_ACP_MCP_COMMAND=buzz-dev-mcp,/opt/bin/my-domain-mcp

# Number of parallel agent subprocesses (1–32).
# BUZZ_ACP_AGENTS=1
Expand Down
27 changes: 21 additions & 6 deletions crates/buzz-acp/src/config.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -258,8 +258,14 @@ pub struct CliArgs {
)]
pub agent_args: Vec<String>,

#[arg(long, env = "BUZZ_ACP_MCP_COMMAND", default_value = "")]
pub mcp_command: String,
/// MCP server binaries to expose to the agent. Repeat the flag, or pass a
/// comma-separated list via `BUZZ_ACP_MCP_COMMAND`. Empty entries are ignored.
#[arg(
long = "mcp-command",
env = "BUZZ_ACP_MCP_COMMAND",
value_delimiter = ','
)]
pub mcp_commands: Vec<String>,

/// Idle timeout: max seconds of silence before killing a turn.
/// Resets on any agent stdout activity.
Expand Down Expand Up @@ -488,7 +494,9 @@ pub struct Config {
pub relay_url: String,
pub agent_command: String,
pub agent_args: Vec<String>,
pub mcp_command: String,
/// MCP server binaries exposed to the agent, in order. Empty after filtering
/// means the agent gets no MCP servers.
pub mcp_commands: Vec<String>,
pub idle_timeout_secs: u64,
pub max_turn_duration_secs: u64,
pub agents: u32,
Expand Down Expand Up @@ -963,7 +971,14 @@ impl Config {
relay_url: args.relay_url,
agent_command,
agent_args,
mcp_command: args.mcp_command,
// Drop empties: an unset-but-present env var (BUZZ_ACP_MCP_COMMAND="")
// yields one empty entry through the comma delimiter.
mcp_commands: args
.mcp_commands
.into_iter()
.map(|c| c.trim().to_string())
.filter(|c| !c.is_empty())
.collect(),
idle_timeout_secs,
max_turn_duration_secs,
agents: args.agents,
Expand Down Expand Up @@ -1029,7 +1044,7 @@ impl Config {
self.keys.public_key().to_hex(),
self.agent_command,
self.agent_args.join(" "),
self.mcp_command,
self.mcp_commands.join(","),
self.idle_timeout_secs,
self.max_turn_duration_secs,
self.agents,
Expand Down Expand Up @@ -1337,7 +1352,7 @@ mod tests {
relay_url: "ws://localhost:3000".into(),
agent_command: "goose".into(),
agent_args: vec!["acp".into()],
mcp_command: "".into(),
mcp_commands: Vec::new(),
idle_timeout_secs: DEFAULT_IDLE_TIMEOUT_SECS,
max_turn_duration_secs: DEFAULT_MAX_TURN_DURATION_SECS,
agents: 1,
Expand Down
168 changes: 121 additions & 47 deletions crates/buzz-acp/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4139,48 +4139,70 @@ async fn run_models(args: ModelsArgs) -> Result<()> {
}

fn build_mcp_servers(config: &Config) -> Vec<McpServer> {
if config.mcp_command.is_empty() {
if config.mcp_commands.is_empty() {
return vec![];
}
vec![McpServer {
name: std::path::Path::new(&config.mcp_command)
.file_stem()
.and_then(|s| s.to_str())
.unwrap_or("mcp")
.to_string(),
command: config.mcp_command.clone(),
args: vec![],
env: {
let mut env = vec![
EnvVar {
name: "BUZZ_RELAY_URL".into(),
value: config.relay_url.clone(),
},
EnvVar {
name: "BUZZ_PRIVATE_KEY".into(),
// bech32 encoding of a valid secret key is infallible.
// Panic here is correct: injecting a bogus secret would cause
// delayed, hard-to-diagnose agent failures downstream.
value: config
.keys
.secret_key()
.to_bech32()
.expect("secret key bech32 encoding should never fail"),
},
];
// Forward BUZZ_AUTH_TAG (NIP-OA owner attestation credential)
// so the MCP server can attach it to every signed event.
if let Ok(auth_tag) = std::env::var("BUZZ_AUTH_TAG") {
if !auth_tag.is_empty() {
env.push(EnvVar {
name: "BUZZ_AUTH_TAG".into(),
value: auth_tag,
});
}

// Every server gets the same injected env: the relay URL, this agent's secret,
// and the NIP-OA credential if one is set.
let base_env = || {
let mut env = vec![
EnvVar {
name: "BUZZ_RELAY_URL".into(),
value: config.relay_url.clone(),
},
EnvVar {
name: "BUZZ_PRIVATE_KEY".into(),
// bech32 encoding of a valid secret key is infallible.
// Panic here is correct: injecting a bogus secret would cause
// delayed, hard-to-diagnose agent failures downstream.
value: config
.keys
.secret_key()
.to_bech32()
.expect("secret key bech32 encoding should never fail"),
},
];
// Forward BUZZ_AUTH_TAG (NIP-OA owner attestation credential)
// so the MCP server can attach it to every signed event.
if let Ok(auth_tag) = std::env::var("BUZZ_AUTH_TAG") {
if !auth_tag.is_empty() {
env.push(EnvVar {
name: "BUZZ_AUTH_TAG".into(),
value: auth_tag,
});
}
env
},
}]
}
env
};

let mut used: HashSet<String> = HashSet::new();
config
.mcp_commands
.iter()
.map(|command| {
let stem = std::path::Path::new(command)
.file_stem()
.and_then(|s| s.to_str())
.unwrap_or("mcp")
.to_string();
// Server names must be unique — two binaries can share a file stem
// (/a/bin/mcp and /b/bin/mcp), and a duplicate name would make one
// shadow the other in the agent's tool namespace.
let mut name = stem.clone();
let mut n = 2;
while !used.insert(name.clone()) {
name = format!("{stem}-{n}");
n += 1;
}
McpServer {
name,
command: command.clone(),
args: vec![],
env: base_env(),
}
})
.collect()
}

#[cfg(test)]
Expand Down Expand Up @@ -4949,7 +4971,7 @@ mod build_mcp_servers_tests {
relay_url: "ws://localhost:3000".into(),
agent_command: "goose".into(),
agent_args: vec!["acp".into()],
mcp_command: "test-mcp-server".into(),
mcp_commands: vec!["test-mcp-server".into()],
idle_timeout_secs: config::DEFAULT_IDLE_TIMEOUT_SECS,
max_turn_duration_secs: config::DEFAULT_MAX_TURN_DURATION_SECS,
agents: 1,
Expand Down Expand Up @@ -5039,29 +5061,81 @@ mod build_mcp_servers_tests {
#[test]
fn empty_mcp_command_returns_no_servers() {
let mut config = test_config();
config.mcp_command = "".into();
config.mcp_commands = Vec::new();
let servers = build_mcp_servers(&config);
assert!(
servers.is_empty(),
"empty mcp_command should produce no MCP servers"
"no mcp commands should produce no MCP servers"
);
}

#[test]
fn absolute_path_mcp_command_uses_file_stem_as_name() {
let mut config = test_config();
config.mcp_command = "/opt/bin/my-mcp-server".into();
config.mcp_commands = vec!["/opt/bin/my-mcp-server".into()];
let servers = build_mcp_servers(&config);
assert_eq!(servers.len(), 1);
assert_eq!(servers[0].name, "my-mcp-server");
}

#[test]
fn multiple_mcp_commands_all_reach_the_agent() {
// The reason this flag is repeatable: an agent needs buzz-dev-mcp to act in
// Buzz (it shells out to the `buzz` CLI) *and* whatever domain server the
// operator adds. One slot forced an either/or.
let mut config = test_config();
config.mcp_commands = vec!["/opt/bin/buzz-dev-mcp".into(), "/opt/bin/domain-mcp".into()];
let servers = build_mcp_servers(&config);
assert_eq!(servers.len(), 2);
assert_eq!(servers[0].name, "buzz-dev-mcp");
assert_eq!(servers[1].name, "domain-mcp");
assert_eq!(servers[0].command, "/opt/bin/buzz-dev-mcp");
assert_eq!(servers[1].command, "/opt/bin/domain-mcp");
}

#[test]
fn every_mcp_server_gets_the_injected_env() {
let mut config = test_config();
config.mcp_commands = vec!["/a/first-mcp".into(), "/b/second-mcp".into()];
let servers = build_mcp_servers(&config);
assert_eq!(servers.len(), 2);
for server in &servers {
for key in ["BUZZ_RELAY_URL", "BUZZ_PRIVATE_KEY"] {
assert!(
server.env.iter().any(|e| e.name == key),
"{} should be injected into {}",
key,
server.name
);
}
}
}

#[test]
fn colliding_file_stems_get_unique_names() {
// Two distinct binaries can share a file stem. Duplicate server names would
// make one shadow the other in the agent's tool namespace.
let mut config = test_config();
config.mcp_commands = vec![
"/a/bin/mcp".into(),
"/b/bin/mcp".into(),
"/c/bin/mcp".into(),
];
let servers = build_mcp_servers(&config);
assert_eq!(servers.len(), 3);
assert_eq!(servers[0].name, "mcp");
assert_eq!(servers[1].name, "mcp-2");
assert_eq!(servers[2].name, "mcp-3");
let unique: HashSet<&str> = servers.iter().map(|s| s.name.as_str()).collect();
assert_eq!(unique.len(), 3, "server names must be unique");
}

#[test]
fn mcp_command_with_no_stem_falls_back_to_mcp() {
// Path::new("").file_stem() returns None — exercises the unwrap_or("mcp") path.
let mut config = test_config();
config.mcp_command = "".into();
// Empty command returns no servers; test the stem logic directly.
config.mcp_commands = Vec::new();
// No commands returns no servers; test the stem logic directly.
assert_eq!(
std::path::Path::new("")
.file_stem()
Expand All @@ -5071,7 +5145,7 @@ mod build_mcp_servers_tests {
);

// Confirm a non-empty command with no stem (e.g. just a dot) also falls back.
config.mcp_command = ".".into();
config.mcp_commands = vec![".".into()];
let servers = build_mcp_servers(&config);
assert_eq!(servers.len(), 1);
assert_eq!(
Expand Down Expand Up @@ -5115,7 +5189,7 @@ mod error_outcome_emission_tests {
// feed emission under test.
agent_command: "true".into(),
agent_args: vec![],
mcp_command: "test-mcp-server".into(),
mcp_commands: vec!["test-mcp-server".into()],
idle_timeout_secs: config::DEFAULT_IDLE_TIMEOUT_SECS,
max_turn_duration_secs: config::DEFAULT_MAX_TURN_DURATION_SECS,
agents: 1,
Expand Down