diff --git a/hive-ag3nt/Cargo.toml b/hive-ag3nt/Cargo.toml
index c268836..f5bc59d 100644
--- a/hive-ag3nt/Cargo.toml
+++ b/hive-ag3nt/Cargo.toml
@@ -3,6 +3,16 @@ name = "hive-ag3nt"
 edition.workspace = true
 version.workspace = true
 
+[dependencies]
+anyhow.workspace = true
+clap.workspace = true
+hive-sh4re.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+
 [[bin]]
 name = "hive-ag3nt"
 path = "src/bin/hive-ag3nt.rs"
diff --git a/hive-ag3nt/src/bin/hive-ag3nt.rs b/hive-ag3nt/src/bin/hive-ag3nt.rs
index 03c1e0c..021c400 100644
--- a/hive-ag3nt/src/bin/hive-ag3nt.rs
+++ b/hive-ag3nt/src/bin/hive-ag3nt.rs
@@ -1,3 +1,91 @@
-fn main() {
-    println!("hive-ag3nt placeholder");
+use std::path::PathBuf;
+use std::time::Duration;
+
+use anyhow::{Result, bail};
+use clap::{Parser, Subcommand};
+use hive_ag3nt::{DEFAULT_SOCKET, client};
+use hive_sh4re::{AgentRequest, AgentResponse};
+
+#[derive(Parser)]
+#[command(name = "hive-ag3nt", about = "hyperhive sub-agent harness")]
+struct Cli {
+    /// Path to the per-agent MCP socket (bind-mounted from the host).
+    #[arg(long, global = true, default_value = DEFAULT_SOCKET)]
+    socket: PathBuf,
+
+    #[command(subcommand)]
+    cmd: Cmd,
+}
+
+#[derive(Subcommand)]
+enum Cmd {
+    /// Run the long-lived harness loop. Polls inbox; prints messages to stdout.
+    Serve {
+        /// Inbox poll interval in milliseconds.
+        #[arg(long, default_value_t = 1000)]
+        poll_ms: u64,
+    },
+    /// Send a message to another agent.
+    Send { to: String, body: String },
+    /// Pop one message from the inbox.
+    Recv,
+}
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info")),
+        )
+        .init();
+
+    let cli = Cli::parse();
+    match cli.cmd {
+        Cmd::Serve { poll_ms } => serve(&cli.socket, Duration::from_millis(poll_ms)).await,
+        Cmd::Send { to, body } => {
+            let resp = client::request(&cli.socket, AgentRequest::Send { to, body }).await?;
+            render(&resp)?;
+            check(&resp)
+        }
+        Cmd::Recv => {
+            let resp = client::request(&cli.socket, AgentRequest::Recv).await?;
+            render(&resp)?;
+            check(&resp)
+        }
+    }
+}
+
+async fn serve(socket: &std::path::Path, interval: Duration) -> Result<()> {
+    tracing::info!(socket = %socket.display(), "hive-ag3nt serve");
+    loop {
+        match client::request(socket, AgentRequest::Recv).await {
+            Ok(AgentResponse::Message { from, body }) => {
+                tracing::info!(%from, %body, "inbox");
+            }
+            Ok(AgentResponse::Empty) => {}
+            Ok(AgentResponse::Ok) => {
+                tracing::warn!("recv produced Ok (unexpected)");
+            }
+            Ok(AgentResponse::Err { message }) => {
+                tracing::warn!(%message, "recv error");
+            }
+            Err(e) => {
+                tracing::warn!(error = ?e, "recv failed; retrying");
+            }
+        }
+        tokio::time::sleep(interval).await;
+    }
+}
+
+fn render(resp: &AgentResponse) -> Result<()> {
+    println!("{}", serde_json::to_string_pretty(resp)?);
+    Ok(())
+}
+
+fn check(resp: &AgentResponse) -> Result<()> {
+    if let AgentResponse::Err { message } = resp {
+        bail!("{message}");
+    }
+    Ok(())
 }
diff --git a/hive-ag3nt/src/bin/hive-m1nd.rs b/hive-ag3nt/src/bin/hive-m1nd.rs
index 314550c..a63ce9d 100644
--- a/hive-ag3nt/src/bin/hive-m1nd.rs
+++ b/hive-ag3nt/src/bin/hive-m1nd.rs
@@ -1,3 +1,5 @@
 fn main() {
+    // Phase 4 — manager tool surface. For now, a placeholder so the binary
+    // exists and can be referenced from the manager nixos-container template.
     println!("hive-m1nd placeholder");
 }
diff --git a/hive-ag3nt/src/client.rs b/hive-ag3nt/src/client.rs
new file mode 100644
index 0000000..8deb726
--- /dev/null
+++ b/hive-ag3nt/src/client.rs
@@ -0,0 +1,27 @@
+use std::path::Path;
+
+use anyhow::{Context, Result, bail};
+use hive_sh4re::{AgentRequest, AgentResponse};
+use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+use tokio::net::UnixStream;
+
+pub async fn request(socket: &Path, req: AgentRequest) -> Result<AgentResponse> {
+    let stream = UnixStream::connect(socket)
+        .await
+        .with_context(|| format!("connect to {}", socket.display()))?;
+    let (read, mut write) = stream.into_split();
+
+    let mut payload = serde_json::to_string(&req)?;
+    payload.push('\n');
+    write.write_all(payload.as_bytes()).await?;
+    write.flush().await?;
+
+    let mut reader = BufReader::new(read);
+    let mut line = String::new();
+    reader.read_line(&mut line).await?;
+    if line.is_empty() {
+        bail!("server closed connection without responding");
+    }
+    let resp: AgentResponse = serde_json::from_str(line.trim())?;
+    Ok(resp)
+}
diff --git a/hive-ag3nt/src/lib.rs b/hive-ag3nt/src/lib.rs
index 8b13789..3890473 100644
--- a/hive-ag3nt/src/lib.rs
+++ b/hive-ag3nt/src/lib.rs
@@ -1 +1,7 @@
+//! Shared in-container harness code used by both `hive-ag3nt` (agent) and
+//! `hive-m1nd` (manager) binaries.
 
+pub mod client;
+
+/// Default socket path inside the container — bind-mounted by `hive-c0re`.
+pub const DEFAULT_SOCKET: &str = "/run/hive/mcp.sock";
diff --git a/nix/templates/agent-base.nix b/nix/templates/agent-base.nix
index af425b5..fa0b8e6 100644
--- a/nix/templates/agent-base.nix
+++ b/nix/templates/agent-base.nix
@@ -7,9 +7,11 @@
   systemd.services.hive-ag3nt = {
     description = "hive-ag3nt harness";
     wantedBy = [ "multi-user.target" ];
+    after = [ "network.target" ];
     serviceConfig = {
-      ExecStart = "${pkgs.hyperhive}/bin/hive-ag3nt";
-      Type = "oneshot";
+      ExecStart = "${pkgs.hyperhive}/bin/hive-ag3nt serve";
+      Restart = "on-failure";
+      RestartSec = 2;
     };
   };