diff --git a/crates/neuron/src/harness/candle.rs b/crates/neuron/src/harness/candle.rs
index e792bb8..3dc8522 100644
--- a/crates/neuron/src/harness/candle.rs
+++ b/crates/neuron/src/harness/candle.rs
@@ -404,6 +404,19 @@ impl Harness for CandleHarness {
             }
         }
 
+        // Stage 7a-i scaffolds tensor-parallel worker subprocesses but
+        // does not yet route inference through them. Refuse TP loads
+        // for now with a clear marker so the request surface is honest.
+        let tp_size = spec.tensor_parallel.unwrap_or(1);
+        if tp_size > 1 {
+            anyhow::bail!(
+                "tensor_parallel={tp_size} requested for '{}': TP worker \
+                 lifecycle is in place (Stage 7a-i) but TP-aware Qwen3 \
+                 inference lands in Stage 7b; single-GPU loads only for now",
+                spec.model_id
+            );
+        }
+
         let devices = spec.devices.clone().unwrap_or_else(|| vec![0]);
         let device = Self::pick_device(&devices)?;
 
diff --git a/crates/neuron/src/harness/mod.rs b/crates/neuron/src/harness/mod.rs
index 9850fd1..a9cbd3c 100644
--- a/crates/neuron/src/harness/mod.rs
+++ b/crates/neuron/src/harness/mod.rs
@@ -1,6 +1,7 @@
 //! Harness registry — maps harness names to trait implementations.
 
 pub mod candle;
+pub mod tp;
 
 use anyhow::Result;
 use cortex_core::harness::{Harness, HarnessConfig, ModelInfo, ModelSpec};
diff --git a/crates/neuron/src/harness/tp/mod.rs b/crates/neuron/src/harness/tp/mod.rs
new file mode 100644
index 0000000..df85fea
--- /dev/null
+++ b/crates/neuron/src/harness/tp/mod.rs
@@ -0,0 +1,204 @@
+//! Tensor-parallel inference plumbing.
+//!
+//! The leader process (the neuron daemon proper) drives one
+//! subprocess per non-zero NCCL rank — `tokio::process::Command` on
+//! `/proc/self/exe --worker --rank N --tp-size N --cuda-device N` —
+//! and talks to each over a newline-delimited JSON RPC channel on
+//! the worker's stdin/stdout (see `rpc.rs`).
+//!
+//! Sub-staging:
+//!
+//! - **7a-i (this commit):** process lifecycle. `WorkerPool::spawn`
+//!   forks N workers; `ping` round-trips every worker to confirm
+//!   they're alive; `shutdown` cleanly drains and reaps. `Init` /
+//!   `NcclSanityCheck` are stubbed.
+//! - **7a-ii:** real NCCL `Comm` setup via `Init`, sanity check via
+//!   `NcclSanityCheck`. CUDA-gated.
+//! - **7b:** TP-aware Qwen3 inference dispatched through the pool.
+//! - **7c:** crash detection, streaming SSE, graceful unload.
+
+pub mod rpc;
+pub mod worker;
+
+use anyhow::{Context, Result};
+use std::path::{Path, PathBuf};
+use std::process::Stdio;
+use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader, Lines};
+use tokio::process::{Child, ChildStdin, ChildStdout, Command};
+
+use rpc::{WorkerRequest, WorkerResponse};
+
+/// One worker subprocess plus its bidirectional stdio handles.
+struct Worker {
+    rank: u32,
+    /// Captured so the leader can log "spawned rank N on device M" and
+    /// future stages can re-issue Init after a CUDA reset. Unused in
+    /// the Stage 7a-i RPC paths themselves.
+    #[allow(dead_code)]
+    cuda_device: u32,
+    child: Child,
+    stdin: ChildStdin,
+    stdout: Lines<BufReader<ChildStdout>>,
+}
+
+impl Worker {
+    async fn request(&mut self, req: &WorkerRequest) -> Result<WorkerResponse> {
+        let mut line = serde_json::to_string(req).context("serialise WorkerRequest")?;
+        line.push('\n');
+        self.stdin
+            .write_all(line.as_bytes())
+            .await
+            .with_context(|| format!("write request to rank {}", self.rank))?;
+        self.stdin
+            .flush()
+            .await
+            .with_context(|| format!("flush stdin to rank {}", self.rank))?;
+
+        let reply = self
+            .stdout
+            .next_line()
+            .await
+            .with_context(|| format!("read reply from rank {}", self.rank))?
+            .ok_or_else(|| anyhow::anyhow!("rank {} stdout closed before reply", self.rank))?;
+        serde_json::from_str(&reply)
+            .with_context(|| format!("parse reply from rank {}: {reply:?}", self.rank))
+    }
+}
+
+/// A live pool of worker subprocesses. Owns the `Child` handles so
+/// dropping the pool kills the children; explicit `shutdown()` is
+/// the graceful path.
+pub struct WorkerPool {
+    world_size: u32,
+    workers: Vec<Worker>,
+    /// Path to the neuron binary used to launch workers — captured at
+    /// `spawn()` time via `/proc/self/exe` so the workers run the same
+    /// binary the leader is running.
+    exe: PathBuf,
+}
+
+impl WorkerPool {
+    /// Spawn `world_size - 1` worker subprocesses. Rank 0 is the
+    /// leader (in-process) and is *not* spawned here — the leader
+    /// holds rank 0's NCCL Comm and shard in its own address space.
+    ///
+    /// `binary` is the path to the neuron executable to run for each
+    /// worker (production passes `/proc/self/exe`; tests pass the
+    /// sibling-binary path from `env!("CARGO_BIN_EXE_neuron")`).
+    /// `cuda_devices` is one entry per rank including rank 0. Worker
+    /// `i` (rank `i`) gets `cuda_devices[i]` as its `--cuda-device`.
+    pub async fn spawn(binary: &Path, world_size: u32, cuda_devices: &[u32]) -> Result<Self> {
+        if world_size < 2 {
+            anyhow::bail!(
+                "WorkerPool::spawn called with world_size={world_size}; \
+                 use the single-process path for world_size < 2"
+            );
+        }
+        if cuda_devices.len() as u32 != world_size {
+            anyhow::bail!(
+                "expected {world_size} cuda_devices entries, got {}",
+                cuda_devices.len()
+            );
+        }
+        let exe = binary.to_path_buf();
+
+        let mut workers = Vec::with_capacity(world_size as usize - 1);
+        // Rank 0 stays in-process. Spawn ranks 1..world_size.
+        for rank in 1..world_size {
+            let cuda_device = cuda_devices[rank as usize];
+            let mut cmd = Command::new(&exe);
+            cmd.arg("--worker")
+                .arg("--rank")
+                .arg(rank.to_string())
+                .arg("--tp-size")
+                .arg(world_size.to_string())
+                .arg("--cuda-device")
+                .arg(cuda_device.to_string())
+                .stdin(Stdio::piped())
+                .stdout(Stdio::piped())
+                // Inherit stderr so worker tracing surfaces alongside
+                // the leader's journalctl stream.
+                .stderr(Stdio::inherit())
+                .kill_on_drop(true);
+
+            let mut child = cmd
+                .spawn()
+                .with_context(|| format!("spawn worker rank {rank}"))?;
+            let stdin = child
+                .stdin
+                .take()
+                .ok_or_else(|| anyhow::anyhow!("rank {rank}: no stdin handle"))?;
+            let stdout = child
+                .stdout
+                .take()
+                .ok_or_else(|| anyhow::anyhow!("rank {rank}: no stdout handle"))?;
+            let stdout = BufReader::new(stdout).lines();
+
+            workers.push(Worker {
+                rank,
+                cuda_device,
+                child,
+                stdin,
+                stdout,
+            });
+            tracing::info!(rank, cuda_device, "spawned tp worker");
+        }
+
+        Ok(Self {
+            world_size,
+            workers,
+            exe,
+        })
+    }
+
+    /// Ping every worker and return their Pong payloads in rank order.
+    /// Useful right after `spawn` to confirm the lifecycle plumbing is
+    /// intact before kicking off any heavier work.
+    pub async fn ping_all(&mut self) -> Result<Vec<WorkerResponse>> {
+        let mut out = Vec::with_capacity(self.workers.len());
+        for w in &mut self.workers {
+            let resp = w.request(&WorkerRequest::Ping).await?;
+            match &resp {
+                WorkerResponse::Pong { rank, .. } if *rank == w.rank => {}
+                WorkerResponse::Pong { rank, .. } => {
+                    anyhow::bail!("rank mismatch: expected {}, got {rank}", w.rank);
+                }
+                other => anyhow::bail!("expected Pong from rank {}, got {other:?}", w.rank),
+            }
+            out.push(resp);
+        }
+        Ok(out)
+    }
+
+    /// Send `Shutdown` to every worker, await each `Bye`, and reap the
+    /// children. Best-effort — individual worker failures are logged
+    /// but don't abort the rest of the sweep.
+    pub async fn shutdown(mut self) -> Result<()> {
+        for w in &mut self.workers {
+            match w.request(&WorkerRequest::Shutdown).await {
+                Ok(WorkerResponse::Bye) => {}
+                Ok(other) => tracing::warn!(
+                    rank = w.rank,
+                    response = ?other,
+                    "expected Bye on shutdown"
+                ),
+                Err(e) => tracing::warn!(rank = w.rank, error = %e, "shutdown request failed"),
+            }
+        }
+        for w in &mut self.workers {
+            match w.child.wait().await {
+                Ok(status) => tracing::info!(rank = w.rank, %status, "worker exited"),
+                Err(e) => tracing::warn!(rank = w.rank, error = %e, "wait on worker failed"),
+            }
+        }
+        Ok(())
+    }
+
+    pub fn world_size(&self) -> u32 {
+        self.world_size
+    }
+
+    pub fn binary_path(&self) -> &PathBuf {
+        &self.exe
+    }
+}
diff --git a/crates/neuron/src/harness/tp/rpc.rs b/crates/neuron/src/harness/tp/rpc.rs
new file mode 100644
index 0000000..e9fd047
--- /dev/null
+++ b/crates/neuron/src/harness/tp/rpc.rs
@@ -0,0 +1,186 @@
+//! Wire protocol between the neuron leader process and its
+//! `--worker` subprocesses.
+//!
+//! Every frame is one newline-delimited JSON object on the worker's
+//! stdin (request) or stdout (response). Both directions are tagged
+//! sum types from the start so new ops in Stage 7b/7c slot in without
+//! breaking compatibility — no "14 message types and a version field"
+//! drift later. Adding a new variant is the canonical way to evolve
+//! the protocol; existing peers that don't recognise an op return
+//! `WorkerResponse::Error { kind: "unknown_op", .. }`.
+//!
+//! The serialised shape uses `tag = "op"` so a request looks like:
+//!     {"op":"ping"}
+//!     {"op":"init","comm_id":"a1b2..."}
+//! and a response:
+//!     {"op":"pong","rank":0,"world_size":2,"cuda_device":0}
+//!     {"op":"error","kind":"nccl_init_failed","message":"..."}
+
+use serde::{Deserialize, Serialize};
+
+/// Leader → worker. Worker handles one at a time; replies with exactly
+/// one `WorkerResponse` per request.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "op", rename_all = "snake_case")]
+pub enum WorkerRequest {
+    /// Liveness probe. Worker replies with `Pong` containing its own
+    /// identity. Used by the leader to confirm the subprocess is up
+    /// and ready before kicking off any heavier work.
+    Ping,
+
+    /// One-shot NCCL communicator setup. The leader generates the
+    /// `comm_id` once (rank 0 of NCCL), broadcasts it to every worker
+    /// via this message, then every rank (leader included) calls
+    /// `Comm::from_rank` with the same id — NCCL blocks until all
+    /// `world_size` ranks check in. The hex-encoded bytes are the
+    /// canonical `cudarc::nccl::Id::internal()` content.
+    Init {
+        /// Hex-encoded NCCL id bytes (128 bytes → 256 hex chars).
+        comm_id: String,
+    },
+
+    /// Sanity check: after Init, every rank runs an `all_reduce` over
+    /// a sentinel value (`1u32`). The expected sum is `world_size`.
+    /// Worker replies with the observed value so the leader can verify
+    /// the NCCL handshake is genuinely live, not just configured.
+    NcclSanityCheck,
+
+    /// Worker should release resources and exit. Worker replies `Bye`
+    /// and then closes stdout / exits zero. The leader reaps the
+    /// child via the `tokio::process::Child` it kept.
+    Shutdown,
+}
+
+/// Worker → leader. Always exactly one of these per `WorkerRequest`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "op", rename_all = "snake_case")]
+pub enum WorkerResponse {
+    /// Reply to `Ping`. Carries enough identity for the leader to log
+    /// what it actually got back.
+    Pong {
+        rank: u32,
+        world_size: u32,
+        cuda_device: u32,
+    },
+
+    /// Reply to `Init`. Empty payload — success is the absence of
+    /// `Error`. NCCL's internal blocking handshake means by the time
+    /// this comes back, every other rank has also reached
+    /// `Comm::from_rank`.
+    InitOk,
+
+    /// Reply to `NcclSanityCheck`. The observed sum after a single
+    /// `all_reduce(SUM, 1u32)` across all ranks. The leader checks
+    /// this matches `world_size`.
+    NcclSanityResult { observed_sum: u32 },
+
+    /// Reply to `Shutdown`. Worker exits immediately after writing this.
+    Bye,
+
+    /// Any request can produce this instead of its dedicated success
+    /// variant. `kind` is a machine-readable category so the leader
+    /// can branch on failure mode without string-matching `message`.
+    Error {
+        /// Short tag — `nccl_init_failed`, `unknown_op`, etc.
+        kind: String,
+        /// Human-readable detail for logs.
+        message: String,
+    },
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn roundtrip<T>(value: &T) -> T
+    where
+        T: Serialize + for<'de> Deserialize<'de>,
+    {
+        serde_json::from_str(&serde_json::to_string(value).expect("serialise"))
+            .expect("deserialise")
+    }
+
+    #[test]
+    fn request_ping_round_trip() {
+        let req = WorkerRequest::Ping;
+        let wire = serde_json::to_string(&req).unwrap();
+        assert_eq!(wire, r#"{"op":"ping"}"#);
+        match roundtrip(&req) {
+            WorkerRequest::Ping => {}
+            other => panic!("expected Ping, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn request_init_carries_hex_id() {
+        let req = WorkerRequest::Init {
+            comm_id: "deadbeef".into(),
+        };
+        let wire = serde_json::to_string(&req).unwrap();
+        assert_eq!(wire, r#"{"op":"init","comm_id":"deadbeef"}"#);
+    }
+
+    #[test]
+    fn request_shutdown_round_trip() {
+        assert_eq!(
+            serde_json::to_string(&WorkerRequest::Shutdown).unwrap(),
+            r#"{"op":"shutdown"}"#
+        );
+    }
+
+    #[test]
+    fn response_pong_round_trip() {
+        let resp = WorkerResponse::Pong {
+            rank: 1,
+            world_size: 4,
+            cuda_device: 1,
+        };
+        let wire = serde_json::to_string(&resp).unwrap();
+        assert!(wire.contains(r#""op":"pong""#));
+        assert!(wire.contains(r#""rank":1"#));
+        assert!(wire.contains(r#""world_size":4"#));
+        match roundtrip(&resp) {
+            WorkerResponse::Pong {
+                rank,
+                world_size,
+                cuda_device,
+            } => {
+                assert_eq!(rank, 1);
+                assert_eq!(world_size, 4);
+                assert_eq!(cuda_device, 1);
+            }
+            other => panic!("expected Pong, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn response_error_carries_kind_and_message() {
+        let resp = WorkerResponse::Error {
+            kind: "nccl_init_failed".into(),
+            message: "could not bind device".into(),
+        };
+        let wire = serde_json::to_string(&resp).unwrap();
+        assert!(wire.contains(r#""op":"error""#));
+        assert!(wire.contains(r#""kind":"nccl_init_failed""#));
+    }
+
+    #[test]
+    fn response_sanity_result_round_trip() {
+        let resp = WorkerResponse::NcclSanityResult { observed_sum: 4 };
+        match roundtrip(&resp) {
+            WorkerResponse::NcclSanityResult { observed_sum } => {
+                assert_eq!(observed_sum, 4);
+            }
+            other => panic!("expected NcclSanityResult, got {other:?}"),
+        }
+    }
+
+    /// Unknown ops on the wire deserialise to an error rather than
+    /// silently mis-matching — confirms our `serde(tag = "op")`
+    /// configuration rejects unknowns instead of doing fuzzy matching.
+    #[test]
+    fn unknown_op_fails_to_parse() {
+        let result: Result<WorkerRequest, _> = serde_json::from_str(r#"{"op":"explode"}"#);
+        assert!(result.is_err(), "should reject unknown op, got {result:?}");
+    }
+}
diff --git a/crates/neuron/src/harness/tp/worker.rs b/crates/neuron/src/harness/tp/worker.rs
new file mode 100644
index 0000000..88c5fd2
--- /dev/null
+++ b/crates/neuron/src/harness/tp/worker.rs
@@ -0,0 +1,105 @@
+//! Entry point for `neuron --worker`.
+//!
+//! Stage 7a-i: bare RPC loop — `Ping` and `Shutdown` work, `Init` and
+//! `NcclSanityCheck` return `Error{kind = "not_implemented_7a_i"}`.
+//! Stage 7a-ii will replace the latter with real `cudarc::nccl` calls
+//! behind the `cuda` feature.
+//!
+//! The worker reads one newline-delimited JSON `WorkerRequest` from
+//! stdin per loop iteration, dispatches synchronously, and writes
+//! exactly one `WorkerResponse` JSON line to stdout. tracing goes to
+//! stderr so it doesn't collide with the RPC stream.
+
+use anyhow::Result;
+use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+
+use super::rpc::{WorkerRequest, WorkerResponse};
+
+#[derive(Debug, Clone, Copy)]
+pub struct WorkerConfig {
+    pub rank: u32,
+    pub world_size: u32,
+    pub cuda_device: u32,
+}
+
+/// Drive the worker RPC loop until `Shutdown` or EOF on stdin.
+pub async fn run(config: WorkerConfig) -> Result<()> {
+    tracing::info!(
+        rank = config.rank,
+        world_size = config.world_size,
+        cuda_device = config.cuda_device,
+        "tp worker starting"
+    );
+
+    let mut state = WorkerState::new(config);
+    let stdin = tokio::io::stdin();
+    let mut reader = BufReader::new(stdin).lines();
+    let mut stdout = tokio::io::stdout();
+
+    while let Some(line) = reader.next_line().await? {
+        if line.trim().is_empty() {
+            continue;
+        }
+        let req: WorkerRequest = match serde_json::from_str(&line) {
+            Ok(r) => r,
+            Err(e) => {
+                let resp = WorkerResponse::Error {
+                    kind: "bad_request".into(),
+                    message: format!("parse {line:?}: {e}"),
+                };
+                write_response(&mut stdout, &resp).await?;
+                continue;
+            }
+        };
+
+        let resp = state.handle(req).await;
+        let is_bye = matches!(resp, WorkerResponse::Bye);
+        write_response(&mut stdout, &resp).await?;
+        if is_bye {
+            break;
+        }
+    }
+
+    tracing::info!(rank = config.rank, "tp worker exiting");
+    Ok(())
+}
+
+async fn write_response(stdout: &mut tokio::io::Stdout, resp: &WorkerResponse) -> Result<()> {
+    let mut line = serde_json::to_string(resp)?;
+    line.push('\n');
+    stdout.write_all(line.as_bytes()).await?;
+    stdout.flush().await?;
+    Ok(())
+}
+
+/// Per-worker state. In Stage 7a-i this only carries the static
+/// config; 7a-ii adds an `Option<cudarc::nccl::safe::Comm>` populated
+/// by `Init`.
+struct WorkerState {
+    config: WorkerConfig,
+}
+
+impl WorkerState {
+    fn new(config: WorkerConfig) -> Self {
+        Self { config }
+    }
+
+    async fn handle(&mut self, req: WorkerRequest) -> WorkerResponse {
+        match req {
+            WorkerRequest::Ping => WorkerResponse::Pong {
+                rank: self.config.rank,
+                world_size: self.config.world_size,
+                cuda_device: self.config.cuda_device,
+            },
+            WorkerRequest::Init { comm_id: _ } => WorkerResponse::Error {
+                kind: "not_implemented_7a_i".into(),
+                message: "NCCL init lands in Stage 7a-ii (CUDA-gated)".into(),
+            },
+            WorkerRequest::NcclSanityCheck => WorkerResponse::Error {
+                kind: "not_implemented_7a_i".into(),
+                message: "NCCL sanity check lands in Stage 7a-ii (CUDA-gated)".into(),
+            },
+            WorkerRequest::Shutdown => WorkerResponse::Bye,
+        }
+    }
+}
diff --git a/crates/neuron/src/main.rs b/crates/neuron/src/main.rs
index 6a7c691..d3b2ed1 100644
--- a/crates/neuron/src/main.rs
+++ b/crates/neuron/src/main.rs
@@ -1,21 +1,52 @@
 use anyhow::Result;
 use clap::Parser;
-use neuron::{api, config::NeuronConfig, discovery, harness::HarnessRegistry, health, startup};
+use neuron::{
+    api,
+    config::NeuronConfig,
+    discovery,
+    harness::{HarnessRegistry, tp},
+    health, startup,
+};
 use std::sync::Arc;
 use std::time::Instant;
 use tokio::sync::RwLock;
 use tracing_subscriber::EnvFilter;
 
+/// Top-level CLI. The same binary runs as either the public neuron
+/// daemon (default) or a tensor-parallel worker subprocess (when
+/// `--worker` is set) spawned by the leader on the same host.
 #[derive(Parser)]
 #[command(name = "neuron")]
 #[command(about = "Per-node daemon for cortex inference clusters")]
 #[command(version)]
 struct Args {
-    /// Port to listen on (overrides config file).
+    /// Run in tensor-parallel worker mode. The leader process spawns
+    /// one of these per non-zero NCCL rank and drives it over
+    /// newline-delimited JSON on stdin/stdout. Worker mode skips
+    /// discovery, the HTTP listener, and the health poller — it's a
+    /// pure RPC loop.
+    #[arg(long, default_value_t = false)]
+    worker: bool,
+
+    /// NCCL rank for worker mode. Ignored when `--worker` is not set.
+    #[arg(long, default_value_t = 0)]
+    rank: u32,
+
+    /// Total NCCL world size for worker mode. Ignored when `--worker`
+    /// is not set.
+    #[arg(long, default_value_t = 1)]
+    tp_size: u32,
+
+    /// CUDA device index for worker mode. Ignored when `--worker` is
+    /// not set.
+    #[arg(long, default_value_t = 0)]
+    cuda_device: u32,
+
+    /// Port to listen on (overrides config file). Daemon mode only.
     #[arg(short, long)]
     port: Option<u16>,
 
-    /// Path to the neuron config file.
+    /// Path to the neuron config file. Daemon mode only.
     #[arg(short, long, default_value = "neuron.toml")]
     config: String,
 }
@@ -23,6 +54,7 @@ struct Args {
 #[tokio::main]
 async fn main() -> Result<()> {
     tracing_subscriber::fmt()
+        .with_writer(std::io::stderr)
         .with_env_filter(
             EnvFilter::try_from_default_env()
                 .unwrap_or_else(|_| EnvFilter::new("info,neuron=debug")),
@@ -31,6 +63,19 @@ async fn main() -> Result<()> {
 
     let args = Args::parse();
 
+    if args.worker {
+        return tp::worker::run(tp::worker::WorkerConfig {
+            rank: args.rank,
+            world_size: args.tp_size,
+            cuda_device: args.cuda_device,
+        })
+        .await;
+    }
+
+    daemon(args).await
+}
+
+async fn daemon(args: Args) -> Result<()> {
     let cfg = NeuronConfig::load(&args.config).unwrap_or_else(|e| {
         tracing::warn!(path = %args.config, error = %e, "config not found, using defaults");
         NeuronConfig::default()
diff --git a/crates/neuron/tests/tp_worker_lifecycle.rs b/crates/neuron/tests/tp_worker_lifecycle.rs
new file mode 100644
index 0000000..d5c004a
--- /dev/null
+++ b/crates/neuron/tests/tp_worker_lifecycle.rs
@@ -0,0 +1,130 @@
+//! Stage 7a-i: confirm the TP worker subprocess lifecycle round-trips.
+//!
+//! Spawns two worker subprocesses via the leader→worker stdio RPC,
+//! pings each, and cleanly shuts them down. No CUDA required —
+//! `Init` and `NcclSanityCheck` are stubbed in 7a-i, so this test
+//! runs on any host the workspace builds on.
+
+use neuron::harness::tp::{WorkerPool, rpc::WorkerResponse};
+
+/// Path to the neuron binary built by cargo for this test process.
+/// cargo populates `CARGO_BIN_EXE_neuron` at compile time for sibling-
+/// binary tests; production paths in main.rs use `/proc/self/exe`.
+const NEURON_BIN: &str = env!("CARGO_BIN_EXE_neuron");
+
+/// Two workers (so we spawn one subprocess: rank 0 is in-process,
+/// rank 1 is the child). Verify the spawned worker responds to Ping
+/// with its own identity, then shut it down cleanly.
+#[tokio::test]
+async fn test_spawn_ping_shutdown() {
+    // cuda_devices: rank 0 → device 0 (leader, unused here),
+    //               rank 1 → device 1 (worker; not actually opened in 7a-i).
+    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 2, &[0, 1])
+        .await
+        .expect("spawn worker pool");
+
+    let pongs = pool.ping_all().await.expect("ping all workers");
+    assert_eq!(pongs.len(), 1, "expected one Pong (rank 1 only)");
+    match &pongs[0] {
+        WorkerResponse::Pong {
+            rank,
+            world_size,
+            cuda_device,
+        } => {
+            assert_eq!(*rank, 1);
+            assert_eq!(*world_size, 2);
+            assert_eq!(*cuda_device, 1);
+        }
+        other => panic!("expected Pong, got {other:?}"),
+    }
+
+    pool.shutdown().await.expect("clean shutdown");
+}
+
+/// Three workers — exercise the loop in `ping_all` / `shutdown`.
+#[tokio::test]
+async fn test_spawn_three_workers() {
+    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 3, &[0, 1, 2])
+        .await
+        .expect("spawn worker pool");
+
+    let pongs = pool.ping_all().await.expect("ping all workers");
+    assert_eq!(pongs.len(), 2, "expected two Pongs (ranks 1 and 2)");
+    for (i, resp) in pongs.iter().enumerate() {
+        match resp {
+            WorkerResponse::Pong {
+                rank,
+                world_size,
+                cuda_device,
+            } => {
+                let expected_rank = (i + 1) as u32;
+                assert_eq!(*rank, expected_rank);
+                assert_eq!(*world_size, 3);
+                assert_eq!(*cuda_device, expected_rank);
+            }
+            other => panic!("expected Pong, got {other:?}"),
+        }
+    }
+
+    pool.shutdown().await.expect("clean shutdown");
+}
+
+/// 7a-i's Init/NcclSanityCheck handlers return an error rather than
+/// silently no-op, so the leader can tell the difference between
+/// "haven't implemented yet" and "succeeded vacuously". Confirm the
+/// shape so 7a-ii's replacement is a drop-in (same wire op names).
+#[tokio::test]
+async fn test_init_returns_not_implemented_in_7a_i() {
+    use neuron::harness::tp::rpc::WorkerRequest;
+    use std::process::Stdio;
+    use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+    use tokio::process::Command;
+
+    // Spawn a single worker by hand to send Init directly (the pool's
+    // public API doesn't expose Init yet — that lands in 7a-ii).
+    let mut child = Command::new(NEURON_BIN)
+        .arg("--worker")
+        .arg("--rank")
+        .arg("1")
+        .arg("--tp-size")
+        .arg("2")
+        .arg("--cuda-device")
+        .arg("1")
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .kill_on_drop(true)
+        .spawn()
+        .expect("spawn worker");
+
+    let mut stdin = child.stdin.take().expect("stdin");
+    let stdout = child.stdout.take().expect("stdout");
+    let mut lines = BufReader::new(stdout).lines();
+
+    let req = WorkerRequest::Init {
+        comm_id: "ff".repeat(128),
+    };
+    let mut payload = serde_json::to_string(&req).unwrap();
+    payload.push('\n');
+    stdin.write_all(payload.as_bytes()).await.unwrap();
+    stdin.flush().await.unwrap();
+
+    let reply = lines
+        .next_line()
+        .await
+        .expect("read line")
+        .expect("got line");
+    let resp: WorkerResponse = serde_json::from_str(&reply).expect("parse reply");
+    match resp {
+        WorkerResponse::Error { kind, .. } => {
+            assert_eq!(kind, "not_implemented_7a_i");
+        }
+        other => panic!("expected Error{{kind=not_implemented_7a_i}}, got {other:?}"),
+    }
+
+    // Clean shutdown.
+    stdin.write_all(b"{\"op\":\"shutdown\"}\n").await.unwrap();
+    stdin.flush().await.unwrap();
+    let _ = lines.next_line().await; // Bye
+    let _ = child.wait().await;
+}