From 2252c55df8e4db7b40d771e4f242ddcb3024e5c0 Mon Sep 17 00:00:00 2001
From: atlas <atlas@hyperhive.local>
Date: Sun, 27 Sep 2026 04:51:07 +0200
Subject: [PATCH] hive-priv: create agent socket dirs on start; drop
 hyperhive-agents.conf

/etc/tmpfiles.d/hyperhive-agents.conf was a boot-time backstop (#2290)
that pre-created every agent's bind sources. The start preamble already
creates them for every c0re-driven start, and on this host only hive-c0re
starts agent containers. The file was also the reason the socket dir's
owner had to be declared there, which is how it spent its life at
`0777 root root` whenever the uid could not be resolved (#4742).

- hive-priv gains `EnsureAgentSocketDir { name }`, called from
  `set_nspawn_flags` in every start path. It creates
  `/run/hive-agent/<name>` `0751 root:root` with mkdirat relative to an
  O_DIRECTORY|O_NOFOLLOW fd for the parent. An existing entry has to be a
  directory (fstatat AT_SYMLINK_NOFOLLOW); anything else is refused, and a
  directory is left alone. hive-c0re's own create_dir_all went: its /run
  is read-only under ProtectSystem=strict.
- The container's `hive-agent-user-migrate` activation chowns that dir to
  the agent user and sets 0751, the same way it already handles state/ and
  harness/. It refuses a symlink or non-directory there, since `test -d`
  and chmod follow links. No host-side passwd parse, and no window where
  the dir is world-writable.
- `/run/hyperhive/agents/<name>` stays created by hive-c0re itself
  (`ensure_agent_runtime_dir`). It holds the `mcp.sock` that hive-c0re
  binds as hive-core, so it must not become root- or agent-owned.
- The `/run/hive-agent` parent is declared in hive-priv.nix, `0755
  root:root`, instead of hive-gateway's hive-core rule. hive-priv is its
  only writer now, and hive-priv's ReadWritePaths needs it to exist.
- The manager start in `ensure_root_agent` now goes through
  `converge_start_preamble` + `start_with_fallback`. It was a bare start,
  so after a reboot the manager's bind sources existed only because of the
  tmpfiles file, and its limits drop-in did not exist at all.
- Removed: `sync_tmpfiles`, `agent_uid_gid` / `parse_passwd_uid_gid`,
  `priv_client::sync_agent_tmpfiles`, `AgentTmpfilesEntry`, the tmpfiles
  body builder and their tests, plus the three call sites.
- Legacy: hive-priv unlinks the file at every start, ignoring ENOENT.
  `SyncAgentTmpfiles` stays one release as a payload-ignoring variant that
  does the same unlink and returns Ok, for an older hive-c0re.

Salvaged from #4752: the boundary.md correction that nginx only dials,
because ProtectSystem=strict makes its /run read-only.

Behaviour change: a manual `nixos-container start h-<name>` right after a
reboot, before hive-c0re has started that agent, now fails on a missing
bind source instead of starting.

Closes #4742
---
 docs/agent-lifecycle/persistence.md       |   5 +
 docs/networking/gateway.md                |   4 +-
 docs/scheduler/coordinator.md             |   2 +-
 docs/trust-boundary/boundary.md           |  23 +-
 docs/trust-boundary/security.md           |   3 +-
 hive-c0re/src/job_queue/exec.rs           |   3 -
 hive-c0re/src/lifecycle/host_config.rs    |  11 +-
 hive-c0re/src/lifecycle/mod.rs            | 118 +------
 hive-c0re/src/lifecycle/tests.rs          |  44 ---
 hive-c0re/src/main.rs                     |   4 -
 hive-c0re/src/priv_client.rs              |  28 +-
 hive-c0re/src/server.rs                   |   2 -
 hive-c0re/src/workers/auto_update.rs      |  14 +-
 hive-priv-sock/src/lib.rs                 |  58 ++--
 hive-priv/src/main.rs                     | 379 +++++++++++++---------
 nix/agent-modules/user.nix                |  14 +
 nix/checks.nix                            |   4 +
 nix/host-modules/hive-c0re/default.nix    |   6 -
 nix/host-modules/hive-gateway/default.nix |  11 -
 nix/host-modules/hive-priv.nix            |  10 +-
 nix/module-eval/agent-user.nix            |  49 +++
 21 files changed, 370 insertions(+), 422 deletions(-)
 create mode 100644 nix/module-eval/agent-user.nix

diff --git a/docs/agent-lifecycle/persistence.md b/docs/agent-lifecycle/persistence.md
index 3f64dd44..af74b3ec 100644
--- a/docs/agent-lifecycle/persistence.md
+++ b/docs/agent-lifecycle/persistence.md
@@ -551,6 +551,11 @@ container lifetime:
    Online vs NeedsLogin in `login::has_session`. Without the
    chown the existing credentials get silently treated as "no
    session" and the operator re-prompts every boot.
+5. **Hand the socket dir `/run/hive-agent/<name>` to the agent
+   user**, `0751`, not recursive. hive-priv creates it `0751 root`
+   on the host before every start; the harness binds its sockets
+   there as the agent user. Why the mode matters:
+   [`boundary.md`](../trust-boundary/boundary.md#the-per-agent-socket-dir).
 
 The activation script will eventually become unnecessary once no
 operators have legacy root-owned state dirs left to migrate; drop
diff --git a/docs/networking/gateway.md b/docs/networking/gateway.md
index 847f3bd4..f1f4001b 100644
--- a/docs/networking/gateway.md
+++ b/docs/networking/gateway.md
@@ -108,7 +108,9 @@ now set unconditionally for every agent. The mechanism:
    harness's `unlink + bind(2)` cycle on socket replace. Per-agent
    subdir keeps each agent's container blind to siblings' sockets.
 
-   The dir is `0751`, owned by the agent's container uid/gid, so
+   The dir is `0751`, owned by the agent's container uid/gid (hive-priv
+   creates it `0751 root` before each start when missing, and the
+   container's activation hands it to the agent user), so
    nginx reaches `web.sock` through `o=--x` (traverse) and the socket's
    own `0666`. The gateway is one of three principals sharing that dir
    and doesn't own its ownership rules — see
diff --git a/docs/scheduler/coordinator.md b/docs/scheduler/coordinator.md
index b5265d10..0a61cb7c 100644
--- a/docs/scheduler/coordinator.md
+++ b/docs/scheduler/coordinator.md
@@ -73,7 +73,7 @@ Cheap — no build slot:
 | `PauseDrain`         | await the harness reporting `PauseAcknowledged`, bounded timeout; best-effort like `Drain`                                                                                                                                                                                                                                                                                                                                                                                              |
 | `DestroyContainer`   | `nixos-container destroy` + un-registration (drop from the roster, clear the ephemeral runtime dir). Runs downstream of a `Stop`, so deliberately excluded from `takes_container_down` — the container is already down by the time it claims                                                                                                                                                                                                                                            |
 | `PurgeState`         | the `purge = true` half of a destroy: delete the agent's state subvolume (via hive-priv) plus its state/applied dirs. Own node because it's conditional and the irreversible step                                                                                                                                                                                                                                                                                                       |
-| `DestroyBookkeeping` | the post-destroy tail — meta sync, fail pending approvals, drop the power intent, notify the manager, rescan, re-emit the tombstone, resync tmpfiles. Same split rationale as `RebuildBookkeeping`/`Swap`. Its `purge` flag only selects the wording of the approval-failure reason and the manager notification — the destructive work is `PurgeState`'s                                                                                                                               |
+| `DestroyBookkeeping` | the post-destroy tail — meta sync, fail pending approvals, drop the power intent, notify the manager, rescan, re-emit the tombstone. Same split rationale as `RebuildBookkeeping`/`Swap`. Its `purge` flag only selects the wording of the approval-failure reason and the manager notification — the destructive work is `PurgeState`'s                                                                                                                                                |
 | `SetWanted`          | write the durable power intent (`wanted = Up`/`Offline`) as the head node of a power-op DAG, replacing the old pre-submit side effect. Takes the agent lease even though it's a store write, so the intent write and the tail `Reconcile` are atomic per-agent — two racing power ops can't clobber each other's intent before either reconciles                                                                                                                                        |
 | `FinalizeDeploy`     | deploy phase 3 — drop the rollback ref, plant `deployed/<id>`, commit the staged `flake.lock`. The first two git steps are fatal on purpose, so a confirmed-good deploy's outcome and the repo's state can't disagree                                                                                                                                                                                                                                                                   |
 | `ResolveApproval`    | tail of an approval-carrying DAG — resolve the approval row from how the work ended (`AfterAny`, one node emitted per outcome). Agentless: the approval row already names its agent                                                                                                                                                                                                                                                                                                     |
diff --git a/docs/trust-boundary/boundary.md b/docs/trust-boundary/boundary.md
index ce99a0b6..5c74302d 100644
--- a/docs/trust-boundary/boundary.md
+++ b/docs/trust-boundary/boundary.md
@@ -115,12 +115,15 @@ both sockets are `0666`, which is all a dialer needs.
 
 <!-- vale write-good.Passive = NO -->
 
-**Ownership is declared, not repaired.** The tmpfiles.d entry written by
-`SyncAgentTmpfiles` names the uid/gid directly. Don't add a chown
-alongside it: `d` re-applies on every boot _and_ every agent
-spawn/destroy, and reverts any ownership set afterwards the next time
-any agent changes — which is exactly how this dir spent a long time at
-`0777 root root` while a privileged chown appeared to be fixing it.
+**One mechanism creates it, one sets its owner.** Before every start,
+hive-priv creates the dir when missing (`EnsureAgentSocketDir`, `0751
+root:root`, no symlink followed) and leaves an existing one alone. The
+container's own activation (`hive-agent-user-migrate`) then chowns it to
+the agent user and sets `0751`. The container has no user namespace, so
+that uid is the host inode's owner. Don't add a host-side chown or chmod:
+two owners of one path revert each other. Until the container activates,
+the dir is `0751 root`: nothing but root can plant a socket in it, and a
+legacy root-run harness can still bind.
 
 <!-- vale write-good.Passive = YES -->
 
@@ -128,9 +131,11 @@ The mode is load-bearing, not cosmetic. Write permission on a
 _directory_ is what confers the right to unlink its entries, whoever owns
 them, and the sticky bit is the only thing that would restrain that (it
 isn't set here). A world-writable socket dir therefore lets anything
-able to reach the path delete an agent's socket and bind its own — and
-nginx reaches all of `/run/hive-agent` as a plain host path. Dropping
-`o=w` removes that permission rather than qualifying it.
+able to reach the path delete an agent's socket and bind its own. On the
+host, any non-root process whose `/run` is writable can do that, such as
+a login session or dnsmasq. nginx only dials: `ProtectSystem=strict` makes its view
+of `/run` read-only. Dropping `o=w` removes that permission rather than
+qualifying it.
 
 ⚠️ **The gateway's nginx and dnsmasq are host services, next to
 `hive-c0re`** (see `docs/networking/gateway.md`) — there is no namespace between
diff --git a/docs/trust-boundary/security.md b/docs/trust-boundary/security.md
index ca8fe746..6fc974b8 100644
--- a/docs/trust-boundary/security.md
+++ b/docs/trust-boundary/security.md
@@ -290,6 +290,7 @@ known operations; there is no arbitrary command pass-through:
 | `ReadContainerJournal`                                         | `journalctl -M <container> -n <n> [filters...]`                                                                                                                                                                                                                                                                                           |
 | `ReloadGatewayNginx`                                           | `systemctl reload/start/reset-failed nginx` (host unit; the unit name is hard-coded, not a parameter)                                                                                                                                                                                                                                     |
 | `WriteNspawnFlags`                                             | write `/etc/nixos-containers/<container>.conf` (bind-mount list + network isolation vars)                                                                                                                                                                                                                                                 |
+| `EnsureAgentSocketDir`                                         | `mkdirat` `/run/hive-agent/<name>` `0751 root:root` under an `O_NOFOLLOW` parent fd; leaves an existing directory alone, refuses anything else                                                                                                                                                                                            |
 | `WriteResourceLimits`                                          | write `CPUQuota=`/`MemoryMax=`/`CPUWeight=`/`IOWeight=` systemd drop-in for agent container                                                                                                                                                                                                                                               |
 | `RemoveServiceDropin`                                          | remove `container@<name>.service.d/` drop-in on destroy                                                                                                                                                                                                                                                                                   |
 | `DaemonReload`                                                 | `systemctl daemon-reload`                                                                                                                                                                                                                                                                                                                 |
@@ -297,7 +298,7 @@ known operations; there is no arbitrary command pass-through:
 | `WriteAgentMatrixToken`                                        | write `0600` credential file into agent state dir                                                                                                                                                                                                                                                                                         |
 | `RestartMatrixDaemon`                                          | `systemctl --machine=h-<name> restart hive-matrix-daemon.service`                                                                                                                                                                                                                                                                         |
 | `ControlInfraContainer`                                        | `systemctl <action> container@<container>.service` — the `InfraContainer` enum is the allowlist, and serde rejects unknown names at the wire boundary (`hive-c0re` has no variant, so no request can name it)                                                                                                                             |
-| `SyncAgentTmpfiles`                                            | write `/etc/tmpfiles.d/hyperhive-agents.conf` for the agent set, then `systemd-tmpfiles --create`                                                                                                                                                                                                                                         |
+| `SyncAgentTmpfiles`                                            | legacy: unlink `/etc/tmpfiles.d/hyperhive-agents.conf` and return `Ok`; kept one release for an older hive-c0re                                                                                                                                                                                                                           |
 | `SetAgentPaused`                                               | create / remove the `<state>/<name>/harness/paused` marker that parks an agent's turn loop                                                                                                                                                                                                                                                |
 | `WriteAgentGithubToken`                                        | write `0600` `github-token` into agent state dir (same semantics as the forge/matrix token writes)                                                                                                                                                                                                                                        |
 | `WriteAgentExtraForgeAccount` / `DeleteAgentExtraForgeAccount` | write / remove `forge-<label>-token` + a `forge-<label>.json` base-URL sidecar, both `0600`. hive-priv validates `label` as a plain identifier before it reaches the filename — an unchecked one traverses out of the state dir                                                                                                           |
diff --git a/hive-c0re/src/job_queue/exec.rs b/hive-c0re/src/job_queue/exec.rs
index 6bf0bdde..3c24fba5 100644
--- a/hive-c0re/src/job_queue/exec.rs
+++ b/hive-c0re/src/job_queue/exec.rs
@@ -347,9 +347,6 @@ async fn run_destroy_bookkeeping(coord: &Arc<Coordinator>, agent: &str, purge: b
     // roster, so any schedule that still targets the just-destroyed agent now
     // drops that ghost column live (no page reload needed).
     coord.emit_schedules_snapshot();
-    // Update tmpfiles.d to remove the destroyed agent's dirs from the boot-time
-    // pre-creation list. Best-effort: failure is logged only.
-    tokio::spawn(crate::lifecycle::sync_tmpfiles());
 }
 
 /// Emit this agent's rebuild-complete todo. `ok` is not computed — it is which
diff --git a/hive-c0re/src/lifecycle/host_config.rs b/hive-c0re/src/lifecycle/host_config.rs
index cdb855bf..fef9f98f 100644
--- a/hive-c0re/src/lifecycle/host_config.rs
+++ b/hive-c0re/src/lifecycle/host_config.rs
@@ -445,14 +445,11 @@ async fn set_nspawn_flags(
     // gateway sees it. Subdir bind (not socket file) keeps the inode
     // visible after the harness unlinks a stale socket on rebind.
     // Applies to manager and sub-agents alike.
+    // hive-priv creates it (`/run` is read-only to hive-c0re) as `0751 root`
+    // and leaves an existing one alone; the container's activation hands it
+    // to the agent user. Don't chown or chmod it here.
     let socket_dir = crate::agent_sockets::agent_dir_for(agent_name);
-    std::fs::create_dir_all(&socket_dir)
-        .with_context(|| format!("create {}", socket_dir.display()))?;
-    // Ownership is NOT repaired here. The dir's owner + mode are declared by
-    // the tmpfiles.d entry (`SyncAgentTmpfiles`), which is the mechanism that
-    // re-applies on every boot and every spawn — so a chown made here was
-    // silently reverted the next time any agent was spawned or destroyed.
-    // This `create_dir_all` only covers the window before that sync lands.
+    crate::priv_client::ensure_agent_socket_dir(agent_name).await?;
     binds.push(BindMount {
         host_path: socket_dir.to_string_lossy().into_owned(),
         container_path: socket_dir.to_string_lossy().into_owned(),
diff --git a/hive-c0re/src/lifecycle/mod.rs b/hive-c0re/src/lifecycle/mod.rs
index 6843e732..11ab3993 100644
--- a/hive-c0re/src/lifecycle/mod.rs
+++ b/hive-c0re/src/lifecycle/mod.rs
@@ -178,79 +178,6 @@ pub fn network_isolation_from_env() -> Result<hive_priv_sock::NetworkIsolation>
     network_isolation_from_vars(bridge.as_deref(), subnet.as_deref())
 }
 
-/// Read the agent user's `(uid, gid)` from the container's nixos-managed
-/// `/etc/passwd`. Returns `None` when the passwd file cannot be read (not
-/// built yet, or this process cannot reach the path), or when it is read
-/// but holds no usable entry for the agent (missing user, e.g. a legacy
-/// container that still runs as root).
-///
-/// Used by `forge` + `matrix` after writing per-agent state files so
-/// the bind-mounted host file ends up readable by the agent user
-/// without waiting for the next container activation to run the chown
-/// fixup.
-///
-/// Notes:
-///   - Reads the *container-local* passwd at
-///     `/var/lib/nixos-containers/<container>/etc/passwd`, not the host's.
-///     The container's user-namespace shares uids with the host (no
-///     `PrivateUsers`), so the uid is directly usable in host-side
-///     `chown(2)`.
-///   - Best-effort: caller treats `None` as "skip the chown".
-///   - ⚠️ Every `None` is logged with which of those cases produced it: the
-///     tmpfiles caller answers `None` by declaring the agent's socket dir
-///     `0777`, and a fallback that widens a directory has to say why it fired.
-#[must_use]
-pub fn agent_uid_gid(agent_name: &str) -> Option<(u32, u32)> {
-    let container = container_name(agent_name);
-    let passwd_path = format!("/var/lib/nixos-containers/{container}/etc/passwd");
-    let content = match std::fs::read_to_string(&passwd_path) {
-        Ok(content) => content,
-        Err(e) => {
-            tracing::warn!(
-                agent = %agent_name,
-                path = %passwd_path,
-                error = ?e,
-                "agent_uid_gid: cannot read the container's passwd"
-            );
-            return None;
-        }
-    };
-    let ids = parse_passwd_uid_gid(&content, agent_name);
-    if ids.is_none() {
-        tracing::warn!(
-            agent = %agent_name,
-            path = %passwd_path,
-            lines = content.lines().count(),
-            "agent_uid_gid: passwd read but no usable entry for this agent"
-        );
-    }
-    ids
-}
-
-/// Find `user`'s `(uid, gid)` in a `passwd(5)` body.
-///
-/// Separate from the read so it can be tested at all — the caller's half is a
-/// host path no test can stand up.
-fn parse_passwd_uid_gid(content: &str, user: &str) -> Option<(u32, u32)> {
-    for line in content.lines() {
-        let mut parts = line.split(':');
-        // Skip, never abort: an unusable row for this same user must not hide
-        // a usable one below it.
-        if parts.next() != Some(user) {
-            continue;
-        }
-        let _password = parts.next();
-        let (Some(uid), Some(gid)) = (parts.next(), parts.next()) else {
-            continue;
-        };
-        let (Ok(uid), Ok(gid)) = (uid.parse(), gid.parse()) else {
-            continue;
-        };
-        return Some((uid, gid));
-    }
-    None
-}
-
 fn validate(name: &str) -> Result<()> {
     if name.is_empty() {
         bail!("agent name must not be empty");
@@ -509,7 +436,7 @@ pub async fn kill(name: &str) -> Result<()> {
 /// exit polls the unit state for [`START_SETTLE_TIMEOUT`] before concluding
 /// the start actually failed. Every caller (dashboard restart, reconcile
 /// start, the cold-start fallback) gets this truth for free.
-pub async fn start(name: &str) -> Result<()> {
+async fn start(name: &str) -> Result<()> {
     validate(name)?;
     if priv_run("start", name).await.is_ok() {
         return Ok(());
@@ -900,48 +827,9 @@ pub fn agent_names(listed: Result<Vec<String>>) -> Result<Vec<String>> {
         .collect())
 }
 
-/// Sync `/etc/tmpfiles.d/hyperhive-agents.conf` with the currently-known
-/// agent set (from `nixos-container list`). Strips the `h-` prefix to get
-/// logical names. Best-effort: errors are logged but never propagated — a
-/// failed tmpfiles write shouldn't block a spawn or destroy.
-///
-/// Called at hive-c0re startup and after each spawn / destroy so the file
-/// always reflects the live agent set. `systemd-tmpfiles-setup.service`
-/// reads the file at boot (before any container units start), pre-creating
-/// bind-mount source dirs so container@h-* units don't race hive-c0re.
-pub async fn sync_tmpfiles() {
-    let agents = match list().await {
-        Ok(containers) => containers
-            .into_iter()
-            .filter_map(|c| c.strip_prefix(AGENT_PREFIX).map(str::to_owned))
-            .map(|name| {
-                // Resolved here, not in hive-priv: the mapping lives in the
-                // container's /etc/passwd, which is c0re's to read. `None`
-                // until the container's first boot renders it.
-                let (uid, gid) = match agent_uid_gid(&name) {
-                    Some((uid, gid)) => (Some(uid), Some(gid)),
-                    None => (None, None),
-                };
-                hive_priv_sock::AgentTmpfilesEntry { name, uid, gid }
-            })
-            .collect::<Vec<_>>(),
-        Err(e) => {
-            tracing::warn!(error = ?e, "sync_tmpfiles: list failed; skipping");
-            return;
-        }
-    };
-    if let Err(e) = crate::priv_client::sync_agent_tmpfiles(&agents).await {
-        tracing::warn!(error = ?e, "sync_tmpfiles: priv call failed");
-    } else {
-        tracing::debug!(count = agents.len(), "sync_tmpfiles: ok");
-    }
-}
-
 /// Ensure the per-agent runtime directory `/run/hyperhive/agents/<name>`
-/// exists. The directory is also written by `SyncAgentTmpfiles` (run at
-/// boot + spawn/destroy), but explicit creation in start/spawn paths guards
-/// against races where hive-c0re starts a container before tmpfiles.d has
-/// applied the new entry.
+/// exists. It is a bind source and `/run` is a tmpfs, so every start path
+/// creates it before `nixos-container start`.
 ///
 /// Pure filesystem op — no `Coordinator` dependency — so callers that only
 /// need the dir do not have to hold an `Arc<Coordinator>`.
diff --git a/hive-c0re/src/lifecycle/tests.rs b/hive-c0re/src/lifecycle/tests.rs
index 64fdfe48..7114c330 100644
--- a/hive-c0re/src/lifecycle/tests.rs
+++ b/hive-c0re/src/lifecycle/tests.rs
@@ -283,50 +283,6 @@ fn an_unknown_state_is_never_reported_as_failed() {
     }
 }
 
-/// A malformed row for the wanted user must not end the scan.
-///
-/// The old implementation used `?` on the field reads, which are only reached
-/// once the name matches — so a short or unparseable row *for this agent*
-/// returned `None` from the whole function and hid a usable entry below it.
-/// The caller answers that `None` by declaring the socket dir `0777`.
-#[test]
-fn passwd_parse_keeps_scanning_past_a_malformed_row_for_the_same_user() {
-    // The two `atlas` rows above the real one are what matters: a malformed
-    // row for the SAME user is what used to end the scan, so the usable entry
-    // below it was never reached. Rows for other users were always skipped
-    // fine. Both unusable shapes are here because they leave the parser by
-    // different arms — missing fields, and fields that will not parse — and a
-    // mutation run showed the second arm untested when only the first was.
-    let body = "\
-root:x:0:0:System administrator:/root:/bin/sh
-# a comment line, not a passwd entry
-atlas:x
-atlas:x:notanumber:994::/:/bin/sh
-atlas:x:1000:994:atlas:/home/atlas:/bin/sh
-";
-    assert_eq!(parse_passwd_uid_gid(body, "atlas"), Some((1000, 994)));
-    // Control: the same body must NOT answer for a user it does not carry,
-    // or the assertion above is satisfied by any parse at all.
-    assert_eq!(parse_passwd_uid_gid(body, "nobody-here"), None);
-}
-
-/// A matching name with unusable ids is not a match. Without this the parser
-/// could return a partly-parsed row and the caller would chown to it.
-#[test]
-fn passwd_parse_rejects_unusable_ids() {
-    assert_eq!(
-        parse_passwd_uid_gid("atlas:x:notanumber:994::/:/bin/sh", "atlas"),
-        None
-    );
-    assert_eq!(parse_passwd_uid_gid("atlas:x:1000", "atlas"), None);
-    assert_eq!(parse_passwd_uid_gid("", "atlas"), None);
-    // Presence control for the three absences above.
-    assert_eq!(
-        parse_passwd_uid_gid("atlas:x:1000:994::/:/bin/sh", "atlas"),
-        Some((1000, 994))
-    );
-}
-
 /// A failed `nixos-container destroy` whose container is still listed fails,
 /// and keeps hive-priv's error in the chain. The destroy DAG gates the state
 /// purge on this call, so this is the case that must not read as success.
diff --git a/hive-c0re/src/main.rs b/hive-c0re/src/main.rs
index 8855946a..bba09717 100644
--- a/hive-c0re/src/main.rs
+++ b/hive-c0re/src/main.rs
@@ -287,10 +287,6 @@ async fn cmd_serve(
     if let Err(e) = auto_update::ensure_root_agent(&coord).await {
         tracing::warn!(error = ?e, "auto-spawn root agent failed");
     }
-    // Sync /etc/tmpfiles.d/hyperhive-agents.conf so agent runtime dirs are
-    // pre-declared for the next boot. Best-effort background task — a failure
-    // here must not block hive-c0re startup. See lifecycle::sync_tmpfiles.
-    tokio::spawn(crate::lifecycle::sync_tmpfiles());
     // Auto-update in the background — don't block service start.
     // Sub-agent rebuilds can take tens of seconds; we want the admin
     // socket up immediately.
diff --git a/hive-c0re/src/priv_client.rs b/hive-c0re/src/priv_client.rs
index 4ef99842..9311250b 100644
--- a/hive-c0re/src/priv_client.rs
+++ b/hive-c0re/src/priv_client.rs
@@ -8,8 +8,8 @@
 
 use anyhow::{Context as _, Result, bail};
 use hive_priv_sock::{
-    AgentTmpfilesEntry, BindMount, CredentialMount, InfraAction, InfraContainer, JournalQuery,
-    NetworkIsolation, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
+    BindMount, CredentialMount, InfraAction, InfraContainer, JournalQuery, NetworkIsolation,
+    PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
 };
 use std::os::fd::{AsRawFd as _, OwnedFd, RawFd};
 
@@ -306,6 +306,14 @@ pub async fn write_nspawn_flags(
     .await?)
 }
 
+/// See [`PrivRequest::EnsureAgentSocketDir`].
+pub async fn ensure_agent_socket_dir(name: &str) -> Result<()> {
+    ok(call(&PrivRequest::EnsureAgentSocketDir {
+        name: name.to_owned(),
+    })
+    .await?)
+}
+
 pub async fn write_resource_limits(
     container: &str,
     memory_max: &str,
@@ -616,22 +624,6 @@ pub async fn send_agent_snapshot_to_file(
     Ok(stdout)
 }
 
-/// Write `/etc/tmpfiles.d/hyperhive-agents.conf` for `agents` and immediately
-/// apply it with `systemd-tmpfiles --create`. Each entry carries the agent's
-/// container uid/gid so the socket dir's ownership is *declared* here rather
-/// than corrected afterwards. See [`PrivRequest::SyncAgentTmpfiles`].
-///
-/// # Errors
-///
-/// Returns an error if the priv socket call fails, if any agent name is
-/// invalid, or if `systemd-tmpfiles --create` exits non-zero.
-pub async fn sync_agent_tmpfiles(agents: &[AgentTmpfilesEntry]) -> Result<()> {
-    ok(call(&PrivRequest::SyncAgentTmpfiles {
-        agents: agents.to_vec(),
-    })
-    .await?)
-}
-
 /// Parse `(referenced, exclusive)` bytes from `btrfs qgroup show -f --raw`
 /// output (a qgroup row is `<id-with-slash> <rfer> <excl> …`).
 ///
diff --git a/hive-c0re/src/server.rs b/hive-c0re/src/server.rs
index f5faad0c..570dfb83 100644
--- a/hive-c0re/src/server.rs
+++ b/hive-c0re/src/server.rs
@@ -313,8 +313,6 @@ async fn handle_spawn(coord: &Arc<Coordinator>, name: &str) -> Result<HostRespon
                 None,
             )
             .await;
-            // Update tmpfiles.d so the new agent's dirs survive a reboot.
-            tokio::spawn(lifecycle::sync_tmpfiles());
         }
         Err(e) => {
             // Spawn failed: register_agent was never called, so there is
diff --git a/hive-c0re/src/workers/auto_update.rs b/hive-c0re/src/workers/auto_update.rs
index f5422401..ab743bed 100644
--- a/hive-c0re/src/workers/auto_update.rs
+++ b/hive-c0re/src/workers/auto_update.rs
@@ -179,7 +179,19 @@ pub async fn ensure_root_agent(coord: &Arc<Coordinator>) -> Result<()> {
             if let Err(e) = coord.power.set(MANAGER_NAME, crate::power::Wanted::Up) {
                 tracing::warn!(error = ?e, "agent_power: set manager wanted=up failed");
             }
-            if let Err(e) = lifecycle::start(MANAGER_NAME).await {
+            // Through the preamble: after a reboot its bind sources and limits
+            // drop-in in `/run` are gone, and nothing else recreates them.
+            let hive = coord.hive_env();
+            let started = async {
+                let paths = Coordinator::agent_paths(
+                    MANAGER_NAME,
+                    crate::paths::agent_runtime_dir(MANAGER_NAME),
+                )?;
+                let token = lifecycle::converge_start_preamble(MANAGER_NAME, &hive, &paths).await?;
+                lifecycle::start_with_fallback(token).await
+            }
+            .await;
+            if let Err(e) = started {
                 tracing::warn!(error = ?e, "manager start failed");
             }
         }
diff --git a/hive-priv-sock/src/lib.rs b/hive-priv-sock/src/lib.rs
index 8fa577c4..da174db2 100644
--- a/hive-priv-sock/src/lib.rs
+++ b/hive-priv-sock/src/lib.rs
@@ -174,9 +174,10 @@ pub const META_DIR: &str = "/var/lib/hyperhive/meta";
 /// boundary prevents importing across the crate.
 pub const AGENT_STATE_ROOT: &str = "/var/lib/hyperhive/agents";
 
-/// Root of per-agent runtime directories on the host (regenerated each boot
-/// by `hive-priv` tmpfiles.d; not persistent). Used by `hive-priv` when
-/// creating per-agent subdirs via `nsenter` / tmpfiles.
+/// Root of per-agent runtime directories on the host (tmpfs, not
+/// persistent). hive-c0re creates each `<name>` subdir itself in the start
+/// preamble; `hive-priv` only names it in the limits drop-in's
+/// `ConditionPathIsDirectory=`.
 /// Must stay in sync with `hive-c0re::paths::agent_runtime_root()`
 /// (`RUNTIME_ROOT + "/agents"`); the privsep boundary prevents importing
 /// across the crate.
@@ -382,6 +383,16 @@ pub enum PrivRequest {
         load_credentials: Vec<CredentialMount>,
     },
 
+    /// Create the agent's socket dir `/run/hive-agent/<name>` as `0751
+    /// root:root` if it is missing; an existing directory is left as it is.
+    /// It is a bind source, so it has to exist before every container start,
+    /// and `/run` is a tmpfs. The container's own activation hands it to the
+    /// agent user. Called from `lifecycle::set_nspawn_flags`.
+    EnsureAgentSocketDir {
+        /// Logical agent name (validated by `validate_agent_name`).
+        name: String,
+    },
+
     /// Write `/run/systemd/system/container@<container>.service.d/hyperhive-limits.conf`
     /// with `[Service]` carrying `MemoryMax=` / `CPUQuota=` (hard caps) and
     /// `CPUWeight=` / `IOWeight=` (cgroup v2 relative shares, contention-only).
@@ -773,42 +784,11 @@ pub enum PrivRequest {
         parent_snapshot_name: Option<String>,
     },
 
-    /// Write `/etc/tmpfiles.d/hyperhive-agents.conf` for the given agent set
-    /// and immediately apply it with `systemd-tmpfiles --create`. Each entry
-    /// declares the per-agent runtime dirs (`/run/hyperhive/agents/<name>` and
-    /// `/run/hive-agent/<name>`) so systemd recreates them at every boot before
-    /// any container units start — preventing bind-mount source missing errors
-    /// when container@h-* units race hive-c0re after a reboot.
-    ///
-    /// Called at hive-c0re startup and after every agent spawn / destroy.
-    /// Agents are logical names (validated by `validate_agent_name`).
-    SyncAgentTmpfiles {
-        /// One entry per live agent. hive-priv validates each name before
-        /// writing any path component derived from it.
-        agents: Vec<AgentTmpfilesEntry>,
-    },
-}
-
-/// One agent's runtime-dir declaration for `SyncAgentTmpfiles`.
-///
-/// Carries the container uid/gid so the tmpfiles entry can *declare* who owns
-/// `/run/hive-agent/<name>` instead of having it corrected afterwards by a
-/// privileged chown. The two mechanisms used to fight: the tmpfiles line wrote
-/// `0777 root root` and a follow-up `ChownSocketDir` narrowed it, but any
-/// later spawn or destroy re-applied the file and reset *every* agent's dir
-/// back to world-writable.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct AgentTmpfilesEntry {
-    /// Logical agent name (e.g. `"atlas"`, `"ruth"`).
-    pub name: String,
-    /// Container uid/gid of the agent user, when known.
-    ///
-    /// `None` only before the container's `/etc/passwd` has been rendered
-    /// (first boot). The dir must stay writable by the not-yet-identifiable
-    /// harness in that window, so hive-priv falls back to the historical
-    /// permissive mode for that one agent; the next sync tightens it.
-    pub uid: Option<u32>,
-    pub gid: Option<u32>,
+    /// Legacy: an older hive-c0re still sends this. hive-priv only unlinks
+    /// `/etc/tmpfiles.d/hyperhive-agents.conf` and returns `Ok`; the `agents`
+    /// payload that request carries is ignored on decode. Remove once no
+    /// deployed hive-c0re sends it, one release after `EnsureAgentSocketDir`.
+    SyncAgentTmpfiles,
 }
 
 /// Response from the privileged helper.
diff --git a/hive-priv/src/main.rs b/hive-priv/src/main.rs
index 66a15987..c978f4b2 100644
--- a/hive-priv/src/main.rs
+++ b/hive-priv/src/main.rs
@@ -22,10 +22,10 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{Context as _, Result, anyhow, bail};
 use hive_priv_sock::{
-    AGENT_PREFIX, AGENT_RUNTIME_ROOT, AGENT_STATE_ROOT, AgentTmpfilesEntry, BindMount,
-    CredentialMount, InfraAction, InfraContainer, JournalQuery, META_DIR, MIGRATE_STAGING_ROOT,
-    NetworkIsolation, PAUSED_MARKER_FILE, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse,
-    PrivStream, PrivStreamLine, SIBLING_CONTAINERS,
+    AGENT_PREFIX, AGENT_RUNTIME_ROOT, AGENT_STATE_ROOT, BindMount, CredentialMount, InfraAction,
+    InfraContainer, JournalQuery, META_DIR, MIGRATE_STAGING_ROOT, NetworkIsolation,
+    PAUSED_MARKER_FILE, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
+    PrivStreamLine, SIBLING_CONTAINERS,
 };
 use serde::Serialize;
 use tokio::io::{AsyncWriteExt, BufReader};
@@ -49,6 +49,9 @@ async fn main() -> Result<()> {
         .init();
 
     let listener = socket_listener()?;
+    if let Err(e) = remove_legacy_tmpfiles() {
+        tracing::warn!(error = %format!("{e:#}"), "legacy tmpfiles cleanup failed");
+    }
     tracing::info!("hive-priv listening");
 
     loop {
@@ -382,6 +385,8 @@ async fn exec(
             ref load_credentials,
         } => handle_write_nspawn_flags(container, binds, isolation, load_credentials),
 
+        PrivRequest::EnsureAgentSocketDir { ref name } => ensure_agent_socket_dir(name),
+
         PrivRequest::WriteResourceLimits {
             ref container,
             ref memory_max,
@@ -499,7 +504,10 @@ async fn exec(
             .await
         }
 
-        PrivRequest::SyncAgentTmpfiles { ref agents } => sync_agent_tmpfiles(agents).await,
+        PrivRequest::SyncAgentTmpfiles => {
+            remove_legacy_tmpfiles()?;
+            Ok((String::new(), String::new()))
+        }
     }
 }
 
@@ -1147,9 +1155,9 @@ fn remove_service_dropin(container: &str) -> Result<(String, String)> {
 /// The condition causes systemd to *skip* (not *fail*) the unit when the
 /// bind-mount source dir is absent — result is `condition`, which does not
 /// increment the start-limit counter.  This is belt-and-braces on top of
-/// the tmpfiles.d entries written by `SyncAgentTmpfiles`: in the unlikely
-/// event the dir is missing at start time, the unit idles rather than
-/// restart-looping into `start-limit-hit`.
+/// hive-c0re creating the dir in its start preamble: if it is missing at
+/// start time, the unit idles rather than restart-looping into
+/// `start-limit-hit`.
 fn write_resource_limits(
     container: &str,
     memory_max: &str,
@@ -3310,126 +3318,109 @@ fn write_bridge_dns_marker_in(rootfs: &Path, gateway_ip: &str) -> Result<()> {
     .publish()
 }
 
-/// `SyncAgentTmpfiles` — write `/etc/tmpfiles.d/hyperhive-agents.conf` for
-/// the given agent set and immediately apply it with `systemd-tmpfiles --create`.
-///
-/// Each call atomically replaces the file with entries for all current agents,
-/// then creates any missing dirs on the running host. The file survives reboots
-/// and is read by `systemd-tmpfiles-setup.service` (runs in `sysinit.target`,
-/// before any container units can start), so bind-mount source dirs are always
-/// pre-created regardless of whether hive-c0re has reached `ensure_agent_runtime_dir`.
-///
-/// Directories written per agent:
-/// - `/run/hyperhive/agents/<name>` (MCP socket dir, bind-mounted into container
-///   as `/run/hive`)
-/// - `/run/hive-agent/<name>` (web socket dir, bind-mounted into container)
-const TMPFILES_PATH: &str = "/etc/tmpfiles.d/hyperhive-agents.conf";
+/// Written by older hive-priv builds. Nothing writes it now, but left on disk
+/// systemd-tmpfiles would still apply it at every boot.
+const LEGACY_TMPFILES_PATH: &str = "/etc/tmpfiles.d/hyperhive-agents.conf";
 
-/// The `tmpfiles.d` body, built without touching the filesystem.
-///
-/// Split out of `sync_agent_tmpfiles` so the modes below can be asserted: the
-/// caller writes the file and then shells out to `systemd-tmpfiles`, neither of
-/// which a test can do, and a mode nobody can assert is a mode that drifts.
-fn agent_tmpfiles_content(agents: &[AgentTmpfilesEntry]) -> String {
-    use std::fmt::Write as _;
-
-    let mut content =
-        String::from("# managed by hive-c0re — do not edit (regenerated on spawn/destroy)\n");
-    // Parent dirs — created with permissive mode so hive-c0re can make subdirs.
-    // /run/hyperhive itself is also a RuntimeDirectory of hive-c0re.service; the
-    // tmpfiles.d entry here ensures it exists before hive-c0re starts (boot race).
-    //
-    // 0751, not 0750: must match hive-c0re.service's own `RuntimeDirectoryMode`
-    // and `docs/trust-boundary/boundary.md` — the extra `--x` on `other` is what
-    // lets a `hive-admin`-only user (no `hive-core` membership) traverse into the
-    // directory to reach `host.sock`; without it that user gets a permission
-    // denied opening the socket despite correct group membership on the socket
-    // itself. A mismatch here isn't just cosmetic: this line is regenerated and
-    // re-applied on every agent spawn/destroy via `systemd-tmpfiles --create`,
-    // so a stale `0750` here actively re-asserts the old, wrong mode far more
-    // often than a reboot does.
-    content.push_str("d /run/hyperhive 0751 hive-core hive-core -\n");
-    writeln!(content, "d {AGENT_RUNTIME_ROOT} 0755 hive-core hive-core -").ok();
-    // `hive-core`, not root: c0re does the `create_dir_all` for a new agent's
-    // subdir itself, so a root-owned parent EACCESes on the first spawn of a
-    // fresh host. This must stay in step with the identical rule in
-    // `nix/host-modules/hive-gateway/default.nix` — the two files declared
-    // different owners for this one path, and which won depended on the order
-    // systemd happened to read them in.
-    writeln!(content, "d {SOCKET_DIR_ROOT} 0755 hive-core hive-core -").ok();
-    // Per-agent dirs.
-    for entry in agents {
-        let name = &entry.name;
-        writeln!(
-            content,
-            "d {AGENT_RUNTIME_ROOT}/{name} 0755 hive-core hive-core -"
-        )
-        .ok();
-        // The agent's socket dir. Three principals need it and no two share a
-        // group, so the mode has to say so explicitly:
-        //
-        //   owner = the agent user   rwx  binds + unlinks agent.sock/web.sock
-        //   other =                  --x  traverse only, no listing
-        //
-        // "other" covers hive-c0re (dials agent.sock) and the gateway's nginx
-        // (dials web.sock, and has all of /run/hive-agent bind-mounted in).
-        // Both sockets are 0666, so traversal is all they need.
-        //
-        // ⚠️ 0751 is load-bearing, not tidiness. A world-writable socket dir
-        // lets anything that can reach the path unlink an agent's socket and
-        // bind its own, receiving that agent's todos. Why directory-write
-        // confers that and the sticky bit does not save it:
-        // `docs/trust-boundary/boundary.md::the per-agent socket dir`.
-        //
-        // The owner is declared here because `d` re-applies on every sync, so
-        // a chown made anywhere else does not survive the next agent's spawn.
-        if let (Some(uid), Some(gid)) = (entry.uid, entry.gid) {
-            writeln!(content, "d {SOCKET_DIR_ROOT}/{name} 0751 {uid} {gid} -").ok();
-        } else {
-            // Before the container's /etc/passwd exists there is no uid to
-            // name, and the harness must still be able to bind. Keep the old
-            // permissive mode for that agent alone; the next sync (any spawn
-            // or destroy, or c0re restart) resolves the uid and tightens it.
-            tracing::info!(%name, "tmpfiles.d: agent uid unknown, deferring 0751 on socket dir");
-            writeln!(content, "d {SOCKET_DIR_ROOT}/{name} 0777 root root -").ok();
+/// Unlink [`LEGACY_TMPFILES_PATH`]; already absent is success. Run at every
+/// hive-priv start and by the legacy `SyncAgentTmpfiles` request.
+fn remove_legacy_tmpfiles() -> Result<()> {
+    match std::fs::remove_file(LEGACY_TMPFILES_PATH) {
+        Ok(()) => {
+            tracing::info!("removed legacy {LEGACY_TMPFILES_PATH}");
+            Ok(())
         }
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(()),
+        Err(e) => Err(e).with_context(|| format!("remove {LEGACY_TMPFILES_PATH}")),
     }
-    content
 }
 
-async fn sync_agent_tmpfiles(agents: &[AgentTmpfilesEntry]) -> Result<(String, String)> {
-    for entry in agents {
-        validate_agent_name(&entry.name)?;
+/// `EnsureAgentSocketDir` — create `/run/hive-agent/<name>` if missing.
+fn ensure_agent_socket_dir(name: &str) -> Result<(String, String)> {
+    ensure_socket_dir_in(Path::new(SOCKET_DIR_ROOT), name)?;
+    Ok((String::new(), String::new()))
+}
+
+/// Create `<root>/<name>` as a `0751` directory owned by the caller (root in
+/// production), or accept an existing directory untouched.
+/// `name` must be an agent ident; it is validated here, before any path is
+/// built from it.
+///
+/// The result is an nspawn bind source, so a symlink here would bind a host
+/// path of the planter's choosing into the container. Every step is relative
+/// to an `O_DIRECTORY|O_NOFOLLOW` fd for `root`, and an existing entry is
+/// checked with `AT_SYMLINK_NOFOLLOW` and refused unless it is a directory.
+///
+/// An existing directory keeps its owner and mode: the container's activation
+/// hands it to the agent user, and re-asserting root here would undo that on
+/// every start.
+fn ensure_socket_dir_in(root: &Path, name: &str) -> Result<()> {
+    use std::os::unix::fs::OpenOptionsExt as _;
+
+    validate_agent_name(name)?;
+    let path = root.join(name);
+    let c_name = std::ffi::CString::new(name)
+        .with_context(|| format!("agent name {name:?} contains a NUL byte"))?;
+    let root_dir = std::fs::OpenOptions::new()
+        .read(true)
+        .custom_flags(libc::O_DIRECTORY | libc::O_NOFOLLOW | libc::O_CLOEXEC)
+        .open(root)
+        .with_context(|| format!("open (no-follow) {}", root.display()))?;
+    // SAFETY: `root_dir` is an open directory fd and `c_name` NUL-terminated.
+    if unsafe { libc::mkdirat(root_dir.as_raw_fd(), c_name.as_ptr(), 0o751) } == 0 {
+        // mkdirat's mode is masked by the umask; the mode is part of the
+        // contract, so it is set explicitly.
+        // SAFETY: as above.
+        let rc = unsafe {
+            libc::fchmodat(
+                root_dir.as_raw_fd(),
+                c_name.as_ptr(),
+                0o751,
+                libc::AT_SYMLINK_NOFOLLOW,
+            )
+        };
+        if rc != 0 {
+            return Err(std::io::Error::last_os_error())
+                .with_context(|| format!("chmod 0751 {}", path.display()));
+        }
+        tracing::info!(path = %path.display(), "created agent socket dir");
+        return Ok(());
     }
-    let content = agent_tmpfiles_content(agents);
-
-    // Overlapping syncs each stage their own temp, so the last rename wins
-    // with one complete roster rather than a mix of two.
-    publish_file(Path::new(TMPFILES_PATH), content.as_bytes(), 0o644, None)?;
-    tracing::info!(agents = agents.len(), "tmpfiles.d: wrote {TMPFILES_PATH}");
-
-    // Apply immediately so dirs exist on the running host, not just after next boot.
-    let out = Command::new("systemd-tmpfiles")
-        .args(["--create", TMPFILES_PATH])
-        .output()
-        .await
-        .context("systemd-tmpfiles --create")?;
-    if !out.status.success() {
-        let stderr = String::from_utf8_lossy(&out.stderr).trim().to_owned();
-        anyhow::bail!(
-            "systemd-tmpfiles --create failed ({}): {stderr}",
-            out.status
+    let err = std::io::Error::last_os_error();
+    if err.kind() != std::io::ErrorKind::AlreadyExists {
+        return Err(err).with_context(|| format!("create {}", path.display()));
+    }
+    // SAFETY: `stat` is plain old data; the zeroed value is only read after
+    // `fstatat` has filled it in.
+    let mut st: libc::stat = unsafe { std::mem::zeroed() };
+    // SAFETY: as for `mkdirat`; `st` is a valid, writable `stat`.
+    let rc = unsafe {
+        libc::fstatat(
+            root_dir.as_raw_fd(),
+            c_name.as_ptr(),
+            &raw mut st,
+            libc::AT_SYMLINK_NOFOLLOW,
+        )
+    };
+    if rc != 0 {
+        return Err(std::io::Error::last_os_error())
+            .with_context(|| format!("stat (no-follow) {}", path.display()));
+    }
+    if st.st_mode & libc::S_IFMT != libc::S_IFDIR {
+        bail!(
+            "{} exists but is not a directory; refusing it as a bind source",
+            path.display()
         );
     }
-    Ok((String::new(), String::new()))
+    Ok(())
 }
 
 #[cfg(test)]
 mod tests {
     use super::{
-        AgentTmpfilesEntry, BindMount, BoundedRun, OwnedFd, PAUSED_MARKER_FILE, PrivRequest,
-        StagedFile, agent_tmpfiles_content, check_fd_agreement, clear_runner_credentials,
-        contains_secret_shaped_run, describe_forge_admin, ensure_plain_filename, git_overlay_flags,
+        BindMount, BoundedRun, OwnedFd, PAUSED_MARKER_FILE, PrivRequest, StagedFile,
+        check_fd_agreement, clear_runner_credentials, contains_secret_shaped_run,
+        describe_forge_admin, ensure_plain_filename, ensure_socket_dir_in, git_overlay_flags,
         limits_dropin_body, matrix_token_filename, open_dir, open_export_dest, partial_name,
         publish_file, redact_secret_line, remove_marker_in, run_bounded, single_output_path,
         toplevel_attr, validate_account_name, validate_credential_name, validate_snapshot_name,
@@ -3446,49 +3437,6 @@ mod tests {
         }
     }
 
-    /// `/run/hyperhive` is declared twice — here and as `hive-c0re.service`'s
-    /// `RuntimeDirectory`/`RuntimeDirectoryMode`. When the two disagree,
-    /// whichever runs last wins, and the loser's mode is silently re-applied on
-    /// every agent spawn. `0751` is the value the socket's own `0660
-    /// hive-admin` gate depends on: `o=--x` is what lets an admin who is not in
-    /// `hive-core` traverse to it at all.
-    #[test]
-    fn hyperhive_runtime_dir_keeps_the_traversable_mode() {
-        let out = agent_tmpfiles_content(&[]);
-        assert!(
-            out.contains("d /run/hyperhive 0751 hive-core hive-core -\n"),
-            "{out}"
-        );
-        // The specific regression: 0750 denies traversal before host.sock's own
-        // permissions are consulted.
-        assert!(!out.contains("d /run/hyperhive 0750"), "{out}");
-    }
-
-    /// Control for the assertion above: an empty roster still emits the parent
-    /// dirs, and a populated one adds per-agent lines — so a `contains` check
-    /// over this body is reading a body that was actually built.
-    #[test]
-    fn tmpfiles_body_grows_with_the_roster() {
-        let empty = agent_tmpfiles_content(&[]);
-        let one = agent_tmpfiles_content(&[AgentTmpfilesEntry {
-            name: "probe".to_owned(),
-            uid: Some(1234),
-            gid: Some(1234),
-        }]);
-        assert!(one.len() > empty.len(), "empty={empty}\none={one}");
-        assert!(
-            one.contains("d /run/hive-agent/probe 0751 1234 1234 -\n"),
-            "{one}"
-        );
-        // Every per-agent line the builder emits, so no mode here is
-        // unasserted — an unpinned mode is one that drifts.
-        assert!(
-            one.contains("d /run/hyperhive/agents/probe 0755 hive-core hive-core -\n"),
-            "{one}"
-        );
-        assert!(!empty.contains("probe"), "{empty}");
-    }
-
     /// Pins the exact attr path we hand to `nix build` — it has to match
     /// `hive-c0re`'s own `lifecycle::prebuild_toplevel` construction, since
     /// that step's whole point is warming the store for this later build.
@@ -4228,6 +4176,121 @@ mod tests {
         }
     }
 
+    /// A missing socket dir is created `0751`, owned by the caller (root in
+    /// production), which is the mode the container's activation expects to
+    /// hand over and the one dialers traverse.
+    #[test]
+    fn socket_dir_created_0751_when_absent() {
+        use std::os::unix::fs::{MetadataExt as _, PermissionsExt as _};
+        let dir = scratch();
+        ensure_socket_dir_in(&dir, "probe").unwrap();
+        let meta = std::fs::symlink_metadata(dir.join("probe")).unwrap();
+        assert!(meta.is_dir());
+        assert_eq!(meta.permissions().mode() & 0o7777, 0o751);
+        // SAFETY: `geteuid` has no preconditions.
+        assert_eq!(meta.uid(), unsafe { libc::geteuid() });
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// An existing directory keeps its mode and contents: by the second start
+    /// it belongs to the agent user, and a live socket may sit inside it.
+    #[test]
+    fn socket_dir_existing_directory_left_alone() {
+        use std::os::unix::fs::PermissionsExt as _;
+        let dir = scratch();
+        let sub = dir.join("probe");
+        std::fs::create_dir(&sub).unwrap();
+        std::fs::set_permissions(&sub, std::fs::Permissions::from_mode(0o700)).unwrap();
+        std::fs::write(sub.join("web.sock"), "").unwrap();
+
+        ensure_socket_dir_in(&dir, "probe").unwrap();
+        let mode = std::fs::metadata(&sub).unwrap().permissions().mode() & 0o7777;
+        assert_eq!(mode, 0o700, "an existing dir's mode must not be reset");
+        assert!(sub.join("web.sock").exists());
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// A symlink at the leaf would make nspawn bind wherever it points; it is
+    /// refused, whether it points at a directory or nowhere.
+    #[test]
+    fn socket_dir_refuses_symlink_leaf() {
+        use std::os::unix::fs::PermissionsExt as _;
+        let dir = scratch();
+        let elsewhere = dir.join("elsewhere");
+        std::fs::create_dir(&elsewhere).unwrap();
+        std::fs::set_permissions(&elsewhere, std::fs::Permissions::from_mode(0o700)).unwrap();
+        std::os::unix::fs::symlink(&elsewhere, dir.join("probe")).unwrap();
+        std::os::unix::fs::symlink(dir.join("missing"), dir.join("dangling")).unwrap();
+
+        assert!(ensure_socket_dir_in(&dir, "probe").is_err());
+        assert!(ensure_socket_dir_in(&dir, "dangling").is_err());
+        let mode = std::fs::metadata(&elsewhere).unwrap().permissions().mode() & 0o7777;
+        assert_eq!(mode, 0o700, "the symlink target must be untouched");
+        assert!(!dir.join("missing").exists());
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// A regular file where the dir should be is refused, not bound.
+    #[test]
+    fn socket_dir_refuses_non_directory_leaf() {
+        let dir = scratch();
+        std::fs::write(dir.join("probe"), "x").unwrap();
+        let err = ensure_socket_dir_in(&dir, "probe").unwrap_err();
+        assert!(format!("{err:#}").contains("not a directory"), "{err:#}");
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// A symlinked root would place the new dir in a tree of the planter's
+    /// choosing; the `O_NOFOLLOW` open refuses it before anything is created.
+    #[test]
+    fn socket_dir_refuses_symlinked_root() {
+        let dir = scratch();
+        let elsewhere = dir.join("elsewhere");
+        std::fs::create_dir(&elsewhere).unwrap();
+        let root = dir.join("root");
+        std::os::unix::fs::symlink(&elsewhere, &root).unwrap();
+
+        assert!(ensure_socket_dir_in(&root, "probe").is_err());
+        assert!(!elsewhere.join("probe").exists());
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// Names that are not a plain agent ident are refused by validation,
+    /// before anything is created. Runs against a scratch parent, so the
+    /// refusal cannot come from a missing `/run/hive-agent`; `.`/`..` exist
+    /// as directories and `Atlas` would be created, so each bad name gets
+    /// through only if validation is gone. Control: a plain name succeeds
+    /// in the same parent.
+    #[test]
+    fn socket_dir_refuses_bogus_names() {
+        let dir = scratch();
+        for bad in ["", ".", "..", "../etc", "a/b", "Atlas", "a b", "a\0b"] {
+            let err = ensure_socket_dir_in(&dir, bad).unwrap_err();
+            assert!(
+                format!("{err:#}").contains("invalid name"),
+                "agent name {bad:?} must be refused by validation, got: {err:#}"
+            );
+        }
+        assert_eq!(std::fs::read_dir(&dir).unwrap().count(), 0);
+        ensure_socket_dir_in(&dir, "atlas").unwrap();
+        assert!(dir.join("atlas").is_dir());
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// An older hive-c0re's `SyncAgentTmpfiles` still decodes, payload and
+    /// all, so it gets the legacy cleanup rather than a parse error. Control:
+    /// an unknown op does not decode.
+    #[test]
+    fn legacy_sync_agent_tmpfiles_request_still_decodes() {
+        let legacy =
+            r#"{"op":"sync_agent_tmpfiles","agents":[{"name":"atlas","uid":1000,"gid":994}]}"#;
+        assert!(matches!(
+            serde_json::from_str::<PrivRequest>(legacy),
+            Ok(PrivRequest::SyncAgentTmpfiles)
+        ));
+        assert!(serde_json::from_str::<PrivRequest>(r#"{"op":"sync_agent_tmpfilez"}"#).is_err());
+    }
+
     /// The runner-credential clear, in all three states that matter. The
     /// PRESENCE arm is the load-bearing one: an implementation that did nothing
     /// at all would pass the "absent is fine" arm perfectly, and the whole point
diff --git a/nix/agent-modules/user.nix b/nix/agent-modules/user.nix
index 8d620cce..be8bb6e7 100644
--- a/nix/agent-modules/user.nix
+++ b/nix/agent-modules/user.nix
@@ -209,6 +209,20 @@ in
       if [ -d "/agents/$userName/harness" ]; then
         chown -hR "$userName:$userName" "/agents/$userName/harness" 2>/dev/null || true
       fi
+      # The socket dir is bind-mounted from the host, where hive-priv creates
+      # it `0751 root`. The harness binds its sockets here as this user; the
+      # 0751 is what lets hive-c0re and nginx dial them without listing
+      # (docs/trust-boundary/boundary.md, "the per-agent socket dir"). Not
+      # recursive: only the harness writes inside it. `test -d` and `chmod`
+      # follow symlinks, so a link here is refused rather than handed over.
+      socketDir="/run/hive-agent/$userName"
+      if [ -L "$socketDir" ] || { [ -e "$socketDir" ] && [ ! -d "$socketDir" ]; }; then
+        echo "hive-agent-user-migrate: $socketDir is a symlink or not a directory; not handing it to $userName" >&2
+      elif [ -d "$socketDir" ]; then
+        chown -h "$userName:$userName" "$socketDir" \
+          && chmod 0751 "$socketDir" \
+          || echo "hive-agent-user-migrate: could not hand $socketDir to $userName; the harness cannot bind its sockets" >&2
+      fi
       # The proposed-config repo is RW-mounted into the editing agent and
       # owned by it; hive-c0re only pulls from it. Heal
       # it to this user too — same as state/harness. In an agent's own
diff --git a/nix/checks.nix b/nix/checks.nix
index 80ed4874..339d229b 100644
--- a/nix/checks.nix
+++ b/nix/checks.nix
@@ -132,6 +132,10 @@ in
     inherit pkgs self nixosSystem;
     inherit (pkgs) lib;
   };
+  module-eval-agent-user = import ./module-eval/agent-user.nix {
+    inherit pkgs self nixosSystem;
+    inherit (pkgs) lib;
+  };
   module-eval-agent-matrix = import ./module-eval/agent-matrix.nix {
     inherit pkgs self nixosSystem;
     inherit (pkgs) lib;
diff --git a/nix/host-modules/hive-c0re/default.nix b/nix/host-modules/hive-c0re/default.nix
index 840d3535..702fc079 100644
--- a/nix/host-modules/hive-c0re/default.nix
+++ b/nix/host-modules/hive-c0re/default.nix
@@ -19,12 +19,6 @@ let
   # listing: `hive-admin` members reach `host.sock` without root, while the
   # socket's own `0660 hive-admin` gates the connection and the per-agent
   # subdirs keep their own perms.
-  #
-  # hive-priv writes a tmpfiles.d entry for this same path and cannot read
-  # this binding, so the two are kept in step by hand. Divergence is not
-  # cosmetic: tmpfiles then tries to fchmod a directory hive-priv has no
-  # write access to, the whole `--create` run fails, and a single WARN per
-  # sync is the only symptom.
   runtimeDirMode = "0751";
 
   baoDeploy = config.services.hyperhive.deploy.bao;
diff --git a/nix/host-modules/hive-gateway/default.nix b/nix/host-modules/hive-gateway/default.nix
index a492ca48..4eee6f0c 100644
--- a/nix/host-modules/hive-gateway/default.nix
+++ b/nix/host-modules/hive-gateway/default.nix
@@ -212,13 +212,6 @@ in
     # before c0re has run, and pin owner + mode rather than leaving it
     # to whoever creates the path first.
     #
-    # /run/hive-agent — per-agent UDS socket dir, written by c0re's
-    # set_nspawn_flags when agents start. Owned by `hive-core` (the
-    # unprivileged coordinator user): c0re does the
-    # `create_dir_all(/run/hive-agent/<name>)` itself, so a root-owned
-    # parent would EACCES on the very first agent create on a fresh host
-    # (hive-priv only chowns the subdir afterwards, it doesn't make it).
-    #
     # ⚠️ There is deliberately NO rule for /var/lib/hyperhive here. One
     # used to declare it `0755 root root` and could never win:
     # `hive-c0re.service` sets `StateDirectory = "hyperhive"` with
@@ -228,10 +221,6 @@ in
     # as someone having changed the mode. c0re's own unit owns that dir;
     # this module no longer has an opinion about it.
     systemd.tmpfiles.rules = [
-      # Must stay in step with the identical rule hive-priv generates into
-      # /etc/tmpfiles.d/hyperhive-agents.conf — the two used to declare
-      # different owners for this path.
-      "d /run/hive-agent 0755 hive-core hive-core - -"
       # The gateway's own config dir — NOT under /var/lib/hyperhive. c0re
       # writes here, nginx reads here, and neither needs any access to the
       # other's tree: no shared parent to traverse means no group
diff --git a/nix/host-modules/hive-priv.nix b/nix/host-modules/hive-priv.nix
index 48c70b7d..f98573f9 100644
--- a/nix/host-modules/hive-priv.nix
+++ b/nix/host-modules/hive-priv.nix
@@ -20,6 +20,11 @@ let
 in
 {
   config = lib.mkIf config.services.hyperhive.deploy.hive-controller.enable {
+    # The parent of every agent socket dir. hive-priv is its only writer
+    # (`EnsureAgentSocketDir`), and its unit lists it in `ReadWritePaths`,
+    # which fails the unit when the path is missing.
+    systemd.tmpfiles.rules = [ "d /run/hive-agent 0755 root root - -" ];
+
     # Socket unit for hive-priv — the narrow root helper that executes
     # privileged operations on behalf of hive-c0re. Systemd creates and
     # holds `/run/hive/priv.sock` before the first connection arrives.
@@ -108,8 +113,9 @@ in
         # hive-priv must write to at runtime. Each is a confirmed hive-priv
         # write that EROFSes (os error 30) without its carve-out:
         #   /etc/nixos-containers     — <container>.conf (bind mounts, nspawn flags)
-        #   /etc/tmpfiles.d           — sync_tmpfiles' hyperhive-agents.conf write
-        #   /run/hive-agent           — chown/chmod per-agent socket dirs
+        #   /etc/tmpfiles.d           — unlinks the legacy hyperhive-agents.conf;
+        #                               drop once every host has run it
+        #   /run/hive-agent           — creates per-agent socket dirs
         #   /run/systemd              — container@ drop-ins + machined state
         #   /run/lock                 — nixos-container's create/destroy lock file
         #   /run/hive-ci              — register_ci_runner's runner-token write
diff --git a/nix/module-eval/agent-user.nix b/nix/module-eval/agent-user.nix
new file mode 100644
index 00000000..4f3de42d
--- /dev/null
+++ b/nix/module-eval/agent-user.nix
@@ -0,0 +1,49 @@
+# `checks.module-eval-agent-user` — see ./lib.nix for the shared
+# rationale (why this suite exists, naming convention, "evaluates
+# not executes").
+{
+  pkgs,
+  lib,
+  self,
+  nixosSystem,
+}:
+let
+  inherit
+    (import ./lib.nix {
+      inherit
+        pkgs
+        lib
+        self
+        nixosSystem
+        ;
+    })
+    agentWith
+    runGroup
+    ;
+
+  # A named agent, so the rendered script carries a name no default supplies.
+  migrate =
+    (agentWith { services.hyperhive.agent.user.name = "iris"; })
+    .system.activationScripts.hive-agent-user-migrate.text;
+  cases = [
+    {
+      # hive-priv creates the host socket dir `0751 root` and never chowns it,
+      # so this activation is the only thing that lets the harness bind there.
+      name = "the agent's activation hands its socket dir to the agent user at 0751";
+      ok =
+        lib.hasInfix "userName=${lib.escapeShellArg "iris"}" migrate
+        && lib.hasInfix ''socketDir="/run/hive-agent/$userName"'' migrate
+        && lib.hasInfix ''chown -h "$userName:$userName" "$socketDir"'' migrate
+        && lib.hasInfix ''chmod 0751 "$socketDir"'' migrate;
+    }
+    {
+      # `test -d` and `chmod` follow symlinks, so without this refusal ahead
+      # of the chown branch a link at the socket dir is chmodded at its target.
+      name = "the agent's activation refuses a symlinked or non-directory socket dir";
+      ok =
+        lib.hasInfix ''if [ -L "$socketDir" ] || { [ -e "$socketDir" ] && [ ! -d "$socketDir" ]; }; then'' migrate
+        && lib.hasInfix ''elif [ -d "$socketDir" ]; then'' migrate;
+    }
+  ];
+in
+runGroup "agent-user" cases