diff --git a/hive-c0re/src/forge.rs b/hive-c0re/src/forge.rs
index 4fe27d5d..b08440c8 100644
--- a/hive-c0re/src/forge.rs
+++ b/hive-c0re/src/forge.rs
@@ -335,6 +335,7 @@ async fn mint_and_persist_token(name: &str, path: &Path, scopes: &str) -> Result
     std::fs::write(path, format!("{token}\n"))
         .with_context(|| format!("write token to {}", path.display()))?;
     let _ = std::fs::set_permissions(path, std::fs::Permissions::from_mode(0o600));
+    crate::lifecycle::chown_to_agent(name, path, "forge");
     tracing::info!(%name, path = %path.display(), "forge: persisted access token");
     Ok(())
 }
diff --git a/hive-c0re/src/lifecycle.rs b/hive-c0re/src/lifecycle.rs
index c81217a5..e3051517 100644
--- a/hive-c0re/src/lifecycle.rs
+++ b/hive-c0re/src/lifecycle.rs
@@ -92,6 +92,59 @@ pub fn is_manager(name: &str) -> bool {
     name == MANAGER_NAME
 }
 
+/// Read the agent user's `(uid, gid)` from the container's nixos-managed
+/// `/etc/passwd`. Returns `None` when the container hasn't been built
+/// yet, the passwd file is unparseable, or the agent user is missing
+/// (e.g. legacy pre-#658 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 (#673).
+///
+/// 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".
+#[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 = std::fs::read_to_string(&passwd_path).ok()?;
+    for line in content.lines() {
+        let mut parts = line.split(':');
+        let user = parts.next()?;
+        if user != agent_name {
+            continue;
+        }
+        let _ = parts.next()?; // x (password placeholder)
+        let uid: u32 = parts.next()?.parse().ok()?;
+        let gid: u32 = parts.next()?.parse().ok()?;
+        return Some((uid, gid));
+    }
+    None
+}
+
+/// Best-effort `chown(path, agent_uid, agent_gid)`. Resolves the agent's
+/// uid/gid via [`agent_uid_gid`] and shells out to `std::os::unix::fs::chown`.
+/// Silently no-ops when the container isn't built yet (`None` from
+/// [`agent_uid_gid`]) and logs at debug on chown syscall failure — the
+/// activation script in `harness-base.nix` is the steady-state safety
+/// net. Used by per-agent state writers in `forge` + `matrix` so the
+/// agent can read the file without waiting for the next container
+/// rebuild (#673).
+pub fn chown_to_agent(name: &str, path: &Path, subsystem: &str) {
+    let Some((uid, gid)) = agent_uid_gid(name) else {
+        return;
+    };
+    if let Err(e) = std::os::unix::fs::chown(path, Some(uid), Some(gid)) {
+        tracing::debug!(%name, %subsystem, path = %path.display(), error = %e, "chown to agent failed");
+    }
+}
+
 fn validate(name: &str) -> Result<()> {
     if name.is_empty() {
         bail!("agent name must not be empty");
diff --git a/hive-c0re/src/matrix.rs b/hive-c0re/src/matrix.rs
index c695e17c..0e0099b6 100644
--- a/hive-c0re/src/matrix.rs
+++ b/hive-c0re/src/matrix.rs
@@ -274,6 +274,7 @@ pub async fn ensure_user_for(
     std::fs::write(&path, format!("{access_token}\n"))
         .with_context(|| format!("matrix: write token to {}", path.display()))?;
     let _ = std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o600));
+    crate::lifecycle::chown_to_agent(name, &path, "matrix");
     tracing::info!(%name, path = %path.display(), "matrix: registered user + persisted access token");
     Ok(())
 }