diff --git a/docs/integrations/forge.md b/docs/integrations/forge.md
index 4d1ff43e..d7186857 100644
--- a/docs/integrations/forge.md
+++ b/docs/integrations/forge.md
@@ -58,12 +58,13 @@ creates what's missing and re-mints a token that's missing or no longer
 matches the forge. A pass whose store or forge reads fail queues nothing, so
 an outage never rotates every hive's token.
 
-The account name isn't reserved, so an agent may carry the name `hive-<x>` too.
-swarm-controller creates the hive's account with the email
-`hive-<hive>@hives.hyperhive.local` and refuses an account by that name with
-any other email: it never adds an agent's account to the team or mints a
-token for it. An agent's own creation refuses the hive's account the same
-way.
+swarm-controller refuses to create an agent whose name starts with `hive-`:
+these accounts own that prefix. An agent that already had such a
+name keeps it, so the accounts are also told apart by email. swarm-controller
+creates the hive's account with the email `hive-<hive>@hives.hyperhive.local`
+and refuses an account by that name with any other email: it never adds an
+agent's account to the team or mints a token for it. An agent's own forge
+account setup refuses the hive's account the same way.
 
 hive-c0re reads the token with `forge::reader_token()`, under the hive's own
 store certificate, every time it reads the forge. Where the store holds none,
diff --git a/hive-types/src/lib.rs b/hive-types/src/lib.rs
index dc123baf..961e557d 100644
--- a/hive-types/src/lib.rs
+++ b/hive-types/src/lib.rs
@@ -93,6 +93,24 @@ pub fn forge_username_violation(name: &str) -> Option<&'static str> {
     }
 }
 
+/// The prefix no new agent's name may start with. `hive-<hive>` is each hive's
+/// own forge account (`swarm_secret_client::forge::hive_login`), and every
+/// agent's forge account takes the agent's name, so an agent called
+/// `hive-<x>` would claim hive `<x>`'s account.
+pub const RESERVED_AGENT_PREFIX: &str = "hive-";
+
+/// The reserved-prefix rule `name` breaks as a new agent's name, or `None`.
+///
+/// Creation sites only, for the reason given at [`is_reserved_name`]. Agent
+/// names only: hive names are kept off `hive` by
+/// `nix/reserved-hive-fragments.nix`, and the other [`Ident`]s never become a
+/// forge user.
+#[must_use]
+pub fn reserved_agent_prefix_violation(name: &str) -> Option<&'static str> {
+    name.starts_with(RESERVED_AGENT_PREFIX)
+        .then_some("must not start with 'hive-': that prefix is reserved for hive forge accounts")
+}
+
 /// A validated hive identifier: 1-63 chars of `[a-z0-9-]`.
 ///
 /// The single ident type for agent names, forge labels, and matrix / github
@@ -187,7 +205,7 @@ impl<'de> serde::Deserialize<'de> for Ident {
 mod ident_tests {
     use super::{
         FORGE_USERNAME_MAX_LEN, Ident, forge_username_violation, is_reserved_name,
-        parse_reserved_names,
+        parse_reserved_names, reserved_agent_prefix_violation,
     };
 
     #[test]
@@ -302,6 +320,20 @@ mod ident_tests {
         }
     }
 
+    #[test]
+    fn the_hive_prefix_is_refused_for_an_agent() {
+        for bad in ["hive-x", "hive-alpha", "hive-"] {
+            assert!(
+                reserved_agent_prefix_violation(bad).is_some_and(|r| r.contains("hive forge")),
+                "{bad:?}"
+            );
+        }
+        // The accept arm: `hive` elsewhere in the name, or without the dash.
+        for ok in ["hivemind", "xhive-y", "hive", "my-hive", "hives-x"] {
+            assert_eq!(reserved_agent_prefix_violation(ok), None, "{ok:?}");
+        }
+    }
+
     #[test]
     fn round_trips_and_serde_validates() {
         let id = Ident::parse("damocles").unwrap();
diff --git a/swarm-controller/src/forge/hive_token.rs b/swarm-controller/src/forge/hive_token.rs
index 971885b9..d2e0867f 100644
--- a/swarm-controller/src/forge/hive_token.rs
+++ b/swarm-controller/src/forge/hive_token.rs
@@ -10,9 +10,12 @@
 //! token the forge still lists with the same last eight characters and scopes
 //! is kept, and a pass whose reads fail queues nothing.
 //!
-//! ⚠️ The account name is not reserved: an agent may be called `hive-<x>`.
-//! Each side refuses an account carrying the other's email, so neither ever
-//! locks down, joins the team with, or mints a token for the other's account.
+//! A new agent may not be called `hive-<x>`
+//! (`hive_types::reserved_agent_prefix_violation`, checked at agent creation),
+//! which is the primary guard against an agent claiming a hive's account. An
+//! agent created before that rule can still hold such a name, so each side
+//! also refuses an account carrying the other's email: neither ever locks
+//! down, joins the team with, or mints a token for the other's account.
 
 use std::sync::Arc;
 
diff --git a/swarm-controller/src/main.rs b/swarm-controller/src/main.rs
index 0485e43f..17691787 100644
--- a/swarm-controller/src/main.rs
+++ b/swarm-controller/src/main.rs
@@ -1882,6 +1882,9 @@ fn broken_name_rules(
             "agent name {agent:?} {rule}: the forge refuses it as a username"
         ));
     }
+    if let Some(rule) = hive_types::reserved_agent_prefix_violation(agent) {
+        broken.push(format!("agent name {agent:?} {rule}"));
+    }
     broken
 }
 
@@ -3892,6 +3895,32 @@ mod tests {
         }
     }
 
+    /// The `hive-` prefix belongs to hives' forge accounts, so a new agent
+    /// may not take it. `hivemind` and `xhive-y` are the control.
+    #[test]
+    fn a_new_agent_with_the_hive_prefix_is_refused() {
+        let raw = nix_reserved();
+        let mut warnings = Vec::new();
+        let broken = super::broken_name_rules("hive-x", Some(&raw), &mut warnings);
+        let err =
+            super::name_verdict("hive-x", &broken, Ok(false), &mut warnings).expect_err("hive-x");
+        assert!(err.contains("reserved for hive forge accounts"), "{err}");
+        for ok in ["hivemind", "xhive-y"] {
+            let broken = super::broken_name_rules(ok, Some(&raw), &mut warnings);
+            assert!(broken.is_empty(), "{ok:?}: {broken:?}");
+        }
+        assert!(warnings.is_empty(), "{warnings:?}");
+    }
+
+    /// The prefix the rule reserves is the one hive accounts are named with.
+    #[test]
+    fn the_reserved_prefix_is_the_hive_account_prefix() {
+        assert!(
+            swarm_secret_client::forge::hive_login("alpha")
+                .starts_with(hive_types::RESERVED_AGENT_PREFIX)
+        );
+    }
+
     /// Re-creating an agent under its own name must keep working: a name
     /// already in the roster is warned about, never refused.
     #[test]
diff --git a/swarm-secret-client/src/forge.rs b/swarm-secret-client/src/forge.rs
index d6de4a07..cb43aab4 100644
--- a/swarm-secret-client/src/forge.rs
+++ b/swarm-secret-client/src/forge.rs
@@ -40,9 +40,9 @@ pub fn agent_token_path(agent: &str) -> Result<String, Error> {
 /// basic-auth header that token travels in.
 ///
 /// The `hive-` prefix keeps the account reading as a hive's rather than an
-/// agent's, as [`crate::matrix::hive_localpart`] does on the homeserver. It
-/// does not make the name unique: an agent may be called `hive-<x>` too, and
-/// `swarm-controller` tells the two apart by the account's email.
+/// agent's, as [`crate::matrix::hive_localpart`] does on the homeserver.
+/// `swarm-controller` refuses a new agent named with this prefix, and tells an
+/// older one apart from the hive's account by the account's email.
 #[must_use]
 pub fn hive_login(hive: &str) -> String {
     format!("hive-{hive}")