diff --git a/hive-matrix-mcp/src/accounts.rs b/hive-matrix-mcp/src/accounts.rs
index f6be4ca6..9b2a2bf0 100644
--- a/hive-matrix-mcp/src/accounts.rs
+++ b/hive-matrix-mcp/src/accounts.rs
@@ -11,9 +11,11 @@
 //! `hyperhive.matrixAccounts`) and are appended after `main`.
 //!
 //! So a single-account agent (no `HIVE_MATRIX_ACCOUNTS`) gets exactly
-//! `main` — zero config, same behaviour as before. The **primary** is
-//! `main`: it is selected when a tool call omits `account`, so callers
-//! never specify one for the hive account.
+//! `main` — zero config, same behaviour as before: omitting `account` on
+//! a tool call resolves to `main`. When **multiple** accounts are
+//! configured, omitting `account` is rejected with the list of names (see
+//! `Registry::resolve` / `pick_name`) — the agent can't tell more than one
+//! account exists, so it must choose explicitly.
 
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
@@ -202,6 +204,30 @@ pub struct Registry {
     by_name: HashMap<String, Arc<Client>>,
 }
 
+/// Pick which account name a request resolves to, or an error. Pure (no
+/// `Client`) so the ambiguity rule is unit-testable. `names` is the sorted
+/// set of configured account names.
+///
+/// - `Some(name)` → that name (membership is checked by the caller).
+/// - `None` with a single account → the primary (zero-config default).
+/// - `None` with multiple accounts → an error: the agent can't tell more
+///   than one account exists, so force an explicit `account` and list the
+///   choices.
+fn pick_name<'a>(
+    account: Option<&'a str>,
+    primary: &'a str,
+    names: &[&str],
+) -> Result<&'a str, String> {
+    match account {
+        Some(name) => Ok(name),
+        None if names.len() <= 1 => Ok(primary),
+        None => Err(format!(
+            "multiple matrix accounts configured; pass `account` explicitly — available: [{}]",
+            names.join(", ")
+        )),
+    }
+}
+
 impl Registry {
     /// Build an empty registry whose primary is `primary`. Clients are
     /// added with [`Registry::insert`] as each account restores.
@@ -250,23 +276,28 @@ impl Registry {
         out
     }
 
-    /// Resolve a request's `account` to a client. `None` → the primary
-    /// account. Returns a human-readable error (listing known accounts)
-    /// when the name is unknown — surfaced to the agent as a tool error.
+    /// Resolve a request's `account` to a client. `None` selects the
+    /// primary account *only when it's unambiguous* — a single configured
+    /// account. With multiple accounts, omitting `account` is an error
+    /// (see `pick_name`): the agent can't tell more than one exists, so we
+    /// force an explicit choice and list the names. Returns a
+    /// human-readable error (listing known accounts) surfaced to the agent
+    /// as a tool error.
     ///
     /// # Errors
     ///
-    /// Errors when the named account (or the primary, if `None`) has no
-    /// live client — e.g. an unknown name, or the primary failed to
+    /// Errors when `account` is omitted but multiple accounts are
+    /// configured, or when the named account (or the primary, if `None`)
+    /// has no live client — e.g. an unknown name, or the primary failed to
     /// restore at startup.
     pub fn resolve(&self, account: Option<&str>) -> Result<&Arc<Client>, String> {
-        let name = account.unwrap_or(&self.primary);
+        let mut names: Vec<&str> = self.by_name.keys().map(String::as_str).collect();
+        names.sort_unstable();
+        let name = pick_name(account, &self.primary, &names)?;
         self.by_name.get(name).ok_or_else(|| {
-            let mut known: Vec<&str> = self.by_name.keys().map(String::as_str).collect();
-            known.sort_unstable();
             format!(
                 "unknown matrix account {name:?}; available accounts: [{}]",
-                known.join(", ")
+                names.join(", ")
             )
         })
     }
@@ -361,7 +392,28 @@ fn write_accounts_snapshot_inner(
 
 #[cfg(test)]
 mod tests {
-    use super::{AccountStatus, heartbeat_accounts_snapshot, write_accounts_snapshot};
+    use super::{AccountStatus, heartbeat_accounts_snapshot, pick_name, write_accounts_snapshot};
+
+    #[test]
+    fn pick_name_single_account_defaults_to_primary() {
+        assert_eq!(pick_name(None, "main", &["main"]).unwrap(), "main");
+    }
+
+    #[test]
+    fn pick_name_multi_account_omitted_errors_with_menu() {
+        let err = pick_name(None, "main", &["main", "public"]).unwrap_err();
+        assert!(err.contains("pass `account` explicitly"));
+        assert!(err.contains("main"));
+        assert!(err.contains("public"));
+    }
+
+    #[test]
+    fn pick_name_explicit_passes_through_even_with_multiple() {
+        assert_eq!(
+            pick_name(Some("public"), "main", &["main", "public"]).unwrap(),
+            "public"
+        );
+    }
 
     fn sample() -> Vec<AccountStatus> {
         vec![