diff --git a/hive-c0re/src/dashboard.rs b/hive-c0re/src/dashboard.rs
index 2288396d..eda66f71 100644
--- a/hive-c0re/src/dashboard.rs
+++ b/hive-c0re/src/dashboard.rs
@@ -1263,6 +1263,36 @@ mod tests {
         p
     }
 
+    #[test]
+    fn validate_agent_name_accepts_canonical_shapes() {
+        assert!(validate_agent_name("damocles").is_none());
+        assert!(validate_agent_name("hm1nd").is_none());
+        assert!(validate_agent_name("agent-with-dashes").is_none());
+        assert!(validate_agent_name("snake_case").is_none());
+        assert!(validate_agent_name("mixed_2-3").is_none());
+        let max = "a".repeat(63);
+        assert!(validate_agent_name(&max).is_none(), "63-char name should pass");
+    }
+
+    #[test]
+    fn validate_agent_name_rejects_bad_input() {
+        assert!(validate_agent_name("").is_some());
+        let too_long = "a".repeat(64);
+        assert!(validate_agent_name(&too_long).is_some());
+        // Path-traversal attempts.
+        assert!(validate_agent_name("../etc/passwd").is_some());
+        assert!(validate_agent_name("alice/bob").is_some());
+        // Uppercase rejected — canonical lowercase convention.
+        assert!(validate_agent_name("Alice").is_some());
+        // No spaces, dots, special chars.
+        assert!(validate_agent_name("alice bob").is_some());
+        assert!(validate_agent_name("alice.bob").is_some());
+        assert!(validate_agent_name("alice;DROP TABLE messages").is_some());
+        // Non-ASCII (incl. unicode homoglyphs of ASCII dash).
+        assert!(validate_agent_name("damóclès").is_some());
+        assert!(validate_agent_name("alice\u{2013}bob").is_some()); // en-dash
+    }
+
     #[test]
     fn reject_symlinks_below_accepts_plain_dirs_and_files() {
         let root = tmproot("symlink-ok");
@@ -1748,6 +1778,31 @@ async fn post_retry_reminder(
     }
 }
 
+/// Validate that a path-param agent name conforms to the hyperhive
+/// naming whitelist: 1-63 chars of `[a-z0-9_-]`. Rejects empty,
+/// uppercase, slashes, dots, and any non-ASCII (incl. unicode
+/// homoglyphs of dash/underscore). Returns `None` on accept, `Some(reason)`
+/// on reject — caller wraps the reason in a 400 response. Conservative
+/// whitelist matching `nixos-container` basename rules and the existing
+/// agent-name convention across the codebase. (mara nag on #566 — this
+/// is the local fix for `mark-all-read`; general extractor + rollout
+/// across other endpoints tracked at #572.)
+fn validate_agent_name(name: &str) -> Option<&'static str> {
+    if name.is_empty() {
+        return Some("agent name must not be empty");
+    }
+    if name.len() > 63 {
+        return Some("agent name must be 63 characters or fewer");
+    }
+    if !name
+        .bytes()
+        .all(|b| b.is_ascii_lowercase() || b.is_ascii_digit() || b == b'-' || b == b'_')
+    {
+        return Some("agent name must contain only [a-z0-9_-]");
+    }
+    None
+}
+
 /// Operator-driven "clear this agent's inbox" — backs the side-panel
 /// "mark all read" button (#559). Marks every message addressed to the
 /// agent as acked (backfilling `delivered_at` for any still-pending
@@ -1758,6 +1813,9 @@ async fn post_mark_all_read(
     State(state): State<AppState>,
     AxumPath(name): AxumPath<String>,
 ) -> Response {
+    if let Some(reason) = validate_agent_name(&name) {
+        return (StatusCode::BAD_REQUEST, format!("bad agent name: {reason}")).into_response();
+    }
     match state.coord.broker.mark_all_read(&name) {
         Ok(n) => {
             tracing::info!(%name, marked = n, "operator marked all messages read");