diff --git a/crates/service/web/storyteller_web/src/http_server/validations/validate_username.rs b/crates/service/web/storyteller_web/src/http_server/validations/validate_username.rs
index 04532d44b4..022b66c28c 100644
--- a/crates/service/web/storyteller_web/src/http_server/validations/validate_username.rs
+++ b/crates/service/web/storyteller_web/src/http_server/validations/validate_username.rs
@@ -4,6 +4,36 @@ use regex::Regex;
 /// Username may be up to this many characters, but not more.
 pub const USERNAME_MAX_LENGTH: usize = 16;
 
+/// Prefixes a username may not start with (case-insensitively), because each
+/// collides with one of our token / ID namespaces. Keeping usernames clear of
+/// these prevents a username from ever being mistaken for an entity token.
+const RESERVED_USERNAME_PREFIXES: &[(&str, &str)] = &[
+  // user tokens
+  ("user_", "Username can't start with 'user_'."),
+  // wallet tokens
+  ("wallet_", "Username can't start with 'wallet_'."),
+  // stripe customer IDs
+  ("cus_", "Username can't start with 'cus_'."),
+  // media file tokens
+  ("m_", "Username can't start with 'm_'."),
+  // close analogue to media file tokens
+  ("media_", "Username can't start with 'media_'."),
+  // folder tokens
+  ("folder_", "Username can't start with 'folder_'."),
+  // session tokens
+  ("session_", "Username can't start with 'session_'."),
+  // comment tokens
+  ("comment_", "Username can't start with 'comment_'."),
+  // inference job tokens
+  ("jinf_", "Username can't start with 'jinf_'."),
+  // hypothetical job tokens
+  ("job_", "Username can't start with 'job_'."),
+  // audit tokens
+  ("audit_", "Username can't start with 'audit_'."),
+  // character tokens
+  ("character_", "Username can't start with 'character_'."),
+];
+
 pub fn validate_username(username: &str) -> Result<(), String> {
   static USERNAME_REGEX: Lazy<Regex> = Lazy::new(|| {
     Regex::new(r"^[A-Za-z0-9_\-]{3,16}$").expect("should be valid regex")
@@ -21,6 +51,16 @@ pub fn validate_username(username: &str) -> Result<(), String> {
     return Err("invalid username characters".to_string());
   }
 
+  // Reject reserved token/ID prefixes. The comparison is case-insensitive, so
+  // "User_", "USER_", etc. are all rejected. Prefixes are lowercase, so we
+  // lowercase the username once and check against each.
+  let lowercased_username = username.to_lowercase();
+  for (prefix, error_message) in RESERVED_USERNAME_PREFIXES {
+    if lowercased_username.starts_with(prefix) {
+      return Err((*error_message).to_string());
+    }
+  }
+
   Ok(())
 }
 
@@ -43,4 +83,71 @@ mod tests {
     assert!(validate_username("........").is_err());
     assert!(validate_username("really-long-username-that-is-too-long").is_err());
   }
+
+  // ── Reserved token/ID prefixes ──
+
+  #[test]
+  fn reserved_prefixes_are_rejected_with_exact_messages() {
+    // (username, expected error message)
+    let cases = [
+      ("user_alice", "Username can't start with 'user_'."),
+      ("wallet_abc", "Username can't start with 'wallet_'."),
+      ("cus_abc123", "Username can't start with 'cus_'."),
+      ("m_abc", "Username can't start with 'm_'."),
+      ("media_abc", "Username can't start with 'media_'."),
+      ("folder_abc", "Username can't start with 'folder_'."),
+      ("session_abc", "Username can't start with 'session_'."),
+      ("comment_abc", "Username can't start with 'comment_'."),
+      ("jinf_abc", "Username can't start with 'jinf_'."),
+      ("job_abc", "Username can't start with 'job_'."),
+      ("audit_abc", "Username can't start with 'audit_'."),
+      ("character_a", "Username can't start with 'character_'."),
+    ];
+    for (username, expected) in cases {
+      assert_eq!(
+        validate_username(username),
+        Err(expected.to_string()),
+        "username {username:?} should be rejected with {expected:?}",
+      );
+    }
+  }
+
+  #[test]
+  fn reserved_prefixes_are_case_insensitive() {
+    let cases = [
+      ("USER_alice", "Username can't start with 'user_'."),
+      ("User_alice", "Username can't start with 'user_'."),
+      ("Wallet_abc", "Username can't start with 'wallet_'."),
+      ("CUS_abc", "Username can't start with 'cus_'."),
+      ("M_abc", "Username can't start with 'm_'."),
+      ("Media_abc", "Username can't start with 'media_'."),
+      ("FOLDER_abc", "Username can't start with 'folder_'."),
+      ("Session_abc", "Username can't start with 'session_'."),
+      ("Comment_abc", "Username can't start with 'comment_'."),
+      ("JINF_abc", "Username can't start with 'jinf_'."),
+      ("Job_abc", "Username can't start with 'job_'."),
+      ("Audit_abc", "Username can't start with 'audit_'."),
+      ("Character_a", "Username can't start with 'character_'."),
+    ];
+    for (username, expected) in cases {
+      assert_eq!(
+        validate_username(username),
+        Err(expected.to_string()),
+        "username {username:?} should be rejected with {expected:?}",
+      );
+    }
+  }
+
+  #[test]
+  fn reserved_words_are_only_blocked_as_prefixes() {
+    // The reserved tokens are only banned at the START of the username. The
+    // same letters elsewhere, or without the trailing underscore, are fine.
+    assert!(validate_username("xuser_").is_ok());
+    assert!(validate_username("myuser").is_ok());
+    assert!(validate_username("a_user_b").is_ok());
+    assert!(validate_username("muser").is_ok());       // "m" but not "m_"
+    assert!(validate_username("mediafoo").is_ok());    // "media" but not "media_"
+    assert!(validate_username("jobless").is_ok());     // "job" but not "job_"
+    assert!(validate_username("comments").is_ok());    // "comment" but not "comment_"
+  }
 }
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