diff --git a/hive-agent/src/harness_state.rs b/hive-agent/src/harness_state.rs
index 6f7273686..df46d71e3 100644
--- a/hive-agent/src/harness_state.rs
+++ b/hive-agent/src/harness_state.rs
@@ -264,8 +264,52 @@ pub fn context_window_tokens(model: &str) -> u64 {
 mod tests {
     use super::{
         DEFAULT_EFFORT, DEFAULT_MODEL, Source, context_window_tokens, effort_from, model_from,
+        read_harness_state, write_harness_state,
     };
 
+    /// A state file an older harness wrote, with the `active_model` nothing
+    /// reads any more, neither stops the agent reading its flags nor is lost
+    /// or turned into an error by the next write: the file is read as a JSON
+    /// value, not a typed struct, so a leftover field cannot fail to parse.
+    #[test]
+    fn a_leftover_active_model_is_ignored_and_survives_a_write() {
+        // `HYPERHIVE_STATE_DIR` is process-wide; this is the only test that
+        // sets it, so a lock of its own is enough.
+        static LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+        let _lock = LOCK
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner);
+        let dir = tempfile::tempdir().unwrap();
+        let previous = std::env::var_os("HYPERHIVE_STATE_DIR");
+        // SAFETY: serialised by `LOCK`, and restored below.
+        unsafe { std::env::set_var("HYPERHIVE_STATE_DIR", dir.path()) };
+        let file = dir.path().join("hyperhive-harness.json");
+        std::fs::write(
+            &file,
+            r#"{"rate_limited":true,"needs_login":false,"active_model":"opus"}"#,
+        )
+        .unwrap();
+
+        let read = read_harness_state();
+        write_harness_state(false, true);
+        let after = read_harness_state();
+        let stored: serde_json::Value =
+            serde_json::from_str(&std::fs::read_to_string(&file).unwrap()).unwrap();
+
+        // SAFETY: still holding `LOCK`.
+        unsafe {
+            match previous {
+                Some(v) => std::env::set_var("HYPERHIVE_STATE_DIR", v),
+                None => std::env::remove_var("HYPERHIVE_STATE_DIR"),
+            }
+        }
+        assert_eq!(read, (true, false));
+        assert_eq!(after, (false, true));
+        assert_eq!(stored["active_model"], "opus", "the leftover is kept");
+        assert_eq!(stored["rate_limited"], false);
+        assert_eq!(stored["needs_login"], true);
+    }
+
     #[test]
     fn context_window_defaults_per_model_family() {
         assert_eq!(context_window_tokens("claude-haiku-4-5"), 200_000);