diff --git a/crates/maestro-protocol/src/lib.rs b/crates/maestro-protocol/src/lib.rs
index 80f3e79..390c245 100644
--- a/crates/maestro-protocol/src/lib.rs
+++ b/crates/maestro-protocol/src/lib.rs
@@ -163,6 +163,13 @@ impl<'de> Deserialize<'de> for ActivityState {
 /// for a fixed TTL, then resumes inference.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct ReportActivityRequest {
+    /// Parsed strictly, unlike [`ActivityState`]'s own lenient `Deserialize`
+    /// (#110). Leniency exists so a client reading a *newer* daemon's snapshot
+    /// degrades an unrecognised state to `Idle` instead of dropping the whole
+    /// session. A report travelling the other way has no such excuse: an agent
+    /// posting a state this daemon does not know is a bug in the agent, and
+    /// silently recording it as `Idle` would hide that behind a 204.
+    #[serde(deserialize_with = "strict_activity_state")]
     pub state: ActivityState,
     /// Short human string shown next to the state (e.g. an error message).
     /// The daemon caps its length server-side.
@@ -170,6 +177,26 @@ pub struct ReportActivityRequest {
     pub detail: Option<String>,
 }
 
+/// Reject an unknown [`ActivityState`] spelling instead of degrading it to
+/// `Idle`. Used only by [`ReportActivityRequest`]; see the note there.
+fn strict_activity_state<'de, D>(deserializer: D) -> Result<ActivityState, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    use serde::de::Error as _;
+    let s = String::deserialize(deserializer)?;
+    match s.as_str() {
+        "Active" => Ok(ActivityState::Active),
+        "Idle" => Ok(ActivityState::Idle),
+        "AwaitingInput" => Ok(ActivityState::AwaitingInput),
+        "Error" => Ok(ActivityState::Error),
+        other => Err(D::Error::unknown_variant(
+            other,
+            &["Active", "Idle", "AwaitingInput", "Error"],
+        )),
+    }
+}
+
 /// Optional worktree request attached to a `CreateSessionRequest`.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct WorktreeSpec {
diff --git a/docs/SIDEBAR.md b/docs/SIDEBAR.md
index cab0444..f1d5a23 100644
--- a/docs/SIDEBAR.md
+++ b/docs/SIDEBAR.md
@@ -108,3 +108,9 @@ If a client receives an `ActivityState` variant it does not know, it must not
 fail rendering: treat it as `Idle` (neutral/dim, line-2 rules apply as usual)
 until the daemon sends a variant it does know. Rust clients deserialize
 activity leniently for the same reason.
+
+Leniency runs one way only. It covers a client *reading* a newer daemon, where
+dropping the whole session would be worse than dimming one card. A report
+arriving at `POST /api/sessions/:id/activity` is parsed strictly and an unknown
+state is a 4xx: an agent posting a state the daemon does not know is a bug in
+the agent, and answering 204 would hide it.