diff --git a/docs/design/session-framework/publishing-v1.md b/docs/design/session-framework/publishing-v1.md
index c1ebce2..c438ac4 100644
--- a/docs/design/session-framework/publishing-v1.md
+++ b/docs/design/session-framework/publishing-v1.md
@@ -51,6 +51,10 @@ or, with no revision, the newest; `Session.GetSnapshot` takes no parameters and
 latest `CommittedSnapshot`. A revision the session never published is `IDENTITY_MISMATCH`, not
 an empty answer. Nothing on this service mutates, selects a participant or reaches a worker, so
 it is not the controller API section 7 rules out; adding a third method that did would be.
+These two names are **internal and provisional**: they are what the internal boundary needs in
+order to be buildable now, and the later public v2 step is free to rename them, supersede them
+or expose a different repair surface entirely. Nothing about them is browser-facing, and the
+public step does not inherit them by default merely because they landed first.
 
 Descriptor revisions and scope link observations to schemas. Cross-topic ordering is not
 guaranteed; a subscriber receiving an unknown descriptor revision must fetch it through the
diff --git a/services/flysim/crates/fly-session/src/coordinator.rs b/services/flysim/crates/fly-session/src/coordinator.rs
index aa21e8e..fb5ddaa 100644
--- a/services/flysim/crates/fly-session/src/coordinator.rs
+++ b/services/flysim/crates/fly-session/src/coordinator.rs
@@ -61,6 +61,8 @@ pub struct Injections {
     /// Publish this boundary's snapshot with a handle that is not the artifact the snapshot
     /// references: the same name, the same shape, another object.
     pub substituted_published_handle: bool,
+    /// Ask an agent to apply a stimulus kind its published descriptor does not declare.
+    pub undeclared_stimulus: bool,
 }
 
 /// What an injection produced, for a test to assert on.
@@ -2246,13 +2248,23 @@ impl Coordinator {
                 .expect("every agent prepared");
             let outcome = outcomes.get(&agent_id).cloned().unwrap_or_default();
             let next_context = next_contexts.get(&agent_id).cloned().expect("checked");
+            let mut task_stimulations = outcome.stimulations.clone();
+            if self.injections.undeclared_stimulus && self.injections.at_step == k {
+                // A kind outside the agent's published `supportedStimuli`. The declaration is
+                // only worth publishing if the worker enforces it.
+                task_stimulations.push(Stimulus {
+                    id: parse_id(&format!("stim-undeclared-{k}")).expect("a serial makes an Id"),
+                    kind_id: id("arena.undeclared"),
+                    duration_ms: 1.0,
+                });
+            }
             let params = CommitParams {
                 agent_id: agent_id.clone(),
                 prepared_request_id: prepared_request.clone(),
                 next_input: self.sensory_input(observation, k + 1),
                 next_decision_context: next_context,
                 rewards: outcome.rewards.clone(),
-                task_stimulations: outcome.stimulations.clone(),
+                task_stimulations,
             };
             let params = match params.to_json() {
                 Value::Object(m) => m,
diff --git a/services/flysim/crates/fly-session/tests/publishing.rs b/services/flysim/crates/fly-session/tests/publishing.rs
index 52ca5a9..a600717 100644
--- a/services/flysim/crates/fly-session/tests/publishing.rs
+++ b/services/flysim/crates/fly-session/tests/publishing.rs
@@ -37,6 +37,7 @@ both_transports!(
     a_refused_event_batch_is_held_and_counted_not_lost,
     the_query_service_answers_reads_and_nothing_else,
     the_published_descriptor_is_what_the_workers_attested_to,
+    a_stimulus_kind_the_descriptor_does_not_declare_is_refused,
 );
 
 all_modes!(
@@ -608,6 +609,61 @@ async fn the_published_descriptor_is_what_the_workers_attested_to(via: Via) {
     f.shutdown().await;
 }
 
+/// `supportedStimuli` is enforced, not advertised: a kind outside the list the descriptor
+/// publishes is refused before the model is touched, so the declaration is worth reading.
+async fn a_stimulus_kind_the_descriptor_does_not_declare_is_refused(via: Via) {
+    let mut f = started(via).await;
+    let declared = f
+        .harness
+        .coordinator
+        .session_descriptor()
+        .expect("a descriptor")
+        .agents
+        .iter()
+        .find(|a| a.agent_id == fly_a())
+        .expect("fly-a")
+        .supported_stimuli
+        .clone();
+    assert!(!declared.contains(&id("arena.undeclared")), "{declared:?}");
+
+    // A clean transition first, so the refusal is the injection and not the composition.
+    f.harness.coordinator.run(1).await.expect("one clean transition");
+    let before = f.harness.coordinator.stats().advances;
+    f.harness.coordinator.injections = Injections {
+        at_step: 1,
+        undeclared_stimulus: true,
+        ..Injections::default()
+    };
+    let failure = f
+        .harness
+        .coordinator
+        .run(1)
+        .await
+        .expect_err("an undeclared stimulus kind is refused");
+    assert_eq!(failure.error.code, ErrorCode::Unsupported, "{failure:?}");
+    assert_eq!(
+        failure.error.mutation,
+        MutationCertainty::None,
+        "refused before the model is touched"
+    );
+    assert!(
+        failure.error.message.contains("arena.undeclared"),
+        "the refusal names the kind: {}",
+        failure.error.message
+    );
+    assert!(failure.participant.is_some(), "and the participant it came from");
+    assert_eq!(
+        f.harness.coordinator.stats().advances,
+        before,
+        "the refused transition never completed, so no boundary was added"
+    );
+    assert!(
+        f.harness.coordinator.is_fenced(),
+        "a commit that refused after the world moved fences the epoch"
+    );
+    f.shutdown().await;
+}
+
 // ------------------------------------------------------------------------------------------
 // "A committed action is labelled as the transition that just ended, not the one about to
 // start."