fix: treat an empty send model as absent for schema-materializing models

actions.js

@@ -104,7 +104,10 @@ function createActions(runtime) {
     const owner = contextSessionId(ctx);
     const sessionId = targetSessionId(args);
     const content = text(args.message, "message", 65_536);
-    if (args.model !== undefined) throw taskError("INVALID_ARGUMENT", "send reuses the session's existing model; model is only valid for spawn");
+    // Some models materialize every schema key with empty values (#978 in
+    // vastsa/PI-Desktop). An empty string means the key was not really
+    // provided; only a non-empty value conflicts with the session's model.
+    if (args.model !== undefined && args.model !== "") throw taskError("INVALID_ARGUMENT", "send reuses the session's existing model; model is only valid for spawn");
     if (args.kind !== undefined && !["task", "message"].includes(args.kind)) {
       throw taskError("INVALID_ARGUMENT", "kind must be task or message");
     }

test/session-orchestrator.test.mjs

@@ -80,6 +80,11 @@ test("any existing session supports bidirectional messages without reselecting m
   assert.equal(h.modelReads, 0);
   assert.equal(h.calls.at(-1).args[0].notifyOnCompletion, false);
   await assert.rejects(h.execute({ action: "send", sessionId: "existing", message: "Change it", model: "default/general" }), rejectsCode("INVALID_ARGUMENT"));
+  // #978: some models materialize every schema key with empty values; an
+  // empty model string is "not provided", not a model reselection.
+  const emptied = await h.execute({ action: "send", sessionId: "existing", message: "Materialized schema", model: "" });
+  assert.equal(emptied.sessionId, "existing");
+  assert.equal(h.sessions.get("existing").modelKey, "existing/unchanged");
   await assert.rejects(h.execute({ action: "send", sessionId: "existing", message: "Forged completion", kind: "completion" }), rejectsCode("INVALID_ARGUMENT"));
   assert.equal(h.calls.filter((entry) => entry.operation === prefix + "spawn").length, 0);
 });