diff --git a/cf-reactor/file_watcher.c b/cf-reactor/file_watcher.c
index b44730355ea..fa4f6d54809 100644
--- a/cf-reactor/file_watcher.c
+++ b/cf-reactor/file_watcher.c
@@ -82,7 +82,7 @@ typedef struct
 } FileWatcherState;
 
 
-void *FileWatcherStateNew(const Rlist *args)
+void *FileWatcherStateNew(ARG_UNUSED const EvalContext *ctx, const Rlist *args)
 {
     assert(args != NULL);
     const char *path = RlistScalarValue(args);
@@ -93,7 +93,7 @@ void *FileWatcherStateNew(const Rlist *args)
     return fws;
 }
 
-void *FileWatcherSizeStateNew(const Rlist *args)
+void *FileWatcherSizeStateNew(ARG_UNUSED const EvalContext *ctx, const Rlist *args)
 {
     assert(args != NULL && args->next != NULL);
     const char *path = RlistScalarValue(args);
diff --git a/cf-reactor/file_watcher.h b/cf-reactor/file_watcher.h
index b184d3651f0..9c426883da7 100644
--- a/cf-reactor/file_watcher.h
+++ b/cf-reactor/file_watcher.h
@@ -29,8 +29,8 @@
 #include <watcher.h>            /* WatcherCheckResult */
 
 
-void *FileWatcherStateNew(const Rlist *args);
-void *FileWatcherSizeStateNew(const Rlist *args);
+void *FileWatcherStateNew(const EvalContext *ctx, const Rlist *args);
+void *FileWatcherSizeStateNew(const EvalContext *ctx, const Rlist *args);
 void FileWatcherStateDestroy(void *state);
 
 WatcherCheckResult FileWatcherCheckFileDeleted(void *state);
diff --git a/cf-reactor/reactor_transform.c b/cf-reactor/reactor_transform.c
index 565e5a1094f..23650035ef8 100644
--- a/cf-reactor/reactor_transform.c
+++ b/cf-reactor/reactor_transform.c
@@ -132,11 +132,13 @@ static const Rlist *GetThenBundles(const Promise *pp)
 /*****************************************************************************/
 
 /* A constraint of the 'when' body passed to the watcher of an event, with the
- * type of value it accepts */
+ * type of value it accepts. An absent optional constraint isn't passed, so
+ * optional ones must come after the required ones. */
 typedef struct
 {
     const char *name;
     RvalType type;
+    bool optional;
 } EventArg;
 
 typedef struct
@@ -151,13 +153,13 @@ typedef struct
 
 static const EventArg NO_EXTRA_ARGS[] =
 {
-    { NULL, RVAL_TYPE_NOPROMISEE }
+    { NULL, RVAL_TYPE_NOPROMISEE, false }
 };
 
 static const EventArg FILE_SIZE_EXCEEDS_ARGS[] =
 {
-    { REACTOR_SIZE_THRESHOLD, RVAL_TYPE_SCALAR },
-    { NULL, RVAL_TYPE_NOPROMISEE }
+    { REACTOR_SIZE_THRESHOLD, RVAL_TYPE_SCALAR, false },
+    { NULL, RVAL_TYPE_NOPROMISEE, false }
 };
 
 #define EventTypeNew(name, type, args, new_state, destroy_state, check) \
@@ -264,8 +266,9 @@ static bool EventsPromiseGetInterval(const Promise *pp, time_t *interval)
     return true;
 }
 
-static bool ResolveWhenBody(const Promise *pp, WatcherOptions *opt)
+static bool ResolveWhenBody(const EvalContext *ctx, const Promise *pp, WatcherOptions *opt)
 {
+    assert(ctx != NULL);
     assert(pp != NULL);
     assert(opt != NULL);
     // Get the event and its arg
@@ -279,9 +282,20 @@ static bool ResolveWhenBody(const Promise *pp, WatcherOptions *opt)
     }
 
     // If the event has additional args, add them to the list of args
+    NDEBUG_UNUSED bool seen_optional = false;
     for (const EventArg *extra_arg = event->extra_args; extra_arg->name != NULL; extra_arg++)
     {
+        /* The state constructors read args by position. So optional args
+         * must come last: an absent one then only shortens the list, instead
+         * of shifting the args after it. */
+        assert(!seen_optional || extra_arg->optional);
+        seen_optional = seen_optional || extra_arg->optional;
+
         const Constraint *con = PromiseGetConstraint(pp, extra_arg->name);
+        if (con == NULL && extra_arg->optional)
+        {
+            continue;
+        }
         if (con == NULL)
         {
             Log(LOG_LEVEL_ERR,
@@ -336,10 +350,15 @@ static bool ResolveWhenBody(const Promise *pp, WatcherOptions *opt)
         return false;
     }
 
-    opt->state = event->new_state(args);
+    opt->state = event->new_state(ctx, args);
+    RlistDestroy(args);
+    if (opt->state == NULL)
+    {
+        /* The state constructor logs why */
+        return false;
+    }
     opt->check_callback = event->check;
     opt->destroy_state = event->destroy_state;
-    RlistDestroy(args);
     return true;
 }
 
@@ -364,7 +383,7 @@ static PromiseResult KeepEventsPromise(EvalContext *ctx, const Promise *pp)
     }
 
     WatcherOptions opt;
-    if (!ResolveWhenBody(pp, &opt))
+    if (!ResolveWhenBody(ctx, pp, &opt))
     {
         return PROMISE_RESULT_FAIL;
     }
diff --git a/cf-reactor/watcher.h b/cf-reactor/watcher.h
index bf12a91f63c..46f1ef6c137 100644
--- a/cf-reactor/watcher.h
+++ b/cf-reactor/watcher.h
@@ -35,7 +35,11 @@ typedef enum
   WATCHER_CHECK_ERROR,      /* unable to tell whether the event happened */
 } WatcherCheckResult;
 
-typedef void *(*WatcherStateNewFn)(const Rlist *args);
+/* Called on the main thread when the policy is evaluated. ctx must not be
+ * kept in the state: the EvalContext is not thread safe, so the watcher
+ * thread can't use it. Returns NULL, after logging why, if the state can't
+ * be created. */
+typedef void *(*WatcherStateNewFn)(const EvalContext *ctx, const Rlist *args);
 typedef WatcherCheckResult (*WatcherCheckFn)(void *state);
 typedef void (*WatcherStateDestroyFn)(void *state);
 
diff --git a/tests/unit/file_watcher_test.c b/tests/unit/file_watcher_test.c
index 51bdb0ccf4f..9ff463f88bb 100644
--- a/tests/unit/file_watcher_test.c
+++ b/tests/unit/file_watcher_test.c
@@ -28,7 +28,7 @@ static void *NewFileDeletedState(const char *path)
 {
     Rlist *args = NULL;
     RlistAppendScalar(&args, path);
-    void *state = FileWatcherStateNew(args);
+    void *state = FileWatcherStateNew(NULL, args);
     RlistDestroy(args);
     return state;
 }
diff --git a/tests/unit/watcher_test.c b/tests/unit/watcher_test.c
index c297793698f..b74308b8901 100644
--- a/tests/unit/watcher_test.c
+++ b/tests/unit/watcher_test.c
@@ -30,7 +30,7 @@ static WatcherOptions *FileDeletedOptions(const char *path, time_t interval)
     Rlist *args = NULL;
     RlistAppendScalar(&args, path);
     opt = (WatcherOptions) {
-        .state = FileWatcherStateNew(args),
+        .state = FileWatcherStateNew(NULL, args),
         .check_callback = FileWatcherCheckFileDeleted,
         .destroy_state = FileWatcherStateDestroy,
         .interval = interval,