diff --git a/epiphany-pipeline/src/lib.rs b/epiphany-pipeline/src/lib.rs
index b3193a58..c7acff60 100644
--- a/epiphany-pipeline/src/lib.rs
+++ b/epiphany-pipeline/src/lib.rs
@@ -648,8 +648,8 @@ pub fn pipeline_key(document: &PipelineDocument) -> Result<String, PipelineRefus
         // ordinary id, so it composes as a subject like any other; each nesting
         // prepends `resolution.` (11 bytes), so a chain `n` deep over a
         // depth-one local of `L` bytes composes `11 * (n - 1) + L` bytes
-        // against the 64-byte bound in `local`, which is the only depth limit
-        // and needs no guard.
+        // against `LOCAL_MAX` in `local`, which is the only depth limit and
+        // needs no guard.
         D::Resolution(value) => {
             let field = "resolution.subject.id";
             let (subject_root, subject_local) = pipeline_id(field, &value.subject.id.0, value.subject.kind)?;
@@ -1307,6 +1307,18 @@ mod tests {
             "a campaign key is not an instance id"
         );
 
+        // Soul X1 and X11: the reader's root and local are each `dotted_text`
+        // whole, so a trailing dot on either is refused by the reader itself,
+        // not only by the writer that never composes one. A leading dot and
+        // an empty part are the same rule on the local.
+        for malformed in ["c.:target:x", "c:target:x.", "c:target:.x", "c:target:a..b"] {
+            let read = pipeline_id("read_back", malformed, PipelineKind::Target);
+            assert!(
+                matches!(&read, Err(PipelineRefusal::InvalidFormat { field, .. }) if field == "read_back"),
+                "{malformed:?} is not an id of any kind, got {read:?}"
+            );
+        }
+
         // The instance root's own key segment is validated, not merely bounded.
         // Nothing else stands between a `Slug` and a key: a space would compose
         // a store key carrying a byte no `Label` may hold.
diff --git a/tools/eureka-cut6b-mutations.psd1 b/tools/eureka-cut6b-mutations.psd1
index 43c1e34b..678906ce 100644
--- a/tools/eureka-cut6b-mutations.psd1
+++ b/tools/eureka-cut6b-mutations.psd1
@@ -12,8 +12,11 @@
 # pass: N7 and N8 re-anchor Cut 6's M12 and M13 on `key_segment`, N3 is the
 # analogue of Cut 6's M5 on `local`, N6 of Cut 6's M15 on the reader, and N9
 # and N10 are the dotted-root mutations that survived until
-# `dotted_roots_key_and_read_back`. The file holds an `é` literal in
-# `bounds_refuse_in_utf8_bytes`, which is why the harness I/O is byte-exact.
+# `dotted_roots_key_and_read_back`. X1 and X11 are Soul's third pass, the
+# reader trimming a trailing dot from the root or the local, killed by the
+# malformed fixtures in `keys_read_back_as_ids_of_their_kind`. The file holds
+# an `é` literal in `bounds_refuse_in_utf8_bytes`, which is why the harness
+# I/O is byte-exact.
 #
 # Soul's N4 moved the bound in `local` and both 64s in the depth test to 60
 # together and survived: the test restated the literal. `LOCAL_MAX` now names
@@ -169,6 +172,24 @@
             Test = 'tests::dotted_roots_key_and_read_back'
             Old  = '    dotted_text(field, root)?;'
             New  = '    label_text(field, root)?;'
+        },
+        # Soul X1 and X11, the reader's halves of S4: a trailing dot on the
+        # root or on the local was refused only by the writer, which never
+        # composes one, so a reader that trimmed it survived. The malformed
+        # fixtures read through `pipeline_id` kill both.
+        @{
+            Id   = 'X1'
+            Rule = 'The reader''s root check refuses a trailing dot; it does not trim one.'
+            Test = 'tests::keys_read_back_as_ids_of_their_kind'
+            Old  = '    dotted_text(field, root)?;'
+            New  = '    dotted_text(field, root.trim_end_matches(''.''))?;'
+        },
+        @{
+            Id   = 'X11'
+            Rule = 'The reader''s local check refuses a trailing dot; it does not trim one.'
+            Test = 'tests::keys_read_back_as_ids_of_their_kind'
+            Old  = '    dotted_text(field, local)?;'
+            New  = '    dotted_text(field, local.trim_end_matches(''.''))?;'
         }
     )
 }