diff --git a/test/blackbox-tests/test-cases/per-module-lib-deps/menhir-incremental-lib-cmi.t b/test/blackbox-tests/test-cases/per-module-lib-deps/menhir-incremental-lib-cmi.t
new file mode 100644
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+++ b/test/blackbox-tests/test-cases/per-module-lib-deps/menhir-incremental-lib-cmi.t
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+Regression guard for menhir mock-compile and sibling module
+incremental rebuild when a sibling library's interface changes.
+
+When a library declares a menhir grammar whose semantic action
+references types or values from a `(libraries ...)` dep, dune emits
+two compile rules whose deps cover that library's cmis: menhir's
+`--infer` mock-compile (which runs `ocamlc -i` on a generated mock
+module to type-check the grammar's semantic actions) and the regular
+sibling-module compile that also references the dep. Both rules
+must invalidate when the dep's interface changes.
+
+Code under test: `src/dune_rules/menhir/menhir_rules.ml`.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.24)
+  > (using menhir 3.0)
+  > EOF
+
+`dep` is a sibling library exposing a value the parent library's
+menhir grammar will reference. `dep.ml` carries both `value` and
+`extra` from the start so the `.mli` edit step is interface-only.
+
+  $ make_dir_with_dune dep <<EOF
+  > (library (name dep))
+  > EOF
+  $ cat > dep/dep.ml <<EOF
+  > let value = 42
+  > let extra = "x"
+  > EOF
+  $ cat > dep/dep.mli <<EOF
+  > val value : int
+  > EOF
+
+`parser`: a library with a menhir grammar whose semantic action
+references `Dep.value`, plus a sibling module `helper` that also
+references `Dep.value`.
+
+  $ make_dir_with_dune parser <<EOF
+  > (library
+  >  (name parser)
+  >  (libraries dep))
+  > (menhir (modules grammar))
+  > EOF
+  $ cat > parser/grammar.mly <<EOF
+  > %token EOF
+  > %start <int> main
+  > %%
+  > main: EOF { Dep.value }
+  > EOF
+  $ cat > parser/helper.ml <<EOF
+  > let echo () = Dep.value
+  > EOF
+  $ cat > parser/helper.mli <<EOF
+  > val echo : unit -> int
+  > EOF
+
+Initial build.
+
+  $ dune build @check
+
+Case 1: the mock-compile ran. The invocation is recognisable by
+`-i` in its argv; its target is `grammar__mock.mli.inferred`. The
+presence of this event positively asserts the `--infer` code path
+ran:
+
+  $ dune trace cat | jq -s 'include "dune"; [.[] | progMatching("ocamlc") | select(.process_args | index("-i") != null) | .target_files]'
+  [
+    [
+      "_build/default/parser/grammar__mock.mli.inferred"
+    ]
+  ]
+
+The mock's `-I` path includes `dep`'s objdir, so the grammar's
+semantic action type-checks against `dep`:
+
+  $ dune trace cat | jq -s 'include "dune"; [.[] | progMatching("ocamlc") | select(.process_args | index("-i") != null) | .process_args | [.[] | select(startswith("dep/"))]]'
+  [
+    [
+      "dep/.dep.objs/byte"
+    ]
+  ]
+
+The mock-compile rule's declared dep set (no `%{...}` variable
+resolves the menhir-generated mock target, hence the literal path):
+
+  $ dune rules --root . --format=json --deps _build/default/parser/grammar__mock.mli.inferred |
+  > jq -r 'include "dune"; .[] | depsGlobs | "\(.dir_kind) \(.dir) \(.predicate)"'
+  In_build_dir _build/default/dep/.dep.objs/byte *.cmi
+
+Case 2: editing `dep`'s `.mli` to expose `extra` must invalidate
+both consumers — the menhir mock-compile and `helper`'s `.cmo`.
+The `.ml` already carries `extra`, so this is an interface-only
+edit.
+
+  $ cat > dep/dep.mli <<EOF
+  > val value : int
+  > val extra : string
+  > EOF
+
+Second build. Assert independently that each of the two
+invariants holds: the menhir mock-compile rebuilt, and the helper
+`.cmo` rebuilt. The test stays robust whether other helper
+artefacts (cmi/cmti) also rebuild — that is governed by
+cctx-wide-vs-per-module dep filtering and isn't the property
+under guard here.
+
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("grammar__mock\\.mli\\.inferred"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/parser/grammar__mock.mli.inferred"
+      ]
+    }
+  ]
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("/parser__Helper\\.cmo$"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/parser/.parser.objs/byte/parser__Helper.cmo",
+        "_build/default/parser/.parser.objs/byte/parser__Helper.cmt"
+      ]
+    }
+  ]