Known gaps are pinned RDKit writer-parity cases that Grimace does not yet fully mirror. They are executable parity debt: the fixtures record exact RDKit outputs for a pinned RDKit version, and the diagnostic runner keeps those outputs visible until the implementation matches them.

Keep this separate from chemical validity. A known-gap string can be a valid SMILES string for the intended molecule; the gap is that the current Grimace writer language does not yet contain the same string that RDKit emits for the same reviewed writer surface.

Current cluster

The current known gaps are coupled directional-stereo cases. They involve slash/backslash bond-direction tokens whose placement depends on more than the local double bond: ring closures, traversal order, and neighboring directional carriers can all interact.

That is narrower than “stereo is unsupported.” Many stereo cases are already covered by passing exact-support or writer-membership fixtures. The known-gap cluster records the remaining RDKit-compatible directional-token placement work.

Pinned cases

The checked-in fixture is:

It currently contains 16 RDKit 2026.03.1 cases:

Group Cases Why it is hard
Directional ring-closure canonical output 1 Ring-closure spelling and slash/backslash orientation are coupled.
Manual multi-double-bond stereo outputs 4 Neighboring directional carriers interact across more than one double bond.
Manual stereo-atom mutation surfaces 2 RDKit can expose writer surfaces that depend on explicit stereo-atom assignments.
CHEMBL3623347 canonical output 1 Decoder-path membership is pinned, but full all-roots support is too large for the normal suite.
CHEMBL409450 random-vector outputs 8 Deterministic RDKit random-writer outputs exercise the same coupled directional-token family.

The RDKit-source-tree traceability view is RDKit serializer coverage. It maps these fixtures back to RDKit’s own serializer tests and source blocks, including the RDKit #4582 / #3967 cases and manual bond-stereo wrapper tests.

For a concrete failing parity case, see Parity examples.

How diagnostics run

Known gaps are intentionally outside default passing test discovery. Run them explicitly when working on directional-stereo parity:

PYTHONPATH=python:. python3 -m unittest tests.run_known_stereo_gaps -q

The runner is expected to fail until the pinned gaps are fixed. That is why the fixtures are not mixed into the passing writer-membership corpus.

Closing a gap

A known gap is closed only when the matching RDKit output belongs to Grimace’s supported writer language under the same pinned RDKit version and writer surface.

The usual promotion path is:

  1. Implement the missing RDKit-compatible directional-token behavior.
  2. Move the case into the strongest passing fixture family that applies: exact support when the support is small enough, otherwise writer membership or bounded decoder-path membership.
  3. Update the serializer ledger from known-gap to covered.
  4. Keep the fixture version-keyed to the RDKit version that supplied the expected output.

For the broader correctness vocabulary, see Correctness contracts. For the full fixture inventory, see Testing fixtures.