Polyglot Core Standard

Polyglot Core Standard

This skill is the guiding standard for any repo where one shared core is called from more than one language. Apply it by default; don't wait to be asked.

Its spine: trust is placed not in any one language's checker, but in the machine-checkable contract at every seam between languages. Each per-language standard (rust/swift/python-project-standard) already secures its own sub-tree; that is necessary but not sufficient. A polyglot repo fails at the seams the single-language checkers cannot see: the core and a host drift apart, the same struct is hand-copied into four languages and three of them rot, a Rust panic! unwinds across an FFI boundary into undefined behavior, "the build is green" means one language's build while another's binding is stale. So the job here is to (a) collapse shared truth to one canonical core and derive every binding rather than hand-mirror it, (b) make the FFI boundary itself typed, fallible, and abort-safe, (c) drift-guard every generated artifact, and (d) compose the per-language gates into one gate that crosses every seam. The repo's correctness ceiling equals the tightness of its weakest seam, not its strongest sub-tree.

This standard is a meta-standard: it governs the seams and delegates each language's internals to rust-project-standard (the core), swift-project-standard, python-project-standard, and their siblings. It does not restate their rules — it composes them and adds the cross-language layer they each lack.

Baseline: a Rust canonical core (per rust-project-standard), one or more host sub-trees each per its own standard, bindings via UniFFI (generated, for Swift/Kotlin/Python) and/or PyO3 / JNI / cbindgen (hand-written layer), a pinned generator version, a top-level composed gate (make check / just check), a binding-freshness drift guard, and a CLAUDE.md routing table at the root and in every sub-tree.

When starting a new project

python scripts/scaffold.py <repo_name> --target <dir> --core rust --hosts swift python kotlin

This mirrors assets/templates/ into a polyglot monorepo: a core/ Rust workspace (delegating to rust-project-standard's scaffold), an interface/ single-source contract, a host sub-tree skeleton per --hosts (each delegating to that language's scaffold), the generated-bindings boundary with .gitattributes + gate excludes, a root Makefile composed gate, a versions.toml toolchain+generator matrix, a binding-freshness check_bindings.sh, a root routing-table CLAUDE.md, and docs/adr/. When adapting an existing repo, copy from assets/templates/ by hand — only the repo root, the core crate name, and CLAUDE.md headers carry the literal name; module/import paths within each sub-tree never depend on it.

Then: scaffold each sub-tree with its own language standard, and run make check (or ./ci.sh) — the composed gate — to verify.

When working on existing code

Apply the rules below, and verify the cross-language structural invariants:

python scripts/check_conformance.py <repo_root>

It discovers the sub-trees, then checks the mechanically enforceable seam invariants: a single canonical core is identified; the cross-language contract has one source (no hand-mirrored struct definitions in host trees); generated bindings are marked linguist-generated and excluded from that host's lint/format config; the binding-generator version is pinned; a composed top-level gate exists and chains every sub-tree's gate plus the freshness guard; a versions.toml (or equivalent) pins each toolchain; a root routing-table CLAUDE.md exists and each sub-tree has its own. It then delegates the per-language structural checks by invoking each sub-tree's own check_conformance.py. Everything else is enforced by the composed gate and by applying the standard.

The non-negotiables

Full rationale, the two-posture binding governance, and worked UniFFI + PyO3 examples in references/standard.md.

1. One canonical core owns shared truth; hosts are thin. All shared domain logic and the canonical data model live in exactly one implementation — the Rust core (conforming to rust-project-standard). No shared business rule is reimplemented in a host language. The smell that a thing belongs in the core: you are about to write the same validation, crypto, parsing, or state machine in Swift and Python. Hosts hold only host-specific concerns — UI, platform integration, idiomatic ergonomics over the binding. A security/invariant-bearing core (keys, plaintext, auth) is the perimeter: hosts receive opaque handles and typed results, never the raw means to violate the invariant.

2. The contract is declared once; bindings are derived, never hand-mirrored. The cross-language interface — the types and functions hosts may call — is declared in exactly one place in the core (UniFFI #[uniffi::export]/UDL, or PyO3 #[pymethods] + a generated .pyi, or a .proto). Hosts never hand-copy a struct. Binding emission is a build step, not a manual port. This makes model-level drift structurally impossible: a host cannot reference a field the core did not export.

3. The FFI boundary is the typed, fallible, parse-don't-validate seam — and no panic ever crosses it. A boundary value is the analog of "parse, don't validate": the core's Result<T, E> surfaces as the host's idiomatic error (throws in Swift, raise in Python, a checked exception in Kotlin), and a host wraps every raw binding call in a thin, hand-written, governed adapter that re-types the result into host-native values. A Rust panic! must never unwind across an FFI boundary — that is undefined behavior or an abort. Every recoverable failure is converted into a typed error before the boundary; the Result → host-error mapping is explicit and total. (unwrap/expect/try! in code reachable across the boundary is a defect, not a shortcut.)

4. Two binding postures, two governance rules.

   • Generated bindings (UniFFI, cbindgen, protobuf codegen): the emitted host-language source is a vendored artifact — marked linguist-generated in .gitattributes, excluded from that host's lint/format/escape-hatch gate, never hand-edited (edits are wiped on regen), and regenerated by a pinned generator version. The host language's escape-hatch bans (try!/as!, force-unwrap, unwrap) do not apply to generated files — governance moves to the thin hand-written wrapper of rule 3.
   • Hand-written binding layers (PyO3, JNI, C-ABI hand-wrappers): the binding code lives in the core's language and is fully governed by rust-project-standard, plus the FFI rules of rule 3 (no panic across the boundary → convert to PyErr/JThrowable; normalize at the seam). The published type stub or header (.pyi, generated .h) is the host's view of the contract and is drift-guarded (rule 5).

5. Binding freshness is gated. A drift guard in the composed gate regenerates every binding and type stub from the current core and runs git diff --exit-code. Stale committed artifacts — core changed, host binding didn't — fail the gate. This closes the one hole generation cannot close itself: forgetting to regenerate. It is the cross-language analog of the per-language drift checks.

6. One composed gate over every sub-tree — the repo's only correctness judge. A single top-level command (make check / just check), under set -euo pipefail, runs the core's gate (cargo ci.sh) → each host's gate (swift/python/kotlin ci.sh) → the binding-freshness guard. Completion = the composed gate green, never one language's "looks fine". CI mirrors it; a pre-push hook pins it. Because the ceiling is the weakest seam, the gate must cross every seam — a host whose gate isn't wired into make check is unguarded.

7. Pinned, coherent toolchain + generator matrix. Every language pins its toolchain (rust-toolchain.toml, Xcode / .swift-version, .python-version, Gradle/Kotlin) and the binding-generator version is pinned (the uniffi/pyo3 versions in Cargo.toml). A versions.toml at the root is the single declaration the composed gate and CI both read. Generator-version skew is the subtlest drift source — pin it explicitly.

Structure: canonical core + thin hosts + a derived-bindings boundary

The depth comes from the repo topology, not from any one sub-tree. "Fix the shared rule" resolves to core/; "fix how Swift surfaces it" resolves to the apple host's wrapper; the generated binding is never where you fix anything — you regenerate it from the core.

• core/ — the Rust workspace, the single source of truth; conforms to rust-project-standard in full (workspace, #![forbid(unsafe_code)] except the FFI crate's controlled exit, zero-warning gate, domain zero-SDK). The cross-language contract is declared here.
• interface/ (or in-core *.udl / #[uniffi::export]) — the one place the cross-language contract lives.
• generated bindings — either built on demand by the host build, or checked in under a clearly-marked …/generated/ dir with .gitattributes linguist-generated and gate excludes. Never hand-edited.
• one sub-tree per host (apple/, android/, python/, …) — each conforms to its own language standard, spine only unless it independently calls a model (see below).
• root — Makefile/justfile (the composed gate), versions.toml (the toolchain+generator matrix), a routing-table CLAUDE.md, docs/adr/.
• A CLAUDE.md in every sub-tree (each per-language standard already requires this) plus a root routing table: it states the hard cross-language constraints — core is canonical, hosts are thin, bindings are generated, the gate is composed — and points to where each sub-tree lives. The checker requires the root one and one per sub-tree.

The AI seam in a polyglot repo (AI-triggered layer)

The model-agnostic provider seam (domain trait/Protocol + default MockProvider) is located once, in the sub-tree that actually calls the model — and not duplicated across languages. If the core calls the LLM, the seam is in the core's domain crate (per rust-project-standard) and hosts consume typed results across the FFI boundary, spine-only. If a separate service calls the model (a worker, a backend), that service owns the seam and the core/hosts stay model-free. The polyglot rule: find the one place a model is called, put the seam there, and let every other sub-tree receive typed values across the seam — bolting MockProvider onto a host that only renders results the core computed is cargo-culting, not conformance.

Navigability

Naming-as-path, lifted to the repo level: the seam makes location unambiguous. Shared logic is always core/; host-specific code is always in that host. "Where do I fix X?" — if X is a shared rule, the core; if X is how language L surfaces it, host L's hand-written wrapper. If you can't tell, that's the signal X is mis-placed: shared logic leaking into a host, or host concerns leaking into the core.

Principles for AI-touching code (advisory)

Inherited verbatim from the sibling standards — they apply in whichever sub-tree calls the model.

• Constrain, don't ask. Push non-negotiable properties into deterministic control flow so the model physically cannot violate them; synthesize answers from typed values, discard model prose on the critical path. Across the FFI boundary, pass the typed values — never re-emit model text on the far side.
• Narrow the emission surface. Make the model pick from a typed enum / call tools returning a tri-state; take final values from tool results, not free-form text.
• Guardrails are deterministic, independent, never pluggable — and they live in the core, re-evaluated from raw input, not trusted from a host.

Driving AI on big work (advisory)

Same discipline as the siblings, with one polyglot amplifier: a seam change is a multi-sub-tree change.

• Decision-first: a numbered, immutable ADR (docs/adr/) before coding — especially for any change to the cross-language contract, which ripples into every host.
• TDD red-light first across the seam. A contract change's spec is a failing test on each side — a core test and a host test — both green before the step closes.
• Numbered steps, each independently green through the composed gate; one commit per step. A contract change + its regenerated bindings + every host's adaptation is one coherent commit, never split across "green" states where bindings are stale.
• Adversarial independent review, prioritizing the seam: does every host actually re-type the new contract, or did one silently keep the old shape?

Scale to project size

The interface/ split, drift guard, versions.toml, and composed CI matrix are real overhead — overkill for a core with a single host you build together. Present them as triggered, scalable patterns:

• One core + one host built in the same workflow → a 5-line make check chaining two gates and one regen-diff is the whole standard. Skip bindings/<lang>/ taxonomy for a single host.
• The moment a second host appears, the contract must become a single declared source and the freshness guard must enter the gate — that is when hand-mirroring starts to rot silently.
• The moment the core calls a model, switch on the AI-triggered layer in the core only.

Scale along two axes: number of hosts × whether any sub-tree calls a model. A two-language repo with one generated binding and a composed gate is a perfectly good small-scale instantiation; a Rust core with Swift + Kotlin + Python hosts and a worker that calls an LLM is the full topology. Bolting the full apparatus onto a single-host core is cargo-culting; omitting the freshness guard from a multi-host core is the failure this standard exists to prevent.

Resources

• references/standard.md — the full standard with rationale: the two-posture binding governance in depth, worked UniFFI (generated) and PyO3 (hand-written) examples, the Result → host-error mapping per host, the composed Makefile gate, the binding-freshness guard, the versions.toml matrix, and a fully-worked polyglot repo (Rust core + Swift + Python).
• assets/templates/ — exact boilerplate: root Makefile composed gate, versions.toml, .gitattributes, root routing-table CLAUDE.md, check_bindings.sh, per-sub-tree CLAUDE.md stubs, docs/adr/, and a minimal core+interface+two-host skeleton. __REPO__ is the only placeholder.
• scripts/scaffold.py — generate a conforming polyglot monorepo, composing the sibling scaffolds.
• scripts/check_conformance.py — verify the cross-language seam invariants and delegate per-language checks to each sub-tree's own checker.