T81 Foundation

Runtime Semantics Boundary (Foundation vs VM)

This note locks the ownership boundary between normative semantics (t81-foundation) and executable runtime behavior (t81-vm).

Active Runtime Baseline

Active tagged runtime baseline: runtime-contract-v0.5
VM contract version: 2026-02-08-v5
VM contract commit pin (t81-vm/main): 4158a42156a085a2b722205be951576fc01969b9
Contract marker in this repo: contracts/runtime-contract.json

Ownership Split

t81-foundation owns:

Normative semantics in spec/ (language, VM model, ISA meaning, canonical data behavior).
RFC process for semantics changes (spec/rfcs/).
Architectural intent and invariants that execution must preserve.

t81-vm owns:

Executable implementation of VM behavior.
Runtime compatibility artifact and host-facing ABI contract:
- t81-vm/docs/contracts/vm-compatibility.json
- t81-vm/include/t81/vm/c_api.h
Parity/conformance automation and evidence artifacts.

Source-of-Truth Rule

If code behavior and spec/ differ, treat spec/ as semantically authoritative.
If runtime compatibility details differ across repos, treat t81-vm/docs/contracts/vm-compatibility.json as executable compatibility authority.
Any proposed change that affects both semantics and runtime must ship as a linked cross-repo change set.

Required Cross-Repo Fan-Out

Contract-impacting runtime changes must synchronize:

t81-vm contract artifact/tag and ABI headers.
t81-lang compatibility gate and docs.
t81-python bridge compatibility docs/tests.
t81-roadmap migration status/checkpoint artifacts.

High-Risk Mismatch Zones

Opcode semantics and trap classes.
Numeric conversion/quantization behavior claims.
Host ABI signatures and struct layout expectations.
Determinism guarantees used by downstream tooling.

Verification Rule

Run python3 scripts/check-runtime-contract-sync.py whenever runtime-contract versions are promoted.
CI enforces marker drift approval (T81_ALLOW_RUNTIME_CONTRACT_CHANGE=1) for intentional contract marker updates.

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