---
id: PLN-0117
ticket: multi-frontend-frontend-backend-contract
title: Common IRBackend contract specs
status: done
created: 2026-07-15
ref_decisions: [DEC-0043]
tags: [compiler, compiler-general, compiler-pbs, ir, backend, multi-frontend]
---

## Briefing

Worker: Spec Editorial Worker.

Domain owner: `compiler/general`, with explicit propagation to `compiler/pbs`.

DEC-0043 accepts the current `IRBackend` shape as the common frontend-to-backend executable handoff. This plan propagates that decision into specs and conformance docs so the common backend is no longer described as PBS-owned by implication.

## Objective

Update compiler-general and PBS specs to state that `IRBackend` is the language-neutral executable handoff emitted by frontends and consumed by common backend stages, while PBS specs describe only PBS-specific admission and lowering into that handoff.

## Dependencies

- Accepted decision: `DEC-0043`.
- No code implementation is required before this plan.
- PLN-0118 may run after or in parallel, but this plan owns normative wording and spec ownership.

## Scope

Included:

- Update `docs/specs/compiler/20. IRBackend to IRVM Lowering Specification.md` so it is clearly a compiler-general backend contract, not a PBS backend contract.
- Update `docs/specs/compiler/22. Backend Spec-to-Test Conformance Matrix.md` to include the common handoff obligations and the required guardrail tests.
- Update `docs/specs/compiler-languages/pbs/13. Lowering IRBackend Specification.md` so it owns PBS lowering into `IRBackend`, not the common backend contract itself.
- Replace PBS-specific wording in common spec text when the obligation is actually common: executable handoff, lifecycle wrapper, published entrypoint, boot guard, spans, capabilities, host calls, intrinsics.
- Add explicit forbidden-coupling language: common backend specs must not require PBS AST, tokens, parser structures, semantic validators, editorial objects, or equivalent objects from future frontends.
- Preserve existing behavior and conformance obligations.

## Non-Goals

- Do not rename `IRBackend`.
- Do not redesign the IR model.
- Do not change `IRBackend -> IRVM` lowering behavior.
- Do not define serialization.
- Do not resolve synthetic test frontend design beyond referencing the need for guardrails already required by DEC-0043.
- Do not move PBS-specific language rules into compiler-general specs.

## Execution Method

1. Audit current spec ownership.
   - Read `docs/specs/compiler/20. IRBackend to IRVM Lowering Specification.md`.
   - Read `docs/specs/compiler/22. Backend Spec-to-Test Conformance Matrix.md`.
   - Read `docs/specs/compiler-languages/pbs/13. Lowering IRBackend Specification.md`.
   - Mark each `IRBackend` obligation as common backend, PBS lowering, or test/conformance bookkeeping.

2. Rewrite the compiler-general lowering spec.
   - Change the title and introduction if they imply PBS ownership.
   - State that `IRBackend` is the common executable frontend-to-backend handoff.
   - Describe allowed common concepts: source identity, spans, modules, callables, executable functions, instruction metadata, globals, synthetic functions, host/intrinsic metadata, reserved metadata, capabilities.
   - Describe forbidden language-owned inputs at the backend boundary.
   - Express lifecycle, published entrypoint, boot guard, and capability obligations in neutral backend terms.

3. Rewrite the PBS lowering spec boundary.
   - Keep PBS-specific parser, AST, semantic, source admission, and lowering rules in the PBS spec.
   - State that PBS emits the common `IRBackend` contract rather than owning it.
   - Replace any common backend obligation duplicated in PBS with a reference to the compiler-general spec unless PBS-specific lowering detail is required.

4. Update the conformance matrix.
   - Add or adjust rows for DEC-0043 obligations.
   - Reference PLN-0118 tests for direct manual `IRBackend` lowering, backend package independence from `p.studio.compiler.pbs`, and public-contract negative checks.
   - Keep existing PBS conformance rows only where they verify PBS lowering obligations.

5. Cross-check terminology.
   - Search specs for wording that says or implies PBS owns common backend behavior.
   - Leave PBS terms only when they describe PBS source or PBS lowering before the handoff.

## Acceptance Criteria

- [x] Compiler-general specs state that `IRBackend` is the common frontend-to-backend executable handoff.
- [x] Compiler-general specs do not describe common backend behavior as PBS-owned.
- [x] PBS lowering specs describe how PBS emits `IRBackend` and defer common backend obligations to compiler-general specs.
- [x] Forbidden coupling is explicit for PBS AST, tokens, parser structures, semantic objects, editorial objects, and future frontend equivalents.
- [x] Lifecycle, published entrypoint, boot guard, spans, and capabilities are written in neutral terms when they are backend obligations.
- [x] Conformance matrix maps DEC-0043 obligations to tests or planned tests without weakening existing coverage.

## Tests

Validation:

- Run `discussion validate`.
- Run a docs/spec search for `PBS IRBackend`, `PBS backend`, and similar phrases in compiler-general specs; remaining occurrences must be intentional and tied to PBS-specific references.
- If the repository has markdown lint or docs validation, run the established command.

## Affected Artifacts

- `docs/specs/compiler/20. IRBackend to IRVM Lowering Specification.md`
- `docs/specs/compiler/22. Backend Spec-to-Test Conformance Matrix.md`
- `docs/specs/compiler-languages/pbs/13. Lowering IRBackend Specification.md`
- `discussion/workflow/decisions/DEC-0043-common-irbackend-frontend-backend-handoff.md`