Design — MAISON-007: Parser-backed TOML mutations

Metadata

Beads feature root: maison-mol-ywg
Feature slug: maison-007-parser-backed-toml-mutations
Design path: docs/src/features/maison-007-parser-backed-toml-mutations/design.md
Implemented record: docs/src/features/maison-007-parser-backed-toml-mutations/index.md
Base branch: dev
Status: draft
Review priority: P1

Feature Summary

Replace the fragile TOML editor with a formatting-preserving parser-backed mutation layer using pinned tomlkit.

User Intent

A concrete regression proved that config_edit.py can delete an unrelated following section when a table header includes a trailing comment.

Goals

TOML mutation preserves comments, table boundaries, arrays of tables, quoted keys, and CRLF input.
Malformed input fails safely without partial writes.
Authoring commands share one parser-backed mutation layer.

Non-Goals

A broad rewrite of all authoring commands before the mutation layer is stable.
Supporting arbitrary TOML formatting normalization as a side effect.

Operators keep using maison and mise tasks as the command surface. The feature changes the underlying safety, validation, or documentation contract named above without requiring operators to learn an unrelated tool. When behavior is unsafe or unsupported, Maison fails with an actionable message instead of silently continuing.

Requirements

Functional Requirements

Pin tomlkit for mutation scripts through a deterministic repository-controlled runtime path that does not rely on ambient Python site-packages.
Preserve tool:add and tool:remove repairability when mise project tool resolution is broken; parser-backed editing must still work on the --skip-tools path.
Replace section-splicing logic in config_edit.py with parser-backed edits for tools, bootstrap packages/apps, lock entries, and inventory host additions.
Add regression tests for trailing comments, quoted keys, arrays of tables, CRLF input, unrelated following sections, malformed structures, lock blocks, and host inventory additions.
Mutation commands write only after successful parse, edit, validation, and lock reconciliation.

Quality Requirements

Preserve Maison’s Nix/Lix system ownership and mise user ownership boundary.
Prefer tested Python stdlib implementations for stateful behavior. Python task files under .mise/tasks/ are acceptable, including usage comments for argument validation and mise templating or sandboxing where useful.
Do not introduce a Rust helper unless a later design proves Python cannot safely satisfy the filesystem or privilege boundary.
Keep implementation small, reviewable, and covered by behavioral or fault-injection tests before relying on it.

Compatibility and Migration Requirements

Preserve supported platforms: aarch64-darwin, aarch64-linux, and x86_64-linux.
Preserve the absence of Home Manager.
Update existing commands in place rather than introducing parallel legacy paths.
Public Maison remains generic and public-safe; personal/site configuration belongs in a private overlay where relevant.

Existing Context

Maison currently documents a two-layer architecture where Nix/Lix owns privileged system state and mise owns user state. The review of commit ded7bbb745f34f1059930fc48eadafe267399ab2 identified this feature as required work. Existing reader documentation under docs/ describes the target operations and must be reconciled with delivered behavior.

Proposed Design

Introduce a small Python TOML mutation module built on pinned tomlkit. Existing TOML authoring surfaces call typed operations for adding and removing tools, bootstrap packages/apps, lock entries, and inventory hosts rather than editing text sections directly. Shell task transaction wrappers and command names remain unchanged.

The tomlkit runtime must be deterministic and repository-controlled because tool:add and tool:remove intentionally bypass mise project tool resolution when repairing invalid tool configuration. The implementation may vendor the exact reviewed tomlkit runtime or use an equivalently pinned checked-in dependency mechanism, but it must not depend on unpinned ambient site-packages.

The module preserves formatting where tomlkit can round-trip, including comments, quoted keys, table boundaries, arrays of tables, and CRLF newlines covered by tests. Unsupported malformed input is rejected before replacing files. Non-TOML authoring artifacts are outside this feature.

Architecture Consistency

Existing Patterns Reused

Nix/Lix remains the privileged system layer.
mise remains the user command and package/dotfile layer.
maison check remains the repository-wide validation entrypoint.
Python is the preferred home for stateful behavior that needs tests, parsing, locks, manifests, or recovery.

Invariants Preserved

A file, package, service, or preference has exactly one owner.
Intel macOS remains unsupported.
Remote deployment remains split between deploy-rs system-profile handling and Maison repository/user handling.
Public Maison does not contain private infrastructure identity or trusted access material.

New Decisions Introduced

This feature adopts the concrete behavior described in Proposed Design as the supported Maison contract.

Architecture Documentation Changes

Update the architecture and operations pages named in Documentation Impact so current reader-facing docs match the implemented behavior.

Operational Considerations

Operators should receive explicit errors, recovery instructions, and validation evidence for this feature’s failure modes. Recovery docs must distinguish Nix generation behavior, repository/source behavior, user convergence behavior, and external package-manager side effects when those concerns apply.

Documentation Impact

Exact page	Create or update	Planned change	Owning Beads task
docs/src/development/tooling.md	Update	Document the pinned parser-backed TOML mutation runtime for contributors	Implementation tasks
docs/task-reference.md	Update	Document parser-backed transactional TOML mutation guarantees for configuration commands	Implementation tasks
docs/add-a-tool.md	Update	Explain tool/package edits preserve supported TOML formatting and fail without partial writes	Implementation tasks
docs/add-an-app.md	Update	Explain app edits preserve supported TOML formatting and fail without partial writes	Implementation tasks
`docs/src/features/maison-007-parser-backed-toml-mutations/index.md`	Create during close-out	Preserve delivery and audit history	Close-out documentation
`docs/src/planned-features.md`	Update	Track roadmap status and Beads root	Planning
`docs/src/SUMMARY.md`	Update	Register this design and delivered record links	Planning / close-out

Validation Strategy

Focused tests/test_config_edit.py regression corpus for comments, quoted keys, arrays of tables, CRLF input, malformed TOML, lock blocks, host inventory additions, and unrelated sections.
Tests proving the pinned tomlkit runtime is used without ambient site-packages and remains available on the tool:add/tool:remove --skip-tools path.
Behavioral authoring command tests.
mise -E dev run check and uv run scripts/check-docs.py.

Implementation Decomposition

maison-007-parser-backed-toml-mutations toml-contract — Add parser-backed TOML regression corpus.
maison-007-parser-backed-toml-mutations toml-implementation — Replace text-splice TOML editing with pinned tomlkit layer.

Dependencies and Parallelism

This feature follows the Maison review order. Its implementation tasks depend on specification reconciliation. The contract task blocks the implementation task so regression coverage lands first. The two planned implementation tasks are not parallel-safe because both touch the mutation helper, authoring behavior, and related tests/docs.

Rollout and Migration

Roll out by updating tests and docs first, then replacing the existing implementation path. Existing commands keep their names unless a feature explicitly narrows unsafe behavior.

Risks and Tradeoffs

Safety fixes may temporarily reduce permissive behavior that previously appeared to work.
Deployment and recovery changes require fault-injection coverage because ordinary happy-path tests are insufficient.
Keeping shell and mise task compatibility while moving logic into Python may require small adapter changes.

Rejected Alternatives

Broad rewrite before fixing the review findings.
Preserving unsafe legacy paths as supported behavior.
Storing private infrastructure identity in public Maison defaults.

Stateful Maison behavior should move into tested Python implementations. .mise/tasks/ may contain Python task files and may use usage comments, task templating, and sandboxing.
The default privileged deployment account is maison-deploy.
Maison is the reusable vehicle; a tracked private overlay repository is the driver for personal/site configuration.
Fresh setup supports --overlay <git-url-or-path>, prompts only when interactive, and fails clearly when a required overlay is missing in non-interactive mode.
Public examples should be real workable starter configurations without personal infrastructure identity.

Assumptions

The reviewed commit and review summary are authoritative planning evidence for this remediation roadmap.
Feature branches should base on dev.

Design Changes During Planning

The private inventory model changed from untracked local TOML to a tracked private overlay repository to preserve Maison’s durability and replication goals.

Source Material

Maison review summary for commit ded7bbb745f34f1059930fc48eadafe267399ab2.
Current Maison documentation under README.md and docs/.

maison