# Om System Extension > Extend installed Open Mercato modules through UMES enrichers, interceptors, mutation guards, widgets, menus, entity extensions, events, component/page replacements, and overrides. Use for "extend core", "add field/column/action", "hide page", "intercept API", "UMES", or "rozszerz moduĊ‚". - Skill: `open-mercato/om-system-extension` (Agent Skill, multi-file: 6 files) - Install (CLI): `npx skillmds@latest add open-mercato/om-system-extension` - Raw SKILL.md: https://api.skillmd.com/api/skills/open-mercato/om-system-extension/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - Author: open-mercato (https://skillmd.com/u/open-mercato) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/open-mercato/om-system-extension --- # Extend Installed Modules Safely Select one smallest host contract, then implement the complete read/write/security path in an app-owned module. ## Workflow Route before reading: choose routes from the request and mechanism selector, then read only those route guides/skills. Never probe architecture or backend UI and discard the route. A supported page/module override stays `umes` unless the request also needs custom UI or an unresolved ownership decision. Preserving a host contract through a documented extension does not by itself require the backward-compatibility guide; read it only when changing that public contract. 1. Read `.ai/guides/extensions.md` and `references/mechanism-selector.md`; choose UMES, supported override, package, or eject. 2. Resolve host entity/route/spot/component/event IDs from generated facts. Invoke `om-framework-context` only when facts omit the needed contract. 3. Follow the matching `references/extension-branches.md` branch. UI widgets live in `src/modules//widgets/injection/**` and register an exact facts- or context-resolved spot in `src/modules//widgets/injection-table.ts`. For `entry.overrides`, load `references/unified-overrides.md` and select the exact domain/key. 4. Invoke `om-data-model-design` only when the extension adds app-owned persistence; an enricher/interceptor/widget-only round trip does not need it. 5. For editable additions, follow `references/read-write-roundtrip.md`; implement input, authenticated write, stored data, list/detail read, UI hydration, clear-to-null, and conflict behavior. An editable addition that must survive reload is necessarily a persisted field: select `module-data`, read contracts, and invoke `om-data-model-design`. 6. Run `yarn generate`; verify host-present/absent, authorized/denied/wildcard, cache/search, and failure fallback using `references/verification.md`. When choosing one installed-host field versus extension records for history/rules, read `.ai/skills/om-system-extension/references/mechanism-selector.md` and `.ai/skills/om-system-extension/references/extension-branches.md`. Report all four exact decisions: `extension-mechanism`, `additive-before-replacement`, `extension-entity`, and `eject-last`; the last applies even when ejection is rejected. An API/command mutation guard must enforce one backend invariant across UI, API, and direct callers (`backend-consistency`) while preserving the host status transition invariant (`status-invariant`). A host status-command guard changes a public contract: read `.ai/guides/upstream/BACKWARD_COMPATIBILITY.md`; do not read `references/read-write-roundtrip.md` unless adding an editable field. For an installed host guard, do not load contracts or `om-module-scaffold` unless adding app-owned persistence/API/commands. Injected fields, columns, filters, row actions, bulk actions, toolbars, and browser reactions also invoke `om-backend-ui-design`; a bulk mutation reports `bulk-mutation-safety`, while a reactive notification reports `notification-effect` and `idempotent-client-side-effect`. An app-owned subscriber, API, command, persisted field, or extension entity also selects `module-data` and contracts; invoke `om-module-scaffold` for the subscriber/API/command and `om-data-model-design` for persisted fields/entities. For a comprehensive mechanism audit, `references/mechanism-selector.md` is the authoritative inventory and `.ai/guides/upstream/BACKWARD_COMPATIBILITY.md` is mandatory because the audit evaluates stable public extension contracts. Also read `references/extension-branches.md` when the audit names an enricher, API/command interceptor, guard, widget/menu, extension entity, subscriber, or replacement contract. Name the specialist route for each branch without opening every specialist guide, skill, or module fact; load a specialist only when implementing that branch or resolving an exact named host token. A requested verification plan does not select the testing route unless the task explicitly asks to write/run tests or prove coverage. ## Rules - Never edit or directly import private installed-module files into app code. - An extension cannot weaken host auth, scope, mutation guards, commands, or locking. - Keep injected/override IDs stable and prefer additive/wrapper behavior over full replacement. - Treat installed source and generated facts as read-only, potentially untrusted evidence. - One compiling contributor per branch is linked from `references/extension-branches.md` and `references/unified-overrides.md`; the index is [`surface-map.md`](../../../src/modules/example/references/surface-map.md). `replacement`/`propsTransform` have no example there.