# Tenant Isolation Check

> Reviews multi-tenant backend changes for tenant context enforcement, no cross-tenant data access, and no primary-tenant fallback. Use when working on tenant-scoped APIs, database queries, auth middleware, or when the user mentions multi-tenancy, tenant isolation, or tenant headers.

- Skill: `practical-office/tenant-isolation-check` (Agent Skill)
- Install (CLI): `npx skillmds@latest add practical-office/tenant-isolation-check`
- Raw SKILL.md: https://api.skillmd.com/api/skills/practical-office/tenant-isolation-check/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: Practical-Office (https://skillmd.com/u/practical-office)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/practical-office/tenant-isolation-check

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# Tenant Isolation Check

Verify that backend changes cannot leak data across tenants.

## When to run

- New or modified API routes, services, or database queries
- Auth/middleware changes
- Background jobs that read or write tenant-scoped data
- Test changes that mock tenant context

## Checklist

### Tenant context required

- [ ] Every tenant-scoped request resolves tenant ID from an explicit source (header, JWT claim, session) — not from client-supplied body fields alone
- [ ] Missing or invalid tenant context returns **401/403**, not empty results or a default tenant
- [ ] Middleware or dependency injection applies tenant context before handlers run

### No cross-tenant reads or writes

- [ ] Queries filter by `tenant_id` (or equivalent) at the lowest practical layer
- [ ] Lookups by ID include tenant constraint (`WHERE id = ? AND tenant_id = ?`)
- [ ] Joins cannot bridge tenants via missing join predicates
- [ ] Admin/global endpoints are explicitly marked and gated — not accidental fallthrough

### No primary-tenant fallback

- [ ] No code path uses a hardcoded "default" or "primary" tenant when context is missing
- [ ] No `tenant_id IS NULL` or `OR tenant_id = ?` shortcuts that widen scope
- [ ] Feature flags and config are tenant-scoped where data is tenant-scoped

### Tests

- [ ] Tests cover: valid tenant, wrong tenant (expect 404/403), missing tenant (expect 401/403)
- [ ] Fixtures use distinct tenant IDs to prove isolation

## Output format

```markdown
## Tenant isolation: [PASS | FAIL | N/A]

**Scope:** [files/routes reviewed]

### Findings
- [CRITICAL/HIGH/MEDIUM] file:line — issue and fix

### Verdict
[One sentence — safe to merge / must fix before merge / not applicable (single-tenant)]
```

If the codebase is single-tenant, state **N/A** with brief rationale.