Nextjs App Router Data Fetching Review

Next.js App Router Data Fetching Review

Purpose

Review the Server Component / Client Component boundary ('use client' placement and its import graph) and Server Action ('use server') authorization logic in a Next.js App Router codebase, without re-litigating rendering mode, fetch() caching, or component decomposition in every response. This skill exists so bundle-leak risk and Server Action authorization-trust defects stay the focus, and so those adjacent concerns stay out of scope.

When to use

Use this skill when the user asks to:

• review a 'use client'/'use server' boundary in a diff or PR,
• determine whether a Server Action correctly authorizes its caller,
• investigate a report that server-only code or a secret appears to be reaching the browser bundle.

Do not use this skill for:

• pure UI/styling changes with no data-fetching or boundary change,
• Pages Router API routes (pages/api/*) — different security model, not Server Actions; use general API-route review instead,
• rendering-mode selection (static/ISR/dynamic) or fetch() cache-configuration review — that is nextjs-rendering-caching-review,
• component decomposition or state-placement review with no boundary or Server Action involved — that is react-component-architecture-review.

Context7 Documentation Protocol

• Resolve /vercel/next.js with resolve-library-id before citing any Server Component restriction or Server Action security claim.
• Before asserting what is or is not safe to import into a Client Component, or what a Server Action must re-verify, read the repo's package.json to confirm the installed Next.js major version, then call query-docs scoped to that version for "Server Actions security" and "Server Components restrictions." The exact bundling/serialization rules and the availability of the Taint API (experimental_taintObjectReference / experimental_taintUniqueValue, gated behind experimental.taint in next.config) are version-specific.
• If Context7 is unavailable, fall back to the official_docs URLs in this skill's metadata.json and label the claim documentation-based, unverified against current release.

Lean operating rules

• Server Components are the App Router default; 'use client' is an opt-in boundary marker at the top of a file, above imports. Once a file is marked 'use client', everything it imports and directly renders is included in the client bundle — trace that import graph, do not eyeball the single file.
• Treat a Client Component's import graph reaching a server-only module (a DB/ORM client, a module reading a non-NEXT_PUBLIC_-prefixed env var, filesystem access, or any module importing server-only) as a HIGH-severity bundle-leak finding. This is the hard security gate for this skill's boundary half.
• A Server Action's arguments (including FormData) are fully client-controlled, even though the function body runs on the server. Treat any Server Action that derives its authorization decision from its own input parameters or FormData — instead of re-deriving identity from the server-side session (cookies(), an auth()/session helper) — as a HIGH-severity Broken Access Control finding (OWASP Top Ten A01), not a style note.
• Page-level or layout-level authentication/authorization checks do not extend into a Server Action invoked from that page. Each Server Action must independently re-verify session and, for resource-scoped mutations, ownership/role — absence of that re-check is a defect even if an upstream page already checked auth.
• Do not recommend converting a Client Component to a Server Component without first checking it doesn't rely on browser-only APIs, local state, effects, or event handlers — that breaks functionality, it does not fix a boundary defect.
• Do not flag every 'use client' directive as a problem. Only flag it when the boundary is placed higher than the smallest interactive leaf needs, or when its import graph leaks server-only code.
• Never execute, build, or run application code as part of this review; this is a static-review skill (Read/Grep/Glob only).
• Treat any hardcoded API key, token, session secret, or credential found in a Server Action, Client Component, or example data as a HIGH-severity finding requiring immediate escalation, not a boundary note.

References

Load these only when needed:

• Review workflow and findings contract — use for the step-by-step review procedure, the bundle-leak trace method, the authorization decision tree, and the required output shape.
• OWASP A01 — Broken Access Control — load only when a Server Action authorization finding is present, to ground the finding's severity and framing.

Response minimum

Return, at minimum:

• per-boundary table ('use client' file, traced import-graph leak or clean, evidence file:line),
• per-Server-Action table (action, authorization source used, verdict),
• ranked findings with file:line, risk class, and fix,
• the Next.js major version the claims were verified against,
• verdict: approve / approve-with-notes / block,
• evidence level and open questions.