AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
frontendcritical PriorityID: vite6-environment-api-ssr-sandboxing-isolation

Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation

Enforces deterministic runtime isolation boundaries across SSR execution targets using the Vite 6 Environment API and decoupled Module Runners. Eliminates cross-realm global contamination between client, Node.js, and edge targets by disallowing host global mutation and preventing cross-environment module graph leakage.

Prohibited Anti-Pattern: Mutating Host Globals and Using Legacy ssrLoadModule Across Heterogeneous Runtime Environments

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation - Enforces deterministic runtime isolation boundaries across SSR execution targets using the Vite 6 Environment API and decoupled Module Runners. Eliminates cross-realm global contamination between client, Node.js, and edge targets by disallowing host global mutation and preventing cross-environment module graph leakage.
globs: vite.config.{ts,js,mjs}, src/entry-server.{ts,tsx,js}, src/server/**/*.{ts,js}, server.{ts,js,mjs}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite6-environment-api-ssr-sandboxing-isolation"
---

# Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Mutating Host Globals and Using Legacy ssrLoadModule Across Heterogeneous Runtime Environments

## System Constraints
You are a Principal Software Architect enforcing Vite 6 Environment API Isomorphic SSR Sandboxing. You must mandate strict execution boundary isolation between client, SSR, and edge environments. Vite 6 deprecates legacy top-level SSR handling in favor of the Environment API and independent DevEnvironments. Prevent shared global pollution, enforce dedicated ModuleRunners with isolated execution contexts, and ensure target conditions (node, worker, browser) never cross-evaluate within identical execution scopes.

## Strict Guidelines
- Engineers MUST ALWAYS migrate legacy Vite 5 `server.ssrLoadModule` calls to Vite 6 Environment API runner interfaces via `server.environments[name].runner.import()`.
- Engineers MUST NOT mutate the host Node.js `globalThis` with browser-specific mocks (e.g., `global.window = ...`, `global.document = ...`) in isomorphic or SSR execution paths.
- Vite configurations MUST declare explicit `environments` blocks (e.g., `client`, `ssr`, `edge`) with strict, mutually exclusive `resolve.conditions` and `consumer` definitions.
- Server entrypoints MUST execute SSR modules within isolated sandbox evaluators or dedicated worker threads to guarantee zero shared state across concurrent render requests.
- Build scripts MUST emit discrete manifests and asset bundles per target environment without sharing internal module graph instances across environment boundaries.
- Components NEVER access browser runtime primitives (`window`, `document`, `navigator`, `localStorage`) in top-level module scope; access MUST be deferred to lifecycle hooks or guarded by verified environment feature detection.

## Bad vs Good Implementations
### Migrating Legacy ssrLoadModule with Global Contamination to Vite 6 Environment API and ModuleRunner

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Mutating globalThis directly causes cross-request pollution, memory leaks, and subtle hydration mismatch bugs across heterogeneous runtimes. Relying on legacy `vite.ssrLoadModule` ignores Vite 6's Environment API, coupling server evaluation to the default Node runtime instead of respecting isolated environment-specific module runners.
```typescript
import { createServer } from 'vite';
import { JSDOM } from 'jsdom';

// ANTI-PATTERN: Polluting host process globalThis and using legacy ssrLoadModule
const dom = new JSDOM('<!DOCTYPE html><html><body></body></html>');
(globalThis as any).window = dom.window;
(globalThis as any).document = dom.window.document;

export async function renderPage(url: string) {
  const vite = await createServer({
    server: { middlewareMode: true },
    appType: 'custom'
  });

  // Deprecated Vite 5 API lacking isolated environment context
  const { render } = await vite.ssrLoadModule('/src/entry-server.ts');
  return render(url);
}
```

#### Production Standard (Correct)
> **Standard**: Uses Vite 6 Environment API to decouple environment configurations (`ssr` vs `edge`) with isolated conditions. Employs `createServerModuleRunner` to execute code within an environment-isolated execution boundary without polluting host process globals.
```typescript
import { createServer } from 'vite';
import { createServerModuleRunner } from 'vite/module-runner';

export async function createIsolatedServer() {
  const vite = await createServer({
    appType: 'custom',
    server: { middlewareMode: true },
    environments: {
      ssr: {
        resolve: {
          conditions: ['node', 'import']
        },
        keepProcessEnv: false
      },
      edge: {
        resolve: {
          conditions: ['worker', 'webworker', 'edge-light']
        }
      }
    }
  });

  return vite;
}

export async function renderPage(url: string) {
  const vite = await createIsolatedServer();
  const ssrEnvironment = vite.environments.ssr;

  // Use isolated ModuleRunner bound to the target environment
  const runner = createServerModuleRunner(ssrEnvironment);
  const { render } = await runner.import('/src/entry-server.ts');

  // Render executes without polluting root globalThis
  return render(url);
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Vite Runtime Inspector MCP
Monitors Vite dev server module graphs and inspects environment runtime boundaries for SSR leaks.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Vite Environment Validator
CLI tool to statically verify Vite 6 environment isolation configurations and detect host global mutations.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation

Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation defines an industrial-grade behavioral standard for frontend engineering. Enforces deterministic runtime isolation boundaries across SSR execution targets using the Vite 6 Environment API and decoupled Module Runners. Eliminates cross-realm global contamination between client, Node.js, and edge targets by disallowing host global mutation and preventing cross-environment module graph leakage. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Mutating Host Globals and Using Legacy ssrLoadModule Across Heterogeneous Runtime Environments Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftMutating Host Globals and Using Legacy ssrLoadModule Across Heterogeneous Runtime EnvironmentsCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS migrate legacy Vite 5 `server.ssrLoadModule` calls to Vite 6 Environment API runner interfaces via `server.environments[name].runner.import()`.

Engineers MUST ALWAYS migrate legacy Vite 5 `server.ssrLoadModule` calls to Vite 6 Environment API runner interfaces via `server.environments[name].runner.import()`.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST NOT mutate the host Node.js `globalThis` with browser-specific mocks (e.g., `global.window = ...`, `global.document = ...`) in isomorphic or SSR execution paths.

Engineers MUST NOT mutate the host Node.js `globalThis` with browser-specific mocks (e.g., `global.window = ...`, `global.document = ...`) in isomorphic or SSR execution paths.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Vite configurations MUST declare explicit `environments` blocks (e.g., `client`, `ssr`, `edge`) with strict, mutually exclusive `resolve.conditions` and `consumer` definitions.

Vite configurations MUST declare explicit `environments` blocks (e.g., `client`, `ssr`, `edge`) with strict, mutually exclusive `resolve.conditions` and `consumer` definitions.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Server entrypoints MUST execute SSR modules within isolated sandbox evaluators or dedicated worker threads to guarantee zero shared state across concurrent render requests.

Server entrypoints MUST execute SSR modules within isolated sandbox evaluators or dedicated worker threads to guarantee zero shared state across concurrent render requests.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Build scripts MUST emit discrete manifests and asset bundles per target environment without sharing internal module graph instances across environment boundaries.

Build scripts MUST emit discrete manifests and asset bundles per target environment without sharing internal module graph instances across environment boundaries.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
6

Components NEVER access browser runtime primitives (`window`, `document`, `navigator`, `localStorage`) in top-level module scope

Components NEVER access browser runtime primitives (`window`, `document`, `navigator`, `localStorage`) in top-level module scope; access MUST be deferred to lifecycle hooks or guarded by verified environment feature detection.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Vite 6 Environment API: Deterministic Isomorphic SSR Sandboxing & Boundary Isolation critical for AI coding assistants?

Enforces deterministic runtime isolation boundaries across SSR execution targets using the Vite 6 Environment API and decoupled Module Runners. Eliminates cross-realm global contamination between client, Node.js, and edge targets by disallowing host global mutation and preventing cross-environment module graph leakage. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Mutating Host Globals and Using Legacy ssrLoadModule Across Heterogeneous Runtime Environments) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related FRONTEND Rules

View all rules →
frontend
119

SolidStart 1.0 Server Functions & Async Boundaries Production Standard

SolidStart 1.0 relies on the Vinxi bundler to split `server$` closures into independent HTTP POST RPC endpoints. Direct lexical closure over client signals within `server$` scopes or failure to demarcate async boundaries with `<Suspense>` causes silent serialization failures, client-bundle secret leaks, and SSR hydration mismatches. Enforcing strict parameter passing, runtime input validation, and Solid async primitives maintains security and fine-grained reactivity across network boundaries.

frontend
119

Astro 5 Actions & Content Layer Production Standard

Enforces strict type safety and architectural correctness for Astro 5 Server Actions and the zero-waterfall Content Layer API. Prevents runtime contract mismatches, hydration mismatches, and unhandled mutation exceptions across dynamic and prerendered boundaries. Failure to follow these patterns leads to silent data fetching waterfalls, build-time cache corruptions, and unhandled server action exceptions.

frontend
51

Nuxt 4 Nitro Server Engine & Shared Directory Production Standard

Nuxt 4 enforces strict boundary segregation between client UI in the app/ directory, backend endpoints in server/, and universal contracts in shared/. Violating cross-boundary isolation or maintaining module-scoped state in Nitro server handlers causes catastrophic cross-request data corruption and hydration mismatches under concurrent production traffic. Architects must enforce request-isolated H3 event contexts, universal serialization schemas, and strict path-alias boundaries.

1 Rule in Stack

Vite