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frontendcritical PriorityID: vite-6-environment-api-runtime-sandbox

Vite 6 Environment API Deterministic Multi-Runtime Isolation Guardrail

Enforces strict boundary isolation across client, SSR, and edge worker module graphs using the Vite 6 Environment API. Guarantees deterministic module evaluation and eliminates cross-tenant state pollution caused by shared module runner instances and un-isolated module-level singletons during SSR dev and bundling.

Prohibited Anti-Pattern: Cross-Tenant State Bleed via Shared Module Graphs and Legacy ssrLoadModule Singletons

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---
description: Vite 6 Environment API Deterministic Multi-Runtime Isolation Guardrail - Enforces strict boundary isolation across client, SSR, and edge worker module graphs using the Vite 6 Environment API. Guarantees deterministic module evaluation and eliminates cross-tenant state pollution caused by shared module runner instances and un-isolated module-level singletons during SSR dev and bundling.
globs: vite.config.{ts,js,mjs}, src/**/*.{ts,tsx}, server/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite-6-environment-api-runtime-sandbox"
---

# Vite 6 Environment API Deterministic Multi-Runtime Isolation Guardrail

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Cross-Tenant State Bleed via Shared Module Graphs and Legacy ssrLoadModule Singletons

## System Constraints
You are a Principal Software Architect enforcing runtime sandbox invariants with the Vite 6 Environment API. You must eliminate cross-tenant state leaks and unsafe module graph mixing across client, SSR, and edge environments. Mandate explicit environment definitions, runtime condition decoupling, ModuleRunner instantiation boundaries, and strictly forbid mutable module-level state in SSR-targeted code paths.

## Strict Guidelines
- Engineers MUST ALWAYS declare isolated runtime targets under the Vite 6 top-level `environments` configuration key (e.g., `client`, `ssr`, `edge`) with runtime-specific `resolve.conditions` and `consumer` properties.
- Engineers MUST NOT use legacy Vite 5 `viteServer.ssrLoadModule` for multi-tenant SSR execution; they MUST use `createServerModuleRunner` bound explicitly to the target `DevEnvironment`.
- Code executed within SSR and Edge environments MUST NOT maintain mutable top-level module variables (`let`, `new Map()`, `new Set()`) to store request- or tenant-scoped data.
- Services MUST isolate request-scoped state using `AsyncLocalStorage` or explicit per-request context pass-through rather than relying on Node.js module caching invariants.
- Build configs MUST set `environments.<name>.resolve.noExternal: true` for edge worker targets to prevent accidental runtime leakage of unbundled Node.js builtins or CJS host singletons.
- Components NEVER assume global browser singletons (`window`, `document`) exist in shared universal modules without explicit runtime-boundary guards or Vite `import.meta.env.SSR` checks.

## Bad vs Good Implementations
### Vite 6 Multi-Environment Configuration & Module Runner Isolation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy ssrLoadModule evaluates modules within a single global runtime cache, and the module-level variable `cachedTenantContext` creates an immediate cross-tenant race condition under concurrent traffic.
```typescript
import { defineConfig } from 'vite';
import type { ViteDevServer } from 'vite';

// Anti-pattern: Legacy Vite config with shared SSR module space and module-level tenant leak
export default defineConfig({
  ssr: {
    noExternal: ['shared-ui']
  }
});

// server.ts: Top-level mutable singleton leaks across concurrent tenant requests
let cachedTenantContext: { tenantId: string; user: string } | null = null;

export async function handleRequest(server: ViteDevServer, req: any, res: any) {
  cachedTenantContext = { tenantId: req.headers['x-tenant-id'], user: req.headers['x-user'] };
  // ssrLoadModule shares a single module graph across all requests
  const { render } = await server.ssrLoadModule('/src/entry-server.tsx');
  const html = await render(cachedTenantContext);
  res.end(html);
}
```

#### Production Standard (Correct)
> **Standard**: Leverages Vite 6's Environment API with distinct environments (`client`, `ssr`, `edge`) and isolated condition sets. The execution runs via `createServerModuleRunner` bounded by `AsyncLocalStorage`, ensuring absolute memory and tenant isolation.
```typescript
import { defineConfig } from 'vite';
import { createServerModuleRunner } from 'vite/module-runner';
import { AsyncLocalStorage } from 'node:async_hooks';
import type { ViteDevServer } from 'vite';

export interface RequestContext {
  readonly tenantId: string;
  readonly user: string;
}

export const tenantStorage = new AsyncLocalStorage<RequestContext>();

export default defineConfig({
  environments: {
    client: {
      consumer: 'client',
      build: { outDir: 'dist/client' }
    },
    ssr: {
      consumer: 'server',
      resolve: { conditions: ['node', 'import'] },
      build: { outDir: 'dist/ssr' }
    },
    edge: {
      consumer: 'server',
      resolve: { conditions: ['worker', 'edge-light'], noExternal: true },
      build: { outDir: 'dist/edge' }
    }
  }
});

export async function handleRequest(server: ViteDevServer, req: any, res: any) {
  const context: RequestContext = Object.freeze({
    tenantId: String(req.headers['x-tenant-id']),
    user: String(req.headers['x-user'])
  });

  const ssrEnvironment = server.environments.ssr;
  const runner = createServerModuleRunner(ssrEnvironment);

  await tenantStorage.run(context, async () => {
    const entryModule = await runner.import('/src/entry-server.tsx');
    const html = await entryModule.render();
    res.setHeader('Content-Type', 'text/html');
    res.end(html);
  });
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Vite DevTools Inspection Server
Enables inspection of active Vite 6 environments, module graphs, and runtime transforms directly via MCP.
Recommended Agent Skills (askills.dev)

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

Vite Inspect Plugin Skill
CLI tool to inspect Vite module graph transformations, resolve conditions, and runtime environment boundaries.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Vite 6 Environment API Deterministic Multi-Runtime Isolation Guardrail

Vite 6 Environment API Deterministic Multi-Runtime Isolation Guardrail defines an industrial-grade behavioral standard for frontend engineering. Enforces strict boundary isolation across client, SSR, and edge worker module graphs using the Vite 6 Environment API. Guarantees deterministic module evaluation and eliminates cross-tenant state pollution caused by shared module runner instances and un-isolated module-level singletons during SSR dev and bundling. 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: Cross-Tenant State Bleed via Shared Module Graphs and Legacy ssrLoadModule Singletons 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 DriftCross-Tenant State Bleed via Shared Module Graphs and Legacy ssrLoadModule SingletonsCritical
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 declare isolated runtime targets under the Vite 6 top-level `environments` configuration key (e.g., `client`, `ssr`, `edge`) with runtime-specific `resolve.conditions` and `consumer` properties.

Engineers MUST ALWAYS declare isolated runtime targets under the Vite 6 top-level `environments` configuration key (e.g., `client`, `ssr`, `edge`) with runtime-specific `resolve.conditions` and `consumer` properties.

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 use legacy Vite 5 `viteServer.ssrLoadModule` for multi-tenant SSR execution

Engineers MUST NOT use legacy Vite 5 `viteServer.ssrLoadModule` for multi-tenant SSR execution; they MUST use `createServerModuleRunner` bound explicitly to the target `DevEnvironment`.

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

Code executed within SSR and Edge environments MUST NOT maintain mutable top-level module variables (`let`, `new Map()`, `new Set()`) to store request- or tenant-scoped data.

Code executed within SSR and Edge environments MUST NOT maintain mutable top-level module variables (`let`, `new Map()`, `new Set()`) to store request- or tenant-scoped data.

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

Services MUST isolate request-scoped state using `AsyncLocalStorage` or explicit per-request context pass-through rather than relying on Node.js module caching invariants.

Services MUST isolate request-scoped state using `AsyncLocalStorage` or explicit per-request context pass-through rather than relying on Node.js module caching invariants.

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 configs MUST set `environments.<name>.resolve.noExternal

Build configs MUST set `environments.<name>.resolve.noExternal: true` for edge worker targets to prevent accidental runtime leakage of unbundled Node.js builtins or CJS host singletons.

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 assume global browser singletons (`window`, `document`) exist in shared universal modules without explicit runtime-boundary guards or Vite `import.meta.env.SSR` checks.

Components NEVER assume global browser singletons (`window`, `document`) exist in shared universal modules without explicit runtime-boundary guards or Vite `import.meta.env.SSR` checks.

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 Multi-Runtime Isolation Guardrail critical for AI coding assistants?

Enforces strict boundary isolation across client, SSR, and edge worker module graphs using the Vite 6 Environment API. Guarantees deterministic module evaluation and eliminates cross-tenant state pollution caused by shared module runner instances and un-isolated module-level singletons during SSR dev and bundling. 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 (Cross-Tenant State Bleed via Shared Module Graphs and Legacy ssrLoadModule Singletons) 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.

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