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Vite 6 Environment API Cross-Runtime Module Isolation Boundary

Mandates strict runtime module-resolution barriers between Edge, Node.js, and Browser execution graphs using Vite 6's Environment API. Eliminates Node built-in leakage, prevents isomorphic runtime pollution, and guarantees deterministic bundle isolation across heterogeneous target runtimes.

Prohibited Anti-Pattern: Leaking Node.js Built-ins and Shared Module Graph Contamination via Imperfect SSR Separation

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---
description: Vite 6 Environment API Cross-Runtime Module Isolation Boundary - Mandates strict runtime module-resolution barriers between Edge, Node.js, and Browser execution graphs using Vite 6's Environment API. Eliminates Node built-in leakage, prevents isomorphic runtime pollution, and guarantees deterministic bundle isolation across heterogeneous target runtimes.
globs: vite.config.{ts,js,mjs}, src/**/*.{ts,tsx,js,jsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite6-environment-api-runtime-isolation-boundary-vite-6-environme-855"
---

# Vite 6 Environment API Cross-Runtime Module Isolation Boundary

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Leaking Node.js Built-ins and Shared Module Graph Contamination via Imperfect SSR Separation

## System Constraints
You are a Principal Software Architect enforcing runtime module boundary invariants using Vite 6's Environment API. You must mandate strict separation between client, ssr (Node.js), and edge environments, prohibiting accidental leakage of Node built-ins or environment-specific libraries into client-facing bundles.

## Strict Guidelines
- Engineers MUST ALWAYS declare explicit runtime environments via Vite 6's `environments` configuration block (e.g., `client`, `ssr`, and `edge`) instead of legacy monolithic configuration.
- Engineers MUST NOT rely on ambient runtime branch checks like `typeof window === 'undefined'` or `process.env.NODE_ENV` inside shared modules when importing server-exclusive dependencies.
- Code MUST isolate server and edge logic into dedicated subpaths or suffixed modules (`*.server.ts`, `*.edge.ts`) and enforce exports mapping conditions in `package.json`.
- Services MUST configure `resolve.conditions` and `resolve.externalConditions` independently for each target environment inside `vite.config.ts`.
- Components NEVER import Node.js built-in modules (`node:fs`, `node:crypto`, `node:buffer`, `node:path`) in any module reachable by the `client` environment graph.
- Vite plugins MUST target specific environments using the `applyToEnvironment(environment)` lifecycle hook rather than applying blanket transforms across the global module graph.

## Bad vs Good Implementations
### Cross-Runtime Module Graph Contamination via Legacy SSR Setup

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Using ambient checks ('typeof window') alongside a shared module graph causes client bundlers to encounter 'node:crypto', triggering bundle bloat, runtime polyfill crashes, and broken tree-shaking in modern browser/edge contexts.
```typescript
// vite.config.ts (Vite 5 legacy paradigm)
import { defineConfig } from 'vite';

export default defineConfig({
  ssr: {
    noExternal: ['shared-lib'],
  },
  define: {
    'typeof window': JSON.stringify('object'),
  },
});

// src/shared/auth.ts (Isomorphic module leaking Node built-ins)
import crypto from 'node:crypto'; // Critical leak: pulled into client bundle graph

export function generateToken(seed: string): string {
  if (typeof window === 'undefined') {
    return crypto.createHash('sha256').update(seed).digest('hex');
  }
  return btoa(seed);
}
```

#### Production Standard (Correct)
> **Standard**: Decouples build targets using Vite 6's Environment API, declaring discrete target-specific environments (`client`, `ssr`, `edge`) with independent resolution conditions. Eliminates dynamic sniffing and preserves strict isolation boundaries.
```typescript
// vite.config.ts (Vite 6 Environment API)
import { defineConfig } from 'vite';

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

// src/shared/auth.token.ts (Environment-agnostic interface)
export interface TokenProvider {
  generateToken(seed: string): Promise<string> | string;
}

// src/client/auth.client.ts
import type { TokenProvider } from '../shared/auth.token';

export const clientTokenProvider: TokenProvider = {
  generateToken: (seed: string) => btoa(seed),
};

// src/server/auth.server.ts
import { createHash } from 'node:crypto';
import type { TokenProvider } from '../shared/auth.token';

export const nodeTokenProvider: TokenProvider = {
  generateToken: (seed: string) => createHash('sha256').update(seed).digest('hex'),
};
```
Recommended MCP Servers (mcp-serverlist.com)

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

Filesystem Guard
Inspects Vite project configs and project directory trees to audit environment boundaries.
Recommended Agent Skills (askills.dev)

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

Vite Environment Boundary Auditor
CLI skill that parses vite.config.ts and validates Vite 6 environment module-graph definitions against cross-environment leaks.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Vite 6 Environment API Cross-Runtime Module Isolation Boundary

Vite 6 Environment API Cross-Runtime Module Isolation Boundary defines an industrial-grade behavioral standard for frontend engineering. Mandates strict runtime module-resolution barriers between Edge, Node.js, and Browser execution graphs using Vite 6's Environment API. Eliminates Node built-in leakage, prevents isomorphic runtime pollution, and guarantees deterministic bundle isolation across heterogeneous target runtimes. 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: Leaking Node.js Built-ins and Shared Module Graph Contamination via Imperfect SSR Separation 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 DriftLeaking Node.js Built-ins and Shared Module Graph Contamination via Imperfect SSR SeparationCritical
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 explicit runtime environments via Vite 6's `environments` configuration block (e.g., `client`, `ssr`, and `edge`) instead of legacy monolithic configuration.

Engineers MUST ALWAYS declare explicit runtime environments via Vite 6's `environments` configuration block (e.g., `client`, `ssr`, and `edge`) instead of legacy monolithic configuration.

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 rely on ambient runtime branch checks like `typeof window === 'undefined'` or `process.env.NODE_ENV` inside shared modules when importing server-exclusive dependencies.

Engineers MUST NOT rely on ambient runtime branch checks like `typeof window === 'undefined'` or `process.env.NODE_ENV` inside shared modules when importing server-exclusive dependencies.

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 MUST isolate server and edge logic into dedicated subpaths or suffixed modules (`*.server.ts`, `*.edge.ts`) and enforce exports mapping conditions in `package.json`.

Code MUST isolate server and edge logic into dedicated subpaths or suffixed modules (`*.server.ts`, `*.edge.ts`) and enforce exports mapping conditions in `package.json`.

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 configure `resolve.conditions` and `resolve.externalConditions` independently for each target environment inside `vite.config.ts`.

Services MUST configure `resolve.conditions` and `resolve.externalConditions` independently for each target environment inside `vite.config.ts`.

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

Components NEVER import Node.js built-in modules (`node

Components NEVER import Node.js built-in modules (`node:fs`, `node:crypto`, `node:buffer`, `node:path`) in any module reachable by the `client` environment graph.

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

Vite plugins MUST target specific environments using the `applyToEnvironment(environment)` lifecycle hook rather than applying blanket transforms across the global module graph.

Vite plugins MUST target specific environments using the `applyToEnvironment(environment)` lifecycle hook rather than applying blanket transforms across the global module graph.

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 Cross-Runtime Module Isolation Boundary critical for AI coding assistants?

Mandates strict runtime module-resolution barriers between Edge, Node.js, and Browser execution graphs using Vite 6's Environment API. Eliminates Node built-in leakage, prevents isomorphic runtime pollution, and guarantees deterministic bundle isolation across heterogeneous target runtimes. 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 (Leaking Node.js Built-ins and Shared Module Graph Contamination via Imperfect SSR Separation) 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

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