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frontendcritical PriorityID: vite6-environment-api-multi-runtime-ssr-guardrail-vite-6-environme-593

Vite 6 Environment API Multi-Runtime SSR Guardrail

Enforces strict isolation between Client, Node.js, and WinterCG edge runtimes using Vite 6 Environment APIs. Prevents cross-runtime global pollution, invalid module externalization, and cross-boundary state leakage during unified multi-environment SSR bundling.

Prohibited Anti-Pattern: Monolithic SSR configurations leaking Node.js built-ins or browser globals across heterogeneous runtime environments

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---
description: Vite 6 Environment API Multi-Runtime SSR Guardrail - Enforces strict isolation between Client, Node.js, and WinterCG edge runtimes using Vite 6 Environment APIs. Prevents cross-runtime global pollution, invalid module externalization, and cross-boundary state leakage during unified multi-environment SSR bundling.
globs: vite.config.ts, vite.config.*.ts, src/entry-server.ts, src/entry-client.ts, src/entry-edge.ts, src/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite6-environment-api-multi-runtime-ssr-guardrail-vite-6-environme-593"
---

# Vite 6 Environment API Multi-Runtime SSR Guardrail

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Monolithic SSR configurations leaking Node.js built-ins or browser globals across heterogeneous runtime environments

## System Constraints
You are a Principal Software Architect enforcing runtime isolation in Vite 6 multi-runtime SSR applications. You must mandate the use of the Vite 6 Environment API (`environments` configuration block) to declare explicit runtime boundaries for 'client', 'ssr' (Node.js), and 'edge' (WinterCG). Prohibit monolithic `ssr: true` overrides, ban dynamic ambient runtime shims, prevent Node.js built-in module leakage into WinterCG/Browser contexts, and guarantee zero cross-environment global variable pollution.

## Strict Guidelines
- Engineers MUST ALWAYS configure distinct runtimes using Vite 6 `environments` (e.g., `environments.client`, `environments.ssr`, `environments.edge`) rather than relying on legacy top-level `ssr` or conditional plugin hacks.
- Engineers MUST NEVER access ambient Node.js globals (`process.env`, `Buffer`, `__dirname`) in shared or edge-targeted code; access must be gated by environment checks or abstracted through environment-agnostic interfaces.
- Vite configuration MUST explicitly declare `resolve.conditions` per environment (e.g., `['node']` for Node SSR, `['worker', 'workerd']` for WinterCG Edge, `['browser']` for Client).
- Services running in WinterCG edge environments MUST configure `resolve.noExternal: true` or specify concrete package bundles to prevent unresolved Node CJS module leaks.
- Components NEVER attach singleton state directly to global scopes (`globalThis`, `window`, `global`); all runtime-scoped contexts must be instantiated per request via context providers or runtime-managed stores.

## Bad vs Good Implementations
### Vite Configuration: Monolithic SSR vs Vite 6 Multi-Environment API

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy top-level `ssr` treats all server builds as a single runtime target, failing to isolate WinterCG edge runtimes (e.g., Cloudflare Workers, workerd) from Node.js, and injects host `process.env` globals across all targets.
```typescript
import { defineConfig } from 'vite';

// ANTI-PATTERN: Monolithic SSR configuration leaking Node module resolutions
// across both WinterCG edge workers and client bundles.
export default defineConfig({
  ssr: {
    noExternal: ['shared-lib'],
    target: 'node'
  },
  define: {
    'process.env.RUNTIME': JSON.stringify(process.env.RUNTIME || 'node')
  }
});
```

#### Production Standard (Correct)
> **Standard**: Declares explicit, isolated environments via Vite 6 Environment API with specific resolve conditions and externalization policies, preventing Node.js-specific polyfills from contaminating the edge or browser targets.
```typescript
import { defineConfig } from 'vite';

// PRODUCTION INVARIANT: Vite 6 isolated Environment API definitions
export default defineConfig({
  environments: {
    client: {
      build: {
        outDir: 'dist/client'
      },
      resolve: {
        conditions: ['browser']
      }
    },
    ssr: {
      build: {
        outDir: 'dist/ssr'
      },
      resolve: {
        conditions: ['node', 'import']
      }
    },
    edge: {
      build: {
        outDir: 'dist/edge'
      },
      resolve: {
        conditions: ['workerd', 'worker'],
        noExternal: true
      }
    }
  }
});
```

### Runtime Execution: Unsafe Global Reference vs Standardized Environment Check

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Accessing `process` or `window` directly breaks execution in WinterCG runtimes where `process` is absent and `window` is undefined, potentially causing fatal ReferenceErrors.
```typescript
// ANTI-PATTERN: Direct reliance on ambient runtime globals causing runtime exceptions in Edge/Client
export function getSecretKey(name: string): string {
  if (typeof process !== 'undefined' && process.env) {
    return process.env[name] ?? '';
  }
  return (window as unknown as Record<string, string>).__ENV__[name];
}
```

#### Production Standard (Correct)
> **Standard**: Abstracts environment-specific access into explicit providers passed during request lifecycle, maintaining modularity without polluting global state across runtimes.
```typescript
// PRODUCTION INVARIANT: Runtime-isolated configuration resolution
interface RuntimeEnvProvider {
  get(key: string): string | undefined;
}

export function resolveConfig(provider: RuntimeEnvProvider, key: string): string {
  const value = provider.get(key);
  if (value === undefined) {
    throw new Error(`Missing environment configuration: ${key}`);
  }
  return value;
}

export const nodeEnvProvider: RuntimeEnvProvider = {
  get: (key) => (typeof process !== 'undefined' ? process.env[key] : undefined)
};

export const winterCGEnvProvider = (env: Record<string, string>): RuntimeEnvProvider => ({
  get: (key) => env[key]
});
```
Recommended MCP Servers (mcp-serverlist.com)

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

Vite Runtime Inspector MCP
Inspects Vite runtime environment module graphs and detects unresolved external imports across boundary definitions.
Recommended Agent Skills (askills.dev)

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

Vite Multi-Environment Build Analyzer
Audits Vite 6 multi-environment bundle outputs for accidental external module cross-contamination.
Architectural Specification & Invariant Rationale

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

Vite 6 Environment API Multi-Runtime SSR Guardrail defines an industrial-grade behavioral standard for frontend engineering. Enforces strict isolation between Client, Node.js, and WinterCG edge runtimes using Vite 6 Environment APIs. Prevents cross-runtime global pollution, invalid module externalization, and cross-boundary state leakage during unified multi-environment SSR 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: Monolithic SSR configurations leaking Node.js built-ins or browser globals 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 DriftMonolithic SSR configurations leaking Node.js built-ins or browser globals 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 configure distinct runtimes using Vite 6 `environments` (e.g., `environments.client`, `environments.ssr`, `environments.edge`) rather than relying on legacy top-level `ssr` or conditional plugin hacks.

Engineers MUST ALWAYS configure distinct runtimes using Vite 6 `environments` (e.g., `environments.client`, `environments.ssr`, `environments.edge`) rather than relying on legacy top-level `ssr` or conditional plugin hacks.

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 NEVER access ambient Node.js globals (`process.env`, `Buffer`, `__dirname`) in shared or edge-targeted code

Engineers MUST NEVER access ambient Node.js globals (`process.env`, `Buffer`, `__dirname`) in shared or edge-targeted code; access must be gated by environment checks or abstracted through environment-agnostic interfaces.

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 configuration MUST explicitly declare `resolve.conditions` per environment (e.g., `['node']` for Node SSR, `['worker', 'workerd']` for WinterCG Edge, `['browser']` for Client).

Vite configuration MUST explicitly declare `resolve.conditions` per environment (e.g., `['node']` for Node SSR, `['worker', 'workerd']` for WinterCG Edge, `['browser']` for Client).

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 running in WinterCG edge environments MUST configure `resolve.noExternal

Services running in WinterCG edge environments MUST configure `resolve.noExternal: true` or specify concrete package bundles to prevent unresolved Node CJS module leaks.

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 attach singleton state directly to global scopes (`globalThis`, `window`, `global`)

Components NEVER attach singleton state directly to global scopes (`globalThis`, `window`, `global`); all runtime-scoped contexts must be instantiated per request via context providers or runtime-managed stores.

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 Multi-Runtime SSR Guardrail critical for AI coding assistants?

Enforces strict isolation between Client, Node.js, and WinterCG edge runtimes using Vite 6 Environment APIs. Prevents cross-runtime global pollution, invalid module externalization, and cross-boundary state leakage during unified multi-environment SSR 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 (Monolithic SSR configurations leaking Node.js built-ins or browser globals 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.

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