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frontendcritical PriorityID: vite-6-environment-api-multi-runtime-partitioning

Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API

Enforces strict compilation graph boundaries across client, Node.js SSR, and edge-worker environments using the Vite 6 Environment API. Eliminates cross-runtime pollution, prevents leaked server secrets, and guarantees deterministic module resolution across heterogeneous runtimes without relying on legacy dual-bundle hacks.

Prohibited Anti-Pattern: Monolithic Isomorphic Bundling and Global Runtime Flag Pollution

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---
description: Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API - Enforces strict compilation graph boundaries across client, Node.js SSR, and edge-worker environments using the Vite 6 Environment API. Eliminates cross-runtime pollution, prevents leaked server secrets, and guarantees deterministic module resolution across heterogeneous runtimes without relying on legacy dual-bundle hacks.
globs: vite.config.{ts,js,mjs}, src/**/*.{ts,tsx,js,jsx}, server/**/*.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/vite-6-environment-api-multi-runtime-partitioning"
---

# Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Monolithic Isomorphic Bundling and Global Runtime Flag Pollution

## System Constraints
You are a Principal Software Architect enforcing Vite 6 Environment API architectural isolation. Mandate isolated module graphs, runtime-specific target configurations, deterministic resolve conditions, and clean module boundaries between client, SSR, and edge environments.

## Strict Guidelines
- Engineers MUST ALWAYS configure runtime variations using Vite 6 top-level `environments` configuration objects (`environments.client`, `environments.ssr`, `environments.edge`) instead of legacy top-level `ssr` overrides.
- Engineers MUST configure distinct `resolve.conditions` and `resolve.noExternal` definitions per environment to prevent runtime-incompatible package resolution (e.g., Node.js built-ins in browser or workerd contexts).
- Engineers MUST NOT share stateful singleton plugin instances across different environments; plugins with runtime-specific state MUST be instantiated independently inside the respective `environments[name].plugins` array.
- Engineers MUST use `this.environment` in plugin hooks rather than relying on the deprecated `ssr` boolean flag to ascertain target execution context.
- Custom dev servers MUST interact with individual runtime graphs via explicit `server.environments[name]` APIs rather than the monolithic `server.transformRequest` or `server.moduleGraph`.
- Codebases MUST NEVER rely on dynamic runtime checks (e.g., `typeof window !== 'undefined'`) to exclude heavy Node.js or edge-specific libraries from client distribution bundles.

## Bad vs Good Implementations
### Legacy Monolithic SSR vs Vite 6 Environment API Partitioning

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy top-level `ssr` configuration provides binary toggles that cannot handle distinct target runtimes (e.g., Cloudflare Workers alongside Node SSR). Stateful plugins shared at the root leak AST mutations and caches across heterogeneous module graphs.
```typescript
import { defineConfig } from 'vite';

// ANTI-PATTERN: Vite 5 legacy monolithic SSR config.
// Leaks client/ssr plugin instances and fails to isolate edge worker requirements.
export default defineConfig({
  ssr: {
    noExternal: ['shared-ui', 'database-driver'],
    target: 'node'
  },
  plugins: [
    myPlugin() // Reused state across runtimes causes graph pollution
  ],
  define: {
    'process.env.RUNTIME': JSON.stringify('isomorphic')
  }
});
```

#### Production Standard (Correct)
> **Standard**: Uses the Vite 6 `environments` API to strictly partition client, Node SSR, and edge execution boundaries with custom resolution conditions, preventing Node.js modules or edge primitives from corrupting opposing graphs.
```typescript
import { defineConfig } from 'vite';
import { createServer } from 'node:http';

// PRODUCTION STANDARD: Vite 6 Environment API with isolated runtime graphs
export default defineConfig({
  environments: {
    client: {
      build: {
        outDir: 'dist/client'
      },
      resolve: {
        conditions: ['browser', 'import']
      }
    },
    ssr: {
      build: {
        outDir: 'dist/ssr'
      },
      resolve: {
        conditions: ['node', 'import'],
        external: ['node:*', 'better-sqlite3']
      }
    },
    edge: {
      build: {
        outDir: 'dist/edge'
      },
      resolve: {
        conditions: ['worker', 'edge-light', 'import'],
        noExternal: true
      }
    }
  },
  plugins: [
    {
      name: 'isolated-runtime-plugin',
      applyToEnvironment(environment) {
        // Guard execution strictly to non-client environments
        return environment.name !== 'client';
      },
      transform(code, id) {
        // Use Environment API context rather than ssr boolean
        if (this.environment.name === 'edge') {
          return code.replace(/__RUNTIME__/g, '"edge-worker"');
        }
        return code.replace(/__RUNTIME__/g, '"node-ssr"');
      }
    }
  ]
});
```
Recommended MCP Servers (mcp-serverlist.com)

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

Filesystem Guard
Inspect and validate file boundaries and build directory partitions in Vite projects.
Recommended Agent Skills (askills.dev)

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

Vite Build Graph Analyzer
CLI tool to inspect Vite module graph outputs and verify cross-environment bundle isolation.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API

Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API defines an industrial-grade behavioral standard for frontend engineering. Enforces strict compilation graph boundaries across client, Node.js SSR, and edge-worker environments using the Vite 6 Environment API. Eliminates cross-runtime pollution, prevents leaked server secrets, and guarantees deterministic module resolution across heterogeneous runtimes without relying on legacy dual-bundle hacks. 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 Isomorphic Bundling and Global Runtime Flag Pollution 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 Isomorphic Bundling and Global Runtime Flag PollutionCritical
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 runtime variations using Vite 6 top-level `environments` configuration objects (`environments.client`, `environments.ssr`, `environments.edge`) instead of legacy top-level `ssr` overrides.

Engineers MUST ALWAYS configure runtime variations using Vite 6 top-level `environments` configuration objects (`environments.client`, `environments.ssr`, `environments.edge`) instead of legacy top-level `ssr` overrides.

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 configure distinct `resolve.conditions` and `resolve.noExternal` definitions per environment to prevent runtime-incompatible package resolution (e.g., Node.js built-ins in browser or workerd contexts).

Engineers MUST configure distinct `resolve.conditions` and `resolve.noExternal` definitions per environment to prevent runtime-incompatible package resolution (e.g., Node.js built-ins in browser or workerd contexts).

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

Engineers MUST NOT share stateful singleton plugin instances across different environments

Engineers MUST NOT share stateful singleton plugin instances across different environments; plugins with runtime-specific state MUST be instantiated independently inside the respective `environments[name].plugins` array.

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

Engineers MUST use `this.environment` in plugin hooks rather than relying on the deprecated `ssr` boolean flag to ascertain target execution context.

Engineers MUST use `this.environment` in plugin hooks rather than relying on the deprecated `ssr` boolean flag to ascertain target execution context.

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

Custom dev servers MUST interact with individual runtime graphs via explicit `server.environments[name]` APIs rather than the monolithic `server.transformRequest` or `server.moduleGraph`.

Custom dev servers MUST interact with individual runtime graphs via explicit `server.environments[name]` APIs rather than the monolithic `server.transformRequest` or `server.moduleGraph`.

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

Codebases MUST NEVER rely on dynamic runtime checks (e.g., `typeof window !== 'undefined'`) to exclude heavy Node.js or edge-specific libraries from client distribution bundles.

Codebases MUST NEVER rely on dynamic runtime checks (e.g., `typeof window !== 'undefined'`) to exclude heavy Node.js or edge-specific libraries from client distribution bundles.

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 Deterministic Multi-Runtime Module Partitioning via Vite 6 Environment API critical for AI coding assistants?

Enforces strict compilation graph boundaries across client, Node.js SSR, and edge-worker environments using the Vite 6 Environment API. Eliminates cross-runtime pollution, prevents leaked server secrets, and guarantees deterministic module resolution across heterogeneous runtimes without relying on legacy dual-bundle hacks. 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 Isomorphic Bundling and Global Runtime Flag Pollution) 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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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.

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