Environment Awareness

FrontMCP’s environment-awareness system lets you declaratively restrict when entries (tools, resources, prompts, skills, agents) are discoverable and executable, based on the runtime environment. Entries that don’t match the current environment are automatically filtered from discovery and blocked from execution.

This enables platform-specific tools (e.g., Apple Notes on macOS only), runtime-specific resources (e.g., Node.js file system), and environment-gated features (e.g., debug tools in development only).

The `availableWhen` Option

Add availableWhen to any entry’s metadata to constrain its availability:

@Tool({
  name: 'apple_notes_search',
  description: 'Search Apple Notes',
  inputSchema: { query: z.string() },
  availableWhen: { platform: ['darwin'] },
})
class AppleNotesSearchTool extends ToolContext {
  async execute({ query }: { query: string }) {
    // This tool only appears on macOS
  }
}

Matching Semantics

AND across fields — all specified fields must match
OR within arrays — at least one value in an array must match
Omitted fields — unconstrained (matches everything)
Empty array — matches nothing (entry is never available)
No availableWhen — always available (default)

// Matches: Node.js OR Bun, AND production only
availableWhen: {
  runtime: ['node', 'bun'],
  env: ['production'],
}

Available Fields

`platform`

Operating system, matching process.platform values:

Value	OS
`'darwin'`	macOS
`'linux'`	Linux
`'win32'`	Windows
`'freebsd'`	FreeBSD

`runtime`

JavaScript runtime:

Value	Runtime
`'node'`	Node.js
`'bun'`	Bun
`'deno'`	Deno
`'edge'`	Edge runtime (Vercel, CF)
`'browser'`	Browser

`deployment`

Deployment mode:

Value	Description
`'standalone'`	Standard server
`'serverless'`	Serverless (Lambda, Vercel)

`env`

NODE_ENV value:

Value	Description
`'production'`	Production
`'development'`	Development
`'test'`	Testing

Supported Entry Types

availableWhen works on all five entry types:

Tool
Resource
Prompt
Skill
Agent

@Tool({
  name: 'apple_notes_search',
  inputSchema: { query: z.string() },
  availableWhen: { platform: ['darwin'] },
})
class AppleNotesSearchTool extends ToolContext { ... }

@Resource({
  name: 'system-info',
  uri: 'system://info',
  availableWhen: { platform: ['darwin', 'linux'] },
})
class SystemInfoResource extends ResourceContext { ... }

@Prompt({
  name: 'xcode-review',
  arguments: [{ name: 'project', required: true }],
  availableWhen: { platform: ['darwin'] },
})
class XcodeReviewPrompt extends PromptContext { ... }

@Skill({
  name: 'ios-deploy',
  description: 'Deploy to iOS devices',
  instructions: { file: './skills/ios-deploy.md' },
  availableWhen: { platform: ['darwin'] },
})
class IosDeploySkill extends SkillContext { ... }

@Agent({
  name: 'local-build-agent',
  llm: { provider: 'anthropic', model: 'claude-sonnet-4-20250514', apiKey: { env: 'ANTHROPIC_API_KEY' } },
  availableWhen: { runtime: ['node'], deployment: ['standalone'] },
})
class LocalBuildAgent extends AgentContext { ... }

Runtime Context API

Inside execute() methods, use runtime context helpers for imperative checks:

@Tool({
  name: 'system_command',
  inputSchema: { cmd: z.string() },
})
class SystemCommandTool extends ToolContext {
  async execute({ cmd }: { cmd: string }) {
    if (this.isPlatform('win32')) {
      // Windows-specific path
      return await this.runPowershell(cmd);
    }
    // Unix path
    return await this.runBash(cmd);
  }
}

Available Methods

Method	Returns	Description
`this.isPlatform('darwin')`	`boolean`	Check OS platform
`this.isRuntime('node')`	`boolean`	Check JavaScript runtime
`this.isDeployment('serverless')`	`boolean`	Check deployment mode
`this.isEnv('production')`	`boolean`	Check NODE_ENV
`this.runtimeContext`	`RuntimeContext`	Full context object

These methods are available on ToolContext, ResourceContext, PromptContext, and AgentContext.

Multi-Platform Pattern

When building tools that serve the same purpose across platforms, use separate files with availableWhen in each:

tools/
  notes/
    notes.darwin.tool.ts     # Apple Notes (macOS)
    notes.win32.tool.ts      # OneNote (Windows)
    notes.linux.tool.ts      # GNOME Notes (Linux)
    index.ts                 # Re-exports all variants

// tools/notes/notes.darwin.tool.ts
@Tool({
  name: 'notes_search',
  description: 'Search notes on this system',
  inputSchema: { query: z.string() },
  availableWhen: { platform: ['darwin'] },
})
export class AppleNotesSearchTool extends ToolContext {
  async execute({ query }) { /* Apple Notes API */ }
}

// tools/notes/index.ts
export { AppleNotesSearchTool } from './notes.darwin.tool';
export { OneNoteSearchTool } from './notes.win32.tool';
export { GnomeNotesSearchTool } from './notes.linux.tool';

import { AppleNotesSearchTool, OneNoteSearchTool, GnomeNotesSearchTool } from './tools/notes';

@App({
  id: 'notes',
  tools: [AppleNotesSearchTool, OneNoteSearchTool, GnomeNotesSearchTool],
})
class NotesApp {}

The multi-platform file pattern (name.platform.tool.ts) is recommended when you have platform-specific implementations of the same logical capability. For simple cases where a single tool needs a platform check, use this.isPlatform() inside execute() instead.

Error Handling

When a client tries to call a tool that exists but is unavailable in the current environment, the SDK returns an EntryUnavailableError (HTTP 403) with both the constraint and the current context:

{
  "code": -32003,
  "message": "Tool \"apple_notes_search\" is not available in the current environment (requires: {\"platform\":[\"darwin\"]}) (current: {\"platform\":\"linux\",\"runtime\":\"node\",\"deployment\":\"standalone\",\"env\":\"production\"})"
}

This is distinct from a ToolNotFoundError (404), helping clients understand why a tool is inaccessible.

How It Works: Registry-Level Filtering

availableWhen is not the same as authorization or rule-based filtering. It is evaluated at the registry level during server boot, not in HTTP request flows.

Concern	Layer	When	Scope
`availableWhen`	Registry (boot)	Server startup	Process-wide, immutable
Authorization	HTTP flow (request)	Per request	Per session/user
Rule-based filtering	HTTP flow (request)	Per request	Dynamic, policy-driven
`hideFromDiscovery`	Registry (listing)	Per list call	Soft hide (entry still callable)

Key differences:

availableWhen is a hard constraint — filtered entries cannot be listed OR called
It runs at registry initialization, not in HTTP flows — no per-request overhead
The runtime context (OS, runtime, deployment, NODE_ENV) is detected once and cached
Results are logged at boot time for operational visibility

Boot-Time Logging

When entries have availableWhen constraints, the SDK logs a summary at startup:

[ToolRegistry] availability: 10 total, 5 with availableWhen constraint, 3 available, 2 filtered [ctx: platform=darwin, runtime=node, deployment=standalone, env=production]
[ToolRegistry] filtered: "windows_tool" — constraint={"platform":["win32"]}, current=platform=darwin
[ToolRegistry] filtered: "browser_tool" — constraint={"runtime":["browser"]}, current=runtime=node

Empty constraint arrays trigger a warning (likely a configuration bug):

[ToolRegistry] "broken_tool" has empty availableWhen arrays for [platform] — this entry will never be available.

​The availableWhen Option

​Matching Semantics

​Available Fields

​platform

​runtime

​deployment

​env

​Supported Entry Types

​Runtime Context API

​Available Methods

​Multi-Platform Pattern

​Error Handling

​How It Works: Registry-Level Filtering

​Boot-Time Logging

The `availableWhen` Option

Matching Semantics

Available Fields

`platform`

`runtime`

`deployment`

`env`

Supported Entry Types

Runtime Context API

Available Methods

Multi-Platform Pattern

Error Handling

How It Works: Registry-Level Filtering

Boot-Time Logging