---
title: Environment Awareness
slug: features/environment-awareness
icon: microchip
description: Conditionally expose tools, resources, prompts, skills, and agents based on platform, runtime, and environment
keywords: ['frontmcp', 'mcp', 'model context protocol', 'features', 'framework', 'environment', 'build target', 'typescript']
---

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.

<Info>
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).
</Info>

---

## The `availableWhen` Option

Add `availableWhen` to any entry's metadata to constrain its availability:

```ts
@Tool({
  name: 'apple_notes_search',
  description: 'Search Apple Notes',
  inputSchema: { query: z.string() },
  availableWhen: { os: ['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)

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

---

## Available Fields

### `os` (replaces `platform`)

Operating system, matching [`process.platform`](https://nodejs.org/api/process.html#processplatform) values. Renamed from `platform` in issue #417 — the old name still works as a deprecated alias.

| 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`

Coarse deployment mode:

| Value           | Description                  |
| --------------- | ---------------------------- |
| `'standalone'`  | Standard server              |
| `'serverless'`  | Serverless (Lambda, Vercel)  |
| `'distributed'` | Multi-pod HA mode            |
| `'browser'`     | Browser bundle               |

### `provider` (issue #417)

Discriminated deploy provider. Lets `@Tool({ availableWhen: { provider: ['vercel'] } })` express provider-specific rules where `deployment: ['serverless']` is too coarse.

| Value           | Detected via env var                 |
| --------------- | ------------------------------------ |
| `'bare'`        | No discriminating env var (default)  |
| `'docker'`      | `/.dockerenv` exists                 |
| `'vercel'`      | `VERCEL`                             |
| `'lambda'`      | `AWS_LAMBDA_FUNCTION_NAME`           |
| `'cloudflare'`  | `CF_PAGES`                           |
| `'netlify'`     | `NETLIFY`                            |
| `'azure'`       | `AZURE_FUNCTIONS_ENVIRONMENT`        |
| `'gcp'`         | `K_SERVICE`                          |
| `'fly'`         | `FLY_APP_NAME`                       |
| `'render'`      | `RENDER`                             |
| `'railway'`     | `RAILWAY_ENVIRONMENT`                |

Override the detection with `FRONTMCP_PROVIDER=<name>` (useful for tests, Docker images without a discriminating env var, etc.).

### `target` (issue #417)

Build target produced by `frontmcp build --target <x>`. Always `'unknown'` in dev (`frontmcp dev`).

| Value           | Source                                                |
| --------------- | ----------------------------------------------------- |
| `'unknown'`     | Dev mode (no build) or unrecognized value             |
| `'node'`        | `frontmcp build --target node`                        |
| `'distributed'` | `frontmcp build --target distributed`                 |
| `'cli'`         | `frontmcp build --target cli`                         |
| `'vercel'`      | `frontmcp build --target vercel`                      |
| `'lambda'`      | `frontmcp build --target lambda`                      |
| `'cloudflare'`  | `frontmcp build --target cloudflare`                  |
| `'browser'`     | `frontmcp build --target browser`                     |
| `'sdk'`         | `frontmcp build --target sdk`                         |
| `'mcpb'`        | `frontmcp build --target mcpb`                        |

Resolution order: `globalThis.FRONTMCP_BUILD_TARGET` (inlined by the adapter) → `process.env.FRONTMCP_BUILD_TARGET` → `'unknown'`.

### `surface` (issue #417)

Per-call axis — set by the transport adapter / dispatcher on the request `ctx` to discriminate "who is calling this tool." Unlike the other axes (which are constant for the process lifetime), `surface` varies per request.

| Value            | Source                                         |
| ---------------- | ---------------------------------------------- |
| `'mcp'`          | MCP `tools/call` (set by the MCP handler)      |
| `'cli'`          | A `frontmcp <verb>` subcommand router (issue #409) |
| `'agent'`        | Agent-to-tool dispatch (`ExecutionContextBase.callTool`) |
| `'job'`          | Job runner                                     |
| `'http-trigger'` | `@Channel` HTTP triggers                       |

Example — block external MCP calls but allow agent dispatch:

```typescript
@Tool({
  name: 'internal_helper',
  availableWhen: { surface: ['agent'] },
  ...
})
```

### `env`

`NODE_ENV` value:

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

### Structured errors

When a tool exists but its `availableWhen` constraint fails at call time, FrontMCP throws `EntryUnavailableError` with a `missingAxes` array in `data` (issue #417). Clients can surface "this tool isn't reachable because provider=vercel / surface=mcp / …" without parsing prose.

```json
{
  "jsonrpc": "2.0",
  "id": 5,
  "error": {
    "code": -32003,
    "message": "Tool \"deploy_to_lambda\" is not available in the current environment ...",
    "data": {
      "entryType": "Tool",
      "entry": "deploy_to_lambda",
      "missingAxes": ["provider"],
      "constraint": { "provider": ["lambda"] },
      "context": { "provider": "vercel", "os": "linux", "runtime": "node", "surface": "mcp", ... }
    }
  }
}
```

---

## Supported Entry Types

`availableWhen` works on all five entry types:

<Tabs>
  <Tab title="Tool">
    ```ts
    @Tool({
      name: 'apple_notes_search',
      inputSchema: { query: z.string() },
      availableWhen: { os: ['darwin'] },
    })
    class AppleNotesSearchTool extends ToolContext { ... }
    ```
  </Tab>
  <Tab title="Resource">
    ```ts
    @Resource({
      name: 'system-info',
      uri: 'system://info',
      availableWhen: { os: ['darwin', 'linux'] },
    })
    class SystemInfoResource extends ResourceContext { ... }
    ```
  </Tab>
  <Tab title="Prompt">
    ```ts
    @Prompt({
      name: 'xcode-review',
      arguments: [{ name: 'project', required: true }],
      availableWhen: { os: ['darwin'] },
    })
    class XcodeReviewPrompt extends PromptContext { ... }
    ```
  </Tab>
  <Tab title="Skill">
    ```ts
    @Skill({
      name: 'ios-deploy',
      description: 'Deploy to iOS devices',
      instructions: { file: './skills/ios-deploy.md' },
      availableWhen: { os: ['darwin'] },
    })
    class IosDeploySkill extends SkillContext { ... }
    ```
  </Tab>
  <Tab title="Agent">
    ```ts
    @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 { ... }
    ```
  </Tab>
</Tabs>

---

## Runtime Context API

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

```ts
@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

`RuntimeContext` exposes the operating system via the `os` property; `platform` remains as a deprecated alias for backward compatibility (issue #417) and resolves to the same value. New code should read `this.runtimeContext.os` directly.

| Method                            | Returns          | Description                                                                                              |
| --------------------------------- | ---------------- | -------------------------------------------------------------------------------------------------------- |
| `this.runtimeContext.os`          | `string`         | Modern OS axis access — same values as `process.platform` (`'darwin'`, `'linux'`, `'win32'`, …)         |
| `this.isPlatform('darwin')`       | `boolean`        | **Deprecated alias** — checks the legacy `.platform` field. Prefer `this.runtimeContext.os === 'darwin'`. |
| `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 (`os`, `platform`, `runtime`, `deployment`, `env`)                                    |

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
```

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

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

Register all variants — the SDK automatically exposes only the one matching the current platform:

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

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

<Tip>
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.
</Tip>

---

## 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. The `data` payload includes a `missingAxes` array so clients can show a precise reason for the failure without parsing the message string:

```json
{
  "code": -32003,
  "message": "Tool \"apple_notes_search\" is not available in the current environment (missing axes: os) (requires: {\"os\":[\"darwin\"]}) (current: {\"os\":\"linux\",\"runtime\":\"node\",\"deployment\":\"standalone\",\"env\":\"production\"})",
  "data": {
    "entryType": "tool",
    "entry": "apple_notes_search",
    "missingAxes": ["os"],
    "constraint": { "os": ["darwin"] },
    "context": {
      "os": "linux",
      "runtime": "node",
      "deployment": "standalone",
      "env": "production"
    }
  }
}
```

This is distinct from a `ToolNotFoundError` (404), helping clients understand why a tool is inaccessible. See the [Structured errors](#structured-errors) reference (if present) for the full schema.

---

## How It Works: Registry-Level Filtering

<Warning>
`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.
</Warning>

| 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.
```
