アカウント不要で、AIエージェントの呼び出しごとに課金します。 PoW（プルーフ・オブ・ワーク）を解けば無料枠、Lightning決済を行えば有料枠となります。
OpenAIのSora APIでは呼び出しごとの課金はできません。Anthropicの請求はツールに引き継がれません。もし今日MCPサーバーを公開し、自律型エージェントが見つけた場合、あなたが請求を負担することになります。
これがそのゲートウェイです。stdio経由の3つのツール。標準ライブラリのみを使用。
npx -y @powforge/captcha-mcpこれだけです。インストールも設定もAPIキーも不要です。サーバーはstdioで起動し、MCPクライアントを待機します。
Claude Code、Cursor、またはMCP互換ホストに組み込むには、設定に以下を追加してください：
{
"mcpServers": {
"powforge-captcha": {
"command": "npx",
"args": ["-y", "@powforge/captcha-mcp"]
}
}
}または npx @powforge/captcha-mcp --install を実行して設定ブロックを出力してください。
Related MCP server: @bitcoinbenji/mcp
PowForge pow-captchaサービス (captcha.powforge.dev) を3つのMCPツールとしてラップします：
ツール | 目的 |
| 新しいプルーフ・オブ・ワークのパズルを要求します。 |
| 解いたナンスを送信します。5分間有効なHMAC署名付きアクセストークンを返します。 |
| サーバーの健全性、累積統計、L402エンドポイントのメタデータを返します。 |
無料枠では、エージェントに約5〜10秒のCPU時間（SHA-256、デフォルトで先頭14ビットがゼロ）のコストがかかります。有料枠では、L402（RFC 7235 + WWW-Authenticate 内のbolt11インボイス）を介してLightningで3サトシのコストがかかります。
アプローチ | 呼び出しコスト | アカウント必須 | エージェント向け |
APIキー | $0 | はい | いいえ |
OAuth | $0 | はい | いいえ |
Stripe課金 | 高いオーバーヘッド | はい | いいえ |
PoW + L402 | 数秒または3サトシ | いいえ | はい |
エージェントにはメールアドレスがありません。確認リンクをクリックすることも、クレジットカードを入力することもできません。PoW + Lightningは、完全に自律的な呼び出し元に対して機能する唯一の認証プリミティブです。
CAPTCHA_URL を設定して、別のキャプチャバックエンドを指定できます。デフォルトは http://localhost:3077 で、開発用にフルスタックをローカルで実行できます。本番環境では https://captcha.powforge.dev を指定してください。
CAPTCHA_URL=https://captcha.powforge.dev npx @powforge/captcha-mcpcaptcha widget repo をクローンするか、パブリックサービスを実行してください。MCPサーバーは status にリストされているキャプチャエンドポイントへのHTTPアクセスのみを必要とします。
git clone https://github.com/zekebuilds-lab/captcha-mcp
cd captcha-mcp
node src/server.jsstderrに ready と出力され、stdinでJSON-RPCを待機します。
手動でプロトコルのスモークテストを行います：
echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1"}}}' | node src/server.jsserverInfo: { name: "@powforge/captcha-mcp", version: "0.1.0" } を含むJSONレスポンスが表示されるはずです。
エージェントがあなたのサービスにトークンを送信した際、エージェントを信頼せずに検証します：
curl -X POST https://captcha.powforge.dev/api/token/verify \
-H "Content-Type: application/json" \
-d '{"token":"<token-from-verify-tool>"}'{valid: true, method, issued_at, expires_at} または {valid: false, reason} を返します。
@powforge/captcha — 同サービス用のブラウザウィジェット。
@powforge/mcp-l402-gate — L402 + Depth-of-Identityスコアリングを使用してMCPサーバーを保護するExpressミドルウェア。
@powforge/mcp-identity — エージェントのレピュテーションオラクル。初回呼び出し時の不正防止のためにこのゲートと組み合わせてください。
MIT
Request a fresh PoW challenge from the PowForge captcha service. Returns {id, salt, difficulty, signature, instructions}. The agent must find a nonce such that SHA-256(salt + nonce) has at least difficulty leading zero bits, then call the verify tool. Free tier — no payment required.
| Name | Required | Description | Default |
|---|---|---|---|
No parameters | |||
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries full burden. It discloses the tool creates a new challenge (non-idempotent), returns specific fields, requires a subsequent PoW solution, and mentions free tier. Lacks rate limit info but is adequate.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
Two sentences concisely cover purpose, return structure, workflow, and free tier. No redundancies or fluff.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
The tool has no input schema and no output schema, so the description fully covers both: explains that no input is needed and details the returned object fields along with subsequent steps. Sufficient for a simple tool.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
No input parameters exist, so schema coverage is 100%. The description adds value by explaining the return structure, which is not parameter semantics but compensates for missing output schema.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description clearly states the verb 'Request' and resource 'fresh PoW challenge from PowForge captcha service', and distinguishes from sibling 'verify' by mentioning the complete workflow including the call to verify.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description explains when to use (requesting a captcha challenge) and what to do next (find nonce, call verify), but does not explicitly contrast with 'status' or provide when-not-to-use conditions.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Return PowForge captcha server health, lifetime stats (pow_solves, ln_skips, challenges_issued), and L402 endpoint metadata (scope, price_sats, paid endpoint URL). Use this to discover the Lightning skip price before paying, or as a liveness check.
| Name | Required | Description | Default |
|---|---|---|---|
No parameters | |||
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries full burden and adequately lists the return data: health, lifetime stats, and L402 metadata. It implies a read-only operation but could explicitly state no side effects.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is a single dense sentence that is front-loaded with the main purpose. It could be split into two sentences for readability, but it is efficient and contains no fluff.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
Given no output schema, the description explains the return values and use cases adequately. It does not specify response format or error handling, but for a health endpoint, the provided information is sufficient for tool selection.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
The tool has zero parameters and schema coverage is 100% (vacuously). Baseline for zero parameters is 4, and the description adds value by detailing the output fields beyond the empty schema.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description clearly states the tool returns health, lifetime stats, and L402 endpoint metadata. It distinguishes itself from siblings 'challenge' and 'verify' by specifying use cases like discovering the Lightning skip price and liveness check.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description explicitly provides two use cases: discovering the Lightning skip price before paying and as a liveness check. It gives clear context for when to use the tool, though it does not mention when not to use it or alternatives.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Verify a PoW solution. Input: {salt, nonce, id, signature, algo?, difficulty?} from a prior challenge call plus the nonce the agent computed. Returns a 5-minute HMAC-signed token on success, or {valid: false, reason} on failure. Tokens can be re-verified server-side via POST /api/token/verify.
| Name | Required | Description | Default |
|---|---|---|---|
| salt | Yes | Hex salt from the challenge response | |
| nonce | Yes | Nonce string the agent computed | |
| id | Yes | Challenge id from the challenge response | |
| signature | Yes | HMAC signature from the challenge response | |
| algo | No | Hash algorithm. Default: 'sha256' | |
| difficulty | No | Leading zero bits required. Default: matches challenge. |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description must disclose behavior. It explains the return value: a 5-minute HMAC-signed token on success or an error object on failure. It also mentions server-side re-verification. However, it does not cover potential side effects or rate limits.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description consists of three concise sentences: the first states the purpose, the second details input/output, and the third mentions server-side capabilities. It is front-loaded and free of superfluous content.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
Given no output schema, the description adequately explains return values (success token with lifetime, failure reason). It references the sibling 'challenge' and describes the token's re-verification endpoint. However, it lacks detail on possible failure reasons or error codes.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema description coverage is 100%, and the description adds valuable context by listing parameters and stating they come from a prior challenge call, plus the agent-computed nonce. This clarifies the origin and usage beyond the schema descriptions.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description explicitly states 'Verify a PoW solution,' using a specific verb and resource. It clearly distinguishes from sibling tools like 'challenge' by describing input from a prior challenge call and the verification purpose.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description indicates that input comes 'from a prior challenge call,' implying a sequence (call challenge first, then verify). It does not explicitly state when not to use it or contrast with alternatives, but the context is clear.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Recent tool additions, removals, and schema changes observed during successful MCP inspections.
v0.2.0challengestatusverifyScored across 3 tools
Each tool has a distinct purpose: challenge initiates a PoW challenge, status checks server health and metadata, and verify submits the solution. No overlap in functionality.
All tool names are single-word imperative verbs (challenge, status, verify), following a consistent naming pattern.
Three tools are appropriate for a captcha service, covering the core workflow (challenge, verify) and a status check. Not excessive or insufficient.
The core PoW challenge-verify cycle is complete. However, the status tool mentions a Lightning skip option, but no tool to execute it, which is a minor gap.
Pay-per-action access to APIs and MCP tools over Lightning L402 and Base USDC x402.
Monetize any MCP server: x402 paywall, pay-per-call billing in USDC on Base, agent marketplace.
Remote MCP server exposing 330 production AI-agent services for web/data processing, validation, AI utilities, blockchain/crypto utilities, and x402 pay-per-use access.
Hosted MCP server for live public-data APIs and Skills for AI agents.
