Borrowing it
Nothing to install: this file belongs to joshsymonds/savecraft.gg. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/joshsymonds/savecraft.gg/main/.claude/skills/working-on-mcp/SKILL.mdgit clone --depth 1 https://github.com/joshsymonds/savecraft.ggWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/joshsymonds/savecraft.gg/working-on-mcp)<a href="https://agentmods.dev/skills/joshsymonds/savecraft.gg/working-on-mcp"><img src="https://agentmods.dev/badge/skills/joshsymonds/savecraft.gg/working-on-mcp/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/joshsymonds/savecraft.gg/working-on-mcp"><img src="https://agentmods.dev/badge/skills/joshsymonds/savecraft.gg/working-on-mcp.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00071 | $0.00939 |
| Opus 5 | $0.00036 | $0.00469 |
| Sonnet 5 | $0.00014 | $0.00188 |
| Haiku 4.5 | $0.00007 | $0.00094 |
Grade A, and why
working-on-mcp scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 64 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Working on MCP & OAuth
Read docs/mcp.md for tool contracts, OAuth flow, notes, and search architecture.
Read docs/mcp-design.md for cross-platform MCP tool design best practices.
Verification
just test-worker # MCP tests are part of the Worker test suite
Architecture Rules
No SDK. The MCP server is hand-rolled JSON-RPC 2.0 in src/mcp/handler.ts. The official @modelcontextprotocol/sdk depends on ajv/express/hono (CJS, incompatible with workerd). The Cloudflare agents SDK's createMcpHandler ignores the env parameter so tools can't access D1/R2 bindings.
Tool functions are pure. Every tool in src/mcp/tools.ts takes (db, snapshots, userUuid) and returns a ToolResult or ViewToolResult. No side effects, no request objects, testable without the MCP protocol layer.
Two response formats. textResult(data, presentation?) for tools without views (legacy, being migrated). viewResult(structuredContent, narrative) for tools with MCP Apps views — returns { structuredContent, content } where the view widget renders structuredContent and the model uses content for its response. See docs/views.md and the working-on-views skill.
MCP Apps extension. Server declares extensions: { "io.modelcontextprotocol/ui": {} } in the initialize response. Handler auto-wires _meta.ui.resourceUri on tool definitions from views.gen.ts.
Protocol version: 2025-06-18. Transport: Streamable HTTP (POST + JSON responses, not SSE).
OAuth Architecture
The Worker is itself the OAuth 2.1 Authorization Server via @cloudflare/workers-oauth-provider. Clerk is the upstream IdP only — users authenticate via Clerk, but the Worker issues its own opaque access tokens stored in OAUTH_KV.
Key properties:
- AI clients (Claude, ChatGPT, Gemini) never see Clerk. The entire OAuth dance happens against our origin.
- Token validation is a KV lookup — no JWT signature check, no network call.
ctx.props.userUuidflows from Clerk'ssubclaim through to R2 prefix scoping.- Zero skip-Clerk paths in production. Authorize returns 503 if Clerk secrets are missing.
OAUTH_ENDPOINTSconstant insrc/oauth.tsis shared with test helpers.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 10d ago First seen · 64 lines · 71 tokens per session scan A 0d4da6361a3f
working-on-mcp is a skill published in the GitHub repository joshsymonds/savecraft.gg (12 stars, last pushed 1mo ago), licensed Apache-2.0. It adds 71 tokens to every session and 939 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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