feat(mcp): local MCP server — scan_repo + blueprint_scene (proof)

mcp/README.md

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+# RepoLens MCP server
+
+A local [MCP](https://modelcontextprotocol.io) server that exposes RepoLens's repo
+analysis as a tool. An LLM client (Claude Desktop, Cursor, etc.) calls `scan_repo`
+and gets RepoLens's verdict-first JSON back — ready to render as components.
+
+This is a **thin proof**: one tool, GitHub-only, Anthropic-only. It reuses the
+extension's own pipeline (`fetcher.js` → `prompt.js` → `parser.js`); only the
+provider call is MCP-specific.
+
+## What stays true
+
+- **Local only.** Runs over stdio, spawned by your client. No hosted backend.
+- **Bring your own key.** The Anthropic key comes from the environment, never from
+  a server or the extension's storage. Nothing phones home.
+
+## Tools
+
+### `scan_repo({ repo })`
+
+`repo` is `owner/name` or a GitHub URL. Returns the analysis JSON:
+
+```json
+{
+  "repoId": "honojs/hono",
+  "platform": "github",
+  "language": "TypeScript",
+  "license": "MIT",
+  "stars": 21000,
+  "description": "Small, fast web framework for the edges.",
+  "fit": "strong",
+  "health": { "score": 92 },
+  "pros": ["..."],
+  "cons": ["..."],
+  "red_flags": [],
+  "capabilities": ["routing", "middleware", "edge"]
+}
+```
+
+### `blueprint_scene({ repo })`
+
+Maps how the repo is built and returns a laid-out graph — `nodes` (key parts) and
+`edges` (how they relate), with positions — ready for a `<DependencyGraph>`-style
+component. Heavier than `scan_repo`: it reads source and makes two model calls
+(atoms, then lineage).
+
+```json
+{
+  "id": "repo:...", "scope": "blueprint", "repoId": "honojs/hono",
+  "nodes": [{ "id": "app", "label": "Hono app", "kind": "entrypoint", "x": 120, "y": 40 }],
+  "edges": [{ "source": "app", "target": "router", "label": "depends-on" }],
+  "camera": { "x": 0, "y": 0, "zoom": 1 }
+}
+```
+
+## Setup
+
+```bash
+cd mcp
+npm install
+export ANTHROPIC_API_KEY=sk-ant-...     # required
+export ANTHROPIC_MODEL=claude-sonnet-4-6 # optional override
+node server.js                           # speaks MCP over stdio
+```
+
+### Add to Claude Desktop
+
+In `claude_desktop_config.json`:
+
+```json
+{
+  "mcpServers": {
+    "repolens": {
+      "command": "node",
+      "args": ["/absolute/path/to/repolens/mcp/server.js"],
+      "env": { "ANTHROPIC_API_KEY": "sk-ant-..." }
+    }
+  }
+}
+```
+
+## Notes
+
+- Unauthenticated GitHub requests are rate-limited. For heavier use, a `GITHUB_TOKEN`
+  pass-through is a follow-up.
+- Next (follow-ups): `deep_dive` (the plain-English layer), multi-provider, and
+  npm / PyPI / GitLab support; a `GITHUB_TOKEN` pass-through for higher rate limits.

mcp/anthropic.js

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+// Minimal Anthropic Messages API call for the MCP server. The key comes from the
+// environment (ANTHROPIC_API_KEY) — never chrome.storage, never a hosted backend —
+// so the server stays local and bring-your-own-key. Mirrors the extension's
+// callAnthropic shape (background.js): same endpoint, version header, and default model.
+
+const ENDPOINT = 'https://api.anthropic.com/v1/messages';
+const DEFAULT_MODEL = 'claude-sonnet-4-6';
+
+/**
+ * @param {string} prompt - the fully-assembled analysis prompt.
+ * @returns {Promise<string>} the model's text response.
+ */
+export async function callAnthropic(prompt) {
+  const key = process.env.ANTHROPIC_API_KEY;
+  if (!key) throw new Error('ANTHROPIC_API_KEY is not set in the environment');
+  const model = process.env.ANTHROPIC_MODEL || DEFAULT_MODEL;
+
+  const res = await fetch(ENDPOINT, {
+    method: 'POST',
+    headers: {
+      'content-type': 'application/json',
+      'anthropic-version': '2023-06-01',
+      'x-api-key': key,
+    },
+    body: JSON.stringify({
+      model,
+      max_tokens: 4096,
+      messages: [{ role: 'user', content: prompt }],
+    }),
+  });
+
+  if (!res.ok) {
+    const detail = await res.text().catch(() => '');
+    throw new Error(`Anthropic API ${res.status}: ${detail.slice(0, 300)}`);
+  }
+
+  const data = await res.json();
+  const text = (data.content || []).map((b) => b.text || '').join('').trim();
+  if (!text) throw new Error('Anthropic returned an empty response');
+  return text;
+}

mcp/blueprint-scene.js

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+// blueprint_scene tool: a laid-out nodes/edges graph of how a repo is built —
+// ready for a <DependencyGraph>-style component.
+//
+// Pipeline (extension modules, verbatim): fetchRepoData + fetchSource ->
+// atoms prompt/parse -> lineage prompt/parse -> buildBlueprintScene. Two model
+// calls (atoms, then lineage); the optional "feynman" plain-English layer is
+// skipped since the scene only needs atoms + lineage. `facts` is null here —
+// the extension's local "runner" (measured metrics) isn't part of a headless MCP.
+
+import { fetchRepoData } from '../fetcher.js';
+import { fetchSource, buildAtomsPrompt, parseAtoms, buildLineagePrompt, parseLineage } from '../deepdive.js';
+import { buildBlueprintScene } from '../blueprint-adapter.js';
+import { parseRepoInput } from './repo-input.js';
+import { callAnthropic } from './anthropic.js';
+
+export const BLUEPRINT_TOOL = {
+  name: 'blueprint_scene',
+  description:
+    'Map how a GitHub repo is built and return a laid-out graph: nodes (its key parts) and ' +
+    'edges (how they relate), with positions. Use this to visualize a repository as a ' +
+    'dependency / architecture diagram. Heavier than scan_repo (reads source, two model calls).',
+  inputSchema: {
+    type: 'object',
+    properties: { repo: { type: 'string', description: 'A repo as owner/name or a GitHub URL' } },
+    required: ['repo'],
+    additionalProperties: false,
+  },
+  outputSchema: {
+    type: 'object',
+    properties: {
+      id: { type: 'string' },
+      scope: { type: 'string' },
+      repoId: { type: 'string' },
+      title: { type: 'string' },
+      nodes: {
+        type: 'array',
+        items: {
+          type: 'object',
+          properties: {
+            id: { type: 'string' },
+            label: { type: 'string' },
+            kind: { type: 'string' },
+            x: { type: 'number' },
+            y: { type: 'number' },
+          },
+        },
+      },
+      edges: {
+        type: 'array',
+        items: {
+          type: 'object',
+          properties: {
+            source: { type: 'string' },
+            target: { type: 'string' },
+            label: { type: 'string' },
+          },
+        },
+      },
+      camera: { type: 'object' },
+    },
+    required: ['nodes', 'edges'],
+  },
+};
+
+export async function runBlueprintScene(args) {
+  const { platform, repoId } = parseRepoInput(args?.repo);
+  const repoData = await fetchRepoData(platform, repoId);
+  const source = await fetchSource(platform, repoId);
+  const { atoms } = parseAtoms(await callAnthropic(buildAtomsPrompt(repoData, source, null)));
+  const lineage = parseLineage(await callAnthropic(buildLineagePrompt(atoms)));
+  return buildBlueprintScene({ deepDive: { atoms, lineage }, repoId, title: repoId });
+}