Phase 45 slice 2: context7 HTTP bridge for doc drift detection

Bun bridge on :3900 that wraps context7's public API and exposes the
surface gateway consumes for Phase 45 drift checks. Own port so a
failure here never tips over mcp-server on :3700.

Endpoints:
  GET /health                    status + cache stats
  GET /docs/:tool                resolve tool → library_id → fetch
                                 docs → return descriptor
                                 {snippet_hash, last_updated,
                                 source_url, docs_preview, ...}
  GET /docs/:tool/diff?since=X   compare current snippet_hash to X;
                                 returns {drifted: bool, current,
                                 previous, preview if drifted}
  GET /cache                     debug dump of cached entries

Implementation notes:
- 5 minute in-memory cache (context7 rate-limits by IP; gateway
  drift-checks are the hot caller)
- 1500-token slices from context7 (enough for drift-meaningful
  hash, not so much we hammer their API)
- snippet_hash = SHA-256 prefix (16 hex chars) of fetched content
- Library resolution prefers "finalized" state; falls back to top
  result if none finalized

Verified live against context7.com:
- /health                                  → ok, 0 cache, 300s TTL
- /docs/docker                             → library_id /docker/docs,
                                             title "Docker", hash
                                             475a0396ca436bba, last
                                             updated 2026-04-20
- /docs/docker (again)                     → cache hit, 0.37ms
                                             (5400× speedup)
- /docs/docker/diff?since=stale-hash-0000  → drifted=true, preview
                                             included
- /docs/docker/diff?since=<current hash>   → drifted=false, preview
                                             omitted (honest: no
                                             drift to show)

Not yet wired:
- Gateway consumer (Phase 45 slice 3):
  /vectors/playbook_memory/doc_drift/check/{id} calls this bridge
  and updates DocRef.snippet_hash + doc_drift_flagged_at
- Systemd unit (bridge is manual-start for now, same as bot/)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mcp-server/context7_bridge.ts

@@ -0,0 +1,178 @@
+// Phase 45 slice 2 — context7 HTTP bridge.
+//
+// Exposes an HTTP surface the Rust gateway consumes to check external
+// doc drift on playbooks. Wraps context7's public API:
+//
+//   https://context7.com/api/v1/search?query=<name>   → resolve
+//   https://context7.com/api/v1/<lib-id>/cli?type=txt&tokens=N → docs
+//
+// Own port so a failure here never tips over the mcp-server on :3700.
+// Cache is in-memory (5 min TTL) — context7 rate-limits by IP, and
+// gateway drift-checks are the hot caller.
+//
+// Endpoints:
+//   GET /health                    health + cache stats
+//   GET /docs/:tool                resolve + fetch + return descriptor
+//   GET /docs/:tool/diff?since=X   return drift vs content hash X
+//   GET /cache                     dump current cache (debugging)
+
+import { createHash } from "node:crypto";
+
+const PORT = Number(process.env.CONTEXT7_BRIDGE_PORT ?? 3900);
+const CONTEXT7_BASE = (process.env.CONTEXT7_BASE_URL ?? "https://context7.com/api/v1").replace(/\/+$/, "");
+const TOKENS_PER_FETCH = 1500; // enough for a drift-meaningful slice, not so much we hammer
+const CACHE_TTL_MS = 5 * 60 * 1000;
+
+interface CachedDoc {
+  tool: string;
+  library_id: string;         // context7 canonical, e.g. "/docker/docs"
+  title: string;
+  last_updated: string | null;
+  snippet_hash: string;       // SHA-256 of first TOKENS_PER_FETCH of fetched docs
+  docs_preview: string;       // first 400 chars, returned in responses for display
+  retrieved_at: string;       // ISO when WE last fetched
+  source_url: string;         // human-facing context7 URL
+}
+
+const cache = new Map<string, { entry: CachedDoc; at: number }>();
+
+function normalizeTool(s: string): string {
+  return s.trim().toLowerCase();
+}
+
+async function resolveLibraryId(tool: string): Promise<{ id: string; title: string; last_updated: string | null } | null> {
+  const r = await fetch(`${CONTEXT7_BASE}/search?query=${encodeURIComponent(tool)}`, {
+    signal: AbortSignal.timeout(10000),
+  });
+  if (!r.ok) throw new Error(`context7 search ${r.status}: ${await r.text().catch(() => "?")}`);
+  const j = await r.json() as any;
+  const results = Array.isArray(j.results) ? j.results : [];
+  // First result is top-ranked; context7 sorts by its internal score.
+  // Skip "state != finalized" to avoid unstable previews.
+  const pick = results.find((x: any) => x?.state === "finalized") ?? results[0];
+  if (!pick?.id) return null;
+  return {
+    id: String(pick.id),
+    title: String(pick.title ?? tool),
+    last_updated: typeof pick.lastUpdateDate === "string" ? pick.lastUpdateDate : null,
+  };
+}
+
+async function fetchDocsText(libraryId: string): Promise<string> {
+  // Strip the leading slash on library_id so the URL doesn't have a
+  // double slash. context7 returns ids like "/docker/docs" but the
+  // fetch path wants "docker/docs".
+  const cleanId = libraryId.replace(/^\/+/, "");
+  const url = `${CONTEXT7_BASE}/${cleanId}?type=txt&tokens=${TOKENS_PER_FETCH}`;
+  const r = await fetch(url, { signal: AbortSignal.timeout(20000) });
+  if (!r.ok) throw new Error(`context7 fetch ${r.status} on ${cleanId}: ${await r.text().catch(() => "?")}`);
+  return await r.text();
+}
+
+function hashContent(s: string): string {
+  return createHash("sha256").update(s).digest("hex").slice(0, 16);
+}
+
+async function getCurrent(tool: string): Promise<CachedDoc> {
+  const key = normalizeTool(tool);
+  const cached = cache.get(key);
+  if (cached && Date.now() - cached.at < CACHE_TTL_MS) {
+    return cached.entry;
+  }
+  const resolved = await resolveLibraryId(key);
+  if (!resolved) {
+    throw new Error(`no context7 library found for '${tool}'`);
+  }
+  const text = await fetchDocsText(resolved.id);
+  const entry: CachedDoc = {
+    tool: key,
+    library_id: resolved.id,
+    title: resolved.title,
+    last_updated: resolved.last_updated,
+    snippet_hash: hashContent(text),
+    docs_preview: text.slice(0, 400),
+    retrieved_at: new Date().toISOString(),
+    source_url: `https://context7.com${resolved.id.startsWith("/") ? resolved.id : "/" + resolved.id}`,
+  };
+  cache.set(key, { entry, at: Date.now() });
+  return entry;
+}
+
+function jsonResponse(body: unknown, status: number = 200): Response {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { "content-type": "application/json" },
+  });
+}
+
+Bun.serve({
+  port: PORT,
+  hostname: "0.0.0.0",
+  async fetch(req) {
+    const url = new URL(req.url);
+
+    if (url.pathname === "/health") {
+      return jsonResponse({
+        status: "ok",
+        cache_size: cache.size,
+        context7_base: CONTEXT7_BASE,
+        ttl_ms: CACHE_TTL_MS,
+      });
+    }
+
+    if (url.pathname === "/cache") {
+      const dump = Array.from(cache.entries()).map(([k, v]) => ({
+        tool: k,
+        library_id: v.entry.library_id,
+        snippet_hash: v.entry.snippet_hash,
+        retrieved_at: v.entry.retrieved_at,
+        age_ms: Date.now() - v.at,
+      }));
+      return jsonResponse({ entries: dump });
+    }
+
+    // GET /docs/:tool — return current descriptor (fetches + caches)
+    const m1 = url.pathname.match(/^\/docs\/([^/]+)$/);
+    if (m1 && req.method === "GET") {
+      const tool = decodeURIComponent(m1[1]);
+      try {
+        const entry = await getCurrent(tool);
+        return jsonResponse(entry);
+      } catch (e) {
+        return jsonResponse({ error: (e as Error).message }, 404);
+      }
+    }
+
+    // GET /docs/:tool/diff?since=hash — compare current vs recorded hash
+    const m2 = url.pathname.match(/^\/docs\/([^/]+)\/diff$/);
+    if (m2 && req.method === "GET") {
+      const tool = decodeURIComponent(m2[1]);
+      const since = url.searchParams.get("since");
+      if (!since) {
+        return jsonResponse({ error: "query param 'since' (snippet_hash) required" }, 400);
+      }
+      try {
+        const entry = await getCurrent(tool);
+        const drifted = entry.snippet_hash !== since;
+        return jsonResponse({
+          tool: entry.tool,
+          drifted,
+          previous_snippet_hash: since,
+          current_snippet_hash: entry.snippet_hash,
+          library_id: entry.library_id,
+          title: entry.title,
+          last_updated_upstream: entry.last_updated,
+          retrieved_at: entry.retrieved_at,
+          source_url: entry.source_url,
+          docs_preview: drifted ? entry.docs_preview : null,
+        });
+      } catch (e) {
+        return jsonResponse({ error: (e as Error).message }, 404);
+      }
+    }
+
+    return jsonResponse({ error: "not found", paths: ["/health", "/docs/:tool", "/docs/:tool/diff?since=HASH", "/cache"] }, 404);
+  },
+});
+
+console.log(`[context7-bridge] listening on :${PORT} (context7 base: ${CONTEXT7_BASE})`);