fix(webapp): sanitize Prisma P1001 leaks in /api/* responses

Adds an Express middleware that intercepts /api/* responses and, when
the status is 5xx and the body matches a known Prisma P1001 ("Can't
reach database server") leak, rewrites the body to a generic
{"error":"Internal Server Error"} before it reaches the client. The
status code is preserved. Sits outside Remix so it catches leaks the
per-route try/catch from #3664 didn't cover.

Deliberately surgical: one rule (P1001 only), one namespace (/api/*
only). /engine/, /otel/, /realtime/, and UI routes are intentionally
excluded. Expandable later by appending to LEAK_RULES or
MONITORED_PREFIXES.

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

Author: d-cs

.server-changes/universal-api-error-boundary.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+`/api/*` responses no longer leak Prisma "Can't reach database server" (P1001) errors when the database is unreachable — affected responses are rewritten to a generic Internal Server Error before reaching the client.

apps/webapp/app/entry.server.tsx

@@ -289,6 +289,7 @@ process.on("uncaughtException", (error, origin) => {
 singleton("RunEngineEventBusHandlers", registerRunEngineEventBusHandlers);
 singleton("SetupBatchQueueCallbacks", setupBatchQueueCallbacks);
 
+export { apiErrorBoundary } from "./services/apiErrorBoundary.server";
 export { apiRateLimiter } from "./services/apiRateLimit.server";
 export { engineRateLimiter } from "./services/engineRateLimit.server";
 export { runWithHttpContext } from "./services/httpAsyncStorage.server";

apps/webapp/app/services/apiErrorBoundary.server.ts

@@ -0,0 +1,142 @@
+import type { RequestHandler } from "express";
+import { logger } from "./logger.server";
+
+// Last line of defense for leaked error messages in `/api/*` responses.
+//
+// PR #3664 added a per-route try/catch to every `api.v1.*` loader/action so an
+// unhandled throw can no longer reach Remix's default error path and surface
+// `error.message` to the client. This middleware sits one layer further out:
+// when an `/api/*` response ends with status 5xx and the assembled body matches
+// a known leak rule, the body is rewritten to a generic
+// `{"error":"Internal Server Error"}` before it reaches the client. The
+// original status code is preserved — a leaky 503 stays 503 — so status-aware
+// clients keep any nuance the route deliberately set.
+//
+// Remix's express adapter writes the response body via `res.write(chunk)` (not
+// `res.end(chunk)`), so this middleware must buffer writes and evaluate the
+// assembled body at `end` time. SSE responses bypass buffering by content-type
+// inspection; oversized responses bypass via a hard cap so streaming code
+// paths that exceed the cap are not held in memory.
+//
+// Conservative initial rule set: only the patterns we have observed leaking
+// in production are filtered. Mirrors `FINGERPRINT_RULES` in
+// `apps/webapp/sentry.server.ts`. Add a rule here when a new leak is spotted.
+const LEAK_RULES: ReadonlyArray<{ name: string; pattern: RegExp }> = [
+  {
+    // Prisma "Can't reach database server" — surfaces connection strings and
+    // host info when Postgres is unreachable mid-query (the failure mode that
+    // motivated #3664 and the Sentry P1001 fingerprint rule).
+    name: "prisma-p1001",
+    pattern: /\bP1001\b|Can't reach database server/i,
+  },
+];
+
+const SANITIZED_BODY = JSON.stringify({ error: "Internal Server Error" });
+
+// Path prefixes the middleware monitors. Scoped to `/api/*` only — the SDK
+// surface where leaks have actually been reported. `/engine/*`, `/otel/*`,
+// `/realtime/*`, and UI/dashboard routes are intentionally out of scope:
+// they have different traffic profiles, leak shapes, or already self-handle
+// errors. Expand this list reactively when a leak is observed on another
+// namespace.
+const MONITORED_PREFIXES = ["/api/"] as const;
+
+// Error responses are tiny. A buffered response that grows past this cap is
+// not a 5xx error body — bail out of buffering so streaming responses do not
+// pile up in memory.
+const MAX_BUFFER_BYTES = 64 * 1024;
+
+export const apiErrorBoundary: RequestHandler = (req, res, next) => {
+  if (!MONITORED_PREFIXES.some((prefix) => req.path.startsWith(prefix))) {
+    return next();
+  }
+
+  const originalWrite = res.write.bind(res);
+  const originalEnd = res.end.bind(res);
+
+  let chunks: Buffer[] = [];
+  let bufferedBytes = 0;
+  let bypass = false;
+
+  const flushAndBypass = () => {
+    if (bypass) return;
+    bypass = true;
+    if (chunks.length === 0) return;
+    const buffered = Buffer.concat(chunks);
+    chunks = [];
+    bufferedBytes = 0;
+    (originalWrite as (c: Buffer) => boolean)(buffered);
+  };
+
+  const isStreamingContentType = (): boolean => {
+    const ct = String(res.getHeader("content-type") ?? "");
+    if (!ct) return false;
+    return ct.includes("text/event-stream") || ct.includes("application/octet-stream");
+  };
+
+  const toBuffer = (chunk: unknown): Buffer => {
+    if (Buffer.isBuffer(chunk)) return chunk;
+    // Covers Uint8Array (Remix's stream output), Uint16Array, Int32Array,
+    // DataView, etc. — anything backed by an ArrayBuffer. Without this,
+    // `String(chunk)` byte-mangles typed arrays into "85,110,..." strings.
+    if (ArrayBuffer.isView(chunk)) {
+      return Buffer.from(chunk.buffer, chunk.byteOffset, chunk.byteLength);
+    }
+    if (typeof chunk === "string") return Buffer.from(chunk);
+    return Buffer.from(String(chunk));
+  };
+
+  const patchedWrite = (chunk: unknown, ...rest: unknown[]): boolean => {
+    // If headers have already been flushed, we cannot rewrite Content-Type
+    // or status later — sanitization is impossible. Flush + bypass and let
+    // bytes flow through unmodified.
+    if (!bypass && (res.headersSent || isStreamingContentType())) flushAndBypass();
+    if (bypass) {
+      return (originalWrite as (c: unknown, ...r: unknown[]) => boolean)(chunk, ...rest);
+    }
+    if (chunk != null) {
+      const buf = toBuffer(chunk);
+      chunks.push(buf);
+      bufferedBytes += buf.length;
+      if (bufferedBytes > MAX_BUFFER_BYTES) flushAndBypass();
+    }
+    return true;
+  };
+
+  const patchedEnd = (chunk?: unknown, ...rest: unknown[]) => {
+    // Same guard as patchedWrite: if headers were flushed (e.g. via an
+    // explicit `res.flushHeaders()` before end-with-body), we cannot rewrite
+    // the response — flush buffered chunks unchanged and skip sanitization.
+    if (!bypass && res.headersSent) flushAndBypass();
+    if (bypass) {
+      return (originalEnd as (c?: unknown, ...r: unknown[]) => typeof res)(chunk, ...rest);
+    }
+    if (chunk != null) chunks.push(toBuffer(chunk));
+    const body = Buffer.concat(chunks);
+    bypass = true;
+
+    if (res.statusCode >= 500 && body.length > 0) {
+      const text = body.toString("utf8");
+      const matched = LEAK_RULES.find((rule) => rule.pattern.test(text));
+      if (matched) {
+        logger.error("apiErrorBoundary sanitized leaked error response", {
+          rule: matched.name,
+          path: req.path,
+          method: req.method,
+          status: res.statusCode,
+        });
+        res.setHeader("Content-Type", "application/json");
+        res.removeHeader("Content-Length");
+        return (originalEnd as (c?: unknown) => typeof res)(SANITIZED_BODY);
+      }
+    }
+
+    if (body.length > 0) (originalWrite as (c: Buffer) => boolean)(body);
+    return (originalEnd as () => typeof res)();
+  };
+
+  res.write = patchedWrite as unknown as typeof res.write;
+  res.end = patchedEnd as unknown as typeof res.end;
+
+  next();
+};

apps/webapp/server.ts

@@ -119,6 +119,7 @@ if (ENABLE_CLUSTER && cluster.isPrimary) {
   if (process.env.HTTP_SERVER_DISABLED !== "true") {
     const socketIo: { io: IoServer } | undefined = build.entry.module.socketIo;
     const wss: WebSocketServer | undefined = build.entry.module.wss;
+    const apiErrorBoundary: express.RequestHandler = build.entry.module.apiErrorBoundary;
     const apiRateLimiter: RateLimitMiddleware = build.entry.module.apiRateLimiter;
     const engineRateLimiter: RateLimitMiddleware = build.entry.module.engineRateLimiter;
     const runWithHttpContext: RunWithHttpContextFunction = build.entry.module.runWithHttpContext;
@@ -168,6 +169,11 @@ if (ENABLE_CLUSTER && cluster.isPrimary) {
         });
       }
 
+      // Universal error boundary for /api/* responses. Sits outside Remix so
+      // it catches leaks regardless of whether the per-route try/catch from
+      // PR #3664 covered them.
+      app.use(apiErrorBoundary);
+
       app.use(apiRateLimiter);
       app.use(engineRateLimiter);