Vercel Blob

Local emulators. Real API contracts. Part of Mockingbird.

A wip portable Vercel Blob emulator verified with the unmodified @vercel/blob 2.8.0 SDK. Blobs, multipart uploads, pending parts and synthetic download grants use SQLite Collections, the shared clock, namespaces and Timeline. Fixtures contain synthetic data only.

#Install

bun add @crvouga/mockingbird-service-vercel-blob

#Usage

The example uses @vercel/blob@2.8.0. It is an optional consumer dependency, not a runtime dependency of the emulator.

import { del, put } from "@vercel/blob"
import { createServer } from "@crvouga/mockingbird-service-vercel-blob/server"
import { DEFAULT_TOKEN } from "@crvouga/mockingbird-service-vercel-blob"

// Test setup: the emulator, and the two variables the Blob SDK reads.
const server = await createServer()
process.env.VERCEL_BLOB_API_URL = `${server.url}/api/blob`
process.env.BLOB_READ_WRITE_TOKEN = DEFAULT_TOKEN

// The app's own Blob calls, unchanged.
const blob = await put("fixture.txt", "fixture bytes", { access: "public", addRandomSuffix: false })
console.log(await (await fetch(blob.url)).text()) // "fixture bytes"
await del(blob.url)
await server.close()

The SDK's supported endpoint override is VERCEL_BLOB_API_URL (or NEXT_PUBLIC_VERCEL_BLOB_API_URL). An application's BLOB_STORE_BASE_URL must be mapped to that SDK override by the application. For namespace suite, use ${server.url}/__admin/ns/suite/api/blob, an x-mockingbird-namespace: suite header, or register the fixture token with PUT /__admin/credentials. Every namespace starts empty unless fixtures: [{ pathname, bytes: string | number[], contentType?, cacheControlMaxAge?, storeId? }] is supplied. tokens maps synthetic Bearer tokens to store IDs; the default DEFAULT_TOKEN (vercel_blob_rw_fixture_fixture) maps to fixture. Invalid or missing tokens return vendor 403 { error: { code: "forbidden", message: "Access denied" } }.

The SDK accepts strings, Buffer, Blob and streams. Its multipart helper does not accept a raw Uint8Array as a typed PutBody; wrap it in a Blob or Buffer. The emulator's HTTP upload handler preserves raw Uint8Array request bodies as well as strings and arbitrary binary bytes.

#Routes and behavior

put() and copy() return { pathname, url, downloadUrl, contentType, contentDisposition, etag }. The upload reply does not contain size; head() and list metadata contain it. A duplicate pathname without overwrite returns vendor 400 bad_request and leaves the existing bytes intact. Overwrite replaces the object atomically and preserves its URL. Conditional writes/deletes use x-if-match and return 412 precondition_failed on a stale ETag; copy checks its source ETag. Deletes validate all preconditions before any removal. No vendor request-ID deduplication is invented: normal duplicate-path rules apply to repeated puts. ETags use SHA-256, as in Vercel's published emulator; timestamps use the injected clock. Default cache TTL is 2,592,000 seconds.

Pagination defaults to 1,000 objects, sorts by pathname (so file10 precedes file2) and emits hasMore plus a cursor only when another page exists. Emulator cursors use the last pathname, following Vercel's emulator; their encoding is not a claim about production's opaque cursor format. Folded mode returns immediate folders separately from blob entries.

Multipart staging never publishes a partial object. Re-uploading a part number replaces that pending part. Completion verifies part ETags, ascending unique numbers, total size, and the documented minimum 5 MiB/equal sizes for all parts except the last, then publishes atomically. An identical completion replay returns the existing upload result. maxBlobBytes defaults to 20 MiB and can be configured: this is an explicit local resource bound, not a production quota.

Public URLs use the configured publicOrigin or the origin of the upload request. They contain an explicit namespace query and point inside the reserved internal byte transport tree. They work through plain fetch over the served listener or in-process emulator.fetch. With a custom adminPrefix, the byte URLs and every control route relocate together. A Timeline branch caller must also forward x-mockingbird-branch when fetching its public URL. Conditional public reads support If-None-Match / 304. This local URL transport is a stand-in, not a Vercel CDN hostname. The SDK's get() validates the production hostname, so use plain fetch for local downloads.

#Controls and failures

All service controls live beside the shared /__admin/health, /__admin/state, /__admin/clock, /__admin/reset, /__admin/requests, fault engine and Timeline controls. adminKey uses the shared x-mockingbird-admin-key guard when configured. The journal records metadata and touched IDs, never upload bodies.

Synthetic grant URLs expire on the shared clock and return 403 after expiry; ordinary public URLs remain public. These controls do not model a production signing protocol. Presets: conflict, rate_limited (429 / Retry-After), truncated_upload, partial_multipart, server_error and network_reset. Each fires once. Activate partial_multipart after creating an upload and before sending the part to fail; earlier parts remain available for retry.

#API

The portable root exports VercelBlobAPI, createRuntime, document, supportedOperationIds, VERCEL_BLOB_NAMESPACE, VERCEL_BLOB_PRESETS, DEFAULT_TOKEN, DEFAULT_STORE_ID and DEFAULT_CACHE_MAX_AGE, plus the fixture/record/upload/grant/runtime option types. VercelBlobAPI exposes fetch, reset, state, metadata, seed, ensureSeeded, app and sqlite. createRuntime adds the shared MockSurface clock, fault engine, journal, credential registry, state, namespaces and Timeline controls. The Node ./server entry exports createServer, DEFAULT_PORT (8812), serveTarget and server types; mockingbird-vercel-blob serve starts a listener.

#Oracle and verification

The contract follows Vercel's SDK documentation, the published official SDK 2.8.0, and Vercel's official emulator Blob routes at e4cd051. The emulator supplies wire/error/list evidence; it explicitly does not implement multipart, which is verified here with the real SDK and the SDK documentation's multipart rules.

bun test runs SDK, binary acceptance, all parity-enabled JSON operation walks, a divergent instance check, namespace/reset/clock/Timeline and catalog preset tests. The served SDK tests run put → public GET → overwrite → del and manual/automatic multipart against this package, without modifying the SDK or contacting Vercel. Two-part acceptance verifies that failed parts leave earlier parts intact and that only validated completion makes the object visible. bun run parity is cold and read-only: VERCEL_BLOB_API_URL and VERCEL_BLOB_READ_WRITE_TOKEN must name an empty test store. Missing credentials or a nonempty/inaccessible store exit 2, never a passing live result.

#Deliberately not modelled

Private stores and their authorization, client upload token signing, production CDN/domain routing, SDK get() production-host validation bypass, range requests, image optimization, provider quotas, cross-store copies, real billing and dashboards, production opaque cursor encoding and undocumented request-ID deduplication are not modelled. Grants are synthetic clock controls. This package never contacts Vercel during normal operation; only the explicit credentialed, read-only parity command does so.