diff --git a/tests/aws/storage.test.ts b/tests/aws/storage.test.ts
index 64d9269..7aafc62 100644
--- a/tests/aws/storage.test.ts
+++ b/tests/aws/storage.test.ts
@@ -9,6 +9,7 @@ import { AuditQueuePoller } from "../../aws/audit-poller.js";
 import { LocalAssetsFetcher } from "../../aws/local-assets.js";
 import { AwsS3Bucket } from "../../aws/s3-bucket.js";
 import { archiveAuditBatch, AwsSqsQueue } from "../../aws/sqs-queue.js";
+import { OAuthVault } from "../../worker/vault.js";
 
 test("S3 adapter preserves bodies and metadata", async () => {
   const client = new FakeS3Client();
@@ -35,6 +36,24 @@ test("S3 adapter preserves bodies and metadata", async () => {
   assert.equal(client.lastPut?.ServerSideEncryption, "aws:kms");
 });
 
+test("vault caps S3 credential reads without capping the general bucket adapter", async () => {
+  const client = new FakeS3Client();
+  const bucket = new AwsS3Bucket(client as unknown as S3Client, "test-bucket");
+  const huge = "x".repeat(64 * 1024);
+  await bucket.put("oauth/conn.json", huge);
+
+  const object = await bucket.get("oauth/conn.json");
+  assert.ok(object);
+  assert.equal(await object.text(), huge);
+
+  const masterKey = Buffer.alloc(32, 7).toString("base64url");
+  const vault = new OAuthVault(bucket, masterKey);
+  await assert.rejects(
+    () => vault.get("oauth/conn.json", "conn", "principal", "github"),
+    /envelope is too large/u,
+  );
+});
+
 test("SQS adapter serializes JSON and audit consumer reports partial failures", async () => {
   const client = new FakeSqsClient();
   const queue = new AwsSqsQueue(client as unknown as SQSClient, "https://sqs.example/test");
diff --git a/tests/vault.test.ts b/tests/vault.test.ts
new file mode 100644
index 0000000..bfd4425
--- /dev/null
+++ b/tests/vault.test.ts
@@ -0,0 +1,93 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { OAuthVault } from "../worker/vault.js";
+
+const masterKey = Buffer.alloc(32, 7).toString("base64url");
+
+test("vault get rejects oversized envelopes before reading the object body", async () => {
+  let textCalls = 0;
+  const bucket = {
+    async get() {
+      return {
+        size: 1024 * 1024,
+        async text() {
+          textCalls += 1;
+          return "x".repeat(1024 * 1024);
+        },
+      };
+    },
+    async put() {},
+    async delete() {},
+  };
+  const vault = new OAuthVault(bucket as never, masterKey);
+  await assert.rejects(
+    () => vault.get("oauth/conn.json", "conn", "principal", "github"),
+    /envelope is too large/u,
+  );
+  assert.equal(textCalls, 0);
+});
+
+test("vault get rejects a huge body when object size is missing", async () => {
+  const payload = "x".repeat(64 * 1024);
+  const bucket = {
+    async get() {
+      return {
+        size: 0,
+        async text() {
+          return payload;
+        },
+        body: new ReadableStream<Uint8Array>({
+          start(controller) {
+            controller.enqueue(new TextEncoder().encode(payload));
+            controller.close();
+          },
+        }),
+      };
+    },
+    async put() {},
+    async delete() {},
+  };
+  const vault = new OAuthVault(bucket as never, masterKey);
+  await assert.rejects(
+    () => vault.get("oauth/conn.json", "conn", "principal", "github"),
+    /envelope is too large/u,
+  );
+});
+
+test("vault put and get round-trip a max-size secret", async () => {
+  const store = new Map<string, string>();
+  const bucket = {
+    async get(key: string) {
+      const value = store.get(key);
+      if (value === undefined) return null;
+      const bytes = new TextEncoder().encode(value);
+      return {
+        size: bytes.byteLength,
+        async text() {
+          return value;
+        },
+        body: new ReadableStream<Uint8Array>({
+          start(controller) {
+            controller.enqueue(bytes);
+            controller.close();
+          },
+        }),
+      };
+    },
+    async put(key: string, value: string) {
+      store.set(key, value);
+    },
+    async delete(key: string) {
+      store.delete(key);
+    },
+  };
+  const vault = new OAuthVault(bucket as never, masterKey);
+  const secret = "s".repeat(16 * 1024);
+  const key = await vault.put("conn", "principal", "github", secret);
+  assert.equal(key, "oauth/conn.json");
+  assert.equal(await vault.get(key, "conn", "principal", "github"), secret);
+  await assert.rejects(
+    () => vault.put("conn", "principal", "github", `${secret}x`),
+    /OAuth secret is invalid/u,
+  );
+});
diff --git a/worker/vault.ts b/worker/vault.ts
index 7830571..06f1f10 100644
--- a/worker/vault.ts
+++ b/worker/vault.ts
@@ -1,17 +1,32 @@
 type Envelope = { v: 1; iv: string; ciphertext: string; createdAt: string };
+type VaultObject = {
+  readonly size: number;
+  text(): Promise<string>;
+  readonly body?: ReadableStream<Uint8Array>;
+};
+type VaultBucket = {
+  get(key: string): Promise<VaultObject | null>;
+  put(key: string, value: string, options?: {
+    httpMetadata?: { contentType?: string };
+    customMetadata?: Record<string, string>;
+  }): Promise<unknown>;
+  delete(key: string): Promise<unknown>;
+};
 const encoder = new TextEncoder();
+const maxSecretBytes = 16 * 1024;
+const maxEnvelopeBytes = 32 * 1024;
 
 export class OAuthVault {
-  readonly #bucket: R2Bucket;
+  readonly #bucket: VaultBucket;
   readonly #masterKey: string;
 
-  constructor(bucket: R2Bucket, masterKey: string) {
+  constructor(bucket: VaultBucket, masterKey: string) {
     this.#bucket = bucket;
     this.#masterKey = masterKey;
   }
 
   async put(connectionId: string, principalId: string, provider: string, secret: string): Promise<string> {
-    if (!secret || encoder.encode(secret).byteLength > 16 * 1024) throw new Error("OAuth secret is invalid");
+    if (!secret || encoder.encode(secret).byteLength > maxSecretBytes) throw new Error("OAuth secret is invalid");
     const key = vaultKey(connectionId);
     const iv = crypto.getRandomValues(new Uint8Array(12));
     const ciphertext = await crypto.subtle.encrypt({ name: "AES-GCM", iv: source(iv), additionalData: source(aad(connectionId, principalId, provider)) }, await this.#key(), source(encoder.encode(secret)));
@@ -24,7 +39,8 @@ export class OAuthVault {
     if (vaultObjectKey !== vaultKey(connectionId)) throw new Error("vault key does not match connection");
     const object = await this.#bucket.get(vaultObjectKey);
     if (!object) throw new Error("OAuth credential is unavailable");
-    const envelope = JSON.parse(await object.text()) as Envelope;
+    if (object.size > maxEnvelopeBytes) throw new Error("OAuth credential envelope is too large");
+    const envelope = JSON.parse(await readCappedText(object, maxEnvelopeBytes)) as Envelope;
     if (envelope.v !== 1) throw new Error("OAuth credential envelope is unsupported");
     try {
       const plain = await crypto.subtle.decrypt({ name: "AES-GCM", iv: source(decode(envelope.iv)), additionalData: source(aad(connectionId, principalId, provider)) }, await this.#key(), source(decode(envelope.ciphertext)));
@@ -49,3 +65,42 @@ function decode(value: string): Uint8Array {
   return new Uint8Array(Buffer.from(value, "base64url"));
 }
 function source(value: Uint8Array): ArrayBuffer { return Uint8Array.from(value).buffer; }
+
+async function readCappedText(
+  object: { text(): Promise<string>; body?: ReadableStream<Uint8Array> },
+  maxBytes: number,
+): Promise<string> {
+  if (object.body) {
+    return new TextDecoder().decode(await readCappedBytes(object.body, maxBytes));
+  }
+  const text = await object.text();
+  if (encoder.encode(text).byteLength > maxBytes) throw new Error("OAuth credential envelope is too large");
+  return text;
+}
+
+async function readCappedBytes(body: ReadableStream<Uint8Array>, maxBytes: number): Promise<Uint8Array> {
+  const chunks: Uint8Array[] = [];
+  let size = 0;
+  const reader = body.getReader();
+  try {
+    while (true) {
+      const { done, value } = await reader.read();
+      if (done) break;
+      size += value.byteLength;
+      if (size > maxBytes) {
+        await reader.cancel().catch(() => undefined);
+        throw new Error("OAuth credential envelope is too large");
+      }
+      chunks.push(value);
+    }
+  } finally {
+    try { reader.releaseLock(); } catch { /* cancel() already released the lock */ }
+  }
+  const bytes = new Uint8Array(size);
+  let offset = 0;
+  for (const chunk of chunks) {
+    bytes.set(chunk, offset);
+    offset += chunk.byteLength;
+  }
+  return bytes;
+}