diff --git a/.changeset/olive-donkeys-jam.md b/.changeset/olive-donkeys-jam.md
new file mode 100644
index 0000000..682215e
--- /dev/null
+++ b/.changeset/olive-donkeys-jam.md
@@ -0,0 +1,20 @@
+---
+'@haverstack/adapter-api': patch
+---
+
+Stop the change-feed client reconnecting against a refusal that will repeat.
+
+`isFatalFeedError` ended the reconnect loop only for an unrenewable
+credential (401) and an authorization refusal (403). Every other refusal the
+server faulted the request for — a malformed cursor or filter answered
+`400 bad_request`, say — was treated as transient, so `subscribeChanges()`
+retried it with backoff indefinitely, settling into an attempt roughly every
+15 seconds and reporting the same error to `onError` each time. The
+subscriber was never told to stop, and `onReset` never fired, so the
+application had nothing to reconcile from either.
+
+The predicate now decides on the wire status: a `4xx` ends the loop, since
+the reconnect sends the same request and would be refused the same way. A
+`5xx` still reconnects, which is what keeps `timeout` — the answer a server
+gives while shedding query load — from turning a busy server into a
+permanently dead subscription.
diff --git a/docs/spec/wire-format.md b/docs/spec/wire-format.md
index 700ac9a..15f98f1 100644
--- a/docs/spec/wire-format.md
+++ b/docs/spec/wire-format.md
@@ -539,6 +539,8 @@ data: {"reason":"cursor_expired"}
 
 **Reconnection is the client's job, with exponential backoff and jitter.** A server restart otherwise produces a synchronized reconnect stampede from every client it dropped.
 
+**A client stops reconnecting when the answer was `4xx`, and keeps reconnecting when it was `5xx`.** The reconnect sends the same request, so a status faulting that request — a malformed cursor or filter, an unrenewable credential, an authorization refusal — will be answered identically however long the client waits, and backing off only spins. A `5xx` says the server could not serve a request it did not fault, which is the case backoff exists for; `timeout` is the answer a server gives while [shedding query load](#bounding-query-cost) and a client that gave up on it would turn a busy server into a dead subscription. A client that stops reports the error to its subscriber first; the repair is to subscribe again.
+
 ### Permission scoping
 
 **A connection delivers the events its token's session may read, and nothing else.** The predicate is literally `canRead` applied per event — no second vocabulary, no feed-specific ACL. A server subscribes **unscoped** at the storage owner and fans out per connection, filtering each through the `ScopedStack` its token's session names via `Stack.forSession()`, taking the `(principalId, subjectId)` pair whole. Delegated authority is then the ordinary [intersection](./access-control.md#delegation-principal-and-subject), inherited rather than reimplemented.
diff --git a/packages/adapter-api/src/index.ts b/packages/adapter-api/src/index.ts
index 91e4b7f..bea2b9c 100644
--- a/packages/adapter-api/src/index.ts
+++ b/packages/adapter-api/src/index.ts
@@ -16,7 +16,7 @@
  * patchContent()/deleteRecord()/etc.'s expectedVersion option.
  */
 
-import { StackQueryError, StackPermissionError } from '@haverstack/core';
+import { StackError, StackQueryError } from '@haverstack/core';
 import type {
   StackAdapter,
   StackRecord,
@@ -59,6 +59,7 @@ import {
   isRetryableAuthError,
   isValidSeq,
   supportsChangeFeed,
+  WIRE_ERROR_STATUS,
   supportsDidChallenge,
   CHANGE_FRAME_READY,
   CHANGE_FRAME_RECORD,
@@ -484,16 +485,20 @@ const reconnectDelay = (attempt: number): number => {
 const sleep = (ms: number): Promise<void> => new Promise((resolve) => setTimeout(resolve, ms));
 
 /**
- * Whether a feed error is one reconnecting cannot recover. An
- * authentication failure (a 401 whose re-auth did not renew, or no
- * credential to renew with) and an authorization refusal (403) will reject
- * the next connection identically, so retrying only spins. A connection or
- * server error is transient and reconnects. The stream is closed by
- * returning from the pump; the subscriber has already been told via
- * onError.
+ * Whether a feed error is one reconnecting cannot recover. A 4xx faults the
+ * request, and the reconnect sends the same one, so retrying only spins. A
+ * 5xx is the server's own trouble and may clear — a shed-load `timeout`
+ * reconnects. The stream is closed by returning from the pump; the
+ * subscriber has already been told via onError.
  */
-const isFatalFeedError = (err: unknown): boolean =>
-  err instanceof APIAdapterAuthError || err instanceof StackPermissionError;
+const isFatalFeedError = (err: unknown): boolean => {
+  if (err instanceof APIAdapterAuthError) return true;
+  if (err instanceof StackError) {
+    const status = WIRE_ERROR_STATUS[err.code];
+    return status >= 400 && status < 500;
+  }
+  return false;
+};
 
 // -------------------------------------------------------
 // Challenge–response handshake
diff --git a/packages/adapter-api/tests/change-feed.test.ts b/packages/adapter-api/tests/change-feed.test.ts
index 4ab4cc2..4db6a95 100644
--- a/packages/adapter-api/tests/change-feed.test.ts
+++ b/packages/adapter-api/tests/change-feed.test.ts
@@ -11,6 +11,7 @@ import {
   APIAdapterAuthError,
 } from '../src/index.js';
 import { WIRE_PROTOCOL_VERSION } from '@haverstack/wire-types';
+import { StackQueryError, StackTimeoutError } from '@haverstack/core';
 import type { RecordChange } from '@haverstack/core';
 
 const BASE_URL = 'https://stack.example.com';
@@ -486,6 +487,68 @@ describe('reconnection', () => {
     stop();
   });
 
+  // A request the server faulted is the same request on the next
+  // connection, so the loop ends rather than spinning against a verdict
+  // that will not change.
+  test('stops reconnecting after a 4xx the reconnect would only repeat', async () => {
+    vi.useFakeTimers();
+    const adapter = await openAdapter();
+    const first = feed();
+    mockFetch.mockResolvedValueOnce(first.response);
+
+    const onError = vi.fn();
+    const subscription = adapter.subscribeChanges({ onError }, () => {});
+    first.write(READY);
+    const stop = await subscription;
+
+    mockFetch.mockResolvedValueOnce(
+      jsonResponse({ error: { code: 'bad_request', message: 'Invalid cursor' } }, 400),
+    );
+    first.end();
+
+    await vi.advanceTimersByTimeAsync(60_000);
+    await vi.waitFor(() => expect(onError).toHaveBeenCalledOnce());
+    expect(onError.mock.calls[0]![0]).toBeInstanceOf(StackQueryError);
+
+    // No further reconnect: the fetch count holds at discovery + first +
+    // the refused reconnect, even after more time passes.
+    const calls = mockFetch.mock.calls.length;
+    await vi.advanceTimersByTimeAsync(120_000);
+    expect(mockFetch.mock.calls.length).toBe(calls);
+    stop();
+  });
+
+  // 503 is a server shedding load, not a verdict on the request — being
+  // reconnected against is the whole point of it.
+  test('keeps reconnecting after a 503 the server may recover from', async () => {
+    vi.useFakeTimers();
+    const adapter = await openAdapter();
+    const first = feed();
+    mockFetch.mockResolvedValueOnce(first.response);
+
+    const onError = vi.fn();
+    const subscription = adapter.subscribeChanges({ onError }, () => {});
+    first.write(READY);
+    const stop = await subscription;
+
+    mockFetch.mockResolvedValueOnce(
+      jsonResponse({ error: { code: 'timeout', message: 'Shedding load' } }, 503),
+    );
+    const recovered = feed();
+    mockFetch.mockResolvedValueOnce(recovered.response);
+    first.end();
+
+    await vi.advanceTimersByTimeAsync(60_000);
+    await vi.waitFor(() => expect(onError).toHaveBeenCalledOnce());
+    expect(onError.mock.calls[0]![0]).toBeInstanceOf(StackTimeoutError);
+
+    // discovery + first + the 503 + the attempt that reaches the server
+    // again: the subscription outlived the refusal.
+    await vi.advanceTimersByTimeAsync(120_000);
+    await vi.waitFor(() => expect(mockFetch.mock.calls.length).toBe(4));
+    stop();
+  });
+
   test('stops reconnecting once unsubscribed, and aborts the open stream', async () => {
     vi.useFakeTimers();
     const adapter = await openAdapter();