diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0ee2623..b2cef3a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -42,6 +42,13 @@
 
 ### Fixed
 
+- Auth tokens now backdate the JWT `iat` claim by `Client::AUTH_IAT_LEEWAY_SECONDS`
+  (5s). `iat` is a whole-second value (RFC 7519 NumericDate) and the server applies
+  minimal forward leeway, so stamping `iat = Time.now.to_i` caused a small fraction of
+  requests to be rejected with `token used before issue at (iat)` (HTTP 401) whenever the
+  caller's clock was even marginally ahead of the server and the second-truncation landed
+  on a boundary. Backdating keeps the token safely behind the server clock. The legacy
+  `stream-chat-ruby` client never sent `iat`, so upgrades from it newly exposed this.
 - `event_class_for_type` now references `GetStream::Generated::Models::*Event`
   (was `StreamChat::*Event`, which raised `NameError` at runtime). `parse_event`
   resolves known event types correctly.
diff --git a/lib/getstream_ruby/client.rb b/lib/getstream_ruby/client.rb
index 4fc38c3..26dd349 100644
--- a/lib/getstream_ruby/client.rb
+++ b/lib/getstream_ruby/client.rb
@@ -25,6 +25,16 @@ module GetStreamRuby
 
   class Client
 
+    # Backdate the JWT `iat` claim by this many seconds.
+    #
+    # JWT timestamps are whole-second (RFC 7519 NumericDate), so `Time.now.to_i`
+    # truncates to the second. The server applies minimal forward leeway on
+    # `iat`, so stamping `iat = now` lets a small fraction of requests be
+    # rejected ("token used before issue at (iat)") whenever the caller's clock
+    # is even marginally ahead of the server and the truncation lands on a
+    # second boundary. Backdating absorbs that sub-second skew.
+    AUTH_IAT_LEEWAY_SECONDS = 5
+
     attr_reader :configuration
 
     def initialize(config = nil, api_key: nil, api_secret: nil, **options)
@@ -274,7 +284,7 @@ def configure_adapter(connection)
     def generate_auth_header
       JWT.encode(
         {
-          iat: Time.now.to_i,
+          iat: Time.now.to_i - AUTH_IAT_LEEWAY_SECONDS,
           server: true,
         },
         @configuration.api_secret,
diff --git a/spec/auth_token_spec.rb b/spec/auth_token_spec.rb
new file mode 100644
index 0000000..e502325
--- /dev/null
+++ b/spec/auth_token_spec.rb
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'jwt'
+
+# Server auth token (JWT) generation. Guards against the clock-skew 401
+# regression: `iat` is backdated so a caller whose clock is marginally ahead of
+# the server does not get intermittently rejected with
+# "token used before issue at (iat)".
+RSpec.describe 'server auth token' do
+
+  let(:secret) { 's' }
+  let(:client) { GetStreamRuby.manual(api_key: 'k', api_secret: secret) }
+
+  def decode_payload
+    header = client.send(:generate_auth_header)
+    JWT.decode(header, secret, true, algorithm: 'HS256').first
+  end
+
+  it 'signs a server token with the server claim' do
+
+    expect(decode_payload['server']).to be(true)
+
+  end
+
+  it 'backdates iat by AUTH_IAT_LEEWAY_SECONDS to absorb client/server clock skew' do
+
+    before = Time.now.to_i
+    iat = decode_payload['iat']
+    after = Time.now.to_i
+
+    # iat must sit at least the leeway behind "now" at signing time, and never
+    # ahead of it, so the server never sees a future-dated token.
+    expect(iat).to be <= (before - GetStreamRuby::Client::AUTH_IAT_LEEWAY_SECONDS)
+    expect(iat).to be >= (after - GetStreamRuby::Client::AUTH_IAT_LEEWAY_SECONDS - 1)
+
+  end
+
+  it 'keeps the leeway positive so the backdate is actually applied' do
+
+    expect(GetStreamRuby::Client::AUTH_IAT_LEEWAY_SECONDS).to be > 0
+
+  end
+
+end