diff --git a/README.md b/README.md
index 2145cf1f..bb82b09c 100644
--- a/README.md
+++ b/README.md
@@ -390,10 +390,11 @@ $seam = new Seam\Seam(retries: 5);
 $seam = new Seam\Seam(retries: 0);
 ```
 
-#### Using the Guzzle client
+#### Using the underlying client
 
-`$seam->client` is the [Guzzle] client, already carrying the endpoint and
-authorization, so it can be used to reach an endpoint the SDK does not expose.
+`$seam->client` already carries the endpoint, authorization, error mapping,
+and retries, so it can be used to reach an endpoint the SDK does not expose.
+It wraps the [Guzzle] client and implements Guzzle's `ClientInterface`.
 
 [Guzzle]: https://docs.guzzlephp.org/
 
@@ -420,6 +421,40 @@ $client = new GuzzleHttp\Client([
 $seam = Seam\Seam::from_client($client);
 ```
 
+The client is used exactly as given. It does not gain the SDK's error mapping
+or retries, so an API error raises Guzzle's exception rather than
+`Seam\HttpApiError`. To opt in, add the middleware yourself.
+
+#### Adding the Seam middleware to your own client
+
+`Seam\Http\ClientFactory::add_middleware` puts the error mapping and retry
+middleware on a handler stack. Build the client with that stack, and with
+`http_errors` disabled so the error middleware raises instead of Guzzle.
+
+```php
+$handler = GuzzleHttp\HandlerStack::create();
+
+Seam\Http\ClientFactory::add_middleware($handler);
+
+$client = new GuzzleHttp\Client([
+    "base_uri" => "https://connect.getseam.com",
+    "headers" => ["authorization" => "Bearer " . $api_key],
+    "handler" => $handler,
+    "http_errors" => false,
+]);
+
+$seam = Seam\Seam::from_client($client);
+```
+
+Pass `retries` to change how many times a failed request is retried, or `0`
+to disable them:
+
+```php
+Seam\Http\ClientFactory::add_middleware($handler, retries: 0);
+```
+
+Add it once per stack: applying it twice stacks two sets of retries.
+
 #### Serializing URL search params
 
 The Seam API parses URL search params as complex types.
diff --git a/codegen/layouts/seam-client.hbs b/codegen/layouts/seam-client.hbs
index fa1faede..4e26a88f 100644
--- a/codegen/layouts/seam-client.hbs
+++ b/codegen/layouts/seam-client.hbs
@@ -28,7 +28,7 @@ class Seam
 {{/each}}
 
   /**
-   * The Guzzle client this instance makes its requests with.
+   * The client this instance makes its requests with.
    *
    * Query params given as a map and NullValue::NULL sentinels in JSON
    * bodies are serialized with the Seam standard before the request goes
diff --git a/src/Http/ClientFactory.php b/src/Http/ClientFactory.php
index 9ba1451f..77bffdb2 100644
--- a/src/Http/ClientFactory.php
+++ b/src/Http/ClientFactory.php
@@ -51,12 +51,7 @@ public static function create(
             $handler = HandlerStack::create($handler);
         }
 
-        // Unshifted so it sits outside every other middleware and only sees
-        // a response none of them could act on: a redirect is followed
-        // rather than raised, and a retried request is judged by the
-        // response it finally settled on.
-        $handler->unshift(ErrorMiddleware::create(), "seam_error");
-        RetryMiddleware::add($handler, $retries);
+        self::add_middleware($handler, $retries);
 
         $headers = array_merge(
             $auth_headers,
@@ -83,6 +78,26 @@ public static function create(
         );
     }
 
+    /**
+     * Adds the Seam error mapping and retry middleware to a handler stack.
+     *
+     * Build the client with the stack, and with `http_errors` disabled so
+     * the error middleware raises instead of Guzzle.
+     *
+     * @param int|null $retries Defaults to self::DEFAULT_RETRIES.
+     */
+    public static function add_middleware(
+        HandlerStack $handler,
+        ?int $retries = null,
+    ): void {
+        // Unshifted so it sits outside every other middleware and only sees
+        // a response none of them could act on: a redirect is followed
+        // rather than raised, and a retried request is judged by the
+        // response it finally settled on.
+        $handler->unshift(ErrorMiddleware::create(), "seam_error");
+        RetryMiddleware::add($handler, $retries ?? self::DEFAULT_RETRIES);
+    }
+
     /**
      * @return array<string, string>
      */
diff --git a/src/Seam.php b/src/Seam.php
index 4ad10234..3f423eea 100644
--- a/src/Seam.php
+++ b/src/Seam.php
@@ -62,7 +62,7 @@ class Seam
     public WorkspacesClient $workspaces;
 
     /**
-     * The Guzzle client this instance makes its requests with.
+     * The client this instance makes its requests with.
      *
      * Query params given as a map and NullValue::NULL sentinels in JSON
      * bodies are serialized with the Seam standard before the request goes
diff --git a/src/SeamWithoutWorkspace.php b/src/SeamWithoutWorkspace.php
index 69e3cc7b..603ba22e 100644
--- a/src/SeamWithoutWorkspace.php
+++ b/src/SeamWithoutWorkspace.php
@@ -19,7 +19,7 @@
 class SeamWithoutWorkspace
 {
     /**
-     * The Guzzle client this instance makes its requests with.
+     * The client this instance makes its requests with.
      */
     public ClientInterface $client;
 
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index b565c9c1..0f6c5feb 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -65,6 +65,115 @@ public function testFromClientNeedsNoCredentials(): void
         );
     }
 
+    private function foreign_client(array $options = []): \GuzzleHttp\Client
+    {
+        return new \GuzzleHttp\Client(
+            array_merge(
+                [
+                    "base_uri" => $this->endpoint,
+                    "headers" => [
+                        "authorization" =>
+                            "Bearer " . $this->seed["seam_apikey1_token"],
+                    ],
+                ],
+                $options,
+            ),
+        );
+    }
+
+    public function testAnInjectedClientIsUsedAsGiven(): void
+    {
+        $seam = Seam::from_client($this->foreign_client());
+
+        $device = $seam->devices->get($this->seed["august_device_1"]);
+        $this->assertSame($this->seed["august_device_1"], $device->device_id);
+
+        $this->expectException(\GuzzleHttp\Exception\ClientException::class);
+
+        $seam->devices->get("nonexistent-device-id");
+    }
+
+    public function testAddMiddlewareGivesAnInjectedClientSeamErrors(): void
+    {
+        $handler = \GuzzleHttp\HandlerStack::create();
+        ClientFactory::add_middleware($handler);
+
+        $seam = Seam::from_client(
+            $this->foreign_client([
+                "handler" => $handler,
+                "http_errors" => false,
+            ]),
+        );
+
+        $this->expectException(HttpApiError::class);
+
+        $seam->devices->get("nonexistent-device-id");
+    }
+
+    public function testAddMiddlewareGivesAnInjectedClientRetries(): void
+    {
+        $recorder = RecordingClient::repeating(
+            RecordingClient::json(503, [
+                "error" => [
+                    "type" => "unavailable",
+                    "message" => "Service Unavailable",
+                ],
+            ]),
+            times: 5,
+        );
+
+        $handler = \GuzzleHttp\HandlerStack::create(
+            $recorder->guzzle_options()["handler"],
+        );
+        ClientFactory::add_middleware($handler);
+
+        $seam = Seam::from_client(
+            $this->foreign_client([
+                "handler" => $handler,
+                "http_errors" => false,
+            ]),
+        );
+
+        try {
+            $seam->devices->get("d1");
+            $this->fail("Expected an HttpApiError");
+        } catch (HttpApiError) {
+            $this->assertSame(3, $recorder->attempt_count());
+        }
+    }
+
+    public function testAddMiddlewareHonoursARetryCount(): void
+    {
+        $recorder = RecordingClient::repeating(
+            RecordingClient::json(503, [
+                "error" => [
+                    "type" => "unavailable",
+                    "message" => "Service Unavailable",
+                ],
+            ]),
+            times: 5,
+        );
+
+        $handler = \GuzzleHttp\HandlerStack::create(
+            $recorder->guzzle_options()["handler"],
+        );
+        ClientFactory::add_middleware($handler, retries: 0);
+
+        $seam = Seam::from_client(
+            $this->foreign_client([
+                "handler" => $handler,
+                "http_errors" => false,
+            ]),
+        );
+
+        try {
+            $seam->devices->get("d1");
+            $this->fail("Expected an HttpApiError");
+        } catch (HttpApiError) {
+            $this->assertSame(1, $recorder->attempt_count());
+        }
+    }
+
     public function testClientOptionReusesAnotherInstancesClient(): void
     {
         $seam = new Seam(client: $this->seam()->client);