diff --git a/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyRequest.php b/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyRequest.php
index cbb42a9ac46e..d84519cfc60a 100755
--- a/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyRequest.php
+++ b/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyRequest.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of ILIAS, a powerful learning management system
  * published by ILIAS open source e-Learning e.V.
@@ -18,6 +16,8 @@
  *
  *********************************************************************/
 
+declare(strict_types=1);
+
 namespace XapiProxy;
 
 use GuzzleHttp\Client;
@@ -38,6 +38,20 @@ class XapiProxyRequest
     private string $cmdPart2plus = "";
     private bool $checkGetStatements = true;
 
+    /**
+     * The xAPI document resources are mutable, so the specification guards them with optimistic
+     * concurrency (xAPI 1.0.3, "Concurrency"): a content player announces the document revision
+     * it expects, and an LRS must reject a PUT on an already existing document that carries
+     * neither of these headers with 409 Conflict.
+     * @var list<string>
+     */
+    private const CONDITIONAL_REQUEST_HEADERS = ['If-Match', 'If-None-Match'];
+
+    /**
+     * @var list<string>
+     */
+    private const DOCUMENT_RESOURCES = ['activities/state', 'activities/profile', 'agents/profile'];
+
     public function __construct(XapiProxy $xapiproxy)
     {
         $this->dic = $GLOBALS['DIC'];
@@ -279,6 +293,25 @@ private function handleProxy(\Psr\Http\Message\RequestInterface $request, $fakeP
                 $this->xapiproxy->log()->error($this->msg($e->getMessage()));
             }
 
+            $responses['default'] = $this->repeatDocumentWriteWithPrecondition(
+                $httpclient,
+                $request,
+                $responses['default'],
+                $uriDefault,
+                $authDefault,
+                $body,
+                $req_opts
+            );
+            $responses['fallback'] = $this->repeatDocumentWriteWithPrecondition(
+                $httpclient,
+                $request,
+                $responses['fallback'],
+                $uriFallback,
+                $authFallback,
+                $body,
+                $req_opts
+            );
+
             $defaultOk = $this->xapiProxyResponse->checkResponse($responses['default'], $endpointDefault);
             $fallbackOk = $this->xapiProxyResponse->checkResponse($responses['fallback'], $endpointFallback);
 
@@ -318,6 +351,16 @@ private function handleProxy(\Psr\Http\Message\RequestInterface $request, $fakeP
             } catch (\Exception $e) {
                 $this->xapiproxy->log()->error($this->msg($e->getMessage()));
             }
+            $responses['default'] = $this->repeatDocumentWriteWithPrecondition(
+                $httpclient,
+                $request,
+                $responses['default'],
+                $uriDefault,
+                $authDefault,
+                $body,
+                $req_opts
+            );
+
             if ($this->xapiProxyResponse->checkResponse($responses['default'], $endpointDefault)) {
                 try {
                     $this->xapiProxyResponse->handleResponse(
@@ -372,10 +415,98 @@ private function createProxyRequest(\Psr\Http\Message\RequestInterface $request,
             $headers['Connection'] = $request->getHeader('Connection');
         }
 
+        foreach (self::CONDITIONAL_REQUEST_HEADERS as $conditionalHeader) {
+            if ($request->hasHeader($conditionalHeader)) {
+                $headers[$conditionalHeader] = $request->getHeader($conditionalHeader);
+            }
+        }
+
         //$this->xapiproxy->log()->debug($this->msg($body));
 
         $req = new Request(strtoupper($request->getMethod()), $uri, $headers, $body);
 
         return $req;
     }
+
+    /**
+     * TinCanJS, which is bundled with common content players, only sends If-Match on a state
+     * write when the caller supplied the SHA1 of the document it read before, and no precondition
+     * at all otherwise. A specification compliant LRS answers such a write with 409 Conflict. In
+     * that case the current ETag is looked up and the write is repeated once with an If-Match
+     * built from it. An LRS that does not demand a precondition never answers 409 and therefore
+     * never causes the additional roundtrip.
+     * @param array{state: string, value?: \GuzzleHttp\Psr7\Response, reason?: mixed} $response
+     * @param array<string, mixed> $req_opts
+     * @return array{state: string, value?: \GuzzleHttp\Psr7\Response, reason?: mixed}
+     */
+    private function repeatDocumentWriteWithPrecondition(
+        Client $httpclient,
+        \Psr\Http\Message\RequestInterface $request,
+        array $response,
+        Uri $uri,
+        string $auth,
+        string $body,
+        array $req_opts
+    ): array {
+        if ($response['state'] !== 'fulfilled' || $response['value']->getStatusCode() !== 409) {
+            return $response;
+        }
+        if (!$this->requiresSynthesizedPrecondition($request)) {
+            return $response;
+        }
+        $etag = $this->fetchDocumentEtag($httpclient, $request, $uri, $auth, $req_opts);
+        if ($etag === '') {
+            return $response;
+        }
+
+        $this->xapiproxy->log()->debug($this->msg('lrs requires a precondition for ' . $uri . ', repeating request with If-Match: ' . $etag));
+
+        try {
+            /** @var \GuzzleHttp\Psr7\Response $repeated */
+            $repeated = $httpclient->send(
+                $this->createProxyRequest($request, $uri, $auth, $body)->withHeader('If-Match', $etag),
+                $req_opts
+            );
+        } catch (\Exception $e) {
+            $this->xapiproxy->log()->error($this->msg($e->getMessage()));
+            return $response;
+        }
+
+        return ['state' => 'fulfilled', 'value' => $repeated];
+    }
+
+    private function requiresSynthesizedPrecondition(\Psr\Http\Message\RequestInterface $request): bool
+    {
+        return strtoupper($request->getMethod()) === 'PUT'
+            && in_array($this->xapiproxy->cmdParts()[3] ?? '', self::DOCUMENT_RESOURCES, true)
+            && !$request->hasHeader('If-Match')
+            && !$request->hasHeader('If-None-Match');
+    }
+
+    /**
+     * Returns the current ETag of an xAPI document, or an empty string if it does not exist.
+     * Some LRS send an ETag along with the 404 of a missing document, so only a 200 is trusted.
+     * @param array<string, mixed> $req_opts
+     */
+    private function fetchDocumentEtag(
+        Client $httpclient,
+        \Psr\Http\Message\RequestInterface $request,
+        Uri $uri,
+        string $auth,
+        array $req_opts
+    ): string {
+        $headers = ['Authorization' => $auth];
+        if ($request->hasHeader('X-Experience-API-Version')) {
+            $headers['X-Experience-API-Version'] = $request->getHeader('X-Experience-API-Version');
+        }
+
+        try {
+            $probe = $httpclient->send(new Request('GET', $uri, $headers), $req_opts);
+        } catch (\Exception $e) {
+            $this->xapiproxy->log()->error($this->msg($e->getMessage()));
+            return '';
+        }
+
+        return $probe->getStatusCode() === 200 ? $probe->getHeaderLine('ETag') : '';
+    }
 }
diff --git a/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyResponse.php b/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyResponse.php
index 843038b59d67..571ef353ff21 100755
--- a/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyResponse.php
+++ b/components/ILIAS/CmiXapi/classes/XapiProxy/XapiProxyResponse.php
@@ -1,6 +1,5 @@
 <?php
 
-declare(strict_types=1);
 /**
  * This file is part of ILIAS, a powerful learning management system
  * published by ILIAS open source e-Learning e.V.
@@ -17,6 +16,8 @@
  *
  *********************************************************************/
 
+declare(strict_types=1);
+
 namespace XapiProxy;
 
 use Psr\Http\Message\ServerRequestInterface;
@@ -26,6 +27,14 @@
 
 class XapiProxyResponse
 {
+    /**
+     * 304, 409 and 412 are the conditional answers of the xAPI document resources. They are
+     * regular protocol answers which the content has to evaluate on its own, so they are
+     * relayed unchanged instead of being replaced by a generic proxy error.
+     * @var list<int>
+     */
+    private const RELAYED_STATUS_CODES = [200, 204, 304, 404, 409, 412];
+
     //        private $dic;
     private XapiProxy $xapiproxy;
     //private $xapiProxyRequest;
@@ -40,7 +49,7 @@ public function checkResponse(array $response, string $endpoint): bool
     {
         if ($response['state'] == 'fulfilled') {
             $status = $response['value']->getStatusCode();
-            if ($status === 200 || $status === 204 || $status === 404) {
+            if (in_array($status, self::RELAYED_STATUS_CODES, true)) {
                 return true;
             } else {
                 $this->xapiproxy->log()->error("LRS error {$endpoint}: " . $response['value']->getBody());
@@ -241,6 +250,9 @@ public function emit(\GuzzleHttp\Psr7\Response $response): void
             }
         }
 
+        // the content may only read the relayed ETag of a document if it is exposed
+        header('Access-Control-Expose-Headers: ETag, Last-Modified, X-Experience-API-Version', true, $statusCode);
+
         // statusline
         header(sprintf(
             'HTTP/%s %d%s',
diff --git a/components/ILIAS/CmiXapi/resources/xapiproxy.php b/components/ILIAS/CmiXapi/resources/xapiproxy.php
index d86acf0a20c1..34d019a3ca06 100755
--- a/components/ILIAS/CmiXapi/resources/xapiproxy.php
+++ b/components/ILIAS/CmiXapi/resources/xapiproxy.php
@@ -35,7 +35,7 @@
     header('Access-Control-Allow-Origin: ' . $_SERVER["HTTP_ORIGIN"]);
     header('Access-Control-Allow-Credentials: true');
     header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS');
-    header('Access-Control-Allow-Headers: X-Experience-API-Version,Accept,Authorization,Etag,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Mx-ReqToken,X-Requested-With');
+    header('Access-Control-Allow-Headers: X-Experience-API-Version,Accept,Authorization,Etag,Cache-Control,Content-Type,DNT,If-Match,If-Modified-Since,If-None-Match,Keep-Alive,Origin,User-Agent,X-Mx-ReqToken,X-Requested-With');
     exit;
 }
 
@@ -54,7 +54,7 @@
     header('Access-Control-Allow-Origin: ' . $_SERVER["HTTP_ORIGIN"]);
     header('Access-Control-Allow-Credentials: true');
     header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS');
-    header('Access-Control-Allow-Headers: X-Experience-API-Version,Accept,Authorization,Etag,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Mx-ReqToken,X-Requested-With');
+    header('Access-Control-Allow-Headers: X-Experience-API-Version,Accept,Authorization,Etag,Cache-Control,Content-Type,DNT,If-Match,If-Modified-Since,If-None-Match,Keep-Alive,Origin,User-Agent,X-Mx-ReqToken,X-Requested-With');
     exit;
 }
 
diff --git a/components/ILIAS/CmiXapi/tests/XapiProxyRequestTest.php b/components/ILIAS/CmiXapi/tests/XapiProxyRequestTest.php
new file mode 100644
index 000000000000..89b0bf3dec3b
--- /dev/null
+++ b/components/ILIAS/CmiXapi/tests/XapiProxyRequestTest.php
@@ -0,0 +1,77 @@
+<?php
+
+/**
+ * This file is part of ILIAS, a powerful learning management system
+ * published by ILIAS open source e-Learning e.V.
+ *
+ * ILIAS is licensed with the GPL-3.0,
+ * see https://www.gnu.org/licenses/gpl-3.0.en.html
+ * You should have received a copy of said license along with the
+ * source code, too.
+ *
+ * If this is not the case or you just want to try ILIAS, you'll find
+ * us at:
+ * https://www.ilias.de
+ * https://github.com/ILIAS-eLearning
+ *
+ *********************************************************************/
+
+declare(strict_types=1);
+
+use GuzzleHttp\Psr7\Request;
+use ILIAS\DI\Container;
+use PHPUnit\Framework\TestCase;
+use XapiProxy\XapiProxy;
+use XapiProxy\XapiProxyRequest;
+
+class XapiProxyRequestTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        $GLOBALS['DIC'] = new Container();
+    }
+
+    /**
+     * @param array<string, string> $headers
+     * @dataProvider preconditionCases
+     */
+    public function testPreconditionIsOnlySynthesizedForUnconditionalDocumentWrites(
+        string $method,
+        string $resource,
+        array $headers,
+        bool $expected
+    ): void {
+        // XapiProxy declares its own method(), so the stub is configured through expects()
+        $proxy = $this->createMock(XapiProxy::class);
+        $proxy->expects($this->any())->method('cmdParts')->willReturn(['', '', '', $resource, '']);
+
+        $method_under_test = new ReflectionMethod(XapiProxyRequest::class, 'requiresSynthesizedPrecondition');
+        $method_under_test->setAccessible(true);
+
+        $this->assertSame(
+            $expected,
+            $method_under_test->invoke(
+                new XapiProxyRequest($proxy),
+                new Request($method, 'https://ilias.example.org/xapiproxy.php/' . $resource, $headers)
+            )
+        );
+    }
+
+    /**
+     * @return array<string, array{0: string, 1: string, 2: array<string, string>, 3: bool}>
+     */
+    public static function preconditionCases(): array
+    {
+        return [
+            'unconditional state write' => ['PUT', 'activities/state', [], true],
+            'unconditional activity profile write' => ['PUT', 'activities/profile', [], true],
+            'unconditional agent profile write' => ['PUT', 'agents/profile', [], true],
+            'client sends If-Match' => ['PUT', 'activities/state', ['If-Match' => '"abc"'], false],
+            'client sends If-None-Match' => ['PUT', 'activities/state', ['If-None-Match' => '*'], false],
+            'merging POST is not guarded' => ['POST', 'activities/state', [], false],
+            'document read is not guarded' => ['GET', 'activities/state', [], false],
+            'document delete is not guarded' => ['DELETE', 'activities/state', [], false],
+            'statements are immutable' => ['PUT', 'statements', [], false],
+        ];
+    }
+}
diff --git a/components/ILIAS/CmiXapi/tests/XapiProxyResponseTest.php b/components/ILIAS/CmiXapi/tests/XapiProxyResponseTest.php
new file mode 100644
index 000000000000..b9c85f965d5f
--- /dev/null
+++ b/components/ILIAS/CmiXapi/tests/XapiProxyResponseTest.php
@@ -0,0 +1,77 @@
+<?php
+
+/**
+ * This file is part of ILIAS, a powerful learning management system
+ * published by ILIAS open source e-Learning e.V.
+ *
+ * ILIAS is licensed with the GPL-3.0,
+ * see https://www.gnu.org/licenses/gpl-3.0.en.html
+ * You should have received a copy of said license along with the
+ * source code, too.
+ *
+ * If this is not the case or you just want to try ILIAS, you'll find
+ * us at:
+ * https://www.ilias.de
+ * https://github.com/ILIAS-eLearning
+ *
+ *********************************************************************/
+
+declare(strict_types=1);
+
+use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\TestCase;
+use XapiProxy\XapiProxy;
+use XapiProxy\XapiProxyResponse;
+
+class XapiProxyResponseTest extends TestCase
+{
+    /**
+     * @dataProvider statusCodes
+     */
+    public function testConditionalAnswersAreRelayedInsteadOfBeingReplacedByAProxyError(
+        int $status,
+        bool $expected
+    ): void {
+        // XapiProxy declares its own method(), so the stub is configured through expects()
+        $proxy = $this->createMock(XapiProxy::class);
+        $proxy->expects($this->any())->method('log')->willReturn($this->createMock(ilLogger::class));
+
+        $this->assertSame(
+            $expected,
+            (new XapiProxyResponse($proxy))->checkResponse(
+                ['state' => 'fulfilled', 'value' => new Response($status)],
+                'https://lrs.example.org/xapi'
+            )
+        );
+    }
+
+    /**
+     * @return array<string, array{0: int, 1: bool}>
+     */
+    public static function statusCodes(): array
+    {
+        return [
+            'ok' => [200, true],
+            'no content' => [204, true],
+            'not modified' => [304, true],
+            'document does not exist' => [404, true],
+            'precondition required by the lrs' => [409, true],
+            'precondition failed' => [412, true],
+            'bad request' => [400, false],
+            'server error' => [500, false],
+        ];
+    }
+
+    public function testConnectionErrorsRemainAnError(): void
+    {
+        $proxy = $this->createMock(XapiProxy::class);
+        $proxy->expects($this->any())->method('log')->willReturn($this->createMock(ilLogger::class));
+
+        $this->assertFalse(
+            (new XapiProxyResponse($proxy))->checkResponse(
+                ['state' => 'rejected', 'reason' => new Exception('connection refused')],
+                'https://lrs.example.org/xapi'
+            )
+        );
+    }
+}