diff --git a/.changeset/fresh-pears-retry.md b/.changeset/fresh-pears-retry.md
new file mode 100644
index 0000000..3cf5029
--- /dev/null
+++ b/.changeset/fresh-pears-retry.md
@@ -0,0 +1,5 @@
+---
+"posthog-php": patch
+---
+
+Retry capture delivery on transient HTTP errors and respect Retry-After responses.
diff --git a/.github/workflows/sdk-compliance.yml b/.github/workflows/sdk-compliance.yml
new file mode 100644
index 0000000..2285355
--- /dev/null
+++ b/.github/workflows/sdk-compliance.yml
@@ -0,0 +1,21 @@
+name: SDK Compliance Tests
+
+permissions:
+  contents: read
+  packages: read
+  pull-requests: write
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  compliance:
+    name: PostHog SDK compliance tests
+    uses: PostHog/posthog-sdk-test-harness/.github/workflows/test-sdk-action.yml@be8b8d5a3f94a249659844e94832e874f049c1e4
+    with:
+      adapter-dockerfile: "sdk_compliance_adapter/Dockerfile"
+      adapter-context: "."
+      test-harness-version: "0.8.0"
diff --git a/lib/Client.php b/lib/Client.php
index 0fe816b..4e4b40c 100644
--- a/lib/Client.php
+++ b/lib/Client.php
@@ -661,7 +661,7 @@ private function doGetFeatureFlagResult(
 
         if (!$flagWasEvaluatedLocally && !$onlyEvaluateLocally) {
             try {
-                $response = $this->requestFlags($distinctId, $groups, $personProperties, $groupProperties);
+                $response = $this->requestFlags($distinctId, $groups, $personProperties, $groupProperties, false, [$key]);
                 $errors = [];
 
                 if (isset($response['errorsWhileComputingFlags']) && $response['errorsWhileComputingFlags']) {
@@ -1662,24 +1662,12 @@ private function requestFlags(
         $payload = array(
             'api_key' => $this->apiKey,
             'distinct_id' => $distinctId,
+            'groups' => empty($groups) ? (object) [] : $groups,
+            'person_properties' => empty($personProperties) ? (object) [] : $personProperties,
+            'group_properties' => empty($groupProperties) ? (object) [] : $groupProperties,
+            'geoip_disable' => $disableGeoip,
         );
 
-        if (!empty($groups)) {
-            $payload["groups"] = $groups;
-        }
-
-        if (!empty($personProperties)) {
-            $payload["person_properties"] = $personProperties;
-        }
-
-        if (!empty($groupProperties)) {
-            $payload["group_properties"] = $groupProperties;
-        }
-
-        if ($disableGeoip) {
-            $payload["geoip_disable"] = true;
-        }
-
         if ($flagKeys !== null) {
             $payload["flag_keys_to_evaluate"] = array_values($flagKeys);
         }
@@ -2012,7 +2000,7 @@ private function hasExplicitCaptureDistinctId(array &$msg): bool
 
     private function normalizeMessageUuid(array $msg): array
     {
-        if (array_key_exists('uuid', $msg) && !$this->isValidUuid($msg['uuid'])) {
+        if (!array_key_exists('uuid', $msg) || !$this->isValidUuid($msg['uuid'])) {
             $msg['uuid'] = Uuid::v4();
         }
 
diff --git a/lib/HttpClient.php b/lib/HttpClient.php
index 7e3c810..45583f3 100644
--- a/lib/HttpClient.php
+++ b/lib/HttpClient.php
@@ -104,6 +104,7 @@ public function sendRequest(string $path, ?string $payload, array $extraHeaders
         do {
             // open connection
             $ch = curl_init();
+            $responseHeaders = [];
 
             if (null !== $payload) {
                 curl_setopt($ch, CURLOPT_POSTFIELDS, $payload);
@@ -131,9 +132,14 @@ public function sendRequest(string $path, ?string $payload, array $extraHeaders
                 curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
             }
 
-            // Capture response headers if we need to extract ETag
+            // Capture response headers if we need to extract ETag or honor Retry-After.
             if ($includeEtag) {
                 curl_setopt($ch, CURLOPT_HEADER, true);
+            } else {
+                curl_setopt($ch, CURLOPT_HEADERFUNCTION, static function ($ch, string $header) use (&$responseHeaders): int {
+                    $responseHeaders[] = trim($header);
+                    return strlen($header);
+                });
             }
 
             // retry failed requests just once to diminish impact on performance
@@ -155,14 +161,14 @@ public function sendRequest(string $path, ?string $payload, array $extraHeaders
 
                 if ($shouldRetry === false) {
                     break;
-                } elseif (($responseCode >= 500 && $responseCode <= 600) || 429 == $responseCode) {
-                    // If status code is greater than 500 and less than 600, it indicates server error
-                    // Error code 429 indicates rate limited.
-                    // Retry uploading in these cases.
-                    usleep($backoff * 1000);
+                } elseif ($this->isRetryableStatus($responseCode)) {
+                    // Retry transient failures. Honor Retry-After when provided, otherwise use
+                    // exponential backoff starting at 100ms.
+                    $retryAfterMs = $this->retryAfterMilliseconds($responseHeaders);
+                    usleep(($retryAfterMs ?? $backoff) * 1000);
                     $backoff *= 2;
                 } else {
-                    // Do not retry non-5xx/non-429 responses (e.g. 4xx, 413 Payload Too Large,
+                    // Do not retry non-transient responses (e.g. 400, 401, 403, 413,
                     // or responseCode 0 for network errors). PHP sends synchronously in the hosting
                     // app's request path, so broad retries would slow down the host application.
                     break;
@@ -199,6 +205,41 @@ private function executePost($ch, bool $includeEtag = false): HttpResponse
         return new HttpResponse($response, $responseCode, null, $curlErrno);
     }
 
+    protected function isRetryableStatus(int $responseCode): bool
+    {
+        return $responseCode === 408
+            || $responseCode === 429
+            || ($responseCode >= 500 && $responseCode <= 600);
+    }
+
+    /**
+     * @param array<int, string> $headers
+     */
+    protected function retryAfterMilliseconds(array $headers): ?int
+    {
+        foreach ($headers as $header) {
+            if (stripos($header, 'Retry-After:') !== 0) {
+                continue;
+            }
+
+            $value = trim(substr($header, strlen('Retry-After:')));
+            if ($value === '') {
+                return null;
+            }
+
+            if (ctype_digit($value)) {
+                return max(0, (int) $value * 1000);
+            }
+
+            $timestamp = strtotime($value);
+            if ($timestamp !== false) {
+                return max(0, (int) (($timestamp - time()) * 1000));
+            }
+        }
+
+        return null;
+    }
+
     private function handleError($code, $message)
     {
         if (null !== $this->errorHandler) {
diff --git a/lib/QueueConsumer.php b/lib/QueueConsumer.php
index 9f522f8..0db1629 100644
--- a/lib/QueueConsumer.php
+++ b/lib/QueueConsumer.php
@@ -76,7 +76,9 @@ public function __construct($apiKey, $options = array())
         }
 
         if (isset($options["compress_request"])) {
-            $this->compress_request = json_decode($options["compress_request"]);
+            $this->compress_request = is_bool($options["compress_request"])
+                ? $options["compress_request"]
+                : (bool) json_decode($options["compress_request"]);
         }
 
         $this->queue = array();
diff --git a/sdk_compliance_adapter/Dockerfile b/sdk_compliance_adapter/Dockerfile
new file mode 100644
index 0000000..e37fa73
--- /dev/null
+++ b/sdk_compliance_adapter/Dockerfile
@@ -0,0 +1,21 @@
+FROM php:8.3-cli
+
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends git unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=composer:2 /usr/bin/composer /usr/bin/composer
+
+COPY composer.json composer.lock /app/
+COPY lib/ /app/lib/
+COPY bin/ /app/bin/
+
+RUN composer install --no-interaction --prefer-dist --no-dev --no-progress
+
+COPY sdk_compliance_adapter/ /app/sdk_compliance_adapter/
+
+EXPOSE 8080
+
+CMD ["php", "/app/sdk_compliance_adapter/adapter.php"]
diff --git a/sdk_compliance_adapter/README.md b/sdk_compliance_adapter/README.md
new file mode 100644
index 0000000..09d83d6
--- /dev/null
+++ b/sdk_compliance_adapter/README.md
@@ -0,0 +1,12 @@
+# PostHog PHP SDK Compliance Adapter
+
+This adapter wraps the PostHog PHP SDK for the PostHog SDK compliance test harness.
+
+## Local run
+
+```sh
+cd sdk_compliance_adapter
+docker compose up --build --abort-on-container-exit --exit-code-from test-harness
+```
+
+The adapter exposes the standard harness endpoints: `/health`, `/init`, `/capture`, `/flush`, `/state`, and `/reset`.
diff --git a/sdk_compliance_adapter/adapter.php b/sdk_compliance_adapter/adapter.php
new file mode 100644
index 0000000..8d47f04
--- /dev/null
+++ b/sdk_compliance_adapter/adapter.php
@@ -0,0 +1,581 @@
+<?php
+
+// phpcs:ignoreFile -- Compliance adapter is an executable test harness shim.
+declare(strict_types=1);
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use PostHog\Client;
+use PostHog\HttpClient;
+use PostHog\HttpResponse;
+use PostHog\PostHog;
+use PostHog\Uuid;
+
+final class RequestInfo
+{
+    public function __construct(
+        public int $timestampMs,
+        public int $statusCode,
+        public int $retryAttempt,
+        public int $eventCount,
+        public array $uuidList,
+    ) {
+    }
+
+    public function toArray(): array
+    {
+        return [
+            'timestamp_ms' => $this->timestampMs,
+            'status_code' => $this->statusCode,
+            'retry_attempt' => $this->retryAttempt,
+            'event_count' => $this->eventCount,
+            'uuid_list' => $this->uuidList,
+        ];
+    }
+}
+
+final class AdapterState
+{
+    public ?Client $client = null;
+    public int $totalEventsCaptured = 0;
+    public int $totalEventsSent = 0;
+    public int $totalRetries = 0;
+    public ?string $lastError = null;
+    /** @var list<RequestInfo> */
+    public array $requestsMade = [];
+    public int $pendingEvents = 0;
+
+    public function reset(): void
+    {
+        if ($this->client !== null) {
+            $this->discardQueuedEvents($this->client);
+            try {
+                $this->client->shutdown();
+            } catch (Throwable $e) {
+                error_log('[adapter] error shutting down client: ' . $e->getMessage());
+            }
+        }
+
+        $this->client = null;
+        $this->totalEventsCaptured = 0;
+        $this->totalEventsSent = 0;
+        $this->totalRetries = 0;
+        $this->lastError = null;
+        $this->requestsMade = [];
+        $this->pendingEvents = 0;
+    }
+
+    public function recordCaptured(): void
+    {
+        $this->totalEventsCaptured++;
+        $this->pendingEvents++;
+    }
+
+    public function recordRequest(int $statusCode, int $retryAttempt, int $eventCount, array $uuidList): void
+    {
+        $this->requestsMade[] = new RequestInfo(
+            (int) floor(microtime(true) * 1000),
+            $statusCode,
+            $retryAttempt,
+            $eventCount,
+            $uuidList,
+        );
+
+        if ($retryAttempt > 0) {
+            $this->totalRetries++;
+        }
+
+        if ($statusCode === 200) {
+            $this->totalEventsSent += $eventCount;
+            $this->pendingEvents = max(0, $this->pendingEvents - $eventCount);
+        }
+    }
+
+    public function recordError(string $error): void
+    {
+        $this->lastError = $error;
+    }
+
+    private function discardQueuedEvents(Client $client): void
+    {
+        try {
+            $clientReflection = new ReflectionObject($client);
+            $consumerProperty = $clientReflection->getProperty('consumer');
+            $consumerProperty->setAccessible(true);
+            $consumer = $consumerProperty->getValue($client);
+            if (!is_object($consumer)) {
+                return;
+            }
+
+            $consumerReflection = new ReflectionObject($consumer);
+            while (!$consumerReflection->hasProperty('queue')) {
+                $parent = $consumerReflection->getParentClass();
+                if ($parent === false) {
+                    return;
+                }
+                $consumerReflection = $parent;
+            }
+
+            $queueProperty = $consumerReflection->getProperty('queue');
+            $queueProperty->setAccessible(true);
+            $queueProperty->setValue($consumer, []);
+        } catch (Throwable $e) {
+            error_log('[adapter] error discarding queued events: ' . $e->getMessage());
+        }
+    }
+
+    public function toArray(): array
+    {
+        return [
+            'pending_events' => $this->pendingEvents,
+            'total_events_captured' => $this->totalEventsCaptured,
+            'total_events_sent' => $this->totalEventsSent,
+            'total_retries' => $this->totalRetries,
+            'last_error' => $this->lastError,
+            'requests_made' => array_map(static fn (RequestInfo $r): array => $r->toArray(), $this->requestsMade),
+        ];
+    }
+}
+
+final class TrackedHttpClient extends HttpClient
+{
+    public function __construct(
+        private AdapterState $state,
+        private string $trackedHost,
+        private bool $trackedUseSsl = true,
+        private int $trackedMaximumBackoffDuration = 10000,
+        private bool $trackedCompressRequests = false,
+        private bool $trackedDebug = false,
+        private int $trackedCurlTimeoutMilliseconds = 10000,
+    ) {
+        parent::__construct(
+            $trackedHost,
+            $trackedUseSsl,
+            $trackedMaximumBackoffDuration,
+            $trackedCompressRequests,
+            $trackedDebug,
+            null,
+            $trackedCurlTimeoutMilliseconds,
+        );
+    }
+
+    public function sendRequest(string $path, ?string $payload, array $extraHeaders = [], array $requestOptions = []): HttpResponse
+    {
+        $protocol = $this->trackedUseSsl ? 'https://' : 'http://';
+        $backoff = 100;
+        $shouldRetry = $requestOptions['shouldRetry'] ?? true;
+        $shouldVerify = $requestOptions['shouldVerify'] ?? true;
+        $includeEtag = $requestOptions['includeEtag'] ?? false;
+        $timeout = isset($requestOptions['timeout'])
+            ? (int) $requestOptions['timeout']
+            : $this->trackedCurlTimeoutMilliseconds;
+        $retryAttempt = 0;
+        $httpResponse = new HttpResponse(false, 0, null, 0);
+
+        do {
+            $ch = curl_init();
+            $responseHeaders = [];
+
+            if ($payload !== null) {
+                curl_setopt($ch, CURLOPT_POSTFIELDS, $payload);
+            }
+
+            $headers = ['Content-Type: application/json'];
+            if ($this->trackedCompressRequests) {
+                $headers[] = 'Content-Encoding: gzip';
+            }
+
+            curl_setopt($ch, CURLOPT_HTTPHEADER, array_merge($headers, $extraHeaders));
+            curl_setopt($ch, CURLOPT_URL, $protocol . $this->trackedHost . $path);
+            curl_setopt($ch, CURLOPT_RETURNTRANSFER, $shouldVerify);
+            curl_setopt($ch, CURLOPT_TIMEOUT_MS, $shouldVerify ? $timeout : 1);
+            curl_setopt($ch, CURLOPT_CONNECTTIMEOUT_MS, $timeout);
+            if (!$shouldVerify) {
+                curl_setopt($ch, CURLOPT_NOSIGNAL, true);
+                curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
+            }
+            if ($includeEtag) {
+                curl_setopt($ch, CURLOPT_HEADER, true);
+            } else {
+                curl_setopt($ch, CURLOPT_HEADERFUNCTION, static function ($ch, string $header) use (&$responseHeaders): int {
+                    $responseHeaders[] = trim($header);
+                    return strlen($header);
+                });
+            }
+
+            $response = curl_exec($ch);
+            $responseCode = (int) curl_getinfo($ch, CURLINFO_RESPONSE_CODE);
+            $curlErrno = (int) curl_errno($ch);
+            $etag = null;
+
+            if ($includeEtag && $response !== false) {
+                $headerSize = (int) curl_getinfo($ch, CURLINFO_HEADER_SIZE);
+                $rawHeaders = substr((string) $response, 0, $headerSize);
+                $body = substr((string) $response, $headerSize);
+                if (preg_match('/^etag:\s*(.+)$/mi', $rawHeaders, $matches)) {
+                    $etag = trim($matches[1]);
+                }
+                $response = $body;
+            }
+
+            curl_close($ch);
+            $httpResponse = new HttpResponse($response, $responseCode, $etag, $curlErrno);
+
+            if ($path === '/batch/') {
+                [$eventCount, $uuidList] = $this->extractBatchInfo($payload);
+                $this->state->recordRequest($responseCode, $retryAttempt, $eventCount, $uuidList);
+            }
+
+            if ($responseCode === 304) {
+                break;
+            }
+
+            if ($shouldVerify && $responseCode !== 200) {
+                if ($shouldRetry === false) {
+                    break;
+                }
+
+                if ($this->isRetryableStatus($responseCode)) {
+                    $retryAfterMs = $this->retryAfterMilliseconds($responseHeaders);
+                    usleep(($retryAfterMs ?? $backoff) * 1000);
+                    $backoff *= 2;
+                    $retryAttempt++;
+                } else {
+                    break;
+                }
+            } else {
+                break;
+            }
+        } while ($shouldRetry && $backoff < $this->trackedMaximumBackoffDuration);
+
+        return $httpResponse;
+    }
+
+    /** @return array{0:int,1:list<string>} */
+    private function extractBatchInfo(?string $payload): array
+    {
+        if ($payload === null || $payload === '') {
+            return [0, []];
+        }
+
+        $json = $payload;
+        if ($this->trackedCompressRequests) {
+            $decoded = gzdecode($payload);
+            if ($decoded !== false) {
+                $json = $decoded;
+            }
+        }
+
+        $decoded = json_decode($json, true);
+        if (!is_array($decoded) || !isset($decoded['batch']) || !is_array($decoded['batch'])) {
+            return [0, []];
+        }
+
+        $uuidList = [];
+        foreach ($decoded['batch'] as $event) {
+            if (is_array($event) && isset($event['uuid']) && is_string($event['uuid'])) {
+                $uuidList[] = $event['uuid'];
+            }
+        }
+
+        return [count($decoded['batch']), $uuidList];
+    }
+}
+
+function isValidUuid(mixed $uuid): bool
+{
+    return is_string($uuid)
+        && preg_match(
+            '/^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i',
+            $uuid
+        ) === 1;
+}
+
+function jsonResponse($client, int $status, array $payload): void
+{
+    $body = json_encode($payload, JSON_UNESCAPED_SLASHES);
+    if ($body === false) {
+        $status = 500;
+        $body = '{"error":"failed to encode response"}';
+    }
+
+    $reason = [
+        200 => 'OK',
+        400 => 'Bad Request',
+        404 => 'Not Found',
+        405 => 'Method Not Allowed',
+        500 => 'Internal Server Error',
+    ][$status] ?? 'OK';
+
+    fwrite($client, "HTTP/1.1 {$status} {$reason}\r\n");
+    fwrite($client, "Content-Type: application/json\r\n");
+    fwrite($client, 'Content-Length: ' . strlen($body) . "\r\n");
+    fwrite($client, "Connection: close\r\n\r\n");
+    fwrite($client, $body);
+}
+
+function readRequest($client): ?array
+{
+    $requestLine = fgets($client);
+    if ($requestLine === false || trim($requestLine) === '') {
+        return null;
+    }
+
+    $parts = explode(' ', trim($requestLine), 3);
+    if (count($parts) < 2) {
+        return null;
+    }
+
+    $headers = [];
+    while (($line = fgets($client)) !== false) {
+        $line = rtrim($line, "\r\n");
+        if ($line === '') {
+            break;
+        }
+        $headerParts = explode(':', $line, 2);
+        if (count($headerParts) === 2) {
+            $headers[strtolower(trim($headerParts[0]))] = trim($headerParts[1]);
+        }
+    }
+
+    $length = isset($headers['content-length']) ? (int) $headers['content-length'] : 0;
+    $body = '';
+    while (strlen($body) < $length && !feof($client)) {
+        $body .= fread($client, $length - strlen($body));
+    }
+
+    return [
+        'method' => strtoupper($parts[0]),
+        'path' => parse_url($parts[1], PHP_URL_PATH) ?: '/',
+        'body' => $body,
+    ];
+}
+
+function requestJson(array $request): array
+{
+    if ($request['body'] === '') {
+        return [];
+    }
+
+    $decoded = json_decode($request['body'], true);
+    return is_array($decoded) ? $decoded : [];
+}
+
+function normalizeHost(string $host): array
+{
+    $useSsl = true;
+    $normalized = trim($host);
+    if (preg_match('/^http:\/\//i', $normalized) === 1) {
+        $useSsl = false;
+        $normalized = preg_replace('/^http:\/\//i', '', $normalized) ?? $normalized;
+    } elseif (preg_match('/^https:\/\//i', $normalized) === 1) {
+        $useSsl = true;
+        $normalized = preg_replace('/^https:\/\//i', '', $normalized) ?? $normalized;
+    }
+
+    return [$normalized, $useSsl];
+}
+
+function maxBackoffDurationForRetries(int $maxRetries): int
+{
+    if ($maxRetries <= 0) {
+        return 100;
+    }
+
+    return (100 * (2 ** $maxRetries)) + 1;
+}
+
+function handleRequest(array $request, AdapterState $state): array
+{
+    try {
+        if ($request['method'] === 'GET' && $request['path'] === '/health') {
+            return [200, [
+                'sdk_name' => 'posthog-php',
+                'sdk_version' => PostHog::VERSION,
+                'adapter_version' => '1.0.0',
+                'capabilities' => ['capture_v0', 'encoding_gzip'],
+            ]];
+        }
+
+        if ($request['method'] === 'POST' && $request['path'] === '/init') {
+            $data = requestJson($request);
+            $apiKey = isset($data['api_key']) ? trim((string) $data['api_key']) : '';
+            $host = isset($data['host']) ? trim((string) $data['host']) : '';
+            if ($apiKey === '') {
+                return [400, ['error' => 'api_key is required']];
+            }
+            if ($host === '') {
+                return [400, ['error' => 'host is required']];
+            }
+
+            $state->reset();
+            [$normalizedHost, $useSsl] = normalizeHost($host);
+            $flushAt = max(1, (int) ($data['flush_at'] ?? 100));
+            $flushIntervalMs = max(0, (int) ($data['flush_interval_ms'] ?? 5000));
+            $maxRetries = max(0, (int) ($data['max_retries'] ?? 3));
+            $enableCompression = (bool) ($data['enable_compression'] ?? false);
+            $maximumBackoffDuration = maxBackoffDurationForRetries($maxRetries);
+            $timeoutMs = max(1000, (int) ($data['timeout_ms'] ?? 10000));
+
+            $httpClient = new TrackedHttpClient(
+                $state,
+                $normalizedHost,
+                $useSsl,
+                $maximumBackoffDuration,
+                $enableCompression,
+                true,
+                $timeoutMs,
+            );
+
+            $state->client = new Client($apiKey, [
+                'host' => $normalizedHost,
+                'ssl' => $useSsl,
+                'consumer' => 'lib_curl',
+                'batch_size' => $flushAt,
+                'flush_interval_seconds' => $flushIntervalMs / 1000,
+                'maximum_backoff_duration' => $maximumBackoffDuration,
+                'compress_request' => $enableCompression ? 'true' : 'false',
+                'debug' => true,
+                'timeout' => $timeoutMs,
+            ], $httpClient, null, false);
+
+            return [200, ['success' => true]];
+        }
+
+        if ($request['method'] === 'POST' && $request['path'] === '/capture') {
+            if ($state->client === null) {
+                return [400, ['error' => 'SDK not initialized']];
+            }
+
+            $data = requestJson($request);
+            $distinctId = isset($data['distinct_id']) ? (string) $data['distinct_id'] : '';
+            $event = isset($data['event']) ? (string) $data['event'] : '';
+            if ($distinctId === '') {
+                return [400, ['error' => 'distinct_id is required']];
+            }
+            if ($event === '') {
+                return [400, ['error' => 'event is required']];
+            }
+
+            $message = [
+                'distinctId' => $distinctId,
+                'event' => $event,
+                'properties' => (isset($data['properties']) && is_array($data['properties'])) ? $data['properties'] : [],
+            ];
+            if (isset($data['timestamp'])) {
+                $message['timestamp'] = $data['timestamp'];
+            }
+
+            if (!isset($message['uuid']) || !isValidUuid($message['uuid'])) {
+                $message['uuid'] = Uuid::v4();
+            }
+
+            $state->client->capture($message);
+
+            $state->recordCaptured();
+            return [200, ['success' => true, 'uuid' => $message['uuid']]];
+        }
+
+        if ($request['method'] === 'POST' && $request['path'] === '/get_feature_flag') {
+            if ($state->client === null) {
+                return [400, ['error' => 'SDK not initialized']];
+            }
+
+            $data = requestJson($request);
+            $key = isset($data['key']) ? (string) $data['key'] : '';
+            $distinctId = isset($data['distinct_id']) ? (string) $data['distinct_id'] : '';
+            if ($key === '') {
+                return [400, ['error' => 'key is required']];
+            }
+            if ($distinctId === '') {
+                return [400, ['error' => 'distinct_id is required']];
+            }
+
+            $groups = (isset($data['groups']) && is_array($data['groups'])) ? $data['groups'] : [];
+            $personProperties = (isset($data['person_properties']) && is_array($data['person_properties']))
+                ? $data['person_properties']
+                : [];
+            $groupProperties = (isset($data['group_properties']) && is_array($data['group_properties']))
+                ? $data['group_properties']
+                : [];
+            $forceRemote = (bool) ($data['force_remote'] ?? true);
+            $disableGeoip = (bool) ($data['disable_geoip'] ?? false);
+
+            if ($disableGeoip) {
+                $snapshot = $state->client->evaluateFlags(
+                    $distinctId,
+                    $groups,
+                    $personProperties,
+                    $groupProperties,
+                    !$forceRemote,
+                    true,
+                    [$key],
+                );
+                $value = $snapshot->getFlag($key);
+            } else {
+                $value = @$state->client->getFeatureFlag(
+                    $key,
+                    $distinctId,
+                    $groups,
+                    $personProperties,
+                    $groupProperties,
+                    !$forceRemote,
+                    true,
+                );
+            }
+
+            return [200, ['success' => true, 'value' => $value]];
+        }
+
+        if ($request['method'] === 'POST' && $request['path'] === '/flush') {
+            if ($state->client === null) {
+                return [400, ['error' => 'SDK not initialized']];
+            }
+
+            $state->client->flush();
+            return [200, ['success' => true, 'events_flushed' => $state->totalEventsSent]];
+        }
+
+        if ($request['method'] === 'GET' && $request['path'] === '/state') {
+            return [200, $state->toArray()];
+        }
+
+        if ($request['method'] === 'POST' && $request['path'] === '/reset') {
+            $state->reset();
+            return [200, ['success' => true]];
+        }
+
+        return [404, ['error' => 'not found']];
+    } catch (Throwable $e) {
+        $state->recordError($e->getMessage());
+        error_log('[adapter] ' . $e);
+        return [500, ['error' => $e->getMessage()]];
+    }
+}
+
+$server = stream_socket_server('tcp://0.0.0.0:8080', $errno, $errstr);
+if ($server === false) {
+    fwrite(STDERR, "Failed to start server: {$errstr} ({$errno})\n");
+    exit(1);
+}
+
+$state = new AdapterState();
+fwrite(STDERR, "PostHog PHP SDK compliance adapter listening on :8080\n");
+
+while (true) {
+    $client = @stream_socket_accept($server, -1);
+    if ($client === false) {
+        usleep(10000);
+        continue;
+    }
+
+    $request = readRequest($client);
+    if ($request === null) {
+        fclose($client);
+        continue;
+    }
+
+    [$status, $payload] = handleRequest($request, $state);
+    jsonResponse($client, $status, $payload);
+    fclose($client);
+}
diff --git a/sdk_compliance_adapter/docker-compose.yml b/sdk_compliance_adapter/docker-compose.yml
new file mode 100644
index 0000000..878432c
--- /dev/null
+++ b/sdk_compliance_adapter/docker-compose.yml
@@ -0,0 +1,21 @@
+name: posthog-php-sdk-compliance
+
+services:
+  sdk-adapter:
+    build:
+      context: ..
+      dockerfile: sdk_compliance_adapter/Dockerfile
+    networks:
+      - test-network
+
+  test-harness:
+    image: ghcr.io/posthog/sdk-test-harness:0.8.0
+    command: ["run", "--adapter-url", "http://sdk-adapter:8080", "--mock-url", "http://test-harness:8081"]
+    networks:
+      - test-network
+    depends_on:
+      - sdk-adapter
+
+networks:
+  test-network:
+    driver: bridge
diff --git a/test/FeatureFlagLocalEvaluationTest.php b/test/FeatureFlagLocalEvaluationTest.php
index 8d39666..479bc81 100644
--- a/test/FeatureFlagLocalEvaluationTest.php
+++ b/test/FeatureFlagLocalEvaluationTest.php
@@ -38,6 +38,24 @@ public function checkEmptyErrorLogs(): void
         $this->assertTrue(empty($errorMessages), "Error logs are not empty: " . implode("\n", $errorMessages));
     }
 
+    private function assertAndStripBatchUuid(int $callIndex): void
+    {
+        $payload = json_decode($this->http_client->calls[$callIndex]['payload'], true);
+        $this->assertIsArray($payload);
+        $this->assertArrayHasKey('batch', $payload);
+
+        foreach ($payload['batch'] as $index => $event) {
+            $this->assertArrayHasKey('uuid', $event);
+            $this->assertMatchesRegularExpression(
+                '/^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i',
+                $event['uuid']
+            );
+            unset($payload['batch'][$index]['uuid']);
+        }
+
+        $this->http_client->calls[$callIndex]['payload'] = json_encode($payload);
+    }
+
     public function testMatchPropertyEquals(): void
     {
         $prop = [
@@ -1442,6 +1460,7 @@ public function testSimpleFlag()
 
             PostHog::flush();
 
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array(
@@ -1694,7 +1713,7 @@ public function testFeatureFlagsLocalEvaluationForNegatedCohorts()
             array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    'payload' => '{"api_key":"random_key","distinct_id":"some-distinct-id","person_properties":{"distinct_id":"some-distinct-id","region":"USA","other":"thing"}}',
+                    'payload' => '{"api_key":"random_key","distinct_id":"some-distinct-id","groups":{},"person_properties":{"distinct_id":"some-distinct-id","region":"USA","other":"thing"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["beta-feature"]}',
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
diff --git a/test/FeatureFlagTest.php b/test/FeatureFlagTest.php
index b51d067..9c699f3 100644
--- a/test/FeatureFlagTest.php
+++ b/test/FeatureFlagTest.php
@@ -46,6 +46,24 @@ public function checkEmptyErrorLogs(): void
         $this->assertTrue(empty($errorMessages), "Error logs are not empty: " . implode("\n", $errorMessages));
     }
 
+    private function assertAndStripBatchUuid(int $callIndex): void
+    {
+        $payload = json_decode($this->http_client->calls[$callIndex]['payload'], true);
+        $this->assertIsArray($payload);
+        $this->assertArrayHasKey('batch', $payload);
+
+        foreach ($payload['batch'] as $index => $event) {
+            $this->assertArrayHasKey('uuid', $event);
+            $this->assertMatchesRegularExpression(
+                '/^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i',
+                $event['uuid']
+            );
+            unset($payload['batch'][$index]['uuid']);
+        }
+
+        $this->http_client->calls[$callIndex]['payload'] = json_encode($payload);
+    }
+
     public static function decideResponseCases(): array
     {
         return [
@@ -233,7 +251,7 @@ public function testIsFeatureEnabled($response)
                 ),
                 1 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","person_properties":{"distinct_id":"user-id"}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","groups":{},"person_properties":{"distinct_id":"user-id"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["having_fun"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -247,12 +265,13 @@ public function testIsFeatureEnabledCapturesFeatureFlagCalledEventWithAdditional
             $this->setUp(MockedResponses::FLAGS_V2_RESPONSE, personalApiKey: null);
             $this->assertTrue(PostHog::isFeatureEnabled('simple-test', 'user-id'));
             PostHog::flush();
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","person_properties":{"distinct_id":"user-id"}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","groups":{},"person_properties":{"distinct_id":"user-id"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["simple-test"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                         "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                     ),
@@ -275,7 +294,7 @@ public function testWhitespacePersonalApiKeyFallsBackToFlagsEndpoint()
             [
                 [
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","person_properties":{"distinct_id":"user-id"}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","groups":{},"person_properties":{"distinct_id":"user-id"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["simple-test"]}', self::FAKE_API_KEY),
                     "extraHeaders" => [0 => 'User-Agent: posthog-php/' . PostHog::VERSION],
                     "requestOptions" => ["timeout" => 3000, "shouldRetry" => false],
                 ],
@@ -304,7 +323,7 @@ public function testIsFeatureEnabledGroups($response)
                 1 => array(
                     "path" => "/flags/?v=2",
                     "payload" => sprintf(
-                        '{"api_key":"%s","distinct_id":"user-id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"user-id"},"group_properties":{"company":{"$group_key":"id:5"}}}',
+                        '{"api_key":"%s","distinct_id":"user-id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"user-id"},"group_properties":{"company":{"$group_key":"id:5"}},"geoip_disable":false,"flag_keys_to_evaluate":["having_fun"]}',
                         self::FAKE_API_KEY
                     ),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
@@ -332,7 +351,7 @@ public function testGetFeatureFlag($response)
                 ),
                 1 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","person_properties":{"distinct_id":"user-id"}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","groups":{},"person_properties":{"distinct_id":"user-id"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["multivariate-test"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -346,12 +365,13 @@ public function testGetFeatureFlagCapturesFeatureFlagCalledEventWithAdditionalMe
             $this->setUp(MockedResponses::FLAGS_V2_RESPONSE, personalApiKey: null);
             $this->assertEquals("variant-value", PostHog::getFeatureFlag('multivariate-test', 'user-id'));
             PostHog::flush();
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","person_properties":{"distinct_id":"user-id"}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"user-id","groups":{},"person_properties":{"distinct_id":"user-id"},"group_properties":{},"geoip_disable":false,"flag_keys_to_evaluate":["multivariate-test"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -400,7 +420,7 @@ public function testGetFeatureFlagGroups($response)
                 1 => array(
                     "path" => "/flags/?v=2",
                     "payload" => sprintf(
-                        '{"api_key":"%s","distinct_id":"user-id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"user-id"},"group_properties":{"company":{"$group_key":"id:5"}}}',
+                        '{"api_key":"%s","distinct_id":"user-id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"user-id"},"group_properties":{"company":{"$group_key":"id:5"}},"geoip_disable":false,"flag_keys_to_evaluate":["multivariate-test"]}',
                         self::FAKE_API_KEY
                     ),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
diff --git a/test/HttpClientTest.php b/test/HttpClientTest.php
index 29e1cc0..a6361f2 100644
--- a/test/HttpClientTest.php
+++ b/test/HttpClientTest.php
@@ -5,6 +5,15 @@
 use PHPUnit\Framework\TestCase;
 use PostHog\HttpClient;
 
+class RetryAfterHttpClient extends HttpClient
+{
+    /** @param array<int, string> $headers */
+    public function parseRetryAfter(array $headers): ?int
+    {
+        return $this->retryAfterMilliseconds($headers);
+    }
+}
+
 class HttpClientTest extends TestCase
 {
     public function testMaskTokensInUrl(): void
@@ -36,4 +45,24 @@ public function testMaskTokensInUrl(): void
         $result = $httpClient->maskTokensInUrl($url);
         $this->assertEquals('https://example.com/api/flags?token=&other=value', $result);
     }
+
+    public function testRetryAfterMillisecondsParsesSeconds(): void
+    {
+        $httpClient = new RetryAfterHttpClient("app.posthog.com");
+
+        $this->assertSame(3000, $httpClient->parseRetryAfter(['Retry-After: 3']));
+        $this->assertSame(0, $httpClient->parseRetryAfter(['Retry-After: 0']));
+    }
+
+    public function testRetryAfterMillisecondsParsesHttpDate(): void
+    {
+        $httpClient = new RetryAfterHttpClient("app.posthog.com");
+        $retryAt = gmdate('D, d M Y H:i:s \G\M\T', time() + 2);
+
+        $retryAfterMs = $httpClient->parseRetryAfter(['Retry-After: ' . $retryAt]);
+
+        $this->assertNotNull($retryAfterMs);
+        $this->assertGreaterThanOrEqual(0, $retryAfterMs);
+        $this->assertLessThanOrEqual(2000, $retryAfterMs);
+    }
 }
diff --git a/test/PostHogTest.php b/test/PostHogTest.php
index da81bc1..07fcc08 100644
--- a/test/PostHogTest.php
+++ b/test/PostHogTest.php
@@ -78,6 +78,21 @@ private function assertValidUuidV4(string $uuid): void
         );
     }
 
+    private function assertAndStripBatchUuid(int $callIndex): void
+    {
+        $payload = json_decode($this->http_client->calls[$callIndex]['payload'], true);
+        $this->assertIsArray($payload);
+        $this->assertArrayHasKey('batch', $payload);
+
+        foreach ($payload['batch'] as $index => $event) {
+            $this->assertArrayHasKey('uuid', $event);
+            $this->assertValidUuidV4($event['uuid']);
+            unset($payload['batch'][$index]['uuid']);
+        }
+
+        $this->http_client->calls[$callIndex]['payload'] = json_encode($payload);
+    }
+
     private function withEnvApiKey(?string $apiKey, callable $callback): void
     {
         $previousApiKey = getenv(PostHog::ENV_API_KEY);
@@ -961,6 +976,7 @@ public function testCaptureWithSendFeatureFlagsOption(): void
             );
             PostHog::flush();
 
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array (
@@ -1017,6 +1033,7 @@ public function testCaptureWithLocalSendFlags(): void
 
             PostHog::flush();
 
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array (
@@ -1066,6 +1083,7 @@ public function testCaptureWithLocalSendFlagsNoOverrides(): void
 
             PostHog::flush();
 
+            $this->assertAndStripBatchUuid(1);
             $this->assertEquals(
                 $this->http_client->calls,
                 array (
@@ -1280,7 +1298,7 @@ public function testDefaultPropertiesGetAddedProperly(): void
                 ),
                 1 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"some_id","x1":"y1"},"group_properties":{"company":{"$group_key":"id:5","x":"y"},"instance":{"$group_key":"app.posthog.com"}}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"some_id","x1":"y1"},"group_properties":{"company":{"$group_key":"id:5","x":"y"},"instance":{"$group_key":"app.posthog.com"}},"geoip_disable":false,"flag_keys_to_evaluate":["random_key"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -1302,7 +1320,7 @@ public function testDefaultPropertiesGetAddedProperly(): void
             array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"override"},"group_properties":{"company":{"$group_key":"group_override"},"instance":{"$group_key":"app.posthog.com"}}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"override"},"group_properties":{"company":{"$group_key":"group_override"},"instance":{"$group_key":"app.posthog.com"}},"geoip_disable":false,"flag_keys_to_evaluate":["random_key"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -1318,7 +1336,7 @@ public function testDefaultPropertiesGetAddedProperly(): void
             array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"some_id"},"group_properties":{"company":{"$group_key":"id:5"}}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5"},"person_properties":{"distinct_id":"some_id"},"group_properties":{"company":{"$group_key":"id:5"}},"geoip_disable":false,"flag_keys_to_evaluate":["random_key"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -1334,7 +1352,7 @@ public function testDefaultPropertiesGetAddedProperly(): void
             array(
                 0 => array(
                     "path" => "/flags/?v=2",
-                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"some_id","x1":"y1"},"group_properties":{"company":{"$group_key":"id:5","x":"y"},"instance":{"$group_key":"app.posthog.com"}}}', self::FAKE_API_KEY),
+                    "payload" => sprintf('{"api_key":"%s","distinct_id":"some_id","groups":{"company":"id:5","instance":"app.posthog.com"},"person_properties":{"distinct_id":"some_id","x1":"y1"},"group_properties":{"company":{"$group_key":"id:5","x":"y"},"instance":{"$group_key":"app.posthog.com"}},"geoip_disable":false,"flag_keys_to_evaluate":["random_key"]}', self::FAKE_API_KEY),
                     "extraHeaders" => array(0 => 'User-Agent: posthog-php/' . PostHog::VERSION),
                     "requestOptions" => array("timeout" => 3000, "shouldRetry" => false),
                 ),
@@ -1368,6 +1386,7 @@ public function testCaptureWithSendFeatureFlagsFalse(): void
 
             PostHog::flush();
 
+            $this->assertAndStripBatchUuid(1);
             // When send_feature_flags is explicitly false, NO feature flags should be added
             $this->assertEquals(
                 $this->http_client->calls,