diff --git a/composer.json b/composer.json
index 9651b02..98892f9 100644
--- a/composer.json
+++ b/composer.json
@@ -54,7 +54,7 @@
         "test:lint": "@php vendor/bin/pint --config https://raw.githubusercontent.com/DIJ-digital/pint-config/main/pint.json",
         "test:unit": "pest",
         "test:types": "phpstan",
-        "test:refactor": "rector --dry-run",
+        "test:refactor": "rector",
         "test": [
             "@test:lint",
             "@test:type-coverage",
diff --git a/src/Contracts/TransporterInterface.php b/src/Contracts/TransporterInterface.php
index 8234ade..558497a 100644
--- a/src/Contracts/TransporterInterface.php
+++ b/src/Contracts/TransporterInterface.php
@@ -9,34 +9,34 @@
 interface TransporterInterface
 {
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      */
     public function request(string $method, string $uri, array $options = []): ResponseInterface;
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      */
     public function get(string $uri, array $options = []): ResponseInterface;
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      */
     public function post(string $uri, array $options = []): ResponseInterface;
 
     /**
-     * @param array<string, mixed> $data
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $data
+     * @param  array<string, mixed>  $options
      */
     public function postJson(string $uri, array $data = [], array $options = []): ResponseInterface;
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      */
     public function delete(string $uri, array $options = []): ResponseInterface;
 
     /**
-     * @param array<string, mixed> $data
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $data
+     * @param  array<string, mixed>  $options
      */
     public function patchJson(string $uri, array $data = [], array $options = []): ResponseInterface;
 }
diff --git a/src/Langfuse.php b/src/Langfuse.php
index ebe96f9..462498f 100644
--- a/src/Langfuse.php
+++ b/src/Langfuse.php
@@ -12,8 +12,7 @@ public function __construct(
         private readonly TransporterInterface $transporter,
         private readonly string $environment = 'default',
         private readonly string $label = 'latest',
-    ) {
-    }
+    ) {}
 
     public function prompt(): Prompt
     {
@@ -37,4 +36,11 @@ public function score(): Score
             transporter: $this->transporter,
         );
     }
+
+    public function observation(): Observation
+    {
+        return new Observation(
+            transporter: $this->transporter,
+        );
+    }
 }
diff --git a/src/Observation.php b/src/Observation.php
new file mode 100644
index 0000000..27f6955
--- /dev/null
+++ b/src/Observation.php
@@ -0,0 +1,138 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DIJ\Langfuse\PHP;
+
+use DIJ\Langfuse\PHP\Contracts\TransporterInterface;
+use DIJ\Langfuse\PHP\Responses\ObservationListResponse;
+use DIJ\Langfuse\PHP\Responses\ObservationResponse;
+use JsonException;
+
+class Observation
+{
+    public function __construct(private readonly TransporterInterface $transporter) {}
+
+    /**
+     * @throws JsonException
+     */
+    public function get(string $observationId): ObservationResponse
+    {
+        $response = $this->transporter->get(
+            uri: sprintf('/api/public/observations/%s', urlencode($observationId)),
+        );
+
+        /** @var array{
+         *     id: string,
+         *     traceId: string|null,
+         *     type: string,
+         *     name: string|null,
+         *     startTime: string,
+         *     endTime: string|null,
+         *     completionStartTime: string|null,
+         *     model: string|null,
+         *     modelParameters: array<string, mixed>|null,
+         *     input: mixed,
+         *     output: mixed,
+         *     metadata: mixed,
+         *     version: string|null,
+         *     parentObservationId: string|null,
+         *     level: string|null,
+         *     statusMessage: string|null,
+         *     promptId: string|null,
+         *     promptName: string|null,
+         *     promptVersion: int|null,
+         *     usage: array{input: int|null, output: int|null, total: int|null, unit: string|null, inputCost: float|null, outputCost: float|null, totalCost: float|null}|null,
+         *     calculatedInputCost: float|null,
+         *     calculatedOutputCost: float|null,
+         *     calculatedTotalCost: float|null,
+         *     latency: float|null,
+         *     timeToFirstToken: float|null,
+         *     projectId: string,
+         *     createdAt: string,
+         *     updatedAt: string,
+         *     environment: string|null
+         * } $data
+         */
+        $data = json_decode($response->getBody()->getContents(), true, flags: JSON_THROW_ON_ERROR);
+
+        return ObservationResponse::fromArray($data);
+    }
+
+    /**
+     * @throws JsonException
+     */
+    public function list(
+        ?int $page = null,
+        ?int $limit = null,
+        ?string $name = null,
+        ?string $userId = null,
+        ?string $type = null,
+        ?string $traceId = null,
+        ?string $level = null,
+        ?string $parentObservationId = null,
+        ?array $environment = null,
+        ?string $fromStartTime = null,
+        ?string $toStartTime = null,
+        ?string $version = null,
+        ?string $filter = null,
+    ): ObservationListResponse {
+        $response = $this->transporter->get(
+            uri: '/api/public/observations',
+            options: ['query' => array_filter([
+                'page' => $page,
+                'limit' => $limit,
+                'name' => $name,
+                'userId' => $userId,
+                'type' => $type,
+                'traceId' => $traceId,
+                'level' => $level,
+                'parentObservationId' => $parentObservationId,
+                'environment' => $environment,
+                'fromStartTime' => $fromStartTime,
+                'toStartTime' => $toStartTime,
+                'version' => $version,
+                'filter' => $filter,
+            ])]
+        );
+
+        /** @var array{
+         *     data: array<int, array{
+         *         id: string,
+         *         traceId: string|null,
+         *         type: string,
+         *         name: string|null,
+         *         startTime: string,
+         *         endTime: string|null,
+         *         completionStartTime: string|null,
+         *         model: string|null,
+         *         modelParameters: array<string, mixed>|null,
+         *         input: mixed,
+         *         output: mixed,
+         *         metadata: mixed,
+         *         version: string|null,
+         *         parentObservationId: string|null,
+         *         level: string|null,
+         *         statusMessage: string|null,
+         *         promptId: string|null,
+         *         promptName: string|null,
+         *         promptVersion: int|null,
+         *         usage: array{input: int|null, output: int|null, total: int|null, unit: string|null, inputCost: float|null, outputCost: float|null, totalCost: float|null}|null,
+         *         calculatedInputCost: float|null,
+         *         calculatedOutputCost: float|null,
+         *         calculatedTotalCost: float|null,
+         *         latency: float|null,
+         *         timeToFirstToken: float|null,
+         *         projectId: string,
+         *         createdAt: string,
+         *         updatedAt: string,
+         *         environment: string|null
+         *     }>,
+         *     meta: array{page: int, limit: int, totalPages: int, totalItems: int}
+         * } $data
+         */
+        $data = json_decode($response->getBody()->getContents(), true, flags: JSON_THROW_ON_ERROR);
+
+        return ObservationListResponse::fromArray($data);
+    }
+}
diff --git a/src/Responses/ObservationListResponse.php b/src/Responses/ObservationListResponse.php
new file mode 100644
index 0000000..1502af2
--- /dev/null
+++ b/src/Responses/ObservationListResponse.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DIJ\Langfuse\PHP\Responses;
+
+use DIJ\Langfuse\PHP\ValueObjects\MetaData;
+
+readonly class ObservationListResponse
+{
+    /**
+     * @param  array<int, ObservationResponse>  $data
+     */
+    public function __construct(
+        public array $data,
+        public MetaData $meta,
+    ) {}
+
+    /**
+     * @param array{
+     *     data: array<int, array{
+     *         id: string,
+     *         traceId: string|null,
+     *         type: string,
+     *         name: string|null,
+     *         startTime: string,
+     *         endTime: string|null,
+     *         completionStartTime: string|null,
+     *         model: string|null,
+     *         modelParameters: array<string, mixed>|null,
+     *         input: mixed,
+     *         output: mixed,
+     *         metadata: mixed,
+     *         version: string|null,
+     *         parentObservationId: string|null,
+     *         level: string|null,
+     *         statusMessage: string|null,
+     *         promptId: string|null,
+     *         promptName: string|null,
+     *         promptVersion: int|null,
+     *         usage?: array{input: int|null, output: int|null, total: int|null, unit: string|null, inputCost: float|null, outputCost: float|null, totalCost: float|null}|null,
+     *         calculatedInputCost?: float|null,
+     *         calculatedOutputCost?: float|null,
+     *         calculatedTotalCost?: float|null,
+     *         latency?: float|null,
+     *         timeToFirstToken?: float|null,
+     *         projectId: string,
+     *         createdAt: string,
+     *         updatedAt: string,
+     *         environment?: string|null
+     *     }>,
+     *     meta: array{page: int, limit: int, totalPages: int, totalItems: int}
+     * } $data
+     */
+    public static function fromArray(array $data): self
+    {
+        return new self(
+            data: array_map(fn (array $item): ObservationResponse => ObservationResponse::fromArray($item), $data['data']),
+            meta: MetaData::fromArray($data['meta']),
+        );
+    }
+}
diff --git a/src/Responses/ObservationResponse.php b/src/Responses/ObservationResponse.php
new file mode 100644
index 0000000..9345caf
--- /dev/null
+++ b/src/Responses/ObservationResponse.php
@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DIJ\Langfuse\PHP\Responses;
+
+readonly class ObservationResponse
+{
+    /**
+     * @param  array<string, mixed>|null  $modelParameters
+     * @param  array{input: int|null, output: int|null, total: int|null, unit: string|null, inputCost: float|null, outputCost: float|null, totalCost: float|null}|null  $usage
+     */
+    public function __construct(
+        public string $id,
+        public ?string $traceId,
+        public string $type,
+        public ?string $name,
+        public string $startTime,
+        public ?string $endTime,
+        public ?string $completionStartTime,
+        public ?string $model,
+        public ?array $modelParameters,
+        public mixed $input,
+        public mixed $output,
+        public mixed $metadata,
+        public ?string $version,
+        public ?string $parentObservationId,
+        public ?string $level,
+        public ?string $statusMessage,
+        public ?string $promptId,
+        public ?string $promptName,
+        public ?int $promptVersion,
+        public ?array $usage,
+        public ?float $calculatedInputCost,
+        public ?float $calculatedOutputCost,
+        public ?float $calculatedTotalCost,
+        public ?float $latency,
+        public ?float $timeToFirstToken,
+        public string $projectId,
+        public string $createdAt,
+        public string $updatedAt,
+        public ?string $environment = null,
+    ) {}
+
+    /**
+     * @param array{
+     *     id: string,
+     *     traceId: string|null,
+     *     type: string,
+     *     name: string|null,
+     *     startTime: string,
+     *     endTime: string|null,
+     *     completionStartTime: string|null,
+     *     model: string|null,
+     *     modelParameters: array<string, mixed>|null,
+     *     input: mixed,
+     *     output: mixed,
+     *     metadata: mixed,
+     *     version: string|null,
+     *     parentObservationId: string|null,
+     *     level: string|null,
+     *     statusMessage: string|null,
+     *     promptId: string|null,
+     *     promptName: string|null,
+     *     promptVersion: int|null,
+     *     usage?: array{input: int|null, output: int|null, total: int|null, unit: string|null, inputCost: float|null, outputCost: float|null, totalCost: float|null}|null,
+     *     calculatedInputCost?: float|null,
+     *     calculatedOutputCost?: float|null,
+     *     calculatedTotalCost?: float|null,
+     *     latency?: float|null,
+     *     timeToFirstToken?: float|null,
+     *     projectId: string,
+     *     createdAt: string,
+     *     updatedAt: string,
+     *     environment?: string|null
+     * } $data
+     */
+    public static function fromArray(array $data): self
+    {
+        return new self(
+            id: $data['id'],
+            traceId: $data['traceId'],
+            type: $data['type'],
+            name: $data['name'],
+            startTime: $data['startTime'],
+            endTime: $data['endTime'],
+            completionStartTime: $data['completionStartTime'],
+            model: $data['model'],
+            modelParameters: $data['modelParameters'],
+            input: $data['input'],
+            output: $data['output'],
+            metadata: $data['metadata'],
+            version: $data['version'],
+            parentObservationId: $data['parentObservationId'],
+            level: $data['level'],
+            statusMessage: $data['statusMessage'],
+            promptId: $data['promptId'],
+            promptName: $data['promptName'],
+            promptVersion: $data['promptVersion'],
+            usage: $data['usage'] ?? null,
+            calculatedInputCost: $data['calculatedInputCost'] ?? null,
+            calculatedOutputCost: $data['calculatedOutputCost'] ?? null,
+            calculatedTotalCost: $data['calculatedTotalCost'] ?? null,
+            latency: $data['latency'] ?? null,
+            timeToFirstToken: $data['timeToFirstToken'] ?? null,
+            projectId: $data['projectId'],
+            createdAt: $data['createdAt'],
+            updatedAt: $data['updatedAt'],
+            environment: $data['environment'] ?? null,
+        );
+    }
+}
diff --git a/src/Testing/Responses/GetObservationListResponse.php b/src/Testing/Responses/GetObservationListResponse.php
new file mode 100644
index 0000000..7bba8cf
--- /dev/null
+++ b/src/Testing/Responses/GetObservationListResponse.php
@@ -0,0 +1,106 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DIJ\Langfuse\PHP\Testing\Responses;
+
+use GuzzleHttp\Psr7\Response;
+
+class GetObservationListResponse extends Response
+{
+    /**
+     * @param  array<array<string>|string>  $headers
+     */
+    public function __construct(int $status = 200, array $headers = [], string $version = '1.1', ?string $reason = null)
+    {
+        parent::__construct($status, $headers, (string) json_encode($this->payload()), $version, $reason);
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    public function payload(): array
+    {
+        return [
+            'data' => [
+                [
+                    'id' => 'obs-abc123',
+                    'traceId' => 'trace-abc123',
+                    'type' => 'GENERATION',
+                    'name' => 'llm-generation-1',
+                    'startTime' => '2025-01-22T10:30:00.000Z',
+                    'endTime' => '2025-01-22T10:30:02.500Z',
+                    'completionStartTime' => '2025-01-22T10:30:00.500Z',
+                    'model' => 'gpt-4o',
+                    'modelParameters' => ['temperature' => 0.7],
+                    'input' => ['prompt' => 'Hello'],
+                    'output' => ['response' => 'Hi there!'],
+                    'metadata' => null,
+                    'version' => 'v1',
+                    'parentObservationId' => null,
+                    'level' => 'DEFAULT',
+                    'statusMessage' => null,
+                    'promptId' => null,
+                    'promptName' => null,
+                    'promptVersion' => null,
+                    'usage' => [
+                        'input' => 10,
+                        'output' => 25,
+                        'total' => 35,
+                        'unit' => 'TOKENS',
+                        'inputCost' => 0.0001,
+                        'outputCost' => 0.0005,
+                        'totalCost' => 0.0006,
+                    ],
+                    'calculatedInputCost' => 0.0001,
+                    'calculatedOutputCost' => 0.0005,
+                    'calculatedTotalCost' => 0.0006,
+                    'latency' => 2.5,
+                    'timeToFirstToken' => 0.5,
+                    'projectId' => 'proj-abc123',
+                    'createdAt' => '2025-01-22T10:30:00.000Z',
+                    'updatedAt' => '2025-01-22T10:30:02.500Z',
+                    'environment' => 'production',
+                ],
+                [
+                    'id' => 'obs-def456',
+                    'traceId' => 'trace-abc123',
+                    'type' => 'SPAN',
+                    'name' => 'retrieval-span',
+                    'startTime' => '2025-01-22T10:29:58.000Z',
+                    'endTime' => '2025-01-22T10:29:59.500Z',
+                    'completionStartTime' => null,
+                    'model' => null,
+                    'modelParameters' => null,
+                    'input' => ['query' => 'search term'],
+                    'output' => ['results' => [['id' => 1], ['id' => 2]]],
+                    'metadata' => ['source' => 'vector-db'],
+                    'version' => 'v1',
+                    'parentObservationId' => null,
+                    'level' => 'DEFAULT',
+                    'statusMessage' => null,
+                    'promptId' => null,
+                    'promptName' => null,
+                    'promptVersion' => null,
+                    'usage' => null,
+                    'calculatedInputCost' => null,
+                    'calculatedOutputCost' => null,
+                    'calculatedTotalCost' => null,
+                    'latency' => 1.5,
+                    'timeToFirstToken' => null,
+                    'projectId' => 'proj-abc123',
+                    'createdAt' => '2025-01-22T10:29:58.000Z',
+                    'updatedAt' => '2025-01-22T10:29:59.500Z',
+                    'environment' => 'production',
+                ],
+            ],
+            'meta' => [
+                'page' => 1,
+                'limit' => 10,
+                'totalPages' => 1,
+                'totalItems' => 2,
+            ],
+        ];
+
+    }
+}
diff --git a/src/Testing/Responses/GetObservationResponse.php b/src/Testing/Responses/GetObservationResponse.php
new file mode 100644
index 0000000..6935bb5
--- /dev/null
+++ b/src/Testing/Responses/GetObservationResponse.php
@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DIJ\Langfuse\PHP\Testing\Responses;
+
+use GuzzleHttp\Psr7\Response;
+
+class GetObservationResponse extends Response
+{
+    /**
+     * @param  array<array<string>|string>  $headers
+     * @param  array<string, mixed>  $data
+     */
+    public function __construct(int $status = 200, array $headers = [], string $version = '1.1', ?string $reason = null, array $data = [])
+    {
+        parent::__construct($status, $headers, (string) json_encode($this->payload($data)), $version, $reason);
+    }
+
+    /**
+     * @param  array<string, mixed>  $data
+     * @return array<string, mixed>
+     */
+    public function payload(array $data = []): array
+    {
+        return array_merge([
+            'id' => 'obs-abc123',
+            'traceId' => 'trace-abc123',
+            'type' => 'GENERATION',
+            'name' => 'llm-generation',
+            'startTime' => '2025-01-22T10:30:00.000Z',
+            'endTime' => '2025-01-22T10:30:02.500Z',
+            'completionStartTime' => '2025-01-22T10:30:00.500Z',
+            'model' => 'gpt-4o',
+            'modelParameters' => ['temperature' => 0.7, 'max_tokens' => 1000],
+            'input' => ['messages' => [['role' => 'user', 'content' => 'Hello']]],
+            'output' => ['content' => 'Hi there! How can I help you?'],
+            'metadata' => ['key' => 'value'],
+            'version' => 'v1',
+            'parentObservationId' => null,
+            'level' => 'DEFAULT',
+            'statusMessage' => null,
+            'promptId' => 'prompt-123',
+            'promptName' => 'greeting-prompt',
+            'promptVersion' => 1,
+            'usage' => [
+                'input' => 10,
+                'output' => 25,
+                'total' => 35,
+                'unit' => 'TOKENS',
+                'inputCost' => 0.0001,
+                'outputCost' => 0.0005,
+                'totalCost' => 0.0006,
+            ],
+            'calculatedInputCost' => 0.0001,
+            'calculatedOutputCost' => 0.0005,
+            'calculatedTotalCost' => 0.0006,
+            'latency' => 2.5,
+            'timeToFirstToken' => 0.5,
+            'projectId' => 'proj-abc123',
+            'createdAt' => '2025-01-22T10:30:00.000Z',
+            'updatedAt' => '2025-01-22T10:30:02.500Z',
+            'environment' => 'production',
+        ], $data);
+    }
+}
diff --git a/src/Transporters/HttpTransporter.php b/src/Transporters/HttpTransporter.php
index 319cf49..a2b734b 100644
--- a/src/Transporters/HttpTransporter.php
+++ b/src/Transporters/HttpTransporter.php
@@ -21,8 +21,7 @@ class HttpTransporter implements TransporterInterface
 {
     public function __construct(
         public readonly ClientInterface $client
-    ) {
-    }
+    ) {}
 
     /**
      * @throws BadRequestException
@@ -45,7 +44,7 @@ public function request(string $method, string $uri, array $options = []): Respo
     }
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      *
      * @throws BadRequestException
      * @throws ForbiddenException
@@ -61,7 +60,7 @@ public function get(string $uri, array $options = []): ResponseInterface
     }
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      *
      * @throws BadRequestException
      * @throws ForbiddenException
@@ -82,7 +81,7 @@ public function postJson(string $uri, array $data = [], array $options = []): Re
     }
 
     /**
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $options
      *
      * @throws BadRequestException
      * @throws ForbiddenException
@@ -98,8 +97,8 @@ public function delete(string $uri, array $options = []): ResponseInterface
     }
 
     /**
-     * @param array<string, mixed> $data
-     * @param array<string, mixed> $options
+     * @param  array<string, mixed>  $data
+     * @param  array<string, mixed>  $options
      *
      * @throws BadRequestException
      * @throws ForbiddenException
diff --git a/tests/Feature/ObservationTest.php b/tests/Feature/ObservationTest.php
new file mode 100644
index 0000000..4c115a1
--- /dev/null
+++ b/tests/Feature/ObservationTest.php
@@ -0,0 +1,126 @@
+<?php
+
+declare(strict_types=1);
+
+use DIJ\Langfuse\PHP\Langfuse;
+use DIJ\Langfuse\PHP\Responses\ObservationListResponse;
+use DIJ\Langfuse\PHP\Responses\ObservationResponse;
+use DIJ\Langfuse\PHP\Testing\Responses\GetObservationListResponse;
+use DIJ\Langfuse\PHP\Testing\Responses\GetObservationResponse;
+use DIJ\Langfuse\PHP\Transporters\HttpTransporter;
+use DIJ\Langfuse\PHP\ValueObjects\MetaData;
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Middleware;
+use Psr\Http\Message\RequestInterface;
+
+it('can get an observation by id', function (): void {
+    $mock = new MockHandler([
+        new GetObservationResponse(),
+    ]);
+
+    $handlerStack = HandlerStack::create($mock);
+    $client = new Client(['handler' => $handlerStack]);
+
+    $observation = (new Langfuse(new HttpTransporter($client)))
+        ->observation()
+        ->get('obs-abc123');
+
+    expect($observation)->toBeInstanceOf(ObservationResponse::class)
+        ->and($observation->id)->toBe('obs-abc123')
+        ->and($observation->traceId)->toBe('trace-abc123')
+        ->and($observation->type)->toBe('GENERATION')
+        ->and($observation->name)->toBe('llm-generation')
+        ->and($observation->model)->toBe('gpt-4o')
+        ->and($observation->latency)->toBe(2.5)
+        ->and($observation->environment)->toBe('production');
+});
+
+it('can list observations', function (): void {
+    $mock = new MockHandler([
+        new GetObservationListResponse,
+    ]);
+
+    $handlerStack = HandlerStack::create($mock);
+    $client = new Client(['handler' => $handlerStack]);
+
+    $observations = (new Langfuse(new HttpTransporter($client)))
+        ->observation()
+        ->list();
+
+    expect($observations)->toBeInstanceOf(ObservationListResponse::class)
+        ->and($observations->data)->toBeArray()
+        ->and($observations->data)->toHaveCount(2)
+        ->and($observations->data[0])->toBeInstanceOf(ObservationResponse::class)
+        ->and($observations->data[0]->id)->toBe('obs-abc123')
+        ->and($observations->data[0]->type)->toBe('GENERATION')
+        ->and($observations->data[1]->id)->toBe('obs-def456')
+        ->and($observations->data[1]->type)->toBe('SPAN')
+        ->and($observations->meta)->toBeInstanceOf(MetaData::class)
+        ->and($observations->meta->totalItems)->toBe(2);
+});
+
+it('can list observations with filters', function (): void {
+    /** @var array<int, array{request: RequestInterface}> $history */
+    $history = [];
+
+    $mock = new MockHandler([
+        new GetObservationListResponse,
+    ]);
+
+    $handlerStack = HandlerStack::create($mock);
+    $handlerStack->push(Middleware::history($history));
+    $client = new Client(['handler' => $handlerStack]);
+
+    $observations = (new Langfuse(new HttpTransporter($client)))
+        ->observation()
+        ->list(
+            page: 1,
+            limit: 10,
+            traceId: 'trace-abc123',
+            type: 'GENERATION',
+            environment: ['production'],
+        );
+
+    expect($observations)->toBeInstanceOf(ObservationListResponse::class)
+        ->and($observations->data)->toBeArray();
+
+    expect($history)->toHaveCount(1);
+
+    /** @var array{request: RequestInterface} $first */
+    $first = $history[0];
+    /** @var RequestInterface $request */
+    $request = $first['request'];
+    $query = $request->getUri()->getQuery();
+
+    expect($query)->toContain('page=1')
+        ->and($query)->toContain('limit=10')
+        ->and($query)->toContain('traceId=trace-abc123')
+        ->and($query)->toContain('type=GENERATION');
+});
+
+it('can get observation with usage data', function (): void {
+    $mock = new MockHandler([
+        new GetObservationResponse(),
+    ]);
+
+    $handlerStack = HandlerStack::create($mock);
+    $client = new Client(['handler' => $handlerStack]);
+
+    $observation = (new Langfuse(new HttpTransporter($client)))
+        ->observation()
+        ->get('obs-abc123');
+
+    expect($observation->usage)->toBeArray()
+        ->and($observation->usage)->not->toBeEmpty()
+        ->and($observation->calculatedTotalCost)->toBe(0.0006)
+        ->and($observation->promptName)->toBe('greeting-prompt')
+        ->and($observation->promptVersion)->toBe(1);
+
+    /** @var array{input: int, output: int, total: int} $usage */
+    $usage = $observation->usage;
+    expect($usage['input'])->toBe(10)
+        ->and($usage['output'])->toBe(25)
+        ->and($usage['total'])->toBe(35);
+});