diff --git a/src/Messages/DTO/Citation.php b/src/Messages/DTO/Citation.php
new file mode 100644
index 00000000..c9bb0b14
--- /dev/null
+++ b/src/Messages/DTO/Citation.php
@@ -0,0 +1,373 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WordPress\AiClient\Messages\DTO;
+
+use WordPress\AiClient\Common\AbstractDataTransferObject;
+use WordPress\AiClient\Common\Exception\InvalidArgumentException;
+
+/**
+ * Represents a citation or source attribution for generated text.
+ *
+ * Citations anchor a span of generated text to its source, which can be
+ * either a remote URL or an index into the request's documents array.
+ * Offset indices are relative to the owning MessagePart's text property.
+ *
+ * Providers disagree on what their offsets measure, so `type` records which
+ * one produced the citation. Vendor-specific leftovers go in `additionalData`.
+ *
+ * @since n.e.x.t
+ *
+ * @phpstan-type CitationArrayShape array{
+ *     type?: string|null,
+ *     url?: string|null,
+ *     documentIndex?: int|null,
+ *     title?: string|null,
+ *     startIndex?: int|null,
+ *     endIndex?: int|null,
+ *     quotedText?: string|null,
+ *     additionalData?: array<string, mixed>|null
+ * }
+ *
+ * @extends AbstractDataTransferObject<CitationArrayShape>
+ */
+class Citation extends AbstractDataTransferObject
+{
+    public const KEY_TYPE = 'type';
+    public const KEY_URL = 'url';
+    public const KEY_DOCUMENT_INDEX = 'documentIndex';
+    public const KEY_TITLE = 'title';
+    public const KEY_START_INDEX = 'startIndex';
+    public const KEY_END_INDEX = 'endIndex';
+    public const KEY_QUOTED_TEXT = 'quotedText';
+    public const KEY_ADDITIONAL_DATA = 'additionalData';
+
+    /**
+     * @var string|null The provider's location type, e.g. `char_location`.
+     */
+    private ?string $type;
+
+    /**
+     * @var string|null The remote source URL.
+     */
+    private ?string $url;
+
+    /**
+     * @var int|null The index into the request's documents array.
+     */
+    private ?int $documentIndex;
+
+    /**
+     * @var string|null An optional title for the source.
+     */
+    private ?string $title;
+
+    /**
+     * @var int|null The start byte offset into the owning MessagePart's text.
+     */
+    private ?int $startIndex;
+
+    /**
+     * @var int|null The end byte offset into the owning MessagePart's text.
+     */
+    private ?int $endIndex;
+
+    /**
+     * @var string|null The quoted source passage (e.g. Anthropic's cited_text).
+     */
+    private ?string $quotedText;
+
+    /**
+     * @var array<string, mixed>|null Vendor-specific leftovers, keyed by name.
+     */
+    private ?array $additionalData;
+
+    /**
+     * Constructor.
+     *
+     * @since n.e.x.t
+     *
+     * @param string|null $url The remote source URL.
+     * @param int|null $documentIndex The index into the request's documents array.
+     * @param string|null $title An optional title for the source.
+     * @param int|null $startIndex The start byte offset into the owning part's text.
+     * @param int|null $endIndex The end byte offset into the owning part's text.
+     * @param string|null $quotedText The quoted source passage.
+     * @param string|null $type The provider's location type, e.g. `char_location`.
+     * @param array<string, mixed>|null $additionalData Vendor-specific leftovers.
+     * @throws InvalidArgumentException If offsets are negative or misordered, or if
+     *                                  $additionalData is not a string-keyed map.
+     */
+    public function __construct(
+        ?string $url = null,
+        ?int $documentIndex = null,
+        ?string $title = null,
+        ?int $startIndex = null,
+        ?int $endIndex = null,
+        ?string $quotedText = null,
+        ?string $type = null,
+        ?array $additionalData = null
+    ) {
+        if ($documentIndex !== null && $documentIndex < 0) {
+            throw new InvalidArgumentException(
+                sprintf('Citation documentIndex must be non-negative, got %d.', $documentIndex)
+            );
+        }
+
+        if ($startIndex !== null && $startIndex < 0) {
+            throw new InvalidArgumentException(
+                sprintf('Citation startIndex must be non-negative, got %d.', $startIndex)
+            );
+        }
+
+        if ($endIndex !== null && $endIndex < 0) {
+            throw new InvalidArgumentException(
+                sprintf('Citation endIndex must be non-negative, got %d.', $endIndex)
+            );
+        }
+
+        if ($startIndex !== null && $endIndex !== null && $startIndex > $endIndex) {
+            throw new InvalidArgumentException(
+                sprintf(
+                    'Citation startIndex (%d) must be less than or equal to endIndex (%d).',
+                    $startIndex,
+                    $endIndex
+                )
+            );
+        }
+
+        if ($additionalData !== null) {
+            foreach (array_keys($additionalData) as $key) {
+                if (!is_string($key)) {
+                    throw new InvalidArgumentException(
+                        'Citation additionalData must be a map with string keys.'
+                    );
+                }
+            }
+        }
+
+        $this->type = $type;
+        $this->url = $url;
+        $this->documentIndex = $documentIndex;
+        $this->title = $title;
+        $this->startIndex = $startIndex;
+        $this->endIndex = $endIndex;
+        $this->quotedText = $quotedText;
+        $this->additionalData = $additionalData;
+    }
+
+    /**
+     * Gets the provider's location type, verbatim.
+     *
+     * Tells you which coordinate frame the offsets belong to.
+     *
+     * @since n.e.x.t
+     *
+     * @return string|null The location type or null if the provider supplied none.
+     */
+    public function getType(): ?string
+    {
+        return $this->type;
+    }
+
+    /**
+     * Gets vendor-specific leftovers that have no normalized counterpart.
+     *
+     * @since n.e.x.t
+     *
+     * @return array<string, mixed>|null The additional data or null if not set.
+     */
+    public function getAdditionalData(): ?array
+    {
+        return $this->additionalData;
+    }
+
+    /**
+     * Gets the remote source URL.
+     *
+     * @since n.e.x.t
+     *
+     * @return string|null The URL or null if the source is a document reference.
+     */
+    public function getUrl(): ?string
+    {
+        return $this->url;
+    }
+
+    /**
+     * Gets the document index.
+     *
+     * @since n.e.x.t
+     *
+     * @return int|null The document index or null if the source is a URL.
+     */
+    public function getDocumentIndex(): ?int
+    {
+        return $this->documentIndex;
+    }
+
+    /**
+     * Gets the title of the source.
+     *
+     * @since n.e.x.t
+     *
+     * @return string|null The title or null if not available.
+     */
+    public function getTitle(): ?string
+    {
+        return $this->title;
+    }
+
+    /**
+     * Gets the start byte offset into the owning MessagePart's text.
+     *
+     * @since n.e.x.t
+     *
+     * @return int|null The start offset or null if span is not available.
+     */
+    public function getStartIndex(): ?int
+    {
+        return $this->startIndex;
+    }
+
+    /**
+     * Gets the end byte offset into the owning MessagePart's text.
+     *
+     * @since n.e.x.t
+     *
+     * @return int|null The end offset or null if span is not available.
+     */
+    public function getEndIndex(): ?int
+    {
+        return $this->endIndex;
+    }
+
+    /**
+     * Gets the quoted source passage.
+     *
+     * @since n.e.x.t
+     *
+     * @return string|null The quoted text or null if not available.
+     */
+    public function getQuotedText(): ?string
+    {
+        return $this->quotedText;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @since n.e.x.t
+     */
+    public static function getJsonSchema(): array
+    {
+        return [
+            'type' => 'object',
+            'properties' => [
+                self::KEY_TYPE => [
+                    'type' => ['string', 'null'],
+                    'description' => 'The provider\'s location type, e.g. char_location.',
+                ],
+                self::KEY_URL => [
+                    'type' => ['string', 'null'],
+                    'description' => 'The remote source URL.',
+                ],
+                self::KEY_DOCUMENT_INDEX => [
+                    'type' => ['integer', 'null'],
+                    'minimum' => 0,
+                    'description' => 'The index into the request\'s documents array.',
+                ],
+                self::KEY_TITLE => [
+                    'type' => ['string', 'null'],
+                    'description' => 'An optional title for the source.',
+                ],
+                self::KEY_START_INDEX => [
+                    'type' => ['integer', 'null'],
+                    'minimum' => 0,
+                    'description' => 'The start byte offset into the owning MessagePart\'s text.',
+                ],
+                self::KEY_END_INDEX => [
+                    'type' => ['integer', 'null'],
+                    'minimum' => 0,
+                    'description' => 'The end byte offset into the owning MessagePart\'s text.',
+                ],
+                self::KEY_QUOTED_TEXT => [
+                    'type' => ['string', 'null'],
+                    'description' => 'The quoted source passage.',
+                ],
+                self::KEY_ADDITIONAL_DATA => [
+                    'type' => 'object',
+                    'additionalProperties' => true,
+                    'description' => 'Vendor-specific leftovers with no normalized counterpart.',
+                ],
+            ],
+            'additionalProperties' => false,
+        ];
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @since n.e.x.t
+     *
+     * @return CitationArrayShape
+     */
+    public function toArray(): array
+    {
+        $data = [];
+
+        if ($this->type !== null) {
+            $data[self::KEY_TYPE] = $this->type;
+        }
+
+        if ($this->url !== null) {
+            $data[self::KEY_URL] = $this->url;
+        }
+
+        if ($this->documentIndex !== null) {
+            $data[self::KEY_DOCUMENT_INDEX] = $this->documentIndex;
+        }
+
+        if ($this->title !== null) {
+            $data[self::KEY_TITLE] = $this->title;
+        }
+
+        if ($this->startIndex !== null) {
+            $data[self::KEY_START_INDEX] = $this->startIndex;
+        }
+
+        if ($this->endIndex !== null) {
+            $data[self::KEY_END_INDEX] = $this->endIndex;
+        }
+
+        if ($this->quotedText !== null) {
+            $data[self::KEY_QUOTED_TEXT] = $this->quotedText;
+        }
+
+        if ($this->additionalData !== null) {
+            $data[self::KEY_ADDITIONAL_DATA] = $this->additionalData;
+        }
+
+        return $data;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @since n.e.x.t
+     */
+    public static function fromArray(array $array): self
+    {
+        return new self(
+            $array[self::KEY_URL] ?? null,
+            $array[self::KEY_DOCUMENT_INDEX] ?? null,
+            $array[self::KEY_TITLE] ?? null,
+            $array[self::KEY_START_INDEX] ?? null,
+            $array[self::KEY_END_INDEX] ?? null,
+            $array[self::KEY_QUOTED_TEXT] ?? null,
+            $array[self::KEY_TYPE] ?? null,
+            $array[self::KEY_ADDITIONAL_DATA] ?? null
+        );
+    }
+}
diff --git a/src/Messages/DTO/MessagePart.php b/src/Messages/DTO/MessagePart.php
index 228471be..398032c9 100644
--- a/src/Messages/DTO/MessagePart.php
+++ b/src/Messages/DTO/MessagePart.php
@@ -24,6 +24,7 @@
  * @phpstan-import-type FileArrayShape from File
  * @phpstan-import-type FunctionCallArrayShape from FunctionCall
  * @phpstan-import-type FunctionResponseArrayShape from FunctionResponse
+ * @phpstan-import-type CitationArrayShape from Citation
  *
  * @phpstan-type MessagePartArrayShape array{
  *     channel: string,
@@ -32,7 +33,8 @@
  *     text?: string,
  *     file?: FileArrayShape,
  *     functionCall?: FunctionCallArrayShape,
- *     functionResponse?: FunctionResponseArrayShape
+ *     functionResponse?: FunctionResponseArrayShape,
+ *     citations?: array<CitationArrayShape>|null
  * }
  *
  * @extends AbstractDataTransferObject<MessagePartArrayShape>
@@ -46,6 +48,7 @@ class MessagePart extends AbstractDataTransferObject
     public const KEY_FILE = 'file';
     public const KEY_FUNCTION_CALL = 'functionCall';
     public const KEY_FUNCTION_RESPONSE = 'functionResponse';
+    public const KEY_CITATIONS = 'citations';
 
     /**
      * @var MessagePartChannelEnum The channel this message part belongs to.
@@ -82,6 +85,11 @@ class MessagePart extends AbstractDataTransferObject
      */
     private ?FunctionResponse $functionResponse = null;
 
+    /**
+     * @var Citation[]|null Optional citations or source attributions.
+     */
+    private ?array $citations = null;
+
     /**
      * Constructor that accepts various content types and infers the message part type.
      *
@@ -90,12 +98,18 @@ class MessagePart extends AbstractDataTransferObject
      * @param mixed $content The content of this message part.
      * @param MessagePartChannelEnum|null $channel The channel this part belongs to. Defaults to CONTENT.
      * @param string|null $thoughtSignature Optional thought signature for extended thinking.
+     * @param Citation[]|null $citations Optional citations for the content.
      * @throws InvalidArgumentException If an unsupported content type is provided.
      */
-    public function __construct($content, ?MessagePartChannelEnum $channel = null, ?string $thoughtSignature = null)
-    {
+    public function __construct(
+        $content,
+        ?MessagePartChannelEnum $channel = null,
+        ?string $thoughtSignature = null,
+        ?array $citations = null
+    ) {
         $this->channel = $channel ?? MessagePartChannelEnum::content();
         $this->thoughtSignature = $thoughtSignature;
+        $this->citations = $citations;
 
         if (is_string($content)) {
             $this->type = MessagePartTypeEnum::text();
@@ -205,6 +219,18 @@ public function getFunctionResponse(): ?FunctionResponse
         return $this->functionResponse;
     }
 
+    /**
+     * Gets the citations.
+     *
+     * @since n.e.x.t
+     *
+     * @return Citation[]|null The citations or null if not set.
+     */
+    public function getCitations(): ?array
+    {
+        return $this->citations;
+    }
+
     /**
      * {@inheritDoc}
      *
@@ -223,6 +249,12 @@ public static function getJsonSchema(): array
             'description' => 'Thought signature for extended thinking.',
         ];
 
+        $citationsSchema = [
+            'type' => 'array',
+            'items' => Citation::getJsonSchema(),
+            'description' => 'Optional citations or source attributions.',
+        ];
+
         return [
             'oneOf' => [
                 [
@@ -238,6 +270,7 @@ public static function getJsonSchema(): array
                             'description' => 'Text content.',
                         ],
                         self::KEY_THOUGHT_SIGNATURE => $thoughtSignatureSchema,
+                        self::KEY_CITATIONS => $citationsSchema,
                     ],
                     'required' => [self::KEY_TYPE, self::KEY_TEXT],
                     'additionalProperties' => false,
@@ -252,6 +285,7 @@ public static function getJsonSchema(): array
                         ],
                         self::KEY_FILE => File::getJsonSchema(),
                         self::KEY_THOUGHT_SIGNATURE => $thoughtSignatureSchema,
+                        self::KEY_CITATIONS => $citationsSchema,
                     ],
                     'required' => [self::KEY_TYPE, self::KEY_FILE],
                     'additionalProperties' => false,
@@ -266,6 +300,7 @@ public static function getJsonSchema(): array
                         ],
                         self::KEY_FUNCTION_CALL => FunctionCall::getJsonSchema(),
                         self::KEY_THOUGHT_SIGNATURE => $thoughtSignatureSchema,
+                        self::KEY_CITATIONS => $citationsSchema,
                     ],
                     'required' => [self::KEY_TYPE, self::KEY_FUNCTION_CALL],
                     'additionalProperties' => false,
@@ -280,6 +315,7 @@ public static function getJsonSchema(): array
                         ],
                         self::KEY_FUNCTION_RESPONSE => FunctionResponse::getJsonSchema(),
                         self::KEY_THOUGHT_SIGNATURE => $thoughtSignatureSchema,
+                        self::KEY_CITATIONS => $citationsSchema,
                     ],
                     'required' => [self::KEY_TYPE, self::KEY_FUNCTION_RESPONSE],
                     'additionalProperties' => false,
@@ -321,6 +357,13 @@ public function toArray(): array
             $data[self::KEY_THOUGHT_SIGNATURE] = $this->thoughtSignature;
         }
 
+        if ($this->citations !== null) {
+            $data[self::KEY_CITATIONS] = array_map(
+                static fn (Citation $citation) => $citation->toArray(),
+                $this->citations
+            );
+        }
+
         return $data;
     }
 
@@ -339,18 +382,37 @@ public static function fromArray(array $array): self
 
         $thoughtSignature = $array[self::KEY_THOUGHT_SIGNATURE] ?? null;
 
+        $citations = null;
+        if (isset($array[self::KEY_CITATIONS])) {
+            $citations = array_map(
+                static fn (array $citationArray) => Citation::fromArray($citationArray),
+                $array[self::KEY_CITATIONS]
+            );
+        }
+
         // Check which properties are set to determine how to construct the MessagePart
         if (isset($array[self::KEY_TEXT])) {
-            return new self($array[self::KEY_TEXT], $channel, $thoughtSignature);
+            return new self($array[self::KEY_TEXT], $channel, $thoughtSignature, $citations);
         } elseif (isset($array[self::KEY_FILE])) {
-            return new self(File::fromArray($array[self::KEY_FILE]), $channel, $thoughtSignature);
+            return new self(
+                File::fromArray($array[self::KEY_FILE]),
+                $channel,
+                $thoughtSignature,
+                $citations
+            );
         } elseif (isset($array[self::KEY_FUNCTION_CALL])) {
-            return new self(FunctionCall::fromArray($array[self::KEY_FUNCTION_CALL]), $channel, $thoughtSignature);
+            return new self(
+                FunctionCall::fromArray($array[self::KEY_FUNCTION_CALL]),
+                $channel,
+                $thoughtSignature,
+                $citations
+            );
         } elseif (isset($array[self::KEY_FUNCTION_RESPONSE])) {
             return new self(
                 FunctionResponse::fromArray($array[self::KEY_FUNCTION_RESPONSE]),
                 $channel,
-                $thoughtSignature
+                $thoughtSignature,
+                $citations
             );
         } else {
             throw new InvalidArgumentException(
@@ -378,5 +440,11 @@ public function __clone()
         if ($this->functionResponse !== null) {
             $this->functionResponse = clone $this->functionResponse;
         }
+        if ($this->citations !== null) {
+            $this->citations = array_map(
+                static fn (Citation $citation) => clone $citation,
+                $this->citations
+            );
+        }
     }
 }
diff --git a/tests/unit/Messages/DTO/CitationTest.php b/tests/unit/Messages/DTO/CitationTest.php
new file mode 100644
index 00000000..782df5f0
--- /dev/null
+++ b/tests/unit/Messages/DTO/CitationTest.php
@@ -0,0 +1,608 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WordPress\AiClient\Tests\unit\Messages\DTO;
+
+use PHPUnit\Framework\TestCase;
+use WordPress\AiClient\Common\Exception\InvalidArgumentException;
+use WordPress\AiClient\Messages\DTO\Citation;
+
+/**
+ * @covers \WordPress\AiClient\Messages\DTO\Citation
+ */
+class CitationTest extends TestCase
+{
+    /**
+     * Tests creating Citation with URL only.
+     *
+     * @return void
+     */
+    public function testCreateWithUrlOnly(): void
+    {
+        $citation = new Citation('https://example.com/doc');
+
+        $this->assertEquals('https://example.com/doc', $citation->getUrl());
+        $this->assertNull($citation->getDocumentIndex());
+        $this->assertNull($citation->getTitle());
+        $this->assertNull($citation->getStartIndex());
+        $this->assertNull($citation->getEndIndex());
+        $this->assertNull($citation->getQuotedText());
+    }
+
+    /**
+     * Tests creating Citation with document index only.
+     *
+     * @return void
+     */
+    public function testCreateWithDocumentIndexOnly(): void
+    {
+        $citation = new Citation(null, 2);
+
+        $this->assertNull($citation->getUrl());
+        $this->assertEquals(2, $citation->getDocumentIndex());
+        $this->assertNull($citation->getTitle());
+    }
+
+    /**
+     * Tests creating Citation with all fields populated.
+     *
+     * @return void
+     */
+    public function testCreateWithAllFields(): void
+    {
+        $citation = new Citation(
+            'https://example.com/doc',
+            null,
+            'Example Document',
+            10,
+            50,
+            'The quoted passage from the source.'
+        );
+
+        $this->assertEquals('https://example.com/doc', $citation->getUrl());
+        $this->assertNull($citation->getDocumentIndex());
+        $this->assertEquals('Example Document', $citation->getTitle());
+        $this->assertEquals(10, $citation->getStartIndex());
+        $this->assertEquals(50, $citation->getEndIndex());
+        $this->assertEquals('The quoted passage from the source.', $citation->getQuotedText());
+    }
+
+    /**
+     * Tests creating Citation with document index and span.
+     *
+     * @return void
+     */
+    public function testCreateWithDocumentIndexAndSpan(): void
+    {
+        $citation = new Citation(null, 0, 'Report', 5, 20);
+
+        $this->assertNull($citation->getUrl());
+        $this->assertEquals(0, $citation->getDocumentIndex());
+        $this->assertEquals('Report', $citation->getTitle());
+        $this->assertEquals(5, $citation->getStartIndex());
+        $this->assertEquals(20, $citation->getEndIndex());
+        $this->assertNull($citation->getQuotedText());
+    }
+
+    /**
+     * Tests creating Citation with equal start and end index.
+     *
+     * @return void
+     */
+    public function testCreateWithEqualStartAndEndIndex(): void
+    {
+        $citation = new Citation('https://example.com', null, null, 10, 10);
+
+        $this->assertEquals(10, $citation->getStartIndex());
+        $this->assertEquals(10, $citation->getEndIndex());
+    }
+
+    /**
+     * Tests creating Citation with zero start and end index.
+     *
+     * @return void
+     */
+    public function testCreateWithZeroIndices(): void
+    {
+        $citation = new Citation('https://example.com', null, null, 0, 0);
+
+        $this->assertEquals(0, $citation->getStartIndex());
+        $this->assertEquals(0, $citation->getEndIndex());
+    }
+
+    /**
+     * Tests that negative documentIndex throws exception.
+     *
+     * @return void
+     */
+    public function testNegativeDocumentIndexThrowsException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Citation documentIndex must be non-negative, got -1.');
+
+        new Citation(null, -1);
+    }
+
+    /**
+     * Tests that negative startIndex throws exception.
+     *
+     * @return void
+     */
+    public function testNegativeStartIndexThrowsException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Citation startIndex must be non-negative, got -1.');
+
+        new Citation('https://example.com', null, null, -1, 10);
+    }
+
+    /**
+     * Tests that negative endIndex throws exception.
+     *
+     * @return void
+     */
+    public function testNegativeEndIndexThrowsException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Citation endIndex must be non-negative, got -5.');
+
+        new Citation('https://example.com', null, null, 0, -5);
+    }
+
+    /**
+     * Tests that startIndex greater than endIndex throws exception.
+     *
+     * @return void
+     */
+    public function testStartIndexGreaterThanEndIndexThrowsException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage(
+            'Citation startIndex (20) must be less than or equal to endIndex (10).'
+        );
+
+        new Citation('https://example.com', null, null, 20, 10);
+    }
+
+    /**
+     * Tests that startIndex without endIndex does not throw.
+     *
+     * @return void
+     */
+    public function testStartIndexWithoutEndIndex(): void
+    {
+        $citation = new Citation('https://example.com', null, null, 5, null);
+
+        $this->assertEquals(5, $citation->getStartIndex());
+        $this->assertNull($citation->getEndIndex());
+    }
+
+    /**
+     * Tests that endIndex without startIndex does not throw.
+     *
+     * @return void
+     */
+    public function testEndIndexWithoutStartIndex(): void
+    {
+        $citation = new Citation('https://example.com', null, null, null, 20);
+
+        $this->assertNull($citation->getStartIndex());
+        $this->assertEquals(20, $citation->getEndIndex());
+    }
+
+    /**
+     * Tests JSON schema has all expected properties.
+     *
+     * @return void
+     */
+    public function testJsonSchema(): void
+    {
+        $schema = Citation::getJsonSchema();
+
+        $this->assertIsArray($schema);
+        $this->assertEquals('object', $schema['type']);
+        $this->assertArrayHasKey('properties', $schema);
+        $this->assertArrayHasKey(Citation::KEY_URL, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_DOCUMENT_INDEX, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_TITLE, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_START_INDEX, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_END_INDEX, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_QUOTED_TEXT, $schema['properties']);
+        $this->assertFalse($schema['additionalProperties']);
+        // No required fields — all are nullable.
+        $this->assertArrayNotHasKey('required', $schema);
+    }
+
+    /**
+     * Tests JSON schema index properties include minimum constraint.
+     *
+     * @return void
+     */
+    public function testJsonSchemaIndexMinimumConstraint(): void
+    {
+        $schema = Citation::getJsonSchema();
+
+        $this->assertEquals(0, $schema['properties'][Citation::KEY_DOCUMENT_INDEX]['minimum']);
+        $this->assertEquals(0, $schema['properties'][Citation::KEY_START_INDEX]['minimum']);
+        $this->assertEquals(0, $schema['properties'][Citation::KEY_END_INDEX]['minimum']);
+    }
+
+    /**
+     * Tests toArray with URL only.
+     *
+     * @return void
+     */
+    public function testToArrayWithUrlOnly(): void
+    {
+        $citation = new Citation('https://example.com/doc');
+        $array = $citation->toArray();
+
+        $this->assertIsArray($array);
+        $this->assertArrayHasKey(Citation::KEY_URL, $array);
+        $this->assertEquals('https://example.com/doc', $array[Citation::KEY_URL]);
+        $this->assertArrayNotHasKey(Citation::KEY_DOCUMENT_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_TITLE, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_START_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_END_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_QUOTED_TEXT, $array);
+    }
+
+    /**
+     * Tests toArray with document index only.
+     *
+     * @return void
+     */
+    public function testToArrayWithDocumentIndexOnly(): void
+    {
+        $citation = new Citation(null, 3);
+        $array = $citation->toArray();
+
+        $this->assertArrayNotHasKey(Citation::KEY_URL, $array);
+        $this->assertArrayHasKey(Citation::KEY_DOCUMENT_INDEX, $array);
+        $this->assertEquals(3, $array[Citation::KEY_DOCUMENT_INDEX]);
+    }
+
+    /**
+     * Tests toArray with all fields populated.
+     *
+     * @return void
+     */
+    public function testToArrayWithAllFields(): void
+    {
+        $citation = new Citation(
+            'https://example.com/doc',
+            1,
+            'Doc Title',
+            10,
+            50,
+            'Quoted passage'
+        );
+        $array = $citation->toArray();
+
+        $this->assertEquals('https://example.com/doc', $array[Citation::KEY_URL]);
+        $this->assertEquals(1, $array[Citation::KEY_DOCUMENT_INDEX]);
+        $this->assertEquals('Doc Title', $array[Citation::KEY_TITLE]);
+        $this->assertEquals(10, $array[Citation::KEY_START_INDEX]);
+        $this->assertEquals(50, $array[Citation::KEY_END_INDEX]);
+        $this->assertEquals('Quoted passage', $array[Citation::KEY_QUOTED_TEXT]);
+    }
+
+    /**
+     * Tests toArray omits null fields.
+     *
+     * @return void
+     */
+    public function testToArrayOmitsNullFields(): void
+    {
+        $citation = new Citation('https://example.com', null, 'Title');
+        $array = $citation->toArray();
+
+        $this->assertArrayHasKey(Citation::KEY_URL, $array);
+        $this->assertArrayHasKey(Citation::KEY_TITLE, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_DOCUMENT_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_START_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_END_INDEX, $array);
+        $this->assertArrayNotHasKey(Citation::KEY_QUOTED_TEXT, $array);
+    }
+
+    /**
+     * Tests fromArray with URL only.
+     *
+     * @return void
+     */
+    public function testFromArrayWithUrlOnly(): void
+    {
+        $array = [
+            Citation::KEY_URL => 'https://example.com/doc',
+        ];
+
+        $citation = Citation::fromArray($array);
+
+        $this->assertEquals('https://example.com/doc', $citation->getUrl());
+        $this->assertNull($citation->getDocumentIndex());
+        $this->assertNull($citation->getTitle());
+        $this->assertNull($citation->getStartIndex());
+        $this->assertNull($citation->getEndIndex());
+        $this->assertNull($citation->getQuotedText());
+    }
+
+    /**
+     * Tests fromArray with document index only.
+     *
+     * @return void
+     */
+    public function testFromArrayWithDocumentIndexOnly(): void
+    {
+        $array = [
+            Citation::KEY_DOCUMENT_INDEX => 2,
+        ];
+
+        $citation = Citation::fromArray($array);
+
+        $this->assertNull($citation->getUrl());
+        $this->assertEquals(2, $citation->getDocumentIndex());
+    }
+
+    /**
+     * Tests fromArray with all fields.
+     *
+     * @return void
+     */
+    public function testFromArrayWithAllFields(): void
+    {
+        $array = [
+            Citation::KEY_URL => 'https://example.com/doc',
+            Citation::KEY_DOCUMENT_INDEX => 1,
+            Citation::KEY_TITLE => 'Doc Title',
+            Citation::KEY_START_INDEX => 10,
+            Citation::KEY_END_INDEX => 50,
+            Citation::KEY_QUOTED_TEXT => 'Quoted passage',
+        ];
+
+        $citation = Citation::fromArray($array);
+
+        $this->assertEquals('https://example.com/doc', $citation->getUrl());
+        $this->assertEquals(1, $citation->getDocumentIndex());
+        $this->assertEquals('Doc Title', $citation->getTitle());
+        $this->assertEquals(10, $citation->getStartIndex());
+        $this->assertEquals(50, $citation->getEndIndex());
+        $this->assertEquals('Quoted passage', $citation->getQuotedText());
+    }
+
+    /**
+     * Tests fromArray with empty array creates all-null citation.
+     *
+     * @return void
+     */
+    public function testFromArrayWithEmptyArray(): void
+    {
+        $citation = Citation::fromArray([]);
+
+        $this->assertNull($citation->getUrl());
+        $this->assertNull($citation->getDocumentIndex());
+        $this->assertNull($citation->getTitle());
+        $this->assertNull($citation->getStartIndex());
+        $this->assertNull($citation->getEndIndex());
+        $this->assertNull($citation->getQuotedText());
+    }
+
+    /**
+     * Tests round-trip array transformation with URL and span.
+     *
+     * @return void
+     */
+    public function testArrayRoundTripWithUrlAndSpan(): void
+    {
+        $original = new Citation('https://example.com/doc', null, 'Doc Title', 5, 25, 'some text');
+        $array = $original->toArray();
+        $restored = Citation::fromArray($array);
+
+        $this->assertEquals($original->getUrl(), $restored->getUrl());
+        $this->assertEquals($original->getDocumentIndex(), $restored->getDocumentIndex());
+        $this->assertEquals($original->getTitle(), $restored->getTitle());
+        $this->assertEquals($original->getStartIndex(), $restored->getStartIndex());
+        $this->assertEquals($original->getEndIndex(), $restored->getEndIndex());
+        $this->assertEquals($original->getQuotedText(), $restored->getQuotedText());
+    }
+
+    /**
+     * Tests round-trip array transformation with document index.
+     *
+     * @return void
+     */
+    public function testArrayRoundTripWithDocumentIndex(): void
+    {
+        $original = new Citation(null, 4, 'Internal Doc', 0, 100);
+        $array = $original->toArray();
+        $restored = Citation::fromArray($array);
+
+        $this->assertEquals($original->getUrl(), $restored->getUrl());
+        $this->assertEquals($original->getDocumentIndex(), $restored->getDocumentIndex());
+        $this->assertEquals($original->getTitle(), $restored->getTitle());
+        $this->assertEquals($original->getStartIndex(), $restored->getStartIndex());
+        $this->assertEquals($original->getEndIndex(), $restored->getEndIndex());
+        $this->assertNull($restored->getQuotedText());
+    }
+
+    /**
+     * Tests round-trip array transformation with minimal citation.
+     *
+     * @return void
+     */
+    public function testArrayRoundTripMinimal(): void
+    {
+        $original = new Citation('https://example.com');
+        $array = $original->toArray();
+        $restored = Citation::fromArray($array);
+
+        $this->assertEquals($original->getUrl(), $restored->getUrl());
+        $this->assertNull($restored->getDocumentIndex());
+        $this->assertNull($restored->getTitle());
+        $this->assertNull($restored->getStartIndex());
+        $this->assertNull($restored->getEndIndex());
+        $this->assertNull($restored->getQuotedText());
+    }
+
+    /**
+     * Tests that the location type discriminator round-trips verbatim.
+     *
+     * @return void
+     */
+    public function testTypeDiscriminatorRoundTrips(): void
+    {
+        $citation = new Citation(
+            'https://www.example.com/en/',
+            null,
+            'Example Title',
+            null,
+            null,
+            'The source passage, verbatim.',
+            'web_search_result_location'
+        );
+
+        $this->assertEquals('web_search_result_location', $citation->getType());
+
+        $array = $citation->toArray();
+        $this->assertSame('web_search_result_location', $array[Citation::KEY_TYPE]);
+
+        $restored = Citation::fromArray($array);
+        $this->assertEquals('web_search_result_location', $restored->getType());
+        $this->assertEquals('The source passage, verbatim.', $restored->getQuotedText());
+    }
+
+    /**
+     * Tests that a document location type is preserved alongside a document index.
+     *
+     * @return void
+     */
+    public function testDocumentLocationTypeIsPreserved(): void
+    {
+        $citation = new Citation(
+            null,
+            0,
+            'My Document',
+            null,
+            null,
+            'The exact text being cited',
+            'char_location'
+        );
+
+        $this->assertEquals('char_location', $citation->getType());
+        $this->assertEquals(0, $citation->getDocumentIndex());
+    }
+
+    /**
+     * Tests that type defaults to null when the provider supplies none.
+     *
+     * @return void
+     */
+    public function testTypeDefaultsToNull(): void
+    {
+        $citation = new Citation('https://example.com');
+
+        $this->assertNull($citation->getType());
+        $this->assertArrayNotHasKey(Citation::KEY_TYPE, $citation->toArray());
+    }
+
+    /**
+     * Tests that provider-specific values round-trip through additionalData.
+     *
+     * @return void
+     */
+    public function testAdditionalDataRoundTrips(): void
+    {
+        $extras = [
+            'encrypted_index' => 'abc123opaqueblob',
+            'license' => 'CC-BY-4.0',
+        ];
+
+        $citation = new Citation(
+            'https://example.com',
+            null,
+            null,
+            null,
+            null,
+            null,
+            'web_search_result_location',
+            $extras
+        );
+
+        $this->assertSame($extras, $citation->getAdditionalData());
+
+        $restored = Citation::fromArray($citation->toArray());
+        $this->assertSame($extras, $restored->getAdditionalData());
+    }
+
+    /**
+     * Tests that additionalData defaults to null and is omitted when unset.
+     *
+     * @return void
+     */
+    public function testAdditionalDataDefaultsToNull(): void
+    {
+        $citation = new Citation('https://example.com');
+
+        $this->assertNull($citation->getAdditionalData());
+        $this->assertArrayNotHasKey(Citation::KEY_ADDITIONAL_DATA, $citation->toArray());
+    }
+
+    /**
+     * Tests that additionalData rejects a non-string-keyed array.
+     *
+     * @return void
+     */
+    public function testAdditionalDataRejectsIntegerKeys(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('additionalData must be a map with string keys');
+
+        new Citation(
+            'https://example.com',
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            ['opaque-value']
+        );
+    }
+
+    /**
+     * Tests that an empty additionalData map serializes as a JSON object, not an array.
+     *
+     * @return void
+     */
+    public function testEmptyAdditionalDataSerializesAsObject(): void
+    {
+        $citation = new Citation(
+            'https://example.com',
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            []
+        );
+
+        $json = json_encode($citation);
+        $this->assertIsString($json);
+        $this->assertStringContainsString('"additionalData":{}', $json);
+    }
+
+    /**
+     * Tests that the new fields are exposed in the JSON schema.
+     *
+     * @return void
+     */
+    public function testJsonSchemaExposesTypeAndAdditionalData(): void
+    {
+        $schema = Citation::getJsonSchema();
+
+        $this->assertArrayHasKey(Citation::KEY_TYPE, $schema['properties']);
+        $this->assertArrayHasKey(Citation::KEY_ADDITIONAL_DATA, $schema['properties']);
+        $this->assertEquals('object', $schema['properties'][Citation::KEY_ADDITIONAL_DATA]['type']);
+    }
+}
diff --git a/tests/unit/Messages/DTO/MessagePartTest.php b/tests/unit/Messages/DTO/MessagePartTest.php
index de7a4ae0..0e89b7c8 100644
--- a/tests/unit/Messages/DTO/MessagePartTest.php
+++ b/tests/unit/Messages/DTO/MessagePartTest.php
@@ -10,6 +10,7 @@
 use WordPress\AiClient\Common\Contracts\WithArrayTransformationInterface;
 use WordPress\AiClient\Files\DTO\File;
 use WordPress\AiClient\Files\Enums\FileTypeEnum;
+use WordPress\AiClient\Messages\DTO\Citation;
 use WordPress\AiClient\Messages\DTO\MessagePart;
 use WordPress\AiClient\Messages\Enums\MessagePartChannelEnum;
 use WordPress\AiClient\Messages\Enums\MessagePartTypeEnum;
@@ -37,6 +38,7 @@ public function testCreateWithTextContent(): void
         $this->assertNull($part->getFile());
         $this->assertNull($part->getFunctionCall());
         $this->assertNull($part->getFunctionResponse());
+        $this->assertNull($part->getCitations());
     }
 
     /**
@@ -55,6 +57,7 @@ public function testCreateWithFileContent(): void
         $this->assertSame($file, $part->getFile());
         $this->assertNull($part->getFunctionCall());
         $this->assertNull($part->getFunctionResponse());
+        $this->assertNull($part->getCitations());
     }
 
     /**
@@ -630,4 +633,143 @@ public function testJsonSchemaIncludesThoughtSignature(): void
             );
         }
     }
+
+    /**
+     * Tests creating MessagePart with citations.
+     *
+     * @return void
+     */
+    public function testCreateWithCitations(): void
+    {
+        $citations = [
+            new Citation('https://example.com/doc1', null, 'Doc 1', 0, 20),
+            new Citation(null, 2, null, 21, 40, 'quoted text'),
+        ];
+
+        $part = new MessagePart(
+            'Text with citations',
+            MessagePartChannelEnum::content(),
+            null,
+            $citations
+        );
+
+        $this->assertEquals($citations, $part->getCitations());
+        $this->assertCount(2, $part->getCitations());
+    }
+
+    /**
+     * Tests toArray includes citations when set.
+     *
+     * @return void
+     */
+    public function testToArrayIncludesCitations(): void
+    {
+        $citations = [new Citation('https://example.com/doc1', null, 'Doc 1', 0, 15)];
+        $part = new MessagePart(
+            'Text with citations',
+            MessagePartChannelEnum::content(),
+            null,
+            $citations
+        );
+
+        $array = $part->toArray();
+
+        $this->assertArrayHasKey(MessagePart::KEY_CITATIONS, $array);
+        $this->assertIsArray($array[MessagePart::KEY_CITATIONS]);
+        $this->assertCount(1, $array[MessagePart::KEY_CITATIONS]);
+        $this->assertEquals('https://example.com/doc1', $array[MessagePart::KEY_CITATIONS][0][Citation::KEY_URL]);
+        $this->assertEquals('Doc 1', $array[MessagePart::KEY_CITATIONS][0][Citation::KEY_TITLE]);
+        $this->assertEquals(0, $array[MessagePart::KEY_CITATIONS][0][Citation::KEY_START_INDEX]);
+        $this->assertEquals(15, $array[MessagePart::KEY_CITATIONS][0][Citation::KEY_END_INDEX]);
+    }
+
+    /**
+     * Tests fromArray with citations.
+     *
+     * @return void
+     */
+    public function testFromArrayWithCitations(): void
+    {
+        $array = [
+            MessagePart::KEY_CHANNEL => MessagePartChannelEnum::content()->value,
+            MessagePart::KEY_TYPE => MessagePartTypeEnum::text()->value,
+            MessagePart::KEY_TEXT => 'Text with citations',
+            MessagePart::KEY_CITATIONS => [
+                [
+                    Citation::KEY_URL => 'https://example.com/doc1',
+                    Citation::KEY_TITLE => 'Doc 1',
+                    Citation::KEY_START_INDEX => 0,
+                    Citation::KEY_END_INDEX => 19,
+                ],
+                [
+                    Citation::KEY_DOCUMENT_INDEX => 3,
+                    Citation::KEY_QUOTED_TEXT => 'a passage',
+                ],
+            ],
+        ];
+
+        $part = MessagePart::fromArray($array);
+
+        $this->assertNotNull($part->getCitations());
+        $this->assertCount(2, $part->getCitations());
+        $this->assertEquals('https://example.com/doc1', $part->getCitations()[0]->getUrl());
+        $this->assertEquals('Doc 1', $part->getCitations()[0]->getTitle());
+        $this->assertEquals(0, $part->getCitations()[0]->getStartIndex());
+        $this->assertEquals(19, $part->getCitations()[0]->getEndIndex());
+        $this->assertNull($part->getCitations()[1]->getUrl());
+        $this->assertEquals(3, $part->getCitations()[1]->getDocumentIndex());
+        $this->assertEquals('a passage', $part->getCitations()[1]->getQuotedText());
+    }
+
+    /**
+     * Tests round-trip array transformation with citations.
+     *
+     * @return void
+     */
+    public function testArrayRoundTripWithCitations(): void
+    {
+        $citations = [
+            new Citation('https://example.com/doc1', null, 'Doc 1', 0, 10, 'quoted'),
+        ];
+        $original = new MessagePart(
+            'Text with citations',
+            MessagePartChannelEnum::content(),
+            null,
+            $citations
+        );
+
+        $array = $original->toArray();
+        $restored = MessagePart::fromArray($array);
+
+        $this->assertEquals($original->getText(), $restored->getText());
+        $this->assertNotNull($restored->getCitations());
+        $this->assertCount(1, $restored->getCitations());
+        $this->assertEquals('https://example.com/doc1', $restored->getCitations()[0]->getUrl());
+        $this->assertEquals('Doc 1', $restored->getCitations()[0]->getTitle());
+        $this->assertEquals(0, $restored->getCitations()[0]->getStartIndex());
+        $this->assertEquals(10, $restored->getCitations()[0]->getEndIndex());
+        $this->assertEquals('quoted', $restored->getCitations()[0]->getQuotedText());
+    }
+
+    /**
+     * Tests that cloning MessagePart with citations creates independent copies.
+     *
+     * @return void
+     */
+    public function testCloneClonesCitations(): void
+    {
+        $citations = [new Citation('https://example.com/doc1', null, 'Title', 5, 15)];
+        $original = new MessagePart('text', null, null, $citations);
+        $cloned = clone $original;
+
+        $this->assertNotSame($original->getCitations()[0], $cloned->getCitations()[0]);
+        $this->assertEquals(
+            $original->getCitations()[0]->getUrl(),
+            $cloned->getCitations()[0]->getUrl()
+        );
+        $this->assertEquals(
+            $original->getCitations()[0]->getStartIndex(),
+            $cloned->getCitations()[0]->getStartIndex()
+        );
+    }
 }