diff --git a/projects/packages/blaze/changelog/add-ads-1036-campaign-preparer b/projects/packages/blaze/changelog/add-ads-1036-campaign-preparer
new file mode 100644
index 000000000000..f5946d95a5ce
--- /dev/null
+++ b/projects/packages/blaze/changelog/add-ads-1036-campaign-preparer
@@ -0,0 +1,4 @@
+Significance: patch
+Type: changed
+
+Blaze: Extract campaign preparation into a reusable preparer.
diff --git a/projects/packages/blaze/src/abilities/class-blaze-abilities.php b/projects/packages/blaze/src/abilities/class-blaze-abilities.php
index 1b04ff714178..63279936d897 100644
--- a/projects/packages/blaze/src/abilities/class-blaze-abilities.php
+++ b/projects/packages/blaze/src/abilities/class-blaze-abilities.php
@@ -22,6 +22,7 @@
 namespace Automattic\Jetpack\Blaze\Abilities;
 
 use Automattic\Jetpack\Blaze;
+use Automattic\Jetpack\Blaze\Campaign_Preparer;
 use Automattic\Jetpack\Connection\Manager as Jetpack_Connection;
 use Automattic\Jetpack\WP_Abilities\Registrar;
 use WP_REST_Request;
@@ -32,11 +33,9 @@
  */
 class Blaze_Abilities extends Registrar {
 
-	const CATEGORY_SLUG                 = 'blaze-ads';
-	const ABILITY_LIST_CAMPAIGNS        = 'blaze-ads/list-campaigns';
-	const ABILITY_PREPARE_CAMPAIGN      = 'blaze-ads/prepare-campaign';
-	private const DEFAULT_BUDGET_TOTAL  = 50.0;
-	private const DEFAULT_DURATION_DAYS = 7;
+	const CATEGORY_SLUG            = 'blaze-ads';
+	const ABILITY_LIST_CAMPAIGNS   = 'blaze-ads/list-campaigns';
+	const ABILITY_PREPARE_CAMPAIGN = 'blaze-ads/prepare-campaign';
 
 	/**
 	 * Slugs we own — used by `opt_into_woo_mcp` and the double-register guard.
@@ -219,7 +218,7 @@ public static function get_abilities(): array {
 			),
 			self::ABILITY_PREPARE_CAMPAIGN => array(
 				'label'               => __( 'Prepare a Blaze campaign', 'jetpack-blaze' ),
-				'description'         => __( 'Prepare a Blaze advertising campaign proposal for an existing post or product on the site. The ability does not write to the DSP itself. It takes a target plus optional natural-language goal, budget, duration, copy, image, and audience overrides; derives sensible defaults from the target post; bundles the result into a prefill payload; and returns a deep-link the merchant clicks to review and submit in the existing Blaze UI. The merchant reviews, accepts payment / T&C, and submits from inside the Blaze UI — that\'s where the actual DSP write happens.', 'jetpack-blaze' ),
+				'description'         => __( 'Prepare a Blaze advertising campaign proposal for an existing post or product on the site. The ability does not write to the DSP itself. It takes a target plus optional natural-language goal, budget, duration, copy, image, and limited audience overrides; derives sensible defaults from the target post; bundles the result into a prefill payload; and returns a deep-link the merchant clicks to review and submit in the existing Blaze UI. Public audience overrides are limited to country and language codes. Device targeting and interest/topic targeting are not public MCP inputs in v1 because they depend on DSP-supported device IDs and IAB category mappings; use the goal/copy fields for intent and let Blaze defaults or the review UI handle those settings. The merchant reviews, accepts payment / T&C, and submits from inside the Blaze UI — that\'s where the actual DSP write happens.', 'jetpack-blaze' ),
 				'input_schema'        => array(
 					'type'                 => 'object',
 					'required'             => array( 'target_urn' ),
@@ -275,7 +274,7 @@ public static function get_abilities(): array {
 						),
 						'languages'            => array(
 							'type'        => 'array',
-							'description' => __( 'Optional ISO 639-1 language codes to target (e.g. ["en", "es"]). Defaults to all languages when omitted; pass a non-empty array to narrow targeting.', 'jetpack-blaze' ),
+							'description' => __( 'Optional ISO 639-1 language codes supported by Blaze/DSP (e.g. ["en", "es"]). Infer from the user\'s natural language request when clear, but omit when unsure or unsupported. Defaults to all languages when omitted; the merchant can adjust language targeting in the Blaze review UI.', 'jetpack-blaze' ),
 							'items'       => array(
 								'type'      => 'string',
 								'minLength' => 2,
@@ -284,7 +283,7 @@ public static function get_abilities(): array {
 						),
 						'countries'            => array(
 							'type'        => 'array',
-							'description' => __( 'Optional ISO 3166-1 alpha-2 country codes to target (e.g. ["US", "GB"]). Defaults to worldwide when omitted; pass a non-empty array to limit reach to those countries.', 'jetpack-blaze' ),
+							'description' => __( 'Optional ISO 3166-1 alpha-2 country codes to target (e.g. ["US", "GB"]). Infer from the user\'s natural-language location request, but emit country codes rather than localized country names. Defaults to worldwide when omitted; pass a non-empty array to limit reach to those countries.', 'jetpack-blaze' ),
 							'items'       => array(
 								'type'      => 'string',
 								'minLength' => 2,
@@ -408,217 +407,21 @@ public static function list_campaigns( $args = array() ) {
 	public static function prepare_campaign( $args = array() ) {
 		$args = is_array( $args ) ? $args : array();
 
-		$urn = isset( $args['target_urn'] ) ? (string) $args['target_urn'] : '';
-		if ( '' === $urn || ! preg_match( '/^urn:wpcom:post:\d+:(\d+)$/', $urn, $matches ) ) {
-			return new \WP_Error(
-				'blaze_invalid_target_urn',
-				/* translators: %s: the malformed URN value supplied by the caller. */
-				sprintf( __( 'Invalid target_urn %s. Expected format: urn:wpcom:post:<site_id>:<post_id>.', 'jetpack-blaze' ), $urn ),
-				array( 'status' => 400 )
-			);
+		$proposal = Campaign_Preparer::prepare( $args );
+		if ( is_wp_error( $proposal ) ) {
+			return $proposal;
 		}
 
-		$post = get_post( (int) $matches[1] );
-		if ( ! $post ) {
-			return new \WP_Error(
-				'blaze_post_not_found',
-				__( 'Post referenced by target_urn does not exist on this site.', 'jetpack-blaze' ),
-				array( 'status' => 404 )
-			);
-		}
-
-		$prefill     = self::build_prefill_payload( $args, $post );
-		$prefill_url = self::build_prefill_url( $post->ID, $prefill );
-
-		return array(
-			'status'      => 'pending_merchant_review',
-			'message'     => sprintf(
-				/* translators: %s: deep-link URL that opens the Blaze widget pre-populated with the prepared campaign proposal. */
-				__( 'Campaign proposal prepared. The merchant must open the Blaze UI to review, accept payment / T&C, and submit. Open here: %s', 'jetpack-blaze' ),
-				$prefill_url
-			),
-			'prefill_url' => $prefill_url,
-			'prefill'     => $prefill,
-		);
-	}
-
-	/**
-	 * Build the campaign prefill payload from the caller's MCP input and
-	 * the resolved target post. Pure function — no I/O — so it's easy to
-	 * test and easy for callers to inspect in the response.
-	 *
-	 * Caller input (`site_name`, `text_snippet`, `cta_text`, `is_evergreen`)
-	 * overrides the post-derived defaults. Anything we can't pull from
-	 * the post or the input is left out — the widget will fill blanks
-	 * with its own defaults at submission time.
-	 *
-	 * @param array    $args MCP input.
-	 * @param \WP_Post $post The target post.
-	 * @return array
-	 */
-	private static function build_prefill_payload( array $args, $post ): array {
-		$featured_image_id   = (int) get_post_thumbnail_id( $post->ID );
-		$featured_image_url  = $featured_image_id > 0 ? wp_get_attachment_image_url( $featured_image_id, 'full' ) : '';
-		$featured_image_mime = $featured_image_id > 0 ? get_post_mime_type( $featured_image_id ) : '';
-
-		// Sensible default snippet: post excerpt, or first ~200 chars of content stripped of HTML.
-		$default_snippet = (string) get_the_excerpt( $post );
-		if ( '' === $default_snippet ) {
-			$stripped        = trim( wp_strip_all_tags( (string) $post->post_content ) );
-			$default_snippet = function_exists( 'mb_substr' ) ? mb_substr( $stripped, 0, 200 ) : substr( $stripped, 0, 200 );
-		}
-
-		$payload = array(
-			'target_urn'    => (string) $args['target_urn'],
-			'type'          => (string) $post->post_type,
-			'site_name'     => isset( $args['site_name'] ) && '' !== (string) $args['site_name']
-				? (string) $args['site_name']
-				: (string) get_the_title( $post ),
-			'text_snippet'  => isset( $args['text_snippet'] ) && '' !== (string) $args['text_snippet']
-				? (string) $args['text_snippet']
-				: $default_snippet,
-			// Default CTA varies by post type — products get a commerce-flavoured
-			// "Shop Now", everything else gets the neutral "Learn More". The widget
-			// requires a non-empty CTA to clear validation, so we always send one.
-			'cta_text'      => isset( $args['cta_text'] ) && '' !== (string) $args['cta_text']
-				? (string) $args['cta_text']
-				: ( 'product' === (string) $post->post_type ? 'Shop Now' : 'Learn More' ),
-			'target_url'    => (string) get_permalink( $post ),
-			'budget'        => array(
-				'mode'     => 'total',
-				'amount'   => (float) ( $args['budget_total'] ?? self::DEFAULT_BUDGET_TOTAL ),
-				'currency' => self::get_site_currency(),
-			),
-			'duration_days' => (int) ( $args['duration_days'] ?? self::DEFAULT_DURATION_DAYS ),
-			'is_evergreen'  => isset( $args['is_evergreen'] ) ? (bool) $args['is_evergreen'] : true,
-			'objective'     => 'VIEWS',
+		return array_merge(
+			$proposal,
+			array(
+				'message' => sprintf(
+					/* translators: %s: deep-link URL that opens the Blaze widget pre-populated with the prepared campaign proposal. */
+					__( 'Campaign proposal prepared. The merchant must open the Blaze UI to review, accept payment / T&C, and submit. Open here: %s', 'jetpack-blaze' ),
+					$proposal['prefill_url']
+				),
+			)
 		);
-
-		if ( isset( $args['goal'] ) && '' !== (string) $args['goal'] ) {
-			$payload['goal'] = (string) $args['goal'];
-		}
-		if ( isset( $args['revision_instruction'] ) && '' !== (string) $args['revision_instruction'] ) {
-			$payload['revision_instruction'] = (string) $args['revision_instruction'];
-		}
-
-		if ( isset( $args['main_image_url'] ) && '' !== (string) $args['main_image_url'] ) {
-			$payload['main_image'] = array(
-				'url'       => (string) $args['main_image_url'],
-				'mime_type' => isset( $args['main_image_mime_type'] ) && '' !== (string) $args['main_image_mime_type']
-					? (string) $args['main_image_mime_type']
-					: 'image/jpeg',
-			);
-		} elseif ( '' !== $featured_image_url ) {
-			$payload['main_image'] = array(
-				'url'       => $featured_image_url,
-				'mime_type' => $featured_image_mime ? $featured_image_mime : 'image/jpeg',
-			);
-		}
-
-		// Optional targeting overrides. Pass-through normalisation only — the
-		// widget resolves country codes to its internal geo records and the
-		// language codes map straight to the language picker. Empty arrays
-		// are dropped so the widget keeps its "all languages / worldwide"
-		// defaults instead of being forced to an empty selection.
-		if ( isset( $args['languages'] ) && is_array( $args['languages'] ) ) {
-			$languages = array_values(
-				array_filter(
-					array_map( 'strtolower', array_map( 'strval', $args['languages'] ) ),
-					static function ( $code ) {
-						return '' !== $code;
-					}
-				)
-			);
-			if ( ! empty( $languages ) ) {
-				$payload['languages'] = $languages;
-			}
-		}
-		if ( isset( $args['countries'] ) && is_array( $args['countries'] ) ) {
-			$countries = array_values(
-				array_filter(
-					array_map( 'strtoupper', array_map( 'strval', $args['countries'] ) ),
-					static function ( $code ) {
-						return 2 === strlen( $code );
-					}
-				)
-			);
-			if ( ! empty( $countries ) ) {
-				$payload['countries'] = $countries;
-			}
-		}
-
-		return $payload;
-	}
-
-	/**
-	 * Build the deep-link the merchant follows to land in the Blaze
-	 * widget with the campaign form pre-populated.
-	 *
-	 * URL shape: `<admin>/tools.php?page=advertising&blaze_prefill=<base64-json>#!/advertising/posts/promote/post-<id>/<hostname>`.
-	 *
-	 * The base path comes from `Blaze::get_campaign_management_url()` so
-	 * we stay aligned with how Blaze opens the promote-post flow today;
-	 * the `blaze_prefill` query param is the Phase 2 addition the widget
-	 * will read on load. The param goes in the query string (not the
-	 * hash) so it reaches the SPA on initial bootstrap reliably.
-	 *
-	 * @param int   $post_id The target post ID.
-	 * @param array $prefill The prefill payload to encode.
-	 * @return string
-	 */
-	private static function build_prefill_url( int $post_id, array $prefill ): string {
-		$base = '';
-		if ( class_exists( '\Automattic\Jetpack\Blaze' ) && method_exists( '\Automattic\Jetpack\Blaze', 'get_campaign_management_url' ) ) {
-			$url_data = Blaze::get_campaign_management_url( $post_id );
-			if ( is_array( $url_data ) && isset( $url_data['link'] ) ) {
-				$base = (string) $url_data['link'];
-			}
-		}
-		if ( '' === $base ) {
-			$base = admin_url( 'tools.php?page=advertising' );
-		}
-
-		$encoded = self::base64url_encode( (string) wp_json_encode( $prefill, JSON_UNESCAPED_SLASHES ) );
-		$param   = 'blaze_prefill=' . rawurlencode( $encoded );
-
-		// Insert the param before the hash if there is one; otherwise append.
-		$hash_pos = strpos( $base, '#' );
-		if ( false === $hash_pos ) {
-			$separator = ( false !== strpos( $base, '?' ) ) ? '&' : '?';
-			return $base . $separator . $param;
-		}
-
-		$pre_hash  = substr( $base, 0, $hash_pos );
-		$hash      = substr( $base, $hash_pos );
-		$separator = ( false !== strpos( $pre_hash, '?' ) ) ? '&' : '?';
-
-		return $pre_hash . $separator . $param . $hash;
-	}
-
-	/**
-	 * URL-safe base64 encode (RFC 4648 §5). Avoids `+` and `/` characters
-	 * that would otherwise need percent-encoding inside a URL.
-	 *
-	 * @param string $input Bytes to encode.
-	 * @return string
-	 */
-	private static function base64url_encode( string $input ): string {
-		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode -- Encoding a structured prefill payload, not obfuscation.
-		return rtrim( strtr( base64_encode( $input ), '+/', '-_' ), '=' );
-	}
-
-	/**
-	 * Currency code for the prefill payload.
-	 *
-	 * Always USD: the DSP only supports USD for billing, so reading the
-	 * Woo store currency would just produce a misleading number for
-	 * non-USD merchants (the DSP would re-interpret the amount as USD
-	 * regardless). Until the DSP gains multi-currency, normalize here.
-	 *
-	 * @return string ISO 4217 currency code.
-	 */
-	private static function get_site_currency(): string {
-		return 'USD';
 	}
 
 	/**
diff --git a/projects/packages/blaze/src/class-campaign-preparer.php b/projects/packages/blaze/src/class-campaign-preparer.php
new file mode 100644
index 000000000000..e6ab36dbba76
--- /dev/null
+++ b/projects/packages/blaze/src/class-campaign-preparer.php
@@ -0,0 +1,200 @@
+<?php
+/**
+ * Blaze campaign preparation.
+ *
+ * @package automattic/jetpack-blaze
+ */
+
+namespace Automattic\Jetpack\Blaze;
+
+/**
+ * Prepares a structured Blaze campaign proposal for review in the Blaze UI.
+ */
+class Campaign_Preparer {
+
+	private const DEFAULT_BUDGET_TOTAL  = 50.0;
+	private const DEFAULT_DURATION_DAYS = 7;
+
+	/**
+	 * Prepare a Blaze campaign proposal from a target post and optional overrides.
+	 *
+	 * @param array $args Preparation input.
+	 * @return array|\WP_Error
+	 */
+	public static function prepare( array $args ) {
+		$urn = isset( $args['target_urn'] ) ? (string) $args['target_urn'] : '';
+		if ( '' === $urn || ! preg_match( '/^urn:wpcom:post:\d+:(\d+)$/', $urn, $matches ) ) {
+			return new \WP_Error(
+				'blaze_invalid_target_urn',
+				/* translators: %s: the malformed URN value supplied by the caller. */
+				sprintf( __( 'Invalid target_urn %s. Expected format: urn:wpcom:post:<site_id>:<post_id>.', 'jetpack-blaze' ), $urn ),
+				array( 'status' => 400 )
+			);
+		}
+
+		$post = get_post( (int) $matches[1] );
+		if ( ! $post ) {
+			return new \WP_Error(
+				'blaze_post_not_found',
+				__( 'Post referenced by target_urn does not exist on this site.', 'jetpack-blaze' ),
+				array( 'status' => 404 )
+			);
+		}
+
+		$prefill     = self::build_prefill_payload( $args, $post );
+		$prefill_url = self::build_prefill_url( $post->ID, $prefill );
+
+		return array(
+			'status'      => 'pending_merchant_review',
+			'prefill_url' => $prefill_url,
+			'prefill'     => $prefill,
+		);
+	}
+
+	/**
+	 * Build the campaign prefill payload from caller input and the target post.
+	 *
+	 * @param array    $args Preparation input.
+	 * @param \WP_Post $post The target post.
+	 * @return array
+	 */
+	private static function build_prefill_payload( array $args, $post ): array {
+		$featured_image_id   = (int) get_post_thumbnail_id( $post->ID );
+		$featured_image_url  = $featured_image_id > 0 ? wp_get_attachment_image_url( $featured_image_id, 'full' ) : '';
+		$featured_image_mime = $featured_image_id > 0 ? get_post_mime_type( $featured_image_id ) : '';
+
+		$default_snippet = (string) get_the_excerpt( $post );
+		if ( '' === $default_snippet ) {
+			$stripped        = trim( wp_strip_all_tags( (string) $post->post_content ) );
+			$default_snippet = function_exists( 'mb_substr' ) ? mb_substr( $stripped, 0, 200 ) : substr( $stripped, 0, 200 );
+		}
+
+		$payload = array(
+			'target_urn'    => (string) $args['target_urn'],
+			'type'          => (string) $post->post_type,
+			'site_name'     => isset( $args['site_name'] ) && '' !== (string) $args['site_name']
+				? (string) $args['site_name']
+				: (string) get_the_title( $post ),
+			'text_snippet'  => isset( $args['text_snippet'] ) && '' !== (string) $args['text_snippet']
+				? (string) $args['text_snippet']
+				: $default_snippet,
+			'cta_text'      => isset( $args['cta_text'] ) && '' !== (string) $args['cta_text']
+				? (string) $args['cta_text']
+				: ( 'product' === (string) $post->post_type ? 'Shop Now' : 'Learn More' ),
+			'target_url'    => (string) get_permalink( $post ),
+			'budget'        => array(
+				'mode'     => 'total',
+				'amount'   => (float) ( $args['budget_total'] ?? self::DEFAULT_BUDGET_TOTAL ),
+				'currency' => self::get_site_currency(),
+			),
+			'duration_days' => (int) ( $args['duration_days'] ?? self::DEFAULT_DURATION_DAYS ),
+			'is_evergreen'  => isset( $args['is_evergreen'] ) ? (bool) $args['is_evergreen'] : true,
+			'objective'     => 'VIEWS',
+		);
+
+		if ( isset( $args['goal'] ) && '' !== (string) $args['goal'] ) {
+			$payload['goal'] = (string) $args['goal'];
+		}
+		if ( isset( $args['revision_instruction'] ) && '' !== (string) $args['revision_instruction'] ) {
+			$payload['revision_instruction'] = (string) $args['revision_instruction'];
+		}
+
+		if ( isset( $args['main_image_url'] ) && '' !== (string) $args['main_image_url'] ) {
+			$payload['main_image'] = array(
+				'url'       => (string) $args['main_image_url'],
+				'mime_type' => isset( $args['main_image_mime_type'] ) && '' !== (string) $args['main_image_mime_type']
+					? (string) $args['main_image_mime_type']
+					: 'image/jpeg',
+			);
+		} elseif ( '' !== $featured_image_url ) {
+			$payload['main_image'] = array(
+				'url'       => $featured_image_url,
+				'mime_type' => $featured_image_mime ? $featured_image_mime : 'image/jpeg',
+			);
+		}
+
+		if ( isset( $args['languages'] ) && is_array( $args['languages'] ) ) {
+			$languages = array_values(
+				array_filter(
+					array_map( 'strtolower', array_map( 'strval', $args['languages'] ) ),
+					static function ( $code ) {
+						return '' !== $code;
+					}
+				)
+			);
+			if ( ! empty( $languages ) ) {
+				$payload['languages'] = $languages;
+			}
+		}
+		if ( isset( $args['countries'] ) && is_array( $args['countries'] ) ) {
+			$countries = array_values(
+				array_filter(
+					array_map( 'strtoupper', array_map( 'strval', $args['countries'] ) ),
+					static function ( $code ) {
+						return 2 === strlen( $code );
+					}
+				)
+			);
+			if ( ! empty( $countries ) ) {
+				$payload['countries'] = $countries;
+			}
+		}
+
+		return $payload;
+	}
+
+	/**
+	 * Build the Blaze review deep-link with the encoded prefill payload.
+	 *
+	 * @param int   $post_id The target post ID.
+	 * @param array $prefill The prefill payload to encode.
+	 * @return string
+	 */
+	private static function build_prefill_url( int $post_id, array $prefill ): string {
+		$base = '';
+		if ( class_exists( '\Automattic\Jetpack\Blaze' ) && method_exists( '\Automattic\Jetpack\Blaze', 'get_campaign_management_url' ) ) {
+			$url_data = \Automattic\Jetpack\Blaze::get_campaign_management_url( $post_id );
+			if ( is_array( $url_data ) && isset( $url_data['link'] ) ) {
+				$base = (string) $url_data['link'];
+			}
+		}
+		if ( '' === $base ) {
+			$base = admin_url( 'tools.php?page=advertising' );
+		}
+
+		$encoded = self::base64url_encode( (string) wp_json_encode( $prefill, JSON_UNESCAPED_SLASHES ) );
+		$param   = 'blaze_prefill=' . rawurlencode( $encoded );
+
+		$hash_pos = strpos( $base, '#' );
+		if ( false === $hash_pos ) {
+			$separator = ( false !== strpos( $base, '?' ) ) ? '&' : '?';
+			return $base . $separator . $param;
+		}
+
+		$pre_hash  = substr( $base, 0, $hash_pos );
+		$hash      = substr( $base, $hash_pos );
+		$separator = ( false !== strpos( $pre_hash, '?' ) ) ? '&' : '?';
+
+		return $pre_hash . $separator . $param . $hash;
+	}
+
+	/**
+	 * URL-safe base64 encode (RFC 4648 section 5).
+	 *
+	 * @param string $input Bytes to encode.
+	 * @return string
+	 */
+	private static function base64url_encode( string $input ): string {
+		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode -- Encoding a structured prefill payload, not obfuscation.
+		return rtrim( strtr( base64_encode( $input ), '+/', '-_' ), '=' );
+	}
+
+	/**
+	 * Currency code for the prefill payload.
+	 *
+	 * @return string ISO 4217 currency code.
+	 */
+	private static function get_site_currency(): string {
+		return 'USD';
+	}
+}
diff --git a/projects/packages/blaze/tests/php/Campaign_Preparer_Test.php b/projects/packages/blaze/tests/php/Campaign_Preparer_Test.php
new file mode 100644
index 000000000000..9cf6db8afa92
--- /dev/null
+++ b/projects/packages/blaze/tests/php/Campaign_Preparer_Test.php
@@ -0,0 +1,152 @@
+<?php
+/**
+ * Tests for the reusable Blaze campaign preparer.
+ *
+ * @package automattic/jetpack-blaze
+ */
+
+namespace Automattic\Jetpack\Blaze;
+
+use Jetpack_Options;
+use PHPUnit\Framework\Attributes\CoversClass;
+use WorDBless\BaseTestCase;
+use WP_Error;
+
+/**
+ * @covers \Automattic\Jetpack\Blaze\Campaign_Preparer
+ */
+#[CoversClass( Campaign_Preparer::class )]
+class Campaign_Preparer_Test extends BaseTestCase {
+
+	/**
+	 * Synthetic site ID used in target URNs.
+	 *
+	 * @var int
+	 */
+	private const TEST_SITE_ID = 12345;
+
+	/**
+	 * Set up a synthetic connected site ID.
+	 */
+	public function set_up() {
+		Jetpack_Options::update_option( 'id', self::TEST_SITE_ID );
+	}
+
+	/**
+	 * Clear synthetic connection state.
+	 */
+	public function tear_down() {
+		Jetpack_Options::delete_option( 'id' );
+	}
+
+	/**
+	 * Helper: insert a test post and return its ID and target URN.
+	 *
+	 * @param array $overrides Post field overrides.
+	 * @return array
+	 */
+	private function make_test_post( array $overrides = array() ): array {
+		$post_id = wp_insert_post(
+			array_merge(
+				array(
+					'post_title'   => 'Test product page',
+					'post_excerpt' => 'A short summary about the product.',
+					'post_content' => 'Long-form content that would otherwise be the fallback snippet source.',
+					'post_status'  => 'publish',
+					'post_type'    => 'post',
+				),
+				$overrides
+			)
+		);
+
+		return array(
+			'post_id'    => (int) $post_id,
+			'target_urn' => sprintf( 'urn:wpcom:post:%d:%d', self::TEST_SITE_ID, (int) $post_id ),
+		);
+	}
+
+	/**
+	 * The preparer returns reusable structured data and leaves MCP-specific
+	 * human-readable messaging to the ability adapter.
+	 */
+	public function test_prepare_returns_structured_proposal_without_mcp_message() {
+		$ctx = $this->make_test_post();
+
+		$result = Campaign_Preparer::prepare(
+			array(
+				'target_urn'    => $ctx['target_urn'],
+				'budget_total'  => 75,
+				'duration_days' => 10,
+			)
+		);
+
+		$this->assertIsArray( $result );
+		$this->assertSame( 'pending_merchant_review', $result['status'] );
+		$this->assertArrayHasKey( 'prefill_url', $result );
+		$this->assertArrayHasKey( 'prefill', $result );
+		$this->assertArrayNotHasKey( 'message', $result );
+		$this->assertStringContainsString( 'blaze_prefill=', $result['prefill_url'] );
+
+		$prefill = $result['prefill'];
+		$this->assertSame( $ctx['target_urn'], $prefill['target_urn'] );
+		$this->assertSame( 'Test product page', $prefill['site_name'] );
+		$this->assertSame( 'A short summary about the product.', $prefill['text_snippet'] );
+		$this->assertSame( 75.0, $prefill['budget']['amount'] );
+		$this->assertSame( 10, $prefill['duration_days'] );
+	}
+
+	/**
+	 * Caller overrides and targeting hints are normalized by the preparer.
+	 */
+	public function test_prepare_applies_overrides_and_targeting_hints() {
+		$ctx = $this->make_test_post(
+			array(
+				'post_type' => 'product',
+			)
+		);
+
+		$result = Campaign_Preparer::prepare(
+			array(
+				'target_urn'           => $ctx['target_urn'],
+				'budget_total'         => 150,
+				'duration_days'        => 7,
+				'site_name'            => 'Custom heading',
+				'text_snippet'         => 'Custom ad copy.',
+				'goal'                 => 'Drive sales in the UK.',
+				'revision_instruction' => 'Make it more direct.',
+				'main_image_url'       => 'https://example.com/custom.jpg',
+				'main_image_mime_type' => 'image/jpeg',
+				'languages'            => array( 'EN', '', 'es' ),
+				'countries'            => array( 'gb', 'usa', 'FR' ),
+				'is_evergreen'         => false,
+			)
+		);
+
+		$this->assertIsArray( $result );
+		$prefill = $result['prefill'];
+		$this->assertSame( 'Custom heading', $prefill['site_name'] );
+		$this->assertSame( 'Custom ad copy.', $prefill['text_snippet'] );
+		$this->assertSame( 'Shop Now', $prefill['cta_text'] );
+		$this->assertSame( 'Drive sales in the UK.', $prefill['goal'] );
+		$this->assertSame( 'Make it more direct.', $prefill['revision_instruction'] );
+		$this->assertSame( 'https://example.com/custom.jpg', $prefill['main_image']['url'] );
+		$this->assertSame( array( 'en', 'es' ), $prefill['languages'] );
+		$this->assertSame( array( 'GB', 'FR' ), $prefill['countries'] );
+		$this->assertFalse( $prefill['is_evergreen'] );
+		$this->assertSame( 'VIEWS', $prefill['objective'] );
+	}
+
+	/**
+	 * Invalid targets remain hard stops owned by the preparation layer.
+	 */
+	public function test_prepare_returns_error_for_invalid_target_urn() {
+		$result = Campaign_Preparer::prepare(
+			array(
+				'target_urn' => 'not-a-urn',
+			)
+		);
+
+		$this->assertInstanceOf( WP_Error::class, $result );
+		$this->assertSame( 'blaze_invalid_target_urn', $result->get_error_code() );
+	}
+}
diff --git a/projects/packages/blaze/tests/php/abilities/Blaze_Abilities_Test.php b/projects/packages/blaze/tests/php/abilities/Blaze_Abilities_Test.php
index 6bdda6af152a..a57791b5962f 100644
--- a/projects/packages/blaze/tests/php/abilities/Blaze_Abilities_Test.php
+++ b/projects/packages/blaze/tests/php/abilities/Blaze_Abilities_Test.php
@@ -372,6 +372,7 @@ public function test_prepare_campaign_schema_is_minimal_and_renamed() {
 		$abilities = Blaze_Abilities::get_abilities();
 
 		$this->assertArrayHasKey( 'blaze-ads/prepare-campaign', $abilities );
+		$this->assertStringContainsString( 'Device targeting and interest/topic targeting are not public MCP inputs in v1', $abilities[ Blaze_Abilities::ABILITY_PREPARE_CAMPAIGN ]['description'] );
 
 		$schema     = $abilities[ Blaze_Abilities::ABILITY_PREPARE_CAMPAIGN ]['input_schema'];
 		$properties = $schema['properties'];
@@ -389,6 +390,11 @@ public function test_prepare_campaign_schema_is_minimal_and_renamed() {
 		$this->assertArrayHasKey( 'languages', $properties );
 		$this->assertArrayHasKey( 'countries', $properties );
 		$this->assertArrayNotHasKey( 'objective', $properties );
+		$this->assertArrayNotHasKey( 'devices', $properties );
+		$this->assertArrayNotHasKey( 'interests', $properties );
+		$this->assertArrayNotHasKey( 'page_topics', $properties );
+		$this->assertStringContainsString( 'ISO 639-1', $properties['languages']['description'] );
+		$this->assertStringContainsString( 'ISO 3166-1 alpha-2', $properties['countries']['description'] );
 	}
 
 	// --- prepare_campaign: prefill payload + URL ---