diff --git a/agents-api.php b/agents-api.php
index ded0a48..bc942e0 100644
--- a/agents-api.php
+++ b/agents-api.php
@@ -93,6 +93,7 @@
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-transcript-persister.php';
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-null-transcript-persister.php';
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-conversation-compaction.php';
+require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-tool-pair-validator.php';
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-markdown-section-compaction-adapter.php';
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-iteration-budget.php';
 require_once AGENTS_API_PATH . 'src/Runtime/class-wp-agent-conversation-result.php';
diff --git a/composer.json b/composer.json
index 4225695..87b8b19 100644
--- a/composer.json
+++ b/composer.json
@@ -42,6 +42,7 @@
       "php tests/conversation-runner-contracts-smoke.php",
       "php tests/conversation-transcript-lock-smoke.php",
       "php tests/conversation-compaction-smoke.php",
+      "php tests/tool-pair-validator-smoke.php",
       "php tests/markdown-section-compaction-smoke.php",
       "php tests/context-registry-smoke.php",
       "php tests/conversation-loop-smoke.php",
diff --git a/src/Runtime/class-wp-agent-tool-pair-validator.php b/src/Runtime/class-wp-agent-tool-pair-validator.php
new file mode 100644
index 0000000..b1cb5e4
--- /dev/null
+++ b/src/Runtime/class-wp-agent-tool-pair-validator.php
@@ -0,0 +1,213 @@
+<?php
+/**
+ * Tool-call / tool-result pair validator.
+ *
+ * @package AgentsAPI
+ */
+
+namespace AgentsAPI\AI;
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Detects and removes orphan tool_call / tool_result messages in a transcript.
+ *
+ * Provider request shapes (Anthropic-style tool_use/tool_result blocks, OpenAI-style
+ * tool_calls/tool messages) require every tool call to be paired with a result. A
+ * transcript with an orphan tool_call or tool_result is a provider 400 waiting to
+ * happen. This validator gives consumers a substrate-level helper to detect or
+ * scrub such transcripts before dispatch.
+ *
+ * Pairing rule: a tool_call envelope at index `i` matches the next tool_result
+ * envelope at index `j > i` with the same `payload.tool_name`. Matching is FIFO
+ * per tool name — the oldest unmatched tool_call wins, mirroring how providers
+ * resolve tool-use IDs positionally when multiple calls share a name.
+ */
+class WP_Agent_Tool_Pair_Validator {
+
+	public const KIND_ORPHAN_TOOL_CALL   = 'orphan_tool_call';
+	public const KIND_ORPHAN_TOOL_RESULT = 'orphan_tool_result';
+
+	public const EVENT_VALIDATED = 'tool_pair_validated';
+	public const EVENT_PRUNED    = 'tool_pair_pruned';
+
+	/**
+	 * Inspect a message list and return any orphan tool_call / tool_result envelopes.
+	 *
+	 * Returned reports are sorted by ascending message index. Each entry has the
+	 * shape `{ index, kind, type, tool_name }`.
+	 *
+	 * @param array<int, array<string, mixed>> $messages Raw or normalized messages.
+	 * @return array<int, array<string, mixed>> Orphan reports.
+	 */
+	public static function validate( array $messages ): array {
+		$orphans = array();
+		$pending = array();
+
+		foreach ( array_values( $messages ) as $index => $message ) {
+			$envelope = WP_Agent_Message::normalize( $message );
+			$type     = $envelope['type'];
+
+			if ( WP_Agent_Message::TYPE_TOOL_CALL === $type ) {
+				$pending[] = array(
+					'index'     => $index,
+					'tool_name' => self::tool_name( $envelope ),
+				);
+				continue;
+			}
+
+			if ( WP_Agent_Message::TYPE_TOOL_RESULT !== $type ) {
+				continue;
+			}
+
+			$tool_name   = self::tool_name( $envelope );
+			$matched_pos = self::match_pending( $pending, $tool_name );
+
+			if ( null === $matched_pos ) {
+				$orphans[] = array(
+					'index'     => $index,
+					'kind'      => self::KIND_ORPHAN_TOOL_RESULT,
+					'type'      => WP_Agent_Message::TYPE_TOOL_RESULT,
+					'tool_name' => $tool_name,
+				);
+				continue;
+			}
+
+			array_splice( $pending, $matched_pos, 1 );
+		}
+
+		foreach ( $pending as $pending_call ) {
+			$orphans[] = array(
+				'index'     => $pending_call['index'],
+				'kind'      => self::KIND_ORPHAN_TOOL_CALL,
+				'type'      => WP_Agent_Message::TYPE_TOOL_CALL,
+				'tool_name' => $pending_call['tool_name'],
+			);
+		}
+
+		usort(
+			$orphans,
+			static function ( array $a, array $b ): int {
+				return $a['index'] <=> $b['index'];
+			}
+		);
+
+		return $orphans;
+	}
+
+	/**
+	 * Convenience predicate: does the transcript have zero orphans?
+	 *
+	 * @param array<int, array<string, mixed>> $messages Messages.
+	 * @return bool
+	 */
+	public static function is_paired( array $messages ): bool {
+		return array() === self::validate( $messages );
+	}
+
+	/**
+	 * Drop orphan tool_call / tool_result envelopes from the message list.
+	 *
+	 * Non-tool messages and properly paired tool messages are preserved. The
+	 * returned events array follows the same `{type, metadata}` shape used by
+	 * the compaction lifecycle so consumers can forward both through a single
+	 * event sink.
+	 *
+	 * @param array<int, array<string, mixed>> $messages Messages.
+	 * @return array{messages: array<int, array<string, mixed>>, removed: array<int, array<string, mixed>>, events: array<int, array<string, mixed>>}
+	 */
+	public static function prune( array $messages ): array {
+		$source  = array_values( $messages );
+		$orphans = self::validate( $source );
+
+		if ( array() === $orphans ) {
+			return array(
+				'messages' => $source,
+				'removed'  => array(),
+				'events'   => array(
+					self::event(
+						self::EVENT_VALIDATED,
+						array(
+							'total_messages' => count( $source ),
+							'orphan_count'   => 0,
+						)
+					),
+				),
+			);
+		}
+
+		$drop_indices = array();
+		foreach ( $orphans as $orphan ) {
+			$drop_indices[ (int) $orphan['index'] ] = true;
+		}
+
+		$retained = array();
+		foreach ( $source as $index => $message ) {
+			if ( isset( $drop_indices[ $index ] ) ) {
+				continue;
+			}
+			$retained[] = $message;
+		}
+
+		$event = self::event(
+			self::EVENT_PRUNED,
+			array(
+				'total_messages'  => count( $source ),
+				'orphan_count'    => count( $orphans ),
+				'retained_count'  => count( $retained ),
+				'removed_indices' => array_map( 'intval', array_keys( $drop_indices ) ),
+				'orphans'         => $orphans,
+			)
+		);
+
+		return array(
+			'messages' => $retained,
+			'removed'  => $orphans,
+			'events'   => array( $event ),
+		);
+	}
+
+	/**
+	 * Find the FIFO-oldest pending tool_call with the given name.
+	 *
+	 * @param array<int, array<string, mixed>> $pending   Pending list.
+	 * @param string                           $tool_name Tool name to match.
+	 * @return int|null Index in $pending or null when no match.
+	 */
+	private static function match_pending( array $pending, string $tool_name ): ?int {
+		foreach ( $pending as $position => $candidate ) {
+			if ( ( $candidate['tool_name'] ?? '' ) === $tool_name ) {
+				return $position;
+			}
+		}
+
+		return null;
+	}
+
+	/**
+	 * Read a normalized envelope's payload tool name.
+	 *
+	 * @param array<string, mixed> $envelope Normalized envelope.
+	 * @return string
+	 */
+	private static function tool_name( array $envelope ): string {
+		$name = $envelope['payload']['tool_name'] ?? '';
+		return is_string( $name ) ? $name : '';
+	}
+
+	/**
+	 * Build a lifecycle event payload.
+	 *
+	 * @param string               $type Event type.
+	 * @param array<string, mixed> $data Event data.
+	 * @return array<string, mixed>
+	 */
+	private static function event( string $type, array $data ): array {
+		return array(
+			'type'     => $type,
+			'metadata' => $data,
+		);
+	}
+}
diff --git a/tests/tool-pair-validator-smoke.php b/tests/tool-pair-validator-smoke.php
new file mode 100644
index 0000000..6fc3edd
--- /dev/null
+++ b/tests/tool-pair-validator-smoke.php
@@ -0,0 +1,106 @@
+<?php
+/**
+ * Pure-PHP smoke test for the tool-call / tool-result pair validator.
+ *
+ * Run with: php tests/tool-pair-validator-smoke.php
+ *
+ * @package AgentsAPI\Tests
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	define( 'ABSPATH', __DIR__ . '/' );
+}
+
+$failures = array();
+$passes   = 0;
+
+echo "agents-api-tool-pair-validator-smoke\n";
+
+require_once __DIR__ . '/agents-api-smoke-helpers.php';
+agents_api_smoke_require_module();
+
+use AgentsAPI\AI\WP_Agent_Message;
+use AgentsAPI\AI\WP_Agent_Tool_Pair_Validator;
+
+$user_message = array( 'role' => 'user', 'content' => 'hello' );
+$assistant    = array( 'role' => 'assistant', 'content' => 'hi' );
+
+$call_search   = WP_Agent_Message::toolCall( 'searching', 'search', array( 'q' => 'foo' ), 1 );
+$result_search = WP_Agent_Message::toolResult( 'results', 'search', array( 'success' => true, 'tool_data' => array( 'hits' => 0 ) ) );
+
+$call_fetch   = WP_Agent_Message::toolCall( 'fetching', 'fetch', array( 'url' => 'https://example.com' ), 2 );
+$result_fetch = WP_Agent_Message::toolResult( 'fetched', 'fetch', array( 'success' => true, 'tool_data' => array( 'status' => 200 ) ) );
+
+echo "\n[1] Empty transcript has no orphans:\n";
+agents_api_smoke_assert_equals( array(), WP_Agent_Tool_Pair_Validator::validate( array() ), 'empty transcript validates clean', $failures, $passes );
+agents_api_smoke_assert_equals( true, WP_Agent_Tool_Pair_Validator::is_paired( array() ), 'empty transcript is paired', $failures, $passes );
+
+echo "\n[2] Transcript without tool messages has no orphans:\n";
+$plain = array( $user_message, $assistant, $user_message );
+agents_api_smoke_assert_equals( array(), WP_Agent_Tool_Pair_Validator::validate( $plain ), 'plain transcript has no orphans', $failures, $passes );
+agents_api_smoke_assert_equals( true, WP_Agent_Tool_Pair_Validator::is_paired( $plain ), 'plain transcript is paired', $failures, $passes );
+
+echo "\n[3] Properly paired tool_call + tool_result is clean:\n";
+$paired = array( $user_message, $call_search, $result_search, $assistant );
+agents_api_smoke_assert_equals( array(), WP_Agent_Tool_Pair_Validator::validate( $paired ), 'paired call+result has no orphans', $failures, $passes );
+
+echo "\n[4] Tool_call with no matching tool_result is flagged:\n";
+$orphan_call = array( $user_message, $call_search, $assistant );
+$orphans     = WP_Agent_Tool_Pair_Validator::validate( $orphan_call );
+agents_api_smoke_assert_equals( 1, count( $orphans ), 'orphan call produces one report', $failures, $passes );
+agents_api_smoke_assert_equals( 1, $orphans[0]['index'], 'orphan call index points at the call', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::KIND_ORPHAN_TOOL_CALL, $orphans[0]['kind'], 'orphan call kind is correct', $failures, $passes );
+agents_api_smoke_assert_equals( 'search', $orphans[0]['tool_name'], 'orphan call tool_name is preserved', $failures, $passes );
+
+echo "\n[5] Tool_result with no matching tool_call is flagged:\n";
+$orphan_result = array( $user_message, $result_search, $assistant );
+$orphans       = WP_Agent_Tool_Pair_Validator::validate( $orphan_result );
+agents_api_smoke_assert_equals( 1, count( $orphans ), 'orphan result produces one report', $failures, $passes );
+agents_api_smoke_assert_equals( 1, $orphans[0]['index'], 'orphan result index points at the result', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::KIND_ORPHAN_TOOL_RESULT, $orphans[0]['kind'], 'orphan result kind is correct', $failures, $passes );
+agents_api_smoke_assert_equals( 'search', $orphans[0]['tool_name'], 'orphan result tool_name is preserved', $failures, $passes );
+
+echo "\n[6] Multiple interleaved calls match FIFO by tool name:\n";
+$multi = array(
+	$call_search,
+	$call_fetch,
+	$result_fetch,
+	$result_search,
+);
+agents_api_smoke_assert_equals( array(), WP_Agent_Tool_Pair_Validator::validate( $multi ), 'interleaved pairs validate clean', $failures, $passes );
+
+echo "\n[7] Two calls for the same tool with one result leaves the second call orphan:\n";
+$double_call = array( $call_search, $call_search, $result_search );
+$orphans     = WP_Agent_Tool_Pair_Validator::validate( $double_call );
+agents_api_smoke_assert_equals( 1, count( $orphans ), 'double-call leaves one orphan', $failures, $passes );
+agents_api_smoke_assert_equals( 1, $orphans[0]['index'], 'second call is the orphan (FIFO matches first)', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::KIND_ORPHAN_TOOL_CALL, $orphans[0]['kind'], 'orphan kind is tool_call', $failures, $passes );
+
+echo "\n[8] Result for a different tool name does not consume an unrelated pending call:\n";
+$crossed = array( $call_search, $result_fetch );
+$orphans = WP_Agent_Tool_Pair_Validator::validate( $crossed );
+agents_api_smoke_assert_equals( 2, count( $orphans ), 'crossed names produce two orphans', $failures, $passes );
+agents_api_smoke_assert_equals( 0, $orphans[0]['index'], 'first orphan (by index) is the call at 0', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::KIND_ORPHAN_TOOL_CALL, $orphans[0]['kind'], 'first orphan kind is tool_call', $failures, $passes );
+agents_api_smoke_assert_equals( 1, $orphans[1]['index'], 'second orphan is the result at 1', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::KIND_ORPHAN_TOOL_RESULT, $orphans[1]['kind'], 'second orphan kind is tool_result', $failures, $passes );
+
+echo "\n[9] prune() drops orphans and emits a lifecycle event:\n";
+$messy = array( $user_message, $call_search, $result_fetch, $assistant );
+$pruned = WP_Agent_Tool_Pair_Validator::prune( $messy );
+agents_api_smoke_assert_equals( 2, count( $pruned['messages'] ), 'pruned transcript drops both orphans', $failures, $passes );
+agents_api_smoke_assert_equals( 'user', $pruned['messages'][0]['role'], 'first retained message is the user turn', $failures, $passes );
+agents_api_smoke_assert_equals( 'assistant', $pruned['messages'][1]['role'], 'second retained message is the assistant turn', $failures, $passes );
+agents_api_smoke_assert_equals( 2, count( $pruned['removed'] ), 'two orphans are reported as removed', $failures, $passes );
+agents_api_smoke_assert_equals( 1, count( $pruned['events'] ), 'prune emits a single lifecycle event', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::EVENT_PRUNED, $pruned['events'][0]['type'], 'lifecycle event type is tool_pair_pruned', $failures, $passes );
+agents_api_smoke_assert_equals( 2, $pruned['events'][0]['metadata']['orphan_count'], 'event metadata records orphan_count', $failures, $passes );
+
+echo "\n[10] prune() on a clean transcript is a no-op with a validated event:\n";
+$clean_pruned = WP_Agent_Tool_Pair_Validator::prune( $paired );
+agents_api_smoke_assert_equals( 4, count( $clean_pruned['messages'] ), 'clean prune retains all messages', $failures, $passes );
+agents_api_smoke_assert_equals( array(), $clean_pruned['removed'], 'clean prune removes nothing', $failures, $passes );
+agents_api_smoke_assert_equals( WP_Agent_Tool_Pair_Validator::EVENT_VALIDATED, $clean_pruned['events'][0]['type'], 'clean prune emits validated event', $failures, $passes );
+agents_api_smoke_assert_equals( 0, $clean_pruned['events'][0]['metadata']['orphan_count'], 'clean prune event reports orphan_count=0', $failures, $passes );
+
+agents_api_smoke_finish( 'tool-pair-validator', $failures, $passes );