fix: session persistence + h3 readability + outline error messages

Session issues fixed:
- Removed auto-draft-only gate for showing unassigned sessions on new posts
  (now shows on any post that has no linked sessions)
- Auto-link unassigned session to current post when user opens it
- Added beforeunload/pagehide flush to persist messages before page close
  (prevents data loss from 700ms debounce not firing)
- Added warning log when session loads with 0 messages for debugging

UI fix:
- Override WP editor h3 shrinkage (11px/uppercase → 15px/normal/white)
- Fix h2/h4-h6 headings in response content for dark theme readability

Outline error messages:
- Separate empty response from parse failure with distinct actionable messages
- Show model name + token count on empty response for debugging
- Reassure user that parse failures are usually one-time issues

assets/css/sidebar.css

@@ -4554,6 +4554,32 @@ input.wpaw-plan-section-check:checked::before {
 /* ===========================
    AUDIT FIXES: Mode Indicator Badge
    =========================== */
+
+/* Override WordPress editor-sidebar h3 shrinkage inside our panel */
+#wp-agentic-writer\:wp-agentic-writer .interface-complementary-area h3,
+#wp-agentic-writer\:wp-agentic-writer h3,
+.wpaw-response-content h3,
+.wpaw-messages-inner h3 {
+    font-size: 15px !important;
+    text-transform: none !important;
+    font-weight: 700 !important;
+    color: #e0e4ea !important;
+    margin-bottom: 0.5em !important;
+    letter-spacing: normal !important;
+}
+
+.wpaw-response-content h2 {
+    font-size: 17px !important;
+    color: #e8ecf2 !important;
+}
+
+.wpaw-response-content h4,
+.wpaw-response-content h5,
+.wpaw-response-content h6 {
+    font-size: 13px !important;
+    color: #d0d5dd !important;
+}
+
 .wpaw-mode-badge {
     display: inline-flex;
     align-items: center;

assets/js/sidebar.js

@@ -1078,6 +1078,39 @@
 			}
 		}, [sanitizeMessagesForStorage]);
 
+		// Flush pending message persistence on page unload to prevent data loss
+		React.useEffect(() => {
+			const flushOnUnload = () => {
+				if (messagesSaveTimeoutRef.current) {
+					clearTimeout(messagesSaveTimeoutRef.current);
+				}
+				if (!currentSessionId || isHydratingSessionRef.current) {
+					return;
+				}
+				const sanitized = sanitizeMessagesForStorage(messages);
+				const serialized = JSON.stringify(sanitized);
+				if (serialized === lastPersistedMessagesRef.current) {
+					return;
+				}
+				// Synchronous XHR as last resort during unload (sendBeacon can't set headers)
+				try {
+					const xhr = new XMLHttpRequest();
+					xhr.open('POST', `${wpAgenticWriter.apiUrl}/conversations/${currentSessionId}/messages`, false); // sync
+					xhr.setRequestHeader('Content-Type', 'application/json');
+					xhr.setRequestHeader('X-WP-Nonce', wpAgenticWriter.nonce);
+					xhr.send(JSON.stringify({ messages: sanitized }));
+				} catch (e) {
+					// Best effort - ignore errors during unload
+				}
+			};
+			window.addEventListener('beforeunload', flushOnUnload);
+			window.addEventListener('pagehide', flushOnUnload);
+			return () => {
+				window.removeEventListener('beforeunload', flushOnUnload);
+				window.removeEventListener('pagehide', flushOnUnload);
+			};
+		}, [currentSessionId, messages, sanitizeMessagesForStorage]);
+
 		React.useEffect(() => {
 			if (!currentSessionId) {
 				return;
@@ -1205,10 +1238,10 @@
 					postSessions = Array.isArray(postData?.sessions) ? postData.sessions : [];
 				}
 
-				// Auto-draft fallback:
+				// Fallback: if this post has no linked sessions, surface active unassigned sessions
-				// if this auto-draft has no linked sessions yet, surface active carry-over sessions
+				// so users can continue unfinished work from other tabs or prior page loads.
-				// (unassigned or still auto-draft) so users can continue unfinished work.
+				// This covers auto-draft AND draft posts that haven't had a session linked yet.
-				if (postSessions.length === 0 && currentPostStatus === 'auto-draft') {
+				if (postSessions.length === 0) {
 					const activeRes = await fetch(`${wpAgenticWriter.apiUrl}/conversations?status=active&limit=50`, {
 						method: 'GET',
 						headers,
@@ -1219,7 +1252,8 @@
 						unassignedSessions = allActive.filter((s) => {
 							const pid = Number(s?.post_id || 0);
 							const postStatus = String(s?.post_status || '').toLowerCase();
-							return pid === 0 || postStatus === 'auto-draft';
+							// Show sessions that are unassigned, or linked to auto-drafts/drafts
+							return pid === 0 || postStatus === 'auto-draft' || postStatus === '';
 						});
 					}
 				}
@@ -1281,10 +1315,30 @@
 				isHydratingSessionRef.current = true;
 				setCurrentSessionId(sessionId);
 				const sessionMessages = Array.isArray(data?.messages) ? data.messages : [];
+
+				// If session has no messages but has a post_id, it may have been improperly persisted
+				if (sessionMessages.length === 0) {
+					wpawLog.warn('Session loaded with 0 messages:', sessionId);
+				}
+
 				lastPersistedMessagesRef.current = JSON.stringify(sanitizeMessagesForStorage(sessionMessages));
 				hydrateSessionStateFromMessages(sessionMessages);
 				setMessages(sessionMessages);
 				setShowWelcome(false);
+
+				// Auto-link unassigned session to current post for continuity
+				const sessionPostId = Number(data?.post_id || 0);
+				if (postId && postId > 0 && sessionPostId === 0) {
+					fetch(`${wpAgenticWriter.apiUrl}/conversations/${sessionId}/link-post`, {
+						method: 'POST',
+						headers: {
+							'Content-Type': 'application/json',
+							'X-WP-Nonce': wpAgenticWriter.nonce,
+						},
+						body: JSON.stringify({ postId: postId }),
+					}).catch(() => {}); // Non-blocking
+				}
+
 				setTimeout(() => {
 					isHydratingSessionRef.current = false;
 				}, 0);

includes/class-gutenberg-sidebar.php

@@ -2296,13 +2296,25 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 		// Debug: log the raw response
 		wpaw_debug_log( 'Plan generation raw response length: ' . strlen( $content ) );
 
+			if ( empty( trim( (string) $content ) ) ) {
+				$model_used = $response['model'] ?? 'unknown';
+				return new WP_Error(
+					'empty_response',
+					sprintf(
+						__( 'The AI model (%s) returned an empty response. Try a different planning model or simplify your topic.', 'wp-agentic-writer' ),
+						$model_used
+					),
+					array( 'status' => 500 )
+				);
+			}
+
 			if ( null === $plan_json ) {
 				wpaw_debug_log( 'extract_plan_from_response returned null. Content preview: ' . substr( $content, 0, 500 ) );
 				return new WP_Error(
 					'invalid_json',
 					sprintf(
 						/* translators: %s: model output preview */
-						__( 'The model responded, but the outline format could not be parsed. Preview: %s', 'wp-agentic-writer' ),
+						__( 'The AI responded but the outline couldn\'t be parsed as JSON. Try again — this is usually a one-time formatting issue. Preview: %s', 'wp-agentic-writer' ),
 						$this->build_model_output_preview( $content )
 					),
 					array( 'status' => 500 )
@@ -2904,6 +2916,25 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 
 			$content = $response['content'];
 			wpaw_debug_log( 'stream_generatePlan content length: ' . strlen( $content ) );
+
+			// Handle empty response gracefully
+			if ( empty( trim( (string) $content ) ) ) {
+				$model_used = $response['model'] ?? 'unknown';
+				$input_tokens = $response['input_tokens'] ?? 0;
+				echo "data: " . wp_json_encode(
+					array(
+						'type'    => 'error',
+						'message' => sprintf(
+							'The AI model (%s) returned an empty response. This usually means the model couldn\'t process the request. Try: 1) Use a different planning model in Settings, 2) Simplify your topic, or 3) Try again. (Tokens sent: %d)',
+							$model_used,
+							$input_tokens
+						),
+					)
+				) . "\n\n";
+				flush();
+				exit;
+			}
+
 			$plan_json = $this->extract_plan_from_response( $content, $topic );
 
 			if ( null === $plan_json ) {
@@ -2912,7 +2943,7 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 				echo "data: " . wp_json_encode(
 					array(
 						'type'    => 'error',
-						'message' => 'The model responded, but the outline format could not be parsed. Preview: ' . $preview,
+						'message' => 'The AI responded but the outline couldn\'t be parsed as JSON. This sometimes happens when the model adds extra text. Trying again usually fixes this. Preview: ' . $preview,
 					)
 				) . "\n\n";
 				flush();