diff --git a/clients/macos/AGENTS.md b/clients/macos/AGENTS.md
index 7d9c5efe75f..ef0a203f0d6 100644
--- a/clients/macos/AGENTS.md
+++ b/clients/macos/AGENTS.md
@@ -252,6 +252,14 @@ All design system types use the `V` prefix (VButton, VColor, VFont, etc.). Alway
   - **Background**: Use a single `.background { }` with a `ZStack` inside instead of chaining multiple `.background()` calls.
   - **No-op backgrounds**: Never add invisible backgrounds like `.background(Capsule().fill(Color.clear))` — they create layout wrappers with zero visual effect.
 - **No animated insertions in chat `LazyVStack`**: ANY animated insertion/removal in a `LazyVStack` triggers `motionVectors` — an O(n) `sizeThatFits` measurement over ALL children that defeats lazy loading and causes multi-minute hangs. The chat message list uses `.transaction { $0.animation = nil }` to suppress all insertion animations. Do NOT remove that modifier or wrap content mutations in `withAnimation` that flows into the `LazyVStack`. See [`.transaction` docs](https://developer.apple.com/documentation/swiftui/view/transaction(_:)) and [WWDC23: Demystify SwiftUI performance](https://developer.apple.com/videos/play/wwdc2023/10160/).
+- **No `.frame(maxWidth:, alignment:)` inside LazyVStack cell hierarchy**: `.frame(maxWidth:)` creates [`_FlexFrameLayout`](https://developer.apple.com/documentation/swiftui/view/frame(minwidth:idealwidth:maxwidth:minheight:idealheight:maxheight:alignment:)) whose `placement()` queries each child's explicit alignment via [`ViewDimensions.subscript`](https://developer.apple.com/documentation/swiftui/viewdimensions). Nested FlexFrames recurse O(depth × children) per layout pass. **This includes `.frame(maxWidth: X)` with no explicit alignment** — it defaults to `.center`, still triggering the query. See [WWDC23: Demystify SwiftUI performance](https://developer.apple.com/videos/play/wwdc2023/10160/). Safe alternatives:
+  - `.frame(width: exactWidth)` — [`_FrameLayout`](https://developer.apple.com/documentation/swiftui/view/frame(width:height:alignment:)), no alignment query.
+  - `HStack { content; Spacer(minLength: 0) }` — leading alignment without queries.
+  - `HStack { Spacer(minLength: 0); content }` — trailing alignment without queries.
+  - [`.containerRelativeFrame(.horizontal)`](https://developer.apple.com/documentation/swiftui/view/containerrelativeframe(_:alignment:)) — width constraint without FlexFrame.
+  
+  Never trade `HStack+Spacer` for `.frame(alignment:)` in lazy containers — fewer layout nodes is not worth O(n) recursive alignment queries per node.
+- **`AlignmentBarrierLayout` for LazyVStack protection**: Wrap content inside a `ScrollView` that contains a `LazyVStack` with `AlignmentBarrierLayout { ... }` to block `explicitAlignment` queries from cascading into the lazy container. The barrier returns `nil` for all alignment queries while passing through sizing and placement unchanged. This is a defense-in-depth measure — even if `.frame(maxWidth:, alignment:)` modifiers exist above the barrier, the alignment cascade stops at the barrier and never reaches the `LazyVStack`. See [`Layout.explicitAlignment`](https://developer.apple.com/documentation/swiftui/layout/explicitalignment(of:in:proposal:subviews:cache:)-3iqmu) and `clients/shared/DesignSystem/Layout/AlignmentBarrierLayout.swift`.
 - **Gallery**: When adding or modifying a design system primitive/component, update the corresponding Gallery section file (`Gallery/Sections/`) so the visual catalog stays current.
 - **Accessibility**: See `clients/AGENTS.md` § [Accessibility](../AGENTS.md#accessibility) for the full checklist (labels, hidden elements, custom interactions, AppKit panels). All rules there apply to macOS components.
 
diff --git a/clients/macos/vellum-assistant/Features/Chat/AssistantProgressView.swift b/clients/macos/vellum-assistant/Features/Chat/AssistantProgressView.swift
index 13919be4819..de98771f7c4 100644
--- a/clients/macos/vellum-assistant/Features/Chat/AssistantProgressView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/AssistantProgressView.swift
@@ -399,7 +399,6 @@ struct AssistantProgressView: View {
         .buttonStyle(.plain)
         .environment(\.isEnabled, true)
         .padding(EdgeInsets(top: VSpacing.xs, leading: VSpacing.sm, bottom: VSpacing.xs, trailing: VSpacing.sm))
-        .frame(maxWidth: .infinity, alignment: .leading)
     }
 
     @ViewBuilder
@@ -882,9 +881,12 @@ private struct StepDetailRow: View {
         isError: Bool = false
     ) -> some View {
         ZStack(alignment: .topTrailing) {
-            outputTextView(text: text, attributedText: attributedText, isError: isError)
+            // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+            HStack(spacing: 0) {
+                outputTextView(text: text, attributedText: attributedText, isError: isError)
+                Spacer(minLength: 0)
+            }
             .padding(EdgeInsets(top: VSpacing.sm, leading: VSpacing.sm, bottom: VSpacing.sm, trailing: VSpacing.sm + VSpacing.xl))
-            .frame(maxWidth: .infinity, alignment: .leading)
             .background(
                 RoundedRectangle(cornerRadius: VRadius.sm)
                     .fill(VColor.surfaceOverlay.opacity(0.6))
@@ -923,17 +925,22 @@ private struct StepDetailRow: View {
         attributedText: AttributedString?,
         isError: Bool = false
     ) -> some View {
+        // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
         if let attrText = attributedText {
-            Text(attrText)
-                .font(VFont.bodySmallDefault)
-                .frame(maxWidth: .infinity, alignment: .leading)
-                .fixedSize(horizontal: false, vertical: true)
+            HStack(spacing: 0) {
+                Text(attrText)
+                    .font(VFont.bodySmallDefault)
+                    .fixedSize(horizontal: false, vertical: true)
+                Spacer(minLength: 0)
+            }
         } else if let plainText = text {
-            Text(plainText)
-                .font(VFont.bodySmallDefault)
-                .foregroundStyle(isError ? VColor.systemNegativeStrong : VColor.contentSecondary)
-                .frame(maxWidth: .infinity, alignment: .leading)
-                .fixedSize(horizontal: false, vertical: true)
+            HStack(spacing: 0) {
+                Text(plainText)
+                    .font(VFont.bodySmallDefault)
+                    .foregroundStyle(isError ? VColor.systemNegativeStrong : VColor.contentSecondary)
+                    .fixedSize(horizontal: false, vertical: true)
+                Spacer(minLength: 0)
+            }
         }
     }
 
diff --git a/clients/macos/vellum-assistant/Features/Chat/ChatBubble.swift b/clients/macos/vellum-assistant/Features/Chat/ChatBubble.swift
index 6378ac2de3f..e9dff3f7480 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ChatBubble.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ChatBubble.swift
@@ -266,22 +266,21 @@ struct ChatBubble: View, Equatable {
     func bubbleChrome<Content: View>(@ViewBuilder _ content: () -> Content) -> some View {
         let isPlainAssistant = !isUser && !message.isError
         if message.isError {
-            // Error: chrome padding + full-width inner expansion frame.
+            // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+            // .containerRelativeFrame resolves against the ScrollView for full-width error background.
             content()
                 .padding(EdgeInsets(top: VSpacing.md, leading: VSpacing.lg,
                                     bottom: VSpacing.md, trailing: VSpacing.lg))
-                .frame(maxWidth: .infinity)
+                .containerRelativeFrame(.horizontal)
                 .background {
                     bubbleChromeBackground
                 }
-                .frame(maxWidth: .infinity, alignment: isUser ? .trailing : .leading)
         } else if isPlainAssistant {
             // Plain assistant: no chrome padding, no inner frame.
             content()
                 .background {
                     bubbleChromeBackground
                 }
-                .frame(maxWidth: bubbleMaxWidth, alignment: .leading)
         } else {
             // User messages (non-error): chrome padding, no inner frame.
             content()
@@ -290,7 +289,6 @@ struct ChatBubble: View, Equatable {
                 .background {
                     bubbleChromeBackground
                 }
-                .frame(maxWidth: bubbleMaxWidth, alignment: .trailing)
         }
     }
 
@@ -341,12 +339,15 @@ struct ChatBubble: View, Equatable {
         let _ = os_signpost(.event, log: PerfSignposts.log, name: "chatBubbleBody",
                             "id=%{public}s streaming=%d", message.id.uuidString, message.isStreaming ? 1 : 0)
         #endif
-        // Outer VStack ensures a single resolved subview for the parent
-        // LazyVStack, avoiding duplicate .id(message.id) from MessageCellView
-        // that caused incorrect width proposals at narrow window sizes (LUM-688).
-        // The avatar sits outside the inner .compositingGroup() scope so
-        // CAShapeLayer animations (breathing, blink, twitch) are unaffected.
-        VStack(alignment: isUser ? .trailing : .leading, spacing: VSpacing.sm) {
+        // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+        HStack(spacing: 0) {
+            if isUser { Spacer(minLength: 0) }
+            // Outer VStack ensures a single resolved subview for the parent
+            // LazyVStack, avoiding duplicate .id(message.id) from MessageCellView
+            // that caused incorrect width proposals at narrow window sizes (LUM-688).
+            // The avatar sits outside the inner .compositingGroup() scope so
+            // CAShapeLayer animations (breathing, blink, twitch) are unaffected.
+            VStack(alignment: isUser ? .trailing : .leading, spacing: VSpacing.sm) {
             // --- Message content (composited) ---
             VStack(alignment: isUser ? .trailing : .leading, spacing: VSpacing.sm) {
                 if !isUser && cachedHasInterleavedContent {
@@ -425,8 +426,9 @@ struct ChatBubble: View, Equatable {
             if isLatestAssistantMessage && !isUser && !hideInlineAvatar {
                 inlineAvatar
             }
+            }
+            if !isUser { Spacer(minLength: 0) }
         }
-        .frame(maxWidth: .infinity, alignment: isUser ? .trailing : .leading)
         .contentShape(Rectangle())
         .onChange(of: message.contentOrder) { _, _ in recomputeInterleavedContentCache() }
         .onChange(of: message.textSegments) { _, _ in recomputeInterleavedContentCache() }
diff --git a/clients/macos/vellum-assistant/Features/Chat/ChatBubbleInterleavedContent.swift b/clients/macos/vellum-assistant/Features/Chat/ChatBubbleInterleavedContent.swift
index 14cee512dc7..10f4a436bc2 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ChatBubbleInterleavedContent.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ChatBubbleInterleavedContent.swift
@@ -326,25 +326,27 @@ extension ChatBubble {
                 return result
             }()
 
-            AssistantProgressView(
-                toolCalls: groupedToolCalls,
-                isStreaming: isLatestGroup ? message.isStreaming : false,
-                hasText: hasTrailingText,
-                isProcessing: isLatestGroup && isProcessingAfterTools,
-                processingStatusText: isLatestGroup && isProcessingAfterTools ? processingStatusText : nil,
-                streamingCodePreview: isLatestGroup ? message.streamingCodePreview : nil,
-                streamingCodeToolName: isLatestGroup ? message.streamingCodeToolName : nil,
-                decidedConfirmations: groupConfirmations,
-                onRehydrate: onRehydrate,
-                onConfirmationAllow: onConfirmationAllow,
-                onConfirmationDeny: onConfirmationDeny,
-                onAlwaysAllow: onAlwaysAllow,
-                onTemporaryAllow: onTemporaryAllow,
-                activeConfirmationRequestId: activeConfirmationRequestId,
-                progressUIState: $progressUIState
-            )
-            .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
-
+            // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+            HStack(spacing: 0) {
+                AssistantProgressView(
+                    toolCalls: groupedToolCalls,
+                    isStreaming: isLatestGroup ? message.isStreaming : false,
+                    hasText: hasTrailingText,
+                    isProcessing: isLatestGroup && isProcessingAfterTools,
+                    processingStatusText: isLatestGroup && isProcessingAfterTools ? processingStatusText : nil,
+                    streamingCodePreview: isLatestGroup ? message.streamingCodePreview : nil,
+                    streamingCodeToolName: isLatestGroup ? message.streamingCodeToolName : nil,
+                    decidedConfirmations: groupConfirmations,
+                    onRehydrate: onRehydrate,
+                    onConfirmationAllow: onConfirmationAllow,
+                    onConfirmationDeny: onConfirmationDeny,
+                    onAlwaysAllow: onAlwaysAllow,
+                    onTemporaryAllow: onTemporaryAllow,
+                    activeConfirmationRequestId: activeConfirmationRequestId,
+                    progressUIState: $progressUIState
+                )
+                Spacer(minLength: 0)
+            }
         }
     }
 
diff --git a/clients/macos/vellum-assistant/Features/Chat/ChatBubbleToolStatusView.swift b/clients/macos/vellum-assistant/Features/Chat/ChatBubbleToolStatusView.swift
index 216b7e2c770..418dc0625c1 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ChatBubbleToolStatusView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ChatBubbleToolStatusView.swift
@@ -35,35 +35,39 @@ extension ChatBubble {
 
         if hasToolCalls || hasStreamingCode || shouldShowProcessing {
             // Unified progress view handles all tool/streaming/processing states
-            AssistantProgressView(
-                toolCalls: message.toolCalls,
-                isStreaming: message.isStreaming,
-                hasText: hasText,
-                isProcessing: shouldShowProcessing,
-                processingStatusText: shouldShowProcessing ? processingStatusText : nil,
-                streamingCodePreview: message.streamingCodePreview,
-                streamingCodeToolName: message.streamingCodeToolName,
-                decidedConfirmations: effectiveConfirmations,
-                onRehydrate: onRehydrate,
-                onConfirmationAllow: onConfirmationAllow,
-                onConfirmationDeny: onConfirmationDeny,
-                onAlwaysAllow: onAlwaysAllow,
-                onTemporaryAllow: onTemporaryAllow,
-                activeConfirmationRequestId: activeConfirmationRequestId,
-                progressUIState: $progressUIState
-            )
-            .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
+            // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+            HStack(spacing: 0) {
+                AssistantProgressView(
+                    toolCalls: message.toolCalls,
+                    isStreaming: message.isStreaming,
+                    hasText: hasText,
+                    isProcessing: shouldShowProcessing,
+                    processingStatusText: shouldShowProcessing ? processingStatusText : nil,
+                    streamingCodePreview: message.streamingCodePreview,
+                    streamingCodeToolName: message.streamingCodeToolName,
+                    decidedConfirmations: effectiveConfirmations,
+                    onRehydrate: onRehydrate,
+                    onConfirmationAllow: onConfirmationAllow,
+                    onConfirmationDeny: onConfirmationDeny,
+                    onAlwaysAllow: onAlwaysAllow,
+                    onTemporaryAllow: onTemporaryAllow,
+                    activeConfirmationRequestId: activeConfirmationRequestId,
+                    progressUIState: $progressUIState
+                )
+                Spacer(minLength: 0)
+            }
 
             // Inline image previews from completed tool calls (e.g. image generation)
             inlineToolCallImages(from: message.toolCalls)
         } else if !effectiveConfirmations.isEmpty, !inlineToolProgressRenderedInContent {
             // No tool display needed — only show permission chips.
+            // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
             HStack(alignment: .center, spacing: VSpacing.sm) {
                 ForEach(Array(effectiveConfirmations.enumerated()), id: \.offset) { _, confirmation in
                     compactPermissionChip(confirmation)
                 }
+                Spacer(minLength: 0)
             }
-            .frame(maxWidth: .infinity, alignment: .leading)
             .padding(.top, VSpacing.xxs)
         }
     }
diff --git a/clients/macos/vellum-assistant/Features/Chat/ChatMarkdownParser.swift b/clients/macos/vellum-assistant/Features/Chat/ChatMarkdownParser.swift
index 4d3a8fa3e05..71401f51f63 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ChatMarkdownParser.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ChatMarkdownParser.swift
@@ -355,13 +355,14 @@ struct MarkdownTableView: View {
             // Header row
             HStack(spacing: 0) {
                 ForEach(Array(headers.enumerated()), id: \.offset) { _, header in
-                    Text(header)
-                        .font(VFont.labelDefault)
-                        .foregroundStyle(VColor.contentSecondary)
-                        .textSelection(.enabled)
-                        .frame(maxWidth: .infinity, alignment: .leading)
-                        .padding(.horizontal, VSpacing.sm)
-                        .padding(.vertical, VSpacing.sm)
+                    HStack(spacing: 0) {
+                        Text(header)
+                            .font(VFont.labelDefault)
+                            .foregroundStyle(VColor.contentSecondary)
+                            .textSelection(.enabled)
+                        Spacer(minLength: 0)
+                    }
+                    .padding(VSpacing.sm)
                 }
             }
 
@@ -371,10 +372,11 @@ struct MarkdownTableView: View {
             ForEach(Array(rows.enumerated()), id: \.offset) { rowIdx, row in
                 HStack(spacing: 0) {
                     ForEach(Array(row.enumerated()), id: \.offset) { _, cell in
-                        inlineMarkdownCell(cell)
-                            .frame(maxWidth: .infinity, alignment: .leading)
-                            .padding(.horizontal, VSpacing.sm)
-                            .padding(.vertical, VSpacing.sm)
+                        HStack(spacing: 0) {
+                            inlineMarkdownCell(cell)
+                            Spacer(minLength: 0)
+                        }
+                        .padding(VSpacing.sm)
                     }
                 }
                 if rowIdx < rows.count - 1 {
@@ -388,7 +390,8 @@ struct MarkdownTableView: View {
             RoundedRectangle(cornerRadius: VRadius.md)
                 .stroke(VColor.borderBase, lineWidth: 0.5)
         )
-        .frame(maxWidth: maxWidth, alignment: .leading)
+        // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+        .frame(width: maxWidth.isFinite ? maxWidth : nil, alignment: .leading)
     }
 
     private func inlineMarkdownCell(_ text: String) -> some View {
diff --git a/clients/macos/vellum-assistant/Features/Chat/MarkdownSegmentView.swift b/clients/macos/vellum-assistant/Features/Chat/MarkdownSegmentView.swift
index 98c62bb9dcf..ee6850108a6 100644
--- a/clients/macos/vellum-assistant/Features/Chat/MarkdownSegmentView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/MarkdownSegmentView.swift
@@ -71,14 +71,17 @@ struct MarkdownSegmentView: View, Equatable {
                     )
                     #else
                     let attributed = buildCombinedAttributedString(from: runSegments)
-                    Text(attributed)
-                        .font(chatFont)
-                        .lineSpacing(4)
-                        .foregroundStyle(textColor)
-                        .tint(tintColor)
-                        .optionalMaxWidth(maxContentWidth)
-                        .lineLimit(nil)
-                        .fixedSize(horizontal: false, vertical: true)
+                    // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+                    HStack(spacing: 0) {
+                        Text(attributed)
+                            .font(chatFont)
+                            .lineSpacing(4)
+                            .foregroundStyle(textColor)
+                            .tint(tintColor)
+                            .lineLimit(nil)
+                            .fixedSize(horizontal: false, vertical: true)
+                        Spacer(minLength: 0)
+                    }
                     #endif
 
                 case .codeBlock(let language, let code):
@@ -904,12 +907,13 @@ private extension AttributedString {
 // MARK: - Optional Max Width
 
 private extension View {
-    /// Applies `.frame(maxWidth:alignment:)` only when a width is provided.
+    /// Applies a definite `.frame(width:)` only when a width is provided.
     /// When `nil`, no frame is applied — the view shrink-wraps to its content.
+    /// ⚠️ No `.frame(maxWidth:)` — see AGENTS.md.
     @ViewBuilder
     func optionalMaxWidth(_ width: CGFloat?) -> some View {
         if let width {
-            self.frame(maxWidth: width, alignment: .leading)
+            self.frame(width: width, alignment: .leading)
         } else {
             self
         }
diff --git a/clients/macos/vellum-assistant/Features/Chat/MessageCellView.swift b/clients/macos/vellum-assistant/Features/Chat/MessageCellView.swift
index 7cf2bba0ada..5781700e5ff 100644
--- a/clients/macos/vellum-assistant/Features/Chat/MessageCellView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/MessageCellView.swift
@@ -138,11 +138,14 @@ struct MessageCellView: View, Equatable {
 
     @ViewBuilder
     private func thinkingIndicatorRow() -> some View {
-        RunningIndicator(
-            label: anchoredThinkingLabel,
-            showIcon: false
-        )
-        .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
+        // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+        HStack(spacing: 0) {
+            RunningIndicator(
+                label: anchoredThinkingLabel,
+                showIcon: false
+            )
+            Spacer(minLength: 0)
+        }
         .id("thinking-indicator")
     }
 
@@ -231,14 +234,16 @@ struct MessageCellView: View, Equatable {
         }
 
         ForEach(subagentsByParent[message.id] ?? []) { subagent in
-            SubagentEventsReader(
-                store: subagentDetailStore,
-                subagent: subagent,
-                onAbort: { onAbortSubagent?(subagent.id) },
-                onTap: { onSubagentTap?(subagent.id) }
-            )
-                .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
-                .id("subagent-\(subagent.id)")
+            HStack(spacing: 0) {
+                SubagentEventsReader(
+                    store: subagentDetailStore,
+                    subagent: subagent,
+                    onAbort: { onAbortSubagent?(subagent.id) },
+                    onTap: { onSubagentTap?(subagent.id) }
+                )
+                Spacer(minLength: 0)
+            }
+            .id("subagent-\(subagent.id)")
         }
 
         if showAnchoredThinkingIndicator {
diff --git a/clients/macos/vellum-assistant/Features/Chat/MessageListContentView.swift b/clients/macos/vellum-assistant/Features/Chat/MessageListContentView.swift
index 8938680c70f..b7a3583fcde 100644
--- a/clients/macos/vellum-assistant/Features/Chat/MessageListContentView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/MessageListContentView.swift
@@ -107,7 +107,6 @@ struct MessageListContentView: View, Equatable {
             }
             Spacer()
         }
-        .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
         .id("thinking-indicator")
         .transition(.opacity)
     }
@@ -118,7 +117,6 @@ struct MessageListContentView: View, Equatable {
             label: "Compacting context\u{2026}",
             showIcon: false
         )
-        .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
         .id("compacting-indicator")
         .transition(.opacity)
     }
@@ -225,13 +223,16 @@ struct MessageListContentView: View, Equatable {
             }
 
             ForEach(state.orphanSubagents) { subagent in
-                SubagentEventsReader(
-                    store: subagentDetailStore,
-                    subagent: subagent,
-                    onAbort: { onAbortSubagent?(subagent.id) },
-                    onTap: { onSubagentTap?(subagent.id) }
-                )
-                    .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
+                // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
+                HStack(spacing: 0) {
+                    SubagentEventsReader(
+                        store: subagentDetailStore,
+                        subagent: subagent,
+                        onAbort: { onAbortSubagent?(subagent.id) },
+                        onTap: { onSubagentTap?(subagent.id) }
+                    )
+                    Spacer(minLength: 0)
+                }
                     .id("subagent-\(subagent.id)")
                     .transition(.opacity)
             }
@@ -248,7 +249,6 @@ struct MessageListContentView: View, Equatable {
                     TypingIndicatorView()
                     Spacer()
                 }
-                .frame(maxWidth: VSpacing.chatBubbleMaxWidth, alignment: .leading)
                 .id("streaming-without-text-indicator")
                 .transition(.opacity)
             } else if isCompacting && !state.shouldShowThinkingIndicator && !state.canInlineProcessing {
diff --git a/clients/macos/vellum-assistant/Features/Chat/MessageListView.swift b/clients/macos/vellum-assistant/Features/Chat/MessageListView.swift
index 76ac331419a..8d91dd0cdcb 100644
--- a/clients/macos/vellum-assistant/Features/Chat/MessageListView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/MessageListView.swift
@@ -111,12 +111,17 @@ struct MessageListView: View {
         let _ = os_signpost(.event, log: PerfSignposts.log, name: "MessageListView.body")
         #endif
             ScrollView {
-                scrollViewContent
-                    .background(
-                        MessageListScrollObserver { newState in
-                            enqueueScrollGeometryUpdate(newState)
-                        }
-                    )
+                // AlignmentBarrierLayout stops explicitAlignment queries from
+                // cascading into the LazyVStack — see AGENTS.md and
+                // AlignmentBarrierLayout.swift for details.
+                AlignmentBarrierLayout {
+                    scrollViewContent
+                }
+                .background(
+                    MessageListScrollObserver { newState in
+                        enqueueScrollGeometryUpdate(newState)
+                    }
+                )
             }
             .id(conversationId)
             .scrollContentBackground(.hidden)
diff --git a/clients/macos/vellum-assistant/Features/Chat/ThinkingBlockView.swift b/clients/macos/vellum-assistant/Features/Chat/ThinkingBlockView.swift
index 8ec7dadd118..9aa145fd2f9 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ThinkingBlockView.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ThinkingBlockView.swift
@@ -37,6 +37,7 @@ struct ThinkingBlockView: View {
                 Divider()
                     .padding(.horizontal, VSpacing.sm)
 
+                // ⚠️ No .frame(maxWidth:) in LazyVStack cells — see AGENTS.md.
                 MarkdownSegmentView(
                     segments: cachedSegments,
                     isStreaming: isStreaming,
@@ -49,7 +50,6 @@ struct ThinkingBlockView: View {
                     codeTextColor: VColor.contentDefault,
                     codeBackgroundColor: VColor.surfaceBase
                 )
-                .frame(maxWidth: .infinity, alignment: .leading)
                 .padding(VSpacing.sm)
                 .transition(.opacity)
             }
diff --git a/clients/shared/DesignSystem/Layout/AlignmentBarrierLayout.swift b/clients/shared/DesignSystem/Layout/AlignmentBarrierLayout.swift
new file mode 100644
index 00000000000..115e813ee94
--- /dev/null
+++ b/clients/shared/DesignSystem/Layout/AlignmentBarrierLayout.swift
@@ -0,0 +1,51 @@
+import SwiftUI
+
+/// Blocks [`explicitAlignment`](https://developer.apple.com/documentation/swiftui/layout/explicitalignment(of:in:proposal:subviews:cache:)-3iqmu)
+/// queries from cascading into its subtree. Returns `nil` for both horizontal
+/// and vertical alignment guides while passing sizing and placement through unchanged.
+///
+/// Place between any `.frame(maxWidth:, alignment:)` and a `LazyVStack` to
+/// prevent O(depth × children) recursive alignment measurement.
+///
+/// - SeeAlso: [WWDC23 – Demystify SwiftUI performance](https://developer.apple.com/videos/play/wwdc2023/10160/)
+/// - SeeAlso: [`ViewDimensions`](https://developer.apple.com/documentation/swiftui/viewdimensions)
+public struct AlignmentBarrierLayout: Layout {
+    public init() {}
+
+    public func sizeThatFits(
+        proposal: ProposedViewSize,
+        subviews: Subviews,
+        cache: inout ()
+    ) -> CGSize {
+        subviews.first?.sizeThatFits(proposal) ?? .zero
+    }
+
+    public func placeSubviews(
+        in bounds: CGRect,
+        proposal: ProposedViewSize,
+        subviews: Subviews,
+        cache: inout ()
+    ) {
+        subviews.first?.place(at: bounds.origin, proposal: proposal)
+    }
+
+    public func explicitAlignment(
+        of guide: HorizontalAlignment,
+        in bounds: CGRect,
+        proposal: ProposedViewSize,
+        subviews: Subviews,
+        cache: inout ()
+    ) -> CGFloat? {
+        nil
+    }
+
+    public func explicitAlignment(
+        of guide: VerticalAlignment,
+        in bounds: CGRect,
+        proposal: ProposedViewSize,
+        subviews: Subviews,
+        cache: inout ()
+    ) -> CGFloat? {
+        nil
+    }
+}