diff --git a/apps/web/src/components/ChatMarkdown.test.tsx b/apps/web/src/components/ChatMarkdown.test.tsx
index ce2b0f8efa53e8ce957aaafa501e23db8cd42bf1..4e22e68ca902db794625e9ba4bf7cf18d907b626 100644
--- a/apps/web/src/components/ChatMarkdown.test.tsx
+++ b/apps/web/src/components/ChatMarkdown.test.tsx
@@ -694,6 +694,28 @@ describe("ChatMarkdown artifact-template cards", () => {
   });
 });
 
+describe("ChatMarkdown heading levels", () => {
+  it("exposes headings below the host heading without changing their tags", () => {
+    const html = renderToStaticMarkup(
+      <ChatMarkdown
+        cwd="/tmp/project"
+        text={"# Top\n\n## Section\n\n###### Fine print"}
+        headingLevelOffset={3}
+      />,
+    );
+
+    expect(html).toContain('<h1 aria-level="4">Top</h1>');
+    expect(html).toContain('<h2 aria-level="5">Section</h2>');
+    expect(html).toContain('<h6 aria-level="6">Fine print</h6>');
+  });
+
+  it("leaves heading levels alone when the markdown is not nested", () => {
+    const html = renderToStaticMarkup(<ChatMarkdown cwd="/tmp/project" text="# Top" />);
+
+    expect(html).toContain("<h1>Top</h1>");
+  });
+});
+
 describe("shouldUseMarkdownFileBrowserPrimaryAction", () => {
   it("uses the browser when it is the only available primary action", () => {
     expect(
diff --git a/apps/web/src/components/ChatMarkdown.tsx b/apps/web/src/components/ChatMarkdown.tsx
index 4f113f6ed4be14008d271042abc8b13ed690f932..3aeaaa5b8443c3260c320ab8d9e5b8ff4fecb3ba 100644
--- a/apps/web/src/components/ChatMarkdown.tsx
+++ b/apps/web/src/components/ChatMarkdown.tsx
@@ -71,7 +71,11 @@ import React, {
   useState,
   type ReactNode,
 } from "react";
-import type { Components, Options as ReactMarkdownOptions } from "react-markdown";
+import type {
+  Components,
+  ExtraProps as ReactMarkdownExtraProps,
+  Options as ReactMarkdownOptions,
+} from "react-markdown";
 import ReactMarkdown from "react-markdown";
 import { toHtml } from "hast-util-to-html";
 import { createIncrementalMarkdownPlugin } from "../markdown-incremental";
@@ -216,6 +220,10 @@ interface ChatMarkdownProps {
   extraRemarkPlugins?: NonNullable<ReactMarkdownOptions["remarkPlugins"]>;
   /** Renders a `t3-context://` link as a chip; without it the link shows its label as text. */
   renderContextReference?: ((reference: ChatMarkdownContextReference) => ReactNode) | undefined;
+  /** Levels added to each markdown heading in the accessibility tree so the
+      text nests under the heading that introduces it, such as a chat message's
+      author. Rendered tags and their styling are unchanged. */
+  headingLevelOffset?: number | undefined;
 }
 
 export interface ChatMarkdownContextReference {
@@ -2217,6 +2225,7 @@ function useChatMarkdownState({
   imageBaseDir,
   onImageExpand,
   renderContextReference,
+  headingLevelOffset = 0,
 }: ChatMarkdownProps) {
   const { resolvedTheme } = useTheme();
   const [localMediaPreview, setLocalMediaPreview] = useState<ExpandedImagePreview | null>(null);
@@ -2615,6 +2624,7 @@ function useChatMarkdownState({
       expandMedia,
       fileLinkChip,
       renderContextReference,
+      headingLevelOffset,
       imageBaseDir,
       inlineCodeFileLinkMetaByText,
       isStreaming,
@@ -2643,6 +2653,7 @@ function useChatMarkdownState({
       expandMedia,
       fileLinkChip,
       renderContextReference,
+      headingLevelOffset,
       imageBaseDir,
       inlineCodeFileLinkMetaByText,
       isStreaming,
@@ -2679,8 +2690,33 @@ const ChatMarkdownRendererContext = React.createContext<
   ReturnType<typeof useChatMarkdownState>["componentState"]
 >(null!);
 
+// Screen readers take a heading's level from its tag, which would let a `#` in a
+// message outrank the heading placed above it. Override only the exposed level:
+// the tag keeps driving the stylesheet and copy-as-markdown.
+function markdownHeadingRenderer(level: 1 | 2 | 3 | 4 | 5 | 6) {
+  const Tag = `h${level}` as const;
+  return function MarkdownHeading({
+    node: _node,
+    ...props
+  }: ComponentProps<typeof Tag> & ReactMarkdownExtraProps) {
+    const { headingLevelOffset } = use(ChatMarkdownRendererContext);
+    return (
+      <Tag
+        {...props}
+        aria-level={headingLevelOffset > 0 ? Math.min(level + headingLevelOffset, 6) : undefined}
+      />
+    );
+  };
+}
+
 // Keep component types stable when streaming changes the message state.
 const CHAT_MARKDOWN_COMPONENTS = {
+  h1: markdownHeadingRenderer(1),
+  h2: markdownHeadingRenderer(2),
+  h3: markdownHeadingRenderer(3),
+  h4: markdownHeadingRenderer(4),
+  h5: markdownHeadingRenderer(5),
+  h6: markdownHeadingRenderer(6),
   div: function MarkdownDiv({ node, children, ...props }) {
     const { onUseArtifactTemplate } = use(ChatMarkdownRendererContext);
     const artifactTemplate = artifactTemplateFromHastProperties(node?.properties);
diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index d17c03ed32772392bcea25c26ec954d55e1d52f1..568a86721e4c754b1093417ccc15b9a56bb10282 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -1455,6 +1455,16 @@ function UserVideoAttachment({ file }: { readonly file: ChatFileAttachment }) {
   );
 }
 
+// Screen readers skim a transcript by heading, so every message announces its
+// author as one. The thread title in ChatHeader is an <h2>; headings written
+// inside a message are exposed below this level. Visually hidden and excluded
+// from selection so sighted users and copied text are unaffected.
+const MESSAGE_HEADING_LEVEL = 3;
+
+function MessageAuthorHeading({ children }: { children: string }) {
+  return <h3 className="sr-only select-none">{children}</h3>;
+}
+
 function UserTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "message" }> }) {
   const ctx = use(TimelineRowCtx);
   const { onImageExpand, onFileOpen } = ctx;
@@ -1614,6 +1624,7 @@ function UserTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "message"
   return (
     <div className="group flex flex-col items-end gap-1">
       <div className="relative max-w-[80%] rounded-2xl bg-message p-3 text-message-foreground">
+        <MessageAuthorHeading>You</MessageAuthorHeading>
         {(regularImages.length > 0 || userVideos.length > 0) && (
           <div className="mb-2 grid max-w-[210px] grid-cols-2 gap-2">
             {regularImages.map((image) => (
@@ -1845,6 +1856,7 @@ function AssistantTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "mess
   return (
     <>
       <div className="relative min-w-0 px-1 py-0.5">
+        <MessageAuthorHeading>T3 Code</MessageAuthorHeading>
         <AssistantCitationSource
           messageId={row.message.id}
           {...(ctx.threadRef ? { threadRef: ctx.threadRef } : {})}
@@ -1859,6 +1871,7 @@ function AssistantTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "mess
             isStreaming={Boolean(row.message.streaming)}
             lineBreaks={shouldPreserveAssistantLineBreaks(messageText)}
             skills={ctx.skills}
+            headingLevelOffset={MESSAGE_HEADING_LEVEL}
             onUseArtifactTemplate={ctx.onUseArtifactTemplate}
             onImageExpand={ctx.onImageExpand}
           />
@@ -3214,6 +3227,7 @@ const UserMessageBody = memo(function UserMessageBody(props: {
       lineBreaks
       parseRawHtml={false}
       renderContextReference={props.renderContextReference}
+      headingLevelOffset={MESSAGE_HEADING_LEVEL}
     />
   );
 });
diff --git a/apps/web/src/components/chat/ProposedPlanCard.tsx b/apps/web/src/components/chat/ProposedPlanCard.tsx
index 9746857a8cacdccc02c0b9b37a33c7d7a936c447..9fcfaf4d17440188221cc7aa8d3e89175c6eddcb 100644
--- a/apps/web/src/components/chat/ProposedPlanCard.tsx
+++ b/apps/web/src/components/chat/ProposedPlanCard.tsx
@@ -150,7 +150,9 @@ export const ProposedPlanCard = memo(function ProposedPlanCard({
       <div className="flex flex-wrap items-center justify-between gap-3">
         <div className="flex min-w-0 items-center gap-2">
           <Badge variant="secondary">Plan</Badge>
-          <p className="truncate text-sm font-medium text-foreground">{title}</p>
+          {/* Same heading level as the message author headings in the timeline,
+              so a plan's own headings nest beneath it in the outline. */}
+          <h3 className="truncate text-sm font-medium text-foreground">{title}</h3>
         </div>
         <Menu>
           <MenuTrigger
@@ -177,6 +179,7 @@ export const ProposedPlanCard = memo(function ProposedPlanCard({
               cwd={cwd}
               threadRef={threadRef}
               isStreaming={false}
+              headingLevelOffset={3}
             />
           ) : (
             <ChatMarkdown
@@ -184,6 +187,7 @@ export const ProposedPlanCard = memo(function ProposedPlanCard({
               cwd={cwd}
               threadRef={threadRef}
               isStreaming={false}
+              headingLevelOffset={3}
             />
           )}
           {canCollapse && !expanded ? (