diff --git a/packages/dashboard/app/components/MailboxMessageContent.tsx b/packages/dashboard/app/components/MailboxMessageContent.tsx
index 61b1d865a5..3316727f3a 100644
--- a/packages/dashboard/app/components/MailboxMessageContent.tsx
+++ b/packages/dashboard/app/components/MailboxMessageContent.tsx
@@ -1,52 +1,19 @@
 import { memo } from "react";
 import ReactMarkdown from "react-markdown";
 import remarkGfm from "remark-gfm";
-import rehypeRaw from "rehype-raw";
-import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
-import type { Options as SanitizeSchema } from "rehype-sanitize";
 import type { Components } from "react-markdown";
 import type { PluggableList } from "unified";
 import { linkifyReactChildren } from "../utils/filePathLinkify";
-import { MermaidDiagram } from "./MermaidDiagram";
+import { sharedRehypePlugins, createMermaidCodeComponent } from "./markdownPipeline";
 
 /*
-FNXC:Markdown 2026-06-23-03:15:
-GitHub PR/issue bodies + comments (and mailbox/chat) embed raw HTML (`<details>`,
-`<summary>`, `<kbd>`, `<sub>`, tables), HTML comments (`<!-- -->`), and ```mermaid
-blocks. Previously raw HTML was escaped to literal text and mermaid showed as code.
-
-Pipeline (ORDER MATTERS): remark-gfm -> rehype-raw -> rehype-sanitize.
-- rehype-raw parses embedded HTML into the hast tree so it renders as real elements.
-  It also DROPS HTML comments by default, so `<!-- ... -->` never appears in output.
-- rehype-sanitize runs AFTER raw to strip XSS: <script>/<style>/<iframe>, event
-  handlers (onClick etc.), and javascript: URLs. Because these bodies come from
-  GitHub (untrusted), sanitize is mandatory — raw without sanitize would be an XSS
-  hole. Running sanitize last guarantees nothing injected via raw survives.
+FNXC:Markdown 2026-06-23-03:30:
+The sanitize schema, rehype plugin chain (rehype-raw -> rehype-sanitize), and the
+mermaid-aware code component now live in ./markdownPipeline so the task
+description + summary in TaskDetailModal share the exact same XSS posture. This
+component consumes those shared exports; see markdownPipeline.tsx for the rationale.
 */
 
-/*
-FNXC:Markdown 2026-06-23-03:15:
-Sanitize schema = rehype-sanitize defaultSchema (a conservative GitHub-like allow
-list that already permits details/summary/kbd/sub/sup/b/i/em/strong/a/img/code/pre/
-tables/br/hr/blockquote/lists/headings/span/div and strips script/style/event
-handlers/javascript: URLs) EXTENDED to ensure the `className` attribute survives on
-common elements (needed for our `language-*` code fences and styled wrappers). We do
-NOT widen tagNames beyond defaults, so script/style/iframe stay stripped.
-*/
-const mailboxSanitizeSchema: SanitizeSchema = {
-  ...defaultSchema,
-  attributes: {
-    ...defaultSchema.attributes,
-    // Preserve className on code/span/div/pre so language fences + wrapper styling work.
-    code: [...(defaultSchema.attributes?.code ?? []), "className"],
-    span: [...(defaultSchema.attributes?.span ?? []), "className"],
-    div: [...(defaultSchema.attributes?.div ?? []), "className"],
-    pre: [...(defaultSchema.attributes?.pre ?? []), "className"],
-    // `<details open>` disclosure state should round-trip.
-    details: [...(defaultSchema.attributes?.details ?? []), "open"],
-  },
-};
-
 const mailboxMarkdownComponents: Components = {
   p: ({ children, ...props }) => <p {...props}>{linkifyReactChildren(children)}</p>,
   li: ({ children, ...props }) => <li {...props}>{linkifyReactChildren(children)}</li>,
@@ -60,24 +27,9 @@ const mailboxMarkdownComponents: Components = {
       {children}
     </table>
   ),
-  /*
-  FNXC:Markdown 2026-06-23-03:15:
-  Code-block override: a fenced ```mermaid block arrives as `<code class="language-mermaid">`.
-  Render it via <MermaidDiagram>, which lazy-imports mermaid so the heavy library is
-  only pulled in when a diagram is present. All other code (inline + other languages)
-  keeps the default rendering.
-  */
-  code: ({ className, children, ...props }) => {
-    if (className === "language-mermaid") {
-      const chart = String(children ?? "").replace(/\n$/, "");
-      return <MermaidDiagram chart={chart} testId="mailbox-mermaid-diagram" />;
-    }
-    return (
-      <code className={className} {...props}>
-        {children}
-      </code>
-    );
-  },
+  // Code-block override: fenced ```mermaid renders as a diagram; all other code
+  // keeps default rendering. See createMermaidCodeComponent in markdownPipeline.
+  code: createMermaidCodeComponent("mailbox-mermaid-diagram"),
   // Open links in a new tab. Sanitize strips javascript: URLs and event handlers
   // before this runs, so href is safe.
   a: ({ children, ...props }) => (
@@ -88,8 +40,6 @@ const mailboxMarkdownComponents: Components = {
 };
 
 const remarkPlugins: PluggableList = [remarkGfm];
-// Raw must run before sanitize: parse HTML, then strip anything unsafe.
-const rehypePlugins: PluggableList = [rehypeRaw, [rehypeSanitize, mailboxSanitizeSchema]];
 
 interface MailboxMessageContentProps {
   /** Raw message body. Rendered as GitHub-flavored markdown. */
@@ -123,7 +73,7 @@ export const MailboxMessageContent = memo(function MailboxMessageContent({
     <div className={wrapperClass} data-testid={testId}>
       <ReactMarkdown
         remarkPlugins={remarkPlugins}
-        rehypePlugins={rehypePlugins}
+        rehypePlugins={sharedRehypePlugins}
         components={mailboxMarkdownComponents}
       >
         {content}
diff --git a/packages/dashboard/app/components/TaskDetailModal.tsx b/packages/dashboard/app/components/TaskDetailModal.tsx
index b1e9be9e67..d1b47aa5fc 100644
--- a/packages/dashboard/app/components/TaskDetailModal.tsx
+++ b/packages/dashboard/app/components/TaskDetailModal.tsx
@@ -9,6 +9,7 @@ import { useColumnLabel } from "../i18n/labels";
 import ReactMarkdown from "react-markdown";
 import type { Components } from "react-markdown";
 import remarkGfm from "remark-gfm";
+import { sharedRehypePlugins, createMermaidCodeComponent } from "./markdownPipeline";
 import type { Task, TaskDetail, TaskAttachment, Column, ColumnId, MergeResult, Settings, GlobalSettings, AgentLogEntry, Agent, TaskPriority, TaskSourceIssue, WorkflowStepResult, GithubIssueAction } from "@fusion/core";
 import {
   DEFAULT_TASK_PRIORITY,
@@ -79,17 +80,30 @@ function isStringValue(value: unknown): value is string {
   return Object.prototype.toString.call(value) === STRING_OBJECT_TAG;
 }
 
+/*
+FNXC:Markdown 2026-06-23-03:30:
+The task DESCRIPTION (spec/prompt) + SUMMARY render via these components plus the
+shared rehype chain (sharedRehypePlugins) so they gain sanitized raw HTML
+(`<details>`/tables/`<kbd>`), drop HTML comments, and render ```mermaid diagrams —
+matching the shared markdown renderer. They KEEP their `.markdown-body` styling
+(NOT the `.mailbox-markdown` wrapper), so the look is unchanged for normal markdown.
+The file-path linkify `code` renderer is preserved as the fallback for non-mermaid
+code, so links AND html AND mermaid all work together.
+*/
+const markdownLinkifyCodeComponent: NonNullable<Components["code"]> = ({ children, ...props }) => {
+  const text = React.Children.toArray(children).join(EMPTY_MARKDOWN_CHILD_SEPARATOR);
+  const linkedChildren = linkifyFilePaths(text);
+  if (linkedChildren.length === 1 && linkedChildren[0]?.constructor === String) {
+    return <code {...props}>{children}</code>;
+  }
+  return <code {...props}>{linkedChildren}</code>;
+};
+
 const markdownLinkifyComponents: Components = {
   p: ({ children, ...props }) => <p {...props}>{linkifyReactChildren(children)}</p>,
   li: ({ children, ...props }) => <li {...props}>{linkifyReactChildren(children)}</li>,
-  code: ({ children, ...props }) => {
-    const text = React.Children.toArray(children).join(EMPTY_MARKDOWN_CHILD_SEPARATOR);
-    const linkedChildren = linkifyFilePaths(text);
-    if (linkedChildren.length === 1 && linkedChildren[0]?.constructor === String) {
-      return <code {...props}>{children}</code>;
-    }
-    return <code {...props}>{linkedChildren}</code>;
-  },
+  // Mermaid fences render as diagrams; all other code falls through to file-path linkify.
+  code: createMermaidCodeComponent("task-detail-mermaid-diagram", markdownLinkifyCodeComponent),
 };
 
 /**
@@ -3565,7 +3579,7 @@ export function TaskDetailContent({
             <div className="detail-section detail-summary">
               <h4>{t("taskDetail.summary.heading", "Summary")}</h4>
               <div className="markdown-body">
-                <ReactMarkdown remarkPlugins={[remarkGfm]} components={markdownLinkifyComponents}>
+                <ReactMarkdown remarkPlugins={[remarkGfm]} rehypePlugins={sharedRehypePlugins} components={markdownLinkifyComponents}>
                   {task.summary}
                 </ReactMarkdown>
               </div>
@@ -3833,7 +3847,7 @@ export function TaskDetailContent({
               <div className="spec-loading"><LoadingSpinner label={t("taskDetail.spec.loading", "Loading specification…")} /></div>
             ) : workingTask.prompt ? (
               <div className="markdown-body">
-                <ReactMarkdown remarkPlugins={[remarkGfm]} components={markdownLinkifyComponents}>
+                <ReactMarkdown remarkPlugins={[remarkGfm]} rehypePlugins={sharedRehypePlugins} components={markdownLinkifyComponents}>
                   {workingTask.prompt.replace(/^#\s+[^\n]*\n+/, "")}
                 </ReactMarkdown>
               </div>
diff --git a/packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx b/packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx
index 23dde5df20..18da32a49e 100644
--- a/packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx
+++ b/packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx
@@ -4,6 +4,15 @@ FN-6532 made Chat the default TaskDetailModal tab. Tests that assert Definition-
 */
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { render, screen, fireEvent, act, waitFor } from "@testing-library/react";
+
+// FNXC:Markdown 2026-06-23-03:30: Mock the heavy `mermaid` library so the shared
+// markdown pipeline's MermaidDiagram resolves without loading the real renderer.
+vi.mock("mermaid", () => ({
+  default: {
+    initialize: vi.fn(),
+    render: vi.fn().mockResolvedValue({ svg: "<svg data-testid='mock-mermaid-svg'></svg>" }),
+  },
+}));
 import userEvent from "@testing-library/user-event";
 import {
   makeTask,
@@ -57,6 +66,61 @@ describe("TaskDetailModal", () => {
     expect(openFile).toHaveBeenCalledWith("packages/dashboard/app/App.tsx", { line: 12, col: undefined });
   });
 
+  /*
+  FNXC:Markdown 2026-06-23-03:30:
+  The task DESCRIPTION (spec/prompt) + SUMMARY now share the markdown pipeline's
+  rehype-raw -> rehype-sanitize chain, so embedded raw HTML renders as real
+  elements (not literal text), HTML comments drop, <script> is stripped, and
+  ```mermaid fences render diagrams — while keeping `.markdown-body` styling.
+  */
+  it("renders raw HTML and mermaid in the description while stripping unsafe content", async () => {
+    const prompt = [
+      "# Prompt",
+      "",
+      "<details><summary>Disclosure title</summary>Hidden detail body.</details>",
+      "",
+      "<!-- secret comment -->",
+      "",
+      "<script>window.__pwned = true;</script>",
+      "",
+      "```mermaid",
+      "graph TD; A-->B;",
+      "```",
+    ].join("\n");
+
+    const { container } = render(
+      <FileBrowserProvider openFile={vi.fn()}>
+        <TaskDetailModal
+          initialTab="definition"
+          task={makeTask({ prompt })}
+          onClose={noop}
+          onMoveTask={noopMove}
+          onDeleteTask={noopDelete}
+          onMergeTask={noopMerge}
+          onOpenDetail={noopOpenDetail}
+          addToast={noop}
+        />
+      </FileBrowserProvider>,
+    );
+
+    // Raw <details>/<summary> renders as a real disclosure element.
+    const details = container.querySelector(".markdown-body details");
+    expect(details).not.toBeNull();
+    expect(details?.querySelector("summary")?.textContent).toBe("Disclosure title");
+    expect(details?.textContent).toContain("Hidden detail body.");
+
+    // HTML comment is dropped, never shown as literal text.
+    expect(container.textContent).not.toContain("secret comment");
+
+    // <script> is stripped by sanitize: not rendered and never executed.
+    expect(container.querySelector("script")).toBeNull();
+    expect((window as unknown as { __pwned?: boolean }).__pwned).toBeUndefined();
+
+    // ```mermaid fence renders the diagram container (lazy MermaidDiagram).
+    const diagram = await screen.findByTestId("task-detail-mermaid-diagram");
+    expect(diagram).not.toBeNull();
+  });
+
   describe("provenance display", () => {
     it.each([
       ["dashboard_ui", undefined, "Created via Dashboard"],
diff --git a/packages/dashboard/app/components/markdownPipeline.tsx b/packages/dashboard/app/components/markdownPipeline.tsx
new file mode 100644
index 0000000000..ef46591fdd
--- /dev/null
+++ b/packages/dashboard/app/components/markdownPipeline.tsx
@@ -0,0 +1,95 @@
+import type { ReactElement } from "react";
+import rehypeRaw from "rehype-raw";
+import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
+import type { Options as SanitizeSchema } from "rehype-sanitize";
+import type { Components } from "react-markdown";
+import type { PluggableList } from "unified";
+import { MermaidDiagram } from "./MermaidDiagram";
+
+/*
+FNXC:Markdown 2026-06-23-03:30:
+Shared markdown rendering pipeline. Both mailbox/chat bodies and the task
+DESCRIPTION (spec/prompt) + SUMMARY in TaskDetailModal need to render embedded
+raw HTML (`<details>`, `<summary>`, `<kbd>`, `<sub>`, tables), drop HTML comments
+(`<!-- -->`), and render ```mermaid blocks as diagrams. The sanitize schema +
+rehype plugin chain are defined ONCE here so every renderer shares the exact same
+XSS posture instead of duplicating (and drifting on) the allow list.
+
+Pipeline (ORDER MATTERS): remark-gfm (added by the caller) -> rehype-raw -> rehype-sanitize.
+- rehype-raw parses embedded HTML into the hast tree so it renders as real elements.
+  It also DROPS HTML comments by default, so `<!-- ... -->` never appears in output.
+- rehype-sanitize runs AFTER raw to strip XSS: <script>/<style>/<iframe>, event
+  handlers (onClick etc.), and javascript: URLs. Because these bodies can come from
+  GitHub (untrusted), sanitize is mandatory — raw without sanitize would be an XSS
+  hole. Running sanitize last guarantees nothing injected via raw survives.
+*/
+
+/*
+FNXC:Markdown 2026-06-23-03:30:
+Sanitize schema = rehype-sanitize defaultSchema (a conservative GitHub-like allow
+list that already permits details/summary/kbd/sub/sup/b/i/em/strong/a/img/code/pre/
+tables/br/hr/blockquote/lists/headings/span/div and strips script/style/event
+handlers/javascript: URLs) EXTENDED to ensure the `className` attribute survives on
+common elements (needed for our `language-*` code fences and styled wrappers). We do
+NOT widen tagNames beyond defaults, so script/style/iframe stay stripped.
+*/
+export const sharedSanitizeSchema: SanitizeSchema = {
+  ...defaultSchema,
+  attributes: {
+    ...defaultSchema.attributes,
+    // Preserve className on code/span/div/pre so language fences + wrapper styling work.
+    code: [...(defaultSchema.attributes?.code ?? []), "className"],
+    span: [...(defaultSchema.attributes?.span ?? []), "className"],
+    div: [...(defaultSchema.attributes?.div ?? []), "className"],
+    pre: [...(defaultSchema.attributes?.pre ?? []), "className"],
+    // `<details open>` disclosure state should round-trip.
+    details: [...(defaultSchema.attributes?.details ?? []), "open"],
+  },
+};
+
+/**
+ * Shared rehype plugin chain enabling sanitized raw HTML.
+ *
+ * Raw must run before sanitize: parse HTML, then strip anything unsafe.
+ * Pass this as `rehypePlugins` to any ReactMarkdown instance that should render
+ * embedded HTML. The caller supplies `remarkPlugins` (typically `[remarkGfm]`).
+ */
+export const sharedRehypePlugins: PluggableList = [
+  rehypeRaw,
+  [rehypeSanitize, sharedSanitizeSchema],
+];
+
+/**
+ * Factory for a mermaid-aware `code` component.
+ *
+ * FNXC:Markdown 2026-06-23-03:30:
+ * A fenced ```mermaid block arrives as `<code class="language-mermaid">`. Render
+ * it via <MermaidDiagram>, which lazy-imports mermaid so the heavy library is only
+ * pulled in when a diagram is present. All other code (inline + other languages)
+ * falls through to `fallback` (the caller's existing `code` renderer, e.g. file-path
+ * linkify) or default rendering when no fallback is given.
+ *
+ * @param testId data-testid for the rendered diagram (distinct per surface).
+ * @param fallback the caller's `code` component for non-mermaid code.
+ */
+export function createMermaidCodeComponent(
+  testId: string,
+  fallback?: Components["code"],
+): NonNullable<Components["code"]> {
+  return function MermaidAwareCode(props) {
+    const { className, children } = props;
+    if (className === "language-mermaid") {
+      const chart = String(children ?? "").replace(/\n$/, "");
+      return <MermaidDiagram chart={chart} testId={testId} />;
+    }
+    if (fallback) {
+      const Fallback = fallback as (p: typeof props) => ReactElement;
+      return <Fallback {...props} />;
+    }
+    return (
+      <code className={className} {...props}>
+        {children}
+      </code>
+    );
+  };
+}