diff --git a/e2e/agentMarkdown.spec.ts b/e2e/agentMarkdown.spec.ts
index 06ccd9a54..9d7a1a13d 100644
--- a/e2e/agentMarkdown.spec.ts
+++ b/e2e/agentMarkdown.spec.ts
@@ -340,3 +340,23 @@ test("selects through code blocks and tables while keeping copy controls outside
   await copies.last().click();
   expect(await page.evaluate(() => navigator.clipboard.readText())).toBe(flow);
 });
+
+test("keeps mermaid `<style>` rules scoped to the rendered svg after id rewriting", async ({ page }) => {
+  const diagram = [
+    "```mermaid",
+    "%%{init: {'theme': 'base', 'flowchart': {'htmlLabels': false, 'curve': 'basis'}}}%%",
+    "flowchart TB",
+    '    subgraph T1["Tier 1"]',
+    "        A[(Local)] <--> B[(Peer)]",
+    "    end",
+    '    subgraph T2["Tier 2"]',
+    "        C[(Host RAM)]",
+    "    end",
+    '    C -->|"PCIe"| A',
+    "```",
+  ].join("\n");
+  const reply = await openReply(page, `Diagram:\n\n${diagram}`);
+  await expect(reply.locator("[data-mermaid-state='ok'] .mermaid-output svg")).toBeVisible();
+  expect(await css(reply, ".cluster rect", "fill")).not.toBe("rgb(0, 0, 0)");
+  expect(await css(reply, ".flowchart-link", "fill")).toBe("none");
+});
diff --git a/src/features/agents/Markdown.test.tsx b/src/features/agents/Markdown.test.tsx
index 683213ef2..439077167 100644
--- a/src/features/agents/Markdown.test.tsx
+++ b/src/features/agents/Markdown.test.tsx
@@ -33,8 +33,9 @@ beforeEach(() => {
     document.body.append(tmp);
     return {
       svg:
-        `<svg id="${id}"><defs><marker id="${id}-arrow" orient="auto"><path d="M0,0L8,4"/></marker></defs>` +
-        `<path d="M0,4" marker-end="url(#${id}-arrow)"/><text>${text.split("\n")[0]}</text></svg>`,
+        `<svg id="${id}"><style>#${id} .flowchart-link{fill:none;}#${id} [id$="-arrow"] path{fill:#9fa2a8;}</style>` +
+        `<defs><marker id="${id}-arrow" orient="auto"><path d="M0,0L8,4"/></marker></defs>` +
+        `<path class="flowchart-link" d="M0,4" marker-end="url(#${id}-arrow)"/><text>${text.split("\n")[0]}</text></svg>`,
     };
   });
   mockIPC((cmd, args) => (cmd === "fs_read_image" ? readImage((args as { path: string }).path) : null));
@@ -236,11 +237,15 @@ describe("mermaid diagrams in a reply", () => {
     expect(idsOne).toHaveLength(2);
     expect(idsTwo).toHaveLength(2);
     expect(idsOne.filter((id) => idsTwo.includes(id))).toEqual([]);
-    // Every marker reference resolves inside its own copy.
+    // Every marker reference and scoped `<style>` selector resolves inside its own copy.
     for (const root of [one.container, two.container]) {
       const svg = root.querySelector(".mermaid-output svg")!;
       const ref = svg.querySelector("[marker-end]")!.getAttribute("marker-end")!;
       expect(new Set(idsOf(root)).has(/url\(#(.+)\)/.exec(ref)![1]!)).toBe(true);
+      expect(svg.querySelector("style")!.textContent).toContain(
+        `#${svg.getAttribute("id")} .flowchart-link{fill:none;}`,
+      );
+      expect(svg.querySelector("defs [id$='-arrow'] path")).not.toBeNull();
     }
   });
 
diff --git a/src/features/viewers/markdown/mermaid.test.ts b/src/features/viewers/markdown/mermaid.test.ts
index c2584ff32..77c4f0789 100644
--- a/src/features/viewers/markdown/mermaid.test.ts
+++ b/src/features/viewers/markdown/mermaid.test.ts
@@ -14,8 +14,10 @@ const m = vi.hoisted(() => ({
     if (text.includes("bad")) throw new Error("Parse error on line 2:\nbad\n^");
     return {
       svg:
-        `<svg id="${id}"><defs><marker id="${id}-arrow" orient="auto"><path d="M0,0L8,4"/></marker></defs>` +
-        `<path d="M0,4" marker-end="url(#${id}-arrow)"/><text>${text.split("\n")[0]}</text></svg>`,
+        `<svg id="${id}"><style>#${id}{fill:#dad;}#${id} .cluster rect{fill:#2e2f30;}#${id} .flowchart-link{fill:none;}#${id} [id$="-arrow"] path{fill:#9fa2a8;}</style>` +
+        `<defs><marker id="${id}-arrow" orient="auto"><path d="M0,0L8,4"/></marker></defs>` +
+        `<g class="cluster" id="dad"><rect/></g>` +
+        `<path class="flowchart-link" d="M0,4" marker-end="url(#${id}-arrow)"/><text>${text.split("\n")[0]}</text></svg>`,
     };
   }),
 }));
@@ -34,6 +36,13 @@ function preview(md: string): HTMLElement {
 
 const SEQ = "```mermaid\nsequenceDiagram\n    A->B:test\n```\n";
 
+async function renderedSvg(svg: string): Promise<SVGSVGElement> {
+  m.render.mockResolvedValueOnce({ svg });
+  const root = preview(SEQ);
+  await renderMermaidIn(root).done;
+  return root.querySelector("svg")!;
+}
+
 describe("mermaid in the markdown preview", () => {
   beforeEach(() => {
     resetMermaidForTests();
@@ -106,7 +115,7 @@ describe("mermaid in the markdown preview", () => {
     const edited = preview(SEQ.replace("test", "edited"));
     const next = renderMermaidIn(edited, previous);
     expect(next.async).toBe(true);
-    expect(edited.querySelector(".mermaid-output svg")?.textContent).toBe("sequenceDiagram");
+    expect(edited.querySelector(".mermaid-output svg text")?.textContent).toBe("sequenceDiagram");
     await next.done;
     expect(m.render).toHaveBeenCalledTimes(2);
     ids.add(m.render.mock.calls[1]![0]);
@@ -128,17 +137,108 @@ describe("mermaid in the markdown preview", () => {
     const b = second.querySelector("svg")!;
     const idsA = idsOf(a);
     const idsB = idsOf(b);
-    expect(idsA).toHaveLength(2);
-    expect(idsB).toHaveLength(2);
+    expect(idsA).toHaveLength(3);
+    expect(idsB).toHaveLength(3);
     expect(idsA.filter((id) => idsB.includes(id))).toEqual([]);
-    // Every marker reference resolves within its own copy.
+    // Every marker reference, `<style>#<svg-id>` rule and `[id$="-arrow"]` suffix rule
+    // resolves within its own copy without touching hex colours (`#dad`) in `<style>`.
     for (const svg of [a, b]) {
       const own = new Set(idsOf(svg));
+      const rootId = svg.getAttribute("id")!;
       const ref = svg.querySelector("[marker-end]")!.getAttribute("marker-end")!;
       expect(own.has(/url\(#(.+)\)/.exec(ref)![1]!)).toBe(true);
+      const css = svg.querySelector("style")!.textContent!;
+      expect(css).toContain(`#${rootId}{fill:#dad;}`);
+      expect(css).toContain(`#${rootId} .cluster rect{fill:#2e2f30;}`);
+      expect(css).toContain(`#${rootId} .flowchart-link{fill:none;}`);
+      expect(svg.querySelector("defs [id$='-arrow'] path")).not.toBeNull();
     }
   });
 
+  it.each(["", ".node", ":hover", ">g", "+g", "~g", "[role]", ", rect", " .cluster rect", "\nrect"])(
+    "rewrites style selectors followed by %j",
+    async (tail) => {
+      const svg = await renderedSvg(
+        `<svg id="openidea-mermaid-1"><style>#openidea-mermaid-1${tail}{fill:none;}</style></svg>`,
+      );
+      expect(svg.querySelector("style")!.textContent).toBe(`#${svg.id}${tail}{fill:none;}`);
+    },
+  );
+
+  it("rewrites fragment links, quoted URLs and marker ids while preserving suffix selectors", async () => {
+    const id = "openidea-mermaid-1";
+    const marker = `${id}_flowchart-pointEnd`;
+    const svg = await renderedSvg(
+      `<svg id="${id}" aria-labelledby="title desc" aria-describedby="desc">` +
+        `<style>#${id} [id$="-pointEnd"]{fill:#dad;}#${marker}{stroke:#dad;}</style>` +
+        `<title id="title">Title</title><desc id="desc">Description</desc>` +
+        `<marker id="${marker}"/><g id="dad"/>` +
+        `<use href="#${marker}"/><use xlink:href="#${marker}"/>` +
+        `<path marker-end="url(#${marker})"/>` +
+        `<path marker-end="url('#${marker}')"/>` +
+        `<path marker-end='url("#${marker}")'/>` +
+        `<path style="marker-end: url( '#${marker}' );"/>` +
+        `<style>#${id} path{marker-end:url("#${marker}");}</style></svg>`,
+    );
+    const markerId = svg.querySelector("marker")!.id;
+    expect(markerId).toBe(`${svg.id}_flowchart-pointEnd`);
+    expect(svg.querySelector("[id$='-pointEnd']")).not.toBeNull();
+    expect(
+      [...svg.querySelectorAll("use")].map((el) => el.getAttribute("href") ?? el.getAttribute("xlink:href")),
+    ).toEqual([`#${markerId}`, `#${markerId}`]);
+    expect([...svg.querySelectorAll("path[marker-end]")].map((el) => el.getAttribute("marker-end"))).toEqual([
+      `url(#${markerId})`,
+      `url('#${markerId}')`,
+      `url("#${markerId}")`,
+    ]);
+    expect(svg.querySelector("path[style]")!.getAttribute("style")).toBe(`marker-end: url( '#${markerId}' );`);
+    expect([...svg.querySelectorAll("style")].map((el) => el.textContent)).toEqual([
+      `#${svg.id} [id$="-pointEnd"]{fill:#dad;}#${markerId}{stroke:#dad;}`,
+      `#${svg.id} path{marker-end:url("#${markerId}");}`,
+    ]);
+    expect(svg.getAttribute("aria-labelledby")).toBe(
+      `${svg.querySelector("title")!.id} ${svg.querySelector("desc")!.id}`,
+    );
+    expect(svg.getAttribute("aria-describedby")).toBe(svg.querySelector("desc")!.id);
+  });
+
+  it("keeps ids with a shared prefix distinct and leaves unknown references unchanged", async () => {
+    const short = "openidea-mermaid-1";
+    const long = "openidea-mermaid-10";
+    const svg = await renderedSvg(
+      `<svg id="${short}"><g id="${long}"/>` +
+        `<style>#${short}>g,#${long}:hover{fill:none;}#${long}0{fill:none;}</style>` +
+        `<use href="#${short}"/><use href="#${long}"/><use href="#${long}0"/>` +
+        `<path marker-end="url('#${long}')"/></svg>`,
+    );
+    const longId = svg.querySelector("g")!.id;
+    expect(longId).toBe(`${svg.id}0`);
+    expect([...svg.querySelectorAll("use")].map((el) => el.getAttribute("href"))).toEqual([
+      `#${svg.id}`,
+      `#${longId}`,
+      `#${long}0`,
+    ]);
+    expect(svg.querySelector("style")!.textContent).toBe(
+      `#${svg.id}>g,#${longId}:hover{fill:none;}#${long}0{fill:none;}`,
+    );
+    expect(svg.querySelector("path")!.getAttribute("marker-end")).toBe(`url('#${longId}')`);
+  });
+
+  it("leaves ids in ordinary labels and unrelated attributes untouched", async () => {
+    const id = "openidea-mermaid-1";
+    const label = `#${id} ${id} id="${id}" href="#${id}" url(#${id})`;
+    const svg = await renderedSvg(
+      `<svg id="${id}"><text>${label}</text>` +
+        `<g data-id="${id}" data-label="#${id}" title="url(#${id})"/>` +
+        `<a href="https://example.com/#${id}">Link</a></svg>`,
+    );
+    expect(svg.querySelector("text")!.textContent).toBe(label);
+    expect(svg.querySelector("g")!.getAttribute("data-id")).toBe(id);
+    expect(svg.querySelector("g")!.getAttribute("data-label")).toBe(`#${id}`);
+    expect(svg.querySelector("g")!.getAttribute("title")).toBe(`url(#${id})`);
+    expect(svg.querySelector("a")!.getAttribute("href")).toBe(`https://example.com/#${id}`);
+  });
+
   it("re-renders when switching between built-in palette themes of the same light / dark base", async () => {
     const root = document.documentElement;
     const previous = root.dataset.colorTheme;
diff --git a/src/features/viewers/markdown/mermaid.ts b/src/features/viewers/markdown/mermaid.ts
index 3d7f9ac91..bd9ccbbbd 100644
--- a/src/features/viewers/markdown/mermaid.ts
+++ b/src/features/viewers/markdown/mermaid.ts
@@ -120,22 +120,57 @@ let applied = 0;
  * hit reapplies the first render's SVG string. Two copies in one document — the same
  * diagram twice, or one drawn again while the first sits hidden (a closed details, an
  * inactive tab) — collide on their marker ids, and Chromium resolves the `url(#…)`
- * references against the hidden copy, losing arrowheads. Rewrites every id per
- * application: longest first, so an id that extends a shorter one moves out of the way.
+ * references against the hidden copy, losing arrowheads.
+ *
+ * Prefix ids so Mermaid's marker suffix selectors keep matching. Rewrite only
+ * attributes and CSS rules, leaving diagram labels and CSS colours untouched.
  */
 function reid(svg: string): string {
-  const ids = [...new Set([...svg.matchAll(/\bid="([^"]+)"/g)].map((m) => m[1]!))];
-  if (!ids.length) return svg;
-  const suffix = `-c${++applied}`;
-  // References come as `id="…"`, `href="#…"` and `url(#…)`; the lookahead keeps
-  // an id that extends this one untouched.
-  for (const id of ids.sort((a, b) => b.length - a.length)) {
-    svg = svg.replace(
-      new RegExp(`(\\bid="|#|url\\(#)${escapeRegExp(id)}(?="|\\))`, "g"),
-      (_m, prefix: string) => `${prefix}${id}${suffix}`,
-    );
+  const tag = /<[a-zA-Z](?:[^>"']|"[^"]*"|'[^']*')*>/g;
+  const attribute = /(\s)([\w:-]+)(\s*=\s*)(["'])([\s\S]*?)\4/g;
+  const ids = new Set<string>();
+  for (const [markup] of svg.matchAll(tag)) {
+    for (const attr of markup.matchAll(attribute)) if (attr[2] === "id") ids.add(attr[5]!);
   }
-  return svg;
+  if (!ids.size) return svg;
+  const prefix = `c${++applied}-`;
+  const rename = (id: string) => (ids.has(id) ? `${prefix}${id}` : id);
+  const pattern = [...ids]
+    .sort((a, b) => b.length - a.length)
+    .map(escapeRegExp)
+    .join("|");
+  const selector = new RegExp(`#(${pattern})(?=$|[\\s.:>+~\\[,])`, "g");
+  const urls = (value: string) =>
+    value.replace(
+      /(url\(\s*)(["']?)#([^'"\s)]+)(\2\s*\))/g,
+      (_m, start: string, quote: string, id: string, end: string) => `${start}${quote}#${rename(id)}${end}`,
+    );
+  return svg.replace(
+    /(<style\b[^>]*>)([\s\S]*?)(<\/style\s*>)|<[a-zA-Z](?:[^>"']|"[^"]*"|'[^']*')*>/g,
+    (markup, open: string | undefined, css: string | undefined, close: string | undefined) => {
+      if (css !== undefined) {
+        // Only selector lists contain ID selectors; declarations may contain hex colours.
+        const scoped = css.replace(
+          /(^|[{}])([^{}]+)(?=\{)/g,
+          (_m, boundary: string, selectors: string) =>
+            boundary + selectors.replace(selector, (_m, id: string) => `#${rename(id)}`),
+        );
+        return `${open!}${urls(scoped)}${close!}`;
+      }
+      return markup.replace(
+        attribute,
+        (_m: string, space: string, name: string, equal: string, quote: string, value: string) => {
+          if (name === "id") value = rename(value);
+          else if (name === "href" || name === "xlink:href") {
+            if (value.startsWith("#")) value = `#${rename(value.slice(1))}`;
+          } else if (name === "aria-labelledby" || name === "aria-describedby") value = value.replace(/\S+/g, rename);
+          else if (/^(?:style|fill|stroke|filter|clip-path|mask|marker(?:-start|-mid|-end)?|cursor)$/.test(name))
+            value = urls(value);
+          return `${space}${name}${equal}${quote}${value}${quote}`;
+        },
+      );
+    },
+  );
 }
 
 function show(block: HTMLElement, result: Result | null, stale?: Node) {