gui: decode percent-encoded non-ASCII artifact paths

micromark percent-encodes non-ASCII characters in link hrefs, so an
artifact link to a Chinese/Japanese/Korean filename dispatched the encoded
path and the backend 404ed on the literal %E6… name. Decode the path (and
strip a leading slash so it stays workspace-relative) before opening.

surfaces/gui/src/components/Markdown.test.tsx

@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { cleanup, fireEvent, render, screen } from "@testing-library/react";
-import { Markdown, OPEN_ARTIFACT_EVENT } from "./Markdown";
+import { Markdown, OPEN_ARTIFACT_EVENT, normalizeArtifactPath } from "./Markdown";
 
 afterEach(cleanup);
 
@@ -35,4 +35,30 @@ describe("Markdown artifact links", () => {
     render(<Markdown text="[](artifact:out/report.pdf)" />);
     expect(screen.getByTestId("artifact-chip").textContent).toContain("report.pdf");
   });
+
+  // Chinese (and other non-ASCII) filenames: micromark percent-encodes the href; without
+  // decode the backend looks up the literal %E6… path and returns "not found".
+  it("decodes percent-encoded non-ASCII artifact paths before opening", () => {
+    const seen: string[] = [];
+    const listener = (e: Event) => seen.push((e as CustomEvent).detail.path);
+    window.addEventListener(OPEN_ARTIFACT_EVENT, listener);
+
+    render(<Markdown text="📄 [Monthly report](artifact:reports/2026-04_月度报告.md)" />);
+    const chip = screen.getByTestId("artifact-chip");
+    expect(chip.getAttribute("title")).toBe("reports/2026-04_月度报告.md");
+    fireEvent.click(chip);
+    expect(seen).toEqual(["reports/2026-04_月度报告.md"]);
+
+    window.removeEventListener(OPEN_ARTIFACT_EVENT, listener);
+  });
+});
+
+describe("normalizeArtifactPath", () => {
+  it("decodes percent-encoded segments and strips a leading slash", () => {
+    expect(normalizeArtifactPath("reports/2026-04_%E6%9C%88%E5%BA%A6%E6%8A%A5%E5%91%8A.md")).toBe(
+      "reports/2026-04_月度报告.md",
+    );
+    expect(normalizeArtifactPath("/reports/foo.md")).toBe("reports/foo.md");
+    expect(normalizeArtifactPath("reports/ascii.md")).toBe("reports/ascii.md");
+  });
 });

surfaces/gui/src/components/Markdown.tsx

@@ -10,16 +10,36 @@ import { Icon } from "./Icon";
 // the session's artifact list, App un-hides the rail.
 export const OPEN_ARTIFACT_EVENT = "ocw-open-artifact";
 
+/** Normalize an artifact: href path for the session workspace.
+ *
+ * react-markdown / micromark percent-encodes non-ASCII URL characters, so
+ * `artifact:reports/报告.md` becomes `artifact:reports/%E6%8A%A5%E5%91%8A.md`.
+ * The backend looks up the literal filesystem path — encoded names 404 as "not found".
+ * Also strip a single leading `/` so `artifact:/reports/x.md` stays workspace-relative
+ * (Path(workspace) / "/abs" would otherwise escape the workspace root).
+ */
+export function normalizeArtifactPath(raw: string): string {
+  let path = raw;
+  try {
+    path = decodeURIComponent(raw);
+  } catch {
+    // malformed % sequences — keep raw
+  }
+  if (path.startsWith("/")) path = path.replace(/^\/+/, "");
+  return path;
+}
+
 function ArtifactChip({ path, title }: { path: string; title: string }) {
   const { t } = useTranslation();
-  const file = path.split("/").pop() || path;
+  const resolved = normalizeArtifactPath(path);
+  const file = resolved.split("/").pop() || resolved;
   return (
     <button
       className="art-chip"
       data-testid="artifact-chip"
-      title={path}
+      title={resolved}
       onClick={() =>
-        window.dispatchEvent(new CustomEvent(OPEN_ARTIFACT_EVENT, { detail: { path } }))
+        window.dispatchEvent(new CustomEvent(OPEN_ARTIFACT_EVENT, { detail: { path: resolved } }))
       }
     >
       <span className="art-chip-ico">