diff --git a/src/trackChangesModel.ts b/src/trackChangesModel.ts
index 17f6d54..43801d0 100644
--- a/src/trackChangesModel.ts
+++ b/src/trackChangesModel.ts
@@ -145,40 +145,51 @@ function wordMergedMarkdown(beforeRaw: string, afterRaw: string): string {
     .join("");
 }
 
-function safeRender(src: string): string {
-  try {
-    return md.render(src);
-  } catch (err) {
-    const message = err instanceof Error ? err.message : String(err);
-    return `<div class="cw-error">Could not render this block: ${md.utils.escapeHtml(message)}</div>`;
-  }
+export interface RenderOptions {
+  /** Per-block markdown→HTML renderer (test seam). Defaults to the bundled markdown-it. */
+  render?: (src: string) => string;
 }
 
-function renderOp(op: BlockOp): string {
+function defaultRender(src: string): string {
+  return md.render(src);
+}
+
+function chip(message: string): string {
+  return `<div class="cw-error">Could not render this block: ${md.utils.escapeHtml(message)}</div>`;
+}
+
+function renderOp(op: BlockOp, render: (src: string) => string): string {
+  const safe = (src: string): string => {
+    try {
+      return render(src);
+    } catch (err) {
+      return chip(err instanceof Error ? err.message : String(err));
+    }
+  };
   let cls: string;
   let inner: string;
   let badge = "";
   switch (op.kind) {
     case "unchanged":
       cls = "cw-unchanged";
-      inner = safeRender(op.block.raw);
+      inner = safe(op.block.raw);
       break;
     case "added":
       cls = "cw-added";
-      inner = safeRender(op.block.raw);
+      inner = safe(op.block.raw);
       if (op.block.type !== "prose") badge = '<span class="cw-badge">added</span>';
       break;
     case "removed":
       cls = "cw-removed";
-      inner = safeRender(op.block.raw);
+      inner = safe(op.block.raw);
       break;
     case "changed":
       cls = "cw-changed";
       if (op.atomic) {
-        inner = safeRender(op.block.raw); // the NEW block, whole (INV-23)
+        inner = safe(op.block.raw); // the NEW block, whole (INV-23)
         badge = '<span class="cw-badge">changed</span>';
       } else {
-        inner = safeRender(wordMergedMarkdown(op.before.raw, op.block.raw));
+        inner = safe(wordMergedMarkdown(op.before.raw, op.block.raw));
       }
       break;
   }
@@ -186,6 +197,13 @@ function renderOp(op: BlockOp): string {
 }
 
 /** Pure entry point: annotated HTML body for the preview (INV-22). */
-export function renderTrackChanges(baselineText: string, currentText: string): string {
-  return diffBlocks(baselineText, currentText).map(renderOp).join("\n");
+export function renderTrackChanges(
+  baselineText: string,
+  currentText: string,
+  opts: RenderOptions = {},
+): string {
+  const render = opts.render ?? defaultRender;
+  return diffBlocks(baselineText, currentText)
+    .map((op) => renderOp(op, render))
+    .join("\n");
 }
diff --git a/test/trackChangesModel.test.ts b/test/trackChangesModel.test.ts
index b07bc04..d74414f 100644
--- a/test/trackChangesModel.test.ts
+++ b/test/trackChangesModel.test.ts
@@ -122,3 +122,18 @@ describe("renderTrackChanges", () => {
     expect(a).toBe(b);
   });
 });
+
+describe("error chip (PUC-6)", () => {
+  it("a block whose render throws becomes an error chip; the rest still renders", () => {
+    const throwing = (src: string) => {
+      if (src.includes("BOOM")) throw new Error("kaboom");
+      return `<p>${src}</p>`;
+    };
+    const html = renderTrackChanges("Good one.\n\nBOOM here.\n", "Good one.\n\nBOOM here.\n", {
+      render: throwing,
+    });
+    expect(html).toContain("cw-error");
+    expect(html).toContain("kaboom");
+    expect(html).toContain("<p>Good one.</p>"); // the healthy block still rendered
+  });
+});