diff --git a/src/trackChangesModel.ts b/src/trackChangesModel.ts
index ffb44bc..d5399ab 100644
--- a/src/trackChangesModel.ts
+++ b/src/trackChangesModel.ts
@@ -351,6 +351,22 @@ function sentinelsToSpans(html: string): string {
     .split(SENT.human.close).join("</span>");
 }
 
+/**
+ * Color one prose block's HTML by F3 author spans (PUA-sentinel technique,
+ * salvaged from F9 renderAuthorship). `blockStart` is the block's char offset in
+ * the source so spans map correctly. Pure; deterministic.
+ */
+export function colorByAuthor(
+  raw: string,
+  blockStart: number,
+  spans: AuthorSpan[],
+  render: (src: string) => string,
+): string {
+  const overlapping = spans.filter((s) => s.end > blockStart && s.start < blockStart + raw.length);
+  const injected = injectSentinels(raw, blockStart, overlapping);
+  return sentinelsToSpans(render(injected));
+}
+
 /**
  * Pure authorship render (INV-26/28): the CURRENT text with each F3-attributed
  * span colored by author. Prose blocks get inline `<span class="cw-by-*">`;
@@ -379,8 +395,7 @@ export function renderAuthorship(
         if (!badge) return `<div class="cw-blk">${inner}</div>`;
         return `<div class="cw-blk ${badge.cls}"><span class="cw-badge">${badge.label}</span>${inner}</div>`;
       }
-      const injected = injectSentinels(b.raw, b.start, overlapping);
-      return `<div class="cw-blk">${sentinelsToSpans(safe(injected))}</div>`;
+      return `<div class="cw-blk">${colorByAuthor(b.raw, b.start, overlapping, safe)}</div>`;
     })
     .join("\n");
 }
diff --git a/test/trackChangesModel.test.ts b/test/trackChangesModel.test.ts
index 0297ee1..0fb1b49 100644
--- a/test/trackChangesModel.test.ts
+++ b/test/trackChangesModel.test.ts
@@ -1,5 +1,5 @@
-import { describe, it, expect } from "vitest";
-import { splitBlocks, splitBlocksWithRanges, diffBlocks, renderTrackChanges, renderAuthorship, type AuthorSpan } from "../src/trackChangesModel";
+import { describe, it, test, expect } from "vitest";
+import { splitBlocks, splitBlocksWithRanges, diffBlocks, renderTrackChanges, renderAuthorship, colorByAuthor, type AuthorSpan } from "../src/trackChangesModel";
 
 describe("splitBlocks", () => {
   it("splits prose paragraphs on blank lines, dropping empties", () => {
@@ -235,6 +235,17 @@ describe("renderAuthorship", () => {
   });
 });
 
+describe("colorByAuthor", () => {
+  test("colorByAuthor wraps human-authored prose in cw-by-human spans", () => {
+    const raw = "hello world";
+    const spans: AuthorSpan[] = [{ start: 0, end: 5, author: "human" }];
+    const render = (src: string) => `<p>${src}</p>`;
+    const html = colorByAuthor(raw, 0, spans, render);
+    expect(html).toContain('<span class="cw-by-human">hello</span>');
+    expect(html).toContain("world");
+  });
+});
+
 import { renderTrackChanges as rtc2 } from "../src/trackChangesModel";
 
 describe("renderTrackChanges — intra-diagram mermaid (#22)", () => {