diff --git a/src/trackChangesModel.ts b/src/trackChangesModel.ts
index 1eb1765..ffb44bc 100644
--- a/src/trackChangesModel.ts
+++ b/src/trackChangesModel.ts
@@ -10,6 +10,7 @@
  */
 import MarkdownIt from "markdown-it";
 import { diffArrays, diffWords } from "diff";
+import { diffMermaid } from "./mermaidDiff";
 
 export type BlockType = "prose" | "code" | "mermaid";
 
@@ -204,6 +205,24 @@ md.renderer.rules.fence = (tokens, idx, options, env, self) => {
   return defaultFence(tokens, idx, options, env, self);
 };
 
+/** Inner source of a ```mermaid fence (drops the fence lines), or null. */
+function mermaidFenceBody(raw: string): string | null {
+  const lines = raw.split(/\r?\n/);
+  const open = lines[0]?.match(/^(\s*)(`{3,}|~{3,})\s*(\w+)?/);
+  if (!open || open[3]?.toLowerCase() !== "mermaid") return null;
+  const body = lines.slice(1);
+  if (body.length && /^(\s*)(`{3,}|~{3,})/.test(body[body.length - 1])) body.pop();
+  return body.join("\n");
+}
+
+/** F7.1 (#22): legend shown beneath an intra-diagram-diffed mermaid block. */
+const MERMAID_LEGEND =
+  '<div class="cw-mermaid-legend">' +
+  '<span class="cw-leg cw-leg-add">added</span>' +
+  '<span class="cw-leg cw-leg-chg">changed</span>' +
+  '<span class="cw-leg cw-leg-rem">removed</span>' +
+  "</div>";
+
 /** Build a markdown string with inline <ins>/<del> from a word-level prose diff. */
 function wordMergedMarkdown(beforeRaw: string, afterRaw: string): string {
   return diffWords(beforeRaw, afterRaw)
@@ -256,8 +275,19 @@ function renderOp(op: BlockOp, render: (src: string) => string): string {
     case "changed":
       cls = "cw-changed";
       if (op.atomic) {
-        inner = safe(op.block.raw); // the NEW block, whole (INV-23)
-        badge = '<span class="cw-badge">changed</span>';
+        const curBody = op.block.type === "mermaid" ? mermaidFenceBody(op.block.raw) : null;
+        const beforeBody = op.before.type === "mermaid" ? mermaidFenceBody(op.before.raw) : null;
+        const md =
+          curBody !== null && beforeBody !== null
+            ? diffMermaid(beforeBody, curBody)
+            : { kind: "fallback" as const };
+        if (md.kind === "augmented") {
+          // Re-render the augmented diagram as a mermaid fence; add a legend, drop the badge (INV-29).
+          inner = safe("```mermaid\n" + md.source + "\n```") + MERMAID_LEGEND;
+        } else {
+          inner = safe(op.block.raw); // the NEW block, whole (INV-23 / INV-30 fallback)
+          badge = '<span class="cw-badge">changed</span>';
+        }
       } else {
         inner = safe(wordMergedMarkdown(op.before.raw, op.block.raw));
       }
diff --git a/test/trackChangesModel.test.ts b/test/trackChangesModel.test.ts
index ddfc255..0297ee1 100644
--- a/test/trackChangesModel.test.ts
+++ b/test/trackChangesModel.test.ts
@@ -99,14 +99,16 @@ describe("renderTrackChanges", () => {
     expect(html).toContain("cw-badge");
   });
 
-  it('emits <pre class="mermaid"> for a mermaid fence + a changed badge', () => {
+  it('emits <pre class="mermaid"> for a changed flowchart, augmented intra-diagram (#22)', () => {
     const html = renderTrackChanges(
       "```mermaid\nflowchart LR\n a-->b\n```\n",
       "```mermaid\nflowchart LR\n a-->c\n```\n",
     );
     expect(html).toContain('<pre class="mermaid">');
-    expect(html).toContain("a--&gt;c"); // current source, escaped
-    expect(html).toContain("cw-badge");
+    expect(html).toContain("a--&gt;c"); // current source preserved as prefix, escaped
+    // a changed flowchart is now diffed in place (INV-29): styling, not a whole-block badge
+    expect(html).toContain("classDef cwAdded");
+    expect(html).not.toContain('<span class="cw-badge">changed</span>');
   });
 
   it("unchanged doc renders with no marks", () => {
@@ -232,3 +234,33 @@ describe("renderAuthorship", () => {
     expect(renderAuthorship(text, spans)).toBe(renderAuthorship(text, spans));
   });
 });
+
+import { renderTrackChanges as rtc2 } from "../src/trackChangesModel";
+
+describe("renderTrackChanges — intra-diagram mermaid (#22)", () => {
+  it("augments a changed flowchart with styling directives (INV-29)", () => {
+    const base = "```mermaid\nflowchart LR\n A-->B\n```\n";
+    const cur = "```mermaid\nflowchart LR\n A-->B\n B-->C\n```\n";
+    const html = rtc2(base, cur);
+    expect(html).toContain("classDef cwAdded");
+    expect(html).toMatch(/class\s+C\s+cwAdded/);
+    expect(html).toContain('class="cw-mermaid-legend"');
+    // no single whole-block "changed" badge when we augmented
+    expect(html).not.toContain('<span class="cw-badge">changed</span>');
+  });
+
+  it("augments a changed sequence diagram", () => {
+    const base = "```mermaid\nsequenceDiagram\n A->>B: hi\n```\n";
+    const cur = "```mermaid\nsequenceDiagram\n A->>B: hi\n B->>C: on\n```\n";
+    const html = rtc2(base, cur);
+    expect(html).toContain("rect rgb(46, 160, 67)");
+  });
+
+  it("falls back to the v1 badge for an unsupported diagram type (INV-30)", () => {
+    const base = "```mermaid\nclassDiagram\n class A\n```\n";
+    const cur = "```mermaid\nclassDiagram\n class B\n```\n";
+    const html = rtc2(base, cur);
+    expect(html).toContain('<span class="cw-badge">changed</span>');
+    expect(html).not.toContain("classDef cwAdded");
+  });
+});