diff --git a/src/mermaidSequenceDiff.ts b/src/mermaidSequenceDiff.ts
index e3f3ed7..bfcdc9a 100644
--- a/src/mermaidSequenceDiff.ts
+++ b/src/mermaidSequenceDiff.ts
@@ -55,9 +55,49 @@ export function parseSequence(source: string): SeqDiagram {
   return { header: header || "sequenceDiagram", participants: declared, statements };
 }
 
-// Replaced in Task 5 with the real diff/emit.
-export function diffSequence(_beforeSrc: string, currentSrc: string): string {
-  void diffArrays;
-  void CW_COLORS;
-  return currentSrc;
+function hexToRgb(hex: string): string {
+  const h = hex.replace("#", "");
+  const n = parseInt(h, 16);
+  return `${(n >> 16) & 255}, ${(n >> 8) & 255}, ${n & 255}`;
+}
+
+function rectWrap(stmt: string, hex: string): string[] {
+  return [`  rect rgb(${hexToRgb(hex)})`, `    ${stmt}`, `  end`];
+}
+
+export function diffSequence(beforeSrc: string, currentSrc: string): string {
+  const before = parseSequence(beforeSrc);
+  const current = parseSequence(currentSrc);
+
+  const out: string[] = [current.header];
+
+  // Participants: keep current declarations, then re-declare removed ones (ghosts).
+  for (const p of current.participants) out.push(`  participant ${p}`);
+  const curSet = new Set(current.participants);
+  for (const p of before.participants) {
+    if (!curSet.has(p)) out.push(`  participant ${p}`);
+  }
+
+  // Message stream diff via LCS over statements; pair adjacent removed+added as changed.
+  const parts = diffArrays(before.statements, current.statements);
+  for (let n = 0; n < parts.length; n++) {
+    const ch = parts[n];
+    if (!ch.added && !ch.removed) {
+      for (const s of ch.value) out.push(`  ${s}`);
+      continue;
+    }
+    if (ch.removed) {
+      const next = parts[n + 1];
+      const addVals = next?.added ? next.value : [];
+      const paired = Math.min(ch.value.length, addVals.length);
+      for (let k = 0; k < paired; k++) out.push(...rectWrap(addVals[k], CW_COLORS.changed));
+      for (let k = paired; k < ch.value.length; k++) out.push(...rectWrap(ch.value[k], CW_COLORS.removed));
+      for (let k = paired; k < addVals.length; k++) out.push(...rectWrap(addVals[k], CW_COLORS.added));
+      if (next?.added) n++;
+      continue;
+    }
+    // lone added run
+    for (const s of ch.value) out.push(...rectWrap(s, CW_COLORS.added));
+  }
+  return out.join("\n");
 }
diff --git a/test/mermaidSequenceDiff.test.ts b/test/mermaidSequenceDiff.test.ts
index accfc26..25bfcfa 100644
--- a/test/mermaidSequenceDiff.test.ts
+++ b/test/mermaidSequenceDiff.test.ts
@@ -22,3 +22,44 @@ describe("parseSequence", () => {
     expect(d.statements).toEqual(["A->>B: hi"]);
   });
 });
+
+import { diffSequence } from "../src/mermaidSequenceDiff";
+
+const rectCount = (s: string, rgb: string) => s.split(`rect rgb(${rgb})`).length - 1;
+
+describe("diffSequence", () => {
+  it("wraps an added message in a green rect", () => {
+    const before = "sequenceDiagram\n A->>B: hi";
+    const current = "sequenceDiagram\n A->>B: hi\n B->>C: on";
+    const out = diffSequence(before, current);
+    expect(rectCount(out, "46, 160, 67")).toBe(1); // CW_COLORS.added as rgb
+    expect(out).toContain("B->>C: on");
+  });
+
+  it("ghosts a removed message back in a grey rect", () => {
+    const before = "sequenceDiagram\n A->>B: hi\n B->>C: gone";
+    const current = "sequenceDiagram\n A->>B: hi";
+    const out = diffSequence(before, current);
+    expect(out).toContain("B->>C: gone");
+    expect(rectCount(out, "128, 128, 128")).toBe(1);
+  });
+
+  it("re-declares a removed participant so it still appears", () => {
+    const before = "sequenceDiagram\n participant A\n participant B\n participant C\n A->>C: x";
+    const current = "sequenceDiagram\n participant A\n participant B\n A->>B: y";
+    const out = diffSequence(before, current);
+    expect(out).toMatch(/participant C/);
+  });
+
+  it("keeps an unchanged message outside any rect", () => {
+    const doc = "sequenceDiagram\n A->>B: hi";
+    const out = diffSequence(doc, doc);
+    expect(out).not.toContain("rect rgb");
+  });
+
+  it("is deterministic", () => {
+    const before = "sequenceDiagram\n A->>B: hi";
+    const current = "sequenceDiagram\n A->>B: hi\n B->>A: bye";
+    expect(diffSequence(before, current)).toBe(diffSequence(before, current));
+  });
+});