diff --git a/src/mermaidSequenceDiff.ts b/src/mermaidSequenceDiff.ts
index d24ff04..e3f3ed7 100644
--- a/src/mermaidSequenceDiff.ts
+++ b/src/mermaidSequenceDiff.ts
@@ -1,4 +1,63 @@
-// stub, replaced in Tasks 4-5
+/**
+ * mermaidSequenceDiff — F7.1 (#22). Pure sequence-diagram parser + diff/emit.
+ * Mermaid sequence diagrams have NO per-message color directive; the only
+ * per-message hook is the `rect rgb(...) … end` background block. So the emitter
+ * rebuilds the message stream (ghosted-removed messages re-inserted at their
+ * baseline position) wrapping each added/changed/removed message in a one-message
+ * tinted rect, and re-declares removed participants (INV-29/31). Deterministic;
+ * no vscode, no DOM.
+ */
+import { diffArrays } from "diff";
+import { CW_COLORS } from "./mermaidDiff";
+
+export interface SeqDiagram {
+  header: string;
+  participants: string[];
+  /** Message/other statement lines, verbatim & trimmed, in order. */
+  statements: string[];
+}
+
+// `A->>B: text`, plus ->, -->, -->>, -x, --x, -), --) variants. Captures from/to.
+const MSG = /^([A-Za-z0-9_]+)\s*(-{1,2}(?:>>?|x|\)))\s*([A-Za-z0-9_]+)\s*:/;
+const PARTICIPANT = /^(?:participant|actor)\s+([A-Za-z0-9_]+)/;
+
+export function parseSequence(source: string): SeqDiagram {
+  const lines = source.split(/\r?\n/);
+  let header = "";
+  const declared: string[] = [];
+  const seen = new Set<string>();
+  const statements: string[] = [];
+  const addP = (p: string) => {
+    if (!seen.has(p)) {
+      seen.add(p);
+      declared.push(p);
+    }
+  };
+  for (const rawLine of lines) {
+    const line = rawLine.trim();
+    if (line === "" || line.startsWith("%%")) continue;
+    if (!header && /^sequenceDiagram\b/.test(line)) {
+      header = line;
+      continue;
+    }
+    const pm = line.match(PARTICIPANT);
+    if (pm) {
+      addP(pm[1]);
+      continue;
+    }
+    const mm = line.match(MSG);
+    if (mm) {
+      addP(mm[1]);
+      addP(mm[3]);
+    }
+    statements.push(line);
+  }
+  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;
 }
diff --git a/test/mermaidSequenceDiff.test.ts b/test/mermaidSequenceDiff.test.ts
new file mode 100644
index 0000000..accfc26
--- /dev/null
+++ b/test/mermaidSequenceDiff.test.ts
@@ -0,0 +1,24 @@
+import { describe, it, expect } from "vitest";
+import { parseSequence } from "../src/mermaidSequenceDiff";
+
+describe("parseSequence", () => {
+  it("captures explicit participants in order", () => {
+    const d = parseSequence("sequenceDiagram\n participant A\n participant B\n A->>B: hi");
+    expect(d.participants).toEqual(["A", "B"]);
+  });
+
+  it("captures message statements verbatim (trimmed)", () => {
+    const d = parseSequence("sequenceDiagram\n A->>B: hi\n B-->>A: bye");
+    expect(d.statements).toEqual(["A->>B: hi", "B-->>A: bye"]);
+  });
+
+  it("infers participants from messages when not declared", () => {
+    const d = parseSequence("sequenceDiagram\n A->>B: hi\n B->>C: on");
+    expect(d.participants).toEqual(["A", "B", "C"]);
+  });
+
+  it("ignores comments and blank lines", () => {
+    const d = parseSequence("sequenceDiagram\n%% note\n\n A->>B: hi");
+    expect(d.statements).toEqual(["A->>B: hi"]);
+  });
+});