diff --git a/src/mermaidDiff.ts b/src/mermaidDiff.ts
new file mode 100644
index 0000000..35b7eef
--- /dev/null
+++ b/src/mermaidDiff.ts
@@ -0,0 +1,49 @@
+/**
+ * mermaidDiff — F7.1 (#22) dispatcher. Detects a mermaid diagram's type and
+ * routes a baseline→current diff to the matching pure differ, which re-emits the
+ * CURRENT diagram source augmented with mermaid styling directives (INV-29). Any
+ * unsupported type or parser surprise degrades to the v1 whole-block badge
+ * (INV-30) — this function never throws. vscode-free, DOM-free, deterministic.
+ */
+import { diffFlowchart } from "./mermaidFlowchartDiff";
+import { diffSequence } from "./mermaidSequenceDiff";
+
+/** Theme-neutral colors baked into emitted mermaid source (source can't read CSS vars). */
+export const CW_COLORS = {
+  added: "#2ea043",
+  changed: "#d29922",
+  removed: "#808080",
+} as const;
+
+export type DiagramType = "flowchart" | "sequence" | "other";
+
+export interface MermaidDiffAugmented {
+  kind: "augmented";
+  source: string;
+}
+export interface MermaidDiffFallback {
+  kind: "fallback";
+}
+export type MermaidDiffResult = MermaidDiffAugmented | MermaidDiffFallback;
+
+export function detectDiagramType(source: string): DiagramType {
+  for (const line of source.split(/\r?\n/)) {
+    const t = line.trim();
+    if (t === "" || t.startsWith("%%")) continue;
+    if (/^(flowchart|graph)\b/.test(t)) return "flowchart";
+    if (/^sequenceDiagram\b/.test(t)) return "sequence";
+    return "other";
+  }
+  return "other";
+}
+
+export function diffMermaid(beforeSrc: string, currentSrc: string): MermaidDiffResult {
+  const type = detectDiagramType(currentSrc);
+  try {
+    if (type === "flowchart") return { kind: "augmented", source: diffFlowchart(beforeSrc, currentSrc) };
+    if (type === "sequence") return { kind: "augmented", source: diffSequence(beforeSrc, currentSrc) };
+  } catch {
+    return { kind: "fallback" };
+  }
+  return { kind: "fallback" };
+}
diff --git a/src/mermaidFlowchartDiff.ts b/src/mermaidFlowchartDiff.ts
new file mode 100644
index 0000000..fef5f2d
--- /dev/null
+++ b/src/mermaidFlowchartDiff.ts
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+// stub, replaced in Tasks 2-3
+export function diffFlowchart(_beforeSrc: string, currentSrc: string): string {
+  return currentSrc;
+}
diff --git a/src/mermaidSequenceDiff.ts b/src/mermaidSequenceDiff.ts
new file mode 100644
index 0000000..d24ff04
--- /dev/null
+++ b/src/mermaidSequenceDiff.ts
@@ -0,0 +1,4 @@
+// stub, replaced in Tasks 4-5
+export function diffSequence(_beforeSrc: string, currentSrc: string): string {
+  return currentSrc;
+}
diff --git a/test/mermaidDiff.test.ts b/test/mermaidDiff.test.ts
new file mode 100644
index 0000000..f02645d
--- /dev/null
+++ b/test/mermaidDiff.test.ts
@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import { detectDiagramType, diffMermaid } from "../src/mermaidDiff";
+
+describe("detectDiagramType", () => {
+  it("detects flowchart from `flowchart` / `graph` headers", () => {
+    expect(detectDiagramType("flowchart LR\n a-->b")).toBe("flowchart");
+    expect(detectDiagramType("graph TD\n a-->b")).toBe("flowchart");
+    expect(detectDiagramType("  \n%% c\ngraph TD")).toBe("flowchart");
+  });
+  it("detects sequence from `sequenceDiagram`", () => {
+    expect(detectDiagramType("sequenceDiagram\n A->>B: hi")).toBe("sequence");
+  });
+  it("returns `other` for unsupported types", () => {
+    expect(detectDiagramType("classDiagram\n class A")).toBe("other");
+    expect(detectDiagramType("stateDiagram-v2\n s1 --> s2")).toBe("other");
+    expect(detectDiagramType("")).toBe("other");
+  });
+});
+
+describe("diffMermaid", () => {
+  it("falls back for an unsupported diagram type", () => {
+    expect(diffMermaid("classDiagram\n class A", "classDiagram\n class B")).toEqual({ kind: "fallback" });
+  });
+  it("falls back (never throws) when the differ throws", () => {
+    // A deliberately malformed flowchart still degrades gracefully.
+    const r = diffMermaid("flowchart LR", "flowchart LR\n a -->");
+    expect(["augmented", "fallback"]).toContain(r.kind);
+  });
+});