diff --git a/src/core/VaultMutationService.ts b/src/core/VaultMutationService.ts
index 647ce0275..9a47c22cf 100644
--- a/src/core/VaultMutationService.ts
+++ b/src/core/VaultMutationService.ts
@@ -1,4 +1,21 @@
 import type { App, TFile } from "obsidian";
+import type { MdbaseMutationBackend } from "./mdbase/MdbaseMutationBackend";
+
+let mdbaseBackend: MdbaseMutationBackend | null = null;
+
+/**
+ * Install (or remove, with `null`) the mdbase write path. While installed, writes
+ * to files in the mdbase collection go through the runtime as intents; everything
+ * else keeps using Obsidian's APIs.
+ */
+export function setMdbaseMutationBackend(backend: MdbaseMutationBackend | null): void {
+	mdbaseBackend = backend;
+}
+
+/** The installed mdbase write path, if any. */
+export function getMdbaseMutationBackend(): MdbaseMutationBackend | null {
+	return mdbaseBackend;
+}
 
 type FrontmatterMutationApp = {
 	fileManager: {
@@ -54,6 +71,10 @@ export async function processVaultFrontMatterWithinMutation(
 	file: TFile,
 	update: (frontmatter: Record<string, unknown>) => void
 ): Promise<void> {
+	if (mdbaseBackend?.claims(file.path)) {
+		const r = await mdbaseBackend.processFrontMatter(file.path, update);
+		if (r.handled) return;
+	}
 	await app.fileManager.processFrontMatter(file, update);
 }
 
@@ -70,10 +91,22 @@ export async function processVaultFileWithinMutation(
 	file: TFile,
 	update: (content: string) => string
 ): Promise<string> {
+	if (mdbaseBackend?.claims(file.path)) {
+		const r = await mdbaseBackend.process(file.path, update);
+		if (r.handled) return r.value;
+	}
 	return app.vault.process(file, update);
 }
 
 export async function createVaultFile(app: App, path: string, content: string): Promise<TFile> {
+	if (mdbaseBackend?.claims(path)) {
+		const r = await mdbaseBackend.create(path, content);
+		if (r.handled) {
+			const file = app.vault.getFileByPath(path);
+			if (file) return file;
+			throw new Error(`mdbase created ${path}, but Obsidian has not indexed it yet`);
+		}
+	}
 	return app.vault.create(path, content);
 }
 
@@ -82,7 +115,13 @@ export async function createVaultFolder(app: App, path: string): Promise<void> {
 }
 
 export async function modifyVaultFile(app: App, file: TFile, content: string): Promise<void> {
-	await withVaultFileMutation(file, () => app.vault.modify(file, content));
+	await withVaultFileMutation(file, async () => {
+		if (mdbaseBackend?.claims(file.path)) {
+			const r = await mdbaseBackend.modify(file.path, content);
+			if (r.handled) return;
+		}
+		await app.vault.modify(file, content);
+	});
 }
 
 export async function renameVaultFile(app: App, file: TFile, newPath: string): Promise<void> {
diff --git a/src/core/mdbase/MdbaseMutationBackend.ts b/src/core/mdbase/MdbaseMutationBackend.ts
new file mode 100644
index 000000000..d4ad0829e
--- /dev/null
+++ b/src/core/mdbase/MdbaseMutationBackend.ts
@@ -0,0 +1,170 @@
+/**
+ * Route TaskNotes' vault writes through the mdbase runtime when it manages the
+ * vault's collection.
+ *
+ * TaskNotes funnels every write through `VaultMutationService`
+ * (`processFrontMatter`, `process`, `create`, `modify`). When a backend is installed
+ * and claims a file, each write becomes a replica intent, in every collection state
+ * (local only, synced, synced end to end), over one client API:
+ * - a frontmatter callback becomes a **field-level update**: the keys the callback
+ *   set, and the keys it removed, with the values it read as the merge base. Two
+ *   devices editing different fields of one task both win, instead of the last
+ *   whole-file write;
+ * - a whole-content callback, or `modify`, becomes a document replace with the read
+ *   document as its base. The replica merges, or holds the file;
+ * - `create` becomes a record create at the requested path.
+ *
+ * The replica publishes into the vault itself, using the editor fence when the note is
+ * open. Each call resolves once the write has settled (see
+ * {@link MdbaseWriteClient.settle}), so callers that re-read the file afterwards see
+ * the new content, as with Obsidian's own APIs.
+ *
+ * Files outside the collection, or written while no runtime is attached, keep using
+ * Obsidian's APIs unchanged.
+ */
+
+/** A record as TaskNotes needs it, with plain JSON-like frontmatter. */
+export interface MdbaseRecord {
+	readonly id: string;
+	readonly path: string;
+	readonly frontmatter: Record<string, unknown>;
+	/** The whole source, when read with `document`. */
+	readonly document?: string;
+}
+
+/** An accepted write (pending or confirmed). */
+export interface MdbaseWrite {
+	readonly mutation: string;
+}
+
+/**
+ * What the backend needs from the mdbase client. The adapter over
+ * `@mdbase-dev/sdk`'s `MdbaseClient` converts values to and from plain JSON.
+ */
+export interface MdbaseWriteClient {
+	/** The collection-relative path for a vault path, or `null` if outside the collection. */
+	collectionPath(vaultPath: string): string | null;
+	/** Read a record by path, with frontmatter (and the document when asked). */
+	find(path: string, opts: { document: boolean }): Promise<MdbaseRecord | null>;
+	/** Field-level update, with `base` taken from `seen`. */
+	update(
+		seen: MdbaseRecord,
+		changes: { patch: Record<string, unknown>; unset: string[] }
+	): Promise<MdbaseWrite>;
+	/** Replace the whole document, with `seen.document` as the merge base. */
+	replaceDocument(seen: MdbaseRecord, document: string): Promise<MdbaseWrite>;
+	/** Create a record from a complete document at `path`. */
+	create(path: string, document: string): Promise<MdbaseWrite>;
+	/**
+	 * Resolve once the write is settled for TaskNotes' purposes: rejected (throws), or
+	 * published into the vault file (the replica's publish, or the editor fence plus
+	 * Obsidian's save) and indexed by Obsidian's vault (so `getFileByPath` finds a
+	 * created file). It does not wait for log confirmation, so TaskNotes works the
+	 * same offline.
+	 */
+	settle(write: MdbaseWrite, vaultPath: string): Promise<void>;
+}
+
+/** Result of a backend attempt: `false` means "not mine, use Obsidian's API". */
+export type Handled<T> = { handled: true; value: T } | { handled: false };
+
+const NOT_HANDLED = { handled: false } as const;
+
+function isPlainObject(v: unknown): v is Record<string, unknown> {
+	return typeof v === "object" && v !== null && !Array.isArray(v) && !(v instanceof Date);
+}
+
+/** Structural equality for YAML-shaped values (objects compare as key sets). */
+export function yamlEqual(a: unknown, b: unknown): boolean {
+	if (a === b) return true;
+	if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
+	if (Array.isArray(a) && Array.isArray(b)) {
+		return a.length === b.length && a.every((x, i) => yamlEqual(x, b[i]));
+	}
+	if (isPlainObject(a) && isPlainObject(b)) {
+		const ka = Object.keys(a);
+		const kb = Object.keys(b);
+		return ka.length === kb.length && ka.every((k) => Object.prototype.hasOwnProperty.call(b, k) && yamlEqual(a[k], b[k]));
+	}
+	return false;
+}
+
+function clone<T>(v: T): T {
+	return typeof structuredClone === "function" ? structuredClone(v) : (JSON.parse(JSON.stringify(v)) as T);
+}
+
+/** The top-level keys a frontmatter callback set or removed. */
+export function frontmatterDiff(
+	before: Record<string, unknown>,
+	after: Record<string, unknown>
+): { patch: Record<string, unknown>; unset: string[] } {
+	const patch: Record<string, unknown> = {};
+	const unset: string[] = [];
+	for (const [k, v] of Object.entries(after)) {
+		if (v === undefined) {
+			if (k in before) unset.push(k);
+		} else if (!(k in before) || !yamlEqual(before[k], v)) {
+			patch[k] = v;
+		}
+	}
+	for (const k of Object.keys(before)) if (!(k in after)) unset.push(k);
+	return { patch, unset };
+}
+
+/** The mdbase write path for TaskNotes. */
+export class MdbaseMutationBackend {
+	constructor(private readonly client: MdbaseWriteClient) {}
+
+	/** Whether this backend handles writes to `vaultPath`. */
+	claims(vaultPath: string): boolean {
+		return this.client.collectionPath(vaultPath) !== null;
+	}
+
+	/** `fileManager.processFrontMatter`, as a field-level update. */
+	async processFrontMatter(
+		vaultPath: string,
+		update: (frontmatter: Record<string, unknown>) => void
+	): Promise<Handled<void>> {
+		const path = this.client.collectionPath(vaultPath);
+		if (path === null) return NOT_HANDLED;
+		const seen = await this.client.find(path, { document: false });
+		if (!seen) return NOT_HANDLED; // not a record (yet): let Obsidian write it
+		const working = clone(seen.frontmatter);
+		update(working);
+		const changes = frontmatterDiff(seen.frontmatter, working);
+		if (Object.keys(changes.patch).length === 0 && changes.unset.length === 0) {
+			return { handled: true, value: undefined };
+		}
+		const write = await this.client.update(seen, changes);
+		await this.client.settle(write, vaultPath);
+		return { handled: true, value: undefined };
+	}
+
+	/** `vault.process`, as a document replace with the read document as base. */
+	async process(vaultPath: string, update: (content: string) => string): Promise<Handled<string>> {
+		const path = this.client.collectionPath(vaultPath);
+		if (path === null) return NOT_HANDLED;
+		const seen = await this.client.find(path, { document: true });
+		if (!seen || seen.document === undefined) return NOT_HANDLED;
+		const next = update(seen.document);
+		if (next === seen.document) return { handled: true, value: next };
+		const write = await this.client.replaceDocument(seen, next);
+		await this.client.settle(write, vaultPath);
+		return { handled: true, value: next };
+	}
+
+	/** `vault.modify`: a document replace from whatever is current. */
+	async modify(vaultPath: string, content: string): Promise<Handled<void>> {
+		const r = await this.process(vaultPath, () => content);
+		return r.handled ? { handled: true, value: undefined } : NOT_HANDLED;
+	}
+
+	/** `vault.create`. The caller looks up the `TFile` once this resolves. */
+	async create(vaultPath: string, content: string): Promise<Handled<void>> {
+		const path = this.client.collectionPath(vaultPath);
+		if (path === null) return NOT_HANDLED;
+		const write = await this.client.create(path, content);
+		await this.client.settle(write, vaultPath);
+		return { handled: true, value: undefined };
+	}
+}
diff --git a/tests/unit/core/mdbase/MdbaseMutationBackend.test.ts b/tests/unit/core/mdbase/MdbaseMutationBackend.test.ts
new file mode 100644
index 000000000..afe08973a
--- /dev/null
+++ b/tests/unit/core/mdbase/MdbaseMutationBackend.test.ts
@@ -0,0 +1,139 @@
+import type { TFile } from "obsidian";
+import {
+	frontmatterDiff,
+	MdbaseMutationBackend,
+	type MdbaseRecord,
+	type MdbaseWriteClient,
+} from "../../../../src/core/mdbase/MdbaseMutationBackend";
+import {
+	createVaultFile,
+	modifyVaultFile,
+	processVaultFile,
+	processVaultFrontMatter,
+	setMdbaseMutationBackend,
+} from "../../../../src/core/VaultMutationService";
+
+class FakeClient implements MdbaseWriteClient {
+	records = new Map<string, MdbaseRecord>();
+	calls: unknown[] = [];
+	settled: string[] = [];
+	collectionPath(vaultPath: string) {
+		return vaultPath.startsWith("Tasks/") ? vaultPath.slice("Tasks/".length) : null;
+	}
+	async find(path: string, opts: { document: boolean }) {
+		const r = this.records.get(path);
+		if (!r) return null;
+		return opts.document ? r : { ...r, document: undefined };
+	}
+	async update(seen: MdbaseRecord, changes: { patch: Record<string, unknown>; unset: string[] }) {
+		this.calls.push(["update", seen.id, changes]);
+		return { mutation: "m1" };
+	}
+	async replaceDocument(seen: MdbaseRecord, document: string) {
+		this.calls.push(["replace", seen.id, seen.document, document]);
+		return { mutation: "m2" };
+	}
+	async create(path: string, document: string) {
+		this.calls.push(["create", path, document]);
+		return { mutation: "m3" };
+	}
+	async settle(_w: unknown, vaultPath: string) {
+		this.settled.push(vaultPath);
+	}
+}
+
+const task: MdbaseRecord = {
+	id: "r1",
+	path: "t.md",
+	frontmatter: { status: "open", tags: ["a"], scheduled: "2026-10-04", meta: { x: 1 } },
+	document: "---\nstatus: open\n---\nbody\n",
+};
+
+function app() {
+	return {
+		fileManager: { processFrontMatter: jest.fn(async () => {}) },
+		vault: {
+			process: jest.fn(async (_f: TFile, fn: (c: string) => string) => fn("obsidian")),
+			modify: jest.fn(async () => {}),
+			create: jest.fn(async (p: string) => ({ path: p }) as TFile),
+			getFileByPath: jest.fn((p: string) => ({ path: p }) as TFile),
+		},
+	};
+}
+
+describe("frontmatterDiff", () => {
+	it("reports only keys the callback set or removed", () => {
+		expect(frontmatterDiff({ a: 1, b: [1, 2], c: { d: 1 }, e: "x" }, { a: 1, b: [1, 2, 3], c: { d: 1 }, f: true })).toEqual({
+			patch: { b: [1, 2, 3], f: true },
+			unset: ["e"],
+		});
+		expect(frontmatterDiff({ a: 1 }, { a: undefined })).toEqual({ patch: {}, unset: ["a"] });
+	});
+});
+
+describe("VaultMutationService with the mdbase backend", () => {
+	let client: FakeClient;
+	beforeEach(() => {
+		client = new FakeClient();
+		client.records.set("t.md", task);
+		setMdbaseMutationBackend(new MdbaseMutationBackend(client));
+	});
+	afterEach(() => setMdbaseMutationBackend(null));
+
+	it("turns a frontmatter callback into a field-level update", async () => {
+		const a = app();
+		await processVaultFrontMatter(a as never, { path: "Tasks/t.md" } as TFile, (fm) => {
+			fm.status = "done";
+			fm.completedDate = "2026-10-04";
+			delete fm.scheduled;
+			(fm.tags as string[]).push("b");
+		});
+		expect(client.calls).toEqual([
+			["update", "r1", { patch: { status: "done", tags: ["a", "b"], completedDate: "2026-10-04" }, unset: ["scheduled"] }],
+		]);
+		expect(client.settled).toEqual(["Tasks/t.md"]);
+		expect(a.fileManager.processFrontMatter).not.toHaveBeenCalled();
+		expect(task.frontmatter.tags).toEqual(["a"]); // the read view is not mutated
+	});
+
+	it("sends nothing when the callback changes nothing", async () => {
+		await processVaultFrontMatter(app() as never, { path: "Tasks/t.md" } as TFile, () => {});
+		expect(client.calls).toEqual([]);
+	});
+
+	it("turns a content callback into a document replace with the read document as base", async () => {
+		const out = await processVaultFile(app() as never, { path: "Tasks/t.md" } as TFile, (c) => c.replace("body", "BODY"));
+		expect(out).toBe("---\nstatus: open\n---\nBODY\n");
+		expect(client.calls).toEqual([["replace", "r1", task.document, out]]);
+	});
+
+	it("routes modify and create", async () => {
+		const a = app();
+		await modifyVaultFile(a as never, { path: "Tasks/t.md" } as TFile, "new");
+		const f = await createVaultFile(a as never, "Tasks/n.md", "---\ntitle: n\n---\n");
+		expect(f.path).toBe("Tasks/n.md");
+		expect(client.calls).toEqual([
+			["replace", "r1", task.document, "new"],
+			["create", "n.md", "---\ntitle: n\n---\n"],
+		]);
+		expect(a.vault.modify).not.toHaveBeenCalled();
+		expect(a.vault.create).not.toHaveBeenCalled();
+	});
+
+	it("leaves files outside the collection, and unknown records, to Obsidian", async () => {
+		const a = app();
+		await processVaultFrontMatter(a as never, { path: "Other/x.md" } as TFile, (fm) => (fm.a = 1));
+		await processVaultFrontMatter(a as never, { path: "Tasks/unknown.md" } as TFile, (fm) => (fm.a = 1));
+		await createVaultFile(a as never, "Other/y.md", "y");
+		expect(a.fileManager.processFrontMatter).toHaveBeenCalledTimes(2);
+		expect(a.vault.create).toHaveBeenCalledTimes(1);
+		expect(client.calls).toEqual([]);
+	});
+
+	it("without a backend, behaves exactly as before", async () => {
+		setMdbaseMutationBackend(null);
+		const a = app();
+		await processVaultFrontMatter(a as never, { path: "Tasks/t.md" } as TFile, (fm) => (fm.a = 1));
+		expect(a.fileManager.processFrontMatter).toHaveBeenCalledTimes(1);
+	});
+});