diff --git a/README.md b/README.md
index c35075c7..789f6256 100644
--- a/README.md
+++ b/README.md
@@ -274,6 +274,17 @@ command registers the driver in a fresh clone or worktree whose
 falls back to `git merge-file`, and GitHub's merges ignore drivers, so a PR
 shows as conflicting until a local merge from its base is pushed.
 
+The registered driver passes Git's conflict marker length and revision labels
+to `quest resolve`, including the ancestor label for `diff3` conflicts. A direct
+invocation defaults to seven-character markers and `ours`/`base`/`theirs`;
+`--marker-size`, `--ours-label`, `--base-label`, and `--theirs-label` override
+them. Labels must fit on one line. The fallback carries the same Git metadata
+when `quest` is missing or too old to accept those options.
+
+Existing configured drivers are preserved. For a checkout still using the old
+quest-generated command, clear its local `merge.quest.driver` entry and run any
+quest command to register the current one.
+
 ## Wait for pull request activity
 
 `quest wait` blocks until a GitHub PR has a review, comment, review-thread
diff --git a/quest/m1/README.md b/quest/m1/README.md
index c9e22fd0..22e8082b 100644
--- a/quest/m1/README.md
+++ b/quest/m1/README.md
@@ -17,6 +17,5 @@ additional login providers. This milestone does not gate the first release.
 - [ChatGPT approval](/quest/m1/chatgpt-waitlist.md) - resolve hosted-use approval before implementation
 - [Live inbox](/quest/m1/inbox-polish.md) - live agent cards, active quests on the log, runner choice for every job, expiring offers
 - [Worker dev setup](/quest/m1/worker-dev.md) - per-server cookies, seeded raw output, one fake GitHub, each test run alone
-- [Merge driver tuning](/quest/m1/resolve-tuning.md) - measure `quest resolve` on moq's history and match git's markers
 - [Incremental history](/quest/m1/history-pass.md) - each history pass reads only what changed since the last
 - [Gate owns descendants](/quest/m1/gate-owns-descendants.md) - cancelling `quest gate` leaves none of its command's processes behind, on Linux and macOS
diff --git a/quest/m1/resolve-tuning.md b/quest/m1/resolve-tuning.md
deleted file mode 100644
index 9145d17f..00000000
--- a/quest/m1/resolve-tuning.md
+++ /dev/null
@@ -1,20 +0,0 @@
-# [S] Measure and refine the quest merge driver
-
-## Goal
-
-The `quest resolve` merge driver's real effect is measured on moq-dev/moq's
-merge history, and its unresolved conflict markers match git's own labels
-and marker size.
-
-## Plan
-
-Decided 2026-10-24 (user) as follow-ups of
-[kixelated/quest#219](https://github.com/kixelated/quest/pull/219), ranked
-after launch:
-
-- **Replay.** Rerun the plan's replay of 280 merges of `main` into PR
-  branches in moq-dev/moq through the driver, and record how many of the 29
-  quest-only conflicts it resolves and which shapes still conflict.
-- **Marker labels.** Register the driver with git's `%L`, `%X`, and `%Y` and
-  pass them to `git merge-file`, so markers carry git's branch labels and
-  marker size.
diff --git a/src/cli/main.ts b/src/cli/main.ts
index 43fdc2c9..60f5aebb 100644
--- a/src/cli/main.ts
+++ b/src/cli/main.ts
@@ -36,7 +36,7 @@ import { type Hold, type State, finish, land, release, take } from "./hold";
 import { LAND_CHECK, checkQueue, parsePlan, parseResult, pushQueue, resolveQueue } from "./land";
 import { landed } from "./landed";
 import { type Report, moveQuest, removeQuest } from "./move";
-import { resolveFile } from "./resolve";
+import { type ResolveOptions, resolveFile } from "./resolve";
 import { heal, init, uninstall } from "./setup";
 import { Refusal, describe, notUpstream, releaseQuest, startQuest } from "./start";
 import { GUIDE, SKILLS, skill } from "./skills";
@@ -953,8 +953,12 @@ export function main(argv: string[], out: Output): number | Promise<number> {
 		.argument("<ours>", "Our version, overwritten with the result (git's %A)")
 		.argument("<base>", "The common ancestor (git's %O)")
 		.argument("<theirs>", "Their version (git's %B)")
-		.action((ours: string, base: string, theirs: string) => {
-			if (resolveFile(ours, base, theirs) > 0) code = 1;
+		.option("--marker-size <size>", "Conflict marker length (git's %L; default: 7)", positiveInteger("marker-size"))
+		.option("--ours-label <label>", "Our conflict label (git's %X; default: ours)")
+		.option("--base-label <label>", "Ancestor conflict label (git's %S; default: base)")
+		.option("--theirs-label <label>", "Their conflict label (git's %Y; default: theirs)")
+		.action((ours: string, base: string, theirs: string, options: ResolveOptions) => {
+			if (resolveFile(ours, base, theirs, options) > 0) code = 1;
 		});
 
 	program
diff --git a/src/cli/resolve.ts b/src/cli/resolve.ts
index f50c53f2..491d2b6c 100644
--- a/src/cli/resolve.ts
+++ b/src/cli/resolve.ts
@@ -18,20 +18,38 @@ const MARKER_SIZE = 7;
  */
 export const DRIVER =
 	"if quest help resolve >/dev/null 2>&1; " +
-	`then quest resolve %A %O %B; ` +
-	`else git merge-file -L ${LABELS.ours} -L ${LABELS.base} -L ${LABELS.theirs} %A %O %B; fi`;
+	"then quest resolve %A %O %B --marker-size=%L --ours-label=%X --base-label=%S --theirs-label=%Y; " +
+	'quest_merge_status=$?; if [ "$quest_merge_status" -le 1 ]; then exit "$quest_merge_status"; fi; fi; ' +
+	"git merge-file --marker-size=%L -L %X -L %S -L %Y %A %O %B";
+
+export interface ResolveOptions {
+	markerSize?: number;
+	oursLabel?: string;
+	baseLabel?: string;
+	theirsLabel?: string;
+}
 
 /**
  * Merge `theirs` into `ours` from their common `base`, as `git merge-file`
  * does, writing the result to `ours`. Returns how many conflicts remain.
  */
-export function resolveFile(ours: string, base: string, theirs: string): number {
+export function resolveFile(ours: string, base: string, theirs: string, options: ResolveOptions = {}): number {
+	const markerSize = options.markerSize ?? MARKER_SIZE;
+	if (!Number.isSafeInteger(markerSize) || markerSize <= 0)
+		throw new Error("marker size must be a positive safe integer");
+	const labels = {
+		ours: options.oursLabel ?? LABELS.ours,
+		base: options.baseLabel ?? LABELS.base,
+		theirs: options.theirsLabel ?? LABELS.theirs,
+	};
+	if (Object.values(labels).some((label) => /[\r\n]/.test(label)))
+		throw new Error("conflict labels must be single lines");
 	// Markers longer than any run of marker characters in the inputs cannot be mistaken for content.
 	const texts = [ours, base, theirs].map((file) => readFileSync(file, "utf8"));
 	const longest = Math.max(
 		...texts.flatMap((text) => text.split("\n").map((line) => /^([<|=>])\1*/.exec(line)?.[0].length ?? 0)),
 	);
-	const size = Math.max(MARKER_SIZE, longest + 1);
+	const size = Math.max(markerSize, longest + 1);
 
 	const merged = spawnSync(
 		"git",
@@ -41,11 +59,11 @@ export function resolveFile(ours: string, base: string, theirs: string): number
 			"--diff3",
 			`--marker-size=${size}`,
 			"-L",
-			LABELS.ours,
+			labels.ours,
 			"-L",
-			LABELS.base,
+			labels.base,
 			"-L",
-			LABELS.theirs,
+			labels.theirs,
 			ours,
 			base,
 			theirs,
@@ -58,8 +76,8 @@ export function resolveFile(ours: string, base: string, theirs: string): number
 		throw new Error(`git merge-file failed: ${merged.stderr.trim()}`);
 	}
 
-	const resolved = resolveLists(ours, chunks(merged.stdout, size));
-	writeFileSync(ours, render(resolved, MARKER_SIZE, diff3()));
+	const resolved = resolveLists(ours, chunks(merged.stdout, size, labels));
+	writeFileSync(ours, render(resolved, markerSize, diff3(), labels));
 	return resolved.filter((chunk) => !Array.isArray(chunk)).length;
 }
 
diff --git a/src/core/merge.ts b/src/core/merge.ts
index fd2cef43..17134b72 100644
--- a/src/core/merge.ts
+++ b/src/core/merge.ts
@@ -38,14 +38,14 @@ export const LABELS: Record<Side, string> = { ours: "ours", base: "base", theirs
 
 /**
  * Split `git merge-file --diff3` output, written with `size`-character
- * markers and `LABELS`, into chunks.
+ * markers and the supplied labels (defaulting to `LABELS`), into chunks.
  */
-export function chunks(text: string, size: number): Chunk[] {
+export function chunks(text: string, size: number, labels = LABELS): Chunk[] {
 	const marker = (char: string, label?: string) => char.repeat(size) + (label === undefined ? "" : ` ${label}`);
-	const start = marker("<", LABELS.ours);
-	const base = marker("|", LABELS.base);
+	const start = marker("<", labels.ours);
+	const base = marker("|", labels.base);
 	const split = marker("=");
-	const end = marker(">", LABELS.theirs);
+	const end = marker(">", labels.theirs);
 
 	const out: Chunk[] = [];
 	let clean: Clean = [];
@@ -81,7 +81,7 @@ export function chunks(text: string, size: number): Chunk[] {
  * Join chunks back into a file. A conflict is written with standard
  * `size`-character markers, with its base section only when `diff3` is set.
  */
-export function render(chunks: Chunk[], size: number, diff3: boolean): string {
+export function render(chunks: Chunk[], size: number, diff3: boolean, labels = LABELS): string {
 	const lines: string[] = [];
 	for (const chunk of chunks) {
 		if (Array.isArray(chunk)) {
@@ -90,9 +90,9 @@ export function render(chunks: Chunk[], size: number, diff3: boolean): string {
 		}
 		// A conflict's lines keep their own `\r`, so its markers take the same ending.
 		const eol = [...chunk.ours, ...chunk.base, ...chunk.theirs].some((line) => line.endsWith("\r")) ? "\r" : "";
-		lines.push(`${"<".repeat(size)} ${LABELS.ours}${eol}`, ...chunk.ours);
-		if (diff3) lines.push(`${"|".repeat(size)} ${LABELS.base}${eol}`, ...chunk.base);
-		lines.push(`${"=".repeat(size)}${eol}`, ...chunk.theirs, `${">".repeat(size)} ${LABELS.theirs}${eol}`);
+		lines.push(`${"<".repeat(size)} ${labels.ours}${eol}`, ...chunk.ours);
+		if (diff3) lines.push(`${"|".repeat(size)} ${labels.base}${eol}`, ...chunk.base);
+		lines.push(`${"=".repeat(size)}${eol}`, ...chunk.theirs, `${">".repeat(size)} ${labels.theirs}${eol}`);
 	}
 	return lines.join("\n");
 }
diff --git a/test/resolve.test.ts b/test/resolve.test.ts
index e6665f47..50225e1c 100644
--- a/test/resolve.test.ts
+++ b/test/resolve.test.ts
@@ -8,18 +8,23 @@ import { fileURLToPath } from "node:url";
 import { describe, expect, test } from "vitest";
 
 import { chunks, render, resolveLists } from "../src/core";
-import { resolveFile } from "../src/cli/resolve";
+import { type ResolveOptions, resolveFile } from "../src/cli/resolve";
 import { heal, init, uninstall } from "../src/cli/setup";
 import { tempDir } from "./fixture";
 
 /** Merge three versions with the driver; the merged text and how many conflicts remain. */
-function merge(base: string, ours: string, theirs: string): { text: string; conflicts: number } {
+function merge(
+	base: string,
+	ours: string,
+	theirs: string,
+	options: ResolveOptions = {},
+): { text: string; conflicts: number } {
 	const dir = tempDir();
 	const [o, b, t] = ["ours", "base", "theirs"].map((name) => join(dir, name));
 	writeFileSync(o, ours);
 	writeFileSync(b, base);
 	writeFileSync(t, theirs);
-	const conflicts = resolveFile(o, b, t);
+	const conflicts = resolveFile(o, b, t, options);
 	return { text: readFileSync(o, "utf8"), conflicts };
 }
 
@@ -119,6 +124,35 @@ describe("resolve", () => {
 		expect(render(chunks(text, 7), 7, false)).toBe("a\n<<<<<<< ours\nb\n=======\nd\n>>>>>>> theirs\ne\n");
 	});
 
+	test("preserves requested labels and marker length while resolving list conflicts", () => {
+		const base = quest(
+			["a", "b", "c"].map((n) => item(n)),
+			"Base plan.",
+		);
+		const ours = quest(
+			["a", "c"].map((n) => item(n)),
+			"Our plan.",
+		);
+		const theirs = quest(
+			["a", "b", "x", "c"].map((n) => item(n)),
+			"Their plan.",
+		);
+		const result = merge(base, ours, theirs, {
+			markerSize: 11,
+			oursLabel: "HEAD",
+			baseLabel: "ancestor",
+			theirsLabel: "topic's-head",
+		});
+		expect(result.conflicts).toBe(1);
+		expect(result.text).toContain("<<<<<<<<<<< HEAD\n");
+		expect(result.text).toContain(">>>>>>>>>>> topic's-head\n");
+		expect(result.text).toContain(["a", "x", "c"].map((n) => item(n)).join("\n"));
+	});
+
+	test("rejects multiline labels before a conflict could be mistaken for clean text", () => {
+		expect(() => merge("base\n", "ours\n", "theirs\n", { oursLabel: "one\ntwo" })).toThrow("single lines");
+	});
+
 	test("resolves the list and keeps markers for prose", () => {
 		const base = quest(["a", "b", "c"].map((n) => item(n)));
 		const ours = quest(
@@ -176,7 +210,7 @@ function git(dir: string, ...args: string[]): string {
 }
 
 /** A repository whose quest file has diverged on `main` and `branch`. */
-function diverged(): string {
+function diverged(prose = false): string {
 	const dir = tempDir();
 	git(dir, "init", "-q", "-b", "main");
 	git(dir, "config", "user.name", "Quest test");
@@ -187,18 +221,33 @@ function diverged(): string {
 		git(dir, "add", "-A");
 		git(dir, "commit", "-q", "-m", "edit");
 	};
-	write(quest(["a", "b", "c"].map((n) => item(n))));
+	write(
+		quest(
+			["a", "b", "c"].map((n) => item(n)),
+			prose ? "Base plan." : "Plan.",
+		),
+	);
 	git(dir, "switch", "-q", "-c", "branch");
-	write(quest(["a", "b", "x", "c"].map((n) => item(n))));
+	write(
+		quest(
+			(prose ? ["a", "b", "c"] : ["a", "b", "x", "c"]).map((n) => item(n)),
+			prose ? "Our plan." : "Plan.",
+		),
+	);
 	git(dir, "switch", "-q", "main");
-	write(quest(["a", "c"].map((n) => item(n))));
+	write(
+		quest(
+			(prose ? ["a", "b", "c"] : ["a", "c"]).map((n) => item(n)),
+			prose ? "Their plan." : "Plan.",
+		),
+	);
 	git(dir, "switch", "-q", "branch");
 	return dir;
 }
 
 /** Merge `main` into the branch with `path` as PATH. */
-function mergeMain(dir: string, path: string) {
-	return spawnSync("git", ["-C", dir, "merge", "-q", "--no-edit", "main"], {
+function mergeMain(dir: string, path: string, branch = "main") {
+	return spawnSync("git", ["-C", dir, "merge", "-q", "--no-edit", branch], {
 		encoding: "utf8",
 		env: { ...process.env, PATH: path },
 	});
@@ -234,9 +283,46 @@ describe.skipIf(process.platform === "win32")("the merge driver", () => {
 		}
 		const merged = mergeMain(dir, tools);
 		expect(merged.status).toBe(1);
-		expect(readFileSync(join(dir, "quest/README.md"), "utf8")).toContain("<<<<<<< ours\n");
+		expect(readFileSync(join(dir, "quest/README.md"), "utf8")).toContain("<<<<<<< HEAD\n");
 	});
 
+	for (const mode of ["quest", "missing", "older"] as const) {
+		test(`matches native diff3 labels and custom marker size with ${mode} quest`, () => {
+			const dir = diverged(true);
+			init(dir);
+			writeFileSync(join(dir, ".gitattributes"), "quest/** merge=quest conflict-marker-size=13\n");
+			git(dir, "config", "merge.conflictStyle", "diff3");
+			git(dir, "add", "-A");
+			git(dir, "commit", "-q", "-m", "init");
+			git(dir, "branch", "-m", "main", "their'branch");
+			// Capture git's built-in output for exactly these commits, then replay with our driver.
+			const attributes = join(dir, ".git/info/attributes");
+			writeFileSync(attributes, "quest/** merge=text\n");
+			expect(mergeMain(dir, process.env.PATH!, "their'branch").status).toBe(1);
+			const expected = readFileSync(join(dir, "quest/README.md"), "utf8");
+			git(dir, "merge", "--abort");
+			writeFileSync(attributes, "");
+			const tools = tempDir();
+			for (const tool of ["git", "sh"]) {
+				const found = execFileSync("sh", ["-c", `command -v ${tool}`], { encoding: "utf8" }).trim();
+				writeFileSync(join(tools, tool), `#!/bin/sh\nexec '${found}' "$@"\n`);
+				chmodSync(join(tools, tool), 0o755);
+			}
+			if (mode !== "missing") {
+				writeFileSync(
+					join(tools, "quest"),
+					mode === "older"
+						? '#!/bin/sh\nif [ "$1" = help ]; then exit 0; fi\nexit 2\n'
+						: `#!/bin/sh\nexec '${process.execPath}' --config='${bunfig}' '${bin}' "$@"\n`,
+				);
+				chmodSync(join(tools, "quest"), 0o755);
+			}
+			const result = mergeMain(dir, mode === "quest" ? `${tools}:${process.env.PATH}` : tools, "their'branch");
+			expect(result.status, result.stderr).toBe(1);
+			expect(readFileSync(join(dir, "quest/README.md"), "utf8")).toBe(expected);
+		});
+	}
+
 	test("any command registers a driver .gitattributes names, and uninstall removes it", () => {
 		const dir = diverged();
 		writeFileSync(join(dir, ".gitattributes"), "* text=auto\nquest/** merge=quest\n");