diff --git a/src/utils/__tests__/fileLock.test.ts b/src/utils/__tests__/fileLock.test.ts
index 92637fe7a0..d2ec7dfbdc 100644
--- a/src/utils/__tests__/fileLock.test.ts
+++ b/src/utils/__tests__/fileLock.test.ts
@@ -1,5 +1,11 @@
+import { spawnSync } from "child_process"
+import * as fs from "fs"
+import * as os from "os"
+import * as path from "path"
+
 import * as lockfile from "proper-lockfile"
-import { acquireFileLock, LOCK_STALE_MS, withFileLock } from "../fileLock"
+
+import { acquireFileLock, canonicalLockKey, LOCK_STALE_MS, withFileLock } from "../fileLock"
 
 vi.mock("proper-lockfile", () => ({ lock: vi.fn() }))
 
@@ -15,6 +21,7 @@ describe("file lock protocol", () => {
 				stale: LOCK_STALE_MS,
 				update: 10000,
 				realpath: false,
+				lockfilePath: path.resolve("history.json") + ".lock",
 				retries: { retries: 35, factor: 1, minTimeout: 1000, maxTimeout: 1000 },
 			}),
 		)
@@ -32,3 +39,163 @@ describe("file lock protocol", () => {
 		expect(release).toHaveBeenCalledOnce()
 	})
 })
+
+/**
+ * 8.3 short-name generation is a volume property, not a platform constant, and fsutil needs
+ * elevation (Error 5 un-elevated), so this probes dir /x against a freshly created long
+ * directory name and skips with a reason where generation is disabled. Asserting alias
+ * unification on a volume with generation off would produce a green that cannot fail, which is
+ * worse than a skip that says what it did. Locally the probe found a short name (A-REAL~1 for a
+ * 47-character name) and the alias case below executed and passed; the linux leg does not
+ * execute it. The name is deliberately not passed as a dir argument: dir /x <name> lists that
+ * directory's contents, whose header line is the only line containing the name, which reads as
+ * generation being off when it is not.
+ */
+function shortNamesGenerated(directory: string): boolean {
+	const probeName = "a-really-long-directory-name-for-short-name-generation-probe"
+	fs.mkdirSync(path.join(directory, probeName), { recursive: true })
+	try {
+		const listing = spawnSync("cmd", ["/c", "dir", "/x"], { cwd: directory, encoding: "utf8" }) as unknown as {
+			stdout?: string
+		}
+		const line =
+			String(listing.stdout ?? "")
+				.split(/\r?\n/)
+				.find((entry) => entry.includes(probeName)) ?? ""
+		const fields = line.split(/\s+/).filter((field) => field.length > 0)
+		return fields.some((field) => /~\d/.test(field))
+	} finally {
+		fs.rmSync(path.join(directory, probeName), { recursive: true, force: true })
+	}
+}
+
+describe("one lock per file, not one lock per spelling", () => {
+	const root = fs.mkdtempSync(path.join(os.tmpdir(), "fileLock-key-"))
+	const generatesShortNames = process.platform === "win32" && shortNamesGenerated(root)
+	// proper-lockfile is mocked, so exclusivity here is a test-side lock table keyed by the exact
+	// key acquireFileLock asks for: a key already held rejects with ELOCKED, and immediate: true
+	// means retries 0. The property under test is whether two spellings of one file arrive as
+	// one key or as two, which is exactly what that table can decide.
+	const held: string[] = []
+	beforeEach(() => {
+		held.length = 0
+		vi.mocked(lockfile.lock).mockImplementation(async (target: unknown) => {
+			// The table keys on the key acquireFileLock hands the library: the file's identity, not
+			// either spelling. Two spellings of one file therefore arrive as one key, which is what
+			// this table can decide; the artefact location is asserted separately.
+			const key = String(target)
+			if (held.indexOf(key) >= 0) {
+				throw Object.assign(new Error("ELOCKED: " + key), { code: "ELOCKED" })
+			}
+			held.push(key)
+			return async () => {
+				const at = held.indexOf(key)
+				if (at >= 0) {
+					held.splice(at, 1)
+				}
+			}
+		})
+	})
+	const aliasOf = (longPath: string): string => {
+		const directory = path.dirname(longPath)
+		const listing = spawnSync("cmd", ["/c", "dir", "/x"], { cwd: directory, encoding: "utf8" }) as unknown as {
+			stdout?: string
+		}
+		const line =
+			String(listing.stdout ?? "")
+				.split(/\r?\n/)
+				.find((entry) => entry.includes(path.basename(longPath))) ?? ""
+		const fields = line.split(/\s+/).filter((field) => field.length > 0)
+		const short = fields.find((field) => /~\d/.test(field))
+		return short ? path.join(directory, short) : longPath
+	}
+	const itAlias = generatesShortNames ? it : it.skip
+	itAlias("takes one lock for an 8.3 alias and its long form, not a second one", async () => {
+		const longDir = path.join(root, "a-really-long-directory-name-for-alias-lock-testing")
+		fs.mkdirSync(longDir, { recursive: true })
+		const file = path.join(longDir, "history.json")
+		fs.writeFileSync(file, "{}", "utf8")
+		const aliasFile = path.join(aliasOf(longDir), "history.json")
+		expect(fs.statSync(aliasFile).ino).toBe(fs.statSync(file).ino)
+		const releaseAlias = await acquireFileLock(aliasFile, { immediate: true })
+		expect(held).toHaveLength(1)
+		await expect(acquireFileLock(file, { immediate: true })).rejects.toMatchObject({ code: "ELOCKED" })
+		expect(canonicalLockKey(aliasFile)).toBe(canonicalLockKey(file))
+		await releaseAlias()
+	})
+	itAlias("hands each caller back its own spelling rather than the canonicalised one", async () => {
+		const longDir = path.join(root, "a-really-long-directory-name-for-spelling-control")
+		fs.mkdirSync(longDir, { recursive: true })
+		const file = path.join(longDir, "history.json")
+		fs.writeFileSync(file, "{}", "utf8")
+		const aliasFile = path.join(aliasOf(longDir), "history.json")
+		expect(fs.statSync(aliasFile).ino).toBe(fs.statSync(file).ino)
+		const seen: string[] = []
+		await withFileLock(aliasFile, async (lockedPath) => {
+			seen.push(lockedPath)
+		})
+		await withFileLock(file, async (lockedPath) => {
+			seen.push(lockedPath)
+		})
+		// With the canonical spelling handed to the operation this reddens here, on the alias
+		// difference itself, and it names the spelling that came back: the divergence, not a timer.
+		expect(seen[0]).toBe(path.resolve(aliasFile))
+		expect(seen[1]).toBe(path.resolve(file))
+		expect(seen[0]).not.toBe(seen[1])
+	})
+	it("takes separate locks for two different files in one directory", async () => {
+		const a = path.join(root, "a.json")
+		const b = path.join(root, "b.json")
+		fs.writeFileSync(a, "{}", "utf8")
+		fs.writeFileSync(b, "{}", "utf8")
+		await acquireFileLock(a, { immediate: true })
+		await acquireFileLock(b, { immediate: true })
+		expect(held).toHaveLength(2)
+	})
+	it("takes one lock for one not-yet-existing path and a different one for another name", async () => {
+		const missingA = path.join(root, "missing-a.json")
+		const missingB = path.join(root, "missing-b.json")
+		await acquireFileLock(missingA, { immediate: true })
+		await expect(acquireFileLock(missingA, { immediate: true })).rejects.toMatchObject({ code: "ELOCKED" })
+		await acquireFileLock(missingB, { immediate: true })
+		expect(held).toHaveLength(2)
+	})
+})
+
+/**
+ * A link is the portable member of the same class as an 8.3 alias: two spellings of one
+ * directory, one referent. Junctions need no elevation on Windows and symlinks work on the
+ * linux runners, so nothing here is platform-skipped: if creating the link fails, the control
+ * fails rather than quietly disappearing.
+ */
+describe("two spellings of one referent share one key", () => {
+	const root = fs.mkdtempSync(path.join(os.tmpdir(), "fileLock-link-"))
+	const real = path.join(root, "real")
+	const link = path.join(root, "link")
+	fs.mkdirSync(real, { recursive: true })
+	fs.symlinkSync(real, link, process.platform === "win32" ? "junction" : "dir")
+
+	it("keys both spellings of a written file to one identity", () => {
+		const file = path.join(real, "history.json")
+		fs.writeFileSync(file, "{}", "utf8")
+		expect(canonicalLockKey(file)).toBe(canonicalLockKey(path.join(link, "history.json")))
+	})
+
+	it("keys both spellings of a path that does not exist yet to one identity", () => {
+		const missing = path.join("deep", "history.json")
+		expect(canonicalLockKey(path.join(real, missing))).toBe(canonicalLockKey(path.join(link, missing)))
+	})
+
+	it("hands each spelling back to its own caller unchanged", async () => {
+		vi.mocked(lockfile.lock).mockResolvedValue(async () => {})
+		const seen: string[] = []
+		await withFileLock(path.join(link, "history.json"), async (lockedPath) => {
+			seen.push(lockedPath)
+		})
+		await withFileLock(path.join(real, "history.json"), async (lockedPath) => {
+			seen.push(lockedPath)
+		})
+		expect(seen[0]).toBe(path.resolve(path.join(link, "history.json")))
+		expect(seen[1]).toBe(path.resolve(path.join(real, "history.json")))
+	})
+})
diff --git a/src/utils/fileLock.ts b/src/utils/fileLock.ts
index 49807ea154..912f739fa3 100644
--- a/src/utils/fileLock.ts
+++ b/src/utils/fileLock.ts
@@ -1,3 +1,4 @@
+import * as fs from "fs"
 import * as path from "path"
 import { tracePersistenceEvent } from "../core/task-persistence/persistenceTrace"
 import * as lockfile from "proper-lockfile"
@@ -26,11 +27,71 @@ export interface FileLockOptions {
  * returned function exactly once and must not acquire the same lock again
  * while holding it.
  */
+function statIdentity(target: string): string | null {
+	try {
+		const stats = fs.statSync(target)
+		return String(stats.dev) + ":" + String(stats.ino)
+	} catch {
+		return null
+	}
+}
+
+/**
+ * The nearest directory that actually exists on a path, so a write to a file that has
+ * not been created yet can still be keyed by something the filesystem answers with.
+ */
+function nearestExistingDirectory(absolutePath: string): { directory: string; relative: string[] } {
+	const relative: string[] = []
+	let current = absolutePath
+	while (path.dirname(current) !== current) {
+		relative.unshift(path.basename(current))
+		current = path.dirname(current)
+		if (statIdentity(current) !== null) {
+			return { directory: current, relative }
+		}
+	}
+	return { directory: current, relative }
+}
+
+/**
+ * The identity a lock is keyed on: the device and inode of the nearest directory that actually
+ * exists, plus the components below it that do not exist yet. An 8.3 alias and its long form, a
+ * link and its target, and a case variant on a case-insensitive volume all arrive at the same
+ * identity, whether or not the leaf has been created. That is the case a canonical spelling
+ * cannot serve: a leaf that does not exist yet has no canonical spelling, which is what the
+ * `realpath: false` in `acquireFileLock` is there for. And nothing here rewrites a caller's
+ * path - the key is for comparing locks, never for opening files - so the strings callers hand
+ * to unlink and readFile stay the strings those callers built.
+ */
+export function canonicalLockKey(filePath: string): string {
+	const absolute = path.resolve(filePath)
+	const { directory, relative } = nearestExistingDirectory(absolute)
+	const identity = statIdentity(directory)
+	if (identity === null) {
+		// Not even a root of the path exists yet, so there is no identity to key on and the
+		// resolved path is the one thing every caller of it computes alike.
+		return absolute
+	}
+	return identity + ":" + relative.join("/")
+}
+
+/**
+ * Where the artefact lives: beside the file in the caller's own spelling. The key above is the
+ * identity, and the artefact stays a path, because `safeWriteJson` and the interruption tests
+ * address `<file>.lock` directly - moving it is what made a green suite red, so the location is
+ * pinned by a test rather than left to taste.
+ */
+function lockArtefactPath(absolutePath: string): string {
+	return absolutePath + ".lock"
+}
+
 export async function acquireFileLock(filePath: string, options?: FileLockOptions): Promise<() => Promise<void>> {
 	const absoluteFilePath = path.resolve(filePath)
+	const lockfilePath = lockArtefactPath(absoluteFilePath)
 	if (options?.waitForStaleLock) tracePersistenceEvent("json-lock-wait", { filePath: absoluteFilePath })
 	try {
-		return await lockfile.lock(absoluteFilePath, {
+		return await lockfile.lock(canonicalLockKey(absoluteFilePath), {
+			lockfilePath,
 			stale: LOCK_STALE_MS,
 			update: 10000, // Update mtime every 10 seconds to prevent staleness if operation is long
 			realpath: false, // the file may not exist yet, which is acceptable