diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 059389934..3c14d2093 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -28,6 +28,7 @@
 - xAI web search now uses `grok-4.5` (at low reasoning effort) instead of `grok-4.3`.
 
 ### Fixed
+- Fixed a first-use race in `ArtifactManager` where two concurrent `allocatePath`/`save` callers on a fresh instance both re-seeded `#nextId` across the directory-scan yield and allocated the same artifact id, silently overwriting the first artifact (same tool type) or making `artifact://` resolution ambiguous (different tool types). The initial scan is now memoized as a single in-flight promise so all concurrent callers share one initialization and receive distinct ids ([#4091](https://github.com/can1357/oh-my-pi/issues/4091)).
 
 - Fixed discarded `Settings` instances keeping debounced save timers and chained background saves armed; discarding an instance now cancels its pending writes so they cannot race a successor's file locks.
 - Fixed `error.notify` raising a "Stopped with error" toast for provider failures while an auto-retry or async-delivery continuation was pending; the toast now waits for the true terminal settle.
diff --git a/packages/coding-agent/src/session/artifacts.ts b/packages/coding-agent/src/session/artifacts.ts
index 0cd61d9a7..a7e9b661f 100644
--- a/packages/coding-agent/src/session/artifacts.ts
+++ b/packages/coding-agent/src/session/artifacts.ts
@@ -39,7 +39,7 @@ export class ArtifactManager {
 	#nextId = 0;
 	readonly #dir: string;
 	#dirCreated = false;
-	#initialized = false;
+	#initPromise: Promise<void> | null = null;
 
 	/**
 	 * @param dir Directory that will hold artifact files. Created lazily on first save.
@@ -61,10 +61,11 @@ export class ArtifactManager {
 			await fs.mkdir(this.#dir, { recursive: true });
 			this.#dirCreated = true;
 		}
-		if (!this.#initialized) {
-			await this.#scanExistingIds();
-			this.#initialized = true;
-		}
+		// Memoize the first-use scan so it runs exactly once. Concurrent callers
+		// share the in-flight promise instead of each re-seeding #nextId across
+		// the readdir yield in #scanExistingIds (which would hand duplicate ids).
+		this.#initPromise ??= this.#scanExistingIds();
+		await this.#initPromise;
 	}
 
 	/**
diff --git a/packages/coding-agent/test/artifacts-concurrency.test.ts b/packages/coding-agent/test/artifacts-concurrency.test.ts
new file mode 100644
index 000000000..db776e530
--- /dev/null
+++ b/packages/coding-agent/test/artifacts-concurrency.test.ts
@@ -0,0 +1,72 @@
+import { afterEach, describe, expect, it } from "bun:test";
+import * as os from "node:os";
+import * as path from "node:path";
+import { ArtifactManager } from "@oh-my-pi/pi-coding-agent/session/artifacts";
+import { removeSyncWithRetries } from "@oh-my-pi/pi-utils";
+
+describe("ArtifactManager concurrent first-use", () => {
+	const dirs: string[] = [];
+
+	function freshDir(): string {
+		const dir = path.join(os.tmpdir(), `omp-artifacts-${crypto.randomUUID()}`, "session");
+		dirs.push(path.dirname(dir));
+		return dir;
+	}
+
+	afterEach(() => {
+		for (const dir of dirs.splice(0)) {
+			removeSyncWithRetries(dir);
+		}
+	});
+
+	// First-use init (dir scan → #nextId seed) must run exactly once. Two callers
+	// racing a fresh manager both yield inside #scanExistingIds before either
+	// marks init done; if the second re-seeds #nextId after the first consumed an
+	// id, both allocate the same numeric id and the second write clobbers the
+	// first. Same toolType => file overwrite; the first id resolves to B's bytes.
+	it("hands concurrent same-toolType savers distinct ids that each resolve to their own content", async () => {
+		const mgr = new ArtifactManager(freshDir());
+		const [idA, idB] = await Promise.all([mgr.save("CONTENT-A", "bash"), mgr.save("CONTENT-B", "bash")]);
+
+		expect(idA).not.toBe(idB);
+
+		const pathA = await mgr.getPath(idA);
+		const pathB = await mgr.getPath(idB);
+		expect(pathA).not.toBeNull();
+		expect(pathB).not.toBeNull();
+		expect(await Bun.file(pathA as string).text()).toBe("CONTENT-A");
+		expect(await Bun.file(pathB as string).text()).toBe("CONTENT-B");
+	});
+
+	// Different toolTypes turn a duplicate id into two coexisting files
+	// (`{id}.bash.log` + `{id}.async.log`); getPath's startsWith(`${id}.`) then
+	// resolves ambiguously in unspecified readdir order. Distinct ids keep each
+	// artifact:// pointing at the content its caller wrote.
+	it("hands concurrent different-toolType savers distinct ids that each resolve to their own content", async () => {
+		const mgr = new ArtifactManager(freshDir());
+		const [idA, idB] = await Promise.all([mgr.save("BASH-BYTES", "bash"), mgr.save("ASYNC-BYTES", "async")]);
+
+		expect(idA).not.toBe(idB);
+
+		const pathA = await mgr.getPath(idA);
+		const pathB = await mgr.getPath(idB);
+		expect(await Bun.file(pathA as string).text()).toBe("BASH-BYTES");
+		expect(await Bun.file(pathB as string).text()).toBe("ASYNC-BYTES");
+	});
+
+	// The race also re-opens on a fresh manager over a directory that already
+	// holds artifacts (e.g. after a `#artifactManager = null` reset): the scan
+	// seeds from maxId, and concurrent callers must still get ids past it.
+	it("does not reuse ids when racing init over a pre-populated directory", async () => {
+		const dir = freshDir();
+		const seed = new ArtifactManager(dir);
+		await seed.save("OLD", "bash");
+
+		const mgr = new ArtifactManager(dir);
+		const [idA, idB] = await Promise.all([mgr.save("NEW-A", "bash"), mgr.save("NEW-B", "bash")]);
+
+		expect(idA).not.toBe(idB);
+		expect(await Bun.file((await mgr.getPath(idA)) as string).text()).toBe("NEW-A");
+		expect(await Bun.file((await mgr.getPath(idB)) as string).text()).toBe("NEW-B");
+	});
+});