diff --git a/packages/harness/src/attachments/attachment-store.ts b/packages/harness/src/attachments/attachment-store.ts
new file mode 100644
index 0000000..483bf67
--- /dev/null
+++ b/packages/harness/src/attachments/attachment-store.ts
@@ -0,0 +1,19 @@
+import type { ImageAttachment } from "@agent-studio/shared";
+import type { ImageContent } from "@earendil-works/pi-ai";
+import type { Readable } from "node:stream";
+import type { SupportedImageMediaType } from "./image-signatures.js";
+
+export type ImageUpload = {
+  name: string;
+  mediaType: SupportedImageMediaType;
+  stream: Readable;
+};
+
+export interface AttachmentStore {
+  add(sessionId: string, upload: ImageUpload): Promise<ImageAttachment>;
+  get(sessionId: string, attachmentId: string): ImageAttachment | undefined;
+  openNormalized(sessionId: string, attachmentId: string): Promise<Buffer>;
+  remove(sessionId: string, attachmentId: string): Promise<boolean>;
+  removeAll(sessionId: string): Promise<void>;
+  toPiImages(sessionId: string, attachmentIds: string[]): Promise<ImageContent[]>;
+}
diff --git a/packages/harness/src/attachments/file-attachment-store.ts b/packages/harness/src/attachments/file-attachment-store.ts
new file mode 100644
index 0000000..d1154ab
--- /dev/null
+++ b/packages/harness/src/attachments/file-attachment-store.ts
@@ -0,0 +1,262 @@
+import { randomUUID } from "node:crypto";
+import { createWriteStream } from "node:fs";
+import { chmod, lstat, mkdir, readFile, realpath, rename, rm, stat } from "node:fs/promises";
+import { isAbsolute, join, relative, resolve, sep } from "node:path";
+import { Transform, type TransformCallback } from "node:stream";
+import { pipeline } from "node:stream/promises";
+import { ImageAttachmentSchema, type ImageAttachment } from "@agent-studio/shared";
+import type { ImageContent } from "@earendil-works/pi-ai";
+import sharp from "sharp";
+import type { EventStore } from "../events/event-store.js";
+import { HarnessError } from "../errors/harness-error.js";
+import type { AttachmentStore, ImageUpload } from "./attachment-store.js";
+import { assertImageType, extensionForMediaType, type SupportedImageMediaType } from "./image-signatures.js";
+
+type StoredAttachment = { metadata: ImageAttachment; filePath: string };
+
+export type FileAttachmentStoreOptions = {
+  sessionsRoot: string;
+  eventStore?: EventStore;
+  maxBytes?: number;
+  maxWidth?: number;
+  maxHeight?: number;
+  maxPixels?: number;
+  maxAttachmentsPerMessage?: number;
+};
+
+export class FileAttachmentStore implements AttachmentStore {
+  readonly #sessionsRoot: string;
+  readonly #eventStore: EventStore | undefined;
+  readonly #maxBytes: number;
+  readonly #maxWidth: number;
+  readonly #maxHeight: number;
+  readonly #maxPixels: number;
+  readonly #maxAttachmentsPerMessage: number;
+  readonly #bySession = new Map<string, Map<string, StoredAttachment>>();
+
+  public constructor(options: FileAttachmentStoreOptions) {
+    this.#sessionsRoot = resolve(options.sessionsRoot);
+    this.#eventStore = options.eventStore;
+    this.#maxBytes = options.maxBytes ?? 10 * 1024 * 1024;
+    this.#maxWidth = options.maxWidth ?? 8_192;
+    this.#maxHeight = options.maxHeight ?? 8_192;
+    this.#maxPixels = options.maxPixels ?? 25_000_000;
+    this.#maxAttachmentsPerMessage = options.maxAttachmentsPerMessage ?? 4;
+    for (const [name, value] of Object.entries({
+      maxBytes: this.#maxBytes,
+      maxWidth: this.#maxWidth,
+      maxHeight: this.#maxHeight,
+      maxPixels: this.#maxPixels,
+      maxAttachmentsPerMessage: this.#maxAttachmentsPerMessage,
+    })) assertPositiveInteger(value, name);
+  }
+
+  public async add(sessionId: string, upload: ImageUpload): Promise<ImageAttachment> {
+    let uploadPath: string | undefined;
+    let normalizedPath: string | undefined;
+    try {
+      const safeName = validateUploadMetadata(sessionId, upload);
+      const attachmentDirectory = await this.#attachmentDirectory(sessionId);
+      uploadPath = join(attachmentDirectory, `${randomUUID()}.upload`);
+      normalizedPath = join(attachmentDirectory, `${randomUUID()}.normalized`);
+      await pipeline(
+        upload.stream,
+        new ByteLimitTransform(this.#maxBytes),
+        createWriteStream(uploadPath, { flags: "wx", mode: 0o600 }),
+      );
+      const headerAndBody = await readFile(uploadPath);
+      const mediaType = assertImageType(safeName, upload.mediaType, headerAndBody.subarray(0, 12));
+      const sourceMetadata = await sharp(uploadPath, { failOn: "error", limitInputPixels: false }).metadata();
+      assertDimensions(sourceMetadata.width, sourceMetadata.height, this.#maxWidth, this.#maxHeight, this.#maxPixels);
+      await normalizeImage(uploadPath, normalizedPath, mediaType, this.#maxPixels);
+      const imageMetadata = await sharp(normalizedPath, { failOn: "error", limitInputPixels: this.#maxPixels }).metadata();
+      const width = imageMetadata.width;
+      const height = imageMetadata.height;
+      assertDimensions(width, height, this.#maxWidth, this.#maxHeight, this.#maxPixels);
+      const normalizedStat = await stat(normalizedPath);
+      if (normalizedStat.size > this.#maxBytes) {
+        throw new HarnessError("IMAGE_TOO_LARGE", "Normalized image exceeds the configured byte limit");
+      }
+
+      const attachmentId = `att_${randomUUID()}`;
+      const finalPath = join(attachmentDirectory, `${attachmentId}${extensionForMediaType(mediaType)}`);
+      await rename(normalizedPath, finalPath);
+      normalizedPath = undefined;
+      const metadata = ImageAttachmentSchema.parse({
+        attachmentId,
+        sessionId,
+        name: safeName,
+        mediaType,
+        byteSize: normalizedStat.size,
+        width,
+        height,
+        status: "ready",
+      });
+      this.#sessionIndex(sessionId).set(attachmentId, { metadata, filePath: finalPath });
+      this.#eventStore?.append(sessionId, {
+        type: "attachment.uploaded",
+        payload: {
+          attachmentId,
+          name: safeName,
+          mediaType,
+          byteSize: normalizedStat.size,
+          width,
+          height,
+        },
+      });
+      return { ...metadata };
+    } catch (error) {
+      const safeName = typeof upload.name === "string" && upload.name.trim() !== ""
+        ? upload.name.trim().slice(0, 255)
+        : "unnamed-image";
+      this.#eventStore?.append(sessionId, {
+        type: "attachment.rejected",
+        payload: { name: safeName, reason: safeErrorCode(error) },
+      });
+      if (error instanceof HarnessError) throw error;
+      throw new HarnessError("IMAGE_INVALID", "Image could not be decoded");
+    } finally {
+      if (uploadPath !== undefined) await rm(uploadPath, { force: true }).catch(() => undefined);
+      if (normalizedPath !== undefined) await rm(normalizedPath, { force: true }).catch(() => undefined);
+    }
+  }
+
+  public get(sessionId: string, attachmentId: string): ImageAttachment | undefined {
+    const stored = this.#bySession.get(sessionId)?.get(attachmentId);
+    return stored === undefined ? undefined : { ...stored.metadata };
+  }
+
+  public async openNormalized(sessionId: string, attachmentId: string): Promise<Buffer> {
+    const stored = this.#stored(sessionId, attachmentId);
+    return readFile(stored.filePath);
+  }
+
+  public async remove(sessionId: string, attachmentId: string): Promise<boolean> {
+    const session = this.#bySession.get(sessionId);
+    const stored = session?.get(attachmentId);
+    if (stored === undefined) return false;
+    session!.delete(attachmentId);
+    await rm(stored.filePath, { force: true });
+    return true;
+  }
+
+  public async removeAll(sessionId: string): Promise<void> {
+    const session = this.#bySession.get(sessionId);
+    if (session === undefined) return;
+    this.#bySession.delete(sessionId);
+    await Promise.all([...session.values()].map(({ filePath }) => rm(filePath, { force: true })));
+  }
+
+  public async toPiImages(sessionId: string, attachmentIds: string[]): Promise<ImageContent[]> {
+    if (attachmentIds.length > this.#maxAttachmentsPerMessage) {
+      throw new HarnessError("TOO_MANY_ATTACHMENTS", "Message contains too many attachments");
+    }
+    return Promise.all(attachmentIds.map(async (attachmentId) => {
+      const stored = this.#stored(sessionId, attachmentId);
+      const bytes = await readFile(stored.filePath);
+      return { type: "image", data: bytes.toString("base64"), mimeType: stored.metadata.mediaType };
+    }));
+  }
+
+  async #attachmentDirectory(sessionId: string): Promise<string> {
+    if (!/^ses_[a-zA-Z0-9_-]+$/.test(sessionId)) {
+      throw new HarnessError("INVALID_SESSION_ID", "Session ID is invalid");
+    }
+    const canonicalRoot = await realpath(this.#sessionsRoot);
+    const sessionDirectory = join(canonicalRoot, sessionId);
+    const sessionMetadata = await lstat(sessionDirectory).catch(() => undefined);
+    if (sessionMetadata === undefined || !sessionMetadata.isDirectory() || sessionMetadata.isSymbolicLink()) {
+      throw new HarnessError("SESSION_NOT_FOUND", "Session workspace does not exist");
+    }
+    assertContained(canonicalRoot, await realpath(sessionDirectory));
+    const directory = join(sessionDirectory, "attachments");
+    await mkdir(directory, { recursive: true, mode: 0o700 });
+    const canonicalDirectory = await realpath(directory);
+    assertContained(sessionDirectory, canonicalDirectory);
+    return canonicalDirectory;
+  }
+
+  #stored(sessionId: string, attachmentId: string): StoredAttachment {
+    const stored = this.#bySession.get(sessionId)?.get(attachmentId);
+    if (stored === undefined) throw new HarnessError("ATTACHMENT_NOT_FOUND", "Attachment was not found");
+    return stored;
+  }
+
+  #sessionIndex(sessionId: string): Map<string, StoredAttachment> {
+    const current = this.#bySession.get(sessionId);
+    if (current !== undefined) return current;
+    const created = new Map<string, StoredAttachment>();
+    this.#bySession.set(sessionId, created);
+    return created;
+  }
+}
+
+class ByteLimitTransform extends Transform {
+  #bytes = 0;
+
+  public constructor(private readonly maxBytes: number) {
+    super();
+  }
+
+  public override _transform(chunk: Buffer, _encoding: BufferEncoding, callback: TransformCallback): void {
+    this.#bytes += chunk.byteLength;
+    if (this.#bytes > this.maxBytes) {
+      callback(new HarnessError("IMAGE_TOO_LARGE", "Image exceeds the configured byte limit"));
+      return;
+    }
+    callback(null, chunk);
+  }
+}
+
+async function normalizeImage(
+  inputPath: string,
+  outputPath: string,
+  mediaType: SupportedImageMediaType,
+  maxPixels: number,
+): Promise<void> {
+  let pipeline = sharp(inputPath, { failOn: "error", limitInputPixels: maxPixels }).rotate();
+  if (mediaType === "image/jpeg") pipeline = pipeline.jpeg({ quality: 90 });
+  if (mediaType === "image/png") pipeline = pipeline.png();
+  if (mediaType === "image/webp") pipeline = pipeline.webp({ quality: 90 });
+  await pipeline.toFile(outputPath);
+  await chmod(outputPath, 0o600);
+}
+
+function validateUploadMetadata(sessionId: string, upload: ImageUpload): string {
+  if (!/^ses_[a-zA-Z0-9_-]+$/.test(sessionId)) {
+    throw new HarnessError("INVALID_SESSION_ID", "Session ID is invalid");
+  }
+  if (typeof upload.name !== "string" || upload.name.trim().length === 0 || upload.name.trim().length > 255) {
+    throw new HarnessError("INVALID_ATTACHMENT_NAME", "Attachment name is invalid");
+  }
+  return upload.name.trim();
+}
+
+function safeErrorCode(error: unknown): string {
+  return error instanceof HarnessError ? error.code : "IMAGE_INVALID";
+}
+
+function assertContained(root: string, candidate: string): void {
+  const fromRoot = relative(root, candidate);
+  if (fromRoot === "" || (!fromRoot.startsWith(`..${sep}`) && fromRoot !== ".." && !isAbsolute(fromRoot))) return;
+  throw new HarnessError("PATH_OUTSIDE_WORKSPACE", "Attachment path is outside the Session workspace");
+}
+
+function assertPositiveInteger(value: number, name: string): void {
+  if (!Number.isSafeInteger(value) || value < 1) throw new RangeError(`${name} must be a positive integer`);
+}
+
+function assertDimensions(
+  width: number | undefined,
+  height: number | undefined,
+  maxWidth: number,
+  maxHeight: number,
+  maxPixels: number,
+): asserts width is number {
+  if (width === undefined || height === undefined) {
+    throw new HarnessError("IMAGE_INVALID", "Image dimensions could not be determined");
+  }
+  if (width > maxWidth || height > maxHeight || width * height > maxPixels) {
+    throw new HarnessError("IMAGE_DIMENSIONS_EXCEEDED", "Image dimensions exceed the configured limit");
+  }
+}
diff --git a/packages/harness/src/attachments/image-signatures.ts b/packages/harness/src/attachments/image-signatures.ts
new file mode 100644
index 0000000..3dc0363
--- /dev/null
+++ b/packages/harness/src/attachments/image-signatures.ts
@@ -0,0 +1,53 @@
+import { extname } from "node:path";
+import { HarnessError } from "../errors/harness-error.js";
+
+export type SupportedImageMediaType = "image/jpeg" | "image/png" | "image/webp";
+
+const extensions: Record<SupportedImageMediaType, ReadonlySet<string>> = {
+  "image/jpeg": new Set([".jpg", ".jpeg"]),
+  "image/png": new Set([".png"]),
+  "image/webp": new Set([".webp"]),
+};
+
+export function detectImageMediaType(bytes: Uint8Array): SupportedImageMediaType | undefined {
+  if (
+    bytes.length >= 8 &&
+    bytes[0] === 0x89 && bytes[1] === 0x50 && bytes[2] === 0x4e && bytes[3] === 0x47 &&
+    bytes[4] === 0x0d && bytes[5] === 0x0a && bytes[6] === 0x1a && bytes[7] === 0x0a
+  ) return "image/png";
+  if (bytes.length >= 3 && bytes[0] === 0xff && bytes[1] === 0xd8 && bytes[2] === 0xff) {
+    return "image/jpeg";
+  }
+  if (
+    bytes.length >= 12 &&
+    Buffer.from(bytes.subarray(0, 4)).toString("ascii") === "RIFF" &&
+    Buffer.from(bytes.subarray(8, 12)).toString("ascii") === "WEBP"
+  ) return "image/webp";
+  return undefined;
+}
+
+export function assertImageType(
+  name: string,
+  declaredType: string,
+  bytes: Uint8Array,
+): SupportedImageMediaType {
+  if (!(declaredType in extensions)) {
+    throw new HarnessError("IMAGE_TYPE_NOT_ALLOWED", "Only JPEG, PNG, and WebP images are allowed");
+  }
+  const mediaType = declaredType as SupportedImageMediaType;
+  const extension = extname(name).toLowerCase();
+  if (!extensions[mediaType].has(extension)) {
+    throw new HarnessError("IMAGE_TYPE_NOT_ALLOWED", "Filename extension does not match an allowed image type");
+  }
+  const detected = detectImageMediaType(bytes);
+  if (detected === undefined || detected !== mediaType) {
+    throw new HarnessError("IMAGE_SIGNATURE_MISMATCH", "Image bytes do not match the declared type");
+  }
+  return mediaType;
+}
+
+export function extensionForMediaType(mediaType: SupportedImageMediaType): string {
+  if (mediaType === "image/jpeg") return ".jpg";
+  if (mediaType === "image/png") return ".png";
+  return ".webp";
+}
diff --git a/packages/harness/src/index.ts b/packages/harness/src/index.ts
index dd19787..aa1ad20 100644
--- a/packages/harness/src/index.ts
+++ b/packages/harness/src/index.ts
@@ -1,5 +1,8 @@
 export * from "./approvals/approval-broker.js";
 export * from "./approvals/in-memory-approval-broker.js";
+export * from "./attachments/attachment-store.js";
+export * from "./attachments/file-attachment-store.js";
+export * from "./attachments/image-signatures.js";
 export * from "./events/event-store.js";
 export * from "./events/in-memory-event-store.js";
 export * from "./errors/harness-error.js";
diff --git a/packages/harness/tests/attachments.test.ts b/packages/harness/tests/attachments.test.ts
new file mode 100644
index 0000000..33ea670
--- /dev/null
+++ b/packages/harness/tests/attachments.test.ts
@@ -0,0 +1,121 @@
+import { mkdir, mkdtemp, rm } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { Readable } from "node:stream";
+import sharp from "sharp";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import * as harness from "../src/index.js";
+
+type MediaType = "image/jpeg" | "image/png" | "image/webp";
+type Upload = { name: string; mediaType: MediaType; stream: Readable };
+type Attachment = { attachmentId: string; sessionId: string; name: string; mediaType: MediaType; byteSize: number; width: number; height: number; status: "ready" };
+type Store = {
+  add(sessionId: string, upload: Upload): Promise<Attachment>;
+  get(sessionId: string, attachmentId: string): Attachment | undefined;
+  openNormalized(sessionId: string, attachmentId: string): Promise<Buffer>;
+  remove(sessionId: string, attachmentId: string): Promise<boolean>;
+  removeAll(sessionId: string): Promise<void>;
+  toPiImages(sessionId: string, attachmentIds: string[]): Promise<Array<{ type: "image"; data: string; mimeType: string }>>;
+};
+
+let root = "";
+
+beforeEach(async () => {
+  root = await mkdtemp(join(tmpdir(), "agent-studio-attachments-"));
+  await mkdir(join(root, "ses_a"), { recursive: true });
+  await mkdir(join(root, "ses_b"), { recursive: true });
+});
+
+afterEach(async () => {
+  await rm(root, { recursive: true, force: true });
+});
+
+function createStore(options: Record<string, unknown> = {}): Store {
+  const Constructor = Reflect.get(harness, "FileAttachmentStore") as unknown as
+    | (new (options: Record<string, unknown>) => Store)
+    | undefined;
+  expect(Constructor, "FileAttachmentStore must be exported").toBeDefined();
+  return new Constructor!({ sessionsRoot: root, ...options });
+}
+
+function upload(name: string, mediaType: MediaType, bytes: Buffer): Upload {
+  return { name, mediaType, stream: Readable.from(bytes) };
+}
+
+async function image(format: "jpeg" | "png" | "webp", width = 4, height = 3): Promise<Buffer> {
+  return sharp({ create: { width, height, channels: 3, background: "#7c3aed" } })[format]().toBuffer();
+}
+
+describe("FileAttachmentStore", () => {
+  it.each([
+    ["photo.jpg", "image/jpeg", "jpeg"],
+    ["chart.png", "image/png", "png"],
+    ["creative.webp", "image/webp", "webp"],
+  ] as const)("accepts and normalizes %s", async (name, mediaType, format) => {
+    const store = createStore();
+    const item = await store.add("ses_a", upload(name, mediaType, await image(format)));
+
+    expect(item).toMatchObject({ sessionId: "ses_a", name, mediaType, width: 4, height: 3, status: "ready" });
+    expect(item.attachmentId).toMatch(/^att_/);
+    expect((await store.openNormalized("ses_a", item.attachmentId)).byteLength).toBe(item.byteSize);
+  });
+
+  it("rejects declared PNG with JPEG bytes", async () => {
+    await expect(createStore().add("ses_a", upload("fake.png", "image/png", await image("jpeg"))))
+      .rejects.toMatchObject({ code: "IMAGE_SIGNATURE_MISMATCH" });
+  });
+
+  it("rejects unsupported SVG before decoding", async () => {
+    const svg = Buffer.from('<svg xmlns="http://www.w3.org/2000/svg"><rect width="1" height="1"/></svg>');
+    await expect(
+      createStore().add("ses_a", { name: "vector.svg", mediaType: "image/png", stream: Readable.from(svg) }),
+    ).rejects.toMatchObject({ code: "IMAGE_TYPE_NOT_ALLOWED" });
+  });
+
+  it("enforces stream byte and decoded dimension limits", async () => {
+    const png = await image("png", 10, 10);
+    await expect(createStore({ maxBytes: 12 }).add("ses_a", upload("large.png", "image/png", png)))
+      .rejects.toMatchObject({ code: "IMAGE_TOO_LARGE" });
+    await expect(createStore({ maxWidth: 5 }).add("ses_a", upload("wide.png", "image/png", png)))
+      .rejects.toMatchObject({ code: "IMAGE_DIMENSIONS_EXCEEDED" });
+    await expect(createStore({ maxPixels: 50 }).add("ses_a", upload("pixels.png", "image/png", png)))
+      .rejects.toMatchObject({ code: "IMAGE_DIMENSIONS_EXCEEDED" });
+  });
+
+  it("does not expose cross-session attachment metadata or bytes", async () => {
+    const store = createStore();
+    const item = await store.add("ses_a", upload("chart.png", "image/png", await image("png")));
+
+    expect(store.get("ses_b", item.attachmentId)).toBeUndefined();
+    await expect(store.openNormalized("ses_b", item.attachmentId)).rejects.toMatchObject({ code: "ATTACHMENT_NOT_FOUND" });
+  });
+
+  it("maps ready attachments to Pi images, enforces count, and removes idempotently", async () => {
+    const store = createStore({ maxAttachmentsPerMessage: 1 });
+    const first = await store.add("ses_a", upload("one.png", "image/png", await image("png")));
+    const second = await store.add("ses_a", upload("two.png", "image/png", await image("png")));
+
+    const images = await store.toPiImages("ses_a", [first.attachmentId]);
+    expect(images[0]).toMatchObject({ type: "image", mimeType: "image/png" });
+    expect(Buffer.from(images[0]!.data, "base64").byteLength).toBe(first.byteSize);
+    await expect(store.toPiImages("ses_a", [first.attachmentId, second.attachmentId])).rejects.toMatchObject({ code: "TOO_MANY_ATTACHMENTS" });
+    await expect(store.remove("ses_a", first.attachmentId)).resolves.toBe(true);
+    await expect(store.remove("ses_a", first.attachmentId)).resolves.toBe(false);
+    await store.removeAll("ses_a");
+    expect(store.get("ses_a", second.attachmentId)).toBeUndefined();
+  });
+
+  it("emits safe metadata events without image data", async () => {
+    const Store = Reflect.get(harness, "InMemoryEventStore") as new () => {
+      listAfter(sessionId: string): { events: unknown[] };
+    };
+    const events = new Store();
+    const store = createStore({ eventStore: events });
+    await store.add("ses_a", upload("chart.png", "image/png", await image("png")));
+
+    const serialized = JSON.stringify(events.listAfter("ses_a"));
+    expect(serialized).toContain("attachment.uploaded");
+    expect(serialized).not.toContain("base64");
+    expect(serialized).not.toContain("iVBOR");
+  });
+});