diff --git a/src/formats/reader.ts b/src/formats/reader.ts
new file mode 100644
index 0000000..296d453
--- /dev/null
+++ b/src/formats/reader.ts
@@ -0,0 +1,190 @@
+export class FormatError extends Error {
+  public format: string;
+  public field: string;
+  public offset: number;
+
+  constructor(format: string, field: string, offset: number, message: string) {
+    super(message);
+    this.name = 'FormatError';
+    this.format = format;
+    this.field = field;
+    this.offset = offset;
+    Object.setPrototypeOf(this, new.target.prototype);
+  }
+}
+
+export class TruncatedDataError extends FormatError {
+  public needed: number;
+  public available: number;
+
+  constructor(
+    format: string,
+    field: string,
+    offset: number,
+    needed: number,
+    available: number,
+    bufferLength: number
+  ) {
+    super(
+      format,
+      field,
+      offset,
+      `${format}: truncated reading ${field} at offset 0x${offset.toString(16).toUpperCase()}: need ${needed} bytes, only ${available} available (buffer length ${bufferLength})`
+    );
+    this.name = 'TruncatedDataError';
+    this.needed = needed;
+    this.available = available;
+    Object.setPrototypeOf(this, new.target.prototype);
+  }
+}
+
+export class InvalidFieldError extends FormatError {
+  public got: unknown;
+  public expected: unknown;
+
+  constructor(
+    format: string,
+    field: string,
+    offset: number,
+    got: unknown,
+    expected: unknown
+  ) {
+    const gotStr = typeof got === 'number' ? `0x${got.toString(16).toUpperCase()}` : String(got);
+    const expStr = typeof expected === 'number' ? `0x${expected.toString(16).toUpperCase()}` : String(expected);
+    super(
+      format,
+      field,
+      offset,
+      `${format}: invalid field ${field} at offset 0x${offset.toString(16).toUpperCase()} (got ${gotStr}, expected ${expStr})`
+    );
+    this.name = 'InvalidFieldError';
+    this.got = got;
+    this.expected = expected;
+    Object.setPrototypeOf(this, new.target.prototype);
+  }
+}
+
+export function requireBytes(
+  data: Uint8Array,
+  offset: number,
+  length: number,
+  format: string,
+  field: string
+): void {
+  if (offset < 0 || offset + length > data.length) {
+    const available = data.length - offset;
+    throw new TruncatedDataError(
+      format,
+      field,
+      Math.max(0, offset),
+      length,
+      Math.max(0, available),
+      data.length
+    );
+  }
+}
+
+export class ByteReader {
+  private view: DataView;
+  private offset: number = 0;
+  private format: string;
+  private arr: Uint8Array;
+
+  constructor(data: Uint8Array, format: string) {
+    this.arr = data;
+    this.view = new DataView(data.buffer, data.byteOffset, data.byteLength);
+    this.format = format;
+  }
+
+  get remaining(): number {
+    return this.arr.length - this.offset;
+  }
+
+  get eof(): boolean {
+    return this.offset >= this.arr.length;
+  }
+  
+  get position(): number {
+    return this.offset;
+  }
+
+  private check(length: number, field: string): void {
+    if (this.offset + length > this.arr.length || this.offset < 0) {
+      throw new TruncatedDataError(
+        this.format,
+        field,
+        this.offset,
+        length,
+        Math.max(0, this.arr.length - this.offset),
+        this.arr.length
+      );
+    }
+  }
+
+  u8(field: string): number {
+    this.check(1, field);
+    const val = this.view.getUint8(this.offset);
+    this.offset += 1;
+    return val;
+  }
+
+  u16le(field: string): number {
+    this.check(2, field);
+    const val = this.view.getUint16(this.offset, true);
+    this.offset += 2;
+    return val;
+  }
+
+  u32le(field: string): number {
+    this.check(4, field);
+    const val = this.view.getUint32(this.offset, true);
+    this.offset += 4;
+    return val;
+  }
+
+  i16le(field: string): number {
+    this.check(2, field);
+    const val = this.view.getInt16(this.offset, true);
+    this.offset += 2;
+    return val;
+  }
+
+  i32le(field: string): number {
+    this.check(4, field);
+    const val = this.view.getInt32(this.offset, true);
+    this.offset += 4;
+    return val;
+  }
+
+  bytes(n: number, field: string): Uint8Array {
+    if (n < 0) {
+      throw new InvalidFieldError(this.format, field, this.offset, n, ">= 0");
+    }
+    this.check(n, field);
+    const val = this.arr.subarray(this.offset, this.offset + n);
+    this.offset += n;
+    return val;
+  }
+
+  skip(n: number, field: string): void {
+    if (n < 0) {
+      throw new InvalidFieldError(this.format, field, this.offset, n, ">= 0");
+    }
+    this.check(n, field);
+    this.offset += n;
+  }
+
+  seek(off: number, field: string): void {
+    if (off < 0 || off > this.arr.length) {
+      throw new TruncatedDataError(
+        this.format,
+        field,
+        off,
+        1,
+        0,
+        this.arr.length
+      );
+    }
+    this.offset = off;
+  }
+}
diff --git a/tests/reader.test.ts b/tests/reader.test.ts
new file mode 100644
index 0000000..3a26160
--- /dev/null
+++ b/tests/reader.test.ts
@@ -0,0 +1,117 @@
+import { describe, expect, test } from 'vitest';
+import { 
+  FormatError, 
+  TruncatedDataError, 
+  InvalidFieldError, 
+  ByteReader, 
+  requireBytes 
+} from '../src/formats/reader';
+
+describe('ByteReader', () => {
+  test('u8 reads correct value and advances', () => {
+    const data = new Uint8Array([0x01, 0x02]);
+    const reader = new ByteReader(data, 'test');
+    expect(reader.u8('f1')).toBe(0x01);
+    expect(reader.position).toBe(1);
+    expect(reader.remaining).toBe(1);
+    expect(reader.eof).toBe(false);
+    expect(reader.u8('f2')).toBe(0x02);
+    expect(reader.position).toBe(2);
+    expect(reader.remaining).toBe(0);
+    expect(reader.eof).toBe(true);
+  });
+
+  test('u8 throws TruncatedDataError at boundary', () => {
+    const data = new Uint8Array([0x01]);
+    const reader = new ByteReader(data, 'test');
+    reader.u8('f1');
+    try {
+      reader.u8('f2');
+      expect.fail('Should throw');
+    } catch (e: any) {
+      expect(e).toBeInstanceOf(TruncatedDataError);
+      expect(e).toBeInstanceOf(FormatError);
+      expect(e.format).toBe('test');
+      expect(e.field).toBe('f2');
+      expect(e.offset).toBe(1);
+      expect(e.needed).toBe(1);
+      expect(e.available).toBe(0);
+      expect(e.message).toBe('test: truncated reading f2 at offset 0x1: need 1 bytes, only 0 available (buffer length 1)');
+    }
+  });
+
+  test('u16le, u32le, i16le, i32le read correct values', () => {
+    const data = new Uint8Array([0xFE, 0xFF, 0x01, 0x00, 0x00, 0x80]);
+    // i16le of [0xFE, 0xFF] is -2
+    // u16le of [0xFE, 0xFF] is 65534
+    // i32le of [0x01, 0x00, 0x00, 0x80] is -2147483647
+    const r1 = new ByteReader(data, 'test');
+    expect(r1.i16le('f1')).toBe(-2);
+    expect(r1.i32le('f2')).toBe(-2147483647);
+    
+    const r2 = new ByteReader(data, 'test');
+    expect(r2.u16le('f3')).toBe(65534);
+    r2.seek(2, 'f4');
+    expect(r2.u32le('f5')).toBe(0x80000001);
+  });
+
+  test('throws precise TruncatedDataError across types', () => {
+    const data = new Uint8Array([0x01, 0x02, 0x03]);
+    const r = new ByteReader(data, 'test');
+    r.skip(2, 'skip');
+    
+    expect(() => r.u16le('u16')).toThrowError(
+      'test: truncated reading u16 at offset 0x2: need 2 bytes, only 1 available (buffer length 3)'
+    );
+  });
+
+  test('bytes() array methods', () => {
+    const data = new Uint8Array([0, 1, 2, 3, 4]);
+    const r = new ByteReader(data, 'test');
+    
+    expect(r.bytes(2, 'b1')).toEqual(new Uint8Array([0, 1]));
+    expect(r.remaining).toBe(3);
+    
+    r.skip(1, 'skip');
+    expect(r.position).toBe(3);
+    
+    expect(() => r.bytes(4, 'b2')).toThrow(TruncatedDataError);
+    // test invalid lengths
+    expect(() => r.bytes(-1, 'b3')).toThrow(InvalidFieldError);
+  });
+
+  test('seek and skip logic', () => {
+    const data = new Uint8Array([0, 1, 2, 3]);
+    const r = new ByteReader(data, 'test');
+    
+    r.seek(2, 'seek1');
+    expect(r.position).toBe(2);
+    
+    r.seek(4, 'seek2');
+    expect(r.position).toBe(4);
+    
+    expect(() => r.seek(5, 'seek3')).toThrow(TruncatedDataError);
+  });
+});
+
+describe('requireBytes', () => {
+  test('validates direct offsets', () => {
+    const data = new Uint8Array(10);
+    expect(() => requireBytes(data, 0, 10, 'fmt', 'f1')).not.toThrow();
+    expect(() => requireBytes(data, 5, 5, 'fmt', 'f2')).not.toThrow();
+    
+    expect(() => requireBytes(data, 8, 3, 'fmt', 'f3')).toThrowError(
+      'fmt: truncated reading f3 at offset 0x8: need 3 bytes, only 2 available (buffer length 10)'
+    );
+  });
+});
+
+describe('InvalidFieldError', () => {
+  test('formats expected values', () => {
+    const err = new InvalidFieldError('fmt', 'sig', 16, 0x1234, 0xABCD);
+    expect(err.message).toBe('fmt: invalid field sig at offset 0x10 (got 0x1234, expected 0xABCD)');
+    
+    const err2 = new InvalidFieldError('fmt', 'type', 0, 'bad', 'good');
+    expect(err2.message).toBe('fmt: invalid field type at offset 0x0 (got bad, expected good)');
+  });
+});