diff --git a/src/AcDream.Content/Pak/PakFormat.cs b/src/AcDream.Content/Pak/PakFormat.cs
new file mode 100644
index 00000000..f871d451
--- /dev/null
+++ b/src/AcDream.Content/Pak/PakFormat.cs
@@ -0,0 +1,124 @@
+using System;
+using System.Buffers.Binary;
+using System.IO;
+
+namespace AcDream.Content.Pak;
+
+/// <summary>
+/// Fixed 64-byte pak file header. Layout (all integers little-endian):
+/// <code>
+/// offset size field
+/// 0      4    magic 'ACPK' (0x4B504341)
+/// 4      4    formatVersion = 1
+/// 8      4    portalIteration     (DatCollection.Portal.Iteration)
+/// 12     4    cellIteration
+/// 16     4    highResIteration
+/// 20     4    languageIteration
+/// 24     8    tocOffset (u64)
+/// 32     4    tocCount  (u32)
+/// 36     4    bakeToolVersion = 1
+/// 40     24   reserved (zero)
+/// </code>
+/// Spec: docs/superpowers/plans/2026-07-05-mp1b-pak-and-bake.md "Format v1 (normative)".
+/// </summary>
+public struct PakHeader {
+    public const int Size = 64;
+    public const uint MagicValue = 0x4B504341u; // 'ACPK' little-endian
+
+    /// <summary>Always <see cref="MagicValue"/> after <see cref="ReadFrom(ReadOnlySpan{byte})"/>; not settable by callers building a header to write.</summary>
+    public uint Magic { get; private set; } = MagicValue;
+
+    public uint FormatVersion;
+    public uint PortalIteration;
+    public uint CellIteration;
+    public uint HighResIteration;
+    public uint LanguageIteration;
+    public ulong TocOffset;
+    public uint TocCount;
+    public uint BakeToolVersion;
+
+    public PakHeader() { }
+
+    public void WriteTo(Span<byte> dest) {
+        if (dest.Length < Size) throw new ArgumentException($"destination must be at least {Size} bytes", nameof(dest));
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[0..4], MagicValue);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[4..8], FormatVersion);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[8..12], PortalIteration);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[12..16], CellIteration);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[16..20], HighResIteration);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[20..24], LanguageIteration);
+        BinaryPrimitives.WriteUInt64LittleEndian(dest[24..32], TocOffset);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[32..36], TocCount);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[36..40], BakeToolVersion);
+        dest[40..64].Clear(); // reserved, zero
+    }
+
+    public void WriteTo(Stream stream) {
+        Span<byte> buf = stackalloc byte[Size];
+        WriteTo(buf);
+        stream.Write(buf);
+    }
+
+    public static PakHeader ReadFrom(ReadOnlySpan<byte> src) {
+        if (src.Length < Size) throw new ArgumentException($"source must be at least {Size} bytes", nameof(src));
+        var magic = BinaryPrimitives.ReadUInt32LittleEndian(src[0..4]);
+        if (magic != MagicValue) {
+            throw new InvalidDataException($"pak header magic mismatch: expected 0x{MagicValue:X8}, got 0x{magic:X8}");
+        }
+        return new PakHeader {
+            FormatVersion = BinaryPrimitives.ReadUInt32LittleEndian(src[4..8]),
+            PortalIteration = BinaryPrimitives.ReadUInt32LittleEndian(src[8..12]),
+            CellIteration = BinaryPrimitives.ReadUInt32LittleEndian(src[12..16]),
+            HighResIteration = BinaryPrimitives.ReadUInt32LittleEndian(src[16..20]),
+            LanguageIteration = BinaryPrimitives.ReadUInt32LittleEndian(src[20..24]),
+            TocOffset = BinaryPrimitives.ReadUInt64LittleEndian(src[24..32]),
+            TocCount = BinaryPrimitives.ReadUInt32LittleEndian(src[32..36]),
+            BakeToolVersion = BinaryPrimitives.ReadUInt32LittleEndian(src[36..40]),
+        };
+    }
+
+    public static PakHeader ReadFrom(Stream stream) {
+        Span<byte> buf = stackalloc byte[Size];
+        stream.ReadExactly(buf);
+        return ReadFrom((ReadOnlySpan<byte>)buf);
+    }
+}
+
+/// <summary>
+/// One 24-byte TOC entry: <c>key u64, offset u64, length u32, crc32 u32</c>.
+/// Entries in a pak's TOC are sorted ascending by <see cref="Key"/> to allow
+/// binary-search lookup. <see cref="Crc32"/> is a corruption tripwire computed
+/// over the blob bytes; the reader verifies lazily on first access.
+/// </summary>
+public struct PakTocEntry {
+    public const int Size = 24;
+
+    public ulong Key;
+    public ulong Offset;
+    public uint Length;
+    public uint Crc32;
+
+    public void WriteTo(Span<byte> dest) {
+        if (dest.Length < Size) throw new ArgumentException($"destination must be at least {Size} bytes", nameof(dest));
+        BinaryPrimitives.WriteUInt64LittleEndian(dest[0..8], Key);
+        BinaryPrimitives.WriteUInt64LittleEndian(dest[8..16], Offset);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[16..20], Length);
+        BinaryPrimitives.WriteUInt32LittleEndian(dest[20..24], Crc32);
+    }
+
+    public void WriteTo(Stream stream) {
+        Span<byte> buf = stackalloc byte[Size];
+        WriteTo(buf);
+        stream.Write(buf);
+    }
+
+    public static PakTocEntry ReadFrom(ReadOnlySpan<byte> src) {
+        if (src.Length < Size) throw new ArgumentException($"source must be at least {Size} bytes", nameof(src));
+        return new PakTocEntry {
+            Key = BinaryPrimitives.ReadUInt64LittleEndian(src[0..8]),
+            Offset = BinaryPrimitives.ReadUInt64LittleEndian(src[8..16]),
+            Length = BinaryPrimitives.ReadUInt32LittleEndian(src[16..20]),
+            Crc32 = BinaryPrimitives.ReadUInt32LittleEndian(src[20..24]),
+        };
+    }
+}
diff --git a/src/AcDream.Content/Pak/PakKey.cs b/src/AcDream.Content/Pak/PakKey.cs
new file mode 100644
index 00000000..da1a8753
--- /dev/null
+++ b/src/AcDream.Content/Pak/PakKey.cs
@@ -0,0 +1,33 @@
+namespace AcDream.Content.Pak;
+
+/// <summary>
+/// MP1b pak asset-type discriminant — the top byte of a <see cref="PakKey"/>.
+/// Numeric values are a WIRE FORMAT (persisted in every pak's TOC): never
+/// renumber existing members, only append.
+/// Spec: docs/superpowers/specs/2026-07-05-modern-pipeline-design.md §6.2;
+/// plan: docs/superpowers/plans/2026-07-05-mp1b-pak-and-bake.md "Format v1".
+/// </summary>
+public enum PakAssetType : byte {
+    GfxObjMesh = 1,
+    SetupMesh = 2,
+    EnvCellMesh = 3,
+}
+
+/// <summary>
+/// Composes/decomposes the 64-bit pak asset key: <c>type:u8 | fileId:u32 | reserved:u24</c>.
+/// Layout: <c>((ulong)type &lt;&lt; 56) | ((ulong)fileId &lt;&lt; 24)</c> — the low 24 bits are
+/// reserved (variant/zero in v1). Ascending numeric key order equals ascending
+/// (type, fileId) tuple order, which is what makes the pak TOC's binary search
+/// over raw u64 keys valid.
+/// </summary>
+public static class PakKey {
+    public static ulong Compose(PakAssetType type, uint fileId) {
+        return ((ulong)type << 56) | ((ulong)fileId << 24);
+    }
+
+    public static (PakAssetType Type, uint FileId) Decompose(ulong key) {
+        var type = (PakAssetType)(byte)(key >> 56);
+        var fileId = (uint)((key >> 24) & 0xFFFFFFFFu);
+        return (type, fileId);
+    }
+}
diff --git a/tests/AcDream.Content.Tests/PakFormatTests.cs b/tests/AcDream.Content.Tests/PakFormatTests.cs
new file mode 100644
index 00000000..4eb1181d
--- /dev/null
+++ b/tests/AcDream.Content.Tests/PakFormatTests.cs
@@ -0,0 +1,145 @@
+using System.IO;
+using AcDream.Content.Pak;
+
+namespace AcDream.Content.Tests;
+
+public class PakFormatTests {
+    [Fact]
+    public void Header_Size_Is64Bytes() {
+        Assert.Equal(64, PakHeader.Size);
+    }
+
+    [Fact]
+    public void TocEntry_Size_Is24Bytes() {
+        Assert.Equal(24, PakTocEntry.Size);
+    }
+
+    [Fact]
+    public void Header_Magic_IsAcpkLittleEndian() {
+        // 'A'=0x41 'C'=0x43 'P'=0x50 'K'=0x4B — little-endian dword reads
+        // back as 0x4B504341 per the normative spec.
+        Assert.Equal(0x4B504341u, PakHeader.MagicValue);
+    }
+
+    [Fact]
+    public void Header_WriteThenRead_RoundTripsEveryField() {
+        var header = new PakHeader {
+            FormatVersion = 1,
+            PortalIteration = 111,
+            CellIteration = 222,
+            HighResIteration = 333,
+            LanguageIteration = 444,
+            TocOffset = 0x1_0000_0002UL,
+            TocCount = 777,
+            BakeToolVersion = 1,
+        };
+
+        using var ms = new MemoryStream();
+        header.WriteTo(ms);
+        Assert.Equal(PakHeader.Size, ms.Length);
+
+        ms.Position = 0;
+        var readBack = PakHeader.ReadFrom(ms);
+
+        Assert.Equal(PakHeader.MagicValue, readBack.Magic);
+        Assert.Equal(header.FormatVersion, readBack.FormatVersion);
+        Assert.Equal(header.PortalIteration, readBack.PortalIteration);
+        Assert.Equal(header.CellIteration, readBack.CellIteration);
+        Assert.Equal(header.HighResIteration, readBack.HighResIteration);
+        Assert.Equal(header.LanguageIteration, readBack.LanguageIteration);
+        Assert.Equal(header.TocOffset, readBack.TocOffset);
+        Assert.Equal(header.TocCount, readBack.TocCount);
+        Assert.Equal(header.BakeToolVersion, readBack.BakeToolVersion);
+    }
+
+    [Fact]
+    public void Header_ReadFrom_Span_RoundTrips() {
+        var header = new PakHeader {
+            FormatVersion = 1,
+            PortalIteration = 5,
+            CellIteration = 6,
+            HighResIteration = 7,
+            LanguageIteration = 8,
+            TocOffset = 999,
+            TocCount = 3,
+            BakeToolVersion = 1,
+        };
+        Span<byte> buf = stackalloc byte[PakHeader.Size];
+        header.WriteTo(buf);
+        var readBack = PakHeader.ReadFrom((ReadOnlySpan<byte>)buf);
+        Assert.Equal(header.CellIteration, readBack.CellIteration);
+        Assert.Equal(header.TocOffset, readBack.TocOffset);
+    }
+
+    [Fact]
+    public void Header_ReservedBytes_AreZero() {
+        var header = new PakHeader { FormatVersion = 1, BakeToolVersion = 1 };
+        Span<byte> buf = stackalloc byte[PakHeader.Size];
+        header.WriteTo(buf);
+        // offset 40, length 24 per the normative layout
+        for (int i = 40; i < 64; i++) {
+            Assert.Equal(0, buf[i]);
+        }
+    }
+
+    [Fact]
+    public void Header_FieldOffsets_MatchNormativeLayout() {
+        var header = new PakHeader {
+            FormatVersion = 0x11111111,
+            PortalIteration = 0x22222222,
+            CellIteration = 0x33333333,
+            HighResIteration = 0x44444444,
+            LanguageIteration = 0x55555555,
+            TocOffset = 0x6666666677777777UL,
+            TocCount = 0x88888888,
+            BakeToolVersion = 0x99999999,
+        };
+        Span<byte> buf = stackalloc byte[PakHeader.Size];
+        header.WriteTo(buf);
+
+        Assert.Equal(PakHeader.MagicValue, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[0..4]));
+        Assert.Equal(0x11111111u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[4..8]));
+        Assert.Equal(0x22222222u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[8..12]));
+        Assert.Equal(0x33333333u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[12..16]));
+        Assert.Equal(0x44444444u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[16..20]));
+        Assert.Equal(0x55555555u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[20..24]));
+        Assert.Equal(0x6666666677777777UL, System.Buffers.Binary.BinaryPrimitives.ReadUInt64LittleEndian(buf[24..32]));
+        Assert.Equal(0x88888888u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[32..36]));
+        Assert.Equal(0x99999999u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[36..40]));
+    }
+
+    [Fact]
+    public void TocEntry_WriteThenRead_RoundTrips() {
+        var entry = new PakTocEntry {
+            Key = PakKey.Compose(PakAssetType.GfxObjMesh, 0x010002B4u),
+            Offset = 0x4000,
+            Length = 12345,
+            Crc32 = 0xDEADBEEF,
+        };
+        Span<byte> buf = stackalloc byte[PakTocEntry.Size];
+        entry.WriteTo(buf);
+        var readBack = PakTocEntry.ReadFrom((ReadOnlySpan<byte>)buf);
+
+        Assert.Equal(entry.Key, readBack.Key);
+        Assert.Equal(entry.Offset, readBack.Offset);
+        Assert.Equal(entry.Length, readBack.Length);
+        Assert.Equal(entry.Crc32, readBack.Crc32);
+    }
+
+    [Fact]
+    public void TocEntry_FieldOffsets_MatchNormativeLayout() {
+        var entry = new PakTocEntry {
+            Key = 0x1111111122222222UL,
+            Offset = 0x3333333344444444UL,
+            Length = 0x55555555,
+            Crc32 = 0x66666666,
+        };
+        Span<byte> buf = stackalloc byte[PakTocEntry.Size];
+        entry.WriteTo(buf);
+
+        Assert.Equal(0x1111111122222222UL, System.Buffers.Binary.BinaryPrimitives.ReadUInt64LittleEndian(buf[0..8]));
+        Assert.Equal(0x3333333344444444UL, System.Buffers.Binary.BinaryPrimitives.ReadUInt64LittleEndian(buf[8..16]));
+        Assert.Equal(0x55555555u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[16..20]));
+        Assert.Equal(0x66666666u, System.Buffers.Binary.BinaryPrimitives.ReadUInt32LittleEndian(buf[20..24]));
+    }
+}
diff --git a/tests/AcDream.Content.Tests/PakKeyTests.cs b/tests/AcDream.Content.Tests/PakKeyTests.cs
index 8b6b5bfa..ba527ac5 100644
--- a/tests/AcDream.Content.Tests/PakKeyTests.cs
+++ b/tests/AcDream.Content.Tests/PakKeyTests.cs
@@ -1,14 +1,75 @@
+using AcDream.Content.Pak;
+
 namespace AcDream.Content.Tests;
 
-/// <summary>
-/// Task 1 scaffold smoke test — becomes the real PakKey test suite in Task 2.
-/// </summary>
 public class PakKeyTests {
+    [Theory]
+    [InlineData(PakAssetType.GfxObjMesh, 0u)]
+    [InlineData(PakAssetType.GfxObjMesh, 0x01000001u)]
+    [InlineData(PakAssetType.SetupMesh, 0x020019FFu)]
+    [InlineData(PakAssetType.EnvCellMesh, 0xA9B40100u)]
+    [InlineData(PakAssetType.GfxObjMesh, 0xFFFFFFFFu)] // max fileId
+    [InlineData(PakAssetType.EnvCellMesh, 0xFFFFFFFFu)]
+    public void ComposeDecompose_RoundTrips(PakAssetType type, uint fileId) {
+        ulong key = PakKey.Compose(type, fileId);
+        var (decodedType, decodedFileId) = PakKey.Decompose(key);
+        Assert.Equal(type, decodedType);
+        Assert.Equal(fileId, decodedFileId);
+    }
+
     [Fact]
-    public void Smoke_ProjectReferencesContentAssembly() {
-        // Any type from AcDream.Content resolves — proves the ProjectReference
-        // and test-project wiring are correct before Task 2 adds real coverage.
-        var key = new TextureKey();
-        Assert.Equal(0u, key.SurfaceId);
+    public void Compose_LowFourBytesReserved() {
+        // Low 24 bits are reserved (zero in v1) — fileId << 24 must not spill
+        // into them, and the reserved region must actually read back as zero.
+        ulong key = PakKey.Compose(PakAssetType.GfxObjMesh, 0xFFFFFFFFu);
+        Assert.Equal(0u, (uint)(key & 0xFFFFFFu));
+    }
+
+    [Fact]
+    public void Compose_TypeOccupiesTopByte() {
+        ulong key = PakKey.Compose(PakAssetType.SetupMesh, 0u);
+        Assert.Equal((byte)PakAssetType.SetupMesh, (byte)(key >> 56));
+    }
+
+    [Theory]
+    [InlineData(PakAssetType.GfxObjMesh, 1)]
+    [InlineData(PakAssetType.SetupMesh, 2)]
+    [InlineData(PakAssetType.EnvCellMesh, 3)]
+    public void AssetType_NumericValuesAreStable(PakAssetType type, byte expected) {
+        // These values are a wire format — pin them so a future refactor can't
+        // silently renumber the enum and corrupt existing paks.
+        Assert.Equal(expected, (byte)type);
+    }
+
+    [Fact]
+    public void KeyOrdering_MatchesTypeThenFileIdOrdering() {
+        // Ascending key order must equal ascending (type, fileId) tuple order —
+        // this is what makes the TOC's binary search over raw u64 keys valid.
+        var pairs = new (PakAssetType Type, uint FileId)[] {
+            (PakAssetType.GfxObjMesh, 0u),
+            (PakAssetType.GfxObjMesh, 1u),
+            (PakAssetType.GfxObjMesh, 0xFFFFFFFFu),
+            (PakAssetType.SetupMesh, 0u),
+            (PakAssetType.SetupMesh, 0x020019FFu),
+            (PakAssetType.EnvCellMesh, 0u),
+            (PakAssetType.EnvCellMesh, 0xA9B40100u),
+        };
+
+        var keys = pairs.Select(p => PakKey.Compose(p.Type, p.FileId)).ToArray();
+
+        // keys[] as generated must already be strictly ascending since pairs[]
+        // is in ascending tuple order.
+        for (int i = 1; i < keys.Length; i++) {
+            Assert.True(keys[i] > keys[i - 1],
+                $"key[{i}]=0x{keys[i]:X16} should be > key[{i - 1}]=0x{keys[i - 1]:X16} " +
+                $"for tuple order ({pairs[i - 1]}) < ({pairs[i]})");
+        }
+
+        // Sorting the keys numerically must reproduce the same order as sorting
+        // the tuples lexicographically by (Type, FileId).
+        var numericSorted = keys.OrderBy(k => k).ToArray();
+        var tupleSorted = pairs.OrderBy(p => (byte)p.Type).ThenBy(p => p.FileId)
+            .Select(p => PakKey.Compose(p.Type, p.FileId)).ToArray();
+        Assert.Equal(tupleSorted, numericSorted);
     }
 }