diff --git a/tests/unit/test_serial.nim b/tests/unit/test_serial.nim
index 62d518a..6061028 100644
--- a/tests/unit/test_serial.nim
+++ b/tests/unit/test_serial.nim
@@ -1,4 +1,4 @@
-import std/options
+import std/[math, options]
 import unittest
 import results
 import ffi
@@ -20,6 +20,22 @@ type Color = enum
   cGreen
   cBlue
 
+type BytesHolder {.ffi.} = object
+  blob: seq[byte]
+
+type DeepInner {.ffi.} = object
+  tag: string
+  nested: Nested
+
+type ComplexContainer {.ffi.} = object
+  ids: seq[int]
+  names: seq[string]
+  points: seq[Point]
+  maybePoint: Option[Point]
+  maybeNames: Option[seq[string]]
+  flags: seq[Option[int]]
+  blob: seq[byte]
+
 suite "CBOR primitives round-trip":
   test "bool true":
     let bytes = cborEncode(true)
@@ -242,3 +258,322 @@ suite "cborEncodeShared":
         c_free(sd)
     check sl == 1
     check sd[0] == 0x60'u8
+
+suite "CBOR boundaries":
+  test "int8":
+    for v in [low(int8), int8(-1), int8(0), int8(1), high(int8)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, int8).value == v
+
+  test "int16":
+    for v in [low(int16), int16(-1), int16(0), int16(1), high(int16)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, int16).value == v
+
+  test "int32":
+    for v in [low(int32), int32(-1), int32(0), int32(1), high(int32)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, int32).value == v
+
+  test "int64":
+    for v in [low(int64), int64(-1), int64(0), int64(1), high(int64)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, int64).value == v
+
+  test "uint8":
+    for v in [uint8(0), uint8(1), high(uint8)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, uint8).value == v
+
+  test "uint16":
+    for v in [uint16(0), uint16(1), high(uint16)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, uint16).value == v
+
+  test "uint32":
+    for v in [uint32(0), uint32(1), high(uint32)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, uint32).value == v
+
+  test "uint64 (UINT64_MAX)":
+    for v in [uint64(0), uint64(1), high(uint64)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, uint64).value == v
+
+  test "float32 finite values incl. ±FLT_MAX":
+    for v in [float32(0.0), float32(-0.0), float32(1.5), float32(-1.5),
+              float32(3.4028235e38), float32(-3.4028235e38)]:
+      let bytes = cborEncode(v)
+      check cborDecode(bytes, float32).value == v
+
+  test "float64 NaN":
+    let bytes = cborEncode(NaN)
+    let back = cborDecode(bytes, float64)
+    check back.isOk
+    check back.value.classify == fcNan
+
+  test "float32 NaN":
+    let v: float32 = NaN
+    let bytes = cborEncode(v)
+    let back = cborDecode(bytes, float32)
+    check back.isOk
+    check back.value.classify == fcNan
+
+  test "float64 +Inf and -Inf":
+    let posBytes = cborEncode(Inf)
+    check cborDecode(posBytes, float64).value == Inf
+    let negBytes = cborEncode(NegInf)
+    check cborDecode(negBytes, float64).value == NegInf
+
+  test "float32 +Inf and -Inf":
+    let pos: float32 = Inf
+    let neg: float32 = NegInf
+    check cborDecode(cborEncode(pos), float32).value == pos
+    check cborDecode(cborEncode(neg), float32).value == neg
+
+  test "float64 subnormal":
+    # 5e-324 is the smallest positive float64 subnormal (Double.MIN_VALUE).
+    let v = 5e-324
+    let bytes = cborEncode(v)
+    let back = cborDecode(bytes, float64)
+    check back.isOk
+    check back.value == v
+    check back.value.classify == math.fcSubnormal
+
+  test "float32 subnormal (compared by bit pattern)":
+    # The smallest positive float32 subnormal has bit pattern 0x00000001.
+    # `math.classify` widens to float64 and would re-classify this as normal,
+    # so we compare the raw 32-bit bit pattern instead.
+    let v = cast[float32](0x00000001'u32)
+    let bytes = cborEncode(v)
+    let back = cborDecode(bytes, float32)
+    check back.isOk
+    check cast[uint32](back.value) == 0x00000001'u32
+
+suite "CBOR round-trips":
+  test "seq[byte]":
+    let bs: seq[byte] = @[0x00'u8, 0x01'u8, 0x7f'u8, 0x80'u8, 0xff'u8]
+    let bytes = cborEncode(bs)
+    check cborDecode(bytes, seq[byte]).value == bs
+
+  test "empty seq[byte] encodes as CBOR bytes(0)":
+    let bs: seq[byte] = @[]
+    let bytes = cborEncode(bs)
+    # 0x40 = major type 2 (bytes), length 0.
+    check bytes.len == 1
+    check bytes[0] == 0x40'u8
+    check cborDecode(bytes, seq[byte]).value == bs
+
+  test "seq[byte] with embedded NUL bytes":
+    let bs: seq[byte] = @[0x00'u8, 0xde'u8, 0x00'u8, 0xad'u8, 0x00'u8]
+    let bytes = cborEncode(bs)
+    let back = cborDecode(bytes, seq[byte])
+    check back.isOk
+    check back.value == bs
+    check back.value.len == 5
+
+  test "string with embedded NUL keeps length, not C-string termination":
+    let s = "a\0b\0c"
+    check s.len == 5
+    let bytes = cborEncode(s)
+    let back = cborDecode(bytes, string)
+    check back.isOk
+    check back.value.len == 5
+    check back.value == s
+
+  test "object with seq[byte] field":
+    let h = BytesHolder(blob: @[0x00'u8, 0xff'u8, 0x10'u8, 0x00'u8])
+    let bytes = cborEncode(h)
+    let back = cborDecode(bytes, BytesHolder)
+    check back.isOk
+    check back.value.blob == h.blob
+
+  test "seq[Option[int]] mixes some and none":
+    let s = @[some(1), none(int), some(-3), none(int), some(0)]
+    let bytes = cborEncode(s)
+    let back = cborDecode(bytes, seq[Option[int]])
+    check back.isOk
+    check back.value == s
+
+  test "Option[seq[int]] some":
+    let o = some(@[1, 2, 3])
+    let bytes = cborEncode(o)
+    let back = cborDecode(bytes, Option[seq[int]])
+    check back.isOk
+    check back.value == o
+
+  test "Option[seq[int]] none":
+    let o = none(seq[int])
+    let bytes = cborEncode(o)
+    let back = cborDecode(bytes, Option[seq[int]])
+    check back.isOk
+    check back.value == o
+
+  test "three-level struct nesting":
+    let d = DeepInner(tag: "outer",
+                     nested: Nested(label: "mid", point: Point(x: 11, y: 22)))
+    let bytes = cborEncode(d)
+    let back = cborDecode(bytes, DeepInner)
+    check back.isOk
+    check back.value.tag == "outer"
+    check back.value.nested.label == "mid"
+    check back.value.nested.point.x == 11
+    check back.value.nested.point.y == 22
+
+  test "seq[Nested] preserves element order and content":
+    let s = @[
+      Nested(label: "a", point: Point(x: 1, y: 2)),
+      Nested(label: "b", point: Point(x: 3, y: 4)),
+      Nested(label: "c", point: Point(x: 5, y: 6)),
+    ]
+    let bytes = cborEncode(s)
+    let back = cborDecode(bytes, seq[Nested])
+    check back.isOk
+    check back.value.len == 3
+    check back.value[1].label == "b"
+    check back.value[2].point.y == 6
+
+  test "Option[Nested] some/none":
+    let some1 = some(Nested(label: "x", point: Point(x: 7, y: 8)))
+    let back1 = cborDecode(cborEncode(some1), Option[Nested])
+    check back1.isOk
+    check back1.value.isSome
+    check back1.value.get.label == "x"
+
+    let none1 = none(Nested)
+    let back2 = cborDecode(cborEncode(none1), Option[Nested])
+    check back2.isOk
+    check back2.value.isNone
+
+  test "ComplexContainer with mixed nested fields":
+    let c = ComplexContainer(
+      ids: @[1, 2, 3],
+      names: @["alpha", "beta"],
+      points: @[Point(x: 1, y: 2), Point(x: 3, y: 4)],
+      maybePoint: some(Point(x: 9, y: 9)),
+      maybeNames: some(@["a", "b"]),
+      flags: @[some(1), none(int), some(2)],
+      blob: @[0x00'u8, 0xff'u8, 0x42'u8],
+    )
+    let bytes = cborEncode(c)
+    let back = cborDecode(bytes, ComplexContainer)
+    check back.isOk
+    check back.value.ids == c.ids
+    check back.value.names == c.names
+    check back.value.points.len == 2
+    check back.value.points[1].y == 4
+    check back.value.maybePoint.isSome
+    check back.value.maybePoint.get.x == 9
+    check back.value.maybeNames == c.maybeNames
+    check back.value.flags == c.flags
+    check back.value.blob == c.blob
+
+  test "ComplexContainer with all-empty / all-none fields":
+    let c = ComplexContainer(
+      ids: @[],
+      names: @[],
+      points: @[],
+      maybePoint: none(Point),
+      maybeNames: none(seq[string]),
+      flags: @[],
+      blob: @[],
+    )
+    let bytes = cborEncode(c)
+    let back = cborDecode(bytes, ComplexContainer)
+    check back.isOk
+    check back.value.ids.len == 0
+    check back.value.names.len == 0
+    check back.value.points.len == 0
+    check back.value.maybePoint.isNone
+    check back.value.maybeNames.isNone
+    check back.value.flags.len == 0
+    check back.value.blob.len == 0
+
+  # Sizes chosen to cross the 24/256/65536-byte CBOR length-encoding
+  # boundaries and the encoder's internal buffer-grow thresholds.
+
+  test "string >64 KiB":
+    const n = 70_000
+    var big = newString(n)
+    for i in 0 ..< n:
+      big[i] = char(ord('a') + (i mod 26))
+    let bytes = cborEncode(big)
+    let back = cborDecode(bytes, string)
+    check back.isOk
+    check back.value.len == n
+    check back.value == big
+
+  test "seq[byte] >64 KiB with embedded NULs":
+    const n = 70_000
+    var blob = newSeq[byte](n)
+    for i in 0 ..< n:
+      blob[i] = byte(i mod 256)
+    let bytes = cborEncode(blob)
+    let back = cborDecode(bytes, seq[byte])
+    check back.isOk
+    check back.value.len == n
+    check back.value == blob
+
+  test "string >1 MiB":
+    const n = 1_200_000
+    var big = newString(n)
+    for i in 0 ..< n:
+      big[i] = char(ord('a') + (i mod 26))
+    let bytes = cborEncode(big)
+    let back = cborDecode(bytes, string)
+    check back.isOk
+    check back.value.len == n
+    check back.value[0] == big[0]
+    check back.value[n - 1] == big[n - 1]
+    check back.value == big
+
+  test "seq[byte] >1 MiB":
+    const n = 1_200_000
+    var blob = newSeq[byte](n)
+    for i in 0 ..< n:
+      blob[i] = byte(i mod 256)
+    let bytes = cborEncode(blob)
+    let back = cborDecode(bytes, seq[byte])
+    check back.isOk
+    check back.value.len == n
+    check back.value == blob
+
+  test "seq[Point] with 100k elements":
+    const n = 100_000
+    var pts = newSeq[Point](n)
+    for i in 0 ..< n:
+      pts[i] = Point(x: i, y: -i)
+    let bytes = cborEncode(pts)
+    let back = cborDecode(bytes, seq[Point])
+    check back.isOk
+    check back.value.len == n
+    check back.value[0].x == 0
+    check back.value[n - 1].x == n - 1
+    check back.value[n - 1].y == -(n - 1)
+
+suite "CBOR void / null sentinel":
+  ## `CborNullByte` is the wire sentinel used by `ffi_thread_request` to
+  ## carry a "successful but no value" reply (an `.ffi.` proc whose handler
+  ## returns `Result[void, string]`). See `ffi/cbor_serial.nim` and
+  ## `ffi/ffi_thread_request.nim` for the producer side.
+
+  test "CborNullByte equals CBOR null (0xf6)":
+    check CborNullByte == 0xf6'u8
+
+  test "top-level none(T) encodes as the single sentinel byte":
+    let bytes = cborEncode(none(int))
+    check bytes.len == 1
+    check bytes[0] == CborNullByte
+
+  test "decoding the sentinel byte as Option[T] yields none":
+    let bytes = @[CborNullByte]
+    let back = cborDecode(bytes, Option[int])
+    check back.isOk
+    check back.value.isNone
+
+  test "decoding the sentinel as a required object type errors out":
+    # A consumer that expects a real payload but is handed the void sentinel
+    # must fail explicitly rather than silently materializing a zeroed value.
+    let bytes = @[CborNullByte]
+    let back = cborDecode(bytes, Point)
+    check back.isErr