diff --git a/mypy/build.py b/mypy/build.py
index c2e378290274..9b6e4d54d1b1 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -2397,6 +2397,8 @@ def parse_file(self, *, temporary: bool = False) -> None:
                 self.source_hash = compute_hash(source)
 
             self.parse_inline_configuration(source)
+            self.check_for_invalid_options()
+
             self.size_hint = len(source)
             if not cached:
                 self.tree = manager.parse_file(
@@ -2447,6 +2449,13 @@ def parse_inline_configuration(self, source: str) -> None:
             for lineno, error in config_errors:
                 self.manager.errors.report(lineno, 0, error)
 
+    def check_for_invalid_options(self) -> None:
+        if self.options.mypyc and not self.options.strict_bytes:
+            self.manager.errors.set_file(self.xpath, self.id, options=self.options)
+            self.manager.errors.report(
+                1, 0, "Option --strict-bytes cannot be disabled when using mypyc", blocker=True
+            )
+
     def semantic_analysis_pass1(self) -> None:
         """Perform pass 1 of semantic analysis, which happens immediately after parsing.
 
diff --git a/mypy/main.py b/mypy/main.py
index 864fe4b4febf..926e72515d95 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -1371,6 +1371,7 @@ def process_options(
     fscache: FileSystemCache | None = None,
     program: str = "mypy",
     header: str = HEADER,
+    mypyc: bool = False,
 ) -> tuple[list[BuildSource], Options]:
     """Parse command line arguments.
 
@@ -1398,6 +1399,9 @@ def process_options(
 
     options = Options()
     strict_option_set = False
+    if mypyc:
+        # Mypyc has strict_bytes enabled by default
+        options.strict_bytes = True
 
     def set_strict_flags() -> None:
         nonlocal strict_option_set
diff --git a/mypy/typeshed/stubs/librt/librt/strings.pyi b/mypy/typeshed/stubs/librt/librt/strings.pyi
index 89f5b843caf8..241f6a6fba5b 100644
--- a/mypy/typeshed/stubs/librt/librt/strings.pyi
+++ b/mypy/typeshed/stubs/librt/librt/strings.pyi
@@ -5,7 +5,7 @@ from mypy_extensions import i64, u8
 @final
 class BytesWriter:
     def append(self, /, x: int) -> None: ...
-    def write(self, /, b: bytes) -> None: ...
+    def write(self, /, b: bytes | bytearray) -> None: ...
     def getvalue(self) -> bytes: ...
     def truncate(self, /, size: i64) -> None: ...
     def __len__(self) -> i64: ...
diff --git a/mypyc/build.py b/mypyc/build.py
index 757f0a49737a..566612ce9d33 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -189,7 +189,7 @@ def get_mypy_config(
     fscache: FileSystemCache | None,
 ) -> tuple[list[BuildSource], list[BuildSource], Options]:
     """Construct mypy BuildSources and Options from file and options lists"""
-    all_sources, options = process_options(mypy_options, fscache=fscache)
+    all_sources, options = process_options(mypy_options, fscache=fscache, mypyc=True)
     if only_compile_paths is not None:
         paths_set = set(only_compile_paths)
         mypyc_sources = [s for s in all_sources if s.path in paths_set]
diff --git a/mypyc/codegen/emit.py b/mypyc/codegen/emit.py
index 34837a73adbd..236da808c038 100644
--- a/mypyc/codegen/emit.py
+++ b/mypyc/codegen/emit.py
@@ -657,7 +657,7 @@ def emit_cast(
         elif is_bytes_rprimitive(typ):
             if declare_dest:
                 self.emit_line(f"PyObject *{dest};")
-            check = "(PyBytes_Check({}) || PyByteArray_Check({}))"
+            check = "(PyBytes_Check({}))"
             if likely:
                 check = f"(likely{check})"
             self.emit_arg_check(src, dest, typ, check.format(src, src), optional)
diff --git a/mypyc/test-data/commandline.test b/mypyc/test-data/commandline.test
index 392ad3620790..36f7f508daf0 100644
--- a/mypyc/test-data/commandline.test
+++ b/mypyc/test-data/commandline.test
@@ -312,3 +312,13 @@ print(type(Eggs(obj1=pkg1.A.B())["obj1"]).__module__)
 [out]
 B
 pkg2.mod2
+
+[case testStrictBytesRequired]
+# cmd: --no-strict-bytes a.py
+
+[file a.py]
+def f(b: bytes) -> None: pass
+f(bytearray())
+
+[out]
+a.py:1: error: Option --strict-bytes cannot be disabled when using mypyc
diff --git a/mypyc/test-data/fixtures/ir.py b/mypyc/test-data/fixtures/ir.py
index cce7af477d7a..c608a68c26e9 100644
--- a/mypyc/test-data/fixtures/ir.py
+++ b/mypyc/test-data/fixtures/ir.py
@@ -167,7 +167,7 @@ class bytes:
     def __init__(self) -> None: ...
     @overload
     def __init__(self, x: object) -> None: ...
-    def __add__(self, x: bytes) -> bytes: ...
+    def __add__(self, x: bytes | bytearray) -> bytes: ...
     def __mul__(self, x: int) -> bytes: ...
     def __rmul__(self, x: int) -> bytes: ...
     def __eq__(self, x: object) -> bool: ...
@@ -178,8 +178,8 @@ def __getitem__(self, i: int) -> int: ...
     def __getitem__(self, i: slice) -> bytes: ...
     def join(self, x: Iterable[object]) -> bytes: ...
     def decode(self, encoding: str=..., errors: str=...) -> str: ...
-    def translate(self, t: bytes) -> bytes: ...
-    def startswith(self, t: bytes) -> bool: ...
+    def translate(self, t: bytes | bytearray) -> bytes: ...
+    def startswith(self, t: bytes | bytearray) -> bool: ...
     def __iter__(self) -> Iterator[int]: ...
 
 class bytearray:
@@ -189,9 +189,12 @@ def __init__(self) -> None: pass
     def __init__(self, x: object) -> None: pass
     @overload
     def __init__(self, string: str, encoding: str, err: str = ...) -> None: pass
-    def __add__(self, s: bytes) -> bytearray: ...
+    def __add__(self, s: bytes | bytearray) -> bytearray: ...
     def __setitem__(self, i: int, o: int) -> None: ...
+    @overload
     def __getitem__(self, i: int) -> int: ...
+    @overload
+    def __getitem__(self, i: slice) -> bytearray: ...
     def decode(self, x: str = ..., y: str = ...) -> str: ...
     def startswith(self, t: bytes) -> bool: ...
 
diff --git a/mypyc/test-data/irbuild-bytes.test b/mypyc/test-data/irbuild-bytes.test
index 5e7c546eb25a..0c77c4bfbb69 100644
--- a/mypyc/test-data/irbuild-bytes.test
+++ b/mypyc/test-data/irbuild-bytes.test
@@ -261,3 +261,16 @@ L0:
     r0 = CPyBytes_Startswith(a, b)
     r1 = truncate r0: i32 to builtins.bool
     return r1
+
+[case testBytesVsBytearray]
+def bytes_func(b: bytes) -> None: pass
+def bytearray_func(ba: bytearray) -> None: pass
+
+def foo(b: bytes, ba: bytearray) -> None:
+    bytes_func(b)
+    bytearray_func(ba)
+    bytes_func(ba)
+    bytearray_func(b)
+[out]
+main:7: error: Argument 1 to "bytes_func" has incompatible type "bytearray"; expected "bytes"
+main:8: error: Argument 1 to "bytearray_func" has incompatible type "bytes"; expected "bytearray"
diff --git a/mypyc/test-data/run-base64.test b/mypyc/test-data/run-base64.test
index bf8ea4590e5e..c37fca9a23d4 100644
--- a/mypyc/test-data/run-base64.test
+++ b/mypyc/test-data/run-base64.test
@@ -1,5 +1,5 @@
 [case testAllBase64Features_librt_experimental]
-from typing import Any
+from typing import Any, cast
 import base64
 import binascii
 import random
@@ -14,7 +14,7 @@ def test_encode_basic() -> None:
     assert b64encode(b"x") == b"eA=="
 
     with assertRaises(TypeError):
-        b64encode(bytearray(b"x"))
+        b64encode(cast(Any, bytearray(b"x")))
 
 def check_encode(b: bytes) -> None:
      assert b64encode(b) == getattr(base64, "b64encode")(b)
@@ -56,7 +56,7 @@ def test_decode_basic() -> None:
     assert b64decode(b"eA==") == b"x"
 
     with assertRaises(TypeError):
-        b64decode(bytearray(b"eA=="))
+        b64decode(cast(Any, bytearray(b"eA==")))
 
     for non_ascii in "\x80", "foo\u100bar", "foo\ua1234bar":
         with assertRaises(ValueError):
diff --git a/mypyc/test-data/run-bytes.test b/mypyc/test-data/run-bytes.test
index 6e4b57152a4b..f5eb6abfe234 100644
--- a/mypyc/test-data/run-bytes.test
+++ b/mypyc/test-data/run-bytes.test
@@ -79,8 +79,8 @@ def test_concat() -> None:
     assert type(b1) == bytes
     assert type(b2) == bytes
     assert type(b3) == bytes
-    brr1: bytes = bytearray(3)
-    brr2: bytes = bytearray(range(5))
+    brr1 = bytearray(3)
+    brr2 = bytearray(range(5))
     b4 = b1 + brr1
     assert b4 == b'123\x00\x00\x00'
     assert type(brr1) == bytearray
@@ -94,9 +94,9 @@ def test_concat() -> None:
     b5 = brr2 + b2
     assert b5 == bytearray(b'\x00\x01\x02\x03\x04456')
     assert type(b5) == bytearray
-    b5 = b2 + brr2
-    assert b5 == b'456\x00\x01\x02\x03\x04'
-    assert type(b5) == bytes
+    b6 = b2 + brr2
+    assert b6 == b'456\x00\x01\x02\x03\x04'
+    assert type(b6) == bytes
 
 def test_join() -> None:
     seq = (b'1', b'"', b'\xf0')
@@ -217,9 +217,9 @@ def test_startswith() -> None:
     assert test.startswith(bytearray(b'some'))
     assert not test.startswith(bytearray(b'other'))
 
-    test = bytearray(b'some string')
-    assert test.startswith(b'some')
-    assert not test.startswith(b'other')
+    test2 = bytearray(b'some string')
+    assert test2.startswith(b'some')
+    assert not test2.startswith(b'other')
 
 [case testBytesSlicing]
 def test_bytes_slicing() -> None:
@@ -257,34 +257,38 @@ def test_bytes_slicing() -> None:
 [case testBytearrayBasics]
 from typing import Any
 
+from testutil import assertRaises
+
 def test_basics() -> None:
-    brr1: bytes = bytearray(3)
+    brr1 = bytearray(3)
     assert brr1 == bytearray(b'\x00\x00\x00')
     assert brr1 == b'\x00\x00\x00'
     l = [10, 20, 30, 40]
-    brr2: bytes = bytearray(l)
+    brr2 = bytearray(l)
     assert brr2 == bytearray(b'\n\x14\x1e(')
     assert brr2 == b'\n\x14\x1e('
-    brr3: bytes = bytearray(range(5))
+    brr3 = bytearray(range(5))
     assert brr3 == bytearray(b'\x00\x01\x02\x03\x04')
     assert brr3 == b'\x00\x01\x02\x03\x04'
-    brr4: bytes = bytearray('string', 'utf-8')
+    brr4 = bytearray('string', 'utf-8')
     assert brr4 == bytearray(b'string')
     assert brr4 == b'string'
     assert len(brr1) == 3
     assert len(brr2) == 4
 
-def f(b: bytes) -> bool:
-    return True
+def f(b: bytes) -> str:
+    return "xy"
 
 def test_bytearray_passed_into_bytes() -> None:
-    assert f(bytearray(3))
     brr1: Any = bytearray()
-    assert f(brr1)
+    with assertRaises(TypeError, "bytes object expected; got bytearray"):
+        f(brr1)
+    with assertRaises(TypeError, "bytes object expected; got bytearray"):
+        b: bytes = brr1
 
 [case testBytearraySlicing]
 def test_bytearray_slicing() -> None:
-    b: bytes = bytearray(b'abcdefg')
+    b = bytearray(b'abcdefg')
     zero = int()
     ten = 10 + zero
     two = 2 + zero
@@ -318,7 +322,7 @@ def test_bytearray_slicing() -> None:
 from testutil import assertRaises
 
 def test_bytearray_indexing() -> None:
-    b: bytes = bytearray(b'\xae\x80\xfe\x15')
+    b = bytearray(b'\xae\x80\xfe\x15')
     assert b[0] == 174
     assert b[1] == 128
     assert b[2] == 254
@@ -347,10 +351,6 @@ def test_bytes_join() -> None:
     assert b' '.join([b'a', b'b']) == b'a b'
     assert b' '.join([]) == b''
 
-    x: bytes = bytearray(b' ')
-    assert x.join([b'a', b'b']) == b'a b'
-    assert type(x.join([b'a', b'b'])) == bytearray
-
     y: bytes = bytes_subclass()
     assert y.join([]) == b'spook'
 
diff --git a/mypyc/test-data/run-strings.test b/mypyc/test-data/run-strings.test
index 0ae67ed7f1c3..3c5a1f1d31e1 100644
--- a/mypyc/test-data/run-strings.test
+++ b/mypyc/test-data/run-strings.test
@@ -892,7 +892,7 @@ def test_decode_error() -> None:
         pass
 
 def test_decode_bytearray() -> None:
-    b: bytes = bytearray(b'foo\x00bar')
+    b = bytearray(b'foo\x00bar')
     assert b.decode() == 'foo\x00bar'
     assert b.decode('utf-8') == 'foo\x00bar'
     assert b.decode('latin-1') == 'foo\x00bar'
@@ -900,7 +900,7 @@ def test_decode_bytearray() -> None:
     assert b.decode('utf-8' + str()) == 'foo\x00bar'
     assert b.decode('latin-1' + str()) == 'foo\x00bar'
     assert b.decode('ascii' + str()) == 'foo\x00bar'
-    b2: bytes = bytearray(b'foo\x00bar\xbe')
+    b2 = bytearray(b'foo\x00bar\xbe')
     assert b2.decode('latin-1') == 'foo\x00bar\xbe'
     with assertRaises(UnicodeDecodeError):
         b2.decode('ascii')
@@ -910,7 +910,7 @@ def test_decode_bytearray() -> None:
         b2.decode('utf-8')
     with assertRaises(UnicodeDecodeError):
         b2.decode('utf-8' + str())
-    b3: bytes = bytearray(b'Z\xc3\xbcrich')
+    b3 = bytearray(b'Z\xc3\xbcrich')
     assert b3.decode("utf-8") == 'Zürich'
 
 def test_invalid_encoding() -> None:
diff --git a/mypyc/test/test_run.py b/mypyc/test/test_run.py
index 681e15b58844..e37964f0be22 100644
--- a/mypyc/test/test_run.py
+++ b/mypyc/test/test_run.py
@@ -202,6 +202,9 @@ def run_case_step(self, testcase: DataDrivenTestCase, incremental_step: int) ->
         options.use_builtins_fixtures = True
         options.show_traceback = True
         options.strict_optional = True
+        options.strict_bytes = True
+        options.disable_bytearray_promotion = True
+        options.disable_memoryview_promotion = True
         options.python_version = sys.version_info[:2]
         options.export_types = True
         options.preserve_asts = True
diff --git a/mypyc/test/testutil.py b/mypyc/test/testutil.py
index 5e485e58c9b5..de9852496f75 100644
--- a/mypyc/test/testutil.py
+++ b/mypyc/test/testutil.py
@@ -114,6 +114,9 @@ def build_ir_for_single_file2(
     options.export_types = True
     options.preserve_asts = True
     options.allow_empty_bodies = True
+    options.strict_bytes = True
+    options.disable_bytearray_promotion = True
+    options.disable_memoryview_promotion = True
     options.per_module_options["__main__"] = {"mypyc": True}
 
     source = build.BuildSource("main", "__main__", program_text)