diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0820a42c..9e2c4a59 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -131,6 +131,12 @@ archived by series under [docs/changelog/](docs/changelog/); see the
   `the_ffi_crate_builds_no_rust_library` refuses a crate type that would
   turn it off again.
 
+- **Python: type checkers read the package's annotations.** The package
+  shipped no PEP 561 `py.typed` marker, so mypy skipped it as untyped
+  ("missing library stubs or py.typed marker") and every value it returned
+  was `Any` in the application, although nearly every public function is
+  annotated. The wheel now carries the marker, and the wheel build fails
+  if it goes missing.
 - **Python: a Linux box that could no longer advertise recovers on its own.**
   Linux loses track of the controller's advertising when a central-role
   connection completes while advertising is on: the controller keeps
diff --git a/bindings/python/offline_protocol_sdk/py.typed b/bindings/python/offline_protocol_sdk/py.typed
new file mode 100644
index 00000000..e69de29b
diff --git a/bindings/python/pyproject.toml b/bindings/python/pyproject.toml
index 8edcd347..62e779b7 100644
--- a/bindings/python/pyproject.toml
+++ b/bindings/python/pyproject.toml
@@ -24,6 +24,7 @@ classifiers = [
     "Programming Language :: Rust",
     "Topic :: Communications",
     "Topic :: Security :: Cryptography",
+    "Typing :: Typed",
 ]
 
 # Audited dependencies — see plan for supply chain security audit results.
@@ -86,7 +87,9 @@ Issues = "https://github.com/Offline-Protocol/offline-protocol-sdk/issues"
 include = ["offline_protocol_sdk*"]
 
 [tool.setuptools.package-data]
-offline_protocol_sdk = ["*.dylib", "*.so", "*.dll", "libuniffi.*", "uniffi.dll"]
+# `py.typed` is the PEP 561 marker. Without it mypy skips the package as
+# untyped and every annotation in it is lost to the application.
+offline_protocol_sdk = ["py.typed", "*.dylib", "*.so", "*.dll", "libuniffi.*", "uniffi.dll"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
diff --git a/bindings/python/scripts/build-wheel.sh b/bindings/python/scripts/build-wheel.sh
index 72557deb..e0d6cccb 100755
--- a/bindings/python/scripts/build-wheel.sh
+++ b/bindings/python/scripts/build-wheel.sh
@@ -266,12 +266,17 @@ WHEEL="$PY_ROOT/dist/offline_protocol_sdk-$PY_VERSION-py3-none-$PLAT_TAG.whl"
   die "dist/ holds more than the retagged wheel"
 
 # What the wheel has to carry: the library under the loader's name, the
-# bindings that open it, and the licences.
+# bindings that open it, the PEP 561 marker that lets a type checker read
+# them, and the licences.
 python - "$WHEEL" "$LOADER_NAME" <<'PYTHON'
 import sys, zipfile
 wheel, loader = sys.argv[1], sys.argv[2]
 names = set(zipfile.ZipFile(wheel).namelist())
-required = [f"offline_protocol_sdk/{loader}", "offline_protocol_sdk/offline_protocol.py"]
+required = [
+    f"offline_protocol_sdk/{loader}",
+    "offline_protocol_sdk/offline_protocol.py",
+    "offline_protocol_sdk/py.typed",
+]
 missing = [n for n in required if n not in names]
 libraries = [n for n in names if n.endswith((".so", ".dylib", ".dll"))]
 licences = [n for n in names if n.endswith(("/LICENSE", "/EXPORT.md"))]