diff --git a/pyproject.toml b/pyproject.toml
index 3d4d12787..d13255b57 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -146,7 +146,15 @@ types-PyYAML = ">=6.0.12"
 pytest-codspeed = ">=2.2.0"
 
 [tool.pytest.ini_options]
-addopts = '--durations=10 --ignore=singer_sdk/helpers/_simpleeval.py -m "not external"'
+addopts = [
+    "--durations=10",
+    "--ignore=singer_sdk/helpers/_simpleeval.py",
+    "-m",
+    "not external",
+    "-ra",
+    "--strict-config",
+    "--strict-markers",
+]
 filterwarnings = [
     "error",
     "ignore:Could not configure external gitlab tests:UserWarning",
@@ -167,13 +175,16 @@ filterwarnings = [
     # https://github.com/joblib/joblib/pull/1518
     "ignore:Attribute n is deprecated:DeprecationWarning:joblib._utils",
 ]
+log_cli_level = "INFO"
 markers = [
     "external: Tests relying on external resources",
     "windows: Tests that only run on Windows",
     "snapshot: Tests that use pytest-snapshot",
 ]
+minversion = "7"
 testpaths = ["tests"]
 norecursedirs = "cookiecutter"
+xfail_strict = false
 
 [tool.commitizen]
 name = "cz_version_bump"
@@ -257,11 +268,14 @@ DEP002 = [
 ]
 
 [tool.mypy]
+enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 exclude = "tests"
 files = "singer_sdk"
 local_partial_types = true
+strict = false
 warn_redundant_casts = true
 warn_return_any = true
+warn_unreachable = true
 warn_unused_configs = true
 warn_unused_ignores = true
 
@@ -295,7 +309,6 @@ extend-exclude = [
     "*simpleeval*",
 ]
 line-length = 88
-src = ["samples", "singer_sdk", "tests"]
 target-version = "py38"
 
 [tool.ruff.format]
diff --git a/singer_sdk/_singerlib/encoding/_simple.py b/singer_sdk/_singerlib/encoding/_simple.py
index 7ce148fc3..5c8fec549 100644
--- a/singer_sdk/_singerlib/encoding/_simple.py
+++ b/singer_sdk/_singerlib/encoding/_simple.py
@@ -161,9 +161,9 @@ def __post_init__(self) -> None:
         self.type = SingerMessageType.SCHEMA
 
         if isinstance(self.bookmark_properties, (str, bytes)):
-            self.bookmark_properties = [self.bookmark_properties]
+            self.bookmark_properties = [self.bookmark_properties]  # type: ignore[unreachable]
         if self.bookmark_properties and not isinstance(self.bookmark_properties, list):
-            msg = "bookmark_properties must be a string or list of strings"
+            msg = "bookmark_properties must be a string or list of strings"  # type: ignore[unreachable]
             raise ValueError(msg)
 
 
diff --git a/singer_sdk/authenticators.py b/singer_sdk/authenticators.py
index c6478cb92..4c15304e5 100644
--- a/singer_sdk/authenticators.py
+++ b/singer_sdk/authenticators.py
@@ -71,7 +71,7 @@ def __call__(cls, *args: t.Any, **kwargs: t.Any) -> t.Any:  # noqa: ANN401
             A singleton instance of the derived class.
         """
         if cls.__single_instance:
-            return cls.__single_instance
+            return cls.__single_instance  # type: ignore[unreachable]
         single_obj = cls.__new__(cls, None)  # type: ignore[call-overload]
         single_obj.__init__(*args, **kwargs)
         cls.__single_instance = single_obj
@@ -165,7 +165,7 @@ def __init__(
         """
         super().__init__(stream=stream)
         if self.auth_headers is None:
-            self.auth_headers = {}
+            self.auth_headers = {}  # type: ignore[unreachable]
         if auth_headers:
             self.auth_headers.update(auth_headers)
 
@@ -206,11 +206,11 @@ def __init__(
 
         if location == "header":
             if self.auth_headers is None:
-                self.auth_headers = {}
+                self.auth_headers = {}  # type: ignore[unreachable]
             self.auth_headers.update(auth_credentials)
         elif location == "params":
             if self.auth_params is None:
-                self.auth_params = {}
+                self.auth_params = {}  # type: ignore[unreachable]
             self.auth_params.update(auth_credentials)
 
     @classmethod
@@ -255,7 +255,7 @@ def __init__(self, stream: RESTStream, token: str) -> None:
         auth_credentials = {"Authorization": f"Bearer {token}"}
 
         if self.auth_headers is None:
-            self.auth_headers = {}
+            self.auth_headers = {}  # type: ignore[unreachable]
         self.auth_headers.update(auth_credentials)
 
     @classmethod
@@ -314,7 +314,7 @@ def __init__(
         auth_credentials = {"Authorization": f"Basic {auth_token}"}
 
         if self.auth_headers is None:
-            self.auth_headers = {}
+            self.auth_headers = {}  # type: ignore[unreachable]
         self.auth_headers.update(auth_credentials)
 
     @classmethod
diff --git a/singer_sdk/connectors/sql.py b/singer_sdk/connectors/sql.py
index b6a74a976..c9f18cbf3 100644
--- a/singer_sdk/connectors/sql.py
+++ b/singer_sdk/connectors/sql.py
@@ -496,7 +496,7 @@ def discover_catalog_entry(
         # Detect key properties
         possible_primary_keys: list[list[str]] = []
         pk_def = inspected.get_pk_constraint(table_name, schema=schema_name)
-        if pk_def and "constrained_columns" in pk_def:
+        if pk_def and "constrained_columns" in pk_def:  # type: ignore[redundant-expr]
             possible_primary_keys.append(pk_def["constrained_columns"])
 
         # An element of the columns list is ``None`` if it's an expression and is
@@ -953,7 +953,7 @@ def merge_sql_types(
         # Get the generic type class
         for opt in sql_types:
             # Get the length
-            opt_len: int = getattr(opt, "length", 0)
+            opt_len: int | None = getattr(opt, "length", 0)
             generic_type = type(opt.as_generic())
 
             if isinstance(generic_type, type):
diff --git a/singer_sdk/helpers/_batch.py b/singer_sdk/helpers/_batch.py
index b57002e1d..2e4ae4615 100644
--- a/singer_sdk/helpers/_batch.py
+++ b/singer_sdk/helpers/_batch.py
@@ -232,7 +232,7 @@ def __post_init__(self) -> None:
             self.storage = StorageTarget.from_dict(self.storage)
 
         if self.batch_size is None:
-            self.batch_size = DEFAULT_BATCH_SIZE
+            self.batch_size = DEFAULT_BATCH_SIZE  # type: ignore[unreachable]
 
     def asdict(self) -> dict[str, t.Any]:
         """Return a dictionary representation of the message.
diff --git a/singer_sdk/helpers/_catalog.py b/singer_sdk/helpers/_catalog.py
index 90eec4c5e..a8747d30c 100644
--- a/singer_sdk/helpers/_catalog.py
+++ b/singer_sdk/helpers/_catalog.py
@@ -43,7 +43,7 @@ def _pop_deselected_schema(
         schema_at_breadcrumb = schema_at_breadcrumb.get(crumb, {})
 
     if not isinstance(schema_at_breadcrumb, dict):  # pragma: no cover
-        msg = (
+        msg = (  # type: ignore[unreachable]
             "Expected dictionary type instead of "
             f"'{type(schema_at_breadcrumb).__name__}' '{schema_at_breadcrumb}' for "
             f"'{stream_name}' bookmark '{breadcrumb!s}' in '{schema}'"
@@ -123,7 +123,7 @@ def set_catalog_stream_selected(
     """
     breadcrumb = breadcrumb or ()
     if not isinstance(breadcrumb, tuple):  # pragma: no cover
-        msg = (
+        msg = (  # type: ignore[unreachable]
             f"Expected tuple value for breadcrumb '{breadcrumb}'. Got "
             f"{type(breadcrumb).__name__}"
         )
diff --git a/singer_sdk/helpers/_state.py b/singer_sdk/helpers/_state.py
index a910bb71e..fd7dee377 100644
--- a/singer_sdk/helpers/_state.py
+++ b/singer_sdk/helpers/_state.py
@@ -118,7 +118,7 @@ def get_writeable_state_dict(
         ValueError: Raise an error if duplicate entries are found.
     """
     if tap_state is None:
-        msg = "Cannot write state to missing state dictionary."
+        msg = "Cannot write state to missing state dictionary."  # type: ignore[unreachable]
         raise ValueError(msg)
 
     if "bookmarks" not in tap_state:
diff --git a/singer_sdk/mapper.py b/singer_sdk/mapper.py
index a2e7bc956..0214fe9e9 100644
--- a/singer_sdk/mapper.py
+++ b/singer_sdk/mapper.py
@@ -377,7 +377,7 @@ def _eval_type(
             ValueError: If the expression is ``None``.
         """
         if expr is None:
-            msg = "Expression should be str, not None"
+            msg = "Expression should be str, not None"  # type: ignore[unreachable]
             raise ValueError(msg)
 
         default = default or th.StringType()
@@ -564,7 +564,7 @@ def always_true(record: dict) -> bool:
         elif filter_rule is None:
             filter_fn = always_true
         else:
-            msg = (
+            msg = (  # type: ignore[unreachable]
                 f"Unexpected filter rule type '{type(filter_rule).__name__}' in "
                 f"expression {filter_rule!s}. Expected 'str' or 'None'."
             )
@@ -783,9 +783,7 @@ def register_raw_stream_schema(  # noqa: PLR0912, C901
                     key_properties=key_properties,
                     flattening_options=self.flattening_options,
                 )
-            elif stream_def is None or (
-                isinstance(stream_def, str) and stream_def == NULL_STRING
-            ):
+            elif stream_def is None or (stream_def == NULL_STRING):
                 mapper = RemoveRecordTransform(
                     stream_alias=stream_alias,
                     raw_schema=schema,
@@ -800,7 +798,7 @@ def register_raw_stream_schema(  # noqa: PLR0912, C901
                 raise StreamMapConfigError(msg)
 
             else:
-                msg = (
+                msg = (  # type: ignore[unreachable]
                     f"Unexpected stream definition type. Expected str, dict, or None. "
                     f"Got '{type(stream_def).__name__}'."
                 )
diff --git a/singer_sdk/plugin_base.py b/singer_sdk/plugin_base.py
index eb5b39de3..ed37bfb3f 100644
--- a/singer_sdk/plugin_base.py
+++ b/singer_sdk/plugin_base.py
@@ -153,7 +153,7 @@ def __init__(
         elif isinstance(config, dict):
             config_dict = config
         else:
-            msg = f"Error parsing config of type '{type(config).__name__}'."
+            msg = f"Error parsing config of type '{type(config).__name__}'."  # type: ignore[unreachable]
             raise ValueError(msg)
         if parse_env_config:
             self.logger.info("Parsing env var for settings config...")
diff --git a/singer_sdk/streams/core.py b/singer_sdk/streams/core.py
index 51a968313..e65f577f2 100644
--- a/singer_sdk/streams/core.py
+++ b/singer_sdk/streams/core.py
@@ -162,7 +162,7 @@ def __init__(
             elif isinstance(schema, singer.Schema):
                 self._schema = schema.to_dict()
             else:
-                msg = f"Unexpected type {type(schema).__name__} for arg 'schema'."
+                msg = f"Unexpected type {type(schema).__name__} for arg 'schema'."  # type: ignore[unreachable]
                 raise ValueError(msg)
 
         if self.schema_filepath:
@@ -187,7 +187,7 @@ def stream_maps(self) -> list[StreamMap]:
         if self._stream_maps:
             return self._stream_maps
 
-        if self._tap.mapper:
+        if self._tap.mapper:  # type: ignore[truthy-bool]
             self._stream_maps = self._tap.mapper.stream_maps[self.name]
             self.logger.info(
                 "Tap has custom mapper. Using %d provided map(s).",
diff --git a/singer_sdk/streams/graphql.py b/singer_sdk/streams/graphql.py
index 4e5455bc3..22a5fb0cc 100644
--- a/singer_sdk/streams/graphql.py
+++ b/singer_sdk/streams/graphql.py
@@ -71,7 +71,7 @@ def prepare_request_payload(
         query = self.query
 
         if query is None:
-            msg = "Graphql `query` property not set."
+            msg = "Graphql `query` property not set."  # type: ignore[unreachable]
             raise ValueError(msg)
 
         if not query.lstrip().startswith("query"):
diff --git a/singer_sdk/tap_base.py b/singer_sdk/tap_base.py
index d69fa5f38..d246a2790 100644
--- a/singer_sdk/tap_base.py
+++ b/singer_sdk/tap_base.py
@@ -148,7 +148,7 @@ def state(self) -> dict:
             RuntimeError: If state has not been initialized.
         """
         if self._state is None:
-            msg = "Could not read from uninitialized state."
+            msg = "Could not read from uninitialized state."  # type: ignore[unreachable]
             raise RuntimeError(msg)
         return self._state
 
@@ -398,7 +398,7 @@ def load_state(self, state: dict[str, t.Any]) -> None:
                 initialized.
         """
         if self.state is None:
-            msg = "Cannot write to uninitialized state dictionary."
+            msg = "Cannot write to uninitialized state dictionary."  # type: ignore[unreachable]
             raise ValueError(msg)
 
         for stream_name, stream_state in state.get("bookmarks", {}).items():
diff --git a/singer_sdk/testing/legacy.py b/singer_sdk/testing/legacy.py
index a47d3e770..eae4c5b5d 100644
--- a/singer_sdk/testing/legacy.py
+++ b/singer_sdk/testing/legacy.py
@@ -40,7 +40,7 @@ def _test_discovery() -> None:
         catalog1 = _get_tap_catalog(tap_class, config or {})
         # Reset and re-initialize with an input catalog
         tap2: Tap = tap_class(config=config, parse_env_config=True, catalog=catalog1)
-        assert tap2
+        assert tap2  # type: ignore[truthy-bool]
 
     def _test_stream_connections() -> None:
         # Initialize with basic config
diff --git a/singer_sdk/testing/tap_tests.py b/singer_sdk/testing/tap_tests.py
index 5839e0cea..92e0b13e1 100644
--- a/singer_sdk/testing/tap_tests.py
+++ b/singer_sdk/testing/tap_tests.py
@@ -48,7 +48,7 @@ def test(self) -> None:
             catalog=catalog,
             **kwargs,
         )
-        assert tap2
+        assert tap2  # type: ignore[truthy-bool]
 
 
 class TapStreamConnectionTest(TapTestTemplate):
@@ -218,7 +218,7 @@ def test(self) -> None:
         try:
             for v in self.non_null_attribute_values:
                 error_message = f"Unable to parse value ('{v}') with datetime parser."
-                assert datetime_fromisoformat(v), error_message
+                assert datetime_fromisoformat(v), error_message  # type: ignore[truthy-bool]
         except ValueError as e:
             raise AssertionError(error_message) from e
 
diff --git a/singer_sdk/typing.py b/singer_sdk/typing.py
index 6bf8d9527..2910125f8 100644
--- a/singer_sdk/typing.py
+++ b/singer_sdk/typing.py
@@ -189,7 +189,7 @@ def __get__(self, instance: P, owner: type[P]) -> t.Any:  # noqa: ANN401
             The property value.
         """
         if instance is None:
-            instance = owner()
+            instance = owner()  # type: ignore[unreachable]
         return self.fget(instance)
 
 
@@ -1123,13 +1123,10 @@ def to_jsonschema_type(
         type_name = from_type
     elif isinstance(from_type, sa.types.TypeEngine):
         type_name = type(from_type).__name__
-    elif isinstance(from_type, type) and issubclass(
-        from_type,
-        sa.types.TypeEngine,
-    ):
+    elif issubclass(from_type, sa.types.TypeEngine):
         type_name = from_type.__name__
     else:  # pragma: no cover
-        msg = "Expected `str` or a SQLAlchemy `TypeEngine` object or type."
+        msg = "Expected `str` or a SQLAlchemy `TypeEngine` object or type."  # type: ignore[unreachable]
         # TODO: this should be a TypeError, but it's a breaking change.
         raise ValueError(msg)  # noqa: TRY004