diff --git a/openaq/__init__.py b/openaq/__init__.py
index 9cebf52d..61a08327 100644
--- a/openaq/__init__.py
+++ b/openaq/__init__.py
@@ -2,7 +2,7 @@
 
 import logging
 
-__version__ = "1.0.0rc3"
+__version__ = "1.0.0rc4"
 
 
 logger = logging.getLogger(__name__)
@@ -18,6 +18,7 @@
     GatewayTimeoutError,
     HTTPRateLimitError,
     IdentifierOutOfBoundsError,
+    InvalidParameterError,
     NotAuthorizedError,
     NotFoundError,
     RateLimitError,
@@ -34,6 +35,7 @@
     "NotAuthorizedError",
     "NotFoundError",
     "TimeoutError",
+    "InvalidParameterError",
     "ValidationError",
     "GatewayTimeoutError",
     "HTTPRateLimitError",
@@ -43,4 +45,5 @@
     "ServerError",
     "ServiceUnavailableError",
     "BadGatewayError",
+    "__version__",
 ]
diff --git a/openaq/client.py b/openaq/client.py
index 4598eaeb..4c9931af 100644
--- a/openaq/client.py
+++ b/openaq/client.py
@@ -14,6 +14,7 @@
 from pathlib import Path
 from types import TracebackType
 from typing import Mapping
+from urllib.parse import urljoin, urlparse
 
 from openaq import __version__
 from openaq.core.exceptions import ApiKeyMissingError, RateLimitError
@@ -21,9 +22,7 @@
     DEFAULT_LIMITS,
     DEFAULT_TIMEOUT,
     Headers,
-    Limits,
     Response,
-    Timeout,
     Transport,
 )
 from openaq.models.countries import Countries
@@ -83,17 +82,11 @@ class OpenAQ:
         auto_wait: Whether to automatically wait when rate limited. Defaults to
             True.
         base_url: The base URL for the API endpoint.
-        transport: The transport instance for making HTTP requests. For internal
+        _transport: The transport instance for making HTTP requests. For internal
             use.
         rate_limit_override: Override the default rate limit capacity of 60
             requests per minute. Useful for accounts with a higher rate limit.
             Defaults to None.
-        timeout: Timeout configuration for HTTP requests. Defaults to 5 seconds
-            for connection and pool, and 8 seconds for read to account for the
-            API's 6 second processing limit. Pass None for no timeout.
-        limits: Connection pool limits for the HTTP transport. Defaults to 20
-            maximum connections with 10 keepalive connections. Keepalive
-            connections expire after 30 seconds.
 
     Note:
         An API key can either be passed directly to the OpenAQ client class at
@@ -131,9 +124,7 @@ def __init__(
         headers: Mapping[str, str] | None = None,
         auto_wait: bool = True,
         base_url: str = DEFAULT_BASE_URL,
-        transport: Transport | None = None,
-        timeout: float | Timeout | None = DEFAULT_TIMEOUT,
-        limits: Limits = DEFAULT_LIMITS,
+        _transport: Transport | None = None,  # internal use only
         rate_limit_override: int | None = None,
     ) -> None:
         """Initializes the OpenAQ client.
@@ -146,14 +137,8 @@ def __init__(
             auto_wait: Whether to automatically wait when rate limited. Defaults
                 to True.
             base_url: The base URL for the API endpoint.
-            transport: The transport instance for making HTTP requests. For
+            _transport: The transport instance for making HTTP requests. For
                 internal use.
-            timeout: Timeout configuration for HTTP requests. Defaults to 5
-                seconds for connection and 8 seconds for read. Pass None for no
-                timeout.
-            limits: Connection pool limits for the HTTP transport. Defaults to
-                20 maximum connections with 10 keepalive connections expiring
-                after 30 seconds.
             rate_limit_override: Initial rate limit capacity in requests per
                 minute. Defaults to 60 and is corrected automatically from
                 server response headers after the first request.
@@ -163,12 +148,17 @@ def __init__(
                 URL is used.
         """
         self._api_key = _resolve_api_key(api_key)
-        self._base_url = base_url
+        parsed = urlparse(base_url)
+        if not parsed.scheme or not parsed.netloc:
+            raise ValueError(
+                f"Invalid base_url, must be a fully qualified URL: {base_url!r}"
+            )
+        self._base_url = parsed.geturl()
         self._auto_wait = auto_wait
         self._transport = (
-            transport
-            if transport is not None
-            else Transport(timeout=timeout, limits=limits)
+            _transport
+            if _transport is not None
+            else Transport(timeout=DEFAULT_TIMEOUT, limits=DEFAULT_LIMITS)
         )
         self._headers = Headers(headers or {})
 
@@ -181,8 +171,8 @@ def __init__(
             )
 
         self._user_agent = f"openaq-python-{__version__}-{platform.python_version()}"
-        assert self._api_key is not None
-        self._headers["X-API-Key"] = self._api_key
+        if self._api_key:
+            self._headers["X-API-Key"] = self._api_key
         self._headers["User-Agent"] = self._user_agent
         self._headers["Accept"] = ACCEPT_HEADER
 
@@ -251,9 +241,13 @@ def _check_rate_limit(self) -> None:
                 self._wait_for_rate_limit_reset()
                 self._rate_limit_remaining = self._rate_limit_capacity
             else:
-                message = f"Rate limit exceeded. Limit resets in {self._rate_limit_reset_seconds} seconds"
-                logger.error(message)
-                raise RateLimitError(message)
+                logger.error(
+                    "Rate limit exceeded. Limit resets in %s seconds",
+                    self._rate_limit_reset_seconds,
+                )
+                raise RateLimitError(
+                    f"Rate limit exceeded. Limit resets in {self._rate_limit_reset_seconds} seconds"
+                )
         self._rate_limit_remaining -= 1
 
     def _set_rate_limit(self, headers: Headers) -> None:
@@ -286,7 +280,7 @@ def _wait_for_rate_limit_reset(self) -> None:
         """
         wait_seconds = self._rate_limit_reset_seconds
         if wait_seconds > 0:
-            logger.info(f"Rate limit hit. Waiting {wait_seconds} seconds for reset.")
+            logger.info("Rate limit hit. Waiting %s seconds for reset.", wait_seconds)
             time.sleep(wait_seconds)
 
     def _get_int_header(self, headers: Headers, key: str, default: int) -> int:
@@ -349,7 +343,7 @@ def _do(
         """
         self._check_rate_limit()
         request_headers = self._build_request_headers(headers)
-        url = self._base_url + path
+        url = urljoin(self._base_url, path.lstrip("/"))
         data = self._transport.send_request(
             method=method, url=url, params=params, headers=request_headers
         )
diff --git a/openaq/core/transport.py b/openaq/core/transport.py
index 2009ef81..f455dab9 100644
--- a/openaq/core/transport.py
+++ b/openaq/core/transport.py
@@ -1,10 +1,11 @@
-"""Base class and utlity functions for working with client transport."""
+"""Base class and utility functions for working with client transport."""
 
 from __future__ import annotations
 
 import http.client
 import json
 import logging
+import ssl
 import threading
 import time
 import urllib.parse
@@ -336,7 +337,7 @@ def release(self, pc: PooledConnection, *, discard: bool = False) -> None:
 
     def close_all(self) -> None:
         """Closes all idle connections in the pool and resets its state."""
-        with self._lock:
+        with self._has_capacity:
             for q in self._idle.values():
                 for pc in q:
                     try:
@@ -345,6 +346,7 @@ def close_all(self) -> None:
                         pass
             self._idle.clear()
             self._total = 0
+            self._has_capacity.notify_all()
 
 
 def _encode_params(
@@ -434,21 +436,31 @@ def _raw_request(
         for attempt in range(2):
             pc = self._pool.acquire(host, self._pool_timeout)
             try:
-                if self._read_timeout is not None:
-                    if pc.conn.sock is not None:
-                        pc.conn.sock.settimeout(self._read_timeout)
-
                 pc.conn.request(method, path, headers=dict(headers))
-                raw = pc.conn.getresponse()
-
-                # After connect, set socket timeout for the read.
                 if self._read_timeout is not None and pc.conn.sock is not None:
                     pc.conn.sock.settimeout(self._read_timeout)
-
+                raw = pc.conn.getresponse()
                 body = raw.read()
                 resp = Response(raw.status, body, raw.msg)
                 self._pool.release(pc)
                 return resp
+
+            except ssl.SSLCertVerificationError as exc:
+                self._pool.release(pc, discard=True)
+                logger.error(
+                    "SSL certificate verification failed for %s: %s. "
+                    "On macOS, run 'Install Certificates.command' in your Python "
+                    "installation directory to fix this.",
+                    host,
+                    exc,
+                )
+                raise
+
+            except ssl.SSLError as exc:
+                self._pool.release(pc, discard=True)
+                logger.error("SSL error for %s: %s", host, exc)
+                raise
+
             except (OSError, http.client.HTTPException) as exc:
                 self._pool.release(pc, discard=True)
                 if attempt == 1:
@@ -495,7 +507,7 @@ def send_request(
         if parsed.query:
             path = f"{path}?{parsed.query}"
 
-        res = self._raw_request(method, host, path, headers)
+        res = self._raw_request(method.upper(), host, path, headers)
         logger.debug("Received response: %s from %s", res.status_code, url)
         return check_response(res)
 
@@ -504,6 +516,20 @@ def close(self) -> None:
         self._pool.close_all()
 
 
+_HTTP_SATUS_MAP = {
+    HTTPStatus.BAD_REQUEST: BadRequestError,
+    HTTPStatus.NOT_FOUND: NotFoundError,
+    HTTPStatus.REQUEST_TIMEOUT: TimeoutError,
+    HTTPStatus.FORBIDDEN: ForbiddenError,
+    HTTPStatus.UNPROCESSABLE_ENTITY: ValidationError,
+    HTTPStatus.TOO_MANY_REQUESTS: HTTPRateLimitError,
+    HTTPStatus.UNAUTHORIZED: NotAuthorizedError,
+    HTTPStatus.INTERNAL_SERVER_ERROR: ServerError,
+    HTTPStatus.BAD_GATEWAY: BadGatewayError,
+    HTTPStatus.SERVICE_UNAVAILABLE: ServiceUnavailableError,
+}
+
+
 def check_response(res: Response) -> Response:
     """Checks the HTTP response of the request.
 
@@ -528,42 +554,18 @@ def check_response(res: Response) -> Response:
     """
     if res.status_code >= HTTPStatus.OK and res.status_code < HTTPStatus.BAD_REQUEST:
         return res
-    elif res.status_code == HTTPStatus.BAD_REQUEST:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise BadRequestError(res.text)
-    elif res.status_code == HTTPStatus.NOT_FOUND:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise NotFoundError(res.text)
-    elif res.status_code == HTTPStatus.REQUEST_TIMEOUT:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise TimeoutError(res.text)
-    elif res.status_code == HTTPStatus.FORBIDDEN:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise ForbiddenError(res.text)
-    elif res.status_code == HTTPStatus.UNPROCESSABLE_ENTITY:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise ValidationError(res.text)
-    elif res.status_code == HTTPStatus.TOO_MANY_REQUESTS:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise HTTPRateLimitError(res.text)
-    elif res.status_code == HTTPStatus.UNAUTHORIZED:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise NotAuthorizedError(res.text)
-    elif res.status_code == HTTPStatus.INTERNAL_SERVER_ERROR:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise ServerError(res.text)
-    elif res.status_code == HTTPStatus.BAD_GATEWAY:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise BadGatewayError(res.text)
-    elif res.status_code == HTTPStatus.SERVICE_UNAVAILABLE:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise ServiceUnavailableError(res.text)
-    elif res.status_code == HTTPStatus.GATEWAY_TIMEOUT:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
+    if res.status_code == HTTPStatus.GATEWAY_TIMEOUT:
+        logger.error("HTTP %s - %s", res.status_code, res.text)
         raise GatewayTimeoutError(
             "Your request timed out on the server. "
             "Consider reducing the complexity of your request."
         )
-    else:
-        logger.exception(f"HTTP {res.status_code} - {res.text}")
-        raise Exception
+    try:
+        http_status = HTTPStatus(res.status_code)
+    except ValueError:
+        http_status = None
+    exc_class = (
+        _HTTP_SATUS_MAP.get(http_status, ServerError) if http_status else ServerError
+    )
+    logger.error("HTTP %s - %s", res.status_code, res.text)
+    raise exc_class(res.text)
diff --git a/openaq/core/validators.py b/openaq/core/validators.py
index e0caab32..bdc38d8c 100644
--- a/openaq/core/validators.py
+++ b/openaq/core/validators.py
@@ -861,7 +861,7 @@ def validate_datetime_params(
         if date_to:
             if not date_check(date_from) or not date_check(date_to):
                 raise InvalidParameterError(
-                    f"Invalid date_from or date_to, must be either date type or ISO-8601 formatted date string, got {type(date_from)} and {type(date_to)}"
+                    f"Invalid date_from or date_to, must be either datetime.date type or ISO-8601 formatted date string, got {type(date_from)} and {type(date_to)}"
                 )
             date_from_date = to_date(date_from)
             date_to_date = to_date(date_to)
@@ -888,7 +888,7 @@ def validate_datetime_params(
         if datetime_to is not None:
             if not datetime_check(datetime_from) or not datetime_check(datetime_to):
                 raise InvalidParameterError(
-                    f"Invalid datetime_from or datetime_to, must be either datetime type or ISO-8601 formatted string, got {type(datetime_from)} and {type(datetime_to)}"
+                    f"Invalid datetime_from or datetime_to, must be either datetime.datetime type or ISO-8601 formatted string, got {type(datetime_from)} and {type(datetime_to)}"
                 )
             datetime_from_datetime = to_datetime(datetime_from)
             datetime_to_datetime = to_datetime(datetime_to)
@@ -902,7 +902,7 @@ def validate_datetime_params(
         elif datetime_from is not None:
             if not datetime_check(datetime_from):
                 raise InvalidParameterError(
-                    f"Invalid datetime_from, must be either datetime type or ISO-8601 formatted string, got {type(datetime_from)}"
+                    f"Invalid datetime_from, must be either datetime.datetime type or ISO-8601 formatted string, got {type(datetime_from)}"
                 )
             datetime_from_datetime = to_datetime(datetime_from)
             if not datetime_from_lesser_check(datetime_from_datetime):
diff --git a/openaq/models/base.py b/openaq/models/base.py
index 259a4aca..e252f5c5 100644
--- a/openaq/models/base.py
+++ b/openaq/models/base.py
@@ -1,3 +1,5 @@
+"""Base class for OpenAQ API resource models."""
+
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
diff --git a/openaq/models/countries.py b/openaq/models/countries.py
index f7c7340e..a0e7b8c4 100644
--- a/openaq/models/countries.py
+++ b/openaq/models/countries.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the countries endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import CountriesResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/instruments.py b/openaq/models/instruments.py
index 8c95cbb1..a23a7328 100644
--- a/openaq/models/instruments.py
+++ b/openaq/models/instruments.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the instruments endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import InstrumentsResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/licenses.py b/openaq/models/licenses.py
index 78d1f678..65aab921 100644
--- a/openaq/models/licenses.py
+++ b/openaq/models/licenses.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the licenses endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import LicensesResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/locations.py b/openaq/models/locations.py
index 6558a666..e926d697 100644
--- a/openaq/models/locations.py
+++ b/openaq/models/locations.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the locations endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import (
     LatestResponse,
diff --git a/openaq/models/manufacturers.py b/openaq/models/manufacturers.py
index 86261002..e82ffbbe 100644
--- a/openaq/models/manufacturers.py
+++ b/openaq/models/manufacturers.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the manufacturers endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import InstrumentsResponse, ManufacturersResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/measurements.py b/openaq/models/measurements.py
index 751e731b..b1f986ba 100644
--- a/openaq/models/measurements.py
+++ b/openaq/models/measurements.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the measurements endpoints of the OpenAQ API."""
+
 import datetime
 from typing import overload
 
diff --git a/openaq/models/owners.py b/openaq/models/owners.py
index be41ca53..f4476f79 100644
--- a/openaq/models/owners.py
+++ b/openaq/models/owners.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the owners endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import OwnersResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/parameters.py b/openaq/models/parameters.py
index 0d19613f..2c905444 100644
--- a/openaq/models/parameters.py
+++ b/openaq/models/parameters.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the parameters endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import LatestResponse, ParametersResponse
 from openaq.core.types import ParameterType, SortOrder
diff --git a/openaq/models/providers.py b/openaq/models/providers.py
index ea3dc7e3..4aab5e75 100644
--- a/openaq/models/providers.py
+++ b/openaq/models/providers.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the providers endpoints of the OpenAQ API."""
+
 from openaq.core.models import build_query_params
 from openaq.core.responses import ProvidersResponse
 from openaq.core.types import SortOrder
diff --git a/openaq/models/sensors.py b/openaq/models/sensors.py
index 7f59f435..52dfee0b 100644
--- a/openaq/models/sensors.py
+++ b/openaq/models/sensors.py
@@ -1,3 +1,5 @@
+"""Resource model for interacting with the sensors endpoints of the OpenAQ API."""
+
 from openaq.core.responses import SensorsResponse
 from openaq.core.validators import validate_integer_id
 
diff --git a/tests/unit/test_client.py b/tests/unit/test_client.py
index 3bc171fc..a847992b 100644
--- a/tests/unit/test_client.py
+++ b/tests/unit/test_client.py
@@ -36,7 +36,7 @@ def mock_config_file():
 class TestClient:
     @pytest.fixture()
     def setup(self):
-        self.client = OpenAQ(api_key="abc123-def456-ghi789", transport=MockTransport())
+        self.client = OpenAQ(api_key="abc123-def456-ghi789", _transport=MockTransport())
 
     @pytest.fixture()
     def mock_openaq_api_key_env_vars(self):
@@ -46,7 +46,7 @@ def mock_openaq_api_key_env_vars(self):
             yield
 
     def test_transport_property(self, setup):
-        assert isinstance(self.client.transport, MockTransport)
+        assert isinstance(self.client._transport, MockTransport)
         with pytest.raises(AttributeError):
             self.client.transport = MockTransport()
 
@@ -61,7 +61,7 @@ def test_custom_headers(self, setup):
         self.client = OpenAQ(
             api_key="abc123-def456-ghi789",
             base_url="https://mycustom.openaq.org",
-            transport=MockTransport(),
+            _transport=MockTransport(),
         )
         assert self.client.headers["X-API-Key"] == "abc123-def456-ghi789"
 
@@ -69,22 +69,22 @@ def test_client_params(self, setup):
         self.client = OpenAQ(
             api_key="abc123-def456-ghi789",
             base_url="https://mycustom.openaq.org",
-            transport=MockTransport(),
+            _transport=MockTransport(),
         )
         assert self.client._base_url == "https://mycustom.openaq.org"
 
     def test_api_env_var(self, mock_openaq_api_key_env_vars):
-        client = OpenAQ(transport=MockTransport())
+        client = OpenAQ(_transport=MockTransport())
         assert client.api_key == "openaq-1a2b3c4d5e6f7g8h9i0j1k2l3m4n5o6p"
 
     @pytest.mark.usefixtures("mock_config_file")
     def test_api_key_from_config(self):
         if int(platform.python_version_tuple()[1]) >= 11:
-            client = OpenAQ(transport=MockTransport())
+            client = OpenAQ(_transport=MockTransport())
             assert client.api_key == "test_api_key"
         else:
             with pytest.raises(ApiKeyMissingError):
-                client = OpenAQ(transport=MockTransport())
+                client = OpenAQ(_transport=MockTransport())
 
     def test_api_key_arg_override_env_var(self, setup, mock_openaq_api_key_env_vars):
         assert self.client.api_key == "abc123-def456-ghi789"
@@ -97,6 +97,10 @@ def test_api_key_arg_override_env_vars_config(
     ):
         assert self.client.api_key == "abc123-def456-ghi789"
 
+    def test_raises_api_key_missing_error_when_key_is_none(self):
+        with pytest.raises(ApiKeyMissingError):
+            OpenAQ(api_key=None, _transport=MockTransport())
+
     @patch('openaq.client.datetime')
     @patch('time.sleep')
     @patch('openaq.client.logger')
@@ -111,7 +115,7 @@ def test_wait_for_rate_limit_reset_waits_when_positive(
 
         mock_sleep.assert_called_once_with(5)
         mock_logger.info.assert_called_once_with(
-            "Rate limit hit. Waiting 5 seconds for reset."
+            "Rate limit hit. Waiting %s seconds for reset.", 5
         )
 
     @patch('openaq.client.datetime')
@@ -169,7 +173,7 @@ def test_context_manager_exit_closes_even_with_exception(self, setup):
     def test_blocks_after_custom_limit(self):
         client = OpenAQ(
             api_key="abc123-def456-ghi789",
-            transport=MockTransport(),
+            _transport=MockTransport(),
             auto_wait=False,
             rate_limit_override=5,
         )
@@ -184,7 +188,7 @@ def test_allows_exactly_override_requests(self):
         limit = 10
         client = OpenAQ(
             api_key="abc123-def456-ghi789",
-            transport=MockTransport(),
+            _transport=MockTransport(),
             auto_wait=False,
             rate_limit_override=limit,
         )
@@ -328,27 +332,46 @@ def test_do_raises_before_sending_when_rate_limited(self, setup):
 
     def test_default_timeout_applied_to_transport(self):
         client = OpenAQ(api_key="abc123-def456-ghi789")
-        assert client.transport._connect_timeout == DEFAULT_TIMEOUT.connect
-        assert client.transport._read_timeout == DEFAULT_TIMEOUT.read
-
-    def test_custom_timeout_passed_to_transport(self):
-        custom_timeout = Timeout(10.0, read=15.0)
-        client = OpenAQ(api_key="abc123-def456-ghi789", timeout=custom_timeout)
-        assert client.transport._connect_timeout == custom_timeout.connect
-        assert client.transport._read_timeout == custom_timeout.read
+        assert client._transport._connect_timeout == DEFAULT_TIMEOUT.connect
+        assert client._transport._read_timeout == DEFAULT_TIMEOUT.read
 
     def test_default_limits_applied_to_transport(self):
         client = OpenAQ(api_key="abc123-def456-ghi789")
-        assert client.transport._pool._max_total == DEFAULT_LIMITS.max_connections
+        assert client._transport._pool._max_total == DEFAULT_LIMITS.max_connections
         assert (
-            client.transport._pool._max_idle == DEFAULT_LIMITS.max_keepalive_connections
+            client._transport._pool._max_idle
+            == DEFAULT_LIMITS.max_keepalive_connections
         )
 
-    def test_custom_limits_passed_to_transport(self):
-        custom_limits = Limits(max_connections=5, max_keepalive_connections=2)
-        client = OpenAQ(api_key="abc123-def456-ghi789", limits=custom_limits)
-        assert client.transport._pool._max_total == 5
-        assert client.transport._pool._max_idle == 2
+    def test_do_does_not_produce_double_slash_in_url(self, setup):
+        """Leading slash on path should not create double slash when base_url has trailing slash."""
+        mock_response = MagicMock()
+        mock_response.headers = Headers({})
+        self.client._transport.send_request = Mock(return_value=mock_response)
+
+        self.client._do("get", "/locations/1")
+
+        call_kwargs = self.client._transport.send_request.call_args
+        url = call_kwargs.kwargs["url"]
+        assert "//" not in url.replace(
+            "https://", ""
+        ), f"URL contains double slash: {url}"
+
+    def test_raises_value_error_for_base_url_without_scheme(self):
+        with pytest.raises(ValueError, match="Invalid base_url"):
+            OpenAQ(
+                api_key="abc123-def456-ghi789",
+                base_url="api.openaq.org/v3/",
+                _transport=MockTransport(),
+            )
+
+    def test_raises_value_error_for_base_url_without_netloc(self):
+        with pytest.raises(ValueError, match="Invalid base_url"):
+            OpenAQ(
+                api_key="abc123-def456-ghi789",
+                base_url="https://",
+                _transport=MockTransport(),
+            )
 
 
 def test_tomllib_conditional_import():
diff --git a/tests/unit/test_transport.py b/tests/unit/test_transport.py
index 3a29dee3..42548f98 100644
--- a/tests/unit/test_transport.py
+++ b/tests/unit/test_transport.py
@@ -1,4 +1,5 @@
 import http.client
+import ssl
 import threading
 import time
 from unittest import mock
@@ -290,3 +291,57 @@ def test_raises_after_two_failures(self):
         with acquire, release:
             with pytest.raises(OSError):
                 transport._raw_request("GET", "api.openaq.org", "/v3/locations", {})
+
+    def test_send_request_uppercases_method(self):
+        transport = Transport()
+        transport._raw_request = mock.Mock(
+            return_value=Response(200, b'{}', http.client.HTTPMessage())
+        )
+
+        transport.send_request(
+            "get", "https://api.openaq.org/v3/locations/1", None, Headers()
+        )
+
+        method = transport._raw_request.call_args.args[0]
+        assert method == "GET"
+
+    @pytest.mark.parametrize(
+        "exc",
+        [
+            ssl.SSLCertVerificationError("cert verify failed"),
+            ssl.SSLError("ssl error"),
+        ],
+    )
+    def test_ssl_error_discards_connection_and_raises(self, exc):
+        transport = self.make_transport()
+        raw = make_raw_response()
+        acquire, release, pc = self.patch_pool(transport, raw)
+        pc.conn.request.side_effect = exc
+
+        with acquire, release as mock_release:
+            with pytest.raises(ssl.SSLError):
+                transport._raw_request("GET", "api.openaq.org", "/v3/locations", {})
+            mock_release.assert_called_once_with(pc, discard=True)
+
+    def test_ssl_error_logs(self):
+        transport = self.make_transport()
+        raw = make_raw_response()
+        acquire, release, pc = self.patch_pool(transport, raw)
+        pc.conn.request.side_effect = ssl.SSLCertVerificationError("cert verify failed")
+
+        with acquire, release:
+            with pytest.raises(ssl.SSLCertVerificationError):
+                with mock.patch("openaq.core.transport.logger") as mock_logger:
+                    transport._raw_request("GET", "api.openaq.org", "/v3/locations", {})
+            mock_logger.error.assert_called_once()
+
+    def test_ssl_error_does_not_retry(self):
+        transport = self.make_transport()
+        raw = make_raw_response()
+        acquire, release, pc = self.patch_pool(transport, raw)
+        pc.conn.request.side_effect = ssl.SSLError("SSL error")
+
+        with acquire, release:
+            with pytest.raises(ssl.SSLError):
+                transport._raw_request("GET", "api.openaq.org", "/v3/locations", {})
+            assert pc.conn.request.call_count == 1