diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,17 @@
 # Changelog for http-client
 
+## 0.7.19
+
+* Make mockable via `Network.HTTP.Client.Internal.requestAction` [#554](https://github.com/snoyberg/http-client/pull/554)
+
+## 0.7.18
+
+* Add the `managerSetMaxNumberHeaders` function to the `Client` module to configure `managerMaxNumberHeaders` in `ManagerSettings`.
+
+## 0.7.17
+
+* Add `managerSetMaxHeaderLength` to `Client` to change `ManagerSettings` `MaxHeaderLength`.
+
 ## 0.7.16
 
 * Add `responseEarlyHints` field to `Response`, containing a list of all HTTP 103 Early Hints headers received from the server.
diff --git a/Network/HTTP/Client.hs b/Network/HTTP/Client.hs
--- a/Network/HTTP/Client.hs
+++ b/Network/HTTP/Client.hs
@@ -112,6 +112,8 @@
     , managerSetProxy
     , managerSetInsecureProxy
     , managerSetSecureProxy
+    , managerSetMaxHeaderLength
+    , managerSetMaxNumberHeaders
     , ProxyOverride
     , proxyFromRequest
     , noProxy
@@ -319,6 +321,16 @@
 -- Since 0.4.7
 managerSetProxy :: ProxyOverride -> ManagerSettings -> ManagerSettings
 managerSetProxy po = managerSetInsecureProxy po . managerSetSecureProxy po
+
+-- @since 0.7.17
+managerSetMaxHeaderLength :: Int -> ManagerSettings -> ManagerSettings
+managerSetMaxHeaderLength l manager = manager
+    { managerMaxHeaderLength = Just $ MaxHeaderLength l }
+
+-- @since 0.7.18
+managerSetMaxNumberHeaders :: Int -> ManagerSettings -> ManagerSettings
+managerSetMaxNumberHeaders n manager = manager
+    { managerMaxNumberHeaders = Just $ MaxNumberHeaders n }
 
 -- $example1
 -- = Example Usage
diff --git a/Network/HTTP/Client/Core.hs b/Network/HTTP/Client/Core.hs
--- a/Network/HTTP/Client/Core.hs
+++ b/Network/HTTP/Client/Core.hs
@@ -6,6 +6,7 @@
     , httpNoBody
     , httpRaw
     , httpRaw'
+    , requestAction
     , getModifiedRequestManager
     , responseOpen
     , responseClose
@@ -25,11 +26,13 @@
 import Network.HTTP.Client.Cookies
 import Data.Maybe (fromMaybe, isJust)
 import Data.Time
+import Data.IORef
 import Control.Exception
 import qualified Data.ByteString.Lazy as L
 import Data.Monoid
 import Control.Monad (void)
 import System.Timeout (timeout)
+import System.IO.Unsafe (unsafePerformIO)
 import Data.KeyedPool
 import GHC.IO.Exception (IOException(..), IOErrorType(..))
 
@@ -94,66 +97,86 @@
             now <- getCurrentTime
             return $ insertCookiesIntoRequest req' (evictExpiredCookies cj now) now
         Nothing -> return (req', Data.Monoid.mempty)
-    (timeout', mconn) <- getConnectionWrapper
-        (responseTimeout' req)
-        (getConn req m)
-
-    -- Originally, we would only test for exceptions when sending the request,
-    -- not on calling @getResponse@. However, some servers seem to close
-    -- connections after accepting the request headers, so we need to check for
-    -- exceptions in both.
-    ex <- try $ do
-        cont <- requestBuilder (dropProxyAuthSecure req) (managedResource mconn)
+    res <- makeRequest req m
+    case cookieJar req' of
+        Just _ -> do
+            now' <- getCurrentTime
+            let (cookie_jar, _) = updateCookieJar res req now' cookie_jar'
+            return (req, res {responseCookieJar = cookie_jar})
+        Nothing -> return (req, res)
 
-        getResponse (mMaxHeaderLength m) timeout' req mconn cont
+makeRequest
+    :: Request
+    -> Manager
+    -> IO (Response BodyReader)
+makeRequest req m = do
+    action <- readIORef requestAction
+    action req m
 
-    case ex of
-        -- Connection was reused, and might have been closed. Try again
-        Left e | managedReused mconn && mRetryableException m e -> do
-            managedRelease mconn DontReuse
-            httpRaw' req m
-        -- Not reused, or a non-retry, so this is a real exception
-        Left e -> do
-          -- Explicitly release connection for all real exceptions:
-          -- https://github.com/snoyberg/http-client/pull/454
-          managedRelease mconn DontReuse
-          throwIO e
-        -- Everything went ok, so the connection is good. If any exceptions get
-        -- thrown in the response body, just throw them as normal.
-        Right res -> case cookieJar req' of
-            Just _ -> do
-                now' <- getCurrentTime
-                let (cookie_jar, _) = updateCookieJar res req now' cookie_jar'
-                return (req, res {responseCookieJar = cookie_jar})
-            Nothing -> return (req, res)
+requestAction :: IORef (Request -> Manager -> IO (Response BodyReader))
+{-# NOINLINE requestAction #-}
+requestAction = unsafePerformIO (newIORef action)
   where
-    getConnectionWrapper mtimeout f =
-        case mtimeout of
-            Nothing -> fmap ((,) Nothing) f
-            Just timeout' -> do
-                before <- getCurrentTime
-                mres <- timeout timeout' f
-                case mres of
-                     Nothing -> throwHttp ConnectionTimeout
-                     Just mConn -> do
-                         now <- getCurrentTime
-                         let timeSpentMicro = diffUTCTime now before * 1000000
-                             remainingTime = round $ fromIntegral timeout' - timeSpentMicro
-                         if remainingTime <= 0
-                             then do
-                                 managedRelease mConn DontReuse
-                                 throwHttp ConnectionTimeout
-                             else return (Just remainingTime, mConn)
+    action
+        :: Request
+        -> Manager
+        -> IO (Response BodyReader)
+    action req m = do
+        (timeout', mconn) <- getConnectionWrapper
+            (responseTimeout' req)
+            (getConn req m)
 
-    responseTimeout' req =
-        case responseTimeout req of
-            ResponseTimeoutDefault ->
-                case mResponseTimeout m of
-                    ResponseTimeoutDefault -> Just 30000000
-                    ResponseTimeoutNone -> Nothing
-                    ResponseTimeoutMicro u -> Just u
-            ResponseTimeoutNone -> Nothing
-            ResponseTimeoutMicro u -> Just u
+        -- Originally, we would only test for exceptions when sending the request,
+        -- not on calling @getResponse@. However, some servers seem to close
+        -- connections after accepting the request headers, so we need to check for
+        -- exceptions in both.
+        ex <- try $ do
+            cont <- requestBuilder (dropProxyAuthSecure req) (managedResource mconn)
+
+            getResponse (mMaxHeaderLength m) (mMaxNumberHeaders m) timeout' req mconn cont
+
+        case ex of
+            -- Connection was reused, and might have been closed. Try again
+            Left e | managedReused mconn && mRetryableException m e -> do
+                managedRelease mconn DontReuse
+                action req m
+            -- Not reused, or a non-retry, so this is a real exception
+            Left e -> do
+              -- Explicitly release connection for all real exceptions:
+              -- https://github.com/snoyberg/http-client/pull/454
+              managedRelease mconn DontReuse
+              throwIO e
+            -- Everything went ok, so the connection is good. If any exceptions get
+            -- thrown in the response body, just throw them as normal.
+            Right res -> return res
+      where
+        getConnectionWrapper mtimeout f =
+            case mtimeout of
+                Nothing -> fmap ((,) Nothing) f
+                Just timeout' -> do
+                    before <- getCurrentTime
+                    mres <- timeout timeout' f
+                    case mres of
+                        Nothing -> throwHttp ConnectionTimeout
+                        Just mConn -> do
+                            now <- getCurrentTime
+                            let timeSpentMicro = diffUTCTime now before * 1000000
+                                remainingTime = round $ fromIntegral timeout' - timeSpentMicro
+                            if remainingTime <= 0
+                                then do
+                                    managedRelease mConn DontReuse
+                                    throwHttp ConnectionTimeout
+                                else return (Just remainingTime, mConn)
+
+        responseTimeout' req =
+            case responseTimeout req of
+                ResponseTimeoutDefault ->
+                    case mResponseTimeout m of
+                        ResponseTimeoutDefault -> Just 30000000
+                        ResponseTimeoutNone -> Nothing
+                        ResponseTimeoutMicro u -> Just u
+                ResponseTimeoutNone -> Nothing
+                ResponseTimeoutMicro u -> Just u
 
 -- | The used Manager can be overridden (by requestManagerOverride) and the used
 -- Request can be modified (through managerModifyRequest). This function allows
diff --git a/Network/HTTP/Client/Headers.hs b/Network/HTTP/Client/Headers.hs
--- a/Network/HTTP/Client/Headers.hs
+++ b/Network/HTTP/Client/Headers.hs
@@ -28,8 +28,8 @@
 charPeriod = 46
 
 
-parseStatusHeaders :: Maybe MaxHeaderLength -> Connection -> Maybe Int -> ([Header] -> IO ()) -> Maybe (IO ()) -> IO StatusHeaders
-parseStatusHeaders mhl conn timeout' onEarlyHintHeaders cont
+parseStatusHeaders :: Maybe MaxHeaderLength -> Maybe MaxNumberHeaders -> Connection -> Maybe Int -> ([Header] -> IO ()) -> Maybe (IO ()) -> IO StatusHeaders
+parseStatusHeaders mhl mnh conn timeout' onEarlyHintHeaders cont
     | Just k <- cont = getStatusExpectContinue k
     | otherwise      = getStatus
   where
@@ -91,9 +91,14 @@
             Just (i, "") -> Just i
             _ -> Nothing
 
+    guardMaxNumberHeaders :: Int -> IO ()
+    guardMaxNumberHeaders count = case fmap unMaxNumberHeaders mnh of
+        Nothing -> pure ()
+        Just n -> when (count >= n) $ throwHttp TooManyHeaderFields
+
     parseHeaders :: Int -> ([Header] -> [Header]) -> IO [Header]
-    parseHeaders 100 _ = throwHttp OverlongHeaders
     parseHeaders count front = do
+        guardMaxNumberHeaders count
         line <- connectionReadLine mhl conn
         if S.null line
             then return $ front []
@@ -107,8 +112,8 @@
                         parseHeaders count front
 
     parseEarlyHintHeadersUntilFailure :: Int -> ([Header] -> [Header]) -> IO [Header]
-    parseEarlyHintHeadersUntilFailure 100 _ = throwHttp OverlongHeaders
     parseEarlyHintHeadersUntilFailure count front = do
+        guardMaxNumberHeaders count
         line <- connectionReadLine mhl conn
         if S.null line
             then return $ front []
diff --git a/Network/HTTP/Client/Manager.hs b/Network/HTTP/Client/Manager.hs
--- a/Network/HTTP/Client/Manager.hs
+++ b/Network/HTTP/Client/Manager.hs
@@ -93,6 +93,7 @@
     , managerProxyInsecure = defaultProxy
     , managerProxySecure = defaultProxy
     , managerMaxHeaderLength = Just $ MaxHeaderLength 4096
+    , managerMaxNumberHeaders = Just $ MaxNumberHeaders 100
     }
 
 -- | Create a 'Manager'. The @Manager@ will be shut down automatically via
@@ -133,6 +134,7 @@
                     then httpsProxy req
                     else httpProxy req
             , mMaxHeaderLength = managerMaxHeaderLength ms
+            , mMaxNumberHeaders = managerMaxNumberHeaders ms
             }
     return manager
 
@@ -259,7 +261,9 @@
                     , "\r\n"
                     ]
                 parse conn = do
-                    StatusHeaders status _ _ _ <- parseStatusHeaders (managerMaxHeaderLength ms) conn Nothing (\_ -> return ()) Nothing
+                    let mhl = managerMaxHeaderLength ms
+                        mnh = managerMaxNumberHeaders ms
+                    StatusHeaders status _ _ _ <- parseStatusHeaders mhl mnh conn Nothing (\_ -> return ()) Nothing
                     unless (status == status200) $
                         throwHttp $ ProxyConnectException ultHost ultPort status
                 in tlsProxyConnection
diff --git a/Network/HTTP/Client/Response.hs b/Network/HTTP/Client/Response.hs
--- a/Network/HTTP/Client/Response.hs
+++ b/Network/HTTP/Client/Response.hs
@@ -81,10 +81,10 @@
 
     mergeHeaders :: W.RequestHeaders -> W.RequestHeaders -> W.RequestHeaders
     mergeHeaders lhs rhs = nubBy (\(a, _) (a', _) -> a == a') (lhs ++ rhs)
-    
+
     stripHeaders :: Request -> Request
     stripHeaders r = do
-        case (hostDiffer r, shouldStripOnlyIfHostDiffer) of 
+        case (hostDiffer r, shouldStripOnlyIfHostDiffer) of
             (True, True) -> stripHeaders' r
             (True, False) -> stripHeaders' r
             (False, False) -> stripHeaders' r
@@ -92,7 +92,7 @@
                 -- We need to check if we have omitted headers in previous
                 -- request chain. Consider request chain:
                 --
-                --  1. example-1.com 
+                --  1. example-1.com
                 --  2. example-2.com (we may have removed some headers here from 1)
                 --  3. example-1.com (since we are back at same host as 1, we need re-add stripped headers)
                 --
@@ -114,14 +114,15 @@
         }
 
 getResponse :: Maybe MaxHeaderLength
+            -> Maybe MaxNumberHeaders
             -> Maybe Int
             -> Request
             -> Managed Connection
             -> Maybe (IO ()) -- ^ Action to run in case of a '100 Continue'.
             -> IO (Response BodyReader)
-getResponse mhl timeout' req@(Request {..}) mconn cont = do
+getResponse mhl mnh timeout' req@(Request {..}) mconn cont = do
     let conn = managedResource mconn
-    StatusHeaders s version earlyHs hs <- parseStatusHeaders mhl conn timeout' earlyHintHeadersReceived cont
+    StatusHeaders s version earlyHs hs <- parseStatusHeaders mhl mnh conn timeout' earlyHintHeadersReceived cont
     let mcl = lookup "content-length" hs >>= readPositiveInt . S8.unpack
         isChunked = ("transfer-encoding", CI.mk "chunked") `elem` map (second CI.mk) hs
 
diff --git a/Network/HTTP/Client/Types.hs b/Network/HTTP/Client/Types.hs
--- a/Network/HTTP/Client/Types.hs
+++ b/Network/HTTP/Client/Types.hs
@@ -1,8 +1,9 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveTraversable #-}
-{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RankNTypes #-}
 module Network.HTTP.Client.Types
     ( BodyReader
     , Connection (..)
@@ -39,6 +40,7 @@
     , ResponseTimeout (..)
     , ProxySecureMode (..)
     , MaxHeaderLength (..)
+    , MaxNumberHeaders (..)
     ) where
 
 import qualified Data.Typeable as T (Typeable)
@@ -147,12 +149,14 @@
                    --
                    -- @since 0.5.0
                    | OverlongHeaders
-                   -- ^ Either too many headers, or too many total bytes in a
-                   -- single header, were returned by the server, and the
-                   -- memory exhaustion protection in this library has kicked
-                   -- in.
+                   -- ^ Too many total bytes in the HTTP header were returned
+                   -- by the server.
                    --
                    -- @since 0.5.0
+                   | TooManyHeaderFields
+                   -- ^ Too many HTTP header fields were returned by the server.
+                   --
+                   -- @since 0.7.18
                    | ResponseTimeout
                    -- ^ The server took too long to return a response. This can
                    -- be altered via 'responseTimeout' or
@@ -821,6 +825,17 @@
     --
     -- Since 0.4.7
     , managerMaxHeaderLength :: Maybe MaxHeaderLength
+    -- ^ Configure the maximum size, in bytes, of an HTTP header field.
+    --
+    -- Default: 4096
+    --
+    -- @since 0.7.17
+    , managerMaxNumberHeaders :: Maybe MaxNumberHeaders
+    -- ^ Configure the maximum number of HTTP header fields.
+    --
+    -- Default: 100
+    --
+    -- @since 0.7.18
     }
     deriving T.Typeable
 
@@ -845,9 +860,10 @@
     , mWrapException :: forall a. Request -> IO a -> IO a
     , mModifyRequest :: Request -> IO Request
     , mSetProxy :: Request -> Request
-    , mModifyResponse      :: Response BodyReader -> IO (Response BodyReader)
+    , mModifyResponse :: Response BodyReader -> IO (Response BodyReader)
     -- ^ See 'managerProxy'
     , mMaxHeaderLength :: Maybe MaxHeaderLength
+    , mMaxNumberHeaders :: Maybe MaxNumberHeaders
     }
     deriving T.Typeable
 
@@ -906,4 +922,12 @@
 newtype MaxHeaderLength = MaxHeaderLength
     { unMaxHeaderLength :: Int
     }
-    deriving (Eq, Show)
+    deriving (Eq, Show, Ord, Num, Enum, Bounded, T.Typeable)
+
+-- | The maximum number of header fields.
+--
+-- @since 0.7.18
+newtype MaxNumberHeaders = MaxNumberHeaders
+    { unMaxNumberHeaders :: Int
+    }
+    deriving (Eq, Show, Ord, Num, Enum, Bounded, T.Typeable)
diff --git a/http-client.cabal b/http-client.cabal
--- a/http-client.cabal
+++ b/http-client.cabal
@@ -1,5 +1,5 @@
 name:                http-client
-version:             0.7.16
+version:             0.7.19
 synopsis:            An HTTP client engine
 description:         Hackage documentation generation is not reliable. For up to date documentation, please see: <http://www.stackage.org/package/http-client>.
 homepage:            https://github.com/snoyberg/http-client
diff --git a/test-nonet/Network/HTTP/Client/HeadersSpec.hs b/test-nonet/Network/HTTP/Client/HeadersSpec.hs
--- a/test-nonet/Network/HTTP/Client/HeadersSpec.hs
+++ b/test-nonet/Network/HTTP/Client/HeadersSpec.hs
@@ -23,7 +23,7 @@
                 , "\nignored"
                 ]
         (connection, _, _) <- dummyConnection input
-        statusHeaders <- parseStatusHeaders Nothing connection Nothing (\_ -> return ()) Nothing
+        statusHeaders <- parseStatusHeaders Nothing Nothing connection Nothing (\_ -> return ()) Nothing
         statusHeaders `shouldBe` StatusHeaders status200 (HttpVersion 1 1) mempty
             [ ("foo", "bar")
             , ("baz", "bin")
@@ -37,7 +37,7 @@
                 ]
         (conn, out, _) <- dummyConnection input
         let sendBody = connectionWrite conn "data"
-        statusHeaders <- parseStatusHeaders Nothing conn Nothing (\_ -> return ()) (Just sendBody)
+        statusHeaders <- parseStatusHeaders Nothing Nothing conn Nothing (\_ -> return ()) (Just sendBody)
         statusHeaders `shouldBe` StatusHeaders status200 (HttpVersion 1 1) [] [ ("foo", "bar") ]
         out >>= (`shouldBe` ["data"])
 
@@ -47,7 +47,7 @@
                 ]
         (conn, out, _) <- dummyConnection input
         let sendBody = connectionWrite conn "data"
-        statusHeaders <- parseStatusHeaders Nothing conn Nothing (\_ -> return ()) (Just sendBody)
+        statusHeaders <- parseStatusHeaders Nothing Nothing conn Nothing (\_ -> return ()) (Just sendBody)
         statusHeaders `shouldBe` StatusHeaders status417 (HttpVersion 1 1) [] []
         out >>= (`shouldBe` [])
 
@@ -59,7 +59,7 @@
                 , "result"
                 ]
         (conn, out, inp) <- dummyConnection input
-        statusHeaders <- parseStatusHeaders Nothing conn Nothing (\_ -> return ()) Nothing
+        statusHeaders <- parseStatusHeaders Nothing Nothing conn Nothing (\_ -> return ()) Nothing
         statusHeaders `shouldBe` StatusHeaders status200 (HttpVersion 1 1) [] [ ("foo", "bar") ]
         out >>= (`shouldBe` [])
         inp >>= (`shouldBe` ["result"])
@@ -78,7 +78,7 @@
         callbackResults :: MVar (Seq.Seq [Header]) <- newMVar mempty
         let onEarlyHintHeader h = modifyMVar_ callbackResults (return . (Seq.|> h))
 
-        statusHeaders <- parseStatusHeaders Nothing conn Nothing onEarlyHintHeader Nothing
+        statusHeaders <- parseStatusHeaders Nothing Nothing conn Nothing onEarlyHintHeader Nothing
         statusHeaders `shouldBe` StatusHeaders status200 (HttpVersion 1 1)
             [("Link", "</foo.js>")
             , ("Link", "</bar.js>")
@@ -110,7 +110,7 @@
         callbackResults :: MVar (Seq.Seq [Header]) <- newMVar mempty
         let onEarlyHintHeader h = modifyMVar_ callbackResults (return . (Seq.|> h))
 
-        statusHeaders <- parseStatusHeaders Nothing conn Nothing onEarlyHintHeader Nothing
+        statusHeaders <- parseStatusHeaders Nothing Nothing conn Nothing onEarlyHintHeader Nothing
         statusHeaders `shouldBe` StatusHeaders status200 (HttpVersion 1 1)
             [("Link", "</foo.js>")
             , ("Link", "</bar.js>")
diff --git a/test-nonet/Network/HTTP/Client/ResponseSpec.hs b/test-nonet/Network/HTTP/Client/ResponseSpec.hs
--- a/test-nonet/Network/HTTP/Client/ResponseSpec.hs
+++ b/test-nonet/Network/HTTP/Client/ResponseSpec.hs
@@ -16,7 +16,7 @@
 
 spec :: Spec
 spec = describe "ResponseSpec" $ do
-    let getResponse' conn = getResponse Nothing Nothing req (dummyManaged conn) Nothing
+    let getResponse' conn = getResponse Nothing Nothing Nothing req (dummyManaged conn) Nothing
         req = parseRequest_ "http://localhost"
     it "basic" $ do
         (conn, _, _) <- dummyConnection
diff --git a/test-nonet/Network/HTTP/ClientSpec.hs b/test-nonet/Network/HTTP/ClientSpec.hs
--- a/test-nonet/Network/HTTP/ClientSpec.hs
+++ b/test-nonet/Network/HTTP/ClientSpec.hs
@@ -31,6 +31,9 @@
 notWindows x = x
 #endif
 
+crlf :: S.ByteString
+crlf = "\r\n"
+
 main :: IO ()
 main = hspec spec
 
@@ -323,3 +326,24 @@
       case parseRequest "https://o_O:18446744072699450606" of
         Left _ -> pure () :: IO ()
         Right req -> error $ "Invalid request: " ++ show req
+
+    it "too many header fields" $ do
+        let message = S.concat $
+                ["HTTP/1.1 200 OK", crlf] <> replicate 120 ("foo: bar" <> crlf) <> [crlf, "body"]
+
+        serveWith message $ \port -> do
+            man <- newManager $ managerSetMaxNumberHeaders 120 defaultManagerSettings
+            req <- parseUrlThrow $ "http://127.0.0.1:" ++ show port
+            httpLbs req man `shouldThrow` \e -> case e of
+                HttpExceptionRequest _ TooManyHeaderFields -> True
+                _otherwise -> False
+
+    it "not too many header fields" $ do
+        let message = S.concat $
+                ["HTTP/1.1 200 OK", crlf] <> replicate 120 ("foo: bar" <> crlf) <> [crlf, "body"]
+
+        serveWith message $ \port -> do
+            man <- newManager $ managerSetMaxNumberHeaders 121 defaultManagerSettings
+            req <- parseUrlThrow $ "http://127.0.0.1:" ++ show port
+            res <- httpLbs req man
+            responseBody res `shouldBe` "body"
