diff --git a/http-client.carp b/http-client.carp
index bfd3e68..2eb86ee 100644
--- a/http-client.carp
+++ b/http-client.carp
@@ -233,6 +233,28 @@ apart.
             (set! acc (+ (* acc 16) d)))))
       (if ok (Maybe.Just acc) (Maybe.Nothing))))
 
+  (hidden bws?)
+  (private bws?)
+  (defn bws? [c] (or (= c \space) (= c \tab)))
+
+  (hidden parse-size-line)
+  (private parse-size-line)
+  ; chunk-size = 1*HEXDIG, then chunk-ext = *( BWS ";" ... ) per RFC 9112 §7.1
+  ; trailing BWS without a chunk-ext is accepted too
+  (defn parse-size-line [line]
+    (let-do [len (String.length line)
+             digits 0
+             i 0]
+      (while (and (< digits len)
+                  (>= (hex-nibble (String.char-at line digits)) 0))
+        (set! digits (Int.inc digits)))
+      (set! i digits)
+      (while (and (< i len) (bws? (String.char-at line i))) (set! i (Int.inc i)))
+      (if (and (> digits 0)
+               (or (= i len) (and (< i len) (= \; (String.char-at line i)))))
+        (parse-hex &(String.byte-slice line 0 digits))
+        (Maybe.Nothing))))
+
   (hidden crlf-at?)
   (private crlf-at?)
   (defn crlf-at? [s i]
@@ -340,17 +362,14 @@ apart.
                                @"malformed chunked body: unterminated chunk size line"
                                @"malformed chunked body: missing terminating zero-size chunk")))
               (set! running false))
-            (let [line &(String.byte-slice (buf s) p eol)
-                  semi (String.index-of line \;)
-                  hex &(String.trim
-                    &(if (= semi -1) @line (String.byte-slice line 0 semi)))]
-              (match (parse-hex hex)
+            (let [line &(String.byte-slice (buf s) p eol)]
+              (match (parse-size-line line)
                 (Maybe.Nothing)
                   (do
                     (fail-parse! s
                                  (fmt
                                    "malformed chunked body: invalid chunk size '%s'"
-                                   hex))
+                                   line))
                     (set! running false))
                 (Maybe.Just size)
                   (if (= size 0)
diff --git a/test/http-client.carp b/test/http-client.carp
index 47ef8eb..844e195 100644
--- a/test/http-client.carp
+++ b/test/http-client.carp
@@ -735,6 +735,38 @@
     &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-hex-prefix"))
     "a 0x-prefixed chunk size is rejected")
 
+  (assert-equal test
+    "malformed chunked body: invalid chunk size ' 5'"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-leading-space"))
+    "a space before the chunk size is rejected")
+
+  (assert-equal test
+    "200 [hello]"
+    &(status-and-body
+      (Client.get "http://127.0.0.1:8791/chunked-trailing-space"))
+    "a space after the chunk size is allowed")
+
+  (assert-equal test
+    "malformed chunked body: invalid chunk size '\t5'"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-leading-tab"))
+    "a tab before the chunk size is rejected")
+
+  (assert-equal test
+    "malformed chunked body: invalid chunk size '5\r'"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-stray-cr"))
+    "a stray CR after the chunk size is rejected")
+
+  (assert-equal test
+    "malformed chunked body: invalid chunk size ' 0 '"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-padded-zero"))
+    "a zero-size chunk with a leading space does not end the body")
+
+  (assert-equal test
+    "200 [hello]"
+    &(status-and-body
+      (Client.get "http://127.0.0.1:8791/chunked-zero-trailing-space"))
+    "a zero-size chunk with a trailing space ends the body")
+
   (assert-equal test
     "malformed chunked body: truncated chunk data"
     &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-truncated"))
@@ -767,6 +799,16 @@
     &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-ext"))
     "a chunk extension after the size is ignored")
 
+  (assert-equal test
+    "200 [hello]"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-ext-space"))
+    "a space between the size and a chunk extension is allowed")
+
+  (assert-equal test
+    "200 [hello]"
+    &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-ext-tab"))
+    "a tab between the size and a chunk extension is allowed")
+
   (assert-equal test
     "200 [hello]"
     &(status-and-body (Client.get "http://127.0.0.1:8791/chunked-trailer"))
diff --git a/test/server.py b/test/server.py
index 5c70938..639e29f 100755
--- a/test/server.py
+++ b/test/server.py
@@ -209,6 +209,18 @@ def _route(self):
             return self._chunked_raw(b"5\r\nhello\r\n")
         if path == "/chunked-missing-crlf":
             return self._chunked_raw(b"5\r\nhelloXX0\r\n\r\n")
+        if path == "/chunked-leading-space":
+            return self._chunked_raw(b" 5\r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-trailing-space":
+            return self._chunked_raw(b"5 \r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-leading-tab":
+            return self._chunked_raw(b"\t5\r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-stray-cr":
+            return self._chunked_raw(b"5\r\r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-padded-zero":
+            return self._chunked_raw(b"5\r\nhello\r\n 0 \r\n\r\n")
+        if path == "/chunked-zero-trailing-space":
+            return self._chunked_raw(b"5\r\nhello\r\n0 \r\n\r\n")
 
         # a chunk size above 16 MiB, whose data is never sent, so the size
         # line alone decides the outcome
@@ -218,6 +230,10 @@ def _route(self):
         # a well-formed body with a chunk extension and a trailer section
         if path == "/chunked-ext":
             return self._chunked_raw(b"5;name=value\r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-ext-space":
+            return self._chunked_raw(b"5 ;name\r\nhello\r\n0\r\n\r\n")
+        if path == "/chunked-ext-tab":
+            return self._chunked_raw(b"5\t;name\r\nhello\r\n0\r\n\r\n")
         if path == "/chunked-trailer":
             return self._chunked_raw(
                 b"5\r\nhello\r\n0\r\nX-Checksum: abc\r\nX-More: 1\r\n\r\n"