diff --git a/internal/pep508/requirement.go b/internal/pep508/requirement.go
index 7f7a49f..751d1a7 100644
--- a/internal/pep508/requirement.go
+++ b/internal/pep508/requirement.go
@@ -143,25 +143,8 @@ func parseRequirementDetails(t *Tokenizer, req *RawRequirement) error {
 		if t.peek(End) {
 			return nil
 		}
-		// Consume horizontal whitespace and any immediately following line
-		// breaks. PEP 508 requires whitespace after a URL; a newline is valid
-		// (common in "defensively multiline" Requires-Dist metadata) and must
-		// be consumed here so a following "; marker" clause is recognized.
 		if !t.consume(WS) {
-			// No horizontal whitespace found; check for a line break.
-			if t.pos < len(t.source) && (t.source[t.pos] == '\n' || t.source[t.pos] == '\r') {
-				// Consume the line break(s).
-				for t.pos < len(t.source) && (t.source[t.pos] == '\n' || t.source[t.pos] == '\r') {
-					t.pos++
-				}
-			} else {
-				return t.NewSyntaxError("Expected whitespace after URL")
-			}
-		} else {
-			// Consumed horizontal whitespace; also consume any following line breaks.
-			for t.pos < len(t.source) && (t.source[t.pos] == '\n' || t.source[t.pos] == '\r') {
-				t.pos++
-			}
+			return t.NewSyntaxError("Expected whitespace after URL")
 		}
 		if t.peek(End) {
 			return nil
diff --git a/internal/pep508/requirement_test.go b/internal/pep508/requirement_test.go
index 801f194..b166257 100644
--- a/internal/pep508/requirement_test.go
+++ b/internal/pep508/requirement_test.go
@@ -171,20 +171,41 @@ func TestParseRequirement_URLContainingSemicolon_NoMarker(t *testing.T) {
 	assert.Nil(t, req.Marker)
 }
 
-// --- URL terminates at any whitespace, not just space/tab ---
-
-func TestParseRequirement_URLThenNewlineThenMarker_Splits(t *testing.T) {
-	// A defensively-multiline value: the "@ url" clause ends in a newline
-	// rather than a space before "; marker". The URL rule must treat the
-	// newline as a terminator (it is the exact complement of the WS rule,
-	// \s+) so the marker clause is not absorbed into the URL token.
-	req := parseRequirementString(t, "foo @ https://x/y\n; python_version > \"3\"")
-	assert.Equal(t, "https://x/y", req.URL)
-	require.NotNil(t, req.Marker)
-	cmp, ok := req.Marker.(*CompareExpr)
-	require.True(t, ok)
-	assert.Equal(t, EnvVar{Name: "python_version"}, cmp.Lhs)
-	assert.Equal(t, Literal{Value: "3"}, cmp.Rhs)
+// --- URL terminates at horizontal whitespace (space/tab) only, matching
+// upstream's own URL rule ([^ \t]+) - NOT at a line break. This was checked
+// against a real packaging 4eb0753dba8fcaaac8eb75463374e448f0931558 install:
+// Requirement("name @ https://example.com\n").url == "https://example.com\n"
+// (the line break is embedded in the url, not rejected). An older version of
+// this test asserted the opposite (line break treated as a terminator, so
+// a following "; marker" clause still parsed) - that pinned a divergence
+// from upstream, introduced when WS was narrowed to horizontal-only ([ \t]+)
+// without updating the URL rule to match; it is corrected below.
+
+func TestParseRequirement_URLForm_BareLineBreak_EmbeddedInURL(t *testing.T) {
+	// With nothing following the line break, the whole thing (URL bytes
+	// plus break) has no space/tab in it, so it is ONE greedy URL token and
+	// the requirement parses successfully - matching upstream exactly.
+	for _, lb := range []string{"\n", "\r", "\r\n"} {
+		t.Run(escapeForName(lb), func(t *testing.T) {
+			req := parseRequirementString(t, "foo @ https://x/y"+lb)
+			assert.Equal(t, "https://x/y"+lb, req.URL)
+			assert.Nil(t, req.Marker)
+		})
+	}
+}
+
+func TestParseRequirement_URLForm_LineBreakBeforeMarker_SwallowsSemicolonIntoURL(t *testing.T) {
+	// The line break is not a delimiter, so it - and the following ";" -
+	// are absorbed into the URL token, right up to the next space. That
+	// consumes the marker clause's required ";", leaving the marker's own
+	// text (which contains spaces) as unparsable trailing garbage.
+	for _, lb := range []string{"\n", "\r", "\r\n"} {
+		t.Run(escapeForName(lb), func(t *testing.T) {
+			tok := NewTokenizer("foo @ https://x/y" + lb + `; python_version > "3"`)
+			_, err := ParseRequirement(tok)
+			require.Error(t, err, "line break then marker should not parse (semicolon is swallowed into the URL)")
+		})
+	}
 }
 
 // --- grammar asymmetry: url form requires wsp+ before ";" marker ---
@@ -205,6 +226,19 @@ func TestParseRequirement_URLForm_SpaceBeforeMarker_Splits(t *testing.T) {
 	require.NotNil(t, req.Marker)
 }
 
+// A trailing horizontal space or tab after the URL (nothing else follows)
+// still terminates the URL correctly and parses, mirroring upstream's
+// test_trailing_horizontal_whitespace (which only covers the name form).
+func TestParseRequirement_URLForm_AllowsTrailingHorizontalWhitespace(t *testing.T) {
+	for _, ws := range []string{" ", "\t", " \t"} {
+		t.Run(escapeForName(ws), func(t *testing.T) {
+			req := parseRequirementString(t, "foo @ https://x/y"+ws)
+			assert.Equal(t, "https://x/y", req.URL)
+			assert.Nil(t, req.Marker)
+		})
+	}
+}
+
 // --- combinations ---
 
 func TestParseRequirement_ExtrasAndVersionSpecAndMarker(t *testing.T) {
diff --git a/internal/pep508/tokenizer.go b/internal/pep508/tokenizer.go
index eeae072..0a959d6 100644
--- a/internal/pep508/tokenizer.go
+++ b/internal/pep508/tokenizer.go
@@ -228,16 +228,14 @@ var tokenRules = map[Kind]*regexp.Regexp{
 	// leftover "=2.0" rather than at the fusion point, which is a worse error
 	// message for the same correct verdict.
 	Specifier: regexp.MustCompile(`\A(?:===\s*[^\s,;)]+|(?:==|~=|!=|<=|>=|<|>)\s*[^\s,;)<>=~]+)`),
-	// URL: a greedy run of non-whitespace characters - the exact
-	// complement of the WS rule above (\s+) - deliberately not bounded by
-	// ";" (see the URL Kind doc comment). Upstream packaging's rule
-	// excludes only space ([^ ]+); this port is stricter, treating all
-	// whitespace (including tab and newlines) as a terminator. That's
-	// safe because a valid URL never contains whitespace, and it avoids
-	// a stray newline (e.g. in a defensively-multiline Requires-Dist
-	// value) being absorbed into the URL along with a following "; marker"
-	// clause.
-	URL: regexp.MustCompile(`\A\S+`),
+	// URL: a greedy run of non-horizontal-whitespace characters, matching
+	// upstream packaging's own URL rule ([^ \t]+) exactly - it stops only
+	// at a space or tab, not at a line break. It is deliberately not
+	// bounded by ";" either (see the URL Kind doc comment): a URL can
+	// itself contain a semicolon (e.g. a query string), so PEP 508 relies
+	// on mandatory whitespace - not a ban on ";" - to separate a URL from
+	// a following "; marker" clause.
+	URL: regexp.MustCompile(`\A[^ \t]+`),
 }
 
 // Tokenizer performs context-sensitive lexing over a PEP 508 source string.