diff --git a/runtime/pb/json.lua b/runtime/pb/json.lua
index f1838ca00b5eeeebc48d0a190e74703b7ef14994..e6a2ea6670f452c0cc5accd907521bb25bafac9d 100644
--- a/runtime/pb/json.lua
+++ b/runtime/pb/json.lua
@@ -573,7 +573,7 @@         if type(v) == 'number' then return string.format('%.0f', v) end
         return tostring(v)
     end
     if proto_type == 'uint32' then return v end
-    if proto_type == 'bytes' then return digest.base64_encode(v) end
+    if proto_type == 'bytes' then return digest.base64_encode(v, {nowrap = true}) end
     if proto_type == 'float' or proto_type == 'double' then
         return v  -- handled by encode_json_number
     end
@@ -740,7 +740,7 @@         -- keeps round-trips through user-registered types stable without
         -- forcing every embedded payload into a Wkt shape).
         local obj = {}
         if type_url ~= '' then obj['@type'] = type_url end
-        if bytes ~= '' then obj.value = digest.base64_encode(bytes) end
+        if bytes ~= '' then obj.value = digest.base64_encode(bytes, {nowrap = true}) end
         return obj
     end
     local inner = desc.decode and desc.decode(bytes)
diff --git a/test/any_fieldmask_test.lua b/test/any_fieldmask_test.lua
index af0ad602eef6305ce04ca009c51b2615f01d67fd..696c7dc3dee68ef190ccad4c3f903b63437e47da 100644
--- a/test/any_fieldmask_test.lua
+++ b/test/any_fieldmask_test.lua
@@ -127,6 +127,17 @@     t.assert_equals(back.extension.type_url, 'type.opaque/Foo')
     t.assert_equals(back.extension.value, '\x01\x02\x03')
 end
 
+gaj.test_json_opaque_value_base64_unwrapped = function()
+    -- Same canonical-base64 rule as plain bytes fields: the Any opaque
+    -- fallback must not emit MIME-style line wraps inside the JSON string.
+    local opaque = {type_url = 'type.opaque/Big', value = string.rep('A', 64)}
+    local enc = pb.json.encode(hello.Event_descriptor, {extension = opaque})
+    t.assert_not_str_contains(enc, '\n')
+    t.assert_not_str_contains(enc, '\\n')
+    local back = pb.json.decode(hello.Event_descriptor, enc)
+    t.assert_equals(back.extension.value, opaque.value)
+end
+
 -- ---------------------------------------------------------------------------
 -- End-to-end: Event message round-trip with extension + update_mask.
 -- ---------------------------------------------------------------------------
diff --git a/test/json_test.lua b/test/json_test.lua
index e59045702bad5890a1d61ca238190686f81f5afa..50d3b0755891491bba40dddde731fe005332a16e 100644
--- a/test/json_test.lua
+++ b/test/json_test.lua
@@ -43,6 +43,21 @@     local p2 = pb.json.decode(hello.Person_descriptor, pb.json.encode(hello.Person_descriptor, p))
     t.assert_equals(p2.avatar, p.avatar)
 end
 
+g.test_bytes_base64_unwrapped_long_payload = function()
+    -- Canonical proto3 JSON requires RFC 4648 base64 with no line wrapping.
+    -- Tarantool's digest.base64_encode defaults to MIME-style 76-char wrap,
+    -- so any payload past ~57 bytes used to emit a `\n` inside the JSON
+    -- string and break grpc-gateway / protojson consumers.
+    local p = {avatar = string.rep('A', 64)}
+    local enc = pb.json.encode(hello.Person_descriptor, p)
+    t.assert_not_str_contains(enc, '\n', 'no raw newline anywhere in JSON output')
+    t.assert_not_str_contains(enc, '\\n', 'no escaped newline in base64 token')
+    local obj = reparse(enc)
+    t.assert_not_str_contains(obj.avatar, '\n', 'base64 token is a single line')
+    local p2 = pb.json.decode(hello.Person_descriptor, enc)
+    t.assert_equals(p2.avatar, p.avatar)
+end
+
 g.test_repeated_packed_scalar = function()
     local p = {lucky_numbers = {1, 2, 3}}
     local obj = reparse(pb.json.encode(hello.Person_descriptor, p))