diff --git a/src/box/iproto.cc b/src/box/iproto.cc
index a08c8c5cbadd1aa9cbd96b47fc2332e5117ae223..7a1d3bfd094bbdcb0fbcf9bdbae9727a3ab16c6c 100644
--- a/src/box/iproto.cc
+++ b/src/box/iproto.cc
@@ -670,6 +670,11 @@ iproto_connection_input_buffer(struct iproto_connection *con)
 		 */
 		return NULL;
 	}
+	/* Update buffer size if readahead has changed. */
+	if (new_ibuf->start_capacity != iproto_readahead) {
+		ibuf_destroy(new_ibuf);
+		ibuf_create(new_ibuf, cord_slab_cache(), iproto_readahead);
+	}
 
 	ibuf_reserve_xc(new_ibuf, to_read + con->parse_size);
 	/*
diff --git a/test/box/net.box.result b/test/box/net.box.result
index 6351898b3a94142e17ba8c5db7ecc4239d02e2a4..b800531b4892cbd51d16278b57c45b606fd25b2d 100644
--- a/test/box/net.box.result
+++ b/test/box/net.box.result
@@ -3452,3 +3452,52 @@ box.schema.user.revoke('guest', 'read,write', 'space', '_space')
 box.schema.user.revoke('guest', 'create', 'space')
 ---
 ...
+--
+-- gh-3958 updating box.cfg.readahead doesn't affect existing connections.
+--
+readahead = box.cfg.readahead
+---
+...
+box.cfg{readahead = 128}
+---
+...
+s = box.schema.space.create("test")
+---
+...
+_ = s:create_index("pk")
+---
+...
+box.schema.user.grant("guest", "read,write", "space", "test")
+---
+...
+-- connection is created with small readahead value,
+-- make sure it is updated if box.cfg.readahead is changed.
+c = net.connect(box.cfg.listen)
+---
+...
+box.cfg{readahead = 100 * 1024}
+---
+...
+box.error.injection.set("ERRINJ_WAL_DELAY", true)
+---
+- ok
+...
+pad = string.rep('x', 8192)
+---
+...
+for i = 1, 5 do c.space.test:replace({i, pad}, {is_async = true}) end
+---
+...
+box.error.injection.set("ERRINJ_WAL_DELAY", false)
+---
+- ok
+...
+test_run:wait_log('default', 'readahead limit is reached', 1024, 0.1)
+---
+...
+s:drop()
+---
+...
+box.cfg{readahead = readahead}
+---
+...
diff --git a/test/box/net.box.test.lua b/test/box/net.box.test.lua
index 6c527f506a9568cb2a54471fd2a7669f7b570c0d..9e5ecfa0dd0d55019cbe945f82e52eb7810b90cd 100644
--- a/test/box/net.box.test.lua
+++ b/test/box/net.box.test.lua
@@ -1397,3 +1397,30 @@ box.schema.func.drop('do_long')
 box.schema.user.revoke('guest', 'write', 'space', '_schema')
 box.schema.user.revoke('guest', 'read,write', 'space', '_space')
 box.schema.user.revoke('guest', 'create', 'space')
+
+--
+-- gh-3958 updating box.cfg.readahead doesn't affect existing connections.
+--
+readahead = box.cfg.readahead
+
+box.cfg{readahead = 128}
+
+s = box.schema.space.create("test")
+_ = s:create_index("pk")
+box.schema.user.grant("guest", "read,write", "space", "test")
+
+-- connection is created with small readahead value,
+-- make sure it is updated if box.cfg.readahead is changed.
+c = net.connect(box.cfg.listen)
+
+box.cfg{readahead = 100 * 1024}
+
+box.error.injection.set("ERRINJ_WAL_DELAY", true)
+pad = string.rep('x', 8192)
+for i = 1, 5 do c.space.test:replace({i, pad}, {is_async = true}) end
+box.error.injection.set("ERRINJ_WAL_DELAY", false)
+
+test_run:wait_log('default', 'readahead limit is reached', 1024, 0.1)
+
+s:drop()
+box.cfg{readahead = readahead}