diff --git a/test/app-luatest/fiber_slice_test.lua b/test/app-luatest/fiber_slice_test.lua
index b67f83357f44b..77729be6d801e 100644
--- a/test/app-luatest/fiber_slice_test.lua
+++ b/test/app-luatest/fiber_slice_test.lua
@@ -32,7 +32,7 @@ g.test_fiber_slice = function()
         overhead = clock.monotonic() - overhead
         local start_time = clock.monotonic()
         local _, err = fib:join()
-        t.assert_equals(tostring(err), "fiber slice is exceeded")
+        t.assert_equals(err.message, "fiber slice is exceeded")
         local time_elapsed = clock.monotonic() - start_time - overhead
         t.assert_almost_equals(time_elapsed, expected_timeout, delta)
     end
diff --git a/test/app-luatest/http_client_test.lua b/test/app-luatest/http_client_test.lua
index eb7fd452267fa..cca9eb2afff73 100644
--- a/test/app-luatest/http_client_test.lua
+++ b/test/app-luatest/http_client_test.lua
@@ -485,7 +485,7 @@ g.test_headers = function(cg)
     local headers = {[hname] = hvalue}
     local ok, err = pcall(http.post, http, url, nil,
                           merge(opts, {headers = headers}))
-    t.assert_equals({ok, tostring(err)}, {false, exp_err},
+    t.assert_equals({ok, err.message}, {false, exp_err},
                     '8193 KiB header: error')
 end
 
diff --git a/test/app-tap/console.test.lua b/test/app-tap/console.test.lua
index 748014ade66f2..5da6c1c4982f9 100755
--- a/test/app-tap/console.test.lua
+++ b/test/app-tap/console.test.lua
@@ -133,13 +133,13 @@ box.cfg{
 -- Connect to iproto console (CALL)
 client:write(string.format("require('console').connect('/')\n"))
 -- error: Connection is not established
-test:ok(yaml.decode(client:read(EOL))[1].error:find('not established'),
+test:ok(yaml.decode(client:read(EOL))[1].error.message:find('not established'),
     'remote network error')
 
 client:write(string.format("require('console').connect('%s')\n",
     IPROTO_SOCKET))
 -- error: Execute access is denied for user 'guest' to function 'dostring
-test:ok(yaml.decode(client:read(EOL))[1].error:find('denied'),
+test:ok(yaml.decode(client:read(EOL))[1].error.message:find('denied'),
     'remote access denied')
 
 -- create user
@@ -147,7 +147,7 @@ box.schema.user.create('test', { password = 'pass' })
 client:write(string.format("require('console').connect('test:pass@%s')\n",
     IPROTO_SOCKET))
 -- error: Execute access denied for user 'test' to function 'dostring
-test:ok(yaml.decode(client:read(EOL))[1].error:find('denied'),
+test:ok(yaml.decode(client:read(EOL))[1].error.message:find('denied'),
     'remote access denied')
 
 -- Add permissions to execute for `test`
diff --git a/test/app-tap/gh-5013-fiber-cancel.test.lua b/test/app-tap/gh-5013-fiber-cancel.test.lua
index 7efb6ab5d09b6..231c09d3b7ab8 100755
--- a/test/app-tap/gh-5013-fiber-cancel.test.lua
+++ b/test/app-tap/gh-5013-fiber-cancel.test.lua
@@ -22,5 +22,5 @@ fiber.yield()
 print(require('yaml').encode(result))
 
 test:ok(result.res == false, 'expected result is false')
-test:ok(tostring(result.err) == 'fiber is cancelled', 'fiber cancellation should be reported')
+test:ok(result.err.message == 'fiber is cancelled', 'fiber cancellation should be reported')
 os.exit(test:check() and 0 or 1)
diff --git a/test/app-tap/logger.test.lua b/test/app-tap/logger.test.lua
index b5ba28fd1cbd1..714a9ed3f6d81 100755
--- a/test/app-tap/logger.test.lua
+++ b/test/app-tap/logger.test.lua
@@ -7,7 +7,7 @@ test:plan(119)
 
 local function test_invalid_cfg(cfg_method, cfg, name, expected)
     local _, err = pcall(cfg_method, cfg)
-    test:ok(tostring(err):find(expected) ~= nil, name)
+    test:ok(err.message:find(expected) ~= nil, name)
 end
 
 local function test_allowed_types(cfg_method, cfg, name, allowed_types)
@@ -205,11 +205,11 @@ test_invalid_cfg(box.cfg, {log_nonblock = true},
 -- Test invalid values for setters
 
 _, err = pcall(log.log_format, {})
-test:ok(tostring(err):find('should be of type string') ~= nil,
+test:ok(err.message:find('should be of type string') ~= nil,
         "invalid format setter value type")
 
 _, err = pcall(log.level, {})
-test:ok(tostring(err):find('should be one of types number, string') ~= nil,
+test:ok(err.message:find('should be one of types number, string') ~= nil,
         "invalid format setter value type")
 
 -- Change format and levels.
diff --git a/test/app-tap/popen.test.lua b/test/app-tap/popen.test.lua
index 3b6d9469f914a..7d657e24eafd5 100755
--- a/test/app-tap/popen.test.lua
+++ b/test/app-tap/popen.test.lua
@@ -96,7 +96,7 @@ local function test_trivial_echo_output(test)
     -- Sending a signal using a closed handle gives an error.
     local exp_err = 'popen: attempt to operate on a closed handle'
     local ok, err = pcall(ph.signal, ph, popen.signal.SIGTERM)
-    test:is_deeply({ok, err.type, tostring(err)},
+    test:is_deeply({ok, err.type, err.message},
                    {false, 'IllegalParams', exp_err},
                    'signal() on closed handle gives an error')
 end
@@ -247,7 +247,7 @@ local function test_read_timeout(test)
     -- Read and get a timeout error.
     local exp_err = 'timed out'
     local res, err = ph:read({timeout = 0.1})
-    test:is_deeply({res, err.type, tostring(err)}, {nil, 'TimedOut', exp_err},
+    test:is_deeply({res, err.type, err.message}, {nil, 'TimedOut', exp_err},
                    'timeout error')
 
     -- Write and read after the timeout error.
diff --git a/test/app-tap/uri.test.lua b/test/app-tap/uri.test.lua
index 18b37d132337a..7381d7966a3f6 100755
--- a/test/app-tap/uri.test.lua
+++ b/test/app-tap/uri.test.lua
@@ -90,10 +90,10 @@ local function test_parse(test)
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     u, error = uri.parse("")
     test:isnil(u, "invalid uri", u)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     u, error = uri.parse("://")
     test:isnil(u, "invalid uri", u)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 end
 
 local function test_format(test)
@@ -245,13 +245,13 @@ local function test_parse_uri_query_params(test)
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     u, error = uri.parse({ "/tmp/unix.sock", default_params = {q = "v"} })
     test:isnil(u, "invalid uri", u)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Multiple URIs is not allowed in `parse` method,
     -- use `parse_many` instead.
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     u, error = uri.parse({ "/tmp/unix.sock, /tmp/unix.sock"})
     test:isnil(u, "invalid uri", u)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 end
 
 local function test_parse_uri_set_with_query_params(test)
@@ -314,16 +314,16 @@ local function test_parse_uri_set_with_query_params(test)
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     uri_set , error= uri.parse_many("/tmp/unix.sock,,/tmp/unix.sock")
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     uri_set, error = uri.parse_many("/tmp/unix.sock, ,/tmp/unix.sock")
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     uri_set, error = uri.parse_many("/tmp/unix.sock,, /tmp/unix.sock")
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     uri_set, error = uri.parse_many("/tmp/unix.sock ,,/tmp/unix.sock")
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 
 
     -- Check that we can't parse string with multiple URIs,
@@ -331,7 +331,7 @@ local function test_parse_uri_set_with_query_params(test)
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     local u, error = uri.parse("/tmp/unix.sock, /tmp/unix.sock")
     test:isnil(u, "invalid uri", u)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 end
 
 local function test_parse_uri_set_from_lua_table(test)
@@ -575,35 +575,35 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
     expected_errmsg = "Incorrect type for URI: should be string, number or table"
     uri_set, error = uri.parse_many(function() end)
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Invalid type of value for numerical key
     expected_errmsg = "Incorrect type for URI: should be string, number or table"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", function() end})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Invalid type of value for string keys
     expected_errmsg = "Invalid URI table: expected " ..
                       "{uri = string, params = table}" .. " or " ..
                       "{string, params = table}"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", uri = function() end})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Incorrect type for URI parameters: should be a table"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", params = function() end})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Incorrect type for URI parameters: should be a table"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", params = ""})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", default_params = ""})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", default_params = ""})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 
     -- Mix "uri=" and numerical keys is banned
     expected_errmsg = "Invalid URI table: expected " ..
@@ -611,13 +611,13 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
                       "{string, params = table}"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", uri = "/tmp/unix.sock"})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Several URIs in one string is allowed only when the
     -- passed as a single string.
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     uri_set, error = uri.parse_many({"/tmp/unix.sock, /tmp/unix.sock"})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- "params" table is allowed only for single URI
     expected_errmsg = "URI parameters are not allowed for multiple URIs"
     uri_set, error = uri.parse_many({
@@ -625,7 +625,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         params = {q1 = "v1"}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- "params" table is not allowed with nested tables
     expected_errmsg = "URI parameters are not allowed for multiple URIs"
     uri_set, error = uri.parse_many({
@@ -633,17 +633,17 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         params = {q1 = "v1"}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- "default_params" table is not allowed in nested URI tables
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({{"/tmp/unix.sock", default_params = {}}})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- "default_params" table is not allowed for single URI
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({"/tmp/unix.sock", default_params = {}})
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Only one URI is allowed in nested tables
     expected_errmsg = "Invalid URI table: expected " ..
                       "{uri = string, params = table}" .. " or " ..
@@ -653,7 +653,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         default_params = {q = "v"}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Nested URI tables is not allowed in nested tables
     expected_errmsg = "Invalid URI table: expected "..
                       "{uri = string, params = table}" .. " or " ..
@@ -662,7 +662,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         {"/tmp/unix.sock", {}}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Nested URI table without URI is now allowed
     expected_errmsg = "Invalid URI table: expected "..
                       "{uri = string, params = table}" .. " or " ..
@@ -672,7 +672,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         { params = {q = "v"} }
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Only string key types are allowed in "params" and
     -- "default_params" table
     expected_errmsg = "Incorrect type for URI parameter name: " ..
@@ -682,14 +682,14 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         params = {"v"},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({
         "/tmp/unix.sock",
         default_params = {"v"},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Invalid type of values in "params" and
     -- "default_params" table
     expected_errmsg = "Incorrect type for URI parameter value: " ..
@@ -699,14 +699,14 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         params = {q = function() end},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({
         "/tmp/unix.sock",
         default_params = {q = function() end},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Incorrect type for URI parameter value: "..
                       "should be string or number"
     uri_set, error = uri.parse_many({
@@ -714,14 +714,14 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         params = {q = {function() end}},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     expected_errmsg = "Default URI parameters are not allowed for single URI"
     uri_set, error = uri.parse_many({
         "/tmp/unix.sock",
         default_params = {q = {function() end}},
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Invalid uri string in URIs table
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     uri_set, error = uri.parse_many({
@@ -729,7 +729,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         "://"
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Invalid uri in nested URI table
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     uri_set, error = uri.parse_many({
@@ -737,7 +737,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         {"://"}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
     -- Same as previous but with "uri=" syntax
     expected_errmsg = "Incorrect URI: expected host:service or /unix.socket"
     uri_set, error = uri.parse_many({
@@ -745,7 +745,7 @@ local function test_parse_invalid_uri_set_from_lua_table(test)
         {uri = "://"}
     })
     test:isnil(uri_set, "invalid uri", uri_set)
-    test:is(tostring(error), expected_errmsg, "error message")
+    test:is(error.message, expected_errmsg, "error message")
 end
 
 local test = tap.test("uri")
diff --git a/test/box-luatest/gh_6085_limit_iteration_in_space_test.lua b/test/box-luatest/gh_6085_limit_iteration_in_space_test.lua
index 38e2fa45dc3d6..d6d0ef6d907e1 100644
--- a/test/box-luatest/gh_6085_limit_iteration_in_space_test.lua
+++ b/test/box-luatest/gh_6085_limit_iteration_in_space_test.lua
@@ -33,7 +33,7 @@ g.test_limit_iteration = function()
             local fib = fiber.new(fiber_f)
             fib:set_joinable(true)
             local _, err = fib:join()
-            t.assert_equals(tostring(err), "fiber slice is exceeded")
+            t.assert_equals(err.message, "fiber slice is exceeded")
         end
 
         fiber.set_max_slice(0.2)
@@ -92,7 +92,7 @@ g.test_limit_on_sigurg = function()
     os.execute('kill -URG ' .. tonumber(pid))
     local _, err = future:wait_result(1.5)
     local end_time = clock.monotonic()
-    t.assert_equals(tostring(err), "fiber slice is exceeded")
+    t.assert_equals(err.message, "fiber slice is exceeded")
     -- Must end before slice is over.
     t.assert(end_time - start_time < 3)
 end
diff --git a/test/box-luatest/gh_7939_fix_meamleak_on_tuple_encode_test.lua b/test/box-luatest/gh_7939_fix_meamleak_on_tuple_encode_test.lua
index 4b08dcb188050..1e4a4d7ae31e0 100644
--- a/test/box-luatest/gh_7939_fix_meamleak_on_tuple_encode_test.lua
+++ b/test/box-luatest/gh_7939_fix_meamleak_on_tuple_encode_test.lua
@@ -176,12 +176,12 @@ g.test_misc_region_leaks = function(cg)
             }
         ');]])
         t.assert_equals(ret, nil)
-        t.assert_equals(tostring(err), errmsg)
+        t.assert_equals(err.message, errmsg)
         t.assert_equals(before, mem_used())
 
         ret, err = box.execute('SELECT ?;', {{trouble}})
         t.assert_equals(ret, nil)
-        t.assert_equals(tostring(err), errmsg)
+        t.assert_equals(err.message, errmsg)
         t.assert_equals(before, mem_used())
     end)
 end
diff --git a/test/box-luatest/gh_8802_box_session_push_deprecation_test.lua b/test/box-luatest/gh_8802_box_session_push_deprecation_test.lua
index 804499ade56e9..948ab3fc989e1 100644
--- a/test/box-luatest/gh_8802_box_session_push_deprecation_test.lua
+++ b/test/box-luatest/gh_8802_box_session_push_deprecation_test.lua
@@ -65,7 +65,7 @@ g.test = function(cg)
         t.assert_equals(ok, compat.box_session_push_deprecation:is_old(),
                         'box.session.push status')
         if not ok then
-            t.assert_equals(tostring(err), 'box.session.push is deprecated',
+            t.assert_equals(err.message, 'box.session.push is deprecated',
                             'box.session.push error')
         end
     end)
@@ -84,7 +84,7 @@ g.test = function(cg)
         t.assert_equals(ok, compat.box_session_push_deprecation:is_old(),
                         'box.session.push status')
         if not ok then
-            t.assert_equals(tostring(err), 'box.session.push is deprecated',
+            t.assert_equals(err.message, 'box.session.push is deprecated',
                             'box.session.push error')
         end
     end)
diff --git a/test/box-luatest/graceful_shutdown_test.lua b/test/box-luatest/graceful_shutdown_test.lua
index bf7bf033b062a..4acb030324a2d 100644
--- a/test/box-luatest/graceful_shutdown_test.lua
+++ b/test/box-luatest/graceful_shutdown_test.lua
@@ -98,7 +98,7 @@ g.test_discarded_requests_completed = function()
     g.server:stop()
     local res, err = fut:result()
     t.assert_not(res)
-    t.assert_equals(tostring(err), 'Response is discarded')
+    t.assert_equals(err.message, 'Response is discarded')
     t.assert_equals(conn.state, 'error')
     t.assert_equals(conn.error, 'Peer closed')
     conn:close()
@@ -122,7 +122,7 @@ g.test_hung_requests_aborted = function()
     g.server:stop()
     local res, err = fut:result()
     t.assert_not(res)
-    t.assert_equals(tostring(err), 'Peer closed')
+    t.assert_equals(err.message, 'Peer closed')
     t.assert_equals(conn.state, 'error')
     t.assert_equals(conn.error, 'Peer closed')
     conn:close()
@@ -161,7 +161,7 @@ g.test_new_requests_not_allowed = function()
     t.assert_equals(conn.error, 'Peer closed')
     local res, err = fut:result()
     t.assert_not(res)
-    t.assert_equals(tostring(err), 'Peer closed')
+    t.assert_equals(err.message, 'Peer closed')
     conn:close()
 end
 
@@ -504,7 +504,7 @@ g.test_graceful_shutdown_not_supported_by_client = function()
     g.server:stop()
     local res, err = fut:result()
     t.assert_not(res)
-    t.assert_equals(tostring(err), 'Peer closed')
+    t.assert_equals(err.message, 'Peer closed')
     conn:close()
 end
 
diff --git a/test/box-luatest/net_box_test.lua b/test/box-luatest/net_box_test.lua
index 1eb9175dc7365..ec37a99d79da5 100644
--- a/test/box-luatest/net_box_test.lua
+++ b/test/box-luatest/net_box_test.lua
@@ -348,12 +348,12 @@ g.test_sync_request_in_trigger = function()
     local function trigger_cb()
         local status, err
         status, err = pcall(c.call, c, 'box.session.user', {timeout = 5})
-        ch:put(status or tostring(err))
+        ch:put(status or err.message)
         local fut = c:call('box.session.user', {}, {is_async = true})
         status, err = pcall(fut.wait_result, fut, 5)
-        ch:put(status or tostring(err))
+        ch:put(status or err.message)
         status, err = pcall(function() for _, _ in fut:pairs(5) do end end)
-        ch:put(status or tostring(err))
+        ch:put(status or err.message)
     end
     local function check()
         local msg = 'Synchronous requests are not allowed in net.box trigger'
diff --git a/test/box-luatest/sending_arbitrary_iproto_packets_test.lua b/test/box-luatest/sending_arbitrary_iproto_packets_test.lua
index d63a69fddf5f3..61795c9d1be73 100644
--- a/test/box-luatest/sending_arbitrary_iproto_packets_test.lua
+++ b/test/box-luatest/sending_arbitrary_iproto_packets_test.lua
@@ -116,10 +116,10 @@ g.test_box_iproto_send_errors = function(cg)
         s:close()
         local _, err = pcall(function() box.iproto.send(sid, {}) end)
         if err ~= nil then
-            t.assert_str_contains(tostring(err), 'Session is closed')
+            t.assert_str_contains(err.message, 'Session is closed')
         else
             err = _G.test_channel:get()
-            t.assert_str_contains(tostring(err), 'Session is closed')
+            t.assert_str_contains(err.message, 'Session is closed')
         end
     end)
 end
diff --git a/test/box-tap/gh-5307-key_def-part-count-check.test.lua b/test/box-tap/gh-5307-key_def-part-count-check.test.lua
index d9458c95ad9fa..5fbb3d14b247a 100755
--- a/test/box-tap/gh-5307-key_def-part-count-check.test.lua
+++ b/test/box-tap/gh-5307-key_def-part-count-check.test.lua
@@ -20,7 +20,7 @@ mytest:ok(ok and res < 0, "Simple inequality")
 -- Should fail
 local exp_err = "Invalid key part count (expected [0..1], got 9)"
 ok, res = pcall(kd.compare_with_key, kd, {1}, {1, 2, 3, 4, 5, 6, 7, 8, 9})
-mytest:is_deeply({ok, tostring(res)}, {false, exp_err},
+mytest:is_deeply({ok, res.message}, {false, exp_err},
     "Invalid key part count")
 
 os.exit(mytest:check() and 0 or 1)
diff --git a/test/box-tap/gh-5450-merger-wrong-key-def.test.lua b/test/box-tap/gh-5450-merger-wrong-key-def.test.lua
index 033f42da4c1e3..6447eaaf38e74 100755
--- a/test/box-tap/gh-5450-merger-wrong-key-def.test.lua
+++ b/test/box-tap/gh-5450-merger-wrong-key-def.test.lua
@@ -20,7 +20,7 @@ local exp_err = "Field 6 has type 'string' in one index, but type " ..
 -- format.
 local sources = {}
 local ok, err = pcall(merger.new, key_def, sources)
-test:is_deeply({ok, tostring(err)}, {false, exp_err},
+test:is_deeply({ok, err.message}, {false, exp_err},
                'unable to create a tuple format')
 
 os.exit(test:check() and 0 or 1)
diff --git a/test/box-tap/gh-6198-max-cnt-of-tuple-fields-insert-overflow-err-msg.test.lua b/test/box-tap/gh-6198-max-cnt-of-tuple-fields-insert-overflow-err-msg.test.lua
index 4021dc7441650..3ee5ecfdb0a75 100755
--- a/test/box-tap/gh-6198-max-cnt-of-tuple-fields-insert-overflow-err-msg.test.lua
+++ b/test/box-tap/gh-6198-max-cnt-of-tuple-fields-insert-overflow-err-msg.test.lua
@@ -12,7 +12,7 @@ local t = box.tuple.new({1}):update({{'=', 1, 1}})
 local expected_err_msg = 'Tuple field count limit reached: see ' ..
                          'box.schema.FIELD_MAX'
 local ok, observed_err_msg = pcall(t.update, t, {{'!', #t, 1}})
-test:is_deeply({ok, tostring(observed_err_msg)}, {false, expected_err_msg},
+test:is_deeply({ok, observed_err_msg.message}, {false, expected_err_msg},
                'unable to insert into a tuple which size equals to ' ..
                'box.schema.FIELD_MAX')
 box.error.injection.set('ERRINJ_TUPLE_FIELD_COUNT_LIMIT', -1)
diff --git a/test/box-tap/gh-6295-assert-on-wrong-id.test.lua b/test/box-tap/gh-6295-assert-on-wrong-id.test.lua
index add2df4290554..35a8b4daa724b 100755
--- a/test/box-tap/gh-6295-assert-on-wrong-id.test.lua
+++ b/test/box-tap/gh-6295-assert-on-wrong-id.test.lua
@@ -13,22 +13,22 @@ box.cfg{}
 local _priv = box.space._priv
 local errmsg = "Function '1000000' does not exist"
 ok, res = pcall(_priv.replace, _priv, {1, 2, 'function', 1000000, box.priv.A})
-test:is_deeply({ok, tostring(res)}, {false, errmsg}, "Proper error is returned")
+test:is_deeply({ok, res.message}, {false, errmsg}, "Proper error is returned")
 
 errmsg = "Sequence '1000000' does not exist"
 ok, res = pcall(_priv.replace, _priv, {1, 2, 'sequence', 1000000, box.priv.A})
-test:is_deeply({ok, tostring(res)}, {false, errmsg}, "Proper error is returned")
+test:is_deeply({ok, res.message}, {false, errmsg}, "Proper error is returned")
 
 errmsg = "Space '1000000' does not exist"
 ok, res = pcall(_priv.replace, _priv, {1, 2, 'space', 1000000, box.priv.A})
-test:is_deeply({ok, tostring(res)}, {false, errmsg}, "Proper error is returned")
+test:is_deeply({ok, res.message}, {false, errmsg}, "Proper error is returned")
 
 errmsg = "User '1000000' is not found"
 ok, res = pcall(_priv.replace, _priv, {1, 2, 'user', 1000000, box.priv.A})
-test:is_deeply({ok, tostring(res)}, {false, errmsg}, "Proper error is returned")
+test:is_deeply({ok, res.message}, {false, errmsg}, "Proper error is returned")
 
 errmsg = "Role '1000000' is not found"
 ok, res = pcall(_priv.replace, _priv, {1, 2, 'role', 1000000, box.priv.A})
-test:is_deeply({ok, tostring(res)}, {false, errmsg}, "Proper error is returned")
+test:is_deeply({ok, res.message}, {false, errmsg}, "Proper error is returned")
 
 os.exit(test:check() and 0 or 1)
diff --git a/test/box-tap/key_def.test.lua b/test/box-tap/key_def.test.lua
index 22e91be543eb4..7a75d7188b180 100755
--- a/test/box-tap/key_def.test.lua
+++ b/test/box-tap/key_def.test.lua
@@ -214,8 +214,10 @@ for _, case in ipairs(key_def_new_cases) do
                 ffi.istype('struct key_def', res)
             test:ok(ok, case[1])
         else
-            local err = tostring(res) -- cdata -> string
-            test:is_deeply({ok, err}, {false, case.exp_err}, case[1])
+            if type(res) == 'cdata' then
+                res = res.message
+            end
+            test:is_deeply({ok, res}, {false, case.exp_err}, case[1])
         end
     end
 end
@@ -290,7 +292,7 @@ test:test('extract_key()', function(test)
     })
     local ok, err = pcall(key_def.extract_key, key_def,
         box.tuple.new({'foo'}))
-    test:is_deeply({ok, tostring(err)}, {false, exp_err},
+    test:is_deeply({ok, err.message}, {false, exp_err},
         'short tuple with a non-nullable part (case 1)')
 
     -- Same as before, but a max fieldno is over tuple:len() + 1.
@@ -302,7 +304,7 @@ test:test('extract_key()', function(test)
     })
     local ok, err = pcall(key_def.extract_key, key_def,
         box.tuple.new({'foo'}))
-    test:is_deeply({ok, tostring(err)}, {false, exp_err},
+    test:is_deeply({ok, err.message}, {false, exp_err},
         'short tuple with a non-nullable part (case 2)')
 
     -- Same as before, but with another key def options:
@@ -316,7 +318,7 @@ test:test('extract_key()', function(test)
     })
     local ok, err = pcall(key_def.extract_key, key_def,
         box.tuple.new({'foo'}))
-    test:is_deeply({ok, tostring(err)}, {false, exp_err},
+    test:is_deeply({ok, err.message}, {false, exp_err},
         'short tuple with a non-nullable part (case 3)')
 
     -- A tuple has a field that does not match corresponding key
@@ -329,7 +331,7 @@ test:test('extract_key()', function(test)
         {type = 'string', fieldno = 3},
     })
     local ok, err = pcall(key_def.extract_key, key_def, {'one', 'two', 3})
-    test:is_deeply({ok, tostring(err)}, {false, exp_err},
+    test:is_deeply({ok, err.message}, {false, exp_err},
         'wrong field type')
 
     local key_def = key_def_lib.new({
@@ -340,7 +342,7 @@ test:test('extract_key()', function(test)
     })
     local ok, err = pcall(key_def.extract_key, key_def,
                           box.tuple.new({1, 1, 22}))
-    test:is_deeply({ok, tostring(err)},
+    test:is_deeply({ok, err.message},
                 {false, 'Tuple field [1]a required by space format is missing'},
                 'invalid JSON structure')
     test:is_deeply(key_def:extract_key({{a=1, b=2}, 1}):totable(),
@@ -393,7 +395,7 @@ test:test('compare()', function(test)
         {type = 'array', fieldno = 2, is_nullable = true},
     })
     local ok, err = pcall(key_def.compare, key_def, {'aa', {}}, {'bb', box.NULL})
-    test:is_deeply({ok, tostring(err)}, {false, cmp_err}, 'no composite comparison')
+    test:is_deeply({ok, err.message}, {false, cmp_err}, 'no composite comparison')
 end)
 
 -- Case: compare_with_key().
@@ -418,13 +420,13 @@ test:test('compare_with_key()', function(test)
         {type = 'map', fieldno = 2},
     })
     local ok, err = pcall(key_def.compare_with_key, key_def, {'aa', {}}, {'bb', box.NULL})
-    test:is_deeply({ok, tostring(err)}, {false, cmp_err}, 'no composite comparison')
+    test:is_deeply({ok, err.message}, {false, cmp_err}, 'no composite comparison')
 
     -- Unserializable key.
     local exp_err = "unsupported Lua type 'function'"
     local key = {function() end}
     local ok, err = pcall(key_def_b.compare_with_key, key_def_b, tuple_a, key)
-    test:is_deeply({ok, tostring(err)}, {false, exp_err}, 'unserializable key')
+    test:is_deeply({ok, err.message}, {false, exp_err}, 'unserializable key')
 end)
 
 -- Case: totable().
diff --git a/test/box-tap/merger.test.lua b/test/box-tap/merger.test.lua
index f22aaa5c5b85f..b012ac7f4bed6 100755
--- a/test/box-tap/merger.test.lua
+++ b/test/box-tap/merger.test.lua
@@ -593,7 +593,9 @@ end
 for _, case in ipairs(bad_merger_select_calls) do
     local merger_inst = merger.new(key_def, case.sources)
     local ok, err = pcall(merger_inst.select, merger_inst, case.opts)
-    err = tostring(err) -- cdata -> string
+    if not ok and type(err) == 'cdata' then
+        err = err.message -- cdata -> string
+    end
     test:is_deeply({ok, err}, {false, case.exp_err}, case[1])
 end
 
diff --git a/test/box-tap/several-listening-sockets.test.lua b/test/box-tap/several-listening-sockets.test.lua
index 4ad5c98c72813..37be44602675b 100755
--- a/test/box-tap/several-listening-sockets.test.lua
+++ b/test/box-tap/several-listening-sockets.test.lua
@@ -99,9 +99,9 @@ local uri_table, errmsg
 local function test_error_message(uri_table, errmsg)
     errmsg = "Incorrect value for option 'listen': " .. errmsg
     local _, err = pcall(box.cfg, {listen = uri_table})
-    test:is(tostring(err), errmsg, "error message")
+    test:is(err.message, errmsg, "error message")
     _, err = pcall(box.cfg, {listen = uri_table})
-    test:is(tostring(err), errmsg, "error message")
+    test:is(err.message, errmsg, "error message")
 end
 
 -- Incorrect URI: expected host:service or /unix.socket
diff --git a/test/box/errinj.test.lua b/test/box/errinj.test.lua
index 78047408d658e..2695d2d2c5736 100644
--- a/test/box/errinj.test.lua
+++ b/test/box/errinj.test.lua
@@ -290,7 +290,7 @@ cn:ping()
 errinj.set('ERRINJ_PORT_DUMP', true)
 ok, ret = pcall(cn.space._space.select, cn.space._space)
 assert(not ok)
-assert(string.match(tostring(ret), 'Failed to allocate'))
+assert(string.match(ret.message, 'Failed to allocate'))
 errinj.set('ERRINJ_PORT_DUMP', false)
 cn:close()
 box.schema.user.revoke('guest', 'read', 'space', '_space')
diff --git a/test/box/func_reload.result b/test/box/func_reload.result
index eaa07dccad017..d25069af3bb69 100644
--- a/test/box/func_reload.result
+++ b/test/box/func_reload.result
@@ -244,7 +244,7 @@ os.setenv("TMPDIR", "/dev/null")
 _, err = pcall(box.schema.func.reload, "reload")
 ---
 ...
-tostring(err):gsub(': [/%w]+:', ':')
+err.message:gsub(': [/%w]+:', ':')
 ---
 - 'failed to create unique dir name: Not a directory'
 - 1
diff --git a/test/box/func_reload.test.lua b/test/box/func_reload.test.lua
index 3a0bc721e19f9..aa305cd1e7392 100644
--- a/test/box/func_reload.test.lua
+++ b/test/box/func_reload.test.lua
@@ -86,7 +86,7 @@ box.schema.func.reload("non-existing")
 -- path for DSO copy
 os.setenv("TMPDIR", "/dev/null")
 _, err = pcall(box.schema.func.reload, "reload")
-tostring(err):gsub(': [/%w]+:', ':')
+err.message:gsub(': [/%w]+:', ':')
 os.setenv("TMPDIR", nil)
 
 box.schema.func.drop("reload.test_reload")
diff --git a/test/box/net.box_long-poll_input_gh-3400.result b/test/box/net.box_long-poll_input_gh-3400.result
index 57c6757ec27f0..90aa9b5ede93e 100644
--- a/test/box/net.box_long-poll_input_gh-3400.result
+++ b/test/box/net.box_long-poll_input_gh-3400.result
@@ -45,7 +45,7 @@ test_run:wait_cond(function() return f:status() == 'dead' end)
 ---
 - true
 ...
-assert(tostring(result) == 'Peer closed')
+assert(result.message == 'Peer closed')
 ---
 - true
 ...
diff --git a/test/box/net.box_long-poll_input_gh-3400.test.lua b/test/box/net.box_long-poll_input_gh-3400.test.lua
index cf3a6b459f1d5..2cda8cf0014d8 100644
--- a/test/box/net.box_long-poll_input_gh-3400.test.lua
+++ b/test/box/net.box_long-poll_input_gh-3400.test.lua
@@ -23,5 +23,5 @@ end)
 pcall(c._request, c, 'INJECT', nil, nil, nil, '\x80')
 test_run:cmd("setopt delimiter ''");
 test_run:wait_cond(function() return f:status() == 'dead' end)
-assert(tostring(result) == 'Peer closed')
+assert(result.message == 'Peer closed')
 c:close()
diff --git a/test/engine/errinj_ddl.result b/test/engine/errinj_ddl.result
index 631aa5fc68c04..794513d64c196 100644
--- a/test/engine/errinj_ddl.result
+++ b/test/engine/errinj_ddl.result
@@ -347,7 +347,7 @@ assert(not ok)
 ---
 - true
 ...
-assert(tostring(err):find('Duplicate key') ~= nil)
+assert(err.message:find('Duplicate key') ~= nil)
 ---
 - true
 ...
diff --git a/test/engine/errinj_ddl.test.lua b/test/engine/errinj_ddl.test.lua
index dff72aec82cbf..219d56ea79a23 100644
--- a/test/engine/errinj_ddl.test.lua
+++ b/test/engine/errinj_ddl.test.lua
@@ -160,7 +160,7 @@ test_run:cmd("setopt delimiter ''");
 errinj.set("ERRINJ_BUILD_INDEX_DELAY", true)
 ok, err = pcall(s.create_index, s, 'sk', {parts = {2, 'unsigned'}})
 assert(not ok)
-assert(tostring(err):find('Duplicate key') ~= nil)
+assert(err.message:find('Duplicate key') ~= nil)
 
 ch:get()
 
diff --git a/test/interactive_tarantool.lua b/test/interactive_tarantool.lua
index 5243800b0d731..30a0d19877049 100644
--- a/test/interactive_tarantool.lua
+++ b/test/interactive_tarantool.lua
@@ -33,7 +33,7 @@ function mt._start_stderr_logger(self)
             local chunk, err = self.ph:read({stderr = true})
             if chunk == nil then
                 log.warn(('%s: got error, exitting: %s'):format(
-                    fiber_name, tostring(err)))
+                    fiber_name, err.message))
                 break
             end
             if chunk == '' then
@@ -123,7 +123,7 @@ function mt.read_until_prompt(self, opts)
 
     local stderr, err = self.ph:read({timeout = 0.05, stderr = true})
     if stderr ~= "" and not (stderr == nil and
-                             tostring(err) == "timed out") then
+                             err.message == "timed out") then
         error(("Unexpected stderr output: %s"):format(stderr))
     end
 
diff --git a/test/justrun.lua b/test/justrun.lua
index 953b53fefe90a..dbe79f3c151f4 100644
--- a/test/justrun.lua
+++ b/test/justrun.lua
@@ -17,7 +17,7 @@ local function collect_stderr(ph)
             local chunk, err = ph:read({stderr = true})
             if chunk == nil then
                 log.warn(('%s: got error, exiting: %s'):format(
-                    fiber_name, tostring(err)))
+                    fiber_name, err.message))
                 break
             end
             if chunk == '' then
diff --git a/test/sql-tap/gh2250-trigger-chain-limit.test.lua b/test/sql-tap/gh2250-trigger-chain-limit.test.lua
index 53c726405f084..032a5d7e390fe 100755
--- a/test/sql-tap/gh2250-trigger-chain-limit.test.lua
+++ b/test/sql-tap/gh2250-trigger-chain-limit.test.lua
@@ -75,12 +75,11 @@ for _, table_count in ipairs({30, 31}) do
 
     local function check(sql)
         local _, msg = pcall(function () test:execsql(sql) end)
-        msg = tostring(msg)
         test:do_test(sql,
                      function()
                          return true
                      end,
-                     table_count <= 30 or msg == 'Maximum number of chained trigger activations exceeded.')
+                     table_count <= 30 or msg.message == 'Maximum number of chained trigger activations exceeded.')
     end
 
     -- Exceed check for UPDATE
diff --git a/test/vinyl-luatest/gh_7240_abort_yielding_dml_test.lua b/test/vinyl-luatest/gh_7240_abort_yielding_dml_test.lua
index ccb6374113f7a..de008a7d9d245 100644
--- a/test/vinyl-luatest/gh_7240_abort_yielding_dml_test.lua
+++ b/test/vinyl-luatest/gh_7240_abort_yielding_dml_test.lua
@@ -40,7 +40,7 @@ g.test_abort_yielding_dml = function(cg)
         local ch = fiber.channel(1)
         fiber.create(function()
             local ok, err = pcall(s.insert, s, {2, 20})
-            ch:put(ok or tostring(err))
+            ch:put(ok or err.message)
         end)
         -- The insert operation blocks on disk read to check uniqueness.
         t.assert_is(ch:get(0.1), nil)