diff --git a/src/nvim/api/private/dispatch.c b/src/nvim/api/private/dispatch.c
index dec2b6c185..c08225bbfc 100644
--- a/src/nvim/api/private/dispatch.c
+++ b/src/nvim/api/private/dispatch.c
@@ -40,7 +40,8 @@ MsgpackRpcRequestHandler msgpack_rpc_get_handler_for(const char *name,
     map_get(String, MsgpackRpcRequestHandler)(methods, m);
 
   if (!rv.fn) {
-    api_set_error(error, kErrorTypeException, "Invalid method: %s",
+    api_set_error(error, kErrorTypeException, "Invalid method: %.*s",
+                  m.size > 0 ? m.size : sizeof("<empty>"),
                   m.size > 0 ? m.data : "<empty>");
   }
   return rv;
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index f3e883de02..ecc0ede4a4 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -162,7 +162,7 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
   dictitem_T *const di = tv_dict_find(dict, key.data, (ptrdiff_t)key.size);
 
   if (di == NULL) {
-    api_set_error(err, kErrorTypeValidation, "Key '%s' not found", key.data);
+    api_set_error(err, kErrorTypeValidation, "Key not found: %s", key.data);
     return (Object)OBJECT_INIT;
   }
 
@@ -191,13 +191,12 @@ Object dict_set_var(dict_T *dict, String key, Object value, bool del,
   }
 
   if (key.size == 0) {
-    api_set_error(err, kErrorTypeValidation,
-                  "Empty variable names aren't allowed");
+    api_set_error(err, kErrorTypeValidation, "Key name is empty");
     return rv;
   }
 
   if (key.size > INT_MAX) {
-    api_set_error(err, kErrorTypeValidation, "Key length is too high");
+    api_set_error(err, kErrorTypeValidation, "Key name is too long");
     return rv;
   }
 
@@ -220,7 +219,7 @@ Object dict_set_var(dict_T *dict, String key, Object value, bool del,
     // Delete the key
     if (di == NULL) {
       // Doesn't exist, fail
-      api_set_error(err, kErrorTypeValidation, "Key does not exist: %s",
+      api_set_error(err, kErrorTypeValidation, "Key not found: %s",
                     key.data);
     } else {
       // Return the old value
@@ -284,9 +283,7 @@ Object get_option_from(void *from, int type, String name, Error *err)
                                       type, from);
 
   if (!flags) {
-    api_set_error(err,
-                  kErrorTypeValidation,
-                  "Invalid option name \"%s\"",
+    api_set_error(err, kErrorTypeValidation, "Invalid option name: '%s'",
                   name.data);
     return rv;
   }
@@ -303,15 +300,14 @@ Object get_option_from(void *from, int type, String name, Error *err)
       rv.data.string.data = stringval;
       rv.data.string.size = strlen(stringval);
     } else {
-      api_set_error(err,
-                    kErrorTypeException,
-                    "Unable to get value for option \"%s\"",
+      api_set_error(err, kErrorTypeException,
+                    "Failed to get value for option '%s'",
                     name.data);
     }
   } else {
     api_set_error(err,
                   kErrorTypeException,
-                  "Unknown type for option \"%s\"",
+                  "Unknown type for option '%s'",
                   name.data);
   }
 
@@ -336,24 +332,20 @@ void set_option_to(uint64_t channel_id, void *to, int type,
   int flags = get_option_value_strict(name.data, NULL, NULL, type, to);
 
   if (flags == 0) {
-    api_set_error(err,
-                  kErrorTypeValidation,
-                  "Invalid option name \"%s\"",
+    api_set_error(err, kErrorTypeValidation, "Invalid option name '%s'",
                   name.data);
     return;
   }
 
   if (value.type == kObjectTypeNil) {
     if (type == SREQ_GLOBAL) {
-      api_set_error(err,
-                    kErrorTypeException,
-                    "Unable to unset option \"%s\"",
+      api_set_error(err, kErrorTypeException, "Cannot unset option '%s'",
                     name.data);
       return;
     } else if (!(flags & SOPT_GLOBAL)) {
       api_set_error(err,
                     kErrorTypeException,
-                    "Cannot unset option \"%s\" "
+                    "Cannot unset option '%s' "
                     "because it doesn't have a global value",
                     name.data);
       return;
@@ -370,7 +362,7 @@ void set_option_to(uint64_t channel_id, void *to, int type,
     if (value.type != kObjectTypeBoolean) {
       api_set_error(err,
                     kErrorTypeValidation,
-                    "Option \"%s\" requires a boolean value",
+                    "Option '%s' requires a Boolean value",
                     name.data);
       return;
     }
@@ -378,17 +370,15 @@ void set_option_to(uint64_t channel_id, void *to, int type,
     numval = value.data.boolean;
   } else if (flags & SOPT_NUM) {
     if (value.type != kObjectTypeInteger) {
-      api_set_error(err,
-                    kErrorTypeValidation,
-                    "Option \"%s\" requires an integer value",
+      api_set_error(err, kErrorTypeValidation,
+                    "Option '%s' requires an integer value",
                     name.data);
       return;
     }
 
     if (value.data.integer > INT_MAX || value.data.integer < INT_MIN) {
-      api_set_error(err,
-                    kErrorTypeValidation,
-                    "Value for option \"%s\" is outside range",
+      api_set_error(err, kErrorTypeValidation,
+                    "Value for option '%s' is out of range",
                     name.data);
       return;
     }
@@ -396,9 +386,8 @@ void set_option_to(uint64_t channel_id, void *to, int type,
     numval = (int)value.data.integer;
   } else {
     if (value.type != kObjectTypeString) {
-      api_set_error(err,
-                    kErrorTypeValidation,
-                    "Option \"%s\" requires a string value",
+      api_set_error(err, kErrorTypeValidation,
+                    "Option '%s' requires a string value",
                     name.data);
       return;
     }
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index e78b8c776d..fed20a272a 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -502,7 +502,7 @@ Integer nvim_strwidth(String text, Error *err)
   FUNC_API_SINCE(1)
 {
   if (text.size > INT_MAX) {
-    api_set_error(err, kErrorTypeValidation, "String length is too high");
+    api_set_error(err, kErrorTypeValidation, "String is too long");
     return 0;
   }
 
@@ -559,7 +559,7 @@ void nvim_set_current_dir(String dir, Error *err)
   FUNC_API_SINCE(1)
 {
   if (dir.size >= MAXPATHL) {
-    api_set_error(err, kErrorTypeValidation, "Directory string is too long");
+    api_set_error(err, kErrorTypeValidation, "Directory name is too long");
     return;
   }
 
@@ -1136,14 +1136,14 @@ Array nvim_call_atomic(uint64_t channel_id, Array calls, Error *err)
     if (calls.items[i].type != kObjectTypeArray) {
       api_set_error(err,
                     kErrorTypeValidation,
-                    "All items in calls array must be arrays");
+                    "Items in calls array must be arrays");
       goto validation_error;
     }
     Array call = calls.items[i].data.array;
     if (call.size != 2) {
       api_set_error(err,
                     kErrorTypeValidation,
-                    "All items in calls array must be arrays of size 2");
+                    "Items in calls array must be arrays of size 2");
       goto validation_error;
     }
 
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 958e2f68fc..271e196103 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -309,7 +309,7 @@ describe('api/buf', function()
       eq(1, funcs.exists('b:lua'))
       curbufmeths.del_var('lua')
       eq(0, funcs.exists('b:lua'))
-      eq({false, 'Key does not exist: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
+      eq({false, 'Key not found: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
       curbufmeths.set_var('lua', 1)
       command('lockvar b:lua')
       eq({false, 'Key is locked: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
diff --git a/test/functional/api/tabpage_spec.lua b/test/functional/api/tabpage_spec.lua
index 260a91a80c..c49091db02 100644
--- a/test/functional/api/tabpage_spec.lua
+++ b/test/functional/api/tabpage_spec.lua
@@ -34,7 +34,7 @@ describe('api/tabpage', function()
       eq(1, funcs.exists('t:lua'))
       curtabmeths.del_var('lua')
       eq(0, funcs.exists('t:lua'))
-      eq({false, 'Key does not exist: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
+      eq({false, 'Key not found: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
       curtabmeths.set_var('lua', 1)
       command('lockvar t:lua')
       eq({false, 'Key is locked: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index ae8a8488d4..5261f57ca7 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -24,13 +24,25 @@ describe('API', function()
   before_each(clear)
 
   it('validates requests', function()
-    expect_err('Invalid method: bogus',
+    -- RPC
+    expect_err('Invalid method: bogus$',
                request, 'bogus')
-    expect_err('Invalid method: … の り 。…',
+    expect_err('Invalid method: … の り 。…$',
                request, '… の り 。…')
-    expect_err('Invalid method: <empty>',
+    expect_err('Invalid method: <empty>$',
                request, '')
-    expect_err("can't serialize object",
+
+    -- Non-RPC: rpcrequest(v:servername) uses internal channel.
+    expect_err('Invalid method: … の り 。…$',
+               request, 'nvim_eval',
+               [=[rpcrequest(sockconnect('pipe', v:servername, {'rpc':1}), '… の り 。…')]=])
+    expect_err('Invalid method: bogus$',
+               request, 'nvim_eval',
+               [=[rpcrequest(sockconnect('pipe', v:servername, {'rpc':1}), 'bogus')]=])
+
+    -- XXX: This must be the last one, else next one will fail:
+    --      "Packer instance already working. Use another Packer ..."
+    expect_err("can't serialize object$",
                request, nil)
   end)
 
@@ -158,7 +170,7 @@ describe('API', function()
     end)
 
     it("VimL error: returns error details, does NOT update v:errmsg", function()
-      expect_err('E121: Undefined variable: bogus', request,
+      expect_err('E121: Undefined variable: bogus$', request,
                  'nvim_eval', 'bogus expression')
       eq('', eval('v:errmsg'))  -- v:errmsg was not updated.
     end)
@@ -173,7 +185,7 @@ describe('API', function()
     end)
 
     it("VimL validation error: returns specific error, does NOT update v:errmsg", function()
-      expect_err('E117: Unknown function: bogus function', request,
+      expect_err('E117: Unknown function: bogus function$', request,
                  'nvim_call_function', 'bogus function', {'arg1'})
       expect_err('E119: Not enough arguments for function: atan', request,
                  'nvim_call_function', 'atan', {})
@@ -182,11 +194,11 @@ describe('API', function()
     end)
 
     it("VimL error: returns error details, does NOT update v:errmsg", function()
-      expect_err('E808: Number or Float required', request,
+      expect_err('E808: Number or Float required$', request,
                  'nvim_call_function', 'atan', {'foo'})
-      expect_err('Invalid channel stream "xxx"', request,
+      expect_err('Invalid channel stream "xxx"$', request,
                  'nvim_call_function', 'chanclose', {999, 'xxx'})
-      expect_err('E900: Invalid channel id', request,
+      expect_err('E900: Invalid channel id$', request,
                  'nvim_call_function', 'chansend', {999, 'foo'})
       eq('', eval('v:exception'))
       eq('', eval('v:errmsg'))  -- v:errmsg was not updated.
@@ -198,7 +210,7 @@ describe('API', function()
           throw 'wtf'
         endfunction
       ]])
-      expect_err('wtf', request,
+      expect_err('wtf$', request,
                  'nvim_call_function', 'Foo', {})
       eq('', eval('v:exception'))
       eq('', eval('v:errmsg'))  -- v:errmsg was not updated.
@@ -212,7 +224,7 @@ describe('API', function()
         endfunction
       ]])
       -- E740
-      expect_err('Function called with too many arguments', request,
+      expect_err('Function called with too many arguments$', request,
                  'nvim_call_function', 'Foo', too_many_args)
     end)
   end)
@@ -248,23 +260,23 @@ describe('API', function()
 
     it('validates args', function()
       command('let g:d={"baz":"zub","meep":[]}')
-      expect_err('Not found: bogus', request,
+      expect_err('Not found: bogus$', request,
                  'nvim_call_dict_function', 'g:d', 'bogus', {1,2})
-      expect_err('Not a function: baz', request,
+      expect_err('Not a function: baz$', request,
                  'nvim_call_dict_function', 'g:d', 'baz', {1,2})
-      expect_err('Not a function: meep', request,
+      expect_err('Not a function: meep$', request,
                  'nvim_call_dict_function', 'g:d', 'meep', {1,2})
-      expect_err('E117: Unknown function: f', request,
+      expect_err('E117: Unknown function: f$', request,
                  'nvim_call_dict_function', { f = '' }, 'f', {1,2})
-      expect_err('Not a function: f', request,
+      expect_err('Not a function: f$', request,
                  'nvim_call_dict_function', "{ 'f': '' }", 'f', {1,2})
-      expect_err('dict argument type must be String or Dictionary', request,
+      expect_err('dict argument type must be String or Dictionary$', request,
                  'nvim_call_dict_function', 42, 'f', {1,2})
-      expect_err('Failed to evaluate dict expression', request,
+      expect_err('Failed to evaluate dict expression$', request,
                  'nvim_call_dict_function', 'foo', 'f', {1,2})
-      expect_err('dict not found', request,
+      expect_err('dict not found$', request,
                  'nvim_call_dict_function', '42', 'f', {1,2})
-      expect_err('Invalid %(empty%) function name', request,
+      expect_err('Invalid %(empty%) function name$', request,
                  'nvim_call_dict_function', "{ 'f': '' }", '', {1,2})
     end)
   end)
@@ -337,7 +349,7 @@ describe('API', function()
       eq(1, funcs.exists('g:lua'))
       meths.del_var('lua')
       eq(0, funcs.exists('g:lua'))
-      eq({false, 'Key does not exist: lua'}, meth_pcall(meths.del_var, 'lua'))
+      eq({false, "Key not found: lua"}, meth_pcall(meths.del_var, 'lua'))
       meths.set_var('lua', 1)
       command('lockvar lua')
       eq({false, 'Key is locked: lua'}, meth_pcall(meths.del_var, 'lua'))
@@ -948,7 +960,7 @@ describe('API', function()
       }
       local status, err = pcall(meths.call_atomic, req)
       eq(false, status)
-      ok(err:match(' All items in calls array must be arrays of size 2') ~= nil)
+      ok(err:match('Items in calls array must be arrays of size 2') ~= nil)
       -- call before was done, but not after
       eq(1, meths.get_var('avar'))
 
@@ -958,7 +970,7 @@ describe('API', function()
       }
       status, err = pcall(meths.call_atomic, req)
       eq(false, status)
-      ok(err:match('All items in calls array must be arrays') ~= nil)
+      ok(err:match('Items in calls array must be arrays') ~= nil)
       eq({2,3}, meths.get_var('bvar'))
 
       req = {
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index d404ef5426..27d7aa11b4 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -169,7 +169,7 @@ describe('api/win', function()
       eq(1, funcs.exists('w:lua'))
       curwinmeths.del_var('lua')
       eq(0, funcs.exists('w:lua'))
-      eq({false, 'Key does not exist: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))
+      eq({false, 'Key not found: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))
       curwinmeths.set_var('lua', 1)
       command('lockvar w:lua')
       eq({false, 'Key is locked: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))