diff --git a/CHANGELOG.md b/CHANGELOG.md
index 00d55d8d1..4499d432d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -41,6 +41,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
 - irc: fix warning on creation of irc.msgbuffer option when the server name contains upper case letters ([#2281](https://github.com/weechat/weechat/issues/2281))
 - irc: display a warning for each unknown or invalid server option in commands /connect and /server
 - relay/api: fix crash when an invalid HTTP request is received from a client
+- relay/api: return HTTP error 404 instead of 400 when the buffer is not found in resources completion and input
 
 ## Version 4.7.1 (2025-08-16)
 
diff --git a/src/plugins/relay/api/relay-api-protocol.c b/src/plugins/relay/api/relay-api-protocol.c
index 38428f178..4e318a33a 100644
--- a/src/plugins/relay/api/relay-api-protocol.c
+++ b/src/plugins/relay/api/relay-api-protocol.c
@@ -797,7 +797,7 @@ RELAY_API_PROTOCOL_CALLBACK(input)
             {
                 relay_api_msg_send_error_json (
                     client,
-                    RELAY_HTTP_400_BAD_REQUEST, NULL,
+                    RELAY_HTTP_404_NOT_FOUND, NULL,
                     "Buffer \"%s\" not found",
                     ptr_buffer_name);
                 cJSON_Delete (json_body);
@@ -819,7 +819,7 @@ RELAY_API_PROTOCOL_CALLBACK(input)
                 {
                     relay_api_msg_send_error_json (
                         client,
-                        RELAY_HTTP_400_BAD_REQUEST, NULL,
+                        RELAY_HTTP_404_NOT_FOUND, NULL,
                         "Buffer \"%lld\" not found",
                         (long long)cJSON_GetNumberValue (json_buffer_id));
                     cJSON_Delete (json_body);
@@ -924,7 +924,7 @@ RELAY_API_PROTOCOL_CALLBACK(completion)
             {
                 relay_api_msg_send_error_json (
                     client,
-                    RELAY_HTTP_400_BAD_REQUEST, NULL,
+                    RELAY_HTTP_404_NOT_FOUND, NULL,
                     "Buffer \"%s\" not found",
                     ptr_buffer_name);
                 cJSON_Delete (json_body);
@@ -946,7 +946,7 @@ RELAY_API_PROTOCOL_CALLBACK(completion)
                 {
                     relay_api_msg_send_error_json (
                         client,
-                        RELAY_HTTP_400_BAD_REQUEST, NULL,
+                        RELAY_HTTP_404_NOT_FOUND, NULL,
                         "Buffer \"%lld\" not found",
                         (long long)cJSON_GetNumberValue (json_buffer_id));
                     cJSON_Delete (json_body);
diff --git a/src/plugins/relay/api/weechat-relay-api.yaml b/src/plugins/relay/api/weechat-relay-api.yaml
index 06e61cacf..cb15c671f 100644
--- a/src/plugins/relay/api/weechat-relay-api.yaml
+++ b/src/plugins/relay/api/weechat-relay-api.yaml
@@ -614,6 +614,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        '404':
+          description: Buffer not found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
       security:
         - password: []
   /completion:
@@ -677,6 +683,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        '404':
+          description: Buffer not found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
       security:
         - password: []
   /ping:
diff --git a/tests/unit/plugins/relay/api/test-relay-api-protocol.cpp b/tests/unit/plugins/relay/api/test-relay-api-protocol.cpp
index 08f55daa9..4430319e9 100644
--- a/tests/unit/plugins/relay/api/test-relay-api-protocol.cpp
+++ b/tests/unit/plugins/relay/api/test-relay-api-protocol.cpp
@@ -622,7 +622,6 @@ TEST(RelayApiProtocolWithClient, CbCompletion)
 
     /* error: no body */
     test_client_recv_http ("POST /api/completion", NULL, NULL);
-    WEE_CHECK_HTTP_CODE(400, "Bad Request");
     STRCMP_EQUAL("HTTP/1.1 400 Bad Request\r\n"
                  "Access-Control-Allow-Origin: *\r\n"
                  "Content-Type: application/json; charset=utf-8\r\n"
@@ -636,8 +635,7 @@ TEST(RelayApiProtocolWithClient, CbCompletion)
                            NULL,
                            "{\"buffer_name\": \"invalid\", "
                            "\"command\": \"test\"}");
-    WEE_CHECK_HTTP_CODE(400, "Bad Request");
-    STRCMP_EQUAL("HTTP/1.1 400 Bad Request\r\n"
+    STRCMP_EQUAL("HTTP/1.1 404 Not Found\r\n"
                  "Access-Control-Allow-Origin: *\r\n"
                  "Content-Type: application/json; charset=utf-8\r\n"
                  "Content-Length: 40\r\n"
@@ -717,7 +715,7 @@ TEST(RelayApiProtocolWithClient, CbInput)
                            NULL,
                            "{\"buffer_name\": \"invalid\", "
                            "\"command\": \"/print test\"}");
-    STRCMP_EQUAL("HTTP/1.1 400 Bad Request\r\n"
+    STRCMP_EQUAL("HTTP/1.1 404 Not Found\r\n"
                  "Access-Control-Allow-Origin: *\r\n"
                  "Content-Type: application/json; charset=utf-8\r\n"
                  "Content-Length: 40\r\n"