diff --git a/apisix/patch.lua b/apisix/patch.lua
index 31383b770128..4d6956720136 100644
--- a/apisix/patch.lua
+++ b/apisix/patch.lua
@@ -414,20 +414,22 @@ function _M.patch()
     end
 
     -- Patch ngx.req.set_body_data to invalidate the parsed request body cache
-    -- in api_ctx. This ensures that body lookups and post_arg.* variable
-    -- lookups after a body rewrite always reflect the new body content.
+    -- in api_ctx. This ensures that body lookups, get_post_args(), and post_arg.* /
+    -- post_arg_* variable lookups after a body rewrite always reflect the new body content.
     local _orig_set_body_data = ngx.req.set_body_data
     ngx.req.set_body_data = function(data)
         local api_ctx = ngx.ctx.api_ctx
-        if api_ctx and api_ctx._request_body_table then
+        if api_ctx then
             api_ctx._request_body_table = nil
             api_ctx._request_body_type = nil
+            api_ctx.req_post_args = nil
             local var = api_ctx.var
             local cache = var and var._cache
             if cache then
                 local keys_to_clear = {}
                 for key in pairs(cache) do
-                    if type(key) == "string" and key:sub(1, 9) == "post_arg." then
+                    if type(key) == "string" and
+                        (key:sub(1, 9) == "post_arg." or key:sub(1, 9) == "post_arg_") then
                         keys_to_clear[#keys_to_clear + 1] = key
                     end
                 end
diff --git a/t/core/request.t b/t/core/request.t
index a99311aa5499..672ec6c246f1 100644
--- a/t/core/request.t
+++ b/t/core/request.t
@@ -611,3 +611,58 @@ header: new
 body: nil
 err: request size 17 is greater than the maximum size 8 allowed
 kind: too_large
+
+
+
+=== TEST 20: set_body_data invalidates req_post_args and post_arg.* / post_arg_* cache
+--- config
+    location /t {
+        content_by_lua_block {
+            local core = require("apisix.core")
+            local ngx_ctx = ngx.ctx
+            local api_ctx = ngx_ctx.api_ctx
+            if api_ctx == nil then
+                api_ctx = core.tablepool.fetch("api_ctx", 0, 32)
+                ngx_ctx.api_ctx = api_ctx
+            end
+
+            core.ctx.set_vars_meta(api_ctx)
+
+            -- 1. Read post args using core.request.get_post_args
+            local args1 = core.request.get_post_args(api_ctx)
+            ngx.say("args1 foo: ", args1["foo"])
+
+            -- 2. Read post_arg.foo and post_arg_foo variables via api_ctx.var (populates var._cache)
+            ngx.say("var post_arg.foo: ", api_ctx.var["post_arg.foo"])
+            ngx.say("var post_arg_foo: ", api_ctx.var["post_arg_foo"])
+
+            -- 3. Rewrite body using ngx.req.set_body_data
+            ngx.req.set_body_data("foo=updated&bar=new")
+
+            -- 4. Re-read post args: should reflect updated body
+            local args2 = core.request.get_post_args(api_ctx)
+            ngx.say("args2 foo: ", args2["foo"])
+            ngx.say("args2 bar: ", args2["bar"])
+
+            -- 5. Re-read post_arg variables: should reflect updated body
+            ngx.say("updated var post_arg.foo: ", api_ctx.var["post_arg.foo"])
+            ngx.say("updated var post_arg.bar: ", api_ctx.var["post_arg.bar"])
+            ngx.say("updated var post_arg_foo: ", api_ctx.var["post_arg_foo"])
+            ngx.say("updated var post_arg_bar: ", api_ctx.var["post_arg_bar"])
+        }
+    }
+--- request
+POST /t
+foo=initial&bar=original
+--- more_headers
+Content-Type: application/x-www-form-urlencoded
+--- response_body
+args1 foo: initial
+var post_arg.foo: initial
+var post_arg_foo: initial
+args2 foo: updated
+args2 bar: new
+updated var post_arg.foo: updated
+updated var post_arg.bar: new
+updated var post_arg_foo: updated
+updated var post_arg_bar: new