diff --git a/spec/02-integration/01-helpers/03-http_mock_spec.lua b/spec/02-integration/01-helpers/03-http_mock_spec.lua
index b1250c1febc..6ffb0770cde 100644
--- a/spec/02-integration/01-helpers/03-http_mock_spec.lua
+++ b/spec/02-integration/01-helpers/03-http_mock_spec.lua
@@ -27,7 +27,7 @@ for _, tls in ipairs {true, false} do
           resp_body = true
         }
       }))
-      
+
       assert(mock:start())
     end)
 
@@ -220,6 +220,32 @@ describe("http_mock config", function()
 
     assert(pl_file.access_time(pid_filename) ~= nil, "mocking not in the correct place")
   end)
+
+  it("init_by_lua_block inject", function ()
+    local mock = assert(http_mock.new(nil, {
+      ["/test"] = {
+        access = [[
+          ngx.print(test_value)
+        ]],
+      },
+    }, {
+      init = [[
+        -- Test that the mock is injected
+        test_value = "hello world"
+      ]]
+    }))
+    mock:start()
+    finally(function()
+      assert(mock:stop())
+    end)
+
+    local client = mock:get_client()
+    local res = assert(client:send({
+      path = "/test"
+    }))
+    assert.response(res).has.status(200)
+    assert.same(res:read_body(), "hello world")
+  end)
 end)
 
 local function remove_volatile_headers(req_t)
diff --git a/spec/helpers/http_mock.lua b/spec/helpers/http_mock.lua
index 5319a06d577..91fc85c6121 100644
--- a/spec/helpers/http_mock.lua
+++ b/spec/helpers/http_mock.lua
@@ -61,6 +61,7 @@ end
 -- @tparam[opt=false] bool opts.log_opts.collect_resp whether to log responses
 -- @tparam[opt=false] bool opts.log_opts.collect_resp_body whether to log response bodies
 -- @tparam[opt=true] bool opts.log_opts.collect_err: whether to log errors
+-- @tparam[opt] string opts.init: the lua code injected into the init_by_lua_block
 -- @treturn http_mock a mock instance
 -- @treturn string the port the mock server listens to
 -- @usage
@@ -160,6 +161,7 @@ function http_mock.new(listens, routes, opts)
     listens = listens,
     routes = routes,
     directives = directives,
+    init = opts.init,
     log_opts = log_opts,
     logs = {},
     tls = opts.tls,
diff --git a/spec/helpers/http_mock/template.lua b/spec/helpers/http_mock/template.lua
index 093bc0d334d..510cfad8c8c 100644
--- a/spec/helpers/http_mock/template.lua
+++ b/spec/helpers/http_mock/template.lua
@@ -24,8 +24,8 @@ events {
 http {
   lua_shared_dict mock_logs $(shm_size);
 
-# if log_opts.err then
   init_by_lua_block {
+# if log_opts.err then
     -- disable warning of global variable
     local g_meta = getmetatable(_G)
     setmetatable(_G, nil)
@@ -41,16 +41,12 @@ http {
       table.insert(err_t, {err, debug.traceback("", 3)})
     end
 
-    function assert(truthy, err) -- luacheck: ignore
-      if truthy then
-        return truthy
-      end
-
-      if ngx.ctx then
+    function assert(truthy, err, ...) -- luacheck: ignore
+      if not truthy and ngx.ctx then
         insert_err(err)
       end
 
-      return original_assert(truthy, err)
+      return original_assert(truthy, err, ...)
     end
 
     original_error = error -- luacheck: ignore
@@ -66,9 +62,12 @@ http {
     err_patched = true -- luacheck: ignore
 
     setmetatable(_G, g_meta)
+# end
+# if init then
+$(init)
+# end
   }
 
-# end
   server {
     listen 0.0.0.0:$(debug.port);
     server_name mock_debug;
@@ -142,7 +141,7 @@ http {
         local method = ngx.req.get_method()
         local uri = ngx.var.request_uri
         local headers = ngx.req.get_headers(nil, true)
-        
+
 
         ngx.req.read_body()
         local body