fix(mcp-bridge): encode stderr notifications as JSON

apisix/plugins/mcp-bridge.lua

@@ -109,9 +109,25 @@ local function on_connect(conf, ctx)
                     local line, _
                     line, _, stderr_partial = proc:stderr_read_line()
                     if line then
-                        local ok, err = server.transport:send(
-                           '{"jsonrpc":"2.0","method":"notifications/stderr","params":{"content":"'
-                            .. (stderr_partial and stderr_partial .. line or line) .. '"}}')
+                        local content = stderr_partial and stderr_partial .. line or line
+                        local msg, enc_err = core.json.encode({
+                            jsonrpc = "2.0",
+                            method = "notifications/stderr",
+                            params = {
+                                content = content,
+                            },
+                        })
+                        if not msg then
+                            msg = core.json.encode({
+                                jsonrpc = "2.0",
+                                method = "notifications/stderr",
+                                params = {
+                                    content = "failed to encode stderr content: " .. (enc_err or "unknown"),
+                                },
+                            })
+                        end
+
+                        local ok, err = server.transport:send(msg)
                         if not ok then
                             core.log.info("session ", server.session_id,
                                           " exit, failed to send response message: ", err)

t/plugin/mcp-bridge.t

@@ -57,3 +57,40 @@ property "command" is required
 property "command" validation failed: wrong type: expected string, got number
 done
 property "args" validation failed: wrong type: expected array, got string
+
+
+
+=== TEST 2: stderr content with JSON special characters keeps the notification well-formed
+--- config
+    location /t {
+        content_by_lua_block {
+            local core = require("apisix.core")
+            local pipe = require("ngx.pipe")
+
+            -- a subprocess line that contains double quotes, a backslash and the
+            -- "}}" sequence, all of which break naive string concatenation
+            local payload = [[Error: invalid input "x\y" }}]]
+
+            local proc = assert(pipe.spawn({"sh", "-c", [[printf '%s\n' "$1" 1>&2]], "sh", payload}))
+            proc:set_timeouts(nil, 1000, 1000)
+            local line = assert(proc:stderr_read_line())
+
+            -- build the notification the same way the plugin does
+            local msg = core.json.encode({
+                jsonrpc = "2.0",
+                method = "notifications/stderr",
+                params = {
+                    content = line,
+                },
+            })
+
+            local decoded, err = core.json.decode(msg)
+            if not decoded then
+                ngx.say("not valid json: ", err)
+                return
+            end
+            ngx.say("valid json: ", decoded.params.content == payload)
+        }
+    }
+--- response_body
+valid json: true