From 147ba69968b84fbb4e235e4c37d7827cd046d93e Mon Sep 17 00:00:00 2001
From: Marco Sadjadi <marco@buildmymcpserver.local>
Date: Thu, 28 May 2026 21:39:11 +0200
Subject: [PATCH] fix(runner): alias params/input to args so tool
 implementations don't ReferenceError

Auth chain finally landed but tool calls crashed in the wetter server
with "Error: params is not defined". The MCP SDK passes the validated
tool args as a single parameter; our template names that parameter
`args` but the model frequently writes `params.location` / `input.x`
because that's how OpenAPI and JSON-RPC reference docs read.

Two-sided fix:
- render.ts wraps every implementation with `const params = args; const
  input = args;` inside the try block. Whichever alias the model
  picked, the variable resolves to the same validated object.
- SYSTEM_PROMPT now states the variable name EXPLICITLY ("variable
  named EXACTLY `args`, e.g. args.location") so new generations stop
  drifting on that detail.

Existing wetter runner needs a rebuild to pick up the alias shim.
---
 apps/generator/src/lib/render.ts | 9 +++++++++
 packages/llm/src/index.ts        | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/apps/generator/src/lib/render.ts b/apps/generator/src/lib/render.ts
index 179a104..9705179 100644
--- a/apps/generator/src/lib/render.ts
+++ b/apps/generator/src/lib/render.ts
@@ -38,6 +38,15 @@ function renderTool(tool: ToolSpec): string {
     inputSchema: ${schemaShape},
   },
   async (args) => {
+    // The MCP SDK passes the validated tool arguments as the single
+    // parameter. Models trained on OpenAPI / JSON-RPC examples reach
+    // for "params" instead of "args", and "input" shows up too — bind
+    // every common alias to the same object so the generated body
+    // works whichever name the model picked. Without this the runner
+    // crashes with "ReferenceError: params is not defined" at the
+    // first tool call (verified in prod with the wetter server).
+    const params = args;
+    const input = args;
     try {
       ${tool.implementation}
     } catch (err) {
diff --git a/packages/llm/src/index.ts b/packages/llm/src/index.ts
index 1c2c546..a62f155 100644
--- a/packages/llm/src/index.ts
+++ b/packages/llm/src/index.ts
@@ -15,7 +15,7 @@ Output ONE JSON object (no markdown, no prose, no code fences) with this exact s
       "inputSchema": {
         "param_name": { "type": "string|number|boolean|array|object", "description": "short", "required": true }
       },
-      "implementation": "async TS body, return { content: [{ type:'text', text:'...' }] }; secrets via process.env; HTTP via globalThis.fetch with AbortSignal.timeout(10000); try/catch -> { content:[{type:'text',text:'Error: ...'}], isError:true }; no eval/Function/child_process; no imports."
+      "implementation": "async TS body. The tool's validated arguments arrive in the variable named EXACTLY `args` (e.g. args.location, args.query). Return { content: [{ type:'text', text:'...' }] }. Secrets via process.env. HTTP via globalThis.fetch with AbortSignal.timeout(10000). Wrap external calls in try/catch and return { content:[{type:'text',text:'Error: ...'}], isError:true } on failure. No eval/Function/child_process. No import statements."
     }
   ],
   "resources": [],