Spot the error in how this MCP server reports a tool failure

MCP has two kinds of errors, and the snippet mixes them up. A **protocol error** uses the JSON-RPC `error` field. It means the request itself was broken, like an unknown method or malformed JSON. The tool never even ran. A tool error is different. Here the `query_database` tool ran fine, it just hit a missing table. That is a normal business outcome, not a transport failure. So it must come back as a successful response with a `result`, and inside that result the `isError` flag is set to `true` with the message in the `content` array. The snippet wrongly puts the SQL failure in the `error` field. That tells the host the protocol broke. The model handling the tool call may never see the message, because it looks for failures inside the tool result, not in the JSON-RPC envelope. The fix is to return the same text inside `result.content` with `isError: true`. Then the model reads the error, can explain it to the user, and can retry with a different query.

The bug is a layer confusion. The server reports a tool logic failure using the JSON-RPC `error` field, which is reserved for protocol faults. MCP draws a hard line between two error classes. A protocol error means the request was malformed or the method does not exist. That belongs in the top-level `error` object. A tool error means the tool executed but its work failed, like a SQL error, a missing file, or an upstream timeout. A missing `users` table is squarely a tool error. Tool errors must ride back inside a normal successful response. The correct shape returns a `result` with `isError: true` and the message in the `content` array. The model handling the call inspects that flag, surfaces the message to the user, and can retry intelligently. The failure mode here is silent. When a tool failure arrives as a JSON-RPC `error`, many host implementations treat it as a transport fault. The message never reaches the model where it looks for tool output. The agent cannot self-correct, so it stalls or guesses instead of reading the real reason and adjusting.