## Problem/Motivation `OutputDefinition` defaults `required` to `TRUE`, and `validateOutputs()` reports a required output that was never set. Nothing calls it. A tool can return `ExecutableResult::success()` without one of its required outputs, and the result stays a success. `ToolBase::execute()` only copies the context values that match a declared output into `$this->outputs`: ```php if ($this->result->isSuccess()) { if ($provided_definitions = $this->getOutputDefinitions()) { $this->outputs = []; foreach ($this->result->getContextValues() as $context_name => $value) { if (isset($provided_definitions[$context_name])) { $this->setOutputValue($context_name, $value); } } } } ``` Inputs get the opposite treatment: `execute()` rejects invalid inputs before `doExecute()` runs. Outputs are the tool author's contract with the caller, and today that contract is not checked. This matters now that `normalizeOutputDefinitions()` emits JSON Schema for outputs and invokers advertise it. In mcp_server_tool_bridge work item [3618724](https://git.drupalcode.org/project/mcp_server_tool_bridge/-/work_items/3618724) I left `required` off the `data` object in the MCP `outputSchema`, because declaring an output required there would let a tool that skips it return a result that violates the schema it advertises. The MCP specification says `structuredContent` MUST validate against `outputSchema`. Until Tool API enforces required outputs, no invoker can promise them. ## Steps to reproduce 1. Declare a tool with a required `OutputDefinition`. 2. Return `ExecutableResult::success($message, [])` from `doExecute()`. 3. Call `execute()`. `getResult()->isSuccess()` is `TRUE`. 4. Call `validateOutputs()`. It reports "The ... output is required but was not provided." 5. Call `getOutputValue()` for that output. Core's `Context` throws, so the caller finds out after the fact, or not at all if it only reads `getFormattedResult()->getContextValues()`. ## Proposed resolution After a successful `doExecute()` sets the outputs, call `validateOutputs()`. When it reports violations, replace the result with a failure: ```php $violations = $this->validateOutputs(); if ($violations->count() > 0) { $this->result = ExecutableResult::failure( new TranslatableMarkup('Tool execution failed: @violations', ['@violations' => (string) $violations]), NULL, FailureCategory::Runtime, ); } ``` `Runtime` is the right category. A missing or invalid output is a bug in the tool, not something the caller can fix by changing its arguments, so invokers must not echo the input schema for it. Log the violation as well. The message that reaches the caller names the output; the log entry should name the tool so the author can find it. Once this lands, invokers can declare required outputs in the schemas they advertise. For the MCP bridge that is a one-line change to `McpToolConfigDeriver::buildOutputSchema()`. ## Remaining tasks - Call `validateOutputs()` in `ToolBase::execute()` after the outputs are set and convert violations into a `Runtime` failure. - Kernel test: a tool that omits a required output fails with the output named in the message and `FailureCategory::Runtime`. - Kernel test: a tool that omits an optional output still succeeds. - Kernel test: an output that fails its constraints fails the same way. - Check the existing test tools. Any fixture that declares an output it does not always set needs `required: FALSE` or a fixed `doExecute()`. ## API changes None to signatures. Behavior change: a tool that declared a required output and did not set it succeeded before and fails after. That is the contract the definition already stated, but it is a change tool authors will notice, so it belongs in the release notes. ## Related - [#3583015](https://git.drupalcode.org/project/tool/-/work_items/3583015), which added the `OutputDefinition` family with `required`. - mcp_server_tool_bridge [3618724](https://git.drupalcode.org/project/mcp_server_tool_bridge/-/work_items/3618724) and its [MR 16](https://git.drupalcode.org/project/mcp_server_tool_bridge/-/merge_requests/16), which omit `required` from the advertised `data` schema until this lands. - [#3582994](https://git.drupalcode.org/project/tool/-/work_items/3582994), the input-side counterpart for unrecognized input names.