## Problem/Motivation `tools/list` never carries an `outputSchema` for a bridged tool, and `tools/call` returns a result whose shape is nowhere described. An MCP client receives a text blob to parse and a `structuredContent` envelope with no schema, and has to guess a structure that Tool API already knows exactly. Tool API has had typed outputs since 1.0.0-beta9: `OutputDefinition` and its map, list, and entity variants, and `ToolDefinitionSerializer::normalizeOutputDefinitions()`, which emits JSON Schema for them the same way `normalizeInputSchema()` does for inputs. The bridge uses the input half and drops the output half. Where it happens: - `McpToolConfigDeriver::createDerivativeDefinition()` builds `inputSchema` with `buildInputSchema()`, but passes `outputSchema: $base_plugin_definition->outputSchema`, which is the base plugin's `NULL` for every derivative. - `ToolApi::execute()` returns `['success' => ..., 'message' => ..., 'data' => $formatted->getContextValues()]`, so the wire shape is an envelope around the tool's outputs. Nothing describes the envelope or its `data`. This matters most for tools with structured outputs: a value object with nested members, or a list of them, comes out as a `data` object whose keys and types the client cannot know in advance. ## Steps to reproduce 1. Expose any Tool API tool with declared outputs through an `mcp_tool_config` entity. `tool_test`'s `greeting_tool` (one string output, `message`) is enough. 2. Call `tools/list`. The tool has `inputSchema` and no `outputSchema`. 3. Call `tools/call`. The result has `content` with the JSON as text and `structuredContent` with `success`, `message`, and `data`, and nothing says what `data` contains. ## Proposed resolution 1. In `McpToolConfigDeriver`, build `outputSchema` the same way as `inputSchema`: create the tool with `McpServerToolBridgeInvoker`, so `ToolOutputDefinitionNormalizeEvent` subscribers apply and entity outputs are described as handle strings, and call `normalizeOutputDefinitions()`. 2. Describe the envelope, not the bare outputs, because that is what `structuredContent` carries: ```json { "type": "object", "properties": { "success": {"type": "boolean"}, "message": {"type": "string"}, "data": { "type": "object", "properties": { ...normalizeOutputDefinitions()... }, "required": [ ...required outputs... ] } }, "required": ["success", "message", "data"] } ``` `input_schema`, which `execute()` adds on a correctable failure, is optional in the envelope schema. 3. Make every `tools/call` result conform. The MCP specification says that when a tool declares `outputSchema`, `structuredContent` MUST validate against it, including on failure. Today a failure returns `'data' => []`, which encodes as a JSON array, not the object the schema promises. Encode an empty `data` as `{}`. This is the same class of problem as #3604059, where empty `properties` encoded as `[]`. 4. Decide whether a tool with no declared outputs gets an `outputSchema` with an empty `data` object or no `outputSchema` at all. The spec makes `outputSchema` optional, and declaring one commits every response to it, so omitting it for output-less tools is the safer default. ## Remaining tasks - Implement the deriver change and the envelope schema. - Fix empty `data` encoding on the failure paths in `ToolApi`. - Kernel test: `tools/list` carries `outputSchema` for `greeting_tool` with `data.properties.message` of type `string`; `tools/call` result validates against it, for a success and for an access-denied failure. - Test a multiple entity output to confirm the invoker-aware normalization describes it as handle strings, matching what `getFormattedResult()` returns. ## API changes None to the bridge's own PHP API. On the wire, bridged tools gain `outputSchema`, and failure results change `data` from `[]` to `{}`. ## Related - Tool API work item 3518120 and its draft MR !162, where typed outputs from value objects and return shapes are generated. Without `outputSchema` on this side, none of that reaches an MCP client. - Tool API #3583015, which added the `OutputDefinition` family. - #3613896, which integrated `normalizeInputSchema()` and left outputs. - #3618716, `isError` on the same envelope. - #3604059, empty `properties` encoding as `[]`.