MCP: a missing, misnamed or mistyped tool argument answers only "An error occurred invoking '<tool>'."; an unknown tool name answers "Access denied ... Required permission: none"

[Justinvolved]

Describe the bug

When a tools/call request does not match the tool's inputSchema, the MCP server returns a tool error whose only text is An error occurred invoking '<tool>'. It does not name the missing member, the unexpected member or the type mismatch. The same text appears for a required member that is absent, for a member sent under the wrong name, and for a value of the wrong type. An agent cannot tell a malformed call from a server fault, so it retries blindly or concludes the tool is broken.

Two related message problems:

• A tool name the server does not register answers Access denied. MCP configuration '<name>' is not allowed to call tool '<tool>'. Required permission: none. Allowed permission: none. instead of an unknown-tool error. This happens for an invented name as well, so it reads as a permission problem when the tool does not exist.
• A member the schema does not declare is silently ignored and the call reports success (for example update_orders with orderDate, see the separate report on order dates).

To Reproduce

1. tools/list. Note that get_assortment_relations requires assortmentId, get_paragraphs_by_page_id requires pageId (integer) and get_pages_by_ids requires pageIds (integer array).
2. tools/call get_assortment_relations {}.
3. tools/call get_paragraphs_by_page_id {"id": <existing page id>} (wrong member name).
4. tools/call get_paragraphs_by_page_id {"pageId": "abc"} (wrong type).
5. tools/call get_pages_by_ids {"id": <existing page id>} (wrong member name).
6. Control: tools/call get_paragraphs_by_page_id {"pageId": <same id>} and get_pages_by_ids {"pageIds": [<same id>]} return data.
7. tools/call get_product_by_id {"id": "<product id>"} (not registered; get_products_by_ids is) and tools/call no_such_tool_xyz {}.

Observed (DW 10.28.11, MCP add-in 0.6.0-BETA, a configuration with Full access):

get_assortment_relations {}                      -> isError: true, "An error occurred invoking 'get_assortment_relations'."
get_paragraphs_by_page_id {"id":8517}            -> isError: true, "An error occurred invoking 'get_paragraphs_by_page_id'."
get_paragraphs_by_page_id {"pageId":"abc"}       -> isError: true, "An error occurred invoking 'get_paragraphs_by_page_id'."
get_pages_by_ids {"id":8517}                     -> isError: true, "An error occurred invoking 'get_pages_by_ids'."
save_user_groups {"items":[...]}  (schema: groups) -> isError: true, "An error occurred invoking 'save_user_groups'."
get_paragraphs_by_page_id {"pageId":8517}        -> {"count":1,"data":[...]}
get_pages_by_ids {"pageIds":[8517]}              -> {"count":1,"data":[{"id":8517,...}]}

get_product_by_id {"id":"<product id>"} -> "An error occurred invoking 'get_product_by_id': Access denied. MCP configuration '<name>' is not allowed to call tool 'get_product_by_id'. Required permission: none. Allowed permission: none."
no_such_tool_xyz {}                    -> same text for 'no_such_tool_xyz'

Expected behavior

• A call that fails schema validation returns JSON-RPC -32602 Invalid params (or a tool error) that names the member: missing required argument 'assortmentId', unknown argument 'id'; expected 'pageId', 'pageId' must be an integer.
• An unregistered tool name returns an unknown-tool error (JSON-RPC -32602 "Unknown tool", as the MCP specification suggests), not an access denial with "Required permission: none".
• A member the schema does not declare is rejected, or at least reported back, rather than ignored while the call reports success.

Environment

• Dynamicweb 10.28.11 (.NET 10 hosting)
• MCP add-in 0.6.0-BETA over HTTP JSON-RPC (/admin/mcp), API-key configuration with Full access