A custom argument validator that raises something other than
ValidationError escaped Tool.run unwrapped, losing the "Error executing
tool" prefix and the UnexpectedToolError type. It is now wrapped as a
crash, and an MCPError raised there still passes through.
A ResourceError (usually ResourceNotFoundError from ctx.read_resource)
that escapes a tool body is now classified like a ToolError, since it is
the same anticipated outcome resources/read logs at INFO. An
UnexpectedResourceError escaping a tool stays a crash.
MCPServer.read_resource is now the single place a resource crash is
wrapped (plus create_resource for templates), so the built-in Resource
types let the original exception propagate to direct callers.
Also: trimmed raise-site comments in favour of the exception docstrings,
reworded the ToolError and ResourceError docstrings, documented the
FunctionResource/FileResource.read change in migration.md, corrected the
uri-templates tip and example, and pinned the new cases in tests
(including a wire test for ResourceNotFoundError from a static resource).
A crashing tool used to leave no server-side trace: _handle_call_tool
turned the exception into an is_error result before the dispatcher
boundary could log it, so a KeyError('id') reached the model as "'id'"
and its traceback existed nowhere. Resources logged once and prompts
twice. Tool.run also re-wrapped a deliberate ToolError, so nothing
downstream could tell an anticipated failure from a crash.
Tool.run now validates arguments first (a schema rejection is a plain
ToolError chained to the ValidationError) and runs the body under an
except ladder that keeps the distinction in the type: a deliberate
ToolError stays a ToolError, anything else becomes the new
UnexpectedToolError. Both keep the "Error executing tool X: " text, so
results are byte-identical. Resources get the matching
UnexpectedResourceError, raised by whichever layer first sees the
foreign exception so __cause__ is always the original.
_log_handler_exception in server.py is the one place tools and
resources are logged: INFO without a traceback for ToolError and
ResourceError (deliberate, unknown name, bad arguments, not found),
ERROR with the traceback for anything else. get_prompt stops logging,
leaving the dispatcher boundary's record as the only one.
ResourceError raised from a static resource now passes through to the
client as it already did from a template.
Max Isbey