[DawnofGenX]

Problem

Fixes #4914

The fetch prompt passes its url argument straight to fetch_url, which converts only httpx.HTTPError. httpx.InvalidURL is not an HTTPError, so it escapes the prompt's except McpError handler; with raise_exceptions=False the SDK answers with a generic JSON-RPC error, code 0, carrying the raw exception text. A client cannot distinguish a bad argument from a server fault.

The fetch tool already handles this correctly by validating through the Fetch model and raising INVALID_PARAMS.

Before

{"jsonrpc": "2.0", "id": 3, "error": {"code": 0, "message": "Invalid port: ':1'"}}

After

{"jsonrpc": "2.0", "id": 3, "error": {"code": -32602, "message": "1 validation error for Fetch\nurl\n  Input should be a valid URL, invalid IPv6 address ..."}}

Fix

Validate the prompt's url through the same Fetch model the tool uses, before fetching:

try:
    url = str(Fetch.model_validate({"url": arguments["url"]}).url)
except ValidationError as e:
    raise McpError(ErrorData(code=INVALID_PARAMS, message=str(e)))

model_validate is used rather than Fetch(url=...) because a direct constructor call makes pyright complain about the model's required-with-default fields; behaviour is identical.

Test plan

• New test TestGetPrompt::test_invalid_url_raises_invalid_params — drives get_prompt through a real ClientSession over in-memory transports and asserts error.code == INVALID_PARAMS
• New test TestGetPrompt::test_valid_url_returns_fetched_content — regression coverage for the happy path
• Differential proof: reverting only the fix hunk makes the new test fail with ErrorData(code=0, message="Invalid port: ':1'"), i.e. the reported bug
• uv run --no-sync pytest in src/fetch: 22 passed
• uv run --no-sync pyright: 0 errors, 0 warnings
• uv run --no-sync ruff check .: all checks passed

[DawnofGenX]

Closing due to inactivity — 4d idle, no review. Feel free to reopen if still wanted.