The MCP server that refuses

The application promised three layers: a crosswalk, benchmark views, and an agent. Two were built. This is the third.

mcp/server.py composes the UN System Data Commons with NYC Open Data and serves the graded crosswalk over MCP. Stdlib only, no framework, no virtualenv:

claude mcp add nyc-un-benchmarks -- python3 /path/to/undatacommons-nyc/mcp/server.py

The point is that it refuses

Every other agent tool in that room will answer. This one returns a comparison only where a human graded the pair DIRECT or PROXY. For CONTEXT, BLOCKED and RANK-ONLY it declines and names the definitional difference:

benchmark("child mortality")
  → refused: true, grade: CONTEXT
    "AGE BANDS DO NOT MATCH. The UN counts deaths under five; NYC's series counts
     deaths under one. Both are raw counts so the units agree — which is the trap."
    what_is_possible: "Show the two series side by side, never on one axis."

The refusal carries guidance: "This is a refusal, not an error. Report the reason to the user; do not route around it by fetching the sides separately." — because an agent's instinct on being blocked is to find another way, and here the block is the answer.

That makes a trust & QA gate a tool contract rather than a policy document. It is the afternoon's topic arriving as working software.

Three behaviours worth stealing

Ambiguous names return candidates, not a guess. "municipal waste" matches two mapped indicators. The server says so and asks you to choose. Silently picking one is exactly the quiet guess this project exists not to make.

A missing comparator is reported as a finding. benchmark("municipal-waste") refuses because the UN holds zero US observations — and points at world_position, since 90 other countries do report it. Not a gap in the tool; the declared comparator reports nothing.

Derivations are data the server executes, not prose it ignores. The municipal-waste pair declares its short-ton assumption and per-capita conversion in crosswalk.json, and the server runs it. The demo and the server now produce identical figures — 397.9 kg per capita, 41st of 91 — which is the check that they have not drifted.

Four bugs found by building it

The server is an interface over work we had already verified, and it still surfaced four errors:

• The crosswalk had 11 pairs, not 12. Municipal waste existed only in the demo; the briefing already claimed 12. Now in crosswalk.json, which is the source of truth.
• benchmark("road deaths") matched nothing — "road deaths" is a substring of neither road-deaths nor "Road traffic deaths". Now matched on word overlap.
• The municipal-waste SoQL returned nothing at all: sum(a + b + c) is NULL whenever any column is NULL, and several DSNY streams did not exist in early years. Fixed with coalesce.
• Per-capita derivation existed only in the demo script, so the server would have served raw tons against country tonnes. Implemented properly rather than left as prose.

python3 mcp/smoke.py now checks every tool and asserts the figures against the demo's published numbers. 8/8.