Incorporates two enhancements from upstream uscensusbureau/us-census-bureau-data-api-mcp, adapted to this fork's existing response-shape and error machinery rather than applied verbatim. Upstream's #145 is stacked on #144, so both arrive together.

uscensusbureau#145 — ACS 1-year population threshold (upstream PR)

ACS 1-year estimates only exist for areas of 65,000+ people; smaller areas return a bare 400 with no explanation — a common source of confusion for the model.

• buildApiErrorMessage now detects acs/acs1 datasets and, on a 400, leads with the threshold explanation + an acs/acs5 suggestion before the generic discovery-tool guidance. Folded into our existing error helper instead of upstream's standalone inline branch.
• Tool description gains a concise threshold note, consistent with our caveats-led description style.
• Detector narrowed to acs/acs1 (upstream also matched bare acs1, which risks false positives).

uscensusbureau#144 — token management (upstream PR)

Upstream replaces pretty-printed JSON (JSON.stringify(data, null, 2)) with compact output to save ~30–40% of response tokens — relevant to our token-constrained 4o/Copilot consumer.

• Our response-format.ts already replaced pretty-printed JSON in resolve-geography-fips and search-data-tables with numbered Record blocks, which exceeds feat: add content flattener for improved token management uscensusbureau/us-census-bureau-data-api-mcp#144's goal — left unchanged.
• fetch-dataset-geography was the only tool still emitting indented JSON; switched to compact. Its payload (ParsedGeographyEntry[]) contains none of the fields upstream's flattener strips, so the standalone content-flattener.ts helper is intentionally not vendored (it would add a competing abstraction for zero extra benefit here).

Testing

• tsc --noEmit clean.
• 35 affected unit tests pass, including a new test for the ACS-1yr 400 path.
• Integration tests require a Docker Postgres not available in this environment; the pure unit suites (mocked DB/fetch) were run in isolation.

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