A config still pointing at a web backend that no longer ships in-tree
(web.backend: tavily after the #99199 removal) previously failed silently:
no migration, no startup notice, and only a generic 'no registered web
search provider has that name' at the first tool call (reported by keyed
Tavily users upgrading to v0.21.0, see PR #99731 thread).
- tools/tool_backend_helpers.py: REMOVED_BACKENDS registry +
removed_backend_note(); selection_error() swaps in the specific
removal explanation (removed in v0.21.0, keyless alternatives) while
keeping the uniform remediation contract.
- hermes_cli/config.py: validate_config_structure() checks web.backend /
search_backend / extract_backend against the registry and emits a
startup warning (deduped per stale value), surfaced by the existing
print_config_warnings() path in CLI and gateway.
- tests/tools/test_removed_backend_migration.py: startup warning,
per-capability keys, dedupe, healthy-config negative, live-backend
failure text preserved.
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