René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[FEATURE] MCP: fix empty-result + nullable-render bugs, add registry discovery/pull tools","articleBody":"Two fixes and two new tools, bundled because they form one coherent unit: the\nfixes harden the result path that the new tools depend on, and together they\nclose the discover -\u003e pull -\u003e query loop for agents (today an agent can only\nuse providers a human has already pulled from a shell).\n\n### Fix 1 - empty result sets reported as extraction failures\n`extractQueryResults` (`internal/stackql/mcpbackend/mcp_service_stackql.go`)\nreturns `(rv, ok \u0026\u0026 len(rv) \u003e 0)`, and the caller maps a false second return to\n`fmt.Errorf(\"failed to extract query results\")`. A zero-row result is a valid\nempty list. Any `list_*` / `run_select_query` returning no rows errors instead\nof rendering empty. Repro: fresh approot, no providers pulled, `list_providers`\n-\u003e \"failed to extract query results\".\nFix: return `(rv, ok)`; let the renderer's `**no results**` branch handle empty.\n\n### Fix 2 - literal/expression columns render as Go nullable wrappers\n`RenderTable`/`dataRow` and `RenderKV` (`pkg/mcp_server/render/render.go`)\nformat with `fmt.Sprintf(\"%v\", v)` assuming scalars. Literal columns carry a\npointer-to-nullable-wrapper, so cells render as `\u0026{ok true}`. Repro:\n`run_select_query` `SELECT 1 as n, 'ok' as status` -\u003e `| \u0026{ok true} | \u0026{1 true} |`.\nProvider-backed columns are unaffected (unwrapped upstream).\nFix: unwrap helper (check valid, take value) applied before formatting in\n`dataRow` and the `RenderKV` loop.\n\n### Feature 1 - list_registry tool\nLets an agent see providers available in the registry to pull, distinct from\nlist_providers (which shows only pulled/installed providers).\n- Renderer: Table. Inputs: optional `provider` (lists versions for that\n  provider; omitted lists all available).\n- New interrogator method `GetRegistryList(provider string)` building\n  `REGISTRY LIST [\u003cprovider\u003e]`; backend `ListRegistry` routes through\n  `runPreprocessedQueryJSON`.\n- Registered with `selectGate(\"list_registry\")` - read-only, allowed in all\n  modes.\n\n### Feature 2 - pull_providers tool\nLets an agent install a provider into the approot cache so subsequent queries\nresolve.\n- Renderer: KV ({messages, timestamp}). Inputs: `provider` (required),\n  `version` (optional).\n- New interrogator method `GetRegistryPull(provider, version string)` building\n  `REGISTRY PULL \u003cprovider\u003e [\u003cversion\u003e]`; backend `PullProvider` routes through\n  `execQuery`.\n- Gating decision (flagged for review): pulling fetches remote content and\n  writes local state but does not touch any cloud control/data plane. Proposed\n  default: allow in all modes (it is not a cloud mutation). If a human-in-the-\n  loop is wanted, classify it as a new `registry_pull` query class gated like\n  lifecycle (refused in read_only, elicitation-prompted in safe). Defaulting to\n  allow keeps autonomous agent flows unblocked.\n\n### Why CI didn't catch the fixes\nNo test asserts on an empty result set, and query/metadata tests assert on\nstructuredContent JSON + row counts, never on rendered cell values; every test\nSELECT hits a real provider table, so no test runs the literal/no-FROM shape\nthat exposes the wrapper.\n\n### Tests added (test/robot/functional/mcp.robot + render_test.go)\n- Empty-result `list_providers`/SELECT renders cleanly, no error.\n- Literal SELECT cell equals `ok`, does not contain `\u0026{`.\n- Unit: nullable wrapper value renders as unwrapped scalar.\n- `list_registry` returns a non-empty available-provider set (against the test\n  registry).\n- `pull_providers` for a known provider returns a timestamp; the provider is\n  then visible via `list_providers`.\n\n### Impact / risk\nLow. Fixes are additive on the read path; provider-backed output unchanged. New\ntools are additive. Only open decision is the pull_providers gating class.","author":{"url":"https://github.com/jeffreyaven","@type":"Person","name":"jeffreyaven"},"datePublished":"2026-05-29T06:15:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":12},"url":"https://github.com/661/stackql/issues/661"}