fix(mcp): keep last-known-good tools instead of flashing red on a transient failure

[waleedlatif1]

Summary

A connected MCP server with cached tools was showing red "Failed to discover MCP tools" on a single transient discovery probe — even though the backend stored status was connected with a valid tool count (verified in the DB while the UI showed red). It made healthy servers look like they "fail all the time" as probes oscillated against the origin's intermittent stalls.

Two coordinated client-side display fixes:

• useMcpToolsQuery no longer discards React Query's retained data on error — it kept the last successful tools around and we were throwing them away (if (result.data && !result.isError) → if (result.data)). The per-server error still rides in toolsStateByServer.
• showDiscoveryError only hard-reds when there are genuinely no tools to show (tools.length === 0). A populated, connected server stays on its tool count through a transient blip; a persistent failure still reads as failed via the stored connectionStatus (gated by MAX_CONSECUTIVE_FAILURES).

Validated against primary sources before changing (I'd over-corrected this exact condition in #5829): the MCP spec's discover-once + tools/list_changed model, plus the actual behavior of Claude Code (which shipped this same fix as a bug removal — a transient failure no longer flags a healthy server), LibreChat (returns already-fetched tools rather than discarding; guards against overwriting with empty), OpenCode (if (!listed) return, leaves tools intact), and VS Code (persistent tool cache). None blanks a populated server on a transient tools/list failure over a live connection. This supersedes the #5829 over-correction.

Type of Change

• Bug fix (UX / correctness of display)

Testing

• New useMcpToolsQuery test: last-known-good tools survive a failing refetch (with the per-server error still exposed). 16 MCP query + settings tests green; tsc + biome clean.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		Jul 22, 2026 6:16am

[cursor]

PR Summary

Low Risk
Client-side MCP display and query aggregation only; no auth or API contract changes, with new unit tests locking behavior.

Overview
Fixes MCP settings and workflow tooling flashing red or dropping tools when a connected server’s latest tools/list probe fails but the backend still reports connected.

useMcpToolsQuery now keeps React Query’s retained per-server tool lists through a failed refetch while stored connectionStatus stays healthy, and only removes them from the aggregate once status is error or disconnected. Per-server errors still surface via toolsStateByServer.

MCP server list rows only treat a discovery error as a hard failure (showDiscoveryError) when there are no tools to show; servers with a last-known tool count keep the normal label until persistent failure updates status.

Tests cover transient refetch failure (tools preserved + error recorded) and persistent failure (stale tools cleared after list refresh shows error).

^{Reviewed by Cursor Bugbot for commit b65d49c. Configure here.}

[greptile-apps]

Greptile Summary

This PR keeps cached MCP tools visible during transient discovery failures. The main changes are:

• Preserve React Query's last successful tool data after a failed refetch.
• Show discovery errors only when no cached tools are available.
• Remove cached tools from the aggregate when the stored connection status indicates failure.
• Add tests for transient and persistent discovery failures.

Confidence Score: 5/5

No additional blocking issue qualifies for this follow-up review.

• The latest changes address the previously reported stale-tool behavior.
• No separate production failure requiring another comment was identified.

Important Files Changed

Filename	Overview
apps/sim/hooks/queries/mcp.ts	Preserves cached tools during transient failures and filters them using the stored server status.
apps/sim/app/workspace/[workspaceId]/settings/components/mcp/mcp.tsx	Keeps the tool count visible when a transient discovery error occurs and cached tools remain.
apps/sim/hooks/queries/mcp.test.tsx	Adds coverage for retaining cached tools after a failed refetch and dropping them after a stored failure status.

_{Reviews (2): Last reviewed commit: "fix(mcp): drop stale tools once a server..." | Re-trigger Greptile}

[waleedlatif1]

@greptile

[waleedlatif1]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit b65d49c. Configure here.}