[waleedlatif1]

Summary

• An MCP server whose hostname no longer resolves (e.g. an expired tunnel URL) made /api/mcp/tools/discover answer 500 and log at error: McpDnsResolutionError from the SSRF pre-check fell through isMcpServerUnavailableError and categorizeError
• isMcpServerUnavailableError now treats it as server-unavailable: discover answers 422 + warn, v2 GET /api/v2/mcp-servers/{id}/tools answers its documented 503
• categorizeError maps it to 502 (Server hostname could not be resolved) and McpSsrfError / McpDomainNotAllowedError to 403, matching test-connection and server lifecycle; reaches discover refresh, servers/[id]/refresh, and MCP tool execution
• Messages stay generic constants; no hostname or address reaches the caller. Failure status, negative cache, and the discovery retry predicate are unchanged (they never keyed on these helpers)

Type of Change

• Bug fix

Testing

• Regression cases in discover/route.test.ts (DNS → 422, SSRF and domain refusal → 403) and v2/mcp-servers/[mcpServerId]/tools/route.test.ts (DNS → 503); all four fail on the pre-fix code (500) and pass with the fix
• Targeted vitest: app/api/mcp/tools, app/api/mcp/servers/[id]/refresh, app/api/v2/mcp-servers, lib/internal/mcp, lib/mcp/utils.test.ts
• biome, check:api-validation:strict, check:comment-hygiene, check:test-patterns; full suite, type-check, and audits run in CI

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• 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
docs	Skipped		Oct 23, 2026 7:58pm UTC

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

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[greptile-apps]

Confidence Score: 5/5

[Medium impact] This PR appears safe to merge.

Summary

Classifies MCP DNS failures and blocked server URLs without returning their raw error messages.

• MCP hostname and egress errors get distinct responses.

No actionable issues found. Tests were inspected but not run.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  DNS[McpDnsResolutionError] --> Unavailable[isMcpServerUnavailableError]
  Unavailable --> Discover[Discovery GET: 422]
  Unavailable --> V2[V2 tool listing: 503]
  DNS --> Categorize[categorizeError]
  Blocked[McpSsrfError or McpDomainNotAllowedError] --> Categorize
  Categorize --> DNSResponse[DNS: 502 with generic message]
  Categorize --> PolicyResponse[Egress refusal: 403 with generic message]

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_{Reviews (1) · Last reviewed commit: "fix(mcp): classify unresolvable and egre..." · Reviewed by Greptile}