# AN-2026-0864: MCP TypeScript SDK v2 client connect() fails with SdkError ERA_NEGOTIATION_FAILED or "Version negotiation failed: the server requires authorization (HTTP 401)" when the protocol-era probe fails

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- id: AN-2026-0864 (version 1; cite https://breakwire.org/claim/AN-2026-0864/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://ts.sdk.modelcontextprotocol.io/v2/troubleshooting#:~:text=ERA_NEGOTIATION_FAILED>
- more sources: <https://ts.sdk.modelcontextprotocol.io/v2/migration/support-2026-07-28> <https://github.com/modelcontextprotocol/typescript-sdk/releases/tag/%40modelcontextprotocol%2Fserver%402.0.0>
- event date: 2026-07-27
- services: mcp-typescript-sdk, mcp, nodejs
- affected: npm:@modelcontextprotocol/server >=2.0.0; npm:@modelcontextprotocol/client >=2.0.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The troubleshooting page says SdkError: ERA_NEGOTIATION_FAILED means connect() found no protocol era both sides speak or the probe was cut short. A pinned version the server does not offer ("the server did not offer pinned protocol version ... via server/discover (no fallback in pin mode)") never falls back: drop the pin or use versionNegotiation mode 'auto'. A modern-only supportedProtocolVersions list removes the legacy fallback ("this client supports no pre-2026-07-28 protocol version to fall  [✓0 ✗0]
1. The page says "Version negotiation probe failed: the server answered with an unusable reply" means something in front of the server answered the probe with a 2xx that is not a usable reply (check the URL; pass connect(transport, { prior: { kind: 'legacy' } }) or mode 'legacy'), and "the server answered the probe with HTTP 5xx" is a server/proxy failure, not era evidence.  [✓0 ✗0]
2. The page says "Version negotiation failed: the server requires authorization (HTTP 401)" is SdkHttpError CLIENT_HTTP_AUTHENTICATION when no authProvider is configured (pass authProvider: { token: async () => key } or an OAuthClientProvider), and "Version negotiation failed: the server denied access (HTTP 403)" (CLIENT_HTTP_FORBIDDEN) is an access refusal, not a protocol mismatch.  [✓0 ✗0]

## Fix
```text
Use `versionNegotiation: { mode: 'auto' }` (the default falls back to the 2025 initialize handshake) unless a hard failure is wanted; keep a pre-2026 entry in supportedProtocolVersions; for 401 pass an `authProvider`; for an unusable probe reply check the URL or use mode 'legacy'.
```

## Error signatures
- substring: `ERA_NEGOTIATION_FAILED` (troubleshooting page heading (SdkError code))
- substring: `Version negotiation failed: the server did not offer pinned protocol version` (troubleshooting page example output)
- substring: `Version negotiation failed: the server requires authorization (HTTP 401)` (troubleshooting page heading)
- substring: `Version negotiation failed: the server denied access (HTTP 403)` (troubleshooting page)
- substring: `Version negotiation probe failed: the server answered with an unusable reply` (troubleshooting page)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0864/reports_

## History
- v1 2026-11-05 by wire-editor: filed from primary sources opened 2026-11-04 (MCP TS SDK v2 upgrade guide, troubleshooting, SDK source at server@2.4.0); strings reproduced on 2.4.0