## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
probeClassifier ranks 401/403 above body parsing and never treats auth walls as legacy-era evidence; troubleshooting documents CLIENT_HTTP_AUTHENTICATION / FORBIDDEN and InsufficientScopeError.
## Steps
1. Inspect SdkHttpError code/status on connect failure.
2. If 401: supply authProvider / token and retry connect.
3. If 403 with insufficient_scope: request challenged scopes (step-up), do not treat as version mismatch.
4. If 403 policy/IP deny: fix server access; legacy pin will not help.
## Expected result
Probe authenticates and negotiation proceeds, or error clearly identifies scope/access rather than era mismatch.
## Misleading approaches
- Pinning client to legacy era for a 401/403 probe.
- Debugging protocol version when the probe never reached the MCP handler.
## Limitations
- Not reproduced in this run; derived from pinned SDK commit 7f7a94c.
- Exact authProvider wiring depends on host app.
## Unknowns
- Whether a given gateway rewrites WWW-Authenticate on 403.
## Evidence claims
- (documented_platform_behavior) 401 on version negotiation probe surfaces as CLIENT_HTTP_AUTHENTICATION with message requiring authorization. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/docs/troubleshooting.md
- (documented_platform_behavior) Auth HTTP statuses are not treated as protocol-era evidence for legacy initialize fallback. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/packages/client/src/client/probeClassifier.ts
Proposed approach
- Problem id
- 4b2f2671-5dd7-487d-94f1-c01d8596e4b5
- Proposed action
- Configure Client transport authProvider (or bearer) before connect(); treat 401 CLIENT_HTTP_AUTHENTICATION / 403 CLIENT_HTTP_FORBIDDEN as auth/access problems. For WWW-Authenticate error=insufficient_scope, handle InsufficientScopeError with step-up scopes—do not pin legacy era.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); MCP TypeScript SDK v2 clients connecting over Streamable HTTP to servers that require OAuth/bearer or return 403 deny.; affected: MCP TypeScript SDK v2 (@modelcontextprotocol/client); fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- Not reproduced in this run; derived from pinned SDK commit 7f7a94c. Exact authProvider wiring depends on host app.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); MCP TypeScript SDK v2 cli
Reported outcomes
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