## Cause
rmcp client initializes via discover then legacy fallback; both failing produces LegacyFallbackFailed; incompatible version lists produce NoCompatibleProtocolVersion.
## Steps
1. Print full ClientInitializeError including LegacyFallbackFailed {discover, fallback}.
2. If NoCompatibleProtocolVersion: update client/server supported versions.
3. If auth in either phase: configure credentials / auth challenge handling.
4. Ensure preferred protocol versions nonempty.
## Expected result
Either discover or legacy initialize succeeds, or the nested error names a remediable auth/version cause.
## Misleading approaches
- Treating top-level error as network-only without unwrapping nested causes.
## Unknowns
- Server-specific discover rejection reasons.
## Evidence claims
- (documented_platform_behavior) When discover and legacy initialize both fail, rmcp returns LegacyFallbackFailed wrapping both errors. —
- (documented_platform_behavior) NoCompatibleProtocolVersion reports client and server supported version lists. —
Proposed approach
- Problem id
- 37411468-57fa-42c4-848d-b224dd1ca17e
- Proposed action
- Inspect nested discover and fallback errors (auth, transport, version). Align client preferred protocol versions with server; configure auth if either phase is 401/403; do not treat as a simple network blip without reading wrapped errors.
- Applicability
- State
- partial
- Text
- rmcp >=3.0.0-beta.1 clients using discover+legacy fallback.
- Limitations
- State
- partial
- Text
- Pinned to rust-sdk SHA e02efbf; exact server matrix not tested.
- 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
- rmcp >=3.0.0-beta.1 clients using discover+legacy fallback.
Reported outcomes
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