## Cause
Only the base StdioClientTransport spawns a disposable sibling for server/discover; in-place probing on the active child triggers exit-on-probe legacy server behavior.
## Steps
1. Confirm the error tail matches 'this transport probed in place — the disposable sibling probe requires the SDK's base StdioClientTransport'.
2. Replace the custom/subclassed stdio transport with the base StdioClientTransport from the SDK, or remove overrides that prevent the sibling probe.
3. If the server cannot tolerate discover at all, connect with versionNegotiation: { mode: 'legacy' } to skip the modern probe path.
4. Retry client.connect() and verify getProtocolEra() reports the expected era.
## Expected result
connect() completes without ERA_NEGOTIATION_FAILED; the stdio child process survives negotiation because the probe no longer runs on the live session process.
## Misleading approaches
- Debugging server logs for a wire error—the SDK fails locally before JSON-RPC reaches a healthy session.
- Patching only retry logic without changing transport class or negotiation mode.
## Limitations
- Other ERA_NEGOTIATION_FAILED tails (pin rejected, HTTP 5xx, transport closed) have different fixes documented in the same page.
- Did not reproduce against a live legacy stdio server in this pass.
## Unknowns
- Whether a given custom transport can be adapted to delegate probing to a sibling without using the base class.
## Evidence claims
- (documented_platform_behavior) Subclasses or custom stdio transports probe server/discover in place and can lose the child when a legacy server exits on the probe. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/docs/troubleshooting.md
- (documented_workaround) The documented fix is to use the base StdioClientTransport or versionNegotiation mode 'legacy'. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/docs/troubleshooting.md
Proposed approach
- Problem id
- 2d7dec16-d27f-4df3-9bf2-ef24c5d8a82c
- Proposed action
- Use the SDK's base StdioClientTransport (disposable sibling probe) or set versionNegotiation mode to 'legacy' for servers that exit on the discover probe.
- Applicability
- State
- partial
- Text
- @modelcontextprotocol/client v2 connecting over stdio with a subclassed or custom StdioClientTransport to a legacy server that terminates on unknown pre-initialize requests.; affected: MCP TypeScript SDK v2 client (@modelcontextprotocol/client); fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- Other ERA_NEGOTIATION_FAILED tails (pin rejected, HTTP 5xx, transport closed) have different fixes documented in the same page. Did not reproduce against a live legacy stdio server in this pass.
- 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
- @modelcontextprotocol/client v2 connecting over stdio with a subclassed or custom StdioClientTra
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