The mechanism
A migration should begin with captured, redacted behavior from the existing client and server. Identify protocol era, transport and optional features before changing code. Do not mix a modern request envelope with a legacy session lifecycle and hope the SDK smooths it over.
The 2026 edition introduces per-request metadata, explicit result types, discovery, modern input continuation and cache hints, while removing or deprecating several older patterns. A migration plan should name each affected assumption and the tests that establish replacement behavior.
Maintain legacy support only for a concrete compatibility requirement. A fallback path increases the test matrix and can conceal configuration mistakes. Prefer an explicit, observable selection of era over a chain of silent retries.
Worked example
This is a migration checklist, not a script. It is safe to review without connecting to an existing host. The official change log is the authoritative source for exact requirements; SDK release notes determine how those requirements appear in your package.
initialize/session assumptions -> per-request metadata and supported-version handling
old server-initiated requests -> MRTR continuation
standalone HTTP GET stream -> explicit subscription request
implicit result completion -> resultType handling
uncategorized result caching -> TTL + scope + authorization context
roots/sampling/logging usage -> review deprecation and replacement designPractice: predict, inspect, explain
Offline exercise. Take the chapter 2 compatibility worksheet and add two clients: modern-only and legacy-only. Define expected outcomes against a modern server and a retained legacy server. Include unsupported version, unknown method and timeout cases in the stdio discovery probe.
Expected observation: a test report should show which peer combinations are supported and which fail clearly. A legacy fallback is not evidence that modern semantics were exercised. Preserve one known-good transcript per supported combination with tokens and private content removed.
Troubleshooting and trade-offs
If migration “works” only after ignoring errors, stop and compare the exact contract. A modern unsupported-version response identifies a modern peer; do not automatically switch it to legacy initialization. If an old dependency controls the transport, upgrade through its documented migration path and retest. Avoid marketing the latest protocol until the executed matrix supports that statement.
Interview practice
Why can silent fallback hide defects?
It can make a broken modern path appear successful by using a different protocol era. Observability and explicit compatibility tests reveal which contract actually ran.
What should survive the migration?
Authorized business behavior, data boundaries and user-visible outcomes, backed by equivalent positive and negative tests. The wire and lifecycle details may change.
Completion check
Produce a peer matrix with supported, rejected and untested cells, plus evidence for each supported cell.
Sources and version notes
This edition targets MCP 2026-07-28, checked 6 October 2026. SDK examples are version-sensitive and labelled when not executed. Synthetic fixtures are learning material, not protocol conformance evidence.
- Official documentation: Changelog
- Official documentation: Deprecated
- Official documentation: Versioning
- Official documentation: Stdio
Make the understanding yours.
Use the completion check above. Mark this chapter when you can explain the mechanism and its limits.
Self-assessed reading progress. This does not certify that a lab ran or a system is secure.