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An MCP server can stop working after an SDK update for two different reasons: your application code may no longer match the SDK’s API, or the client and server may no longer agree on protocol or transport behavior. Those are separate layers, so diagnose them separately. First identify the exact SDK version your process loaded, then check the language-specific migration guide and the protocol version the client and server negotiated.
Why did my MCP server stop working after I updated the SDK?
“The SDK changed” is not a diagnosis. A major SDK release can rename or remove classes, imports, helpers, and exception types even when the MCP protocol used by your client and server has not changed. Separately, a client and server can disagree about a protocol revision, handshake, session, capability, or transport. A failure that looks silent may be an import error, a stale type assumption, changed runtime behavior, or a connection-level mismatch.
The MCP SDK beta announcement by TypeScript SDK Lead Felix Weinberger, Python SDK Lead Max Isbey, and Lead Maintainer Den Delimarsky makes the distinction explicit: “Those are new major versions, so moving your own code onto them is a breaking change, and one you can take on your own schedule; it is separate from anything that happens on July 28.” The SDK major migration and the protocol publication date are not the same event. MCP SDK beta announcement, June 29, 2026.
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How to check which MCP SDK version your server actually loaded
- Inspect the resolved dependency, not just the manifest. Check your package manager’s lockfile and the version reported by the running environment or deployment image. A version range in a manifest does not prove which release was installed. Compare local, CI, and production resolutions if they differ.
- Identify the language and SDK major. Record whether the process uses Python, TypeScript, Go, C#, or another SDK, and whether it is on a v1 or v2 line. Migration behavior is language-specific; a rename documented for Python is not evidence of the same change in TypeScript or another language.
- Compare imports and APIs with that SDK’s official migration guide. Look for removed imports, renamed symbols, module moves, helper changes, exception changes, and altered defaults. Use the guide for the exact language and major version.
- Check the negotiated protocol era and transport. Establish what revision the client requested or discovered, what the server supports, and whether the connection used a legacy handshake, modern discovery, HTTP+SSE, or another supported transport.
- Reproduce the combinations you claim to support. Test the deployed client/server pair, then test the relevant legacy and modern combinations. Capture startup output, connection errors, protocol responses, and logs so the failure can be assigned to a layer rather than guessed.
Did the SDK rename an import or change the protocol?
Use the first observable failure to choose where to look, but do not treat it as proof by itself. A failure before the server starts usually points toward imports or application APIs. A server that starts but fails during connection needs protocol, transport, authorization, and network checks. A connection that succeeds but behaves differently may indicate changed application assumptions or behavior. The dependency version, logs, and a reproduction determine the cause.
#1 Best Overall
| Evidence | Likely layer to inspect | What to verify |
|---|---|---|
| Import or symbol not found at startup | SDK application API | Resolved SDK major, import path, renamed or removed symbol, and migration guide for that language. |
| Type-check or exception-handling failure | SDK application API | Changed types, method signatures, context access, and exception names. |
| Handshake or protocol request failure | Wire protocol or negotiation | Client and server protocol revisions, negotiation mode, server capabilities, and transport behavior. |
| Authorization, network, timeout, or unusable response | Connection and transport | Transport configuration and the specific error. Do not assume that every discovery failure means the server is legacy. |
| Connection succeeds, but runtime behavior changes | Application assumptions or behavior | Version-specific defaults and behavior documented by the relevant SDK, then compare a minimal reproduction. |
Python v2: concrete API changes to check
The official Python SDK migration guide documents several breaking v1-to-v2 changes. These examples apply to Python; they should not be projected onto other language SDKs.
- The high-level server class changed from
FastMCPtoMCPServer. The oldmcp.server.fastmcpimport path was removed, not kept as a deprecation alias. - Modules moved from
mcp.server.fastmcp.*tomcp.server.mcpserver.*. - Context access changed from
ctx.fastmcptoctx.mcp_server. get_context()was removed; the migration guide directs applications to declare aContextparameter instead.- The base exception changed from
FastMCPErrortoMCPServerError.
Search application code, tests, and plugin integrations for the old names, not just the server’s main entry point. An import that worked under the old major may fail immediately under the new one because the old path is gone rather than deprecated.
Rank #2
TypeScript v2: protocol negotiation depends on mode
The TypeScript v2 migration guide describes different connection behavior by configuration. In that guide, the default Client.connect() uses the legacy 2025 initialize handshake. Modern protocol negotiation is opt-in rather than an automatic behavior to assume in every setup. See the TypeScript v2 migration guide.
| Configuration in the guide | Behavior described | Diagnostic implication |
|---|---|---|
Default Client.connect() |
Uses the legacy 2025 initialize handshake. | A v2 SDK package does not by itself prove that the modern protocol negotiation path is active. |
mode: 'auto' |
Probes with server/discover and can fall back to the 2025 handshake in supported situations. |
Check the probe response and error category; fallback is conditional, not a response to every failure. |
{ pin: '2026-07-28' } |
Pins the modern revision and does not fall back; it rejects against a legacy-only server. | Confirm that the server supports the pinned revision before using this mode. |
The guide says network outages, HTTP authorization errors, server errors, unusable 2xx responses, and certain timeouts are surfaced as errors according to transport and configuration; they are not all treated as proof that a server is legacy. Separate a discovery failure from a confirmed legacy response before attributing the problem to protocol age.
Why does my MCP client connect to an older server but fail against a newer one?
Version labels alone do not explain the connection. The TypeScript v1 documentation describes v1.x as the maintenance line implementing MCP through 2025-11-25, and points to separate v2 @modelcontextprotocol/server and @modelcontextprotocol/client packages for the 2026-07-28 specification. That packaging and protocol distinction is specific to the TypeScript documentation. See the TypeScript SDK documentation.
Compare the precise client and server SDK versions, protocol revision, transport, and negotiation mode. For example, a client configured to pin 2026-07-28 is expected by the TypeScript v2 guide to reject a legacy-only server, whereas its default connection path uses the legacy handshake. A successful connection to one server and a failure against another is therefore a reason to inspect the actual pair and configuration, not evidence that every newer server is incompatible.
C# provides a separate, language-specific example: its v2 release notes describe probing server/discover and falling back to legacy initialize under documented circumstances, while surfacing several modern-server error codes. The notes also say stable, non-deprecated 1.x APIs continue to work without modification in compatible connections. Do not infer that all SDKs share C#’s fallback rules. See the C# SDK release notes.
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No. The MCP project’s 2026-07-28 specification announcement says the publication was not a switch-off for previous protocol implementations. At publication, Roots, Sampling, and Logging were deprecated but would continue to work for at least twelve months; new implementations were advised not to adopt them. The announcement also gave legacy HTTP+SSE a year-long offramp. Those are policy durations stated in that announcement, not measured compatibility statistics; check current project guidance before relying on a specific end date.
Best Value
The same announcement listed TypeScript, Python, Go, and C# as Tier 1 SDKs speaking the new revision at publication, with Rust support in beta. This is a publication-time status, not a guarantee about every later package release or deployment.
Quick Recap
How to make an SDK migration safer
- Keep the resolved SDK version reproducible in development, CI, and deployment; review lockfile changes rather than relying only on a broad version range.
- For a major upgrade, follow the migration guide for the actual language and package, and update imports, context use, types, and exception handling as applicable.
- Test both API compatibility and wire compatibility. A code-level migration test does not establish that legacy clients can negotiate with the server, and a successful handshake does not establish that application code uses the new API correctly.
- Write down the client/server versions and protocol revisions your production deployment supports, including whether negotiation is legacy, automatic, or pinned where the SDK documents those options.
- When an error occurs, preserve the startup trace and connection details. Classify it as an import/type error, protocol or transport error, authorization/network error, or changed runtime behavior before changing configuration.
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