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Versioning and Compatibility changedspecification/2026-07-28/basic/versioning

Nearest release: v2.1.284, published 4 hours before upstream edited the page. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.

Upstream edited this page at 28 Sep 2026 21:57 UTC, give or take a minute or two: the time comes from Anthropic’s own sitemap rather than from a commit. This site recorded the change at 28 Sep 2026 22:07 UTC.

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159159The following matrix summarizes the expected outcome of every combination of
160160client and server era:
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162| Client | Server | Outcome |
163| -------- | -------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
164| Modern | Modern | Works. `server/discover` is optional; version mismatches surface as `UnsupportedProtocolVersionError` and the client retries with a mutually supported version. |
165| Modern | Legacy | Fails. The server may reject the request with an implementation-defined error, stay silent, or even process an era-ambiguous method under legacy semantics. On stdio, clients **SHOULD** send `server/discover` first to fail deterministically; the client then surfaces an actionable error to the user. |
166| Dual-era | Modern | Works. The stdio probe returns a `DiscoverResult` (or `UnsupportedProtocolVersionError`); on HTTP, the first modern request succeeds or returns a modern error. The client stays modern. |
167| Dual-era | Legacy | Works. stdio: the probe returns a non-modern error or times out, and the client falls back to `initialize`. HTTP: the modern request returns a `4xx` without a recognized modern error body, and the client falls back to `initialize` (and possibly further to the deprecated HTTP+SSE transport). |
168| Legacy | Modern | Fails. stdio: the server rejects `initialize` with a JSON-RPC error; the exact code is implementation-defined (`initialize` is an unknown method and the request also lacks the required `_meta` fields). HTTP: the request is missing the required headers and is rejected per [server validation](/specification/2026-07-28/basic/transports/streamable-http#server-validation) with `400 Bad Request` (a client on the deprecated HTTP+SSE transport fails at its opening `GET` instead). Legacy clients have no fall-forward mechanism. |
169| Legacy | Dual-era | Works. The server answers `initialize` and serves the client according to the negotiated legacy revision. |
170| Legacy | Legacy | Works according to the legacy revision; out of scope for this document. |
162| Client | Server | Outcome |
163| - | - | - |
164| Modern | Modern | Works. `server/discover` is optional; version mismatches surface as `UnsupportedProtocolVersionError` and the client retries with a mutually supported version. |
165| Modern | Legacy | Fails. The server may reject the request with an implementation-defined error, stay silent, or even process an era-ambiguous method under legacy semantics. On stdio, clients **SHOULD** send `server/discover` first to fail deterministically; the client then surfaces an actionable error to the user. |
166| Dual-era | Modern | Works. The stdio probe returns a `DiscoverResult` (or `UnsupportedProtocolVersionError`); on HTTP, the first modern request succeeds or returns a modern error. The client stays modern. |
167| Dual-era | Legacy | Works. stdio: the probe returns a non-modern error or times out, and the client falls back to `initialize`. HTTP: the modern request returns a `4xx` without a recognized modern error body, and the client falls back to `initialize` (and possibly further to the deprecated HTTP+SSE transport). |
168| Legacy | Modern | Fails. stdio: the server rejects `initialize` with a JSON-RPC error; the exact code is implementation-defined (`initialize` is an unknown method and the request also lacks the required `_meta` fields). HTTP: the request is missing the required headers and is rejected per [server validation](/specification/2026-07-28/basic/transports/streamable-http#server-validation) with `400 Bad Request` (a client on the deprecated HTTP+SSE transport fails at its opening `GET` instead). Legacy clients have no fall-forward mechanism. |
169| Legacy | Dual-era | Works. The server answers `initialize` and serves the client according to the negotiated legacy revision. |
170| Legacy | Legacy | Works according to the legacy revision; out of scope for this document. |
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172172A dual-era **server** selects its behavior from how the client opens:
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