Tighten draft protocol negotiation boundaries (#1692)
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
This commit is contained in:
Stephen Halter
GitHub
parent
a8d374b280
commit
fd38f51e27
6 changed files with 1755 additions and 589 deletions
|
|
@ -175,7 +175,7 @@ Here's an example implementation of how a console application might handle elici
|
|||
[MRTR](xref:mrtr) is the SEP-2322 mechanism for server-driven input requests, finalized in protocol revision `2026-07-28`. In that revision, the server-to-client `elicitation/create` request method is removed; the recommended way to ask the user for input from a server handler is to throw <xref:ModelContextProtocol.Protocol.InputRequiredException> and let the SDK emit an <xref:ModelContextProtocol.Protocol.InputRequiredResult> on the wire.
|
||||
|
||||
> [!IMPORTANT]
|
||||
> `ElicitAsync` throws `InvalidOperationException("Elicitation is not supported in stateless mode.")` whenever the server is running stateless — which includes every Streamable HTTP server under `2026-07-28` once that revision is forced to stateless-only in a future PR. Stdio servers and current-protocol stateful Streamable HTTP servers continue to work via the legacy server-to-client `elicitation/create` request flow. For code that needs to run on stateless servers — including all `2026-07-28` Streamable HTTP servers going forward — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
> `ElicitAsync` throws `InvalidOperationException("Elicitation is not supported in stateless mode.")` whenever the server is running stateless — including Streamable HTTP requests served under `2026-07-28` with `Stateless = true`. Stdio servers and initialize-handshake stateful Streamable HTTP sessions continue to work via the initialize-era server-to-client `elicitation/create` request flow; an HTTP server set to `Stateless = false` refuses `2026-07-28` so dual-path clients can fall back before using that flow. For code that needs to run on stateless servers — including `2026-07-28` Streamable HTTP — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
|
||||
For example:
|
||||
|
||||
|
|
|
|||
|
|
@ -33,7 +33,7 @@ MRTR is useful when:
|
|||
|
||||
## Opting in
|
||||
|
||||
MRTR activates when both peers negotiate protocol revision **`2026-07-28`**. The C# SDK client prefers `2026-07-28` by default — it probes with `server/discover` and falls back to a legacy `initialize` handshake only when the server doesn't support it. Servers accept `2026-07-28` automatically when a client offers it. No experimental flags are required; pinning `ProtocolVersion` to a legacy revision opts back out.
|
||||
MRTR activates when both peers negotiate protocol revision **`2026-07-28`**. The C# SDK client prefers `2026-07-28` by default — it probes with `server/discover` and falls back to an `initialize` handshake only when the server doesn't support it. Stateless HTTP servers accept `2026-07-28` automatically when a client offers it; HTTP servers configured with `Stateless = false` refuse that revision with `UnsupportedProtocolVersion` so dual-path clients can fall back to a session-capable revision. No experimental flags are required; pinning `ProtocolVersion` to an initialize-capable revision opts back out.
|
||||
|
||||
```csharp
|
||||
// Client — the SDK prefers 2026-07-28 (and therefore MRTR) by default.
|
||||
|
|
@ -283,4 +283,4 @@ The SDK supports `InputRequiredException` across two protocol revisions and two
|
|||
|
||||
Under the current protocol revision (`2025-06-18` and earlier), stdio and stateful Streamable HTTP keep `ClientCapabilities` populated, so the legacy methods work normally and remain the recommended way to do one-shot client interactions. Under `2026-07-28`, the spec removes those request methods from Streamable HTTP entirely; the SDK still allows the legacy methods on `2026-07-28` stdio sessions because stdio is implicitly single-process / stateful and the client handler is wired up regardless of negotiated revision. `InputRequiredException` is the way to write tools that work on every supported configuration.
|
||||
|
||||
Because `2026-07-28` removes `Mcp-Session-Id` (SEP-2567) and the `initialize` handshake (SEP-2575), Streamable HTTP runs statelessly whenever a client speaks `2026-07-28`. The `Stateful` row for `2026-07-28` in the compatibility matrix above therefore applies only to stdio — a server explicitly set to `Stateless = false` still serves `2026-07-28` requests without a session and creates a legacy session only when an older client falls back to `initialize`.
|
||||
Because `2026-07-28` removes `Mcp-Session-Id` (SEP-2567) and the `initialize` handshake (SEP-2575), Streamable HTTP can serve that revision only through the stateless path. The `Stateful` row for `2026-07-28` in the compatibility matrix above therefore applies to stdio and other non-HTTP stateful sessions; an HTTP server explicitly set to `Stateless = false` refuses `2026-07-28` with `UnsupportedProtocolVersion` and creates a session only when an older client falls back to `initialize`.
|
||||
|
|
|
|||
|
|
@ -109,7 +109,7 @@ server.RegisterNotificationHandler(
|
|||
[MRTR](xref:mrtr) is the SEP-2322 mechanism for server-driven input requests, finalized in protocol revision `2026-07-28`. In that revision, the server-to-client `roots/list` request method is removed; the recommended way to ask the client for its roots from a server handler is to throw <xref:ModelContextProtocol.Protocol.InputRequiredException> and let the SDK emit an <xref:ModelContextProtocol.Protocol.InputRequiredResult> on the wire.
|
||||
|
||||
> [!IMPORTANT]
|
||||
> `RequestRootsAsync` throws `InvalidOperationException("Roots are not supported in stateless mode.")` whenever the server is running stateless — which includes every Streamable HTTP server under `2026-07-28` once that revision is forced to stateless-only in a future PR. Stdio servers and current-protocol stateful Streamable HTTP servers continue to work via the legacy server-to-client `roots/list` request flow. For code that needs to run on stateless servers — including all `2026-07-28` Streamable HTTP servers going forward — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
> `RequestRootsAsync` throws `InvalidOperationException("Roots are not supported in stateless mode.")` whenever the server is running stateless — including Streamable HTTP requests served under `2026-07-28` with `Stateless = true`. Stdio servers and initialize-handshake stateful Streamable HTTP sessions continue to work via the initialize-era server-to-client `roots/list` request flow; an HTTP server set to `Stateless = false` refuses `2026-07-28` so dual-path clients can fall back before using that flow. For code that needs to run on stateless servers — including `2026-07-28` Streamable HTTP — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
|
||||
For example:
|
||||
|
||||
|
|
|
|||
|
|
@ -126,7 +126,7 @@ Sampling requires the client to advertise the `sampling` capability. This is han
|
|||
[MRTR](xref:mrtr) is the SEP-2322 mechanism for server-driven input requests, finalized in protocol revision `2026-07-28`. In that revision, the server-to-client `sampling/createMessage` request method is removed; the recommended way to ask the client to sample from a server handler is to throw <xref:ModelContextProtocol.Protocol.InputRequiredException> and let the SDK emit an <xref:ModelContextProtocol.Protocol.InputRequiredResult> on the wire.
|
||||
|
||||
> [!IMPORTANT]
|
||||
> `SampleAsync` and `AsSamplingChatClient` throw `InvalidOperationException("Sampling is not supported in stateless mode.")` whenever the server is running stateless — which includes every Streamable HTTP server under `2026-07-28` once that revision is forced to stateless-only in a future PR. Stdio servers and current-protocol stateful Streamable HTTP servers continue to work via the legacy server-to-client `sampling/createMessage` request flow. For code that needs to run on stateless servers — including all `2026-07-28` Streamable HTTP servers going forward — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
> `SampleAsync` and `AsSamplingChatClient` throw `InvalidOperationException("Sampling is not supported in stateless mode.")` whenever the server is running stateless — including Streamable HTTP requests served under `2026-07-28` with `Stateless = true`. Stdio servers and initialize-handshake stateful Streamable HTTP sessions continue to work via the initialize-era server-to-client `sampling/createMessage` request flow; an HTTP server set to `Stateless = false` refuses `2026-07-28` so dual-path clients can fall back before using that flow. For code that needs to run on stateless servers — including `2026-07-28` Streamable HTTP — throw `InputRequiredException` from your handler instead. It works under both protocols and both session modes.
|
||||
|
||||
For example:
|
||||
|
||||
|
|
|
|||
|
|
@ -28,11 +28,11 @@ When sessions are enabled (`Stateless = false`), the server creates and tracks a
|
|||
|
||||
## Forward and backward compatibility
|
||||
|
||||
The `Stateless` property is the single most important setting for forward-proofing your MCP server. The default is now `Stateless = true` (sessions disabled), which is the forward-compatible setting for the `2026-07-28` protocol revision and beyond. Stateless servers still respond to legacy clients on `2025-11-25` and earlier — the SDK keeps the `initialize` + `Mcp-Session-Id` handshake available for those clients — but they cannot use the session-dependent features ([unsolicited notifications](#how-streamable-http-delivers-messages), resource subscriptions, per-client isolation). Server-to-client requests are the exception: [elicitation](xref:elicitation) — and the now-deprecated [sampling](xref:sampling) and [roots](xref:roots) — can run statelessly through [MRTR](xref:mrtr), though MRTR requires the unratified `2026-07-28` revision and is far less widely supported than session-based requests. We recommend every server set `Stateless` explicitly rather than relying on the default:
|
||||
The `Stateless` property is the single most important setting for forward-proofing your MCP server. The default is now `Stateless = true` (sessions disabled), which is the forward-compatible setting for the `2026-07-28` protocol revision and beyond. Stateless servers still respond to clients on `2025-11-25` and earlier — the SDK keeps the `initialize` + `Mcp-Session-Id` handshake available for those clients — but they cannot use the session-dependent features ([unsolicited notifications](#how-streamable-http-delivers-messages), resource subscriptions, per-client isolation). Server-to-client requests are the exception: [elicitation](xref:elicitation) — and the now-deprecated [sampling](xref:sampling) and [roots](xref:roots) — can run statelessly through [MRTR](xref:mrtr), though MRTR requires the unratified `2026-07-28` revision and is far less widely supported than session-based requests. We recommend every server set `Stateless` explicitly rather than relying on the default:
|
||||
|
||||
- **`Stateless = true`** — the current default and the forward-compatible choice. Your server opts out of sessions entirely and the `Mcp-Session-Id` header is never sent or honored. The `2026-07-28` protocol revision drops the `initialize` handshake and `Mcp-Session-Id` from the wire format entirely, so this is the only configuration that lets the server respond to `2026-07-28` clients without falling back to legacy handling. If you don't need [unsolicited notifications](#how-streamable-http-delivers-messages), server-to-client requests, or session-scoped state, this is the setting to use today.
|
||||
- **`Stateless = true`** — the current default and the forward-compatible choice. Your server opts out of sessions entirely and the `Mcp-Session-Id` header is never sent or used. The `2026-07-28` protocol revision drops the `initialize` handshake and `Mcp-Session-Id` from the wire format entirely, so this is the only configuration that lets the server respond to `2026-07-28` clients without falling back to initialize-handshake handling. If you don't need [unsolicited notifications](#how-streamable-http-delivers-messages), server-to-client requests, or session-scoped state, this is the setting to use today.
|
||||
|
||||
- **`Stateless = false`** — the right choice when your server depends on sessions for [unsolicited notifications](#how-streamable-http-delivers-messages), resource subscriptions, or per-client isolation, none of which work without a session. Setting this explicitly protects your server from a future default change, and the [MCP specification requires](https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#streamable-http) that clients use sessions when a server's `initialize` response includes an `Mcp-Session-Id` header, so compliant clients always honor your server's session. Server-to-client requests no longer force a session: [elicitation](xref:elicitation) — and the now-deprecated [sampling](xref:sampling) and [roots](xref:roots) — can run statelessly through [MRTR](xref:mrtr) (see [Stateless alternatives for server-to-client interactions](#stateless-alternatives-for-server-to-client-interactions)). MRTR is only as available as the unratified `2026-07-28` revision, however, so keep a session if you need server-to-client requests against clients that don't speak it. Note that even with `Stateless = false`, a `2026-07-28` request is still served without a session because the protocol has no session header; the stateful path activates only when a client falls back to a legacy revision.
|
||||
- **`Stateless = false`** — the right choice when your server depends on sessions for [unsolicited notifications](#how-streamable-http-delivers-messages), resource subscriptions, or per-client isolation, none of which work without a session. Setting this explicitly protects your server from a future default change, and the [MCP specification requires](https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#streamable-http) that clients use sessions when a server's `initialize` response includes an `Mcp-Session-Id` header, so compliant clients always honor your server's session. Server-to-client requests no longer force a session: [elicitation](xref:elicitation) — and the now-deprecated [sampling](xref:sampling) and [roots](xref:roots) — can run statelessly through [MRTR](xref:mrtr) (see [Stateless alternatives for server-to-client interactions](#stateless-alternatives-for-server-to-client-interactions)). MRTR is only as available as the unratified `2026-07-28` revision, however, so keep a session if you need server-to-client requests against clients that don't speak it. Note that with `Stateless = false`, a `2026-07-28` request is refused with `UnsupportedProtocolVersion`; the stateful path activates only when a client falls back to an initialize-capable revision.
|
||||
|
||||
<!-- mlc-disable-next-line -->
|
||||
> [!TIP]
|
||||
|
|
@ -42,18 +42,18 @@ The `Stateless` property is the single most important setting for forward-proofi
|
|||
|
||||
The `2026-07-28` protocol revision goes further than `Stateless = true`: it removes the `initialize` handshake (SEP-2575) and the `Mcp-Session-Id` header (SEP-2567) from the wire format entirely. Clients bootstrap by sending `server/discover` instead, and every request carries the negotiated protocol version in the `MCP-Protocol-Version` HTTP header (HTTP transport) or the `_meta.io.modelcontextprotocol/protocolVersion` JSON-RPC field (every transport).
|
||||
|
||||
**Server side.** With `Stateless = true` (the default), the SDK already meets `2026-07-28` on the wire. Any HTTP POST that arrives with the `2026-07-28` `MCP-Protocol-Version` header is routed through the stateless path automatically — no session is created, no `Mcp-Session-Id` is returned, and the `GET` and `DELETE` endpoints are not mapped. Legacy clients that still send `initialize` on the same endpoint continue to work in stateless mode for the lifetime of that single POST. With `Stateless = false`, the server still falls back to legacy session creation when the client speaks `2025-11-25` or earlier — but a `2026-07-28` request (which carries no session) on a stateful server is refused with a `-32022 UnsupportedProtocolVersion` error, so a dual-era client downgrades to the legacy `initialize` handshake and obtains a session. A `2026-07-28` request that carries an `Mcp-Session-Id` is always rejected, since the revision has no session concept.
|
||||
**Server side.** With `Stateless = true` (the default), the SDK already meets `2026-07-28` on the wire. Any HTTP POST that arrives with the `2026-07-28` `MCP-Protocol-Version` header is routed through the stateless path automatically — no session is created, no `Mcp-Session-Id` is returned, and the `GET` and `DELETE` endpoints are not mapped. Clients that still send `initialize` on the same endpoint continue to work in stateless mode for the lifetime of that single POST. With `Stateless = false`, the server still creates HTTP sessions when the client speaks `2025-11-25` or earlier — but a `2026-07-28` request on a stateful server is refused with a `-32022 UnsupportedProtocolVersion` error, so a dual-path client downgrades to the `initialize` handshake and obtains a session. If a `2026-07-28` request carries an `Mcp-Session-Id`, the server ignores the header and still does not echo or mint a session ID for that request.
|
||||
|
||||
**Stateful options marked obsolete.** Because Streamable HTTP no longer supports sessions starting with the `2026-07-28` revision, the stateful-only knobs on <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions> — `IdleTimeout`, `MaxIdleSessionCount`, `EventStreamStore`, `SessionMigrationHandler`, and `PerSessionExecutionContext` — are now marked `[Obsolete]` with diagnostic `MCP9006` to signal that they only apply to legacy-protocol back-compat. You can still set them — the warning is informational — and they continue to govern stateful behavior for legacy clients.
|
||||
**Stateful options marked obsolete.** Because Streamable HTTP no longer supports sessions starting with the `2026-07-28` revision, the stateful-only knobs on <xref:ModelContextProtocol.AspNetCore.HttpServerTransportOptions> — `IdleTimeout`, `MaxIdleSessionCount`, `EventStreamStore`, `SessionMigrationHandler`, and `PerSessionExecutionContext` — are now marked `[Obsolete]` with diagnostic `MCP9006` to signal that they only apply to initialize-handshake back-compat. You can still set them — the warning is informational — and they continue to govern stateful behavior for initialize-capable clients.
|
||||
|
||||
**Client side — automatic fallback.** Clients automatically probe `2026-07-28` first and fall back to the `initialize` handshake when the server doesn't support it:
|
||||
|
||||
- **HTTP**: the client sends its first request with the `2026-07-28` `MCP-Protocol-Version` header. If the server returns HTTP `400` with anything other than a structured `-32022` / `-32021` / `-32020` JSON-RPC error, the client switches to the legacy `initialize` flow on the same endpoint.
|
||||
- **stdio**: the client sends a `server/discover` probe with a 5-second timeout. A `DiscoverResult` confirms `2026-07-28`; a `-32022` error with a `supported` payload triggers a retry at the highest mutually-supported version; anything else — including a timeout — falls back to legacy `initialize` on the same stdin/stdout. The SDK does not relaunch the server process.
|
||||
- **HTTP**: the client sends its first request with the `2026-07-28` `MCP-Protocol-Version` header. If the server returns HTTP `400` with anything other than a structured `-32022` / `-32021` / `-32020` JSON-RPC error, the client switches to the `initialize` flow on the same endpoint.
|
||||
- **stdio**: the client sends a `server/discover` probe with a 5-second timeout. A `DiscoverResult` confirms `2026-07-28`; a `-32022` error with a `supported` payload triggers a retry at the highest mutually-supported initialize-capable version; anything else — including a timeout — falls back to `initialize` on the same stdin/stdout. The SDK does not relaunch the server process.
|
||||
|
||||
The era is cached per <xref:ModelContextProtocol.Client.IClientTransport> instance, so the probe cost is paid only on the first connect.
|
||||
|
||||
**Opting out of fallback.** Pin <xref:ModelContextProtocol.Client.McpClientOptions.ProtocolVersion> to `2026-07-28` when you want the client to refuse to fall back. A non-null `ProtocolVersion` is also treated as the minimum, so the connect call throws an <xref:ModelContextProtocol.McpException> instead of silently degrading to a legacy revision. This is useful for strict-modern production code and for tests that need to assert `2026-07-28`-only behavior. To try several versions yourself, leave `ProtocolVersion` unset (the default) or retry the connection with a different value.
|
||||
**Opting out of fallback.** Pin <xref:ModelContextProtocol.Client.McpClientOptions.ProtocolVersion> to `2026-07-28` when you want the client to refuse to fall back. A non-null `ProtocolVersion` is also treated as the minimum, so the connect call throws an <xref:ModelContextProtocol.McpException> instead of silently degrading to an initialize-capable revision. This is useful for strict `2026-07-28` production code and for tests that need to assert `2026-07-28`-only behavior. To try several versions yourself, leave `ProtocolVersion` unset (the default) or retry the connection with a different value.
|
||||
|
||||
```csharp
|
||||
var clientOptions = new McpClientOptions
|
||||
|
|
|
|||
|
|
@ -183,7 +183,7 @@ app.MapMcp();
|
|||
app.Run();
|
||||
```
|
||||
|
||||
By default, the HTTP transport uses **stateful sessions** — the server assigns an `Mcp-Session-Id` to each client and tracks session state in memory. For most servers, **stateless mode is recommended** instead. It simplifies deployment, enables horizontal scaling without session affinity, and avoids issues with clients that don't send the `Mcp-Session-Id` header. We recommend setting `Stateless` explicitly (rather than relying on the current default) for [forward compatibility](xref:stateless#forward-and-backward-compatibility). See [Sessions](xref:stateless) for a detailed guide on when to use stateless vs. stateful mode, configure session options, and understand [cancellation and disposal](xref:stateless#cancellation-and-disposal) behavior during shutdown.
|
||||
By default, the HTTP transport runs **statelessly** — the server does not assign an `Mcp-Session-Id` or track transport session state in memory. This simplifies deployment, enables horizontal scaling without session affinity, and matches the `2026-07-28` Streamable HTTP wire format. Set `Stateless = false` explicitly when your server needs stateful sessions for unsolicited notifications, resource subscriptions, or per-client isolation. See [Sessions](xref:stateless) for a detailed guide on when to use stateless vs. stateful mode, configure session options, and understand [cancellation and disposal](xref:stateless#cancellation-and-disposal) behavior during shutdown.
|
||||
|
||||
#### Host name validation
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue