## Summary
`ClientSessionOptions.protocolVersion` was unexported and documented as
test-only, so external consumers of `Client.Connect` could not request a
specific protocol revision.
This exports the field as `ProtocolVersion` with the same semantics:
- empty → request the latest supported version (unchanged default)
- non-empty → send that version in initialize/discover
- existing negotiate-down path is unchanged
## Usage
```go
cs, err := client.Connect(ctx, transport, &mcp.ClientSessionOptions{
ProtocolVersion: "2025-11-25",
})
```
## Testing
- `go test ./mcp/ -count=1` (pass)
Fixes#1113
Co-authored-by: arimu1 <19286898+arimu1@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
This PR adds the protocol version `2026-07-28` to the list of supported
protocol versions and fixes legacy tests hardcoding the legacy protocol
version for those use cases.
To guarantee a better compatibility with legacy server, enable the
fallback to the legacy initialize rpc in case of any error returned by
the server, in case of `server/discover` request.
## Summary
Part 3 of the SEP-2575 ("Make MCP Stateless") implementation. Builds on
PR #975 which landed the client-side discover/fallback flow and the
per-request `_meta` plumbing. This PR adds the **server-side**
counterparts:
- `server/discover` handler (replaces the
stub),ProtocolVersionSupporter`
- transport hook for per-transport version filtering,
- transport-agnostic version validation and method-removal enforcement
in `ServerSession.handle`,
- HTTP-level wire compliance for SEP-2575 error codes/status codes,
- bug fix in `internal/jsonrpc2`,`processResult` with an `MCPGODEBUG`
opt-out for backward compatibility.
Implements the client half of the SEP-2575 / `2026-06-30` protocol. This
CL is client-only; server-side `server/discover` is still a
`MethodNotFound` stub.
### Changes
- **`server/discover` probe in `Client.Connect()`** (`client.go`). When
the caller opts into `protocolVersion >= 2026-06-30`, the client probes
the server with `server/discover` before falling back to the legacy
`initialize`
handshake.
- **Per-request `_meta` injection** (`shared.go`, `injectRequestMeta`).
For`>= 2026-06-30` sessions every outgoing request gets the SEP-2575
triple (`protocolVersion`, `clientInfo`, `clientCapabilities`).
- **Server-side fix**: `ServerSession.handle` no longer pre-populates
`state.InitializeParams` from a `server/discover` request's `_meta`, so
a subsequent fallback `initialize` is not rejected as a duplicate.
### Fallback rules
Client falls back to the legacy `initialize` handshake when discover
returns:
- `MethodNotFound` (-32601),
- `UnsupportedProtocolVersionError` (-32004),
- an HTTP `Bad Request` (substring-matched on the error message, to
interop
with pre-2026-06-30 servers).
Fixes#966
This implements
[SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions)
and resolves#777.
## Summary of changes
- Add `Extensions` field to `ServerCapabilities` and
`ClientCapabilities` structs per
[SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions)
- Add `AddExtension` helper methods on both types that normalize nil
settings to empty maps.
- [The spec requires an object, not
null](https://modelcontextprotocol.io/community/seps/2133-extensions#negotiation:~:text=an%20empty%20object%20indicates%20no%20settings.)
- Include shallow-copy support via `maps.Clone` in the existing
`clone()` methods
- also include shallow-copy support for `Experimental` field.
- Add comprehensive tests: unit tests for nil-safety, capability
preservation, and over-the-wire round-trip
## What's tested
- [x] `go test ./mcp/...` passes
- [x] Server conformance tests pass (30 scenarios, 40 assertions, 0
failures)
- [x] Client conformance tests match existing baseline (auth scenarios
are known failures)
Expose the jsonrpc.Error type to allow access to underlying JSON-RPC
error codes. Also, expose common JSON-RPC error codes in the jsonrpc
package, and MCP error codes in the mcp package.
Also, implement support for the MCP URL elicitation required error
(-32042), enabling servers to request out-of-band user authorization and
clients to automatically handle these requests with retry logic.
Server-side:
- Add CodeURLElicitationRequired constant
- Add URLElicitationRequiredError() constructor.
Client-side:
- Add urlElicitationMiddleware to intercept URLElicitationRequired
errors and retry calls.
- Update callElicitationCompleteHandler to signal waiting operations.
- Fix capability advertisement to support both form and URL modes.
Fixes#452Fixes#623
This CL enables the server session to see what capabilities the client
has by introducing the InitializeParams() function.
This CL also adds a test to ensure ClientCapabilities is accurate.
Fixes#141
Add a test that checks for JSON null on the wire, and fix two categories
of bugs leading to null:
1. In many places, we could pass typed nil where we shouldn't have. Add
an orZero helper to avoid typed nills.
2. When listing roots, we should avoid nil.
Fixesgolang/go#74105
Change-Id: I061420487dbfcbf9016cb94b39bd72270e871c0e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682675
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Iterators return pointers instead of values, to be consistent.
Move tests out of example file.
Factor out iterator tests.
Change-Id: Icb14edf99d738de9d9dc08c1169057cb3dc5894b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680295
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
CallToolParams.Arguments is any.
Clients can send whatever they want.
Tool authors who create their own ToolHandler (bypassing NewTool)
will receive the arguments as a map[string]any which has been
validated against the input schema.
Tool authors who call NewTool can choose the type that the arguments
unmarshal into.
There is no double-unmarshaling and no downcasting.
There is no way for tool authors to avoid the unmarshal, and no way to
avoid the validation unless the omit an input schema, which is probably
bad for LLMs. If tool authors want this optimization, we can provide
it later.
Change-Id: Ieb201e56c7fb8a23bb1a93b27e946630ec9e79ad
Reviewed-on: https://go-review.googlesource.com/c/tools/+/679595
TryBot-Bypass: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Add a test to verify that the toWireParams helper maps all fields of
CallToolParams.
Change-Id: I81c28791734d60c793c76809c6b93d27e78c3c36
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
This CL streamlines the client side pagination logic by adding a generic
helper that handles pagination. This also simplifies the existing tools
iterator method.
Change-Id: If4304deda1ccd22d0c4580d76deaefc64e5dfeb9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
