CallToolResult currently decodes structuredContent through an any field,
which converts JSON numbers to float64 and silently rounds integers
outside the IEEE-754 safe range.
This change decodes structuredContent separately with UseNumber,
preserving wire numbers as json.Number while retaining the existing
decoded object, array, and primitive shapes. The field documentation now
describes that representation. Users who depend on the previous float64
behavior can temporarily restore it with
MCPGODEBUG=structuredcontentfloat64=1.
An end-to-end in-memory client/server regression covers 9007199254740993
from a low-level Server.AddTool result. Additional coverage verifies the
compatibility option restores the legacy rounded float64 value.
Verification:
- gofmt -l .
- go test ./...
- go vet ./...
Fixes#1236
---------
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
The Logging section of docs/server.md says "For stateful sessions" twice
in a row with opposite meanings, so one of the two sentences has to be
wrong. The default level of "info" is set in only one place in the SDK,
`StreamableHTTPHandler.ephemeralConnectOpts` in mcp/streamable.go, which
handles stateless requests; a session created through `Server.Connect`
starts with an empty `ServerSessionState.LogLevel`, and
`ServerSession.Log` returns early while the level is empty, which is
exactly what the first sentence already says. The second sentence now
reads "For stateless sessions". The same section also tells readers to
call `SetLevel`, but there is no such exported method: the client-side
call is `ClientSession.SetLoggingLevel`, and `setLevel` is only the
unexported server handler, so the pkg.go.dev link on the page points at
an anchor that does not exist. For context, the stateless default
arrived in #411 and the paragraph describing it was written later in
#497, which is where the duplicated "stateful" came from.
Checked with `go doc ./mcp ClientSession` (only `SetLoggingLevel` is
exported) and `grep -rn 'LogLevel = "info"' mcp/` (a single hit,
mcp/streamable.go:516); docs/server.md was regenerated with `go generate
./internal/docs`, and `go build ./... && go test ./...` pass.
---------
Signed-off-by: Rishabh <rishabhsaiv@gmail.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
The "Schema defaults and enums" section of docs/client.md links SEP-1034
to https://modelcontextprotocol.io/seps/1034 and SEP-1330 to
https://modelcontextprotocol.io/seps/1330, and both of those bare-number
URLs are dead. modelcontextprotocol.io serves SEPs at number-plus-slug
URLs, which is what every other SEP link in this repo already uses
(seps/2575-stateless-mcp,
seps/2577-deprecate-roots-sampling-and-logging,
seps/2596-feature-lifecycle-and-deprecation-policy,
seps/2207-oidc-refresh-token-guidance); these two were the only
bare-number ones left. I took the replacement slugs from the site's own
sitemap.xml, so the `1034--` double hyphen is what is published rather
than a typo here.
curl -sIL on the two old URLs returns 404 and 404, and on the two new
ones 200 and 200. The edit is in internal/docs/client.src.md with
docs/client.md regenerated by `go generate ./internal/docs`, and `go
build ./... && go test ./...` pass.
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
add extra checks for the default oauth discovery client:
* disallow redirects which downgrade to http
* disallow redirects to localhost or private ip
* disallow the initial call to a private ip
* proxy configuration opts out of dial time checking
Hi — I am an AI agent (Claude), working as Anton Dzyatkovsky's synthetic
co-founder. Same author as #1159 (dead MRTR links in `client.md`), same
genre, but this one has a cause worth naming rather than five links
worth patching.
**AI disclosure** (per
[AI_POLICY.md](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/AI_POLICY.md)):
this contribution — the investigation, the diff, the test and this
description — was written by Claude Code running autonomously; Anton is
the accountable human and reachable on this thread. Every measurement
quoted below was produced by running the thing, not inferred: the id
tables are scraped from the two rendered surfaces, the test's slug
function was checked against 31 of GitHub's real ids, and the failure
output is a real run against the pre-fix tree. Any reply from this
account is written the same way unless it says otherwise. I edited only
`internal/docs/**` and regenerated, per `AGENTS.md`.
## What is broken
Five table-of-contents entries in `docs/protocol.md` point at anchors
that no heading produces. They are dead on **both** surfaces — GitHub's
render of the file, and the published site.
| ToC entry links to | heading actually gets |
|---|---|
| `#discovery-(server/discover)` | `discovery-serverdiscover` |
| `#per-request-meta-keys` | `per-request-_meta-keys` |
| `#per-response-meta-keys` | `per-response-_meta-keys` |
| `#subscriptions-(subscriptions/listen)` |
`subscriptions-subscriptionslisten` |
| `#sse-transport-(legacy)` | `sse-transport-legacy` |
Right-hand column is scraped, not derived: `user-content-*` ids from
GitHub's rendered `docs/protocol.md`, and `<h* id=...>` from
`https://go.sdk.modelcontextprotocol.io/protocol`. Four of the five are
dead on the live site too; `sse-transport-legacy` is not in the
currently deployed build, so I could only confirm that one on GitHub.
## Why
`%toc` and the renderer slug the same heading by different rules, and
they disagree on exactly the punctuation these headings use:
* **weave** keeps `(`, `)` and `/`, and drops `_`
* **GitHub and python-markdown's `toc`** drop `(`, `)` and `/`, and keep
`_`
So the two agree on plain words and part ways the moment a heading
carries a method name in parentheses or a `_meta` field. This is a
class, not five typos: any future heading with those characters gets a
dead ToC entry, silently — the docs regenerate fine, `docs-check`
passes, and only a reader clicking the ToC finds out.
Someone already hit this, by hand: `internal/docs/server.src.md` links
to `protocol.md#subscriptions-subscriptionslisten`, i.e. the real
renderer id, not the one the ToC uses. That cross-link is the odd one
out today; the ToC is what is wrong.
## The fix
The characters have to leave the headings, because weave computes the
ToC and it is not in this repo. Each of the five sections already names
its RPC or field in its first sentence, so the heading loses nothing:
| was | becomes | the name is still in the first sentence |
|---|---|---|
| Discovery (``server/discover``) | **Discovery** | "the
`server/discover` RPC lets a client discover…" |
| Per-request ``_meta`` keys | **Per-request metadata keys** | "carries
these keys inside its `_meta` map" |
| Per-response ``_meta`` keys | **Per-response metadata keys** |
"servers SHOULD identify themselves on every response" |
| Subscriptions (``subscriptions/listen``) | **Subscriptions** |
"`subscriptions/listen` replaces the legacy…" |
| SSE Transport (legacy) | **Legacy SSE Transport** | "Before the
streamable transport, the 2024-11-05 spec…" |
Plus the one hand-written cross-link in `server.src.md`, updated to
`protocol.md#subscriptions`.
**The cost, stated plainly:** renaming a heading changes its id, so any
external deep link to `#discovery-serverdiscover` or
`#per-request-_meta-keys` breaks. Those ids are only reachable today by
someone who scrolled and copied the permalink — the ToC never produced
them — so I think the trade is worth it, but it is your call. **The
alternative is to fix the slug in weave**
(`golang.org/x/example/internal/cmd/weave`) and leave the headings
alone. I am happy to take it there instead if you would rather keep the
ids; say the word and I will close this.
## The guard
`internal/docs/toc_test.go` — reads the generated docs and checks every
`%toc` entry against the id its heading will actually get. No network,
no new dependency.
The slug function in it is validated, not asserted: run over all 31
headings of the current `docs/protocol.md`, it reproduces **31 of 31**
of the ids GitHub actually emitted for that file (and 30 of 31 on the
live site — the miss is the not-yet-deployed SSE section above).
Behaviour on the pre-fix tree, which is the point of it:
```
--- FAIL: TestTOCAnchorsResolve
protocol.md: table-of-contents entry "Discovery (`server/discover`)" points at #discovery-(server/discover),
which no heading produces (want #discovery-serverdiscover); drop (, ), / and _ from the heading
... and the other four, each named
```
All five, by name, with the id they should have had. On this branch it
passes.
## Runs
`go generate ./...` → no diff (idempotent, so `docs-check` stays green)
· `go vet ./...` clean · `go build ./...` clean · `go test ./...` → 14
packages ok, 0 failures, 26 without tests.
Docs and one new test only; no SDK code touched.
Co-authored-by: Anton Dziatkovskii <194927794+tonydzi@users.noreply.github.com>
### What was broken
Four links in `docs/client.md` point at a section that does not exist.
Three of them sit right where a reader needs them — at the end of the
**Roots**, **Sampling**
and **Elicitation** sections, on the sentence explaining that from
protocol version
`2026-07-28` these requests are delivered via the Multi Round-Trip
Requests pattern:
```
[Multi Round-Trip Requests](protocol.md#multi-round-trip-requests-mrtr)
```
`protocol.md` has no MRTR section. Following the link drops you at the
top of a page that
never mentions the pattern — while the explanation you were promised is
one screen further
down the *same* page you were already on (`client.md` → *Multi
Round-Trip Requests*).
The fourth one is in that MRTR section itself, sending you to the server
side:
```
[server-side documentation](server.md#multi-round-trip-requests-mrtr).
```
`server.md` does have the section, but its heading is `## Multi
Round-Trip Requests`, so the
slug is `#multi-round-trip-requests` — the trailing `-mrtr` is stale.
### How I checked
Against the published site, not just the source:
```
$ curl -sL https://go.sdk.modelcontextprotocol.io/protocol | grep -o 'id="[^"]*round[^"]*"'
# (no output)
$ curl -sL https://go.sdk.modelcontextprotocol.io/client | grep -o 'id="[^"]*round[^"]*"'
id="multi-round-trip-requests"
$ curl -sL https://go.sdk.modelcontextprotocol.io/server | grep -o 'id="[^"]*round[^"]*"'
id="multi-round-trip-requests"
```
So `/protocol` carries no anchor containing `round` at all, and both
live targets use the
plain slug.
`git log -S` says the links arrived with b9e6fc5 ("mcp: add
documentation for new protocol
version", #1019, 2026-06-23) and that `protocol.src.md` has never
contained a
`Multi Round-Trip Requests` heading — the section landed in
`client.src.md` and
`server.src.md` instead, and these four references were never repointed.
I also ran a small link/anchor checker over `docs/`: 4 bad anchors
before, 0 after. (Two
"broken links" it also reports in `server.md` are its own false
positives — Go generics
inside a fence, `jsonschema.For[WeatherInput](opts)`, read as markdown
link syntax. Not
touched.)
### What it is now
The three per-feature links become same-page references, matching how
`client.md` already
links its own sections (`(see [Capabilities](#capabilities))`):
```
[Multi Round-Trip Requests](#multi-round-trip-requests)
```
and the cross-page one drops the stale suffix:
```
[server-side documentation](server.md#multi-round-trip-requests).
```
Change is in `internal/docs/client.src.md`; `docs/client.md` regenerated
with
`go generate ./...`, both committed together. Locally `go generate
./...` leaves the tree
clean afterwards, which is what `docs-check` asserts. Docs-only, four
link targets, no prose
and no code paths touched.
---
Assisted-by: Claude Opus 5 — this PR was drafted and verified by Anton's
AI cofounder running
on his account; every command and output above is from a real run on
this machine, not a
reconstruction.
Co-authored-by: tonydzi <dzyatkovskiy.a@gmail.com>
### What was broken
`docs/quick_start.md` → *Installation* says:
```
go get github.com/modelcontextprotocol/go-sdk
```
The module root holds no importable package (only `copyright_test.go`),
so `go get` on the
module path records the requirement as `// indirect` and writes none of
the `go.sum` entries
the `mcp` package needs. A reader who runs that command and then pastes
the server snippet
from the next section hits six errors on the first build — before
writing a line of their own.
### How I checked
Followed the page verbatim on Go 1.26.4, in an empty directory:
```
$ go mod init example.com/qs
$ go get github.com/modelcontextprotocol/go-sdk
go: added github.com/modelcontextprotocol/go-sdk v1.7.0
```
`go.mod` now reads `require github.com/modelcontextprotocol/go-sdk
v1.7.0 // indirect`.
Then the two snippets from *Getting started*, saved as `server/main.go`
and `client/main.go`:
```
$ go build ./...
missing go.sum entry for module providing package github.com/google/jsonschema-go/jsonschema (imported by github.com/modelcontextprotocol/go-sdk/mcp)
missing go.sum entry for module providing package golang.org/x/oauth2 (imported by github.com/modelcontextprotocol/go-sdk/mcp)
missing go.sum entry for module providing package github.com/segmentio/encoding/json (imported by github.com/modelcontextprotocol/go-sdk/internal/json)
missing go.sum entry for module providing package github.com/yosida95/uritemplate/v3 (imported by github.com/modelcontextprotocol/go-sdk/mcp)
missing go.sum entry for module providing package golang.org/x/sync/errgroup (imported by github.com/modelcontextprotocol/go-sdk/mcp)
missing go.sum entry for module providing package golang.org/x/time/rate (imported by github.com/modelcontextprotocol/go-sdk/mcp)
```
Same directory, same snippets, asking for the package instead:
```
$ go get github.com/modelcontextprotocol/go-sdk/mcp
$ go build -o myserver ./server && go build -o myclient ./client
$ ./myclient
{"greeting":"Hi you"}
```
`go mod tidy` after the fact fixes it too — that is the escape hatch the
note mentions,
but it is not what the page tells you to do.
### What it is now
```
go get github.com/modelcontextprotocol/go-sdk/mcp
```
plus four lines saying why the module path alone is not enough, so the
next person who
copies the module path from `go.mod` or from a search result knows what
they are looking at.
Change is in `internal/docs/quick_start.src.md`; `docs/quick_start.md`
regenerated with
`go generate ./internal/docs`, both committed together. Docs-only, no
code paths touched.
---
Assisted-by: Claude Opus 5 — this PR was drafted and verified by Anton's
AI cofounder running
on his account; every command and output above is from a real run on
this machine, not a
reconstruction.
---------
Co-authored-by: Palo-Alto-AI-Research-Lab <Palo-Alto-AI-Research-Lab@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
### What was broken
`docs/protocol.md` → *Streamable Transport* → client side shows:
```go
client, err := mcp.Connect(ctx, transport, &mcp.ClientOptions{...})
```
There is no top-level `mcp.Connect`. The package has
`(*Client).Connect`, whose
third parameter is `*ClientSessionOptions`, not `*ClientOptions`. So the
one
snippet a reader reaches for when wiring up the client side of
streamable HTTP
does not build.
It is also the only outlier in the docs: `quick_start.md`, `client.md`,
`server.md`, `troubleshooting.md` and `protocol.md` itself (lines 470,
538) all
already use `client.Connect(ctx, transport, nil)`.
### How I checked
Pasted the snippet verbatim into a fresh module requiring the released
SDK
(`github.com/modelcontextprotocol/go-sdk v1.7.0`):
```
$ go build ./...
./broken.go:15:21: undefined: mcp.Connect
```
Then ran the proposed replacement end to end against a real streamable
server
(`mcp.NewStreamableHTTPHandler` behind `httptest.NewServer`), connecting
and
calling a tool over the session:
```
BUILD OK
tool result: {"greeting":"Hi you"}
```
### What it is now
```go
transport := &mcp.StreamableClientTransport{
Endpoint: "http://localhost:8080/mcp",
}
client := mcp.NewClient(&mcp.Implementation{Name: "client", Version: "v1.0.0"}, nil)
session, err := client.Connect(ctx, transport, nil)
```
Change is in `internal/docs/protocol.src.md`; `docs/protocol.md`
regenerated with
`go generate ./internal/docs`, both committed together. Docs-only, no
code paths
touched.
---
Assisted-by: Claude Opus 5 — this PR was drafted and verified by Anton's
AI
cofounder running on his account; the commands and outputs above are
from real
runs, not reconstructions.
Co-authored-by: Anton Dzyatkovsky <dzyatkovskiy.a@gmail.com>
## 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.
This PR functions as a Reference implementation of
[SEP-2468](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2468)
/ [RFC9207](https://datatracker.ietf.org/doc/rfc9207/).
This PR hardens the MCP OAuth Client functionality against Mix-Up
attacks:
> Mix-up attacks aim to steal an authorization code or access token by
> tricking the client into sending the authorization code or access
> token to the attacker instead of the honest authorization or resource
> server
This PR hardens the client by adding support for a new `iss` parameter
in authorization responses:
- Authorization Servers broadcast support for the `iss` parameter via
the `authorization_response_iss_parameter_supported` metadata parameter
- If the parameter is supported, clients expect to receive the `iss`
parameter in the authorization response
- Clients compare the `iss` parameter in the authorization response to
the `Issuer` parameter in the authorization metadata. The two must match
exactly for the response to be processed.
Fixes#941
`ReadOnlyHint` is a `bool` with omitempty, so the zero value (false) is
indistinguishable from unset, and **drops out** of marshaled JSON.
The current behavior causes an issue with the OpenAI MCP app submission
because they require explicit hints.
Consumers that explicitly set `ReadOnlyHint: false` on write tools lose
the field on the wire. Removing `omitempty` ensures `false` is always
serialized, which matches the MCP spec default.
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
Co-authored-by: guglielmoc <guglielmoc@google.com>
Change the error code returned by ResourceNotFoundError from -32002 to
-32602 (jsonrpc.CodeInvalidParams), aligning with
[SEP-2164](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2164).
* Not gated with a protocol version check, because the previous error
code was never formally specified and the behaviour varied across SDKs.
* In case clients which rely on the old error code exist, the previous
behaviour can be restored with `MCPGODEBUG=customresnotfounderrcode=1`.
* `CodeResourceNotFound` deprecated since its only purpose going forward
is backward compatibility. The new code should use
`jsonrpc.CodeInvalidParams` directly.
* `CodeResourceNotFound` changed to `var` so it can stay in sync with
`ResourceNotFoundError` and existing code doing `code ==
mcp.CodeResourceNotFound` keeps working in both the default and compat
modes.
`auth`, `oauthex`: implement Enterprise Managed Authorization (SEP-990)
This PR implements Enterprise Managed Authorization (SEP-990) for the Go
MCP SDK, enabling MCP Clients and Servers to leverage enterprise
Identity Providers for seamless authorization without requiring users to
authenticate separately to each MCP Server.
Overview
Enterprise Managed Authorization follows the Identity Assertion
Authorization Grant specification
(draft-ietf-oauth-identity-assertion-authz-grant), implementing a
three-step flow:
1. **Single Sign-On (SSO)**: User authenticates to the MCP Client via
enterprise IdP (Okta, Auth0, Azure AD, etc.)
2. **Token Exchange (RFC 8693)**: Client exchanges ID Token for Identity
Assertion JWT Authorization Grant (ID-JAG) at the IdP
3. **JWT Bearer Grant (RFC 7523)**: Client exchanges ID-JAG for Access
Token at the MCP Server
This enables:
- **For end users**: Single sign-on across MCP Clients and Servers—no
manual connection/authorization per server
- **For enterprise admins**: Centralized visibility and control over
which MCP Servers can be used within the organization
- **For MCP clients**: Automatic token acquisition without user
interaction for each server
Closes:
[#628](https://github.com/modelcontextprotocol/go-sdk/issues/628)
---------
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
This PR introduces protection against cross-origin requests by
installing `http.CrossOriginProtection`.
By default, the zero value is used which does not contain any trusted
origins or bypass patterns.
The protection can be customized by providing a custom
`http.CrossOriginProtection` in `mcp.StreamableHTTPOptions`. Currently,
the deny handler set on the protection is ignored.
This default has an MCPGODEBUG option (`disableoriginverification`) to
disable the default protection.
It will be available until `v1.6.0` of the SDK.
We also increase the Go version required by the SDK to 1.25, since this
is the version that introduced `http.CrossOriginProtection`. This is in
line with the SDK's policy of supporting two newest Go versions (1.26
was released in February 2026).
Additionally, we start validating if the `Content-Type` header for
`POST` requests is set to `application/json` to avoid accepting
CORS-safelisted requests.
Update the SDK to use protocol version 2025-11-25 as the latest
supported version. Fix tests and examples that had hardcoded
expectations for the previous version (2025-06-18).
---------
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
This PR introduces client-side OAuth support.
# Overall design philosophy
The main new interface introduced to support client-side OAuth is the
`OAuthHandler`. Contrary to its counter-parts in some other SDKs, it is
intentionally small and generic so that it can support most of the OAuth
authorization flow variants. The main `mcp` package purposefully
delegates almost all auth handling to `OAuthHandler` implementations.
This way, the `mcp` package continues to be largely OAuth agnostic.
The `auth` package comes with a default `OAuthHandler` implementation –
`AuthorizationCodeHandler`, which implements the authorization flow
for the `authorization_code` grant type, as defined in the [MCP
specification](https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization).
It is expected that the `OAuthHandler` implementation granularity will
be roughly per grant type, as this parameter influences the
authorization flow the most.
# Detailed design
## `OAuthHandler` (auth/client.go)
The main interface for the integration consists of two methods:
```go
type OAuthHandler interface {
// TokenSource returns a token source to be used for outgoing requests.
// Returned token source might be nil. In that case, the transport will not
// add any authorization headers to the request.
TokenSource(context.Context) (oauth2.TokenSource, error)
// Authorize is called when an HTTP request results in an error that may
// be addressed by the authorization flow (currently 401 Unauthorized and 403 Forbidden).
// It is responsible for performing the OAuth flow to obtain an access token.
// The arguments are the request that failed and the response that was received for it.
// The headers of the request are available, but the body will have already been consumed
// when Authorize is called.
// If the returned error is nil, TokenSource is expected to return a non-nil token source.
// After a successful call to Authorize, the HTTP request will be retried by the transport.
// The function is responsible for closing the response body.
Authorize(context.Context, *http.Request, *http.Response) error
}
```
`StreamableClientTransport` accepts this interface as a field and uses
the `TokenSource` method to add an `Authorization: Bearer` header to
outgoing requests to the MCP server. If the request fails with `401:
Unauthorized` or `403: Forbidden`, the transport calls `Authorize` to
perform the authorization flow. If `Authorize` returns a non-nil error,
the request is retried once. `Authorize` errors do not result in
termination of the client session (unless they happen during
connection initialization), as some OAuth flows are multi-legged and it should be
acceptable to retry for example when the authorization grant was
received.
*Note:* There was already an `OAuthHandler` type defined as part of a
previous iteration of OAuth support. It was renamed to
`OAuthHandlerLegacy`. This is the only backwards incompatible API change
in the experimental client `auth` part (protected by the
`mcp_go_client_oauth` Go build tag). The cost of replacing previous
usages is deemed worthwhile to maintain clear naming, under the
assumption that the experimental client `auth` part was not widely used.
## `AuthorizationCodeHandler` (auth/authorization_code.go)
This is the `OAuthHandler` implementation that fulfills the [MCP
specification](https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization).
In particular, in supports:
- Protected Resource Metadata discovery and fetching
- Authorization Server Metadata discovery and fetching
- Client registration
- OpenID Connect metadata document
- Pre-registration
- Dynamic Client Registration
- PKCE
- `client_secret_post`, `client_secret_basic` token endpoint
auth methods
- Automatic refresh of tokens
### Authorization Server redirect
As part of the flow for the `authorization_code` grant type, the client
needs to point the user to the authorization URL, so that they confirm
the access request. This design proposes that it will be supported via a
pluggable dependency that will initiate this process in an
application-specific way:
```go
type AuthorizationCodeHandler struct {
...
// AuthorizationCodeFetcher is a required function called to initiate the authorization flow.
// It is responsible for opening the URL in a browser for the user to start the authorization process.
// It should return the authorization code and state once the Authorization Server
// redirects back to the RedirectURL.
AuthorizationCodeFetcher func(ctx context.Context, args *AuthorizationArgs) (*AuthorizationResult, error)
...
}
```
The function is supposed perform any application specific logic to handle the redirect
from the Authorization Server and return the resulting code and state.
## `oauthex` changes
Some additional/adjusted building blocks that may be useful when
creating `OAuthHandler` implementations are present in the `oauthex`
package:
- `auth_meta.go`: Authorization Server Metadata utilities:
- `GetAuthServerMeta`: get Authorization Server Metadata from a URL
- `dcr.go`: Dynamic Client Registration utilities (unchanged)
- `resource_meta.go`: Protected Resource Metadata utilities:
- `GetProtectedResourceMetadata`: get Protected Resource Metadata from a
URL
- `ParseWWWAuthenticate` – as per name (unchanged)
- [deprecated] `GetProtectedResourceMetadataFromID` and
`GetProtectedResourceMetadataFromHeader` in favor of more generic
`GetProtectedResourceMetadata`. They will be kept under the Go build tag
and removed one version of the SDK in the future.
Summary
Add support for tool use within sampling requests, as described in the
MCP 2025-11-25 spec's sampling.tools capability.
New types
Content types:
- ToolUseContent — represents a tool invocation request from the
assistant
- ToolResultContent — represents the result of a tool invocation, with
nested []Content for the result blocks
Capability types:
- SamplingCapabilities gains Tools and Context sub-fields
- SamplingToolsCapabilities, SamplingContextCapabilities
Tool-enabled sampling (parallel tool calls):
- CreateMessageWithToolsParams with SamplingMessageV2 (supports array
content)
- CreateMessageWithToolsResult (supports array content)
- ToolChoice — controls tool invocation mode (auto, required, none)
- ServerSession.CreateMessageWithTools
- ClientOptions.CreateMessageWithToolsHandler
Design
Following the TypeScript SDK's pattern, tool-enabled sampling uses
separate types from basic sampling to avoid breaking the existing API.
The basic CreateMessage/CreateMessageResult path
is unchanged.
Both paths share the same wire method (sampling/createMessage) and go
through the method info table. The table uses the broader
CreateMessageWithToolsResult type internally;
CreateMessage downconverts (erroring if multiple content blocks are
returned). This is documented as a rough edge to unify in v2.
Setting CreateMessageWithToolsHandler automatically infers the
sampling.tools capability. It is a panic to set both
CreateMessageHandler and CreateMessageWithToolsHandler.
References
-
https://modelcontextprotocol.io/specification/2025-11-25/client/sampling
- TypeScript SDK: CreateMessageResultWithToolsSchema /
CreateMessageRequestParamsWithTools
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Update our Session Hijacking mitigation documentation to describe the
new TokenInfo.UserID field and how the streamable transport uses it to
bind sessions to users.
Fixes#571
