405 Method Not Allowed responses MUST include an Allow header listing
supported methods, per RFC 9110 Section 15.5.6. This fixes issues with
strict HTTP gateways (like Apigee) that treat 405 responses without an
Allow header as malformed, returning 502 Bad Gateway errors.
## Changes
- **SSEHandler**: Add `Allow: GET, POST` header for unsupported methods
- **StreamableHTTPHandler**: Add Allow header for GET-without-session case:
- Stateless mode: `Allow: POST, DELETE` (GET is never valid)
- Stateful mode: `Allow: GET, POST, DELETE` (GET is valid once you have
a session)
- Add tests to verify Allow header presence in all 405 responses
## RFC 9110 Reference
[Section 15.5.6 (405 Method Not
Allowed)](https://httpwg.org/specs/rfc9110.html#status.405):
> The origin server MUST generate an Allow header field in a 405
response containing a list of the target resource's currently supported
methods.
## Testing
All existing tests pass, plus new tests added:
- `TestSSE405AllowHeader` - verifies SSE handler compliance
- `TestStreamable405AllowHeader` - verifies Streamable handler
compliance in both stateful and stateless modes
---------
Co-authored-by: omgitsads <4619+omgitsads@users.noreply.github.com>
mcp: add DisableStandaloneSSE option to control standalone SSE stream
Currently, StreamableClientTransport always establishes a standalone SSE
connection after initialization to receive server-initiated messages.
While this is useful for receiving notifications like
ToolListChangedNotification, it causes problems in several real-world
scenarios:
1. **Unnecessary resource usage**: Many use cases (for me), especially
in scientific computing scenarios, only require simple request-response
communication. Clients don't need server-initiated notifications, making
the persistent SSE connection wasteful.
2. **Server compatibility issues**: Some third-party MCP servers don't
properly handle GET requests for SSE streams. When the client
automatically tries to establish the SSE connection, it fails or hangs,
blocking the entire connection process. This is particularly problematic
when using third-party servers that cannot be modified. This issue is
similar to what was discussed in issue #634.
3. **Lack of user control**: The MCP specification states that the
standalone SSE stream is optional ("The client MAY issue an HTTP GET"),
but the current SDK implementation doesn't provide a way to opt out.
This implementation is also inconsistent with other MCP SDKs like
github.com/mark3labs/mcp-go, which provides getListeningEnabled control
(defaults to false, requires explicit enablement).
Fixes#728
Co-authored-by: zhaoxin <zhaoxin@pjlab.org.cn>
Issue encountered: the MCP server request was unexpectedly redirected to
an HTML page. Because the client didn’t validate the response
Content-Type, it still entered handleSSE, which then kept treating the
HTML as an SSE stream and repeatedly re-requesting the same
page—creating an infinite request loop.
To be consistent with logging level, use a named string type for icon
themes. This improves readability / discoverability, and is more
consistent.
Updates #552
For #725
When POST requests in the streamableClientConn return a transient error,
return this error to the caller rather than permanently breaking the
connection.
This is achieved by using the special sentinel ErrRejected error to the
jsonrpc2 layer. In doing so, the change revealed a pre-existing bug:
ErrRejected had the same code as ErrConnectionClosing, and
jsonrpc2.WireError implements errors.Is, so the two sentinel values
could be conflated. This is fixed by using a new internal code.
There's more to do for #683: we should also retry transient errors in
handleSSE.
For #683
Change CloseSSEStream from func(time.Duration) to
func(CloseSSEStreamArgs) for forward compatibility, allowing new fields
to be added without breaking existing callers.
Debounce and batch notifications for changes in server feature sets
(adding or removing tools, prompts, resources, or resource templates).
This PR handles server-side change notifications only. We could do the
same for client-side notifications (e.g. roots changed), but at the
moment we don't need to.
Fixes#649.
I encountered these logs, and they were distracting and noisy. We
shouldn't log to stderr unless configured by the user. Thread through
the logger.
Also, use ensureLogger in the streamable client to simplify call sites.
Updates #708
Updates #684
I am seeing a bug where connections with a Streamable HTTP client are
not reused at all. This makes performance terrible, since each request
establishes a new TCP connection.
I was able to root cause this down to behavior of the `processStream`
function. This sets up a bufio.NewScanner, and waits to get data. Once
we get a message, we abort and *close the connection*.
At this point, there are two paths:
* The bufio reader read the *full* response body to EOF, and then we
closed. This case is fine; we can re-use the idle connection.
* The bufio reader *did not* real the full response body. It may have
read the entire actual data that *would be sent*, but since its
streaming happened to not yet be delivered. In this case, we call
`body.Close()` before an EOF, which causes the HTTP client to not
attempt to re-use the connection.
In my case, the server I am using reliable sends the response over 2 TCP
packets (one with the event, and the other with the closure of the
`chunked` encoding (`0`)). This causes the body to never be fully read.
This PR fixes that case by unconditionally reading the full body before
we close the connection. With this change, I am able to reliably re-use
connections for future requests.
Replace closure-based deliver and closeLocked fields with explicit
state:
- pendingJSONMessages: signals JSON mode and collects messages
- w: indicates stream is claimed by HTTP request
- done: signals stream completion (non-nil implies w is non-nil)
- lastIdx, supportsPrimeClose, logger: moved from closures to fields
Add stream methods to consolidate logic:
- close: sends close event and closes done channel
- release: releases stream from HTTP request for resumption
- deliverLocked: writes/stores messages, handles request accounting
Factor out hangResponse to document HTTP response lifecycle.
Inline writeEvent to eliminate confusing pointer-based index
management.
After a deep dive with the team, document areas of the codebase
identified as unclear. For now, this is a documentation-only commit (apart
from one switch of jsonrpc2.Request.ID.IsValid->Request.IsCall). Special
TODO(cleanup) comments are added for additional code changes in
follow-up commits.
Also, include high level doc comments for server and client transports,
in new streamable_server.go and streamable_client.go. Eventually, the
server and client transport logic should be migrated to these files, as
streamable.go has gotten too large. However, we can do that at a future
date, to minimize diff.
The prime and close SSE events (SEP-1699) were added in protocol version
2025-11-25. Only send these events when the client negotiates that
version or later.
- In servePOST, extract protocol version from InitializeParams for
initialize requests, otherwise use the Mcp-Protocol-Version header
- In serveGET/acquireStream, read protocol version from header
- Only set closeLocked callback when prime/close is supported
- Add unexported protocolVersion field to ClientSessionOptions for
testing
Fixes#686
Add a UserID field to auth.TokenInfo that TokenVerifiers can populate
from JWT "sub" claims or token introspection. The streamable HTTP
transport uses this to bind sessions to users, rejecting requests where
the user ID doesn't match the session's original user.
Fixes#589
After walking through our handling of streamable client context
cancellation (due to encountering a shutdown deadlock), I think I've
settled on a more coherent strategy for handling call cancellation:
- In our call handler, retire the request if the call exits due to
cancellation: the caller will never see the actual result anyway.
- In connectSSE, use the actual request context (the same context used
in Write) for the client request, so that it terminates when the context
is cancelled. Thread through the initialization context for the
standalone SSE request.
Also, a couple minor improvements:
- Use a detached context for the background context of the client
connection. We want to preserve context values (see #513), but it is not
right to cancel the connection after Connect has already returned, if
the context times out.
- Don't use Last-Event-ID != "" as the signal for whether the connectSSE
call is initial: if the standalone SSE stream disconnects without an
event ID, we'll still reconnect it, and don't want to do so without a
delay.
+ tests, updating the streamable client connection test harness to
accomodate the new aspects being exercised.
Fixes#662
This adds a simple schema and checks that the content returned by an
elicitation handler is validated against the schema. Previously, we had
no test coverage checking if the content was validated (i.e., we would
see no test failures if we removed content validation against the
schema).
The bulk of the actual validation logic is handled by the jsonschema
library so adding comprehensive tests covering different schema-features
is redundant, we simply need to see that some validation is happening.
See https://github.com/modelcontextprotocol/go-sdk/issues/625 for more
details on the motivation of this change.
Fixes#625
---------
Co-authored-by: Markus Kusano <kusano@google.com>
This CL implements support for SEP-1699, adding the following new API:
- Event.Retry corresponds to the 'retry' field of a server sent event.
This is used to signal client retry behavior.
- RequestExtra.CloseStream is set by the streamable transport to allow
closing a stream.
The streamable server transport is updated to set CloseStream for calls,
allowing server operations or middleware to close the underlying stream.
If the configured 'reconnectAfter' delay is set, a 'retry' message is
sent prior to terminating the stream.
The streamable client transport is updated to read the configured delay.
Fixes#630
This change updates the MCP client stdio JSON decoder to correctly
handle
both Unix (`\n`) and Windows (`\r\n`) line endings.
Fixes#664
Co-authored-by: qysz <i_zf@qq.com>
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
Add initial support for URL mode elicitation, by introducing the new
types, as well as both server and client-side validation.
A follow-up CL will add support for ErrURLElicitationRequired.
For #623
Implement SEP 986, enforcing restrictions on tool names. Since we're
already at v1 we couldn't turn this into a panic, so use an error log
instead. This is consistent with the typescript SDK. A note is left in
our 'rough edges' doc to panic in v2.
Fixes#621
Fixes#636
## Problem
The StreamableClientTransport tries to JSON-decode ALL SSE events,
including ping events sent by servers for keep-alive purposes. This
causes errors like 'invalid character 'p' looking for beginning of
value' when encountering non-JSON event data.
## Solution
Modified processStream to check evt.Name and skip events that are not
'message' events (or unnamed events, which default to 'message' per SSE
spec).
## Changes
- Added event name check in processStream before attempting JSON decode
- Added test case for ping event filtering
- Added test for scanning multiple events including ping events
## Testing
- All existing tests pass
- New tests verify ping events are properly filtered
- Tested with DeepWiki MCP server which sends ping events
mcp/protocol: Implement SEP-973
- Define Icon structure, which includes source, mimeType and sizes.
- sizes is any array of strings. refer
https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1531
- Support setting websiteUrl, icons for mcp.Implementation
- Support setting icons for mcp.Prompt
- Support setting icons for mcp.Tool
- Support setting icons for mcp.Resource
Fixes#552
Some MCP servers do not support SSE, but when receiving a request, they
return 400 Bad Request or 404 Not Found instead of the standard 405
Method Not Allowed. The TypeScript and Python SDK ignore this error,
while the Go SDK terminates the session. This patch addresses the issue
in the non-strict mode.
Fixes#610
---------
Signed-off-by: Xie Zhihao <zhihao.xie@bytedance.com>
If a stateful streamable session fails to initialize, it is unusable.
Avoid allocating resources by closing the session immediately.
Also:
- Fix a bug where InitializeParams is not guarded with its mutex.
- Fix a resource leak where sessions are persisted with session id "",
even though they are unaddressable.
- Add a relevant benchmark, and update tests.
Fixes#578
Add a timeout option for the streamable HTTP handler that automatically
cleans up idle sessions.
Also, fix a bug in the streamable client, where we hang on a request
even though the client can never get a response (because the HTTP
request terminated without a response or Last-Event-Id).
Fixes#499
Address longstanding TODOs to clean up stream metadata after streams are
complete:
- Interpret a missing stream as a complete stream.
- Remove the request->stream mapping when the request completes. Any
writes after that point will fail as there can be no recipient.
Tested using examples/client/loadtest, and updating
TestStreamableTransports to check in-band and out-of-band behavior.
This CL implements two changes that must be logically connected.
The first is to provide a mechanism for disabling stream replay.
Previously, our documentation stated that if
StreamableServerTransport.EventStore was nil, a default in-memory event
store would be used. In that case, there would be no way to completely
disable stream replay, and as described in #576, there are many cases
where replay is undesirable.
But of course changing the behavior of the transport zero value
implicitly affects its default behavior, and so this CL also implements
the proposal #580: stream replay is disabled by default.
Implementing this change required a significant refactoring, as
previously we were relying on the event store for serialized message
delivery from the JSON-RPC layer to the MCP layer: the connection would
write to the event store, and independently the stream (be it an
incoming POST or replay GET) would iterate and serve messages in the
stream.
In order to achieve the goals of this CL, it was necessary to decouple
storage from delivery. The 'stream' abstraction now tracks a delivery
callback that writes straight to the HTTP response. It would have been
convenient to store the ongoing http.ResponseWriter directly in the
stream (this is how typescript does it), but due to the design of our
EventStore API, only the HTTP handler knows the next event index, so a
'deliver' abstraction was an unfortunate requirement (suggestions for
how to further simplify this are welcome).
More simplification is possible: in particular, as a result of this
refactoring it should be entirely possible to clean up streams once
we've received all responses. Any replay would only need access to the
EventStore, if at all. This is left to a follow-up CL, to limit this
already significant change.
Furthermore, a nice consequence of this refactoring is that, when not
using event storage, servers can get synchronous feedback that message
delivery failed, which should avoid unnecessary work. We can
additionally cancel ongoing requests on early client termination, but
that is also left to a follow-up CL.
Throughout, the terminology 'standalone SSE stream' replaced 'hanging
GET' when referring to the non-replay GET request issued by the client.
This is consistent with other SDKs.
Fixes#576
Updates #580
This PR fixes goroutine leaks in all unit tests.
To find the leaks I integrated https://github.com/uber-go/goleak and I
suggest to keep using it to catch any future regressions.
I used the folowing script to more easily find individual tests which
were leaking goroutines:
```
for t in $(go test -run=Nothing -list=. ./mcp | grep -v ok); do go test ./mcp -run="$t" > /dev/null && echo -n . || echo -e "\nDetected leak: $t"; done
```
PR 501 added logging to the streamable server, but didn't pass a non-nil
logger to the streamable transport (an oversight). However, tests still
passed due to net/http's panic recovery.
Update test servers to enforce that handlers do not panic, and fix the
panic by:
1. Passing along the configured logger to the session transport.
2. Using ensureLogger in Connect, since we technically allow
StreamableServerTransports to be constructed by the user.
Fixes#556
mcp/server.go: implement server‑side logging
- Thread a *slog.Logger through server paths to improve debuggability
- Introduce no‑op discard handler + ensureLogger so loggers are never
nil
- Use ensured loggers in Server
- address todo comments related to adding logging support
All tests are passing and no functionality should change with this PR
Fixes https://github.com/modelcontextprotocol/go-sdk/issues/170
This commit adds ability to create a mcp.Transport with specific
io.ReadCloser and io.WriteCloser.
Ii is needed when you need create an mcp.Client for mcp.Server launched
in separate process and you need to communicate with it with it's own
stdin/stdout.
