Simplify variant dispatch and session by replacing per-method
boilerplate with generic handlers backed by reflect.
## Motivation and Context
The dispatcher and backend session had separate handler functions for
every MCP method (ListTools, ListResources, CallTool, Subscribe, etc.),
each following an identical pattern: extract params, inject metadata,
call inner handler, cast result. This meant ~250 lines of near-identical
code across dispatch.go and session.go, and every new MCP method
required adding another copy.
## How Has This Been Tested?
All existing unit and integration tests pass (go test -v ./...)
## Breaking Changes
None. All changes are internal to the variants package. Public API is
unchanged.
## Types of changes
Refactor (non-breaking change which fixes an issue)
## Summary
- Switched variant dispatch from `ClientSession` RPC calls to direct
inner server handler invocation via a new `backendSession` type,
preserving `context.Context` values (e.g. `RequestContext`) that were
previously lost crossing the in-memory transport boundary
- Added `backendSession` abstraction in `session.go` that mirrors the
`mcp.ClientSession` API but calls the inner server's handler chain
directly, forwarding `RequestExtra` (token info, HTTP headers, etc.)
- Captured the inner server's handler chain via receiving middleware
(`captureMCPMethodHandler`) in `backend.go`, reused across all
connections
- Simplified `innerConnection` to hold `*backendSession` + cleanup;
in-memory transport/client kept alive only for notification forwarding
(progress, logging)
- Refactored `getSession` → `getConnection` returning `*innerConnection`
to reduce indirection in dispatch code
- Moved `injectVariantMeta` calls to after type assertion and nil checks
in all dispatch handlers for consistency and nil-safety
## Test plan
- [x] Existing variant dispatch tests pass (`go test ./variants/...`)
- [x] Verify context values (e.g. `RequestContext`) are available in
tool handlers when invoked through variant dispatch
- [x] Verify `RequestExtra` fields (token info, HTTP headers) are
forwarded to inner server handlers
Summary
Builds on the types and architecture from the previous PR to deliver a
working variant-aware MCP
server that routes requests to inner mcp.Server instances based on
per-request _meta variant selection,
enabling a single MCP endpoint to serve multiple tool/resource/prompt
variants simultaneously.
```go
// Create standard mcp.Server instances — one per variant
codeReviewServer := mcp.NewServer(&mcp.Implementation{Name:
"devplatform", Version: "v1.0.0"}, nil)
mcp.AddTool(codeReviewServer, &mcp.Tool{Name: "list_pull_requests",
...}, handler)
mcp.AddTool(codeReviewServer, &mcp.Tool{Name: "get_diff", ...}, handler)
pmServer := mcp.NewServer(&mcp.Implementation{Name: "devplatform",
Version: "v1.0.0"}, nil)
mcp.AddTool(pmServer, &mcp.Tool{Name: "list_issues", ...}, handler)
mcp.AddTool(pmServer, &mcp.Tool{Name: "create_issue", ...}, handler)
// Register them as variants on a single server
vs := variants.NewServer(&mcp.Implementation{Name: "devplatform",
Version: "v1.0.0"}).
WithVariant(variants.ServerVariant{
ID: "code-review",
Description: "PR and code review operations",
Status: variants.Stable,
}, codeReviewServer, 0).
WithVariant(variants.ServerVariant{
ID: "project-management",
Description: "Issue and project tracking operations",
Status: variants.Stable,
}, pmServer, 1)
// Future: register variants backed by remote MCP servers
// WithHTTPVariant(v, mcpServer, priority) — variant backed by an
mcp.Server exposed over HTTP
// WithRemoteVariant(v, "https://...", priority) — variant backed by a
remote MCP endpoint
// Serve over HTTP for multiple concurrent clients
handler := variants.NewStreamableHTTPHandler(vs, nil)
http.ListenAndServe(":8080", handler)
// Or serve over stdio for single-client use:
// vs.Run(ctx, &mcp.StdioTransport{})
```
Key additions
- Backend abstraction (inMemoryBackend) — creates in-memory transport pairs between the variant server and
inner mcp.Server instances, with notification forwarding for progress and logging events.
- Request dispatcher (dispatch.go) — handles routing for tools/list, tools/call, resources/list,
resources/read, resources/subscribe, prompts/list, prompts/get, and completion/complete methods.
Implements variant-scoped cursor wrapping to correctly paginate across multiple inner servers.
- Session middleware (session.go) — manages per-session inner connections in stateful mode and shared
connections in stateless HTTP mode, ensuring proper lifecycle management.
- Initialize enrichment — injects availableVariants into the server's capabilities response so clients can
discover which variants are available.
- Capability discovery — merges inner server capabilities via union to present a unified capability set to
clients.
- NewStreamableHTTPHandler — provides an HTTP handler for multi-client serving over Streamable HTTP
transport.
- Integration tests (server_test.go, 458 lines) — covers end-to-end variant selection, backward
compatibility (no variant specified), HTTP transport, and stateless modes.
- Examples — stdio-based (examples/server/variants/main.go) and HTTP-based
(examples/server/variants-http/main.go) example servers demonstrating the API.
- README — rewritten with API documentation and architecture diagrams.
TODO (subsequent PRs):
- Unit Testing
- More integration tests
Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net>
