c01fd188d2Summary
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>
108 lines
4.6 KiB
Go
108 lines
4.6 KiB
Go
// Example: A developer productivity platform (à la GitHub) that exposes
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// different tool sets for different agent types using MCP server variants.
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//
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// Variants:
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// - code-review: PR operations, diffs, and review comments
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// - project-management: issue tracking, labels, and assignments
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// - security-readonly: security scanning alerts and advisories (read-only)
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// - ci-automation: CI/CD workflow management and dispatch
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//
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// Run over stdio:
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//
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// go run .
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package main
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import (
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"context"
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"log"
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"github.com/modelcontextprotocol/go-sdk/mcp"
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"github.com/modelcontextprotocol/experimental-ext-variants/go/sdk/examples/server/exampletools"
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"github.com/modelcontextprotocol/experimental-ext-variants/go/sdk/variants"
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)
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func main() {
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// Code review variant: PR-focused tools for code review agents
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codeReviewServer := mcp.NewServer(&mcp.Implementation{Name: "devplatform", Version: "v1.0.0"}, nil)
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mcp.AddTool(codeReviewServer, &mcp.Tool{
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Name: "list_pull_requests",
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Description: "List open pull requests, optionally filtered by author",
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}, exampletools.ListPullRequests)
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mcp.AddTool(codeReviewServer, &mcp.Tool{
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Name: "get_diff",
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Description: "Get the diff for a pull request, including changed files and line counts",
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}, exampletools.GetDiff)
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mcp.AddTool(codeReviewServer, &mcp.Tool{
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Name: "add_review_comment",
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Description: "Post a review comment on a specific line of a pull request",
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}, exampletools.AddReviewComment)
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// Project management variant: issue-focused tools for PM agents
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pmServer := mcp.NewServer(&mcp.Implementation{Name: "devplatform", Version: "v1.0.0"}, nil)
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mcp.AddTool(pmServer, &mcp.Tool{
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Name: "list_issues",
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Description: "List issues, optionally filtered by state and labels",
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}, exampletools.ListIssues)
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mcp.AddTool(pmServer, &mcp.Tool{
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Name: "create_issue",
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Description: "Create a new issue with title, body, and optional labels",
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}, exampletools.CreateIssue)
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mcp.AddTool(pmServer, &mcp.Tool{
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Name: "add_label",
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Description: "Add labels to an existing issue",
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}, exampletools.AddLabel)
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// Security variant: read-only security scanning tools
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securityServer := mcp.NewServer(&mcp.Implementation{Name: "devplatform", Version: "v1.0.0"}, nil)
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mcp.AddTool(securityServer, &mcp.Tool{
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Name: "list_security_alerts",
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Description: "List code scanning alerts for a repository",
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}, exampletools.ListSecurityAlerts)
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mcp.AddTool(securityServer, &mcp.Tool{
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Name: "get_advisory",
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Description: "Get details of a security advisory",
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}, exampletools.GetAdvisory)
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// CI/CD variant: workflow management tools for automation agents
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ciServer := mcp.NewServer(&mcp.Implementation{Name: "devplatform", Version: "v1.0.0"}, nil)
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mcp.AddTool(ciServer, &mcp.Tool{
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Name: "list_workflow_runs",
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Description: "List recent workflow runs for a repository",
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}, exampletools.ListWorkflowRuns)
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mcp.AddTool(ciServer, &mcp.Tool{
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Name: "trigger_workflow",
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Description: "Trigger a workflow dispatch event",
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}, exampletools.TriggerWorkflow)
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vs := variants.NewServer(&mcp.Implementation{Name: "devplatform", Version: "v1.0.0"}).
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WithVariant(variants.ServerVariant{
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ID: "code-review",
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Description: "Pull request and code review operations. Includes diff viewing, review comments, approval workflows, and merge controls.",
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Hints: map[string]string{"domain": "code-review", "accessLevel": "read-write"},
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Status: variants.Stable,
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}, codeReviewServer, 0).
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WithVariant(variants.ServerVariant{
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ID: "project-management",
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Description: "Issue and project tracking operations. Includes issue CRUD, labels, milestones, assignments, and project boards.",
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Hints: map[string]string{"domain": "project-management", "accessLevel": "read-write"},
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Status: variants.Stable,
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}, pmServer, 1).
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WithVariant(variants.ServerVariant{
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ID: "security-readonly",
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Description: "Security scanning and vulnerability management. Read-only access to code scanning alerts, secret detection, and security advisories.",
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Hints: map[string]string{"domain": "security", "accessLevel": "readonly"},
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Status: variants.Stable,
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}, securityServer, 2).
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WithVariant(variants.ServerVariant{
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ID: "ci-automation",
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Description: "CI/CD workflow management. Trigger runs, monitor jobs, manage deployments designed for automation agents.",
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Hints: map[string]string{"domain": "ci-cd", "accessLevel": "automation"},
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Status: variants.Stable,
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}, ciServer, 3)
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ctx := context.Background()
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if err := vs.Run(ctx, &mcp.StdioTransport{}); err != nil {
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log.Fatal(err)
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}
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}
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