mcp-best-practices

Build, secure, and optimize production MCP servers with the TypeScript SDK (spec 2025-11-25, SDK v1.29 / v2 beta). Use when building or reviewing MCP servers or tools - covering transports, tool and schema design, error handling, security and OAuth, performance, known SDK bugs, content vs structuredContent delivery, v2 migration, MCP Apps, extensions, and the Registry.

Decision reference for building production MCP servers with the TypeScript SDK. Not a tutorial - assumes you already have a working server and need to make it correct, fast, and secure.

Quick Reference

Component	Current	Next
Spec	2025-11-25 (specification)	2026-07-28 Release Candidate, locked 2026-05-21: stateless/sessionless overhaul (see "Spec 2026-07-28 RC Direction")
TS SDK (stable)	v1.29.0 (@modelcontextprotocol/sdk)	v2 beta published
TS SDK (v2)	Beta (2.0.0-beta.3 on npm, 2026-07-09; the latest dist-tag points at the beta, so a plain npm install resolves to the prerelease): /server, /client, /core, /hono, /express, /node, /fastify, /codemod (+ /server-legacy, deprecated, frozen at beta.2)	Stable ships with the final spec on 2026-07-28
JSON Schema	2020-12 default (explicit $schema supported)	-
Transport	Streamable HTTP (remote), stdio (local)	SSE + WebSocket removed in v2
Extensions	MCP Apps (Stable, SEP-1865), Auth Extensions (official)	Domain-specific WGs
Registry	Preview with v0.1 API freeze since 2025-10-24 (registry)	GA pending

v1 imports (production today):

typescript

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

v2 imports (v2 beta, installable now; stable pending 2026-07-28):

typescript

import { McpServer } from "@modelcontextprotocol/server";
import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/server";

Canonical SDK docs: ts.sdk.modelcontextprotocol.io (v1) and /v2/ (tutorial, troubleshooting, generated API reference). Test servers with the MCP Inspector; debugging guide at docs/tools/debugging.

Server Setup

Transport Decision

Scenario	Transport	Key Config
Remote, stateless (K8s, CF Workers)	WebStandardStreamableHTTPServerTransport	sessionIdGenerator: undefined, enableJsonResponse: true
Remote, stateful (long tasks, SSE)	WebStandardStreamableHTTPServerTransport	sessionIdGenerator: () => randomUUID()
Local CLI / Claude Desktop	StdioServerTransport	Default
Legacy SSE clients	SSE removed in v2 - migrate to Streamable HTTP	-

Stateless Pattern (recommended for remote deployment)

Per-request server+transport creation is the canonical pattern. Maintainer @ihrpr confirms: "each transport should have an instance of MCPServer" (#343). Sharing instances leaks cross-client data (GHSA-345p-7cg4-v4c7).

typescript

app.post("/mcp", async (c) => {
  const server = new McpServer({ name: "my-server", version: "1.0.0" });
  // Register tools, resources, prompts...
  registerTools(server);

  const transport = new WebStandardStreamableHTTPServerTransport({
    sessionIdGenerator: undefined,   // stateless - no session tracking
    enableJsonResponse: true,        // JSON responses, no SSE streaming
  });

  // All tools/resources must be registered before connect() (#893)
  try {
    await server.connect(transport);
    return transport.handleRequest(c.req.raw);
  } finally {
    await transport.close();
    await server.close();
  }
});

What to hoist to module level (don't recreate per request):

• Zod schemas (they never change)
• Annotation objects ({ readOnlyHint: true, ... })
• Tool description strings
• Payment configs, upstream API clients

The McpServer itself must be per-request, but its constant inputs should not be.

If you only route POST (the common stateless layout), answer GET /mcp with an explicit 405 Method Not Allowed - the spec requires 405 when no SSE stream is offered, and the official TS client treats 405 as the benign no-stream signal, while an empty 200 sends it into a reconnect storm. See references/transport-patterns.md.

For deep dive on transports, sessions, HTTP/2 gotchas, and K8s deployment: see references/transport-patterns.md

Framework Integration

Hono (web-standard):

typescript

import { Hono } from "hono";
const app = new Hono();
app.post("/mcp", handleMcpRequest);  // WebStandardStreamableHTTPServerTransport
app.get("/mcp", handleMcpSse);       // Optional: SSE for server notifications
app.delete("/mcp", handleMcpDelete); // Optional: session termination

Cloudflare Workers: Same pattern - WebStandardStreamableHTTPServerTransport works natively in Workers runtime.

Express/Node (v2): Use @modelcontextprotocol/express middleware with NodeStreamableHTTPServerTransport (wraps the Web Standard transport for IncomingMessage/ServerResponse).

Tool Design

Registration API

v1 (current stable) - server.tool(name, description, zodShape, annotations, handler). Positional overloads are ambiguous; same fields as v2 below minus outputSchema.

v2 (migration target) - registerTool() with config object:

typescript

server.registerTool("search_docs", {
  title: "Document Search",
  description: "Search documents by keyword or phrase",
  inputSchema: z.object({
    query: z.string().describe("Search query"),
    max_results: z.number().optional().describe("Max results (default 20)"),
  }),
  outputSchema: z.object({
    results: z.array(z.object({ id: z.string(), text: z.string() })),
    has_more: z.boolean(),
  }),
  annotations: { readOnlyHint: true, destructiveHint: false, idempotentHint: true, openWorldHint: true },
}, async ({ query, max_results }) => {
  const result = await fetchDocs(query, max_results);
  return {
    // Both channels carry IDENTICAL bytes. Divergent payloads = the text block
    // silently vanishes on Claude Code/Codex/Copilot. See "Tool Result Delivery" below.
    structuredContent: result,
    content: [{ type: "text", text: JSON.stringify(result) }],
  };
});

Naming

Spec (2025-11-25, SHOULD-level recommendations, not MUSTs): names SHOULD be 1-128 chars, case-sensitive. Allowed: A-Za-z0-9_-.

DO: search_docs, get_user_profile, admin.tools.list DON'T: search (too generic, collides across servers), Search Docs (spaces not allowed)

Service-prefix your tools (github_*, jira_*) when multiple servers are active - LLMs confuse generic names across servers.

Schema Rules

.describe() on every field - this is what LLMs use for argument generation.

For complete Zod-to-JSON-Schema conversion rules, what breaks silently, outputSchema/structuredContent patterns: see references/tool-schema-guide.md

Critical bugs (detail + status in the Known SDK Bugs table below; conversion deep-dive in the reference):

• z.union()/z.discriminatedUnion() silently produce empty schemas on all released v1 (#1643) - use flat z.object() + z.enum() discriminator.
• Raw JSON Schema objects throw at registration since v1.28 (#1596); z.transform() is silently stripped (#702).
• Client AJV rejects unstripped structuredContent extras (Zod v4 emits additionalProperties: false): .parse() upstream data before assigning, or .passthrough() for intentional extras.

Annotations

All are optional hints (untrusted from untrusted servers per spec):

Annotation	Default	Meaning
readOnlyHint	false	Tool doesn't modify its environment
destructiveHint	true	May perform destructive updates (only when readOnly=false)
idempotentHint	false	Repeated calls with same args have no additional effect
openWorldHint	true	Interacts with external entities (APIs, web)

Set them accurately - clients use them for consent prompts and auto-approval decisions.

The "Lethal Trifecta": combining (1) access to private data + (2) exposure to untrusted content + (3) external communication ability creates data-theft conditions (demonstrated via a malicious calendar event + MCP calendar server + code-execution tool). Design tool sets so no single agent holds all three.

Other Tool-Definition Fields (2025-11-25)

• icons (SEP-973): tools, resources, prompts, and implementations can carry icons: [{ src, mimeType, sizes }] for client UI display.
• execution.taskSupport: per-tool negotiation of task-augmented execution - "forbidden" (default), "optional", or "required". Pairs with the Tasks extension (see Extensions below).
• listChanged capability + notifications/tools/list_changed: declare tools: { listChanged: true } and emit the notification when the tool set changes at runtime - required plumbing if you adopt the dynamic tool loading strategy from "Token Bloat Mitigation".

Tool Result Delivery: content vs structuredContent

The footgun: when a tool returns BOTH a text content block and structuredContent, several major clients (Claude Code, Codex CLI, VS Code Copilot, Goose) silently drop the text block and forward only structuredContent to the model. If the two payloads differ, the human-readable one vanishes. This is client behavior the spec does not constrain - not an SDK transform. Don't return both channels expecting both to reach the model.

Empirically tested - Claude Code 2.1.165 (MCP 2025-11-25)

Measured with claude -p --output-format=stream-json, reading the exact tool_result the model received:

Tool returns	What the model receives
One text block, no structuredContent	text verbatim
content: [] + structuredContent	JSON.stringify(structuredContent) as a string in the content slot - works
text block + structuredContent	text block silently dropped; structuredContent wins
text + structuredContent + outputSchema	same - outputSchema makes zero difference
two text blocks, no structuredContent	both preserved verbatim

structuredContent is not a separate typed channel to the model on Claude Code - it is stringified into the standard tool_result content slot, so it costs the same tokens as the equivalent JSON-as-text. It does not buy cheaper or out-of-band structured data.

Intentional, per Anthropic maintainer (anthropics/claude-code#9962): structuredContent support landed in Claude Code v2.0.21 and "we made structuredContent the default when both formats are present... optimizing for agent performance." Reproduced across unrelated servers (Laravel, Roblox Studio, YouTube) - host-side precedence, not a server bug.

What the spec actually says (2025-11-25)

• content is required on CallToolResult (content: ContentBlock[]); structuredContent? and isError? are optional. An empty content: [] is schema-valid.
• Backwards-compat SHOULD (the only relevant normative line): "a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block."
• No precedence rule. The spec never says which field a client should prefer when both are present (Discussion #1563; clarification in flight via SEP-1624 -> SEP-2200). That gap is the documented root cause of client divergence.
• outputSchema: servers MUST produce conforming structuredContent; clients SHOULD (not MUST) validate it.

The official TypeScript SDK passes both fields through verbatim on server and client (the only mutation is optional outputSchema validation, which can throw). Any stringify-into-content you observe is the host harness, not the SDK.

Cross-client behavior (the matrix above is Claude Code only)

Client	When both content + structuredContent present
Claude Code CLI, OpenAI Codex CLI, VS Code Copilot, Goose	shadow - only structuredContent reaches the model (text dropped)
Cursor, Claude.ai web, ChatGPT MCP connector	prefer content / surface both to the model
Google ADK (framework)	forwards both by default; content-only is opt-in

VS Code maintainer's framing (microsoft/vscode#290063): "structuredContent actually should not be presented to the model, its use case is PTC [programmatic tool calling]." Clients disagree on enforcing that, so portable servers can't rely on it either way. (Non-Claude-Code rows come from issue trackers/maintainer statements, not the stream-json harness - treat exact delivery as client-version-dependent.)

The rule for server authors

• DON'T return divergent content and structuredContent (e.g. a rendered ASCII table as text + different JSON as structured). On shadowing clients the text silently disappears and only the JSON reaches the model.
• DO, if you emit structuredContent, mirror the same bytes into a text block: content: [{ type: "text", text: JSON.stringify(payload) }]. This is the spec's backwards-compat SHOULD. Shadowing clients use the structured copy; others fall back to the identical text - either way the model gets the data. Mirroring does not double tokens on shadowing clients (they drop the text).
• PREFER one channel per tool / per mode. For a human-readable rendering (table, summary) to reach the model, return it as text only, no structuredContent - or expose a format: "table" | "json" arg (table -> text-only; json -> JSON mirrored into both channels). Both are empirically valid on Claude Code and keep one channel per call.
• outputSchema gates client-side validation only; it does not make the text block survive on shadowing clients.

Beyond Text: Content Types

content blocks are not text-only: the spec defines image, audio, resource_link, and embedded resource blocks, all supporting optional annotations (audience, priority, lastModified). Resource links returned by tools are not guaranteed to appear in resources/list. For image-returning tools, don't inline full-resolution base64 - see references/tool-schema-guide.md for content types and the preview + URL pattern.

Error Handling

Two distinct mechanisms with different LLM visibility:

Type	LLM Sees It?	Use For
Tool error (isError: true in CallToolResult)	Yes - enables self-correction	Input validation, API failures, business logic errors
Protocol error (JSON-RPC error response)	Maybe - clients MAY expose	Unknown tool, malformed request, server crash

Per SEP-1303 (merged into spec 2025-11-25): input validation errors MUST be tool execution errors, not protocol errors. The LLM needs to see "date must be in the future" to self-correct.

typescript

// DO: Tool execution error - LLM can self-correct
return {
  isError: true,
  content: [{ type: "text", text: "Date must be in the future. Current date: 2026-03-25" }],
};

// DON'T: Protocol error for validation - LLM can't see this
throw new McpError(ErrorCode.InvalidParams, "Invalid date");

Known SDK behavior: When the SDK converts an McpError thrown from a tool handler into a CallToolResult, the error.data field is dropped. If you embed structured data in McpError's data field, it may not reach the client. The x402/MPP MCP ecosystem standardized on isError: true tool results with structuredContent for this reason. (One exception: code -32042 "Payment Required" survives McpServer end-to-end with error.data intact - see references/error-handling.md.)

For full error taxonomy, code examples, and payment error patterns: see references/error-handling.md

Resources and Instructions

Server Instructions

Set in the initialization response - acts as a system-level hint to the LLM about how to use your server:

typescript

const server = new McpServer({
  name: "docs-api",
  version: "1.0.0",
  instructions: "Knowledge base API. Use search_docs for full-text search, get_doc for retrieval by ID. All tools are read-only.",
});

Resource Registration

Expose documentation or structured data via docs:// URI scheme:

typescript

server.resource("search-operators", "docs://search-operators", {
  title: "Search Operators Guide",
  description: "Supported search operators and syntax",
  mimeType: "text/markdown",
}, async () => ({
  contents: [{ uri: "docs://search-operators", text: operatorsMarkdown }],
}));

Other Server Primitives

Beyond tools, the spec (2025-11-25) defines primitives a production server often needs. All are optional capabilities negotiated at initialization; a server that omits them still conforms.

Primitive	Methods	When you need it
Prompts	prompts/list, prompts/get (registerPrompt)	Reusable, parameterized prompt templates users invoke by name (slash-commands, canned workflows). Args are completable.
Resource Templates	resources/templates/list (RFC 6570 URI templates)	Parameterized resources - docs://{id} instead of enumerating every static URI. Template variables are completable.
Pagination	opaque cursor param + nextCursor in result, on every */list	Large tool/resource/prompt catalogs. The cursor is opaque - never parse or synthesize it; loop until nextCursor is absent. Distinct from in-tool offset/limit args.
Completions	completion/complete	Argument autocomplete for prompt args and resource-template variables. Return ranked candidates with hasMore/total hints.
Cancellation	notifications/cancelled	Client aborts an in-flight long request by id. Honor it via the handler's abort signal (extra.signal v1 / ctx.mcpReq.signal v2) - stop work, release resources.

Pagination is the one most servers actually hit first: a tools/list (or resources/list) with 50+ entries should paginate rather than dump everything in one response.

Performance

Module-Level Caching

The McpServer must be per-request, but everything else can be shared:

typescript

// Module-level (created once)
const SCHEMAS = {
  search: z.object({ query: z.string().describe("Search query") }),
  fetch: z.object({ id: z.string().describe("Resource ID") }),
};
const READ_ONLY_ANNOTATIONS = {
  readOnlyHint: true, destructiveHint: false, idempotentHint: true, openWorldHint: true,
} as const;

// Per-request (created each time)
function createMcpServer(ctx: Context) {
  const server = new McpServer({ name: "my-server", version: "1.0.0" });
  server.tool("search", "Search", SCHEMAS.search, READ_ONLY_ANNOTATIONS, handler);
  return server;
}

Token Bloat Mitigation

Tool definitions consume context window before any conversation starts. GitHub MCP: 20,444 tokens for 80 tools (SEP-1576).

Strategies:

1. 5-15 tools per server - community sweet spot. Split beyond that.
2. Outcome-oriented tools - bundle multi-step operations into single tools (e.g., track_order(email) not get_user + list_orders + get_status).
3. Response granularity - return curated results, not raw API dumps. 800-token user object vs 20-token summary.
4. outputSchema + structuredContent - typed output for programmatic/PTC clients. Caveat: on shadowing clients (Claude Code et al.) structuredContent is stringified into the model's context at the same token cost as text - it is not a free out-of-band channel. See "Tool Result Delivery: content vs structuredContent".
5. Dynamic tool loading - register only relevant tool subsets based on request context (e.g., ?tools=search,fetch query parameter). Pair with the listChanged capability if the set changes mid-session.
6. Progressive tool discovery / code mode - clients with large catalogs increasingly use a search_tools meta-tool and programmatic tool calling (code mode), where structuredContent is consumed outside the model context. Both are documented in client best practices - design curated, well-described tools so these flows work.

Result-Size Budgets (per-client caps)

Clients silently truncate large tool results. Budget for the strictest client you target:

Client	Default cap	Configurable
Claude Code	25,000 tokens (warning at 10k)	MAX_MCP_OUTPUT_TOKENS env; per-tool _meta["anthropic/maxResultSizeChars"] up to 500,000 chars
OpenAI Codex CLI	10,000 bytes on byte-policy models (includes the JSON envelope)	tool_output_token_limit config
Gemini CLI	40,000 chars (head 20% / tail 80% trim; full output saved to a file)	settings; 0 or negative disables

Enforce your own cap server-side - see "Result-Size Budgets and Truncation" in references/tool-schema-guide.md. Two rules worth stating here: never truncate isError results (payment/auth challenges must survive intact), and treat client budgets as per-connection properties - accept them as URL query params (?max_chars=, alongside ?tools=) rather than growing every tool schema with override args.

No-Parameter Tools

For tools with no inputs, use explicit empty schema:

typescript

inputSchema: { type: "object" as const, additionalProperties: false }

Security

Top Threats (real-world incidents, 2025-2026)

Attack	Example	Mitigation
Tool poisoning	Hidden instructions in descriptions (WhatsApp MCP, Apr 2025)	Review tool descriptions; clients should display them
Supply chain	Malicious npm packages (Smithery breach, Oct 2025)	Pin versions, audit dependencies
Command injection	child_process.exec with unsanitized input (CVE-2025-53967)	Never interpolate user input into shell commands
Stdio config injection	User-controlled input reaches StdioServerParameters without sanitization (OX Security disclosure, 2026-04-15)	Sanitize stdio config inputs in client code; prefer first-party servers; treat by Anthropic as "by design" - not patched in SDK
Cross-server shadowing	Malicious server overrides legitimate tool names	Service-prefix tool names; validate tool sources
Token theft	Over-privileged PATs with broad scopes	Minimal scopes; OAuth 2.1 Resource Indicators (RFC 8707)
Token passthrough	Server accepts/forwards tokens not issued for it	Validate audience claim; never transit client tokens to upstream APIs
SSRF	Malicious OAuth metadata URLs targeting internal services	HTTPS enforcement, block private IPs, validate redirect targets
Confused deputy	Proxy server consent cookies exploited via DCR	Per-client consent before forwarding to third-party auth
Session hijacking	Stolen/guessed session IDs for impersonation	Cryptographically random IDs, bind to user identity, never use for auth
Cross-client response leak	Shared McpServer/transport reused across clients (CVE-2026-25536, affects v1.10.0-1.25.3)	Require SDK ≥ v1.26.0; per-request server+transport
UriTemplate ReDoS	Malicious URI patterns (CVE-2026-0621)	Upgrade to v1.25.2+ / v2.0.0-alpha.1+

Server-Side Requirements (spec normative)

• Validate all inputs at tool boundaries
• Implement access controls per user/session
• Rate limit tool invocations
• Sanitize outputs before returning to client
• Validate Origin header - respond 403 for invalid origins (2025-11-25 requirement)
• Require MCP-Protocol-Version header on all requests after initialization (spec 2025-06-18+)
• Bind local servers to localhost (127.0.0.1) only

Auth (OAuth 2.1)

MCP normatively requires OAuth 2.1 (draft-ietf-oauth-v2-1-13; "Authorization servers MUST implement OAuth 2.1"), not 2.0 - PKCE mandatory, implicit flow removed. Servers are OAuth 2.1 Resource Servers; clients MUST send Resource Indicators (RFC 8707) binding tokens to your server.

• Validate audience - reject tokens not issued for your server (passthrough is forbidden)
• PKCE S256, short-lived tokens, minimal scopes (elevate via WWW-Authenticate challenges)
• Use a tested validation library (Keycloak, Auth0, ...) - don't roll your own; never log Authorization headers/tokens/secrets

For full security attack/mitigation patterns and auth implementation details: see references/security-auth.md

Known SDK Bugs

Issue	Severity	Status	Workaround
#1643 - z.union()/z.discriminatedUnion() silently dropped	High	Fixed in v2 line (PR #1796); v1.x backport PR #2017 still open	Use flat z.object() + z.enum() - bug present on all released v1
#1699 - Transport closure stack overflow (15-25+ concurrent)	High	Fixed in PR #1788 (closed 2026-04-02)	Upgrade to ≥ v1.29.0 / v2 beta
#1619 - HTTP/2 + SSE Content-Length error	Medium	Closed (reclassified to upstream @hono/node-server#266)	Use enableJsonResponse: true or avoid HTTP/2 upstream
#893 - Dynamic registration after connect blocked	Medium	Open	Register all tools/resources before connect()
#1596 - Plain JSON Schema silently dropped	Fixed	v1.28.0	Upgrade to v1.28+
Client AJV strict rejects unstripped structuredContent extras	High	Behavior, not bug	Server .parse() upstream data before returning, or use .passthrough()
GHSA-345p-7cg4-v4c7 / CVE-2026-25536 - Shared instances leak cross-client data	Critical	Fixed v1.26.0	Require ≥ v1.26.0 (or v2.0.0-alpha.1+); per-request server+transport
CVE-2026-0621 - UriTemplate ReDoS	Medium	Fixed v1.25.2 / v2.0.0-alpha.1	Upgrade

V2 Migration

For comprehensive migration guide with all breaking changes and before/after code: see references/v2-migration.md

Key breaking changes:

1. Package split: @modelcontextprotocol/sdk -> @modelcontextprotocol/server + /client + /core
2. ESM-first (CJS builds restored in beta.2), Node.js 20+ (Bun/Deno supported)
3. Zod v4 required (or any Standard Schema library)
4. McpError -> ProtocolError (from @modelcontextprotocol/core)
5. extra parameter -> structured ctx with ctx.mcpReq
6. server.tool() -> registerTool() (config object, not positional args)
7. SSE server transport removed (clients can still connect to legacy SSE servers)
8. @modelcontextprotocol/hono and @modelcontextprotocol/express middleware packages
9. DNS rebinding protection enabled by default for localhost servers

v1.x gets 6 more months of support after v2 stable ships. No rush, but write new code with v2 patterns in mind.

Spec 2026-07-28 RC Direction (post-2025-11-25)

2025-11-25 is the latest released revision (everything above targets it). The next revision is a locked Release Candidate: "The release candidate is locked as of May 21, 2026. The final specification will be published on July 28, 2026" (RC announcement); the changelog still renders under "draft". The TS SDK v2 beta implements the 2026-07-28 wire contract - multi-round-trip requests, subscriptions/listen, server/discover, header routing (#2286). Stable v2 ships alongside the final spec on 2026-07-28.

Decision-relevant shifts:

• MCP becomes stateless and sessionless. The draft removes the initialize/notifications/initialized handshake and the Mcp-Session-Id header entirely (SEP-2575, SEP-2567). Every request carries its protocol version, client identity, and capabilities in _meta. Servers needing cross-call state return a server-minted handle from a creation tool and accept it as an ordinary tool argument - not protocol sessions. This validates the skill's existing "prefer stateless" stance; do not build new servers on session affinity.
• server/discover RPC replaces initialize-time negotiation for advertising versions/capabilities/identity (SEP-2575).
• subscriptions/listen replaces the HTTP GET stream and resources/subscribe/resources/unsubscribe - one long-lived POST-response stream with opt-in notification types (SEP-2575). Servers SHOULD emit an SSE comment keep-alive (:\r\n); clients MUST ignore SSE comment lines (#2954). The same SEP-2575 pass removes ping, logging/setLevel, and notifications/roots/list_changed (log level moves per-request into _meta).
• SSE resumability is removed - the Last-Event-ID header and SSE event IDs leave Streamable HTTP (SEP-2575); clients MUST re-issue an interrupted request as a new request with a new ID. Don't build new replay/event-store infrastructure.
• Server-initiated requests are only allowed while the server is actively processing a client request (SEP-2260) - earlier revisions recommended this; the RC makes it required.
• Elicitation cleanup: notifications/elicitation/complete and URL-mode elicitationId (both introduced in 2025-11-25) are removed; sampling's includeContext values "thisServer"/"allServers" are reclassified Deprecated (SEP-2596).
• Multi Round-Trip Requests (MRTR) replace server-initiated requests (roots/list, sampling/createMessage, elicitation/create): a tool returns inputRequests; the client answers with inputResponses on the next call (SEP-2322). All results now carry a required resultType (complete | input_required); an input_required result is an InputRequiredResult carrying inputRequests.
• Error-code allocation policy + renumbering (PR #2907): -32000..-32019 implementation-defined, -32020..-32099 reserved for the spec. HeaderMismatch -32001->-32020, MissingRequiredClientCapability -32003->-32021, UnsupportedProtocolVersion -32004->-32022; resource-not-found settles on -32602.
• OpenTelemetry trace-context propagation via _meta (traceparent, tracestate, baggage) for cross-service tracing (SEP-414).
• Formal feature lifecycle (Active/Deprecated/Removed, 12-month minimum window, deprecated registry, SEP-2596). Under it, Roots, Sampling, and Logging are now formally Deprecated (SEP-2577, the advisory deprecation above made formal) and the HTTP+SSE transport is reclassified Deprecated (SEP-2596).
• Auth: OAuth 2.0 Dynamic Client Registration is deprecated in favor of Client ID Metadata Documents (CIMD) (PR #2858); clients MUST validate a present iss (RFC 9207, SEP-2468) and key persisted credentials by issuer (SEP-2352). Further hardening: clients declare their OIDC application_type at registration (SEP-837), OIDC-flavored refresh-token guidance (SEP-2207), scope accumulation during step-up (SEP-2350), and .well-known discovery-suffix clarification (SEP-2351) - see references/security-auth.md.
• Caching: list/read results gain required ttlMs + cacheScope (public/private) via a CacheableResult interface (SEP-2549); tools SHOULD be returned in deterministic order for prompt-cache hits.
• HTTP: Streamable HTTP POSTs require Mcp-Method/Mcp-Name headers, with optional x-mcp-header to mirror tool params into headers (SEP-2243).
• Schemas loosen: inputSchema/outputSchema accept any JSON Schema 2020-12 keywords (with $ref resolution), and structuredContent may be any JSON value (SEP-2106).

The content vs structuredContent dual-delivery footgun is unchanged in the RC - the backwards-compat SHOULD persists and no precedence rule landed, so the guidance above still holds.

Ecosystem gates around the RC: a Standards-Track SEP can no longer reach Final without a matching scenario in the conformance suite (SEP-2484), which also scores official SDKs for the SDK tier system; a Server Card working group is standardizing .well-known/mcp.json server self-description.

Extensions

MCP extensions are optional, strictly additive capabilities on top of the core protocol. Both sides negotiate support during initialization via extensions in capabilities.

Identifiers: {vendor-prefix}/{extension-name}. Official: io.modelcontextprotocol/*. Third-party: reversed domain (e.g., com.example/my-ext).

Official Extensions

Extension	Identifier	Purpose
MCP Apps	io.modelcontextprotocol/ui	Interactive HTML UIs in chat (charts, forms, dashboards)
OAuth Client Credentials	io.modelcontextprotocol/oauth-client-credentials	Machine-to-machine auth (CI/CD, daemons, server-to-server)
Enterprise-Managed Auth	io.modelcontextprotocol/enterprise-managed-authorization	Centralized access control via enterprise IdP

Client support: Claude (web + Desktop), ChatGPT, VS Code Copilot, Goose, Postman, MCPJam, Microsoft 365 Copilot, Cursor, Archestra.AI, and PostHog Code all support MCP Apps (client matrix). Among auth extensions, Enterprise-Managed Authorization reached Stable (2026-06-18) with Archestra.AI as the first client shipping it; OAuth Client Credentials is still Draft with no client adoption yet.

For MCP Apps architecture, ext-apps SDK, and build patterns: see references/mcp-apps.md For extensions system, auth extensions, and MCP Registry: see references/extensions-registry.md

Server Capabilities Beyond Tools

Capability	Purpose	v2 API
Elicitation	Request structured user input mid-tool	ctx.mcpReq.elicitInput()
Sampling	Request LLM completion from client	ctx.mcpReq.requestSampling()
Tasks	Long-running ops with lifecycle management	Official extension (SEP-2663)
Progress	Incremental progress on requests	ctx.mcpReq.sendProgress()

Deprecation / status notice: SEP-2577 (final, 2026-05-15) advisory-deprecates Roots, Sampling, and Logging - no wire changes, features stay functional for 1+ year, but design new servers without them. Tasks moved out of the core 2025-11-25 spec (the experimental tasks feature there is removed) into the official io.modelcontextprotocol/tasks extension (SEP-2663, final, 2026-05-15; redesigned in the draft): a server may answer tools/call with an async task handle the client polls via tasks/get + tasks/update (tasks/cancel to abort). The draft redesign drops the blocking tasks/result and tasks/list methods and allows servers to return task handles unsolicited.