Protocol

Transports

Transport	Entry point	Use case
Streamable HTTP	McpAction (PSR-15)	Production: an MCP client connects over the network with a shared-secret header.
stdio	McpServeCommand (mcp:serve)	Local development: the client spawns the command as a subprocess and speaks JSON-RPC over stdin/stdout.

Both are served from the same Mcp\Server instance built by McpServerFactory — a tool class registers identically for either transport.

Streamable HTTP

McpAction::handle() runs the SDK's StreamableHttpTransport against the current PSR-7 request. Requests are POST/GET/DELETE/OPTIONS; a session is created on initialize and identified afterward by the Mcp-Session-Id response header, which the client echoes back on every subsequent call. Behind that:

• CORS, DNS-rebinding protection, and protocol-version middleware wrap every call when allowed_hosts is configured — the SDK's default host allow-list (localhost, 127.0.0.1, [::1]) always applies; production deployments behind a real domain add it to allowed_hosts.
• The response may switch to Server-Sent Events framing mid-call whenever the handler suspends to send a progress/log notification or a sampling/ elicitation round trip — clients must handle both plain-JSON and SSE-framed bodies.

stdio

McpServeCommand extends Symfony\Component\Console\Command\Command directly and runs the SDK's stdio transport over the process's own stdin/ stdout. There is no HTTP request, so SharedSecretMiddleware never runs and the resolved client id is always null for interceptors and caching — see Security.

Sessions

An MCP Streamable HTTP session spans several HTTP requests: initialize first, then tools/call (and friends) carrying the returned Mcp-Session-Id. The SDK's default in-memory session store would lose the session between PHP-FPM workers, so this package binds Session\PrivateFileSessionStore by default — file-based, owner-only (0700 directory, 0600 files), under an application-specific directory derived from server_name. For multi-host deployments, rebind SessionStoreInterface to a shared PSR-16-backed store (Mcp\Server\Session\Psr16SessionStore).

Sessions are also bound to the client that created them when a client identity is configured — see Security: session ownership for the exact enforcement.

Capabilities

The SDK negotiates capabilities during initialize based on what the built Server actually has: tools, resources (with resources.subscribe whenever any resource exists), prompts, and completion. See Capabilities for how each is declared and served.

Protocol version

The served MCP revision comes from the SDK (2025-11-25 under the current ~0.7.0 pin) and is not negotiated — initialize answers with this revision regardless of what the client requested. Pin a specific revision with protocol_version in params; an unsupported value fails at config-load time, not on the first request. Do not hardcode a revision anywhere in application code that talks to this server — read it from the SDK's own ProtocolVersion enum, exactly like Testing\McpTester does, so a test client and the server can never silently disagree.