88831b46f6
Move the documentation that had grown to 3000+ lines in README.md onto https://ruby.sdk.modelcontextprotocol.io and slim the README down to a quick start and feature overview, following the Python SDK layout: - Add 21 server pages as the docs/_server/ collection (overview plus transports, discovery, tools, prompts, resources, roots, sampling, elicitation, multi round-trip results, notifications, notification subscriptions, cancellation, progress, ping, completions, logging, pagination, server context, configuration, and custom methods), 8 client pages as the docs/_client/ collection (overview, transports, lifecycle, multi round-trip results, cancellation, ping, pagination, and authorization), and 3 extension pages as the docs/_extensions/ collection (overview, capability extensions, and MCP Apps, following the Extensions Overview recommendation that SDK documentation list the supported extensions) - one page per topic, ordered to match the sidebar of the 2026-07-28 specification, with the client-side APIs (pinging, cancelling, and paginating from MCP::Client) documented under Building Clients - Add top-level Examples and Protocol Versions pages after Installation, linking the runnable examples in examples/ and summarizing the supported protocol versions, the era model, and client negotiation - Render the three sections as just-the-docs collections, which list every page beneath a plain category heading in the sidebar with no folding, and serve every page at an extensionless URL under /server/, /client/, and /extensions/, matching the sibling SDK documentation sites - Replace docs/building-servers.md and docs/building-clients.md with redirects to the new section overview pages via jekyll-redirect-from, redirect the previously published /installation.html to its extensionless URL, fold their content that was missing from README.md into the new pages, and update the Tool argument keys reference comment in lib/mcp/server.rb to the relocated Tools page - Reduce README.md to the badges, a compact feature overview in the Python SDK style, installation instructions, a stdio server and client quick start, and a License section, using absolute URLs for the remaining repository links so they resolve on rubygems.org - Adapt formatting where GitHub rendering habits break on the site: convert the numbered "three ways to define" lists into headings, since kramdown restarts numbering at 1 when code blocks split list items, convert GitHub-style alerts into just-the-docs callouts with the SEP-2260 server-to-client association note raised to a red warning, merge the duplicated Exception Reporting and Configuration Block Data sections into the Configuration page, and relocate the Streamable HTTP settings that were nested under the Logging section into the Transports page - Refresh the migrated content against the current implementation: correct stale claims and broken samples the README carried, note on each session-era feature how it relates to the modern lifecycle of MCP 2026-07-28, and point spec links at the latest revision, keeping deliberate 2025-11-25 pins for pages the modern revision removed - Style the site after the Rails API documentation palette (red links and accents on neutral surfaces) with matching light and dark color schemes, center the sidebar and content as one block, add a Previous/Next footer pager following the sidebar order, open external links in a new tab, and serve the MCP logo as the favicon Every code block and heading from the previous README was verified to have a home in the new docs pages or the slimmed README before the reduction; a few samples were corrected rather than copied, as noted above.
111 lines
3.9 KiB
Markdown
111 lines
3.9 KiB
Markdown
---
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layout: default
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title: Server Context
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nav_order: 19
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---
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# Server Context
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The `server_context` is a user-defined hash that is passed into the server instance and made available to [tool](/server/tools/) and [prompt](/server/prompts/) calls.
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It can be used to provide contextual information such as authentication state, user IDs, or request-specific data.
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**Type:**
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```ruby
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server_context: { [String, Symbol] => Any }
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```
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**Example:**
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```ruby
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server = MCP::Server.new(
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name: "my_server",
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server_context: { user_id: current_user.id, request_id: request.uuid }
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)
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```
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This hash is then passed as the `server_context` keyword argument to tool and prompt calls.
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Note that the exception reporter does not receive this user-defined hash, and instrumentation
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callbacks omit it unless you opt in with `instrument_server_context`.
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See the [Configuration](/server/configuration/) page for the arguments they receive.
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## Request-specific `_meta` Parameter
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The MCP protocol supports a special [`_meta` parameter](https://modelcontextprotocol.io/specification/latest/basic#general-fields) in requests that allows clients to pass request-specific metadata. The server automatically extracts this parameter and makes it available to tools and prompts as a nested field within the `server_context`.
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{: .note }
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> `_meta` is only merged when `server_context` is a `Hash` (or `nil`, in which case a new `{ _meta: ... }` hash is synthesized).
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> If you assign a non-`Hash` value to `server_context`, `_meta` is not merged and tools will not see it
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> under `server_context[:_meta]`. Keep `server_context` as a `Hash` if your tools need access to `_meta`.
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**Access Pattern:**
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When a client includes `_meta` in the request params, it becomes available as `server_context[:_meta]`:
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```ruby
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class MyTool < MCP::Tool
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def self.call(message:, server_context:)
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# Access provider-specific metadata
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session_id = server_context.dig(:_meta, :session_id)
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request_id = server_context.dig(:_meta, :request_id)
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# Access server's original context
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user_id = server_context.dig(:user_id)
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MCP::Tool::Response.new([{
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type: "text",
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text: "Processing for user #{user_id} in session #{session_id}"
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}])
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end
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end
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```
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**Client Request Example:**
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A `tools/call` request carrying `_meta`, as read by the tool above:
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```json
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{
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"jsonrpc": "2.0",
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"id": 1,
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"method": "tools/call",
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"params": {
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"name": "my_tool",
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"arguments": { "message": "Hello" },
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"_meta": {
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"session_id": "abc123",
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"request_id": "req_456"
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}
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}
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}
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```
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### Distributed Tracing
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Per SEP-414, the keys `traceparent`, `tracestate`, and `baggage` are reserved un-prefixed `_meta` keys for propagating
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[W3C Trace Context](https://www.w3.org/TR/trace-context/) across MCP requests. The SDK guarantees these keys pass through
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incoming request `_meta` untouched, and exposes their names as constants on `MCP::TraceContext` (`TRACEPARENT_META_KEY`,
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`TRACESTATE_META_KEY`, `BAGGAGE_META_KEY`, and `META_KEYS`). The SDK does not depend on OpenTelemetry; bridge the values
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to your tracing system yourself:
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```ruby
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class TracedTool < MCP::Tool
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def self.call(message:, server_context:)
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traceparent = server_context.dig(:_meta, :traceparent)
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# Hand traceparent/tracestate/baggage to your tracing library
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# (e.g. the opentelemetry-ruby gems) to continue the caller's trace.
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MCP::Tool::Response.new([{ type: "text", text: "ok" }])
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end
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end
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```
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On the client side, every request method (`call_tool`, `read_resource`, `get_prompt`, `complete`, `ping`, and the `list_*` methods)
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accepts a `meta:` keyword to inject these keys into the outgoing request, so trace context can flow on every request:
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```ruby
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meta = { MCP::TraceContext::TRACEPARENT_META_KEY => "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01" }
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client.call_tool(tool: tool, arguments: { message: "Hello" }, meta: meta)
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client.read_resource(uri: "file:///report.txt", meta: meta)
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```
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