json-rpc-server

08 — MCP

Model Context Protocol is Anthropic’s standard for exposing tools and resources to LLM clients (Claude Desktop, your own LLM-driven agents, etc.). The bundle exposes any RPC method as an MCP tool without duplicate code paths.

Two endpoints

Path	Method	Returns
`/mcp/tools`	GET	`{"tools": [{name, description, roles, inputSchema, outputSchema?, annotations?}]}`
`/mcp/call`	POST	`{"content": [...], "structuredContent": ...}`

/mcp/call body shape:

{ "name": "user.get", "arguments": { "email": "x@y" } }

Enabling MCP

MCP is off by default — /mcp/tools and /mcp/call are not registered until you turn it on:

json_rpc_server:
  mcp:
    enabled: true

The Flex recipe ships with mcp.enabled: false for the same reason: most projects don’t consume MCP, and a live /mcp/tools endpoint is a small fingerprinting surface for anonymous callers.

Opting in methods

Once MCP is enabled, only methods with #[Rpc\Mcp] are exposed:

#[Rpc\Method('user.get')]
#[Rpc\Mcp(description: 'Look up a user by email.')]
final class GetUser { /* … */ }

To expose everything except a few:

json_rpc_server:
  mcp:
    expose_all: true
    exclude_prefixes: ['internal.', 'debug.']
    exclude_methods: ['user.delete']

To deny everything except a few:

json_rpc_server:
  mcp:
    whitelist_methods: ['user.get', 'user.list']

Filter priority (first match wins):

exclude_methods — explicit deny
whitelist_methods — explicit allow
#[Rpc\Mcp(enabled: false)] — developer opt-out
Method is deprecated (and no explicit #[Rpc\Mcp]) → hidden
exclude_prefixes — bulk deny
expose_all: true → exposed
#[Rpc\Mcp] present → exposed
Otherwise → hidden

Operator config (exclude_*, whitelist_*) wins over the developer’s attribute — the deployment owner gets the final say.

Disabling MCP entirely

mcp.enabled: false (the default) removes the routes and services. To turn it back off after enabling:

json_rpc_server:
  mcp:
    enabled: false

JsonSchemaBuilder stays available either way, so debug:rpc --openrpc still works.

Input schema

The bundle precomputes a JSON Schema draft-07 fragment for every method’s input at container compile time. /mcp/tools serves these directly — no reflection on each request.

Coverage:

PHP / Symfony source	JSON Schema
`string`, `int`, `float`, `bool`, `array`	`{type: "..."}`
`array` + PHPDoc `list<Dto>` / `Dto[]`	`{type: "array", items: {<Dto object schema>}}` — `items` is a schema object, not `[]`
`array` + PHPDoc `array<string, Dto>`	`{type: "object", additionalProperties: {<Dto schema>}}` — matches JSON object maps in `params` (not JSON arrays)
`?T`	`{type: ["T", "null"]}`
Backed enum	`{type, enum: [...]}`
Plain enum	`{type: "string", enum: [...]}`
`\DateTimeInterface`	depends on `datetime_format` — `string/date-time` or `integer`
`Type\Date`	`{type: "string", format: "date"}`
`#[Assert\Length(min, max)]`	`minLength`, `maxLength`
`#[Assert\Range(min, max)]`	`minimum`, `maximum`
`#[Assert\Positive]`	`exclusiveMinimum: 0`
`#[Assert\Email]`	`format: email`
`#[Assert\Url]`	`format: uri`
`#[Assert\Regex]`	`pattern: ...`
`#[Assert\Choice]`	`enum: [...]`

Unknown constraints are skipped (not guessed).

Output schema

/mcp/tools also ships an outputSchema next to inputSchema when one can be derived — so MCP clients (and the LLMs behind them) know what shape to expect from structuredContent before they call the tool. The same schema is reused as the OpenRPC result.schema so the two contracts stay in lockstep.

Sources, in priority order:

#[Rpc\Method(outputSchema: SomeDto::class)] — schema-ized via the bundle’s JsonSchemaBuilder (same as inputSchema DTO mapping).
#[Rpc\Method(outputSchema: [...])] — a literal JSON Schema array, used as-is. Use this when the response is hand-rolled (array, mixed shapes) and you want clients to see the real keys.
__invoke() return type — auto-detected: scalar → {type: …}, class/enum → JsonSchemaBuilder::fromClass(…).
Otherwise (array, mixed, void, missing) → field is omitted. Clients then see “no advertised shape” instead of a meaningless {type: array} placeholder.

#[Rpc\Method('user.get', outputSchema: UserDto::class)]
#[Rpc\Mcp(description: 'Look up a user by id.')]
final class GetUser
{
    /** @return array<string, mixed> */
    public function __invoke(GetUserRequest $req): array { /* ... */ }
}

The schema is advisory only: it ships as a hint for clients/LLMs and is never validated against the actual response. Drift between the declared shape and what __invoke returns is the developer’s responsibility — the bundle won’t silently coerce or reject anything.

Tool annotations

MCP tools/list[].annotations are advisory hints that clients (and the LLMs behind them) use to decide whether to ask the user before invoking a tool, throttle retries, etc. They never gate execution — security still belongs to roles / authorization.

#[Rpc\Method('user.delete')]
#[Rpc\Mcp(
    description: 'Delete a user by id.',
    title: 'Delete user',
    readOnlyHint: false,
    destructiveHint: true,
    idempotentHint: false,
    openWorldHint: false,
)]
final class DeleteUser { /* ... */ }

Field	Type	Default (MCP spec)	Meaning
`title`	string	—	Human-friendly display label; clients fall back to the method name when absent.
`readOnlyHint`	bool	`false`	True if calling never modifies environment state.
`destructiveHint`	bool	`true`	True if the tool may delete or destructively mutate state. Only meaningful when `readOnlyHint: false`.
`idempotentHint`	bool	`false`	True if repeating the call with identical arguments has no additional effect.
`openWorldHint`	bool	`true`	True if the tool can reach external systems (third-party APIs, internet).

Leaving a field null (the bundle default) means “unset” — the bundle either auto-derives it (see below) or omits it so the client uses the MCP-spec default.

Auto-derivation from `#[Rpc\Cache]`

A method that carries #[Rpc\Cache] is, by definition, a function of its arguments and must not mutate observable state during a cache hit — so the bundle fills:

readOnlyHint: true
idempotentHint: true

whenever neither is set explicitly. Explicit false on the attribute always wins — auto-derivation only fills null slots, never overrides developer intent.

annotations is omitted from the tool entry entirely when no field is set and no auto-derive rule fired, so clients see the spec defaults instead of an empty object.

Result formats

How __invoke output is rendered into MCP content. The default is compact JSON; pick the right shape for your LLM:

#[Rpc\Mcp(format: McpFormat::Toon)]

Format	Wire	When to use
`json` (default)	compact JSON	Most cases.
`pretty_json`	JSON with indentation	Debugging via Claude Desktop.
`markdown`	Markdown table when homogeneous; JSON otherwise	Human-readable summaries.
`plain`	String form of scalars; JSON for structures	One-line scalar results.
`toon`	TOON (token-efficient)	LLM list payloads — 30–50% fewer tokens than JSON.

Plus structuredContent (the normalized object form) is always included alongside content for non-scalar results — MCP spec encourages this so machine-parsing clients don’t have to re-parse the text block.

Format resolution priority

X-Mcp-Format: toon header on the request
?format=toon query parameter
#[Rpc\Mcp(format: McpFormat::Toon)] attribute
json_rpc_server.mcp.default_format bundle config
Default: json

Customising results: McpResultTransformer

When the JSON-RPC response carries fields the LLM shouldn’t see (internal IDs, debug flags, cache keys), implement McpResultTransformer on the handler:

use Knetesin\JsonRpcServerBundle\Mcp\McpResultTransformer;

#[Rpc\Method('user.getById')]
#[Rpc\Mcp]
final class GetById implements McpResultTransformer
{
    public function __invoke(GetByIdRequest $req): UserResponse { /* ... */ }

    public function transformMcpResult(mixed $result): mixed
    {
        // $result is already normalized (array form).
        unset($result['internalDebugFlags'], $result['cacheKey']);
        return $result;
    }
}

Runs after __invoke and after normalization. The JSON-RPC /rpc response is unaffected — only /mcp/call sees the transformed output.

For bulk reshaping across many methods, prefer a custom McpResultFormatter (decorating DefaultMcpResultFormatter).

Description

#[Rpc\Mcp(description: 'Fetch a user profile by email. Returns id, email, name.')]

Falls back to #[Rpc\Method(description: ...)] when omitted.

Rate limiting for MCP

#[Rpc\RateLimit] does not apply to /mcp/call by default — MCP traffic typically comes from a trusted internal agent. Flip on for public MCP:

json_rpc_server:
  mcp:
    apply_rate_limit: true

HTTP statuses

Failure	Status	Body shape
Parse / invalid envelope	400	`{isError: true, error: {...}, content: [text]}`
Method not found / not exposed	404	same
Body too large	413	same
Auth, rate limit, invalid params, internal error	200	same

200 for handler-level failures is the MCP convention — clients check isError: true in the body, not the HTTP status.

Connecting Claude Desktop

{
  "mcpServers": {
    "myapp": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-fetch", "https://api.example.com/mcp"]
    }
  }
}

Or use any MCP HTTP transport that calls /mcp/tools and /mcp/call.

TOON format — when it wins

TOON encodes lists of homogeneous flat objects as a tabular form:

users[3]{id,name,email}:
  1,Alice,alice@example.com
  2,Bob,bob@example.com
  3,Carol,carol@example.com

vs JSON:

[{"id":1,"name":"Alice","email":"alice@example.com"},
 {"id":2,"name":"Bob","email":"bob@example.com"},
 {"id":3,"name":"Carol","email":"carol@example.com"}]

For 100 rows × 6 columns the JSON version is ~2× the tokens. Defaults stay JSON because most LLMs round-trip JSON more cleanly; toggle to toon for read-heavy listing methods explicitly.

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