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Handle a _meta that is not an object without raising - #650
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MCP requires `_meta` to be an object. When a request's `_meta` is a string, an array or a number, three `dig(:_meta, ...)` calls raised TypeError instead of answering a client error: - `validate_modern_headers` (Streamable HTTP, 2026-07-28): every method answered HTTP 500 -32603. `RequestEnvelope.parse!` already treats a non-object `_meta` as empty to answer -32602, but the header check runs first. - `call_tool`: legacy `tools/call` answered -32603 Internal error. - `build_server_context`: legacy `prompts/get` answered -32603. A non-object `_meta` carries no protocol version and no progress token, so these reads now skip it. The 2026-07-28 path answers HTTP 400 -32602, and legacy `tools/call` and `prompts/get` ignore the value, as legacy `tools/list` and `RequestEnvelope.modern?` already do.
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## Motivation and Context `_meta` is an object on every request (`MetaObject` in the schema). After #650, a String, Array, or number there no longer raises from `dig`, and the modern lifecycle answers -32602 because the envelope keys are missing; the legacy lifecycle, though, went on to run the handler. That left two gaps. The value still reached `server_context[:_meta]`, where the `server_context.dig(:_meta, ...)` the server context page shows raises `TypeError` on a String or an Array, so a handler written as documented still turned the request into -32603. And the TypeScript and Python servers refuse such a request as invalid params on both lifecycles, through schema validation of `params` and their inbound checks, so the Ruby server answered differently from both. The request is now refused with -32602 "Invalid params: _meta must be an object" before the envelope is read and before any handler runs, on both lifecycles. A notification handed to a handler is dropped the same way, without a response, as the TypeScript server drops a notification with invalid params; `notifications/cancelled` is served before the check and reads no `_meta`. A `null` `_meta` reads as absent, as it does for the Python server's request model. The server context page states the rule, since it tells handlers to read `_meta` from `server_context`. ## How Has This Been Tested? test/mcp/server_test.rb and test/mcp/server/transports/streamable_http_transport_test.rb. Before the fix, legacy `tools/call` with `_meta: "not-an-object"` ran the tool, and a tool reading `server_context.dig(:_meta, :key)` answered -32603 "Internal error" with "Internal error calling tool meta_reader" as the data. ## Breaking Changes A legacy-lifecycle request whose `_meta` is not an object now answers -32602. Released versions served such a request for methods that never read `_meta`, such as `tools/list`, and answered -32603 for `tools/call` and `prompts/get`.
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Motivation and Context
MCP requires
_metato be an object, but a request whose_metais a string, an array or a numbermakes the server raise
TypeErrorfromdiginstead of answering a client error:MCP-Protocol-Version: 2026-07-28:validate_modern_headerscallsparams.dig(:_meta, ...), and the catch-all inhandle_modernanswers HTTP 500-32603for every method.RequestEnvelope.parse!already treats a non-object_metaas empty to answer-32602, but the header check runs first.call_toolandbuild_server_contextcalldig(:_meta, :progressToken),so
tools/callandprompts/getanswer-32603Internal error.tools/listalready ignores the value.A non-object
_metacarries no protocol version and no progress token, so these reads now skip it:-32602from envelope validation;tools/callandprompts/getignore it, astools/listdoes and asRequestEnvelope.modern?does.Similar to #609 / #616 (a malformed
Acceptmember answered 500).How Has This Been Tested?
test/mcp/server/transports/streamable_http_transport_test.rb: a modern POST with a string or array_metaanswers 400 with-32602.test/mcp/server_test.rb:tools/callandprompts/getwith a string or array_metarun normally.-32603"Internal error calling tool").rake testandrake rubocoppass.Breaking Changes
None. Requests with an object
_metaare handled as before. Only requests that raised now get a client error (2026-07-28) or are served (legacy).Types of changes
Checklist