Overview changedspecification/draft/basic/index
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126126
127127MCP defines the following error codes:
128128
129| Code | Name |
130| -------- | ----------------------------------------------------------------------------------------------------- |
131| `-32020` | [`HeaderMismatch`](/specification/draft/schema#headermismatcherror) |
129| Code | Name |
130| - | - |
131| `-32020` | [`HeaderMismatch`](/specification/draft/schema#headermismatcherror) |
132132| `-32021` | [`MissingRequiredClientCapability`](/specification/draft/schema#missingrequiredclientcapabilityerror) |
133| `-32022` | [`UnsupportedProtocolVersion`](/specification/draft/schema#unsupportedprotocolversionerror) |
133| `-32022` | [`UnsupportedProtocolVersion`](/specification/draft/schema#unsupportedprotocolversionerror) |
134134
135135Codes defined by earlier protocol versions remain reserved and will not be
136136reused. Implementations of this protocol version **MUST NOT** emit these codes:
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345345
346346The following `_meta` keys are reserved by this specification:
347347
348| Key | Description | Defined in |
349| -------------------------------------------- | ----------------------------------------------------------- | ------------------------------------------------------------------ |
350| `progressToken` | Opts the request into progress notifications | [Progress](/specification/draft/basic/patterns/progress) |
351| `io.modelcontextprotocol/protocolVersion` | Protocol version for a request | Per-request protocol fields (below) |
352| `io.modelcontextprotocol/clientInfo` | Client name and version | Per-request protocol fields (below) |
353| `io.modelcontextprotocol/clientCapabilities` | Client capabilities relevant to a request | Per-request protocol fields (below) |
354| `io.modelcontextprotocol/logLevel` | Minimum log level the server should emit for a request | [Logging](/specification/draft/server/utilities/logging) |
355| `io.modelcontextprotocol/subscriptionId` | Correlates a notification with its originating subscription | [Subscriptions](/specification/draft/basic/patterns/subscriptions) |
356| `traceparent`, `tracestate`, `baggage` | OpenTelemetry trace context propagation | OpenTelemetry trace context (below) |
348| Key | Description | Defined in |
349| - | - | - |
350| `progressToken` | Opts the request into progress notifications | [Progress](/specification/draft/basic/patterns/progress) |
351| `io.modelcontextprotocol/protocolVersion` | Protocol version for a request | Per-request protocol fields (below) |
352| `io.modelcontextprotocol/clientInfo` | Client name and version | Per-request protocol fields (below) |
353| `io.modelcontextprotocol/clientCapabilities` | Client capabilities relevant to a request | Per-request protocol fields (below) |
354| `io.modelcontextprotocol/logLevel` | Minimum log level the server should emit for a request | [Logging](/specification/draft/server/utilities/logging) |
355| `io.modelcontextprotocol/subscriptionId` | Correlates a notification with its originating subscription | [Subscriptions](/specification/draft/basic/patterns/subscriptions) |
356| `traceparent`, `tracestate`, `baggage` | OpenTelemetry trace context propagation | OpenTelemetry trace context (below) |
357357
358358Official [extensions](/specification/draft/basic/versioning#extension-negotiation)
359359define additional `_meta` keys under the `io.modelcontextprotocol/` prefix, and
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368368prior connection state. See
369369[Versioning and Compatibility][lifecycle] for version negotiation rules.
370370
371| Key | Type | Required | Description |
372| -------------------------------------------- | -------------------- | -------- | --------------------------------------------------------- |
373| `io.modelcontextprotocol/protocolVersion` | `string` | Yes | Protocol version for this request (e.g., `"2026-07-28"`) |
374| `io.modelcontextprotocol/clientInfo` | `Implementation` | No | Client name and version |
375| `io.modelcontextprotocol/clientCapabilities` | `ClientCapabilities` | Yes | Client capabilities relevant to this request |
376| `io.modelcontextprotocol/logLevel` | `LoggingLevel` | No | Minimum log level the server should emit for this request |
371| Key | Type | Required | Description |
372| - | - | - | - |
373| `io.modelcontextprotocol/protocolVersion` | `string` | Yes | Protocol version for this request (e.g., `"2026-07-28"`) |
374| `io.modelcontextprotocol/clientInfo` | `Implementation` | No | Client name and version |
375| `io.modelcontextprotocol/clientCapabilities` | `ClientCapabilities` | Yes | Client capabilities relevant to this request |
376| `io.modelcontextprotocol/logLevel` | `LoggingLevel` | No | Minimum log level the server should emit for this request |
377377
378378A request missing any required field is malformed; the server **MUST** reject it with
379379JSON-RPC error code `-32602` (Invalid params). On HTTP, the response status **MUST** be
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395395every result's `_meta`, unless specifically configured not to do so, to
396396identify themselves without relying on any prior connection state:
397397
398| Key | Type | Required | Description |
399| ------------------------------------ | ---------------- | -------- | ----------------------- |
400| `io.modelcontextprotocol/serverInfo` | `Implementation` | No | Server name and version |
398| Key | Type | Required | Description |
399| - | - | - | - |
400| `io.modelcontextprotocol/serverInfo` | `Implementation` | No | Server name and version |
401401
402402<Note>
403403 `io.modelcontextprotocol/clientInfo` and `io.modelcontextprotocol/serverInfo`
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