Overview changedspecification/2026-07-28/basic/index
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2626
2727### Requests
2828
29[Requests](/specification/2026-07-28/schema#jsonrpcrequest) are sent from the client to the server, to initiate an operation.
29[Requests](/specification/2026-07-28/schema.md#jsonrpcrequest) are sent from the client to the server, to initiate an operation.
3030
3131```typescript theme={null}
3232{
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5050
5151#### Result Responses
5252
53[Result responses](/specification/2026-07-28/schema#jsonrpcresultresponse) are sent when the operation completes successfully.
53[Result responses](/specification/2026-07-28/schema.md#jsonrpcresultresponse) are sent when the operation completes successfully.
5454
5555```typescript theme={null}
5656{
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8282
8383#### Error Responses
8484
85[Error responses](/specification/2026-07-28/schema#jsonrpcerrorresponse) are sent when the operation fails or encounters an error.
85[Error responses](/specification/2026-07-28/schema.md#jsonrpcerrorresponse) are sent when the operation fails or encounters an error.
8686
8787```typescript theme={null}
8888{
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117117 **MUST NOT** assume any specific meaning for these codes.
118118* **`-32020` to `-32099` — reserved for the MCP specification.** Error codes
119119 in this sub-range are defined exclusively by the MCP specification and
120 recorded in the [schema](/specification/2026-07-28/schema). Implementations
120 recorded in the [schema](/specification/2026-07-28/schema.md). Implementations
121121 **MUST NOT** emit any code from this sub-range that is not defined by this
122122 specification and **MUST** use defined codes only with their specified
123123 meanings.
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152152
153153### Notifications
154154
155[Notifications](/specification/2026-07-28/schema#jsonrpcnotification) are sent from the client to the server or vice versa, as a one-way message.
155[Notifications](/specification/2026-07-28/schema.md#jsonrpcnotification) are sent from the client to the server or vice versa, as a one-way message.
156156The receiver **MUST NOT** send a response.
157157
158158```typescript theme={null}
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169169
170170### Message Patterns
171171
172The Model Context Protocol (MCP) supports several [Message Patterns](/specification/2026-07-28/basic/patterns) that define how clients and servers interact:
172The Model Context Protocol (MCP) supports several [Message Patterns](/specification/2026-07-28/basic/patterns.md) that define how clients and servers interact:
173173
1741. **[Request and Response](/specification/2026-07-28/basic/patterns#request-and-response)**: A client sends a request to the server, and the server responds with a result or error.
1752. **[Multi Round-Trip Requests (MRTR)](/specification/2026-07-28/basic/patterns#multi-round-trip-requests)**: A server requires additional client input (sampling, elicitation, or roots) to complete a request.
1763. **[Subscribe and Notify](/specification/2026-07-28/basic/patterns#subscribe-and-notify)**: A client subscribes to a stream of notifications from the server, which are sent as they occur.
1741. **[Request and Response](/specification/2026-07-28/basic/patterns.md#request-and-response)**: A client sends a request to the server, and the server responds with a result or error.
1752. **[Multi Round-Trip Requests (MRTR)](/specification/2026-07-28/basic/patterns.md#multi-round-trip-requests)**: A server requires additional client input (sampling, elicitation, or roots) to complete a request.
1763. **[Subscribe and Notify](/specification/2026-07-28/basic/patterns.md#subscribe-and-notify)**: A client subscribes to a stream of notifications from the server, which are sent as they occur.
177177
178178## Statelessness
179179
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212212> [!NOTE]
213213>
214214> For a walkthrough of how the per-request model maps to SDK code, see the
215> [Architecture guide](/docs/2026-07-28/learn/architecture#example).
215> [Architecture guide](/docs/2026-07-28/learn/architecture.md#example).
216216
217217## Auth
218218
219MCP provides an [Authorization](/specification/2026-07-28/basic/authorization) framework for use with HTTP.
219MCP provides an [Authorization](/specification/2026-07-28/basic/authorization.md) framework for use with HTTP.
220220Implementations using an HTTP-based transport **SHOULD** conform to this specification,
221221whereas implementations using STDIO transport **SHOULD NOT** follow this specification,
222222and instead retrieve credentials from the environment.
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345345
346346| Key | Description | Defined in |
347347| - | - | - |
348| `progressToken` | Opts the request into progress notifications | [Progress](/specification/2026-07-28/basic/patterns/progress) |
348| `progressToken` | Opts the request into progress notifications | [Progress](/specification/2026-07-28/basic/patterns/progress.md) |
349349| `io.modelcontextprotocol/protocolVersion` | Protocol version for a request | Per-request protocol fields (below) |
350350| `io.modelcontextprotocol/clientInfo` | Client name and version | Per-request protocol fields (below) |
351351| `io.modelcontextprotocol/clientCapabilities` | Client capabilities relevant to a request | Per-request protocol fields (below) |
352| `io.modelcontextprotocol/logLevel` | Minimum log level the server should emit for a request | [Logging](/specification/2026-07-28/server/utilities/logging) |
353| `io.modelcontextprotocol/subscriptionId` | Correlates a notification with its originating subscription | [Subscriptions](/specification/2026-07-28/basic/patterns/subscriptions) |
352| `io.modelcontextprotocol/logLevel` | Minimum log level the server should emit for a request | [Logging](/specification/2026-07-28/server/utilities/logging.md) |
353| `io.modelcontextprotocol/subscriptionId` | Correlates a notification with its originating subscription | [Subscriptions](/specification/2026-07-28/basic/patterns/subscriptions.md) |
354354| `traceparent`, `tracestate`, `baggage` | OpenTelemetry trace context propagation | OpenTelemetry trace context (below) |
355355
356Official [extensions](/specification/2026-07-28/basic/versioning#extension-negotiation)
356Official [extensions](/specification/2026-07-28/basic/versioning.md#extension-negotiation)
357357define additional `_meta` keys under the `io.modelcontextprotocol/` prefix, and
358358third-party extensions use their own vendor prefix.
359359In both cases the keys are specified in the extension's documentation.
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409409the server **MUST** include `io.modelcontextprotocol/subscriptionId` in `_meta` so the
410410client can correlate the notification with the originating subscription request.
411411
412[lifecycle]: /specification/2026-07-28/basic/versioning
412[lifecycle]: /specification/2026-07-28/basic/versioning.md
413413
414[subscriptions-listen]: /specification/2026-07-28/basic/patterns/subscriptions
414[subscriptions-listen]: /specification/2026-07-28/basic/patterns/subscriptions.md
415415
416416**OpenTelemetry trace context:**
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