Introduce implementation information to the initialize phase (#192)

* Introduce implementation information to the initialize phase

This introduces a way for Clients and Agents to provide information about themselves to either party.

This can be used to show users which implementation and it's version to users, or be used for logging/metrics purposes to figure out the source of usage.

This is being introduced as an optional field for now, so it is backwards compatible, but it seems like a likely candidate to be made into a required field for v2 so both sides can count on this information being available.

Copying roughly the exact same pattern as MCP here: https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle#initialization

* Start a v2 document

* Fix clippy

Author: benbrandt

docs/protocol/initialization.mdx

@@ -28,6 +28,8 @@ Before a Session can be created, Clients **MUST** initialize the connection by c
 - The latest [protocol version](#protocol-version) supported
 - The [capabilities](#client-capabilities) supported
 
+They **SHOULD** also provide a name and version to the Agent.
+
 ```json
 {
   "jsonrpc": "2.0",
@@ -41,12 +43,17 @@ Before a Session can be created, Clients **MUST** initialize the connection by c
         "writeTextFile": true
       },
       "terminal": true
+    },
+    "clientInfo": {
+      "name": "my-client",
+      "title": "My Client",
+      "version": "1.0.0"
     }
   }
 }
 ```
 
-The Agent **MUST** respond with the chosen [protocol version](#protocol-version) and the [capabilities](#agent-capabilities) it supports:
+The Agent **MUST** respond with the chosen [protocol version](#protocol-version) and the [capabilities](#agent-capabilities) it supports. It **SHOULD** also provide a name and version to the Client as well:
 
 ```json
 {
@@ -66,6 +73,11 @@ The Agent **MUST** respond with the chosen [protocol version](#protocol-version)
         "sse": true
       }
     },
+    "agentInfo": {
+      "name": "my-agent",
+      "title": "My Agent",
+      "version": "1.0.0"
+    },
     "authMethods": []
   }
 }
@@ -174,6 +186,29 @@ Note: This transport has been deprecated by the MCP spec.
 
 </ResponseField>
 
+## Implementation Information
+
+Both Clients and Agents **SHOULD** provide information about their implementation in the `clientInfo` and `agentInfo` fields respectively. Both take the following three fields:
+
+<ParamField path="name" type="string">
+  Intended for programmatic or logical use, but can be used as a display name
+  fallback if title isn’t present.
+</ParamField>
+
+<ParamField path="title" type="string">
+  Intended for UI and end-user contexts — optimized to be human-readable and
+  easily understood. If not provided, the name should be used for display.
+</ParamField>
+
+<ParamField path="version" type="string">
+  Version of the implementation. Can be displayed to the user or used for
+  debugging or metrics purposes.
+</ParamField>
+
+<Info>
+  Note: in future versions of the protocol, this information will be required.
+</Info>
+
 ---
 
 Once the connection is initialized, you're ready to [create a session](./session-setup) and begin the conversation with the Agent.

docs/protocol/schema.mdx

@@ -90,6 +90,12 @@ See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/ini
 
     - Default: `{"fs":{"readTextFile":false,"writeTextFile":false},"terminal":false}`
 
+</ResponseField>
+<ResponseField name="clientInfo" type={<><span><a href="#implementation">Implementation</a></span><span> | null</span></>} >
+  Information about the Client name and version sent to the Agent.
+
+Note: in future versions of the protocol, this will be required.
+
 </ResponseField>
 <ResponseField name="protocolVersion" type={<a href="#protocolversion">ProtocolVersion</a>} required>
   The latest protocol version supported by the client.
@@ -115,6 +121,12 @@ See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/ini
 
     - Default: `{"loadSession":false,"mcpCapabilities":{"http":false,"sse":false},"promptCapabilities":{"audio":false,"embeddedContext":false,"image":false}}`
 
+</ResponseField>
+<ResponseField name="agentInfo" type={<><span><a href="#implementation">Implementation</a></span><span> | null</span></>} >
+  Information about the Agent name and version sent to the Client.
+
+Note: in future versions of the protocol, this will be required.
+
 </ResponseField>
 <ResponseField name="authMethods" type={<><span><a href="#authmethod">AuthMethod</a></span><span>[]</span></>} >
   Authentication methods supported by the agent.
@@ -1491,6 +1503,31 @@ An HTTP header to set when making requests to the MCP server.
   The value to set for the HTTP header.
 </ResponseField>
 
+## <span class="font-mono">Implementation</span>
+
+Describes the name and version of an MCP implementation, with an optional
+title for UI representation.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="name" type={"string"} required>
+  Intended for programmatic or logical use, but can be used as a display
+name fallback if title isn’t present.
+</ResponseField>
+<ResponseField name="title" type={"string | null"} >
+  Intended for UI and end-user contexts — optimized to be human-readable
+and easily understood.
+
+If not provided, the name should be used for display.
+
+</ResponseField>
+<ResponseField name="version" type={"string"} required>
+  Version of the implementation. Can be displayed to the user or used
+for debugging or metrics purposes.
+</ResponseField>
+
 ## <span class="font-mono">McpCapabilities</span>
 
 MCP capabilities supported by the agent

docs/v2-changes.md

@@ -0,0 +1,9 @@
+# Proposal for changes in an ACP v2
+
+A WIP document to keep track of ideas and proposals for breaking changes that would require a version bump in the protocol.
+
+## Proposed Changes
+
+- Make `clientInfo` and `agentInfo` required in the `initialize` request.
+
+## Agreed on Changes

rust/agent.rs

@@ -29,6 +29,11 @@ pub struct InitializeRequest {
     /// Capabilities supported by the client.
     #[serde(default)]
     pub client_capabilities: ClientCapabilities,
+    /// Information about the Client name and version sent to the Agent.
+    ///
+    /// Note: in future versions of the protocol, this will be required.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub client_info: Option<Implementation>,
     /// Extension point for implementations
     #[serde(skip_serializing_if = "Option::is_none", rename = "_meta")]
     pub meta: Option<serde_json::Value>,
@@ -54,11 +59,34 @@ pub struct InitializeResponse {
     /// Authentication methods supported by the agent.
     #[serde(default)]
     pub auth_methods: Vec<AuthMethod>,
+    /// Information about the Agent name and version sent to the Client.
+    ///
+    /// Note: in future versions of the protocol, this will be required.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub agent_info: Option<Implementation>,
     /// Extension point for implementations
     #[serde(skip_serializing_if = "Option::is_none", rename = "_meta")]
     pub meta: Option<serde_json::Value>,
 }
 
+/// Describes the name and version of an MCP implementation, with an optional
+/// title for UI representation.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+pub struct Implementation {
+    /// Intended for programmatic or logical use, but can be used as a display
+    /// name fallback if title isn’t present.
+    name: String,
+    /// Intended for UI and end-user contexts — optimized to be human-readable
+    /// and easily understood.
+    ///
+    /// If not provided, the name should be used for display.
+    title: Option<String>,
+    /// Version of the implementation. Can be displayed to the user or used
+    /// for debugging or metrics purposes.
+    version: String,
+}
+
 // Authentication
 
 /// Request parameters for the authenticate method.

rust/bin/generate.rs

@@ -17,7 +17,7 @@ struct AgentOutgoingMessage(JsonRpcMessage<OutgoingMessage<AgentSide, ClientSide
 #[schemars(extend("x-docs-ignore" = true))]
 struct ClientOutgoingMessage(JsonRpcMessage<OutgoingMessage<ClientSide, AgentSide>>);
 
-#[expect(dead_code, clippy::large_enum_variant)]
+#[expect(dead_code)]
 #[derive(JsonSchema)]
 #[serde(untagged)]
 enum AcpTypes {

schema/schema.json

@@ -958,6 +958,25 @@
       "required": ["name", "value"],
       "type": "object"
     },
+    "Implementation": {
+      "description": "Describes the name and version of an MCP implementation, with an optional\ntitle for UI representation.",
+      "properties": {
+        "name": {
+          "description": "Intended for programmatic or logical use, but can be used as a display\nname fallback if title isn’t present.",
+          "type": "string"
+        },
+        "title": {
+          "description": "Intended for UI and end-user contexts — optimized to be human-readable\nand easily understood.\n\nIf not provided, the name should be used for display.",
+          "type": ["string", "null"]
+        },
+        "version": {
+          "description": "Version of the implementation. Can be displayed to the user or used\nfor debugging or metrics purposes.",
+          "type": "string"
+        }
+      },
+      "required": ["name", "version"],
+      "type": "object"
+    },
     "InitializeRequest": {
       "description": "Request parameters for the initialize method.\n\nSent by the client to establish connection and negotiate capabilities.\n\nSee protocol docs: [Initialization](https://agentclientprotocol.com/protocol/initialization)",
       "properties": {
@@ -975,6 +994,17 @@
           },
           "description": "Capabilities supported by the client."
         },
+        "clientInfo": {
+          "anyOf": [
+            {
+              "$ref": "#/$defs/Implementation"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "description": "Information about the Client name and version sent to the Agent.\n\nNote: in future versions of the protocol, this will be required."
+        },
         "protocolVersion": {
           "$ref": "#/$defs/ProtocolVersion",
           "description": "The latest protocol version supported by the client."
@@ -1007,6 +1037,17 @@
           },
           "description": "Capabilities supported by the agent."
         },
+        "agentInfo": {
+          "anyOf": [
+            {
+              "$ref": "#/$defs/Implementation"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "description": "Information about the Agent name and version sent to the Client.\n\nNote: in future versions of the protocol, this will be required."
+        },
         "authMethods": {
           "default": [],
           "description": "Authentication methods supported by the agent.",