Fix Kotlin reducer dropping session/mcpServerStateChanged

clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt

@@ -82,6 +82,7 @@ import com.microsoft.agenthostprotocol.generated.StateActionSessionInputComplete
 import com.microsoft.agenthostprotocol.generated.StateActionSessionInputRequested
 import com.microsoft.agenthostprotocol.generated.StateActionSessionIsArchivedChanged
 import com.microsoft.agenthostprotocol.generated.StateActionSessionIsReadChanged
+import com.microsoft.agenthostprotocol.generated.StateActionSessionMcpServerStateChanged
 import com.microsoft.agenthostprotocol.generated.StateActionSessionMetaChanged
 import com.microsoft.agenthostprotocol.generated.StateActionSessionModelChanged
 import com.microsoft.agenthostprotocol.generated.StateActionSessionPendingMessageRemoved
@@ -1051,6 +1052,60 @@ public fun sessionReducer(state: SessionState, action: StateAction): SessionStat
         }
     }
 
+    is StateActionSessionMcpServerStateChanged -> {
+        // Full-replacement of an MCP server customization's `state` + `channel`,
+        // located by id. Mirrors the canonical TS reducer (and the Go/Rust/Swift
+        // ports): a top-level McpServer entry is matched first (hosts MAY surface
+        // MCP servers directly at the top level); otherwise the search descends
+        // into container children. A no-op when no customization carries the id,
+        // or when the matched id belongs to a non-MCP customization type.
+        val a = action.value
+        val list = state.customizations
+        if (list == null) {
+            state
+        } else {
+            val topIdx = list.indexOfFirst { customizationId(it) == a.id }
+            if (topIdx >= 0) {
+                val entry = list[topIdx]
+                if (entry !is CustomizationMcpServer) {
+                    state
+                } else {
+                    val updated = list.toMutableList()
+                    updated[topIdx] = CustomizationMcpServer(
+                        entry.value.copy(state = a.state, channel = a.channel),
+                    )
+                    state.copy(customizations = updated)
+                }
+            } else {
+                var changed = false
+                val updated = list.map { container ->
+                    val children = customizationChildren(container)
+                    if (children == null) {
+                        container
+                    } else {
+                        val childIdx = children.indexOfFirst { childCustomizationId(it) == a.id }
+                        if (childIdx < 0) {
+                            container
+                        } else {
+                            val child = children[childIdx]
+                            if (child !is ChildCustomizationMcpServer) {
+                                container
+                            } else {
+                                changed = true
+                                val newChildren = children.toMutableList()
+                                newChildren[childIdx] = ChildCustomizationMcpServer(
+                                    child.value.copy(state = a.state, channel = a.channel),
+                                )
+                                withCustomizationChildren(container, newChildren)
+                            }
+                        }
+                    }
+                }
+                if (!changed) state else state.copy(customizations = updated)
+            }
+        }
+    }
+
     // ── Truncation ────────────────────────────────────────────────────────
 
     is StateActionSessionTruncated -> {

types/test-cases/reducers/159-session-mcpserverstatechanged-upserts-top-level-server.json

@@ -0,0 +1,57 @@
+{
+  "description": "session/mcpServerStateChanged replaces the state and channel of a top-level McpServer customization",
+  "reducer": "session",
+  "initial": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "mcpServer",
+        "id": "mcp-1",
+        "uri": "file:///workspace/.mcp/servers.json",
+        "name": "Filesystem",
+        "enabled": true,
+        "state": { "kind": "starting" }
+      }
+    ]
+  },
+  "actions": [
+    {
+      "type": "session/mcpServerStateChanged",
+      "id": "mcp-1",
+      "state": { "kind": "ready" },
+      "channel": "mcp://filesystem"
+    }
+  ],
+  "expected": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "mcpServer",
+        "id": "mcp-1",
+        "uri": "file:///workspace/.mcp/servers.json",
+        "name": "Filesystem",
+        "enabled": true,
+        "state": { "kind": "ready" },
+        "channel": "mcp://filesystem"
+      }
+    ]
+  }
+}

types/test-cases/reducers/160-session-mcpserverstatechanged-upserts-container-child.json

@@ -0,0 +1,87 @@
+{
+  "description": "session/mcpServerStateChanged replaces the state and channel of an McpServer customization nested inside a container",
+  "reducer": "session",
+  "initial": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "plugin",
+        "id": "plugin-a",
+        "uri": "https://plugins.example/a",
+        "name": "Plugin A",
+        "enabled": true,
+        "children": [
+          {
+            "type": "skill",
+            "id": "skill-1",
+            "uri": "https://plugins.example/a#skills/lint",
+            "name": "lint"
+          },
+          {
+            "type": "mcpServer",
+            "id": "mcp-child",
+            "uri": "https://plugins.example/a#mcp/search",
+            "name": "Search",
+            "enabled": true,
+            "state": { "kind": "starting" }
+          }
+        ]
+      }
+    ]
+  },
+  "actions": [
+    {
+      "type": "session/mcpServerStateChanged",
+      "id": "mcp-child",
+      "state": { "kind": "ready" },
+      "channel": "mcp://search"
+    }
+  ],
+  "expected": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "plugin",
+        "id": "plugin-a",
+        "uri": "https://plugins.example/a",
+        "name": "Plugin A",
+        "enabled": true,
+        "children": [
+          {
+            "type": "skill",
+            "id": "skill-1",
+            "uri": "https://plugins.example/a#skills/lint",
+            "name": "lint"
+          },
+          {
+            "type": "mcpServer",
+            "id": "mcp-child",
+            "uri": "https://plugins.example/a#mcp/search",
+            "name": "Search",
+            "enabled": true,
+            "state": { "kind": "ready" },
+            "channel": "mcp://search"
+          }
+        ]
+      }
+    ]
+  }
+}

types/test-cases/reducers/161-session-mcpserverstatechanged-noop-unknown-id.json

@@ -0,0 +1,57 @@
+{
+  "description": "session/mcpServerStateChanged is a no-op when no customization carries the targeted id",
+  "reducer": "session",
+  "initial": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "mcpServer",
+        "id": "mcp-1",
+        "uri": "file:///workspace/.mcp/servers.json",
+        "name": "Filesystem",
+        "enabled": true,
+        "state": { "kind": "ready" },
+        "channel": "mcp://filesystem"
+      }
+    ]
+  },
+  "actions": [
+    {
+      "type": "session/mcpServerStateChanged",
+      "id": "mcp-does-not-exist",
+      "state": { "kind": "stopped" }
+    }
+  ],
+  "expected": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "mcpServer",
+        "id": "mcp-1",
+        "uri": "file:///workspace/.mcp/servers.json",
+        "name": "Filesystem",
+        "enabled": true,
+        "state": { "kind": "ready" },
+        "channel": "mcp://filesystem"
+      }
+    ]
+  }
+}

types/test-cases/reducers/162-session-mcpserverstatechanged-noop-non-mcp-id.json

@@ -0,0 +1,76 @@
+{
+  "description": "session/mcpServerStateChanged is a no-op when the targeted id belongs to a non-McpServer customization (a top-level container or one of its non-MCP children)",
+  "reducer": "session",
+  "initial": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "plugin",
+        "id": "plugin-a",
+        "uri": "https://plugins.example/a",
+        "name": "Plugin A",
+        "enabled": true,
+        "children": [
+          {
+            "type": "skill",
+            "id": "skill-1",
+            "uri": "https://plugins.example/a#skills/lint",
+            "name": "lint"
+          }
+        ]
+      }
+    ]
+  },
+  "actions": [
+    {
+      "type": "session/mcpServerStateChanged",
+      "id": "plugin-a",
+      "state": { "kind": "ready" },
+      "channel": "mcp://nope"
+    },
+    {
+      "type": "session/mcpServerStateChanged",
+      "id": "skill-1",
+      "state": { "kind": "ready" },
+      "channel": "mcp://nope"
+    }
+  ],
+  "expected": {
+    "summary": {
+      "resource": "copilot:/test-session",
+      "provider": "copilot",
+      "title": "Test Session",
+      "status": 1,
+      "createdAt": 1000,
+      "modifiedAt": 1000
+    },
+    "lifecycle": "ready",
+    "turns": [],
+    "customizations": [
+      {
+        "type": "plugin",
+        "id": "plugin-a",
+        "uri": "https://plugins.example/a",
+        "name": "Plugin A",
+        "enabled": true,
+        "children": [
+          {
+            "type": "skill",
+            "id": "skill-1",
+            "uri": "https://plugins.example/a#skills/lint",
+            "name": "lint"
+          }
+        ]
+      }
+    ]
+  }
+}