diff --git a/platform/completion/common/src/protocol/RpcCompletionCommand.kt b/platform/completion/common/src/protocol/RpcCompletionCommand.kt
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+++ b/platform/completion/common/src/protocol/RpcCompletionCommand.kt
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+// Copyright 2000-2026 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.platform.completion.common.protocol
+
+import com.intellij.openapi.editor.impl.EditorId
+import com.intellij.platform.project.ProjectId
+import kotlinx.serialization.Serializable
+
+/**
+ * A command sent frontend->backend over the single ordered [RpcCompletionService.commandFlow].
+ *
+ * This replaces the previously separate per-call RPCs (`closeSession`, `releaseRequests`, `prefixChanged`) and the per-lookup
+ * `streamLookupEvents` flow. Those transports had no mutual ordering guarantee, so a session teardown (`CloseSession`) could overtake
+ * the lookup's terminal [RpcLookupElementEvent.ItemSelected] on the backend and tear the completion session down before the forwarded
+ * finishing action inserted the chosen item (wrong / missing insertion). Delivering every command on one ordered flow makes emission
+ * order the delivery order, so `CloseSession` can never be processed before the `ItemSelected` that precedes it.
+ *
+ * Symmetric to [RpcCompletionSessionEvent] carried by [RpcCompletionService.responseFlow] in the opposite direction.
+ */
+@Serializable
+sealed interface RpcCompletionCommand {
+
+  /** A lookup mirror event (state change / item selected / cancel). Carries its own request/project id. */
+  @Serializable
+  data class LookupEvent(
+    val event: RpcLookupElementEvent,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("LookupEvent") {
+      field("event", event)
+    }
+  }
+
+  /** Closes a completion session on the backend, disposing the session and all of its request sessions. */
+  @Serializable
+  data class CloseSession(
+    val sessionId: RpcCompletionSessionId,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("CloseSession") {
+      field("sessionId", sessionId)
+    }
+  }
+
+  /** Releases backend request sessions the frontend dropped from its cache (no longer reusable). */
+  @Serializable
+  data class ReleaseRequests(
+    val sessionId: RpcCompletionSessionId,
+    val requestIds: List<RpcCompletionRequestId>,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("ReleaseRequests") {
+      field("sessionId", sessionId)
+      field("requestIds", requestIds)
+    }
+  }
+
+  /** Notifies the backend that the prefix of the given request changed. */
+  @Serializable
+  data class PrefixChanged(
+    val requestId: RpcCompletionRequestId,
+    val update: RpcPrefixUpdate,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("PrefixChanged") {
+      field("requestId", requestId)
+      field("update", update)
+    }
+  }
+
+  /**
+   * Opens a completion session on the backend (spans a single lookup and a series of requests). Idempotent; also
+   * created lazily by the first [StartRequest], so an explicit open just sets up session-scoped state eagerly.
+   */
+  @Serializable
+  data class OpenSession(
+    val sessionId: RpcCompletionSessionId,
+    val editorId: EditorId,
+    val projectId: ProjectId,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("OpenSession") {
+      field("sessionId", sessionId)
+      field("editorId", editorId)
+    }
+  }
+
+  /**
+   * Starts a completion request within a session. The response streams back on
+   * [com.intellij.platform.completion.common.split.RpcCompletionService.responseFlow], tagged with the request's
+   * session id. Sent only after the response flow is established, so the backend response channel exists when it runs.
+   */
+  @Serializable
+  data class StartRequest(
+    val request: RpcCompletionRequest,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("StartRequest") {
+      field("request", request)
+    }
+  }
+
+  /**
+   * Requests ModCommand computation for a completion item; the backend responds with
+   * [RpcCompletionResponseEvent.ModCommandResults] on the response flow.
+   */
+  @Serializable
+  data class ScheduleModCommand(
+    val requestId: RpcCompletionRequestId,
+    val completionItemId: RpcCompletionItemId,
+  ) : RpcCompletionCommand {
+    override fun toString(): String = buildToString("ScheduleModCommand") {
+      field("requestId", requestId)
+      field("completionItemId", completionItemId)
+    }
+  }
+}