refactor: use assistant role for prunable-tools injection

Switch from user role to assistant role message for injecting
prunable-tools context. This reduces the likelihood of the model
directly addressing the injected content as if responding to
user input, improving the conversational experience.

Author: Tarquinen

lib/messages/prune.ts

@@ -3,8 +3,8 @@ import type { Logger } from "../logger"
 import type { PluginConfig } from "../config"
 import { loadPrompt } from "../prompt"
 import { extractParameterKey, buildToolIdList } from "./utils"
-import { getLastUserMessage, isMessageCompacted } from "../shared-utils"
-import { UserMessage } from "@opencode-ai/sdk"
+import { getLastAssistantMessage, isMessageCompacted } from "../shared-utils"
+import { AssistantMessage } from "@opencode-ai/sdk"
 
 const PRUNED_TOOL_INPUT_REPLACEMENT = "[Input removed to save context]"
 const PRUNED_TOOL_OUTPUT_REPLACEMENT =
@@ -101,8 +101,8 @@ export const insertPruneToolContext = (
         return
     }
 
-    const lastUserMessage = getLastUserMessage(messages)
-    if (!lastUserMessage) {
+    const lastAssistantMessage = getLastAssistantMessage(messages)
+    if (!lastAssistantMessage) {
         return
     }
 
@@ -131,30 +131,33 @@ export const insertPruneToolContext = (
         prunableToolsContent = prunableToolsList + nudgeString
     }
 
-    const userMessage: WithParts = {
+    const assistantInfo = lastAssistantMessage.info as AssistantMessage
+    const assistantMessage: WithParts = {
         info: {
             id: SYNTHETIC_MESSAGE_ID,
-            sessionID: lastUserMessage.info.sessionID,
-            role: "user",
+            sessionID: assistantInfo.sessionID,
+            role: "assistant",
+            parentID: assistantInfo.parentID,
+            modelID: assistantInfo.modelID,
+            providerID: assistantInfo.providerID,
             time: { created: Date.now() },
-            agent: (lastUserMessage.info as UserMessage).agent || "build",
-            model: {
-                providerID: (lastUserMessage.info as UserMessage).model.providerID,
-                modelID: (lastUserMessage.info as UserMessage).model.modelID,
-            },
+            tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+            cost: 0,
+            path: assistantInfo.path,
+            mode: assistantInfo.mode,
         },
         parts: [
             {
                 id: SYNTHETIC_PART_ID,
-                sessionID: lastUserMessage.info.sessionID,
+                sessionID: assistantInfo.sessionID,
                 messageID: SYNTHETIC_MESSAGE_ID,
                 type: "text",
                 text: prunableToolsContent,
             },
         ],
     }
 
-    messages.push(userMessage)
+    messages.push(assistantMessage)
 }
 
 export const prune = (

lib/shared-utils.ts

@@ -13,3 +13,13 @@ export const getLastUserMessage = (messages: WithParts[]): WithParts | null => {
     }
     return null
 }
+
+export const getLastAssistantMessage = (messages: WithParts[]): WithParts | null => {
+    for (let i = messages.length - 1; i >= 0; i--) {
+        const msg = messages[i]
+        if (msg.info.role === "assistant") {
+            return msg
+        }
+    }
+    return null
+}