feat: use synthetic assistant messages for non-DeepSeek/Kimi injection

Author: Tarquinen

lib/messages/inject.ts

@@ -8,7 +8,9 @@ import {
     buildToolIdList,
     createSyntheticTextPart,
     createSyntheticToolPart,
+    createSyntheticAssistantMessage,
     isIgnoredUserMessage,
+    isDeepSeekOrKimi,
 } from "./utils"
 import { getFilePathFromParameters, isProtectedFilePath } from "../protected-file-patterns"
 import { getLastUserMessage, isMessageCompacted } from "../shared-utils"
@@ -182,6 +184,7 @@ export const insertPruneToolContext = (
     }
 
     const userInfo = lastUserMessage.info as UserMessage
+    const variant = state.variant ?? userInfo.variant
 
     const lastNonIgnoredMessage = messages.findLast(
         (msg) => !(msg.info.role === "user" && isIgnoredUserMessage(msg)),
@@ -199,12 +202,24 @@ export const insertPruneToolContext = (
         const textPart = createSyntheticTextPart(lastNonIgnoredMessage, combinedContent)
         lastNonIgnoredMessage.parts.push(textPart)
     } else {
-        // Append tool part to existing assistant message. This approach works universally across
-        // models including DeepSeek and Kimi which don't output reasoning parts following an
+        // For non-user message case: push a new synthetic assistant message or append tool part
+        // for DeepSeek/Kimi. DeepSeek and Kimi don't output reasoning parts following an
         // assistant injection containing text parts. Tool parts appended to the last assistant
         // message are the safest way to inject context without disrupting model behavior.
+        const providerID = userInfo.model?.providerID || ""
         const modelID = userInfo.model?.modelID || ""
-        const toolPart = createSyntheticToolPart(lastNonIgnoredMessage, combinedContent, modelID)
-        lastNonIgnoredMessage.parts.push(toolPart)
+
+        if (isDeepSeekOrKimi(providerID, modelID)) {
+            const toolPart = createSyntheticToolPart(
+                lastNonIgnoredMessage,
+                combinedContent,
+                modelID,
+            )
+            lastNonIgnoredMessage.parts.push(toolPart)
+        } else {
+            messages.push(
+                createSyntheticAssistantMessage(lastUserMessage, combinedContent, variant),
+            )
+        }
     }
 }

lib/messages/utils.ts

@@ -13,6 +13,17 @@ const isGeminiModel = (modelID: string): boolean => {
     return lowerModelID.includes("gemini")
 }
 
+export const isDeepSeekOrKimi = (providerID: string, modelID: string): boolean => {
+    const lowerProviderID = providerID.toLowerCase()
+    const lowerModelID = modelID.toLowerCase()
+    return (
+        lowerProviderID.includes("deepseek") ||
+        lowerProviderID.includes("kimi") ||
+        lowerModelID.includes("deepseek") ||
+        lowerModelID.includes("kimi")
+    )
+}
+
 export const createSyntheticUserMessage = (
     baseMessage: WithParts,
     content: string,
@@ -45,6 +56,47 @@ export const createSyntheticUserMessage = (
     }
 }
 
+export const createSyntheticAssistantMessage = (
+    baseMessage: WithParts,
+    content: string,
+    variant?: string,
+): WithParts => {
+    const userInfo = baseMessage.info as UserMessage
+    const now = Date.now()
+    const messageId = generateUniqueId("msg")
+    const partId = generateUniqueId("prt")
+
+    return {
+        info: {
+            id: messageId,
+            sessionID: userInfo.sessionID,
+            role: "assistant" as const,
+            agent: userInfo.agent || "code",
+            parentID: userInfo.id,
+            modelID: userInfo.model.modelID,
+            providerID: userInfo.model.providerID,
+            mode: "default",
+            path: {
+                cwd: "/",
+                root: "/",
+            },
+            time: { created: now, completed: now },
+            cost: 0,
+            tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+            ...(variant !== undefined && { variant }),
+        },
+        parts: [
+            {
+                id: partId,
+                sessionID: userInfo.sessionID,
+                messageID: messageId,
+                type: "text" as const,
+                text: content,
+            },
+        ],
+    }
+}
+
 export const createSyntheticTextPart = (baseMessage: WithParts, content: string) => {
     const userInfo = baseMessage.info as UserMessage
     const partId = generateUniqueId("prt")