feat: add reasoning model support for synthetic message injection

Reasoning models expect their encrypted reasoning parts in assistant messages,
which we cannot generate. This adds detection via chat.params hook and appends
a synthetic user message (<system-context-injection/>) to close the assistant
turn properly when a reasoning model is active.

Author: Tarquinen

index.ts

@@ -1,4 +1,5 @@
 import type { Plugin } from "@opencode-ai/plugin"
+import type { Model } from "@opencode-ai/sdk"
 import { getConfig } from "./lib/config"
 import { Logger } from "./lib/logger"
 import { loadPrompt } from "./lib/prompt"
@@ -26,6 +27,22 @@ const plugin: Plugin = (async (ctx) => {
     })
 
     return {
+        "chat.params": async (
+            input: { sessionID: string; agent: string; model: Model; provider: any; message: any },
+            _output: { temperature: number; topP: number; options: Record<string, any> },
+        ) => {
+            const isReasoning = input.model.capabilities?.reasoning ?? false
+            if (state.isReasoningModel !== isReasoning) {
+                logger.info(
+                    `Reasoning model status changed: ${state.isReasoningModel} -> ${isReasoning}`,
+                    {
+                        modelId: input.model.id,
+                        providerId: input.model.providerID,
+                    },
+                )
+            }
+            state.isReasoningModel = isReasoning
+        },
         "experimental.chat.system.transform": async (
             _input: unknown,
             output: { system: string[] },

lib/messages/prune.ts

@@ -48,6 +48,14 @@ Context management was just performed. Do not use the ${toolName} again. A fresh
 
 const SYNTHETIC_MESSAGE_ID = "msg_01234567890123456789012345"
 const SYNTHETIC_PART_ID = "prt_01234567890123456789012345"
+const SYNTHETIC_USER_MESSAGE_ID = "msg_01234567890123456789012346"
+const SYNTHETIC_USER_PART_ID = "prt_01234567890123456789012346"
+
+// Content for the synthetic user message appended after the assistant message for reasoning models.
+// This is required because reasoning models expect their reasoning parts in assistant messages,
+// and we cannot generate those encrypted/proprietary parts. By closing the assistant turn with
+// a user message, the model sees a complete conversation structure.
+const REASONING_MODEL_USER_MESSAGE_CONTENT = "<system-context-injection/>"
 
 const buildPrunableToolsList = (
     state: SessionState,
@@ -158,6 +166,31 @@ export const insertPruneToolContext = (
     }
 
     messages.push(assistantMessage)
+
+    // For reasoning models, append a synthetic user message to close the assistant turn.
+    // This is required because reasoning models expect their reasoning parts in the last
+    // assistant message, which we cannot generate. The user message signals a complete turn.
+    if (state.isReasoningModel) {
+        const userMessage: WithParts = {
+            info: {
+                id: SYNTHETIC_USER_MESSAGE_ID,
+                sessionID: assistantInfo.sessionID,
+                role: "user",
+                time: { created: Date.now() + 1 },
+            } as any, // Using 'as any' because we're creating a minimal synthetic message
+            parts: [
+                {
+                    id: SYNTHETIC_USER_PART_ID,
+                    sessionID: assistantInfo.sessionID,
+                    messageID: SYNTHETIC_USER_MESSAGE_ID,
+                    type: "text",
+                    text: REASONING_MODEL_USER_MESSAGE_CONTENT,
+                },
+            ],
+        }
+        messages.push(userMessage)
+        logger.debug("Appended synthetic user message for reasoning model")
+    }
 }
 
 export const prune = (

lib/state/state.ts

@@ -55,6 +55,7 @@ export function createSessionState(): SessionState {
         lastToolPrune: false,
         lastCompaction: 0,
         currentTurn: 0,
+        isReasoningModel: false,
     }
 }
 
@@ -73,6 +74,7 @@ export function resetSessionState(state: SessionState): void {
     state.lastToolPrune = false
     state.lastCompaction = 0
     state.currentTurn = 0
+    state.isReasoningModel = false
 }
 
 export async function ensureSessionInitialized(

lib/state/types.ts

@@ -34,4 +34,5 @@ export interface SessionState {
     lastToolPrune: boolean
     lastCompaction: number
     currentTurn: number // Current turn count derived from step-start parts
+    isReasoningModel: boolean // Whether the current model has reasoning capabilities
 }