Refactor: Move prunable tool list injection to user messages

- Split context management injection: core protocol remains in system message, but dynamic tool list moves to user message for better adherence
- Add injectUserMessage support to all provider formats
- Update synthetic prompt with system-reminder to prevent model from referencing invisible injected content

Author: Tarquinen

lib/fetch-wrapper/formats/anthropic.ts

@@ -33,6 +33,15 @@ export const anthropicFormat: FormatDescriptor = {
         return true
     },
 
+    injectUserMessage(body: any, injection: string): boolean {
+        if (!injection || !body.messages) return false
+        body.messages.push({
+            role: 'user',
+            content: [{ type: 'text', text: injection }]
+        })
+        return true
+    },
+
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[] {
         const outputs: ToolOutput[] = []
 

lib/fetch-wrapper/formats/bedrock.ts

@@ -32,6 +32,15 @@ export const bedrockFormat: FormatDescriptor = {
         return true
     },
 
+    injectUserMessage(body: any, injection: string): boolean {
+        if (!injection || !body.messages) return false
+        body.messages.push({
+            role: 'user',
+            content: [{ text: injection }]
+        })
+        return true
+    },
+
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[] {
         const outputs: ToolOutput[] = []
 

lib/fetch-wrapper/formats/gemini.ts

@@ -31,6 +31,15 @@ export const geminiFormat: FormatDescriptor = {
         return true
     },
 
+    injectUserMessage(body: any, injection: string): boolean {
+        if (!injection || !body.contents) return false
+        body.contents.push({
+            role: 'user',
+            parts: [{ text: injection }]
+        })
+        return true
+    },
+
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[] {
         const outputs: ToolOutput[] = []
 

lib/fetch-wrapper/formats/openai-chat.ts

@@ -27,6 +27,12 @@ export const openaiChatFormat: FormatDescriptor = {
         return true
     },
 
+    injectUserMessage(body: any, injection: string): boolean {
+        if (!injection || !body.messages) return false
+        body.messages.push({ role: 'user', content: injection })
+        return true
+    },
+
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[] {
         const outputs: ToolOutput[] = []
 

lib/fetch-wrapper/formats/openai-responses.ts

@@ -23,6 +23,16 @@ export const openaiResponsesFormat: FormatDescriptor = {
         return true
     },
 
+    injectUserMessage(body: any, injection: string): boolean {
+        if (!injection || !body.input) return false
+        body.input.push({
+            type: 'message',
+            role: 'user',
+            content: injection
+        })
+        return true
+    },
+
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[] {
         const outputs: ToolOutput[] = []
 

lib/fetch-wrapper/handler.ts

@@ -1,9 +1,11 @@
 import type { FetchHandlerContext, FetchHandlerResult, FormatDescriptor, PrunedIdData } from "./types"
 import { type PluginState, ensureSessionRestored } from "../state"
 import type { Logger } from "../logger"
-import { buildPrunableToolsList, buildSystemInjection } from "./prunable-list"
+import { buildPrunableToolsList, buildUserInjection } from "./prunable-list"
 import { syncToolCache } from "../state/tool-cache"
+import { loadPrompt } from "../core/prompt"
 
+const SYNTHETIC_INSTRUCTION = loadPrompt("synthetic")
 const PRUNED_CONTENT_MESSAGE = '[Output removed to save context - information superseded or no longer needed]'
 
 function getMostRecentActiveSession(allSessions: any): any | undefined {
@@ -90,10 +92,15 @@ export async function handleFormat(
 
         if (prunableList) {
             const includeNudge = ctx.config.nudge_freq > 0 && ctx.toolTracker.toolResultCount > ctx.config.nudge_freq
-            const systemInjection = buildSystemInjection(prunableList, includeNudge)
-            
-            if (format.injectSystemMessage(body, systemInjection)) {
-                ctx.logger.debug("fetch", `Injected prunable tools list into system message (${format.name})`, {
+            if (format.injectSystemMessage(body, SYNTHETIC_INSTRUCTION)) {
+                modified = true
+            }
+
+            const userInjection = buildUserInjection(prunableList, includeNudge)
+
+            if (format.injectUserMessage && format.injectUserMessage(body, userInjection)) {
+                const nudgeMsg = includeNudge ? " with nudge" : ""
+                ctx.logger.debug("fetch", `Injected prunable tools list${nudgeMsg} into user message (${format.name})`, {
                     ids: numericIds,
                     nudge: includeNudge,
                     toolsSincePrune: ctx.toolTracker.toolResultCount

lib/fetch-wrapper/prunable-list.ts

@@ -4,7 +4,6 @@ import { loadPrompt } from '../core/prompt'
 import type { ToolMetadata } from './types'
 
 const NUDGE_INSTRUCTION = loadPrompt("nudge")
-const SYNTHETIC_INSTRUCTION = loadPrompt("synthetic")
 
 export interface PrunableListResult {
     list: string
@@ -43,17 +42,16 @@ export function buildPrunableToolsList(
     }
 }
 
-export function buildSystemInjection(
+export function buildUserInjection(
     prunableList: string,
     includeNudge: boolean
 ): string {
     if (!prunableList) {
         return ''
     }
 
-    // Always include synthetic instruction, optionally add nudge
-    const parts = [SYNTHETIC_INSTRUCTION, prunableList]
-    
+    const parts = [prunableList]
+
     if (includeNudge) {
         parts.push(NUDGE_INSTRUCTION)
     }

lib/fetch-wrapper/types.ts

@@ -19,6 +19,7 @@ export interface FormatDescriptor {
     detect(body: any): boolean
     getDataArray(body: any): any[] | undefined
     injectSystemMessage(body: any, injection: string): boolean
+    injectUserMessage?(body: any, injection: string): boolean
     extractToolOutputs(data: any[], state: PluginState): ToolOutput[]
     replaceToolOutput(data: any[], toolId: string, prunedMessage: string, state: PluginState): boolean
     hasToolOutputs(data: any[]): boolean

lib/prompts/synthetic.txt

@@ -1,3 +1,9 @@
+<system-reminder>
+The <prunable-tools> list and any pruning nudges are injected by a plugin and are invisible to the user.
+
+IMPORTANT: Your thinking/reasoning blocks must NOT reference, discuss, or address the <prunable-tools> list or any nudges about pruning. The user can see your thinking blocks, and referencing invisible plugin content will confuse them. In your thinking, focus only on the user's task and your problem-solving approach.
+</system-reminder>
+
 <instruction name=context_management_protocol>
 You are operating in a context-constrained environment. You must actively manage your context window using the `prune` tool.