diff --git a/lib/messages/utils.ts b/lib/messages/utils.ts
index 54abbd8..26b2c60 100644
--- a/lib/messages/utils.ts
+++ b/lib/messages/utils.ts
@@ -101,14 +101,4 @@ export function buildToolIdList(messages: WithParts[]): string[] {
         }
     }
     return toolIds
-}
-
-export function getPruneToolIds(numericToolIds: number[], toolIdList: string[]): string[] {
-    const pruneToolIds: string[] = []
-    for (const index of numericToolIds) {
-        if (!isNaN(index) && index >= 0 && index < toolIdList.length) {
-            pruneToolIds.push(toolIdList[index])
-        }
-    }
-    return pruneToolIds
-}
+}
\ No newline at end of file
diff --git a/lib/prompts/tool.txt b/lib/prompts/tool.txt
index 87149f4..2eda4e8 100644
--- a/lib/prompts/tool.txt
+++ b/lib/prompts/tool.txt
@@ -1,7 +1,7 @@
 Prunes tool outputs from context to manage conversation size and reduce noise.
 
 ## IMPORTANT: The Prunable List
-A `<prunable-tools>` list is injected into user messages showing available tool outputs you can prune. Each line has the format `ID: tool, parameter` (e.g., `20: read, /path/to/file.ts`). Use these numeric IDs to select which tools to prune.
+A `<prunable-tools>` list is injected into user messages showing available tool outputs you can prune. Each line has the format `ID: tool, parameter` (e.g., `20: read, /path/to/file.ts`). You MUST only use numeric IDs that appear in this list to select which tools to prune.
 
 ## CRITICAL: When and How to Prune
 
diff --git a/lib/strategies/prune-tool.ts b/lib/strategies/prune-tool.ts
index c162e08..c546363 100644
--- a/lib/strategies/prune-tool.ts
+++ b/lib/strategies/prune-tool.ts
@@ -1,7 +1,7 @@
 import { tool } from "@opencode-ai/plugin"
 import type { SessionState, ToolParameterEntry, WithParts } from "../state"
 import type { PluginConfig } from "../config"
-import { findCurrentAgent, buildToolIdList, getPruneToolIds } from "../messages/utils"
+import { findCurrentAgent, buildToolIdList } from "../messages/utils"
 import { calculateTokensSaved } from "../utils"
 import { PruneReason, sendUnifiedNotification } from "../ui/notification"
 import { formatPruningResultForTool } from "../ui/display-utils"
@@ -70,7 +70,22 @@ export function createPruneTool(
 
             const currentAgent: string | undefined = findCurrentAgent(messages)
             const toolIdList: string[] = buildToolIdList(messages)
-            const pruneToolIds: string[] = getPruneToolIds(numericToolIds, toolIdList)
+
+            // Validate that all numeric IDs are within bounds
+            if (numericToolIds.some(id => id < 0 || id >= toolIdList.length)) {
+                return "Invalid IDs provided. Only use numeric IDs from the <prunable-tools> list."
+            }
+
+            // Check for protected tools (model hallucinated an ID not in the prunable list)
+            for (const index of numericToolIds) {
+                const id = toolIdList[index]
+                const metadata = state.toolParameters.get(id)
+                if (metadata && config.strategies.pruneTool.protectedTools.includes(metadata.tool)) {
+                    return "Invalid IDs provided. Only use numeric IDs from the <prunable-tools> list."
+                }
+            }
+
+            const pruneToolIds: string[] = numericToolIds.map(index => toolIdList[index])
             state.prune.toolIds.push(...pruneToolIds)
 
             const toolMetadata = new Map<string, ToolParameterEntry>()