diff --git a/packages/agent-runtime/src/run-agent-step.ts b/packages/agent-runtime/src/run-agent-step.ts
index d45b5ba8ff..54895a6aa4 100644
--- a/packages/agent-runtime/src/run-agent-step.ts
+++ b/packages/agent-runtime/src/run-agent-step.ts
@@ -45,7 +45,7 @@ import {
   buildUserMessageContent,
   expireMessages,
 } from './util/messages'
-import { countTokensJson } from './util/token-counter'
+import { countTokensJson, safeJsonStringify } from './util/token-counter'
 
 import type { AgentTemplate } from '@codebuff/common/types/agent-template'
 import type { TrackEventFn } from '@codebuff/common/types/contracts/analytics'
@@ -860,7 +860,7 @@ export async function loopAgentSteps(
   // Convert tools to a serializable format for context-pruner token counting
   const toolDefinitions = mapValues(tools, (tool) => ({
     description: tool.description,
-    inputSchema: tool.inputSchema as {},
+    inputSchema: JSON.parse(safeJsonStringify(tool.inputSchema)),
   }))
 
   const additionalToolDefinitionsWithCache = async () => {
diff --git a/packages/agent-runtime/src/util/token-counter.ts b/packages/agent-runtime/src/util/token-counter.ts
index 960a676cd3..1db50d5c74 100644
--- a/packages/agent-runtime/src/util/token-counter.ts
+++ b/packages/agent-runtime/src/util/token-counter.ts
@@ -27,10 +27,34 @@ export function countTokens(text: string): number {
   }
 }
 
+export function safeJsonStringify(obj: any): string {
+  const seen = new WeakSet()
+  return JSON.stringify(obj, (key, value) => {
+    if (typeof value === 'object' && value !== null) {
+      if (seen.has(value)) {
+        return '[Circular]'
+      }
+      seen.add(value)
+    }
+    if (typeof value === 'function') {
+      return value.toString()
+    }
+    return value
+  })
+}
+
 export function countTokensJson(text: string | object): number {
-  return countTokens(JSON.stringify(text))
+  if (typeof text === 'string') {
+    return countTokens(text)
+  }
+  try {
+    return countTokens(JSON.stringify(text))
+  } catch (e) {
+    return countTokens(safeJsonStringify(text))
+  }
 }
 
+
 export function countTokensForFiles(
   files: Record<string, string | null>,
 ): Record<string, number> {