diff --git a/src/agents/dynamic-agent-prompt-builder.ts b/src/agents/dynamic-agent-prompt-builder.ts
index aea8066ac..1a65a120c 100644
--- a/src/agents/dynamic-agent-prompt-builder.ts
+++ b/src/agents/dynamic-agent-prompt-builder.ts
@@ -343,6 +343,23 @@ export function buildAntiPatternsSection(): string {
 ${patterns.join("\n")}`
 }
 
+export function buildToolCallFormatSection(): string {
+  return `## Tool Call Format (CRITICAL)
+
+**ALWAYS use the native tool calling mechanism. NEVER output tool calls as text.**
+
+When you need to call a tool:
+1. Use the tool call interface provided by the system
+2. Do NOT write tool calls as plain text like \`assistant to=functions.XXX\`
+3. Do NOT output JSON directly in your text response
+4. The system handles tool call formatting automatically
+
+**CORRECT**: Invoke the tool through the tool call interface
+**WRONG**: Writing \`assistant to=functions.todowrite\` or \`json\n{...}\` as text
+
+Your tool calls are processed automatically. Just invoke the tool - do not format the call yourself.`
+}
+
 export function buildNonClaudePlannerSection(model: string): string {
   const isNonClaude = !model.toLowerCase().includes('claude')
   if (!isNonClaude) return ""
diff --git a/src/agents/hephaestus/gpt-5-3-codex.ts b/src/agents/hephaestus/gpt-5-3-codex.ts
index 2eb6b2667..7c497b901 100644
--- a/src/agents/hephaestus/gpt-5-3-codex.ts
+++ b/src/agents/hephaestus/gpt-5-3-codex.ts
@@ -17,6 +17,7 @@ import {
   buildOracleSection,
   buildHardBlocksSection,
   buildAntiPatternsSection,
+  buildToolCallFormatSection,
   categorizeTools,
 } from "../dynamic-agent-prompt-builder";
 const MODE: AgentMode = "all";
@@ -127,7 +128,7 @@ export function buildHephaestusPrompt(
   const hardBlocks = buildHardBlocksSection();
   const antiPatterns = buildAntiPatternsSection();
   const todoDiscipline = buildTodoDisciplineSection(useTaskSystem);
-
+  const toolCallFormat = buildToolCallFormatSection();
   return `You are Hephaestus, an autonomous deep worker for software engineering.
 
 ## Identity
@@ -166,6 +167,7 @@ ${hardBlocks}
 
 ${antiPatterns}
 
+${toolCallFormat}
 ## Phase 0 - Intent Gate (EVERY task)
 
 ${keyTriggers}