fix(hephaestus): add tool call format instructions to prevent malformed output

GPT-5.x Codex models occasionally hallucinate malformed tool calls like:
  assistant to=functions.XXX <garbled_unicode>json\n{...}

This is a model-level issue where the model outputs tool calls as text
instead of using the native tool calling mechanism. Add explicit prompt
instructions telling the model to use the tool call interface.

Related: #2190

src/agents/dynamic-agent-prompt-builder.ts

@@ -317,6 +317,23 @@ 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 buildDeepParallelSection(model: string, categories: AvailableCategory[]): string {
   const isNonClaude = !model.toLowerCase().includes('claude')
   const hasDeepCategory = categories.some(c => c.name === 'deep')

src/agents/hephaestus.ts

@@ -16,6 +16,7 @@ import {
   buildOracleSection,
   buildHardBlocksSection,
   buildAntiPatternsSection,
+  buildToolCallFormatSection,
   categorizeTools,
 } from "./dynamic-agent-prompt-builder";
 
@@ -127,7 +128,7 @@ 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}