diff --git a/src/agents/sisyphus-junior/gpt-5-4.ts b/src/agents/sisyphus-junior/gpt-5-4.ts
index d1bd8c177..889a5c8aa 100644
--- a/src/agents/sisyphus-junior/gpt-5-4.ts
+++ b/src/agents/sisyphus-junior/gpt-5-4.ts
@@ -97,7 +97,7 @@ Style:
 1. SEARCH existing codebase for similar patterns/styles
 2. Match naming, indentation, import styles, error handling conventions
 3. Default to ASCII. Add comments only for non-obvious blocks
-4. ${GPT_APPLY_PATCH_GUIDANCE}
+4. For existing files, use the edit tool instead of write/apply_patch. Use write only when creating a new file. Do not use cat or echo for file creation/editing. ${GPT_APPLY_PATCH_GUIDANCE}
 5. Do not chain bash commands with separators - each command should be a separate tool call
 
 ### After Implementation (MANDATORY - DO NOT SKIP)
diff --git a/src/agents/sisyphus-junior/index.test.ts b/src/agents/sisyphus-junior/index.test.ts
index 7da727f30..ffe44032b 100644
--- a/src/agents/sisyphus-junior/index.test.ts
+++ b/src/agents/sisyphus-junior/index.test.ts
@@ -181,6 +181,18 @@ describe("createSisyphusJuniorAgentWithOverrides", () => {
     })
   })
 
+  describe("GPT-5.4 edit protocol", () => {
+    test("GPT-5.4 prompt prefers edit tool over apply_patch for existing files", () => {
+      // given / when
+      const prompt = buildSisyphusJuniorPrompt("openai/gpt-5.4-mini", false)
+
+      // then
+      expect(prompt).toContain("For existing files, use the edit tool instead of write/apply_patch.")
+      expect(prompt).toContain("Use write only when creating a new file.")
+      expect(prompt).not.toContain("Always use apply_patch for manual code edits.")
+    })
+  })
+
   describe("tool safety (task blocked, call_omo_agent allowed)", () => {
     test("task remains blocked, call_omo_agent is allowed via tools format", () => {
       // given