diff --git a/app/git.js b/app/git.js
index 38c5582..39468fe 100644
--- a/app/git.js
+++ b/app/git.js
@@ -203,6 +203,10 @@ export async function mergeInstructionText(existingText, sourceText, relPath, ai
 
   try {
     const aiMerged = await aiMergeAssistant({ relPath, existingText, sourceText, deterministicText: deterministic, requiredBlocks });
+    if (aiMerged == null) {
+      warn(`[merge] ${relPath} AI result unavailable; using deterministic merge`);
+      return deterministic;
+    }
     if (typeof aiMerged === 'string' && validateMergedInstructionText(aiMerged, requiredBlocks)) {
       return normalizeText(aiMerged) === normalizeText(existingText) ? existingText : aiMerged;
     }
diff --git a/app/git.test.js b/app/git.test.js
index 5d92566..e766a0d 100644
--- a/app/git.test.js
+++ b/app/git.test.js
@@ -211,6 +211,15 @@ describe('commitAndPush', () => {
     assert.ok(result.includes('extra block'));
   });
 
+  it('uses deterministic instruction merge when AI returns no usable result', async () => {
+    const aiMergeAssistant = async () => null;
+
+    const result = await mergeInstructionText('repo block', 'source block', 'AGENTS.md', aiMergeAssistant);
+
+    assert.ok(result.includes('repo block'));
+    assert.ok(result.includes('source block'));
+  });
+
   it('exits when AI output drops a block', async () => {
     const originalExit = process.exit;
     let exitCode = null;
diff --git a/app/llm.js b/app/llm.js
index a686053..12a0dd7 100644
--- a/app/llm.js
+++ b/app/llm.js
@@ -111,9 +111,69 @@ export async function chat(systemPrompt, userContent) {
 export async function chatJSON(systemPrompt, userContent) {
   const text = await chat(systemPrompt, userContent);
   try {
-    return JSON.parse(text.trim().replace(/^```[^\n]*\n?/, '').replace(/```$/, '').trim());
+    return JSON.parse(extractJSONText(text));
   } catch (e) {
     line(`[LLM] JSON 解析失敗: ${e.message}`);
     return [];
   }
 }
+
+function stripOuterFence(text) {
+  return String(text)
+    .trim()
+    .replace(/^```[a-zA-Z0-9_-]*\n?/, '')
+    .replace(/```$/, '')
+    .trim();
+}
+
+function extractBalancedJSON(text, startIndex) {
+  const source = String(text);
+  const open = source[startIndex];
+  const close = open === '{' ? '}' : ']';
+  let depth = 0;
+  let inString = false;
+  let escaped = false;
+
+  for (let i = startIndex; i < source.length; i++) {
+    const ch = source[i];
+    if (inString) {
+      if (escaped) {
+        escaped = false;
+      } else if (ch === '\\') {
+        escaped = true;
+      } else if (ch === '"') {
+        inString = false;
+      }
+      continue;
+    }
+    if (ch === '"') {
+      inString = true;
+      continue;
+    }
+    if (ch === open) depth += 1;
+    else if (ch === close) {
+      depth -= 1;
+      if (depth === 0) return source.slice(startIndex, i + 1);
+    }
+  }
+  return null;
+}
+
+function extractJSONText(text) {
+  const stripped = stripOuterFence(text);
+  try {
+    JSON.parse(stripped);
+    return stripped;
+  } catch {}
+
+  for (let i = 0; i < stripped.length; i++) {
+    if (stripped[i] !== '[' && stripped[i] !== '{') continue;
+    const candidate = extractBalancedJSON(stripped, i);
+    if (!candidate) continue;
+    try {
+      JSON.parse(candidate);
+      return candidate;
+    } catch {}
+  }
+  return stripped;
+}
diff --git a/app/llm.test.js b/app/llm.test.js
index be2756a..2024946 100644
--- a/app/llm.test.js
+++ b/app/llm.test.js
@@ -208,6 +208,20 @@ describe('chatJSON', async () => {
     assert.deepEqual(result, [{ level: 'info' }]);
   });
 
+  it('extracts JSON array from surrounding prose', async () => {
+    process.env.OPENAI_API_KEY = 'sk-test';
+    mockAxiosPost([makeOkResponse('**Reviewing findings**\n\n[{"level":"warning","suggestion":"x"}]\n\nDone.')]);
+    const result = await chatJSON('sys', 'user');
+    assert.deepEqual(result, [{ level: 'warning', suggestion: 'x' }]);
+  });
+
+  it('extracts JSON object from surrounding prose', async () => {
+    process.env.OPENAI_API_KEY = 'sk-test';
+    mockAxiosPost([makeOkResponse('**Begin Combine**\n{"merged_text":"repo block\\n\\nsource block"}')]);
+    const result = await chatJSON('sys', 'user');
+    assert.deepEqual(result, { merged_text: 'repo block\n\nsource block' });
+  });
+
   it('returns [] when JSON is invalid', async () => {
     process.env.OPENAI_API_KEY = 'sk-test';
     mockAxiosPost([makeOkResponse('not json')]);