diff --git a/.devcontainer/opencode-seed/commands/to-markdown.md b/.devcontainer/opencode-seed/commands/to-markdown.md
index 8ba74eb..85633fd 100644
--- a/.devcontainer/opencode-seed/commands/to-markdown.md
+++ b/.devcontainer/opencode-seed/commands/to-markdown.md
@@ -5,7 +5,7 @@ agent: build
 
 # to-markdown
 
-Convert source content into Markdown using MarkItDown.
+Convert source content into Markdown using MarkItDown CLI (fallback when MCP is unavailable).
 
 ## Usage
 
diff --git a/.devcontainer/opencode-seed/instructions/markitdown-converter.md b/.devcontainer/opencode-seed/instructions/markitdown-converter.md
index 4bc784a..6eaa4c6 100644
--- a/.devcontainer/opencode-seed/instructions/markitdown-converter.md
+++ b/.devcontainer/opencode-seed/instructions/markitdown-converter.md
@@ -1,6 +1,7 @@
-When users ask to read, summarize, or analyze non-Markdown sources (PDF, DOCX, PPTX, HTML, image-derived text, or URLs), first convert the source with `/to-markdown`.
+When users ask to read, summarize, or analyze non-Markdown sources (PDF, DOCX, PPTX, HTML, image-derived text, or URLs), first convert the source with the `markitdown` MCP tool (`convert_to_markdown`).
 
 Guidelines:
+- Prefer MCP conversion first; use `/to-markdown` only as a fallback when MCP is unavailable.
 - Prefer `/to-markdown <input> <output.md>` for large inputs.
 - If `<input>` is a directory, never use `Read File` on that directory path; run `/to-markdown <input-dir> <output-dir>` instead.
 - For small inputs, `/to-markdown <input>` and return inline Markdown is acceptable.
diff --git a/.devcontainer/opencode-seed/skills/markitdown-converter/SKILL.md b/.devcontainer/opencode-seed/skills/markitdown-converter/SKILL.md
index 507e650..cc1673e 100644
--- a/.devcontainer/opencode-seed/skills/markitdown-converter/SKILL.md
+++ b/.devcontainer/opencode-seed/skills/markitdown-converter/SKILL.md
@@ -17,9 +17,10 @@ Use this skill when you need to transform non-Markdown sources (PDF, Office docs
 ## OpenChamber Usage
 
 1. Explicit command:
+   - Use MCP tool `markitdown.convert_to_markdown(uri)` first
    - `/to-markdown <input> [output.md]`
 2. Natural language:
-   - Ask normally (e.g. "このPDFをMarkdown化して"), then route through `/to-markdown`
+   - Ask normally (e.g. "このPDFをMarkdown化して"), then route through MCP `convert_to_markdown`
 
 ## Notes
 
diff --git a/.devcontainer/setup.sh b/.devcontainer/setup.sh
index ac5975f..fea7b2b 100755
--- a/.devcontainer/setup.sh
+++ b/.devcontainer/setup.sh
@@ -124,6 +124,20 @@ if command -v uv &> /dev/null; then
     fi
 fi
 
+MARKITDOWN_MCP_MARKER="/home/vscode/.local/share/uv/tools/markitdown-mcp/.ecc-installed"
+if command -v uv &> /dev/null; then
+    if ! command -v markitdown-mcp &> /dev/null || [[ ! -f "$MARKITDOWN_MCP_MARKER" ]]; then
+        echo "🧩 markitdown-mcp をインストール中..."
+        if uv tool install --upgrade markitdown-mcp; then
+            mkdir -p "$(dirname "$MARKITDOWN_MCP_MARKER")"
+            touch "$MARKITDOWN_MCP_MARKER"
+            echo "   ✅ markitdown-mcp インストール完了"
+        else
+            echo "   ⚠️  markitdown-mcp インストールに失敗しました"
+        fi
+    fi
+fi
+
 # ECC の設定適用
 echo "   ECC設定を適用中..."
 
@@ -161,6 +175,16 @@ cat > ~/.opencode/opencode.json << 'EOF'
   "model": "anthropic/claude-sonnet-4-5",
   "small_model": "anthropic/claude-haiku-4-5", 
   "default_agent": "build",
+  "mcp": {
+    "markitdown": {
+      "type": "local",
+      "command": [
+        "/home/vscode/.local/bin/markitdown-mcp"
+      ],
+      "enabled": true,
+      "timeout": 30000
+    }
+  },
   "plugin": [
     "./plugins"
   ]
diff --git a/.devcontainer/startup.sh b/.devcontainer/startup.sh
index 6db2f9c..2da12ae 100755
--- a/.devcontainer/startup.sh
+++ b/.devcontainer/startup.sh
@@ -236,6 +236,20 @@ if command -v uv >/dev/null 2>&1; then
     fi
 fi
 
+MARKITDOWN_MCP_MARKER="/home/vscode/.local/share/uv/tools/markitdown-mcp/.ecc-installed"
+if command -v uv >/dev/null 2>&1; then
+    if ! command -v markitdown-mcp >/dev/null 2>&1 || [ ! -f "$MARKITDOWN_MCP_MARKER" ]; then
+        echo "📦 markitdown-mcp を uv でインストール中..."
+        if uv tool install --upgrade markitdown-mcp >/tmp/markitdown-mcp-install.log 2>&1; then
+            mkdir -p "$(dirname "$MARKITDOWN_MCP_MARKER")"
+            touch "$MARKITDOWN_MCP_MARKER"
+            echo "✅ markitdown-mcp インストール完了"
+        else
+            echo "⚠️  markitdown-mcp インストール失敗 (ログ: /tmp/markitdown-mcp-install.log)"
+        fi
+    fi
+fi
+
 if [ -f /home/vscode/.opencode/opencode.json ]; then
     python3 - <<'PY'
 import json
@@ -248,7 +262,18 @@ entry = "instructions/markitdown-converter.md"
 if entry not in instructions:
     instructions.append(entry)
     config["instructions"] = instructions
-    path.write_text(json.dumps(config, indent=2, ensure_ascii=False) + "\n", encoding="utf-8")
+
+mcp = config.get("mcp", {})
+if "markitdown" not in mcp:
+    mcp["markitdown"] = {
+        "type": "local",
+        "command": ["/home/vscode/.local/bin/markitdown-mcp"],
+        "enabled": True,
+        "timeout": 30000,
+    }
+    config["mcp"] = mcp
+
+path.write_text(json.dumps(config, indent=2, ensure_ascii=False) + "\n", encoding="utf-8")
 PY
 fi