feat: add private mcp tunnel tool

docs/cli.mdx

@@ -162,6 +162,7 @@ sandbox-agent api sessions create <SESSION_ID> [OPTIONS]
 | `-m, --model <MODEL>` | Model override |
 | `-v, --variant <VARIANT>` | Model variant |
 | `-A, --agent-version <VERSION>` | Agent version |
+| `--mcp-tunnel-tools <JSON>` | JSON array of MCP tool definitions |
 
 ```bash
 sandbox-agent api sessions create my-session \
@@ -289,6 +290,22 @@ sandbox-agent api sessions reply-permission <SESSION_ID> <PERMISSION_ID> [OPTION
 sandbox-agent api sessions reply-permission my-session perm1 --reply once
 ```
 
+#### Reply to MCP Tunnel Tool Call
+
+```bash
+sandbox-agent api sessions reply-mcp-tunnel <SESSION_ID> <CALL_ID> [OPTIONS]
+```
+
+| Option | Description |
+|--------|-------------|
+| `-o, --output <TEXT>` | Tool output (required) |
+| `--is-error` | Mark tool result as error |
+| `--content <JSON>` | Optional MCP content payload |
+
+```bash
+sandbox-agent api sessions reply-mcp-tunnel my-session call-1 --output "ok"
+```
+
 ---
 
 ## CLI to HTTP Mapping
@@ -308,3 +325,4 @@ sandbox-agent api sessions reply-permission my-session perm1 --reply once
 | `api sessions reply-question` | `POST /v1/sessions/{sessionId}/questions/{questionId}/reply` |
 | `api sessions reject-question` | `POST /v1/sessions/{sessionId}/questions/{questionId}/reject` |
 | `api sessions reply-permission` | `POST /v1/sessions/{sessionId}/permissions/{permissionId}/reply` |
+| `api sessions reply-mcp-tunnel` | `POST /v1/sessions/{sessionId}/mcp-tunnel/calls/{callId}/response` |

docs/openapi.json

@@ -365,6 +365,69 @@
         }
       }
     },
+    "/v1/sessions/{session_id}/mcp-tunnel/calls/{call_id}/response": {
+      "post": {
+        "tags": [
+          "sessions"
+        ],
+        "operationId": "reply_mcp_tunnel_call",
+        "parameters": [
+          {
+            "name": "session_id",
+            "in": "path",
+            "description": "Session id",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "call_id",
+            "in": "path",
+            "description": "MCP call id",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/McpTunnelToolResponseRequest"
+              }
+            }
+          },
+          "required": true
+        },
+        "responses": {
+          "204": {
+            "description": "MCP tool call responded"
+          },
+          "400": {
+            "description": "",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/v1/sessions/{session_id}/messages": {
       "post": {
         "tags": [
@@ -1063,6 +1126,14 @@
             "type": "string",
             "nullable": true
           },
+          "mcpTunnel": {
+            "allOf": [
+              {
+                "$ref": "#/components/schemas/McpTunnelConfig"
+              }
+            ],
+            "nullable": true
+          },
           "model": {
             "type": "string",
             "nullable": true
@@ -1258,6 +1329,56 @@
           "failed"
         ]
       },
+      "McpTunnelConfig": {
+        "type": "object",
+        "required": [
+          "tools"
+        ],
+        "properties": {
+          "tools": {
+            "type": "array",
+            "items": {
+              "$ref": "#/components/schemas/McpTunnelTool"
+            }
+          }
+        }
+      },
+      "McpTunnelTool": {
+        "type": "object",
+        "required": [
+          "name"
+        ],
+        "properties": {
+          "description": {
+            "type": "string",
+            "nullable": true
+          },
+          "inputSchema": {
+            "nullable": true
+          },
+          "name": {
+            "type": "string"
+          }
+        }
+      },
+      "McpTunnelToolResponseRequest": {
+        "type": "object",
+        "required": [
+          "output"
+        ],
+        "properties": {
+          "content": {
+            "nullable": true
+          },
+          "isError": {
+            "type": "boolean",
+            "nullable": true
+          },
+          "output": {
+            "type": "string"
+          }
+        }
+      },
       "MessageRequest": {
         "type": "object",
         "required": [