# `POST /v1/sessions` — Create or fork a session

Part of the [Bkper Managed Agent API](https://bkper.com/docs/api/managed-agent.md). Read its guide for authentication, conventions, and examples.

New session or fork (at a finished reply, or before a user message). Forking alone never sends input. Same key returns the same session.

## Operation contract

```json
{
  "operationId": "createSession",
  "tags": [
    "Sessions"
  ],
  "summary": "Create or fork a session",
  "security": [
    {
      "bearerAuth": []
    }
  ],
  "parameters": [
    {
      "name": "Idempotency-Key",
      "in": "header",
      "required": true,
      "schema": {
        "type": "string",
        "pattern": "^[A-Za-z0-9_-]{1,128}$"
      }
    }
  ],
  "description": "New session or fork (at a finished reply, or before a user message). Forking alone never sends input. Same key returns the same session.",
  "requestBody": {
    "required": true,
    "content": {
      "application/json": {
        "schema": {
          "$ref": "#/components/schemas/CreateSession"
        }
      }
    }
  },
  "responses": {
    "201": {
      "description": "Success",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Session"
          }
        }
      }
    },
    "400": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "401": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "403": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "404": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "409": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "410": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "411": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "413": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    },
    "503": {
      "description": "Rejected; never contains credentials or upstream payloads.",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Error"
          }
        }
      }
    }
  }
}
```

## Schemas

### CreateSession

```json
{
  "type": "object",
  "properties": {
    "title": {
      "type": "string",
      "minLength": 1,
      "maxLength": 200
    },
    "model": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string"
        },
        "thinkingLevel": {
          "type": "string"
        }
      },
      "required": [
        "id"
      ],
      "additionalProperties": false
    },
    "parent": {
      "type": "object",
      "properties": {
        "sessionId": {
          "type": "string",
          "minLength": 1,
          "maxLength": 128
        },
        "entryId": {
          "type": "string",
          "minLength": 1,
          "maxLength": 128
        },
        "kind": {
          "type": "string",
          "const": "fork"
        },
        "position": {
          "description": "Default at: inherit through a finished assistant reply. Before: inherit history before a user message; does not send it.",
          "type": "string",
          "enum": [
            "at",
            "before"
          ]
        }
      },
      "required": [
        "sessionId",
        "entryId",
        "kind"
      ],
      "additionalProperties": false
    },
    "input": {
      "type": "object",
      "properties": {
        "content": {
          "minItems": 1,
          "maxItems": 16,
          "type": "array",
          "items": {
            "oneOf": [
              {
                "type": "object",
                "properties": {
                  "type": {
                    "type": "string",
                    "const": "text"
                  },
                  "text": {
                    "type": "string"
                  }
                },
                "required": [
                  "type",
                  "text"
                ],
                "additionalProperties": false
              },
              {
                "type": "object",
                "properties": {
                  "type": {
                    "type": "string",
                    "const": "file"
                  },
                  "fileId": {
                    "type": "string",
                    "pattern": "^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$",
                    "description": "A file of the caller, from POST /v1/files or a publish."
                  }
                },
                "required": [
                  "type",
                  "fileId"
                ],
                "additionalProperties": false
              }
            ]
          },
          "description": "Text parts (at most 16000 characters in total) and up to 10 file parts; some text or a file. Files unknown to the caller are 404 file_not_found; expired ones 410 file_gone."
        },
        "delivery": {
          "type": "string",
          "enum": [
            "prompt",
            "steer",
            "followUp"
          ]
        },
        "bookContext": {
          "description": "Advisory Book/query context, frozen with this input. Never identity or authorization.",
          "type": "object",
          "properties": {
            "bookId": {
              "type": "string",
              "pattern": "^[A-Za-z0-9_-]{1,128}$"
            },
            "query": {
              "type": "string",
              "maxLength": 4000,
              "pattern": "^[^\\u0000-\\u001f]*$"
            }
          },
          "required": [
            "bookId",
            "query"
          ],
          "additionalProperties": false
        }
      },
      "required": [
        "content"
      ],
      "additionalProperties": false
    },
    "instructions": {
      "description": "Omitted on a fork: the parent's current instructions. Empty: none.",
      "type": "string",
      "maxLength": 16000
    }
  },
  "additionalProperties": false
}
```

### Session

```json
{
  "type": "object",
  "properties": {
    "id": {
      "type": "string"
    },
    "title": {
      "type": "string"
    },
    "createdAt": {
      "type": "string",
      "pattern": "^\\d+$",
      "description": "Milliseconds since the epoch, as a string."
    },
    "updatedAt": {
      "type": "string",
      "pattern": "^\\d+$",
      "description": "Milliseconds since the epoch, as a string."
    },
    "status": {
      "type": "string",
      "description": "Run state. Open: idle | working | stopping."
    },
    "archived": {
      "type": "boolean"
    },
    "parent": {
      "type": "object",
      "properties": {
        "sessionId": {
          "type": "string",
          "minLength": 1,
          "maxLength": 128
        },
        "entryId": {
          "type": "string",
          "minLength": 1,
          "maxLength": 128
        },
        "kind": {
          "type": "string",
          "const": "fork"
        },
        "position": {
          "type": "string",
          "description": "Fork boundary: at inherits history through entryId; before inherits history before it. Open: at | before."
        }
      },
      "required": [
        "sessionId",
        "entryId",
        "kind",
        "position"
      ]
    },
    "model": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string"
        },
        "thinkingLevel": {
          "type": "string"
        }
      },
      "required": [
        "id"
      ]
    },
    "context": {
      "type": "object",
      "properties": {
        "tokens": {
          "type": "number"
        },
        "window": {
          "type": "number"
        }
      },
      "required": [
        "tokens",
        "window"
      ],
      "description": "Current conversation."
    },
    "usage": {
      "type": "object",
      "properties": {
        "inputTokens": {
          "type": "number"
        },
        "outputTokens": {
          "type": "number"
        },
        "cacheReadTokens": {
          "type": "number"
        },
        "cacheWriteTokens": {
          "type": "number"
        },
        "estimatedCost": {
          "type": "string",
          "description": "USD estimate from catalog prices; Bkper AI accounting is authoritative."
        }
      },
      "required": [
        "inputTokens",
        "outputTokens",
        "cacheReadTokens",
        "cacheWriteTokens",
        "estimatedCost"
      ],
      "description": "This conversation only; inherited history is not counted again."
    },
    "properties": {
      "type": "object",
      "propertyNames": {
        "type": "string"
      },
      "additionalProperties": {
        "type": "string"
      }
    },
    "instructions": {
      "description": "Present when set. Without it the agent runs with Bkper's base only.",
      "type": "string",
      "maxLength": 16000
    }
  },
  "required": [
    "id",
    "createdAt",
    "updatedAt",
    "status",
    "archived",
    "model",
    "context",
    "usage",
    "properties"
  ]
}
```

### Error

```json
{
  "type": "object",
  "properties": {
    "error": {
      "type": "object",
      "properties": {
        "code": {
          "type": "string",
          "description": "Stable code, e.g. not_found, idempotency_conflict, session_working, usage_limit_exceeded. Open."
        },
        "type": {
          "description": "Category, e.g. invalid_request_error, authentication_error, usage_limit_error. Open.",
          "type": "string"
        },
        "message": {
          "type": "string"
        }
      },
      "required": [
        "code"
      ]
    }
  },
  "required": [
    "error"
  ]
}
```

## Authentication

```json
{
  "bearerAuth": {
    "type": "http",
    "scheme": "bearer"
  }
}
```