---
title: "Changes to Create Tracer Session"
method: POST
path: "/api/v1/sessions"
---

# Changes to Create Tracer Session

`POST /api/v1/sessions`

> Every recorded change to this operation, newest first.

## Timeline

Changed in 3 of 77 revisions.

- **2026-10-28** `73dfb1001b4b`: 19 info
- **2026-10-16** `1a8ee9358d81`: 4 breaking, 1 info
- **2026-05-28** `d3056703d8de`: 4 info

## Changes

- **2026-10-28** `73dfb1001b4b`: 19 info
  - added the optional property `completion_cost` to the response with the `200` status
  - added the optional property `completion_tokens` to the response with the `200` status
  - added the optional property `error_rate` to the response with the `200` status
  - added the optional property `experiment_progress` to the response with the `200` status
  - added the optional property `feedback_stats` to the response with the `200` status
  - added the optional property `first_token_p50` to the response with the `200` status
  - added the optional property `first_token_p99` to the response with the `200` status
  - added the optional property `last_run_start_time` to the response with the `200` status
  - added the optional property `latency_p50` to the response with the `200` status
  - added the optional property `latency_p99` to the response with the `200` status
  - added the optional property `prompt_cost` to the response with the `200` status
  - added the optional property `prompt_tokens` to the response with the `200` status
  - added the optional property `run_count` to the response with the `200` status
  - added the optional property `run_facets` to the response with the `200` status
  - added the optional property `session_feedback_stats` to the response with the `200` status
  - added the optional property `streaming_rate` to the response with the `200` status
  - added the optional property `test_run_number` to the response with the `200` status
  - added the optional property `total_cost` to the response with the `200` status
  - added the optional property `total_tokens` to the response with the `200` status
- **2026-10-16** `1a8ee9358d81`: 4 breaking, 1 info
  - the `detail/items/` response's property type changed from `object` to `string` for status `422`
  - removed the required property `detail/items/loc` from the response with the `422` status
  - removed the required property `detail/items/msg` from the response with the `422` status
  - removed the required property `detail/items/type` from the response with the `422` status
  - added the new optional request property `tag_value_ids`
- **2026-05-28** `d3056703d8de`: 4 info
  - added the new optional request property `evaluator_keys`
  - added the new optional request property `kicked_off_by`
  - added the new optional request property `num_examples`
  - added the new optional request property `num_repetitions`

---

[Operation](https://skmtc.dev/langchain-ai/apis/langsmith/docs/api/v1/sessions/post.md) - [API](https://skmtc.dev/langchain-ai/apis/langsmith.md) - [Page](https://skmtc.dev/langchain-ai/apis/langsmith/changes/api/v1/sessions/post)