---
title: "Changes to Run Headless Agent"
method: POST
path: "/agents/server/sessions/{session_id}/chat"
---

# Changes to Run Headless Agent

`POST /agents/server/sessions/{session_id}/chat`

> Every recorded change to this endpoint, newest first.

## Timeline

Changed in 9 of 70 revisions.

- **2026-08-28** `4016becf04d6` — 2 info
- **2026-08-25** `e4bf71c6ae16` — 2 breaking, 22 info
- **2026-08-21** `6107b5235c74` — 2 info
- **2026-08-11** `881c8eca3876` — 1 breaking, 10 info
- **2026-07-30** `eea5907029bd` — 2 warning
- **2026-07-14** `74accc51bb16` — 2 breaking, 2 info
- **2026-07-13** `2a6261fc25f6` — 4 info
- **2026-06-24** `3636983878f7` — 1 info
- **2026-04-29** `c81391216eb5` — 1 breaking

## Changes

- **2026-08-28** `4016becf04d6` — 2 info
  - added the new optional request property `oneOf[subschema #1: LegacyChatSubmitMessage]/messages/items/parts/items/anyOf[subschema #2: ReasoningUIPart]/id`
  - added the new optional request property `oneOf[subschema #2: LegacyChatRegenerateMessage]/messages/items/parts/items/anyOf[subschema #2: ReasoningUIPart]/id`
- **2026-08-25** `e4bf71c6ae16` — 2 breaking, 22 info
  - removed `#/components/schemas/ProjectContext, #/components/schemas/TraceContext, #/components/schemas/SessionContext, #/components/schemas/PromptContext, #/components/schemas/PromptVersionContext, #/components/schemas/AgentSpanContext, #/components/schemas/PlaygroundContext, #/components/schemas/CodeEvaluatorContext, #/components/schemas/LlmEvaluatorContext, #/components/schemas/DatasetContext` from the `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property `oneOf` list
  - removed `#/components/schemas/ProjectContext, #/components/schemas/TraceContext, #/components/schemas/SessionContext, #/components/schemas/PromptContext, #/components/schemas/PromptVersionContext, #/components/schemas/AgentSpanContext, #/components/schemas/PlaygroundContext, #/components/schemas/CodeEvaluatorContext, #/components/schemas/LlmEvaluatorContext, #/components/schemas/DatasetContext` from the `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property `oneOf` list
  - mapped value for discriminator key `code_evaluator` changed from `#/components/schemas/CodeEvaluatorContext` to `#/components/schemas/CodeEvaluatorUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `code_evaluator` changed from `#/components/schemas/CodeEvaluatorContext` to `#/components/schemas/CodeEvaluatorUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `dataset` changed from `#/components/schemas/DatasetContext` to `#/components/schemas/DatasetUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `dataset` changed from `#/components/schemas/DatasetContext` to `#/components/schemas/DatasetUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `llm_evaluator` changed from `#/components/schemas/LlmEvaluatorContext` to `#/components/schemas/LlmEvaluatorUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `llm_evaluator` changed from `#/components/schemas/LlmEvaluatorContext` to `#/components/schemas/LlmEvaluatorUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `playground` changed from `#/components/schemas/PlaygroundContext` to `#/components/schemas/PlaygroundUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `playground` changed from `#/components/schemas/PlaygroundContext` to `#/components/schemas/PlaygroundUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `project` changed from `#/components/schemas/ProjectContext` to `#/components/schemas/ProjectUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `project` changed from `#/components/schemas/ProjectContext` to `#/components/schemas/ProjectUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `prompt` changed from `#/components/schemas/PromptContext` to `#/components/schemas/PromptUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `prompt` changed from `#/components/schemas/PromptContext` to `#/components/schemas/PromptUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `prompt_version` changed from `#/components/schemas/PromptVersionContext` to `#/components/schemas/PromptVersionUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `prompt_version` changed from `#/components/schemas/PromptVersionContext` to `#/components/schemas/PromptVersionUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `session` changed from `#/components/schemas/SessionContext` to `#/components/schemas/SessionUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `session` changed from `#/components/schemas/SessionContext` to `#/components/schemas/SessionUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `span` changed from `#/components/schemas/AgentSpanContext` to `#/components/schemas/SpanUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `span` changed from `#/components/schemas/AgentSpanContext` to `#/components/schemas/SpanUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - mapped value for discriminator key `trace` changed from `#/components/schemas/TraceContext` to `#/components/schemas/TraceUIContext` for `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property
  - mapped value for discriminator key `trace` changed from `#/components/schemas/TraceContext` to `#/components/schemas/TraceUIContext` for `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property
  - added `#/components/schemas/ProjectUIContext, #/components/schemas/TraceUIContext, #/components/schemas/SessionUIContext, #/components/schemas/PromptUIContext, #/components/schemas/PromptVersionUIContext, #/components/schemas/SpanUIContext, #/components/schemas/PlaygroundUIContext, #/components/schemas/CodeEvaluatorUIContext, #/components/schemas/LlmEvaluatorUIContext, #/components/schemas/DatasetUIContext` to the `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/` request property `oneOf` list
  - added `#/components/schemas/ProjectUIContext, #/components/schemas/TraceUIContext, #/components/schemas/SessionUIContext, #/components/schemas/PromptUIContext, #/components/schemas/PromptVersionUIContext, #/components/schemas/SpanUIContext, #/components/schemas/PlaygroundUIContext, #/components/schemas/CodeEvaluatorUIContext, #/components/schemas/LlmEvaluatorUIContext, #/components/schemas/DatasetUIContext` to the `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/` request property `oneOf` list
- **2026-08-21** `6107b5235c74` — 2 info
  - request property `oneOf[subschema #1: LegacyChatSubmitMessage]/contexts/items/oneOf[subschema #12: GraphQLContext]/` deprecated
  - request property `oneOf[subschema #2: LegacyChatRegenerateMessage]/contexts/items/oneOf[subschema #12: GraphQLContext]/` deprecated
- **2026-08-11** `881c8eca3876` — 1 breaking, 10 info
  - removed `#/components/schemas/ChatSubmitMessage, #/components/schemas/ChatRegenerateMessage` from the request body `oneOf` list
  - api operation id `run_server_agent_agents_server_sessions__session_id__chat_post` removed and replaced with `legacyServerAgentChat`
  - endpoint deprecated
  - mapped value for key `regenerate-message` changed from `#/components/schemas/ChatRegenerateMessage` to `#/components/schemas/LegacyChatRegenerateMessage` from the request discriminator
  - mapped value for key `submit-message` changed from `#/components/schemas/ChatSubmitMessage` to `#/components/schemas/LegacyChatSubmitMessage` from the request discriminator
  - added `#/components/schemas/LegacyChatSubmitMessage, #/components/schemas/LegacyChatRegenerateMessage` to the request body `oneOf` list
  - added the non-success response with the status `400`
  - added the non-success response with the status `401`
  - added the non-success response with the status `403`
  - added the non-success response with the status `404`
  - added the non-success response with the status `507`
- **2026-07-30** `eea5907029bd` — 2 warning
  - removed the request property `oneOf[subschema #1: ChatSubmitMessage]/contexts/items/oneOf[subschema #2: ProjectContext]/rootSpansOnly`
  - removed the request property `oneOf[subschema #2: ChatRegenerateMessage]/contexts/items/oneOf[subschema #2: ProjectContext]/rootSpansOnly`
- **2026-07-14** `74accc51bb16` — 2 breaking, 2 info
  - removed `#/components/schemas/AssistantMessageMetadata` from the `oneOf[subschema #1: ChatSubmitMessage]/messages/items/metadata` request property `anyOf` list
  - removed `#/components/schemas/AssistantMessageMetadata` from the `oneOf[subschema #2: ChatRegenerateMessage]/messages/items/metadata` request property `anyOf` list
  - added `subschema #1` to the `oneOf[subschema #1: ChatSubmitMessage]/messages/items/metadata` request property `anyOf` list
  - added `subschema #1` to the `oneOf[subschema #2: ChatRegenerateMessage]/messages/items/metadata` request property `anyOf` list
- **2026-07-13** `2a6261fc25f6` — 4 info
  - added the new optional request property `oneOf[subschema #1: ChatSubmitMessage]/messages/items/metadata/anyOf[subschema #1: AssistantMessageMetadata]/turnTraceContext`
  - added the new optional request property `oneOf[subschema #1: ChatSubmitMessage]/turnTraceContext`
  - added the new optional request property `oneOf[subschema #2: ChatRegenerateMessage]/messages/items/metadata/anyOf[subschema #1: AssistantMessageMetadata]/turnTraceContext`
  - added the new optional request property `oneOf[subschema #2: ChatRegenerateMessage]/turnTraceContext`
- **2026-06-24** `3636983878f7` — 1 info
  - endpoint added
- **2026-04-29** `c81391216eb5` — 1 breaking
  - api path removed without deprecation

---

[Operation](https://skmtc.dev/arize-ai/apis/arize-phoenix-rest-api/docs/agents/server/sessions/:session_id/chat/post.md) · [API](https://skmtc.dev/arize-ai/apis/arize-phoenix-rest-api.md) · [Page](https://skmtc.dev/arize-ai/apis/arize-phoenix-rest-api/changes/agents/server/sessions/:session_id/chat/post)