Execute message (SSE)
POST
/v1/threads/{threadId}/messagesExecute a message in the thread context. Returns Server-Sent Events (SSE) stream.
Requires positive budget balance.
Event Types:
response.started- Execution startedresponse.item.completed- Partial result (reasoning, tool call, etc.)response.completed- Execution completestream.completed- Stream endedstream.error- Error occurred
How to call this endpoint
Every ACP API request uses bearer authentication. The examples here show the actual request path, auth header, and body shape that the platform expects.
Path, query, and header parameters
These parameters control which ACP object the endpoint acts on and how the request is processed.
Path parameters
| Name | Location | Type | Required | Description |
|---|---|---|---|---|
| threadId | path | string | Yes | Thread ID |
Query parameters
None.
Header parameters
| Name | Location | Type | Required | Description |
|---|---|---|---|---|
| Idempotency-Key | header | string | No | A client-generated identity for this turn. Retrying with the same key returns the existing execution or Batch handoff instead of scheduling duplicate work. |
Body schema
Content type: application/json · Required
| Field | Type | Required | Description |
|---|---|---|---|
| content | string | Yes | Message content to execute. |
| task | string | No | Legacy alias for content. |
| executionContent | string | No | Optional execution-only content while `content` remains the visible user message. |
| attachments | object[] | No | Attachments to include in the execution context. |
| attachments[].id | string | No | Unique identifier. |
| attachments[].filename | string | No | — |
| attachments[].mimeType | string | No | — |
| attachments[].size | integer | No | — |
| attachments[].type | image | document | No | — |
| attachments[].url | string | No | — |
| attachments[].workspacePath | string | No | — |
| attachments[].integrationSource | string | No | — |
| attachments[].githubRepoFullName | string | No | — |
| attachments[].githubRef | string | No | — |
| attachments[].githubItemPath | string | No | — |
| attachments[].githubSelectionType | string | No | — |
| githubRepo | object | No | — |
| githubRepo.repoFullName | string | No | — |
| githubRepo.repoName | string | No | — |
| githubRepo.branch | string | No | — |
| messageMetadata | object | No | Metadata persisted with the visible user message. |
| quotedSelection | object | No | — |
| quotedSelection.text | string | No | — |
| quotedSelection.sourceType | working_log | run_summary | No | — |
| researchModeEnabled | boolean | No | Enable deep research mode for this execution. |
| truncateAtMessageIndex | integer | No | Rewind the thread to a specific message index before executing. |
| enabledSkills | string[] | No | Built-in and custom Skill IDs enabled for this execution. |
| editMessageId | string | No | Edit and re-run a previous user message. |
| persistFileChanges | boolean | No | Persist file changes from the execution back into the attached computer state. |
| reasoningEffort | minimal | low | medium | high | No | Reasoning intensity used by the execution runtime. |
| mcpServers | object[] | No | MCP server overrides for this execution. |
| mcpServers[].name | string | No | Human-readable name. |
| mcpServers[].type | stdio | http | No | — |
| mcpServers[].command | string | No | For stdio servers |
| mcpServers[].args | string[] | No | — |
| mcpServers[].url | string | No | For HTTP servers |
| mcpServers[].bearerToken | string | No | — |
| mcpServers[].enabled | boolean | No | — |
| envVars | object | No | Environment-variable overrides for this execution. |
| knowledgeContext | object | No | — |
| knowledgeContext.enabled | boolean | No | Disable automatic Knowledge retrieval for this execution when false. |
| knowledgeContext.libraryIds | string[] | No | Restrict retrieval to these accessible libraries. Empty means all eligible libraries. |
| knowledgeContext.limit | integer | No | — |
| knowledgeContext.mode | read | propose | write | No | Maximum Knowledge capability requested for the thread. Explicit attachments default to propose; every read or mutation is still checked against the caller and resource permissions. |
| knowledgeContext.source | string | No | Stable client provenance such as composer or project. |
| knowledgeContext.bindings | object[] | No | Explicit library version snapshots to use for this execution. |
| knowledgeContext.bindings[].libraryId | string | Yes | — |
| knowledgeContext.bindings[].versionId | string | No | — |
| knowledgeContext.bindings[].versionNumber | integer | No | — |
| knowledgeContext.bindings[].fingerprint | string | No | — |
| knowledge | object | No | — |
| knowledge.enabled | boolean | No | Disable automatic Knowledge retrieval for this execution when false. |
| knowledge.libraryIds | string[] | No | Restrict retrieval to these accessible libraries. Empty means all eligible libraries. |
| knowledge.limit | integer | No | — |
| knowledge.mode | read | propose | write | No | Maximum Knowledge capability requested for the thread. Explicit attachments default to propose; every read or mutation is still checked against the caller and resource permissions. |
| knowledge.source | string | No | Stable client provenance such as composer or project. |
| knowledge.bindings | object[] | No | Explicit library version snapshots to use for this execution. |
| knowledge.bindings[].libraryId | string | Yes | — |
| knowledge.bindings[].versionId | string | No | — |
| knowledge.bindings[].versionNumber | integer | No | — |
| knowledge.bindings[].fingerprint | string | No | — |
| queueWhenCapacityUnavailable | boolean | No | Persist the turn in Batches instead of failing when runtime capacity is unavailable. Defaults to true on local appliances and false on cloud deployments. |
What the API returns
Each response code below includes the documented payload shape for the ACP API.
200SSE event streamtext/event-stream
None.
202Message persisted and accepted by Batches because runtime capacity was unavailabletext/event-stream
None.
402Insufficient budgetapplication/json
| Field | Type | Required | Description |
|---|---|---|---|
| error | string | No | — |
| message | string | No | Message text. |
| currentBudget | number | No | — |
409Thread is busy (execution in progress)
None.