> ## Documentation Index > Fetch the complete documentation index at: https://docs.matrices.com/llms.txt > Use this file to discover all available pages before exploring further. # Append a message to a thread ## OpenAPI ````yaml /openapi/platform.v1.json post /v1/threads/{threadId}/messages openapi: 3.0.3 info: description: Public platform API for catalog and proxied data-plane routes. title: Matrices Platform API version: v1 servers: - url: https://api.matrices.com security: [] tags: - description: Authenticated bearer credential context. name: auth x-group: Auth - description: Write-only provider credentials with safe connection metadata. name: connections x-group: Connections - description: Materialized workspace principals and active agent counts. name: identities x-group: Identities - description: Remote MCP server registrations. name: toolsets x-group: Toolsets - description: Named allow-only agent tool permissions. name: roles x-group: Roles - description: Client-executed tool calls. name: tools x-group: Tools - description: Reusable agent behavior profiles. name: profiles x-group: Profiles - description: Workspace-owned agent identities. name: agents x-group: Agents - description: Immutable private image inputs. name: images x-group: Images - description: Public model catalog. name: models x-group: Models - description: Running agent threads. name: threads x-group: Threads - description: Agent schedules. name: schedules x-group: Schedules - description: Workspace append-only feeds. name: feeds x-group: Feeds - description: Agent subscriptions over workspace feeds. name: subscriptions x-group: Subscriptions - description: Agent-private and workspace-shared durable memory notes. name: memory x-group: Memory - description: Thread messages. name: messages x-group: Messages - description: Thread event log data. name: events x-group: Events - description: Runtime hook audit and wait resolution. name: hooks x-group: Hooks paths: /v1/threads/{threadId}/messages: post: tags: - messages summary: Append a message to a thread operationId: AppendMessage parameters: - $ref: '#/components/parameters/ThreadID' - $ref: '#/components/parameters/OptionalIdempotencyKey' requestBody: content: application/json: schema: $ref: '#/components/schemas/AppendMessageRequest' required: true responses: '202': content: application/json: schema: $ref: '#/components/schemas/SubmitMessageResult' description: Accepted message submission. headers: Idempotent-Replayed: $ref: '#/components/headers/IdempotentReplayed' '400': $ref: '#/components/responses/BadRequest' '404': $ref: '#/components/responses/NotFound' '409': $ref: '#/components/responses/Conflict' '413': $ref: '#/components/responses/PayloadTooLarge' '503': $ref: '#/components/responses/RuntimeUnavailable' security: - APIKeyBearer: [] components: parameters: ThreadID: in: path name: threadId required: true schema: type: string OptionalIdempotencyKey: description: >- Identifies one submission attempt. Retrying with the same key returns the original admission with `Idempotent-Replayed: true`. Use a new key after changing any request field. in: header name: Idempotency-Key required: false schema: type: string schemas: AppendMessageRequest: additionalProperties: false properties: content: $ref: '#/components/schemas/MessageContent' mode: $ref: '#/components/schemas/SubmitMode' model: description: >- Optional concrete catalog model id selected from the target agent profile for queue or interrupt submissions; wildcard selectors are not accepted, and model is not accepted with steer. pattern: ^[^*]+$ type: string required: - content type: object SubmitMessageResult: properties: status: $ref: '#/components/schemas/ThreadStatus' threadId: type: string turnId: type: string required: - threadId - turnId - status type: object MessageContent: description: >- Ordered message content, bounded to 512 KiB of encoded JSON. At most 20 image blocks are permitted and image IDs must be unique within the message. items: $ref: '#/components/schemas/MessageContentBlock' maxItems: 40 minItems: 1 type: array SubmitMode: enum: - queue - steer - interrupt type: string ThreadStatus: enum: - running - awaiting - idle - failed - cancelled type: string ErrorResponse: properties: attemptsRemaining: type: integer code: type: string error: type: string message: type: string retryAfterSeconds: type: integer required: - error - message type: object MessageContentBlock: discriminator: mapping: image: '#/components/schemas/MessageImageContentBlock' text: '#/components/schemas/MessageTextContentBlock' propertyName: type oneOf: - $ref: '#/components/schemas/MessageTextContentBlock' - $ref: '#/components/schemas/MessageImageContentBlock' MessageImageContentBlock: additionalProperties: false properties: imageId: $ref: '#/components/schemas/ImageID' type: enum: - image type: string required: - type - imageId type: object MessageTextContentBlock: additionalProperties: false properties: text: description: >- Non-blank UTF-8 text. User messages permit at most 8 KiB total text; assistant messages permit 64 KiB. minLength: 1 pattern: \S type: string type: enum: - text type: string required: - type - text type: object ImageID: description: Opaque identifier for one immutable private image. maxLength: 68 minLength: 68 pattern: ^img_[0-9a-f]{64}$ type: string headers: IdempotentReplayed: description: >- Present with value `true` only when the response is the canonical result of an earlier admission. This immediate-response diagnostic is not persisted for a parked operation result. schema: enum: - 'true' type: string responses: BadRequest: content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' description: Invalid request. NotFound: content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' description: Resource not found. Conflict: content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' description: Request conflicts with the thread's current state. PayloadTooLarge: content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' description: Request payload is too large. RuntimeUnavailable: content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' description: Runtime environment is unavailable or cannot be selected. securitySchemes: APIKeyBearer: scheme: bearer type: http ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.