Partner Filing API errors

Latest version: Version 1.2.0 Beta
Last updated: 10 November 2026
API type: REST
Sandbox base URL: Provided with sandbox access
Production base URL: https://www.simplecompanytax.co.uk/api/v1

Developer documentation

Errors

Errors use {error: {code, message, fields?}, request_id}. Correct fields before retrying an input error. Retry a temporary service error after Retry-After. Do not guess whether an authority accepted an unknown outcome.

Code	Message	HTTP status and endpoint
`INVALID_API_KEY`	A valid Bearer API key is required.	401 `POST /filings/ct600` 401 `POST /filings/accounts` 401 `GET /filings` 401 `GET /filings/{id}` 401 `GET /filings/{id}/documents` 401 `POST /uploads/accounts` 401 `PUT /uploads/accounts` 401 `GET /companies/{number}` 401 `GET /account` 401 `GET /health`
`LIVE_KEYS_NOT_ENABLED`	Live API access is not enabled for this Practice.	403 `POST /filings/ct600` 403 `POST /filings/accounts` 403 `GET /filings` 403 `GET /filings/{id}` 403 `GET /filings/{id}/documents` 403 `POST /uploads/accounts` 403 `PUT /uploads/accounts` 403 `GET /companies/{number}` 403 `GET /account` 403 `GET /health`
`RATE_LIMITED`	The API request limit has been reached.	429 `POST /filings/ct600` 429 `POST /filings/accounts` 429 `GET /filings` 429 `GET /filings/{id}` 429 `GET /filings/{id}/documents` 429 `POST /uploads/accounts` 429 `PUT /uploads/accounts` 429 `GET /companies/{number}` 429 `GET /account` 429 `GET /health`
`SERVICE_UNAVAILABLE`	The API is temporarily unavailable.	503 `POST /filings/ct600` 503 `POST /filings/accounts` 503 `GET /filings` 503 `GET /filings/{id}` 503 `GET /filings/{id}/documents` 503 `POST /uploads/accounts` 503 `PUT /uploads/accounts` 503 `GET /companies/{number}` 503 `GET /account` 503 `GET /health`
`INVALID_REQUEST`	Check the request fields.	400 `POST /filings/ct600` 400 `POST /filings/accounts` 400 `GET /filings` 400 `POST /uploads/accounts` 400 `PUT /uploads/accounts`
`NOT_FOUND`	The resource was not found.	404 `POST /filings/ct600` 404 `POST /filings/accounts` 404 `GET /filings/{id}` 404 `GET /filings/{id}/documents` 404 `PUT /uploads/accounts` 404 `GET /companies/{number}`
`IDEMPOTENCY_KEY_REQUIRED`	Send an Idempotency-Key of 1 to 200 printable characters.	400 `POST /filings/ct600` 400 `POST /filings/accounts`
`IDEMPOTENCY_KEY_CONFLICT`	This idempotency key was already used with a different request.	409 `POST /filings/ct600` 409 `POST /filings/accounts`
`DRAFT_CHANGED`	The saved filing changed. Review it and submit with a new idempotency key.	409 `POST /filings/ct600` 409 `POST /filings/accounts`
`RETRY_NOT_AVAILABLE`	Only an unchanged filing from the same operation and environment can be sent again with retry_of.	400 `POST /filings/ct600` 400 `POST /filings/accounts`
`FILING_SLOT_UNAVAILABLE`	An active Practice plan with an available filing slot is required.	402 `POST /filings/ct600` 402 `POST /filings/accounts`
`INPUT_REQUIRED`	Complete the named fields before filing.	400 `POST /filings/ct600` 400 `POST /filings/accounts`
`HMRC_AUTH_FAILED`	HMRC refused the Government Gateway details. Use fresh credentials and a new idempotency key.	Not declared by a current endpoint
`NOT_SENT_CREDENTIALS_REQUIRED`	The unsent part requires fresh credentials and a new idempotency key.	Not declared by a current endpoint
`TRANSMISSION_OUTCOME_UNKNOWN`	The outcome is not yet known. This part must not be sent again.	Not declared by a current endpoint

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