# Invalid client identifier {0} when trying oauth2/token

**URL:** <https://developer.community.nhs.uk/t/invalid-client-identifier-0-when-trying-oauth2-token/15140>\
**Category:** API Platform\
**Tags:** onboarding, fhir\
**Created:** [17 September 2026 13:26 UTC](https://developer.community.nhs.uk/t/invalid-client-identifier-0-when-trying-oauth2-token/15140 "2026-09-17T13:26:25Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![Clarissa\_John](https://avatars.discourse-cdn.com/v4/letter/c/c57346/32.png) [@Clarissa\_John](https://developer.community.nhs.uk/u/Clarissa_John)\
**Post date:** [2 October 2026 10:31 UTC](https://developer.community.nhs.uk/t/invalid-client-identifier-0-when-trying-oauth2-token/15140/2 "2026-10-02T10:31:20Z")

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Dear @Howard_Lai

Thank you for query on the NHSE API developer community and apologies for the delayed response.  
Please see the below forum posts for information which provides some guidance on this error:

[Invalid client identifier when trying to authenticate for Application-restricted RESTful APIs sandbox environments - API Platform - NHS England Developer Community](https://developer.community.nhs.uk/t/invalid-client-identifier-when-trying-to-authenticate-for-application-restricted-restful-apis-sandbox-environments/8476)

[e-RS FHIR API - Invalid Client Id - PTL INT - API Platform - NHS England Developer Community](https://developer.community.nhs.uk/t/e-rs-fhir-api-invalid-client-id-ptl-int/3814)

If you have reviewed an verified all the actions in the above forum posts and still get Invalid client identifier, please raise a ticket via the **NHS England Developer Help & Support / ServiceNow** [Customer Service Portal - Customer Support](https://www.support.digitalservices.nhs.uk/csm) and include for each environment:

- **Application names** and **dev/int/sandbox Application IDs**
- The **dev/int/sandbox API key** you’re using (first/last 4 chars only in the ticket)
- A **redacted sample JWT payload** (header + body, without signature)
- The **exact cURL or HTTP request** you are sending (with secrets removed)
- Approximate **timestamp** of a failing call:

Thanks,

**NHS England API Platform team**

**Please note: The API Platform team can only address queries relevant to the NHS England API Platform, including security, rate limiting, logging, monitoring and alerting. For any API, CIS2, NHS Login specific queries, please reach out to the relevant API / Service teams.**

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