Custom Action

Expanded Definition

Custom action is best understood as an execution boundary, not just a plugin. It gives a GPT a defined way to call an external API, but the real security question is what the schema permits, what credential is attached, and whether the action can be abused to reach data or functions beyond the intended task. In practice, custom actions sit at the intersection of agentic AI, API governance, and NHI controls, because the action often runs with a secret, token, or delegated OAuth grant that behaves like a non-human identity. NHI Management Group treats this as a governance problem as much as an integration problem, because the action’s scope determines blast radius. For a standards-based lens, NIST Cybersecurity Framework 2.0 is useful for mapping the control implications around access, monitoring, and response. Usage in the industry is still evolving, and definitions vary across vendors when custom actions are blended with tools, plugins, or function calling.

The most common misapplication is assuming the GPT itself is the trust boundary, which occurs when teams review the prompt but not the action schema, credential scope, and downstream API permissions.

Examples and Use Cases

Implementing custom actions rigorously often introduces integration overhead, requiring organisations to balance agent flexibility against tighter API governance, secret handling, and approval workflows.

• A support GPT opens tickets in a service desk system through a custom action that uses a scoped API key limited to ticket creation only.
• A procurement assistant submits purchase requests via OAuth, but the action is restricted so it cannot approve payments or change vendor records.
• A developer copilot queries internal documentation through an action that exposes read-only access, reducing the need to embed broader credentials.
• An operations agent triggers a deployment workflow, but the action is gated by role approval and monitored as a privileged NHI pathway.
• In breach investigations, teams use Ultimate Guide to NHIs to compare the action’s credential scope against their broader NHI inventory and NIST Cybersecurity Framework 2.0 to classify affected access paths.

Why It Matters in NHI Security

Custom actions matter because they convert conversational intent into real system access, and that means every exposed endpoint becomes part of the attack surface. When the action is over-permissioned, a prompt injection or malformed request can push the agent into unintended operations using valid credentials. NHI Management Group notes that 79% of organisations have experienced secrets leaks, with 77% of these incidents resulting in tangible damage, which is especially relevant when action credentials are stored poorly or reused across environments. This is why custom actions should be governed like any other NHI: scoped, logged, rotated, and offboarded. They also need continuous review because the schema itself can become a policy gap if it expands faster than oversight. For an access-control perspective, the action should be treated as a distinct privilege path, not an auxiliary feature. Organisations typically encounter the operational risk only after an agent has sent a harmful request or accessed the wrong system, at which point custom action governance becomes unavoidable to address.

For broader NHI lifecycle controls, the Ultimate Guide to NHIs is the most relevant reference for rotation, visibility, and offboarding expectations. Where teams are formalising their control model, NIST Cybersecurity Framework 2.0 helps translate the concept into access, detect, and respond obligations.

Related resources from NHI Mgmt Group

• What is the 'no prompt means no action' principle in Agentic AI security?
• When should organisations require human approval for an AI agent action?
• What is the difference between flagging and blocking an AI agent action?
• What is the difference between automation and machine action governance?