Merge semantics: only non-null fields in the body override the latest downstream signatory state.

Access and authorization

• Appropriate Open Banking access is required (e.g. mCust in the implementation).
• The caller's partner identifier must be present (SIG_PARTNERID_REQUIRED if missing).
• The customer must exist (SIG_CUSTOMERNOTFOUND) and be allowed for the token (SIG_CUSTOMERACCESS_DENIED).
• The signatory must belong to the requested customer; otherwise the call fails (e.g. access denied or signatory not found, depending on downstream behavior).

Validation

• At least one body property must be present; an empty JSON object is rejected (SIG_VALIDATION_ERROR).
• Email: if emailAddress is sent, it must be a valid format and satisfy uniqueness rules where the platform applies them.
• Phone: after merge, the effective phone number is validated with validate-phone (SIG_PHONE_INVALID if invalid)—this applies even when the number was not changed in the patch but remains the current value.
• Approval limit: when provided, must be non-negative and within digit/scale limits.

Phone is validated against the merged signatory state, not only newly sent fields.