diff --git a/.github/instructions/terraform-modules.instructions.md b/.github/instructions/terraform-modules.instructions.md
index b471a32e..a489cea4 100644
--- a/.github/instructions/terraform-modules.instructions.md
+++ b/.github/instructions/terraform-modules.instructions.md
@@ -132,12 +132,14 @@ Every module must contain:
 | `context.tf` | Tags context (copied from `tags/exports/context.tf`) | Yes |
 | `data.tf` | Data sources (e.g., `data.aws_*`, `data.local_file`) | Only if data sources exist |
 | `locals.tf` | Derived/computed values (naming, defaults) | Only if `locals {}` blocks exist |
+| `validations.tf` | Cross-variable `terraform_data` preconditions | Only if cross-variable constraints exist |
 | `README.md` | Usage documentation with examples | Yes |
 
 **Conditional file guidance:**
 
 - **`data.tf`**: Create this file if the module queries external data (e.g., `data.aws_availability_zones`, `data.aws_ami`). Store all data sources here for clarity.
 - **`locals.tf`**: Create this file only if the module defines `locals {}` blocks for computed values, naming logic, or CIDR calculations. If no locals are needed, omit the file entirely.
+- **`validations.tf`**: Create this file when the module needs to enforce constraints that span multiple input variables (e.g., mutual exclusivity, co-required inputs). Use a `terraform_data` resource with `lifecycle { precondition { ... } }` blocks. See the Cross-Variable Validation section below.
 
 For any newly created module, `context.tf` must come from `infrastructure/modules/tags/exports/context.tf`, and the copied file must reference `source = "../tags"`.
 
@@ -176,6 +178,44 @@ variable "recovery_window_in_days" {
 }
 ```
 
+## Cross-Variable Validation (`validations.tf`)
+
+Terraform `validation` blocks can only reference the variable they are declared on. When a constraint spans multiple variables (e.g., mutual exclusivity, co-required inputs), use a `terraform_data` resource with `lifecycle { precondition { ... } }` blocks in a dedicated `validations.tf` file.
+
+```hcl
+################################################################
+# Input validation
+#
+# Validates cross-variable constraints that cannot be expressed
+# through individual variable validation blocks:
+#
+#   * <constraint 1 description>
+#   * <constraint 2 description>
+################################################################
+
+resource "terraform_data" "validations" {
+  count = module.this.enabled ? 1 : 0
+
+  lifecycle {
+    precondition {
+      condition     = !(var.option_a && var.option_b != null)
+      error_message = "option_a and option_b are mutually exclusive; set only one."
+    }
+    precondition {
+      condition     = !var.enable_feature || var.feature_config != null
+      error_message = "enable_feature requires feature_config to be set."
+    }
+  }
+}
+```
+
+**Rules:**
+
+- Gate with `count = module.this.enabled ? 1 : 0` so checks only run when the module is active.
+- `terraform_data` is a built-in resource requiring no extra provider (available since Terraform 1.4).
+- Add a reference in the `main.tf` header comment: `# Cross-variable input constraints are enforced in validations.tf.`
+- Document the constraints in a `## Validation` section in `README.md`.
+
 ## Outputs
 
 - Include `description` on all outputs.
@@ -220,6 +260,7 @@ Module READMEs should include:
    - Advanced/edge example (e.g., restore-from-snapshot, records-only mode)
 4. **Conventions** section explaining naming, context behaviour, and notable defaults.
 5. **What this module does NOT do** section listing intentional exclusions/guardrails.
+6. **Validation** section (when `validations.tf` exists) listing each precondition constraint in plain English.
 
 READMEs that do not include a `## Usage` section are incomplete and must be updated before merge.
 
diff --git a/.github/skills/terraform-module-patterns.skill.md b/.github/skills/terraform-module-patterns.skill.md
index 5c6f9e93..9d07688e 100644
--- a/.github/skills/terraform-module-patterns.skill.md
+++ b/.github/skills/terraform-module-patterns.skill.md
@@ -185,7 +185,47 @@ variable "meaningful_name" {
 
 - `data.tf` is mandatory only when data sources exist in the module (for example `data.aws_*`, `data.local_file`, `data.external`).
 - `locals.tf` is mandatory only when the module defines one or more `locals {}` blocks.
-- When present, keep all data sources in `data.tf` and all local values in `locals.tf`.
+- `validations.tf` is mandatory when cross-variable constraints exist (mutual exclusivity, co-required inputs). See below.
+- When present, keep all data sources in `data.tf`, all local values in `locals.tf`, and all cross-variable preconditions in `validations.tf`.
+
+## Cross-Variable Validation Pattern (`validations.tf`)
+
+Individual `variable` validation blocks can only reference the variable they are declared on. For constraints spanning multiple variables, use a `terraform_data` resource in `validations.tf`:
+
+```hcl
+################################################################
+# Input validation
+#
+# Validates cross-variable constraints that cannot be expressed
+# through individual variable validation blocks:
+#
+#   * <constraint 1 — e.g., option_a and option_b are mutually exclusive>
+#   * <constraint 2 — e.g., enable_feature requires feature_config>
+################################################################
+
+resource "terraform_data" "validations" {
+  count = module.this.enabled ? 1 : 0
+
+  lifecycle {
+    precondition {
+      condition     = !(var.option_a && var.option_b != null)
+      error_message = "option_a and option_b are mutually exclusive; set only one."
+    }
+    precondition {
+      condition     = !var.enable_feature || var.feature_config != null
+      error_message = "enable_feature requires feature_config to be set."
+    }
+  }
+}
+```
+
+**Key rules:**
+
+- Gate with `count = module.this.enabled ? 1 : 0` — checks only run when the module is active.
+- `terraform_data` is a Terraform built-in; no extra provider required (available since Terraform 1.4, well within `>= 1.13`).
+- `lifecycle { precondition }` on `module` blocks is **not supported** — always use a `terraform_data` resource.
+- Reference `validations.tf` in the `main.tf` header comment: `# Cross-variable input constraints are enforced in validations.tf.`
+- Document each constraint in a `## Validation` section in `README.md`.
 
 ## Locals Pattern (When Locals Are Needed)
 
@@ -314,6 +354,14 @@ module "example" {
 
 * <Notable naming/default behaviours>
 * <Important constraints>
+
+## Validation
+
+*(Include this section only when `validations.tf` exists.)*
+
+The following constraints are enforced at `plan` time via preconditions in `validations.tf`:
+
+* **<Constraint name>**: <Plain-English description of what is checked and why>.
 ```
 
 ## Security Patterns
diff --git a/infrastructure/modules/secrets-manager/README.md b/infrastructure/modules/secrets-manager/README.md
index df6f7eeb..fcb530f6 100644
--- a/infrastructure/modules/secrets-manager/README.md
+++ b/infrastructure/modules/secrets-manager/README.md
@@ -61,11 +61,31 @@ module "api_key" {
 }
 ```
 
+### Random password
+
+```hcl
+module "generated_password" {
+  source = "git::https://github.com/NHSDigital/screening-terraform-modules-aws.git//infrastructure/modules/secrets-manager?ref=<tag>"
+
+  context = module.this.context
+  stack   = "database"
+  name    = "db-password"
+
+  description            = "Auto-generated database password"
+  kms_key_id             = module.rds_kms.key_arn
+  create_random_password = true
+
+  # Optional: customise the generated password
+  random_password_length           = 48
+  random_password_override_special = "!@#$%&*()-_=+[]{}<>:?"
+}
+```
+
 ### Secret with rotation
 
 ```hcl
 module "rotated_password" {
-    source = "git::https://github.com/NHSDigital/screening-terraform-modules-aws.git//infrastructure/modules/secrets-manager?ref=<tag>"
+  source = "git::https://github.com/NHSDigital/screening-terraform-modules-aws.git//infrastructure/modules/secrets-manager?ref=<tag>"
 
   context = module.this.context
   stack   = "database"
@@ -75,6 +95,7 @@ module "rotated_password" {
   ignore_secret_changes = true  # let the rotation Lambda manage the value
 
   enable_rotation     = true
+  rotate_immediately  = true
   rotation_lambda_arn = "arn:aws:lambda:eu-west-2:123456789012:function:rotate-db-secret"
   rotation_rules = {
     automatically_after_days = 30
@@ -90,6 +111,9 @@ module "rotated_password" {
 - `secret_string` is the plaintext secret value (as a string or JSON-encoded object); use Terraform variables and `sensitive = true` to avoid exposure.
 - `ignore_secret_changes` should be set to `true` when enabling rotation so Terraform does not overwrite the rotated value on the next apply.
 - `create_random_password` generates a random password as the secret value; do not set `secret_string` when using this option.
+- `random_password_length` controls the length of the generated password (default `32`; must be 8–4096).
+- `random_password_override_special` overrides the default special character set used during password generation.
+- `rotate_immediately` controls whether rotation fires immediately (`true`) or waits for the next scheduled window (`false`); only relevant when `enable_rotation = true`.
 - `create_policy` defaults to `false`; set to `true` and provide `policy_statements` to attach a resource-based policy.
 - Secret names are derived from `module.this.id` for consistency with other screening modules.
 
@@ -99,7 +123,13 @@ module "rotated_password" {
 - Create rotation Lambda functions; you must create the function separately and provide its ARN via `rotation_lambda_arn`.
 - Populate secret values automatically; you must provide `secret_string` or enable `create_random_password`.
 - Manage secret replicas across regions (use native `aws_secretsmanager_secret_rotation` resources if multi-region replication is required).
-- Retrieve secret values; use `data.aws_secretsmanager_secret_version` in consumer stacks to read secrets.
+
+## Validation
+
+The following constraints are enforced at `plan` time via preconditions in `validations.tf`:
+
+- **Secret value mutual exclusivity**: `secret_string`, `secret_string_wo`, and `create_random_password` are mutually exclusive — setting more than one will produce a clear error before any resource is created.
+- **Rotation completeness**: Setting `enable_rotation = true` requires both `rotation_lambda_arn` and `rotation_rules` to be provided.
 
 <!-- vale off -->
 <!-- markdownlint-disable -->
@@ -114,7 +144,9 @@ module "rotated_password" {
 
 ## Providers
 
-No providers.
+| Name | Version |
+| ---- | ------- |
+| <a name="provider_terraform"></a> [terraform](#provider\_terraform) | n/a |
 
 ## Modules
 
@@ -125,7 +157,9 @@ No providers.
 
 ## Resources
 
-No resources.
+| Name | Type |
+| ---- | ---- |
+| [terraform_data.validations](https://registry.terraform.io/providers/hashicorp/terraform/latest/docs/resources/data) | resource |
 
 ## Inputs
 
@@ -159,9 +193,12 @@ No resources.
 | <a name="input_policy_statements"></a> [policy\_statements](#input\_policy\_statements) | A map of IAM policy statements to attach to the secret policy. Only used when create\_policy is true. | <pre>map(object({<br/>    sid           = optional(string)<br/>    actions       = optional(list(string))<br/>    not_actions   = optional(list(string))<br/>    effect        = optional(string)<br/>    resources     = optional(list(string))<br/>    not_resources = optional(list(string))<br/>    principals = optional(list(object({<br/>      type        = string<br/>      identifiers = list(string)<br/>    })))<br/>    not_principals = optional(list(object({<br/>      type        = string<br/>      identifiers = list(string)<br/>    })))<br/>    condition = optional(list(object({<br/>      test     = string<br/>      values   = list(string)<br/>      variable = string<br/>    })))<br/>  }))</pre> | `{}` | no |
 | <a name="input_project"></a> [project](#input\_project) | ID element. A project identifier, indicating the name or role of the project the resource is for, such as `website` or `api` | `string` | `null` | no |
 | <a name="input_public_facing"></a> [public\_facing](#input\_public\_facing) | Whether this resource is public facing | `bool` | `false` | no |
+| <a name="input_random_password_length"></a> [random\_password\_length](#input\_random\_password\_length) | The length of the randomly-generated password. Only used when create\_random\_password is true. | `number` | `32` | no |
+| <a name="input_random_password_override_special"></a> [random\_password\_override\_special](#input\_random\_password\_override\_special) | Supply your own list of special characters for random password generation. Overrides the default special character set. Only used when create\_random\_password is true. | `string` | `"!@#$%&*()-_=+[]{}<>:?"` | no |
 | <a name="input_recovery_window_in_days"></a> [recovery\_window\_in\_days](#input\_recovery\_window\_in\_days) | Number of days AWS Secrets Manager waits before permanently deleting the secret. Valid values: 0 (immediate deletion) or 7-30. | `number` | `30` | no |
 | <a name="input_regex_replace_chars"></a> [regex\_replace\_chars](#input\_regex\_replace\_chars) | Terraform regular expression (regex) string.<br/>Characters matching the regex will be removed from the ID elements.<br/>If not set, `"/[^a-zA-Z0-9-]/"` is used to remove all characters other than hyphens, letters and digits. | `string` | `null` | no |
 | <a name="input_region"></a> [region](#input\_region) | ID element \_(Rarely used, not included by default)\_.  Usually an abbreviation of the selected AWS region e.g. 'uw2', 'ew2' or 'gbl' for resources like IAM roles that have no region | `string` | `null` | no |
+| <a name="input_rotate_immediately"></a> [rotate\_immediately](#input\_rotate\_immediately) | When enable\_rotation is true, specifies whether to rotate the secret immediately or wait until the next scheduled rotation window. Defaults to immediate rotation when not set. | `bool` | `null` | no |
 | <a name="input_rotation_lambda_arn"></a> [rotation\_lambda\_arn](#input\_rotation\_lambda\_arn) | ARN of the Lambda function that rotates the secret. Required when enable\_rotation is true. | `string` | `""` | no |
 | <a name="input_rotation_rules"></a> [rotation\_rules](#input\_rotation\_rules) | Rotation schedule for the secret. Provide either automatically\_after\_days or a schedule\_expression (cron/rate). Required when enable\_rotation is true. | <pre>object({<br/>    automatically_after_days = optional(number)<br/>    duration                 = optional(string)<br/>    schedule_expression      = optional(string)<br/>  })</pre> | `null` | no |
 | <a name="input_secret_name"></a> [secret\_name](#input\_secret\_name) | Optional explicit name for the secret. When null, the name is derived from context labels via module.this.id. | `string` | `null` | no |
@@ -184,6 +221,7 @@ No resources.
 | <a name="output_secret_arn"></a> [secret\_arn](#output\_secret\_arn) | The ARN of the secret |
 | <a name="output_secret_id"></a> [secret\_id](#output\_secret\_id) | The ID of the secret (same as the ARN) |
 | <a name="output_secret_name"></a> [secret\_name](#output\_secret\_name) | The name of the secret |
+| <a name="output_secret_string"></a> [secret\_string](#output\_secret\_string) | The secret string value. Sensitive — only use where required. |
 | <a name="output_secret_version_id"></a> [secret\_version\_id](#output\_secret\_version\_id) | The unique identifier of the current version of the secret |
 <!-- END_TF_DOCS -->
 <!-- markdownlint-restore -->
diff --git a/infrastructure/modules/secrets-manager/main.tf b/infrastructure/modules/secrets-manager/main.tf
index f7bc1b10..c484a466 100644
--- a/infrastructure/modules/secrets-manager/main.tf
+++ b/infrastructure/modules/secrets-manager/main.tf
@@ -11,6 +11,8 @@
 #
 # Inputs intentionally NOT exposed (hardcoded below):
 #   - block_public_policy → always true; callers cannot override
+#
+# Cross-variable input constraints are enforced in validations.tf.
 ################################################################
 
 module "secret" {
@@ -27,11 +29,13 @@ module "secret" {
   recovery_window_in_days = var.recovery_window_in_days
 
   # Secret value
-  create_random_password   = var.create_random_password
-  secret_string            = var.secret_string
-  secret_string_wo         = var.secret_string_wo
-  secret_string_wo_version = var.secret_string_wo_version
-  ignore_secret_changes    = var.ignore_secret_changes
+  create_random_password           = var.create_random_password
+  random_password_length           = var.random_password_length
+  random_password_override_special = var.random_password_override_special
+  secret_string                    = var.secret_string
+  secret_string_wo                 = var.secret_string_wo
+  secret_string_wo_version         = var.secret_string_wo_version
+  ignore_secret_changes            = var.ignore_secret_changes
 
   # Policy
   create_policy       = var.create_policy
@@ -40,6 +44,7 @@ module "secret" {
 
   # Rotation
   enable_rotation     = var.enable_rotation
+  rotate_immediately  = var.rotate_immediately
   rotation_lambda_arn = var.rotation_lambda_arn
   rotation_rules      = var.rotation_rules
 
diff --git a/infrastructure/modules/secrets-manager/outputs.tf b/infrastructure/modules/secrets-manager/outputs.tf
index 5363e619..b30554bf 100644
--- a/infrastructure/modules/secrets-manager/outputs.tf
+++ b/infrastructure/modules/secrets-manager/outputs.tf
@@ -17,3 +17,9 @@ output "secret_version_id" {
   description = "The unique identifier of the current version of the secret"
   value       = module.secret.secret_version_id
 }
+
+output "secret_string" {
+  description = "The secret string value. Sensitive — only use where required."
+  sensitive   = true
+  value       = module.secret.secret_string
+}
diff --git a/infrastructure/modules/secrets-manager/validations.tf b/infrastructure/modules/secrets-manager/validations.tf
new file mode 100644
index 00000000..0a758f1f
--- /dev/null
+++ b/infrastructure/modules/secrets-manager/validations.tf
@@ -0,0 +1,34 @@
+################################################################
+# Input validation
+#
+# Validates cross-variable constraints that cannot be expressed
+# through individual variable validation blocks:
+#
+#   * secret_string, secret_string_wo, and create_random_password
+#     are mutually exclusive — at most one may be set
+#   * enable_rotation requires both rotation_lambda_arn and
+#     rotation_rules to be provided
+################################################################
+
+resource "terraform_data" "validations" {
+  count = module.this.enabled ? 1 : 0
+
+  lifecycle {
+    precondition {
+      condition     = !(var.create_random_password && var.secret_string != null)
+      error_message = "create_random_password and secret_string are mutually exclusive; set only one."
+    }
+    precondition {
+      condition     = !(var.create_random_password && var.secret_string_wo != null)
+      error_message = "create_random_password and secret_string_wo are mutually exclusive; set only one."
+    }
+    precondition {
+      condition     = !(var.secret_string != null && var.secret_string_wo != null)
+      error_message = "secret_string and secret_string_wo are mutually exclusive; set only one."
+    }
+    precondition {
+      condition     = !var.enable_rotation || (var.rotation_lambda_arn != "" && var.rotation_rules != null)
+      error_message = "enable_rotation requires both rotation_lambda_arn and rotation_rules to be set."
+    }
+  }
+}
diff --git a/infrastructure/modules/secrets-manager/variables.tf b/infrastructure/modules/secrets-manager/variables.tf
index 036f7f8f..2c47affd 100644
--- a/infrastructure/modules/secrets-manager/variables.tf
+++ b/infrastructure/modules/secrets-manager/variables.tf
@@ -9,6 +9,11 @@
 #   - name / name_prefix   → derived from module.this.id via locals.tf
 #   - secret_binary        → not supported; use secret_string
 #   - replica              → replication not required for this use case
+#   - version_stages       → low-value for standard use cases
+################################################################
+
+################################################################
+# Naming
 ################################################################
 
 variable "secret_name" {
@@ -17,6 +22,10 @@ variable "secret_name" {
   description = "Optional explicit name for the secret. When null, the name is derived from context labels via module.this.id."
 }
 
+################################################################
+# Secret configuration
+################################################################
+
 variable "description" {
   type        = string
   default     = null
@@ -39,12 +48,32 @@ variable "recovery_window_in_days" {
   }
 }
 
+################################################################
+# Secret value
+################################################################
+
 variable "create_random_password" {
   type        = bool
   default     = false
   description = "When true, generates a random password as the secret value. When set, secret_string and secret_string_wo should not be set."
 }
 
+variable "random_password_length" {
+  type        = number
+  default     = 32
+  description = "The length of the randomly-generated password. Only used when create_random_password is true."
+  validation {
+    condition     = var.random_password_length >= 8 && var.random_password_length <= 4096
+    error_message = "random_password_length must be between 8 and 4096 inclusive."
+  }
+}
+
+variable "random_password_override_special" {
+  type        = string
+  default     = "!@#$%&*()-_=+[]{}<>:?"
+  description = "Supply your own list of special characters for random password generation. Overrides the default special character set. Only used when create_random_password is true."
+}
+
 variable "secret_string" {
   type        = string
   default     = null
@@ -71,6 +100,10 @@ variable "ignore_secret_changes" {
   description = "When true, Terraform will ignore any changes made to the secret value outside of Terraform (e.g. by a rotation Lambda). Set to true when rotation is enabled."
 }
 
+################################################################
+# Policy
+################################################################
+
 variable "create_policy" {
   type        = bool
   default     = false
@@ -103,12 +136,22 @@ variable "policy_statements" {
   description = "A map of IAM policy statements to attach to the secret policy. Only used when create_policy is true."
 }
 
+################################################################
+# Rotation
+################################################################
+
 variable "enable_rotation" {
   type        = bool
   default     = false
   description = "Whether to enable automatic secret rotation via a Lambda function."
 }
 
+variable "rotate_immediately" {
+  type        = bool
+  default     = null
+  description = "When enable_rotation is true, specifies whether to rotate the secret immediately or wait until the next scheduled rotation window. Defaults to immediate rotation when not set."
+}
+
 variable "rotation_lambda_arn" {
   type        = string
   default     = ""