Internal (unstable)

Hook	Kind	Tier	Since	Summary
boolean_smtp_api_debug	action	C — internal, may change in any release	1.0.0	Fires with diagnostic details after an API-mode send attempt, whatever its outcome.
boolean_smtp_api_debug_response_enabled	filter	C — internal, may change in any release	1.0.0	Filters whether raw debug data may be attached to an admin REST response.
boolean_smtp_debug_attach_api_debug	filter	C — internal, may change in any release	1.0.0	Filters the legacy API debug response toggle.
boolean_smtp_debug_http_enabled	filter	C — internal, may change in any release	1.0.0	Filters whether the outgoing-HTTP debug channel is enabled.
boolean_smtp_debug_http_log_style	filter	C — internal, may change in any release	1.0.0	Filters how outgoing HTTP exchanges are formatted in the `requests` log channel.
boolean_smtp_debug_http_mailer_only	filter	C — internal, may change in any release	1.0.0	Filters whether the outgoing-HTTP debug channel is restricted to mail-provider URLs.
boolean_smtp_debug_http_mailer_url_match	filter	C — internal, may change in any release	1.0.0	Filters whether a URL not matched by the built-in provider list counts as mail-related.
boolean_smtp_debug_log_error	action	C — internal, may change in any release	1.0.0	Fires when a debug session could not be stored.
boolean_smtp_debug_mailer_action_enabled	filter	C — internal, may change in any release	1.0.0	Filters whether the mailer-diagnostics debug channel is enabled.
boolean_smtp_debug_mask_visible_suffix_length	filter	C — internal, may change in any release	1.0.0	Filters the number of trailing characters left visible when a value is masked.
boolean_smtp_debug_output_xhr	filter	C — internal, may change in any release	1.0.0	Filters the legacy API debug response toggle (alias of `boolean_smtp_debug_attach_api_debug`).
boolean_smtp_debug_queries_enabled	filter	C — internal, may change in any release	1.0.0	Filters whether the database-query debug channel is enabled.
boolean_smtp_debug_query_call_site_include_frame	filter	C — internal, may change in any release	1.0.0	Filters whether a backtrace frame may be used as a query's recorded call site.
boolean_smtp_debug_request_enabled	filter	C — internal, may change in any release	1.0.0	Filters whether the incoming-request/REST debug channel is enabled.
boolean_smtp_debug_request_secret_visibility	filter	C — internal, may change in any release	1.0.0	Filters the secret visibility used for REST/POST request payload blocks in the `requests` log channel.
boolean_smtp_debug_scope	filter	C — internal, may change in any release	1.0.0	Filters the debug logging scope.
boolean_smtp_debug_secret_visibility	filter	C — internal, may change in any release	1.0.0	Filters how sensitive values appear in debug logs and API debug responses.
boolean_smtp_debug_sensitive_key	filter	C — internal, may change in any release	1.0.0	Filters whether a data key is treated as sensitive for debug-log redaction.
boolean_smtp_debug_should_log_http	filter	C — internal, may change in any release	1.0.0	Filters whether an outgoing HTTP request/response is recorded to the debug log.
boolean_smtp_log_mailer_resolved_to_error_log	filter	C — internal, may change in any release	1.0.0	Filters whether the resolved mailer diagnostics are written to the plugin log.
boolean_smtp_oauth_debug_ui	filter	C — internal, may change in any release	1.0.0	Filters whether the admin UI shows raw OAuth provider diagnostics.
boolean_smtp_vite_dev_server_url	filter	C — internal, may change in any release	1.0.0	Filters the resolved Vite dev-server URL.

These hooks have no stability promise: they can change or disappear in any release. Each still has its own page like every hook does, but never build on one — check its Fires in section before relying on where it fires.