diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index b2459f3..613d21f 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -242,6 +242,8 @@ Validate: `go test ./internal/domain ./internal/backend`. Add ownership tests
 only for mutable structures whose accidental sharing would affect calls.
 No public `SmokeTest` method or temporary not-implemented API is introduced yet.
 
+**Completed: 2026-10-13.**
+
 ## Stage 2: Load Final Backend Diagnostic Metadata
 
 Scope: `internal/configuration`, `internal/backend`, and `backends.go`.
diff --git a/internal/domain/domain.go b/internal/domain/domain.go
index 78d2c25..686b439 100644
--- a/internal/domain/domain.go
+++ b/internal/domain/domain.go
@@ -180,12 +180,14 @@ type PromptMessageTemplate struct {
 	CacheControl *CacheControl `yaml:"cache_control,omitempty" json:"cache_control,omitempty"`
 }
 
-// Backend describes reusable OpenAI-compatible connection defaults.
+// Backend describes reusable OpenAI-compatible connection defaults and optional
+// diagnostic metadata.
 type Backend struct {
 	ID               string
 	Endpoint         string
 	APIKeyEnv        string
 	ExtraParams      map[string]any
+	SmokeTests       *BackendSmokeTestConfig
 	ConcurrencyLimit int
 	QueueCapacity    int
 	QueueCapacitySet bool
diff --git a/internal/domain/smoke_diagnostics.go b/internal/domain/smoke_diagnostics.go
new file mode 100644
index 0000000..d1fa129
--- /dev/null
+++ b/internal/domain/smoke_diagnostics.go
@@ -0,0 +1,263 @@
+package domain
+
+import (
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+)
+
+// BackendSmokeTestConfig describes optional backend-owned diagnostic defaults.
+type BackendSmokeTestConfig struct {
+	DefaultProfile *string
+	Reachability   *ReachabilityProbeConfig
+	Authentication *AuthenticationProbeConfig
+}
+
+// ReachabilityProbeConfig describes an optional endpoint for reachability tests.
+type ReachabilityProbeConfig struct {
+	Endpoint *string
+}
+
+// AuthenticationProbeConfig describes an optional authentication diagnostic.
+type AuthenticationProbeConfig struct {
+	Kind     *string
+	Endpoint *string
+}
+
+// SmokeTestKind identifies one built-in diagnostic operation.
+type SmokeTestKind string
+
+const (
+	SmokeTestReachability     SmokeTestKind = "reachability"
+	SmokeTestAuthentication   SmokeTestKind = "authentication"
+	SmokeTestTextInference    SmokeTestKind = "text_inference"
+	SmokeTestStructuredOutput SmokeTestKind = "structured_output"
+)
+
+// SmokeTestRequest selects one diagnostic and its execution target.
+type SmokeTestRequest struct {
+	Kind             SmokeTestKind
+	BackendID        string
+	PromptID         string
+	PromptVersion    string
+	ProfileID        string
+	Execution        *ExecutionTargetOverride
+	APIKey           string `json:"-" yaml:"-"`
+	TransientRetries *int
+}
+
+// SmokeTestReport contains factual diagnostic results, not generated content.
+type SmokeTestReport struct {
+	Kind       SmokeTestKind
+	Supported  bool
+	Applicable *bool
+	Completed  bool
+	Reason     SmokeTestReason
+	Target     SmokeTestTarget
+	Facts      SmokeTestFacts
+	Timing     SmokeTestTiming
+	Attempts   []SmokeTestAttempt
+	Usage      ObservedTokenUsage
+	Failure    ExecutionFailureKind
+}
+
+// SmokeTestTarget identifies the selected diagnostic target and its settings.
+type SmokeTestTarget struct {
+	BackendID              string
+	ProfileID              string
+	PromptID               string
+	PromptVersion          string
+	DefaultSourceBackendID string
+	SelectionSource        string
+	Endpoint               string
+	EffectiveSettings      ExecutionTarget
+	SettingsPresence       ExecutionTargetPresence
+}
+
+// SmokeTestFacts contains optional factual observations from a diagnostic.
+type SmokeTestFacts struct {
+	CredentialsSupplied       *bool
+	EndpointReachable         *bool
+	CredentialsAccepted       *bool
+	ResponseReceived          *bool
+	GenerationCompleted       *bool
+	TextReturned              *bool
+	ExpectedTextMatched       *bool
+	StructuredOutputRequested *bool
+	ValidJSON                 *bool
+	SchemaMatched             *bool
+}
+
+// SmokeTestTiming records elapsed diagnostic and phase durations.
+type SmokeTestTiming struct {
+	StartedAt          time.Time
+	Duration           time.Duration
+	CapacityWait       *time.Duration
+	RequestDuration    *time.Duration
+	RetryDelay         *time.Duration
+	EvaluationDuration *time.Duration
+}
+
+// SmokeTestAttempt records one dispatched attempt or an interrupted acquisition.
+type SmokeTestAttempt struct {
+	Dispatched      bool
+	InitialEndpoint string
+	Endpoint        string
+	HTTPStatus      *int
+	Facts           SmokeTestFacts
+	Timing          SmokeTestTiming
+	Failure         ExecutionFailureKind
+	Usage           ObservedTokenUsage
+}
+
+// SmokeTestReason identifies additional factual context for a diagnostic.
+type SmokeTestReason string
+
+const (
+	SmokeTestReasonNone                               SmokeTestReason = ""
+	SmokeTestReasonAuthenticationNotApplicable        SmokeTestReason = "authentication_not_applicable"
+	SmokeTestReasonProbeNotConfigured                 SmokeTestReason = "probe_not_configured"
+	SmokeTestReasonHTTPDiagnosticsUnavailable         SmokeTestReason = "http_diagnostics_unavailable"
+	SmokeTestReasonRequiredCredentialMissing          SmokeTestReason = "required_credential_missing"
+	SmokeTestReasonAuthenticationResponseUnrecognized SmokeTestReason = "authentication_response_unrecognized"
+)
+
+// CloneBackend returns a copy of a backend, including its mutable settings.
+func CloneBackend(backend Backend) (Backend, error) {
+	cloned := backend
+	var err error
+	cloned.ExtraParams, err = jsonvalue.CopyMap(backend.ExtraParams)
+	if err != nil {
+		return Backend{}, err
+	}
+	cloned.SmokeTests = CloneBackendSmokeTestConfig(backend.SmokeTests)
+	return cloned, nil
+}
+
+// CloneBackendSmokeTestConfig returns an independent copy of optional metadata.
+func CloneBackendSmokeTestConfig(config *BackendSmokeTestConfig) *BackendSmokeTestConfig {
+	if config == nil {
+		return nil
+	}
+	cloned := *config
+	cloned.DefaultProfile = cloneString(config.DefaultProfile)
+	if config.Reachability != nil {
+		reachability := *config.Reachability
+		reachability.Endpoint = cloneString(config.Reachability.Endpoint)
+		cloned.Reachability = &reachability
+	}
+	if config.Authentication != nil {
+		authentication := *config.Authentication
+		authentication.Kind = cloneString(config.Authentication.Kind)
+		authentication.Endpoint = cloneString(config.Authentication.Endpoint)
+		cloned.Authentication = &authentication
+	}
+	return &cloned
+}
+
+// CloneSmokeTestRequest returns a copy that owns its optional settings.
+func CloneSmokeTestRequest(request SmokeTestRequest) (SmokeTestRequest, error) {
+	cloned := request
+	cloned.TransientRetries = cloneInt(request.TransientRetries)
+	if request.Execution == nil {
+		return cloned, nil
+	}
+	execution := *request.Execution
+	execution.Temperature = cloneFloat64(request.Execution.Temperature)
+	execution.MaxTokens = cloneInt(request.Execution.MaxTokens)
+	execution.TopP = cloneFloat64(request.Execution.TopP)
+	execution.TimeoutSeconds = cloneInt(request.Execution.TimeoutSeconds)
+	execution.ReasoningEffort = cloneString(request.Execution.ReasoningEffort)
+	var err error
+	execution.ExtraParams, err = jsonvalue.CopyMap(request.Execution.ExtraParams)
+	if err != nil {
+		return SmokeTestRequest{}, err
+	}
+	cloned.Execution = &execution
+	return cloned, nil
+}
+
+// CloneSmokeTestReport returns a copy with independent optional facts and maps.
+func CloneSmokeTestReport(report SmokeTestReport) (SmokeTestReport, error) {
+	cloned := report
+	cloned.Applicable = cloneBool(report.Applicable)
+	cloned.Facts = cloneSmokeTestFacts(report.Facts)
+	cloned.Timing = cloneSmokeTestTiming(report.Timing)
+	var err error
+	cloned.Target.EffectiveSettings.ExtraParams, err = jsonvalue.CopyMap(report.Target.EffectiveSettings.ExtraParams)
+	if err != nil {
+		return SmokeTestReport{}, err
+	}
+	if report.Attempts != nil {
+		cloned.Attempts = make([]SmokeTestAttempt, len(report.Attempts))
+		for index, attempt := range report.Attempts {
+			cloned.Attempts[index] = attempt
+			cloned.Attempts[index].HTTPStatus = cloneInt(attempt.HTTPStatus)
+			cloned.Attempts[index].Facts = cloneSmokeTestFacts(attempt.Facts)
+			cloned.Attempts[index].Timing = cloneSmokeTestTiming(attempt.Timing)
+		}
+	}
+	return cloned, nil
+}
+
+func cloneSmokeTestFacts(facts SmokeTestFacts) SmokeTestFacts {
+	facts.CredentialsSupplied = cloneBool(facts.CredentialsSupplied)
+	facts.EndpointReachable = cloneBool(facts.EndpointReachable)
+	facts.CredentialsAccepted = cloneBool(facts.CredentialsAccepted)
+	facts.ResponseReceived = cloneBool(facts.ResponseReceived)
+	facts.GenerationCompleted = cloneBool(facts.GenerationCompleted)
+	facts.TextReturned = cloneBool(facts.TextReturned)
+	facts.ExpectedTextMatched = cloneBool(facts.ExpectedTextMatched)
+	facts.StructuredOutputRequested = cloneBool(facts.StructuredOutputRequested)
+	facts.ValidJSON = cloneBool(facts.ValidJSON)
+	facts.SchemaMatched = cloneBool(facts.SchemaMatched)
+	return facts
+}
+
+func cloneSmokeTestTiming(timing SmokeTestTiming) SmokeTestTiming {
+	timing.CapacityWait = cloneDuration(timing.CapacityWait)
+	timing.RequestDuration = cloneDuration(timing.RequestDuration)
+	timing.RetryDelay = cloneDuration(timing.RetryDelay)
+	timing.EvaluationDuration = cloneDuration(timing.EvaluationDuration)
+	return timing
+}
+
+func cloneBool(value *bool) *bool {
+	if value == nil {
+		return nil
+	}
+	cloned := *value
+	return &cloned
+}
+
+func cloneDuration(value *time.Duration) *time.Duration {
+	if value == nil {
+		return nil
+	}
+	cloned := *value
+	return &cloned
+}
+
+func cloneFloat64(value *float64) *float64 {
+	if value == nil {
+		return nil
+	}
+	cloned := *value
+	return &cloned
+}
+
+func cloneInt(value *int) *int {
+	if value == nil {
+		return nil
+	}
+	cloned := *value
+	return &cloned
+}
+
+func cloneString(value *string) *string {
+	if value == nil {
+		return nil
+	}
+	cloned := *value
+	return &cloned
+}
diff --git a/internal/domain/smoke_diagnostics_test.go b/internal/domain/smoke_diagnostics_test.go
new file mode 100644
index 0000000..ea87df9
--- /dev/null
+++ b/internal/domain/smoke_diagnostics_test.go
@@ -0,0 +1,110 @@
+package domain
+
+import (
+	"testing"
+	"time"
+)
+
+func TestCloneBackendCopiesDiagnosticMetadataAndProviderValues(t *testing.T) {
+	profile := "profile"
+	reachabilityEndpoint := "https://probe.example/health"
+	authKind := "openrouter_key"
+	authEndpoint := "https://probe.example/key"
+	backend := Backend{
+		ExtraParams: map[string]any{"nested": map[string]any{"value": "original"}},
+		SmokeTests: &BackendSmokeTestConfig{
+			DefaultProfile: &profile,
+			Reachability:   &ReachabilityProbeConfig{Endpoint: &reachabilityEndpoint},
+			Authentication: &AuthenticationProbeConfig{
+				Kind:     &authKind,
+				Endpoint: &authEndpoint,
+			},
+		},
+	}
+
+	cloned, err := CloneBackend(backend)
+	if err != nil {
+		t.Fatal(err)
+	}
+	*cloned.SmokeTests.DefaultProfile = "changed"
+	*cloned.SmokeTests.Reachability.Endpoint = "changed"
+	*cloned.SmokeTests.Authentication.Kind = "changed"
+	*cloned.SmokeTests.Authentication.Endpoint = "changed"
+	cloned.ExtraParams["nested"].(map[string]any)["value"] = "changed"
+
+	if profile != "profile" || reachabilityEndpoint != "https://probe.example/health" ||
+		authKind != "openrouter_key" || authEndpoint != "https://probe.example/key" {
+		t.Fatal("cloned diagnostic metadata shared source pointers")
+	}
+	if got := backend.ExtraParams["nested"].(map[string]any)["value"]; got != "original" {
+		t.Fatalf("source provider parameter changed through clone: %v", got)
+	}
+}
+
+func TestCloneSmokeTestRequestOwnsOptionalSettings(t *testing.T) {
+	temperature := 0.25
+	retries := 2
+	reasoning := "low"
+	request := SmokeTestRequest{
+		APIKey:           "secret",
+		TransientRetries: &retries,
+		Execution: &ExecutionTargetOverride{
+			Temperature:     &temperature,
+			ReasoningEffort: &reasoning,
+			ExtraParams:     map[string]any{"nested": map[string]any{"value": "original"}},
+		},
+	}
+
+	cloned, err := CloneSmokeTestRequest(request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	*cloned.TransientRetries = 3
+	*cloned.Execution.Temperature = 0.5
+	*cloned.Execution.ReasoningEffort = "high"
+	cloned.Execution.ExtraParams["nested"].(map[string]any)["value"] = "changed"
+
+	if retries != 2 || temperature != 0.25 || reasoning != "low" {
+		t.Fatal("cloned request shared optional scalar pointers")
+	}
+	if got := request.Execution.ExtraParams["nested"].(map[string]any)["value"]; got != "original" {
+		t.Fatalf("source provider parameter changed through clone: %v", got)
+	}
+}
+
+func TestCloneSmokeTestReportOwnsMutableValues(t *testing.T) {
+	applicable := true
+	reachable := true
+	status := 200
+	wait := 3 * time.Millisecond
+	report := SmokeTestReport{
+		Applicable: &applicable,
+		Target: SmokeTestTarget{EffectiveSettings: ExecutionTarget{
+			ExtraParams: map[string]any{"nested": map[string]any{"value": "original"}},
+		}},
+		Facts: SmokeTestFacts{EndpointReachable: &reachable},
+		Attempts: []SmokeTestAttempt{{
+			HTTPStatus: &status,
+			Facts:      SmokeTestFacts{EndpointReachable: &reachable},
+			Timing:     SmokeTestTiming{CapacityWait: &wait},
+		}},
+	}
+
+	cloned, err := CloneSmokeTestReport(report)
+	if err != nil {
+		t.Fatal(err)
+	}
+	*cloned.Applicable = false
+	*cloned.Facts.EndpointReachable = false
+	*cloned.Attempts[0].HTTPStatus = 503
+	*cloned.Attempts[0].Facts.EndpointReachable = false
+	*cloned.Attempts[0].Timing.CapacityWait = time.Second
+	cloned.Target.EffectiveSettings.ExtraParams["nested"].(map[string]any)["value"] = "changed"
+
+	if !applicable || !reachable || status != 200 || wait != 3*time.Millisecond {
+		t.Fatal("cloned report shared optional scalar pointers")
+	}
+	if got := report.Target.EffectiveSettings.ExtraParams["nested"].(map[string]any)["value"]; got != "original" {
+		t.Fatalf("source provider parameter changed through clone: %v", got)
+	}
+}