diff --git a/docs/cli.md b/docs/cli.md
index 3b99f32..4c27461 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -47,6 +47,8 @@ Rules:
 - `--campaign` and `--campaign-file` are mutually exclusive.
 - `--session` is not used by `session init`.
 - if both positional `<session_id>` and `--session-id` are provided, values must match.
+- `--previous-session-id` is a strict expectation: the selected session file
+  must contain the same `previous_session_id`.
 - `clean --all` cannot be combined with campaign/session selectors.
 
 ## Session ID Input Rules
diff --git a/docs/config.md b/docs/config.md
index 86fde4f..a7aa622 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -49,7 +49,10 @@ remote state with an unsafe legacy identity must be migrated before use.
 
 ## Validation and Merge Rules
 
-- YAML decode is strict (`KnownFields(true)`): unknown fields fail load.
+- YAML decode is strict (`KnownFields(true)`) and accepts exactly one document:
+  unknown fields or trailing documents fail load.
+- Configured timeout and retry-delay durations must be positive. An omitted
+  artifact timeout continues to inherit its configured Scriptorium timeout.
 - Session files must be concrete; unresolved `{{ ... }}` placeholders fail load.
 - Pipeline defaults are applied before validation.
 - Campaign and session identities must agree.
@@ -150,8 +153,8 @@ Rules:
 | `pipeline.campaigns.root` | string | No | `/usr/local/share/narratio/campaigns` |
 | `pipeline.campaigns.default_campaign_id` | string | No | empty |
 | `pipeline.secrets.env_dir` | string | No | empty |
-| `pipeline.storage.backend` | string | No | empty |
-| `pipeline.storage.s3.bucket` | string | Conditional | required for S3 session-audio and for publish upload when backend is `s3` |
+| `pipeline.storage.backend` | string | No | `local`; supported values are `local` and `s3` (case-insensitive) |
+| `pipeline.storage.s3.bucket` | string | Conditional | required when backend is `s3` and S3 session-audio or publish upload is enabled |
 | `pipeline.storage.s3.root_prefix` | string | No | `dnd` |
 | `pipeline.storage.s3.region` | string | No | empty |
 | `pipeline.storage.s3.endpoint` | string | No | empty |
@@ -316,6 +319,20 @@ For each artifact input `pipeline.scriptorium.artifacts.<name>.inputs.<input_nam
 Audio rules:
 
 - configure local mode (`audio_dir` or `audio_files`) or S3 mode (`audio_s3.prefix`), not both.
+- `audio_s3` requires `pipeline.storage.backend: s3` and a configured S3 bucket.
+
+### Storage backend selection
+
+`local` is the default and disables remote object-store operations. Configure
+`s3` explicitly before supplying `storage.s3`; a populated S3 block does not
+select a backend on its own. Unknown backend names and an S3 block paired with
+`local` are rejected during configuration validation.
+
+### Previous-session expectation
+
+`previous_session_id` is optional in a session file. When a command supplies
+`--previous-session-id`, however, the session file must contain the same value;
+an omitted or different value is rejected before the command performs work.
 
 ## Maintained Examples
 
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index e636ac8..f9802f9 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -745,6 +745,8 @@ effects and give the configuration package focused, non-duplicative tests.
 durations, backend combinations, strict previous expectations, environment
 isolation, defaults, unknown fields, and stable actionable errors.
 
+**Status:** Completed.
+
 ## Stage 22 — Make product configuration truthful and own remote temp files
 
 **Read first:** `audit-findings.md` lines 2521–2552 (COR-024), 3157–3197
diff --git a/examples/pipeline.full.annotated.yml b/examples/pipeline.full.annotated.yml
index 618899c..0bd4293 100644
--- a/examples/pipeline.full.annotated.yml
+++ b/examples/pipeline.full.annotated.yml
@@ -12,7 +12,7 @@ workspace:
 #   env_dir: ./secrets
 
 storage:
-  # Optional storage backend selector; use "s3" for publish + S3 audio workflows.
+  # Defaults to "local". Use "s3" explicitly for publish + S3 audio workflows.
   backend: s3
   s3:
     # Required when using S3 audio or S3 publish uploads.
diff --git a/internal/adapters/storage/factory.go b/internal/adapters/storage/factory.go
index e7adc14..42fa2e8 100644
--- a/internal/adapters/storage/factory.go
+++ b/internal/adapters/storage/factory.go
@@ -14,16 +14,15 @@ func NewObjectStoreFromConfig(ctx context.Context, cfg *config.Config) (ObjectSt
 		return nil, fmt.Errorf("pipeline config is required")
 	}
 
-	if strings.EqualFold(strings.TrimSpace(cfg.Pipeline.Storage.Backend), "s3") {
+	switch strings.ToLower(strings.TrimSpace(cfg.Pipeline.Storage.Backend)) {
+	case config.StorageBackendS3:
 		if cfg.Pipeline.Storage.S3 == nil {
 			return nil, fmt.Errorf("pipeline.storage.s3 is required when pipeline.storage.backend is s3")
 		}
 		return NewS3BackendFromConfig(ctx, *cfg.Pipeline.Storage.S3)
+	case "", config.StorageBackendLocal:
+		return nil, fmt.Errorf("no remote object store backend is configured")
+	default:
+		return nil, fmt.Errorf("unsupported pipeline.storage.backend %q", cfg.Pipeline.Storage.Backend)
 	}
-
-	if cfg.Pipeline.Storage.S3 != nil && strings.TrimSpace(cfg.Pipeline.Storage.S3.Bucket) != "" {
-		return NewS3BackendFromConfig(ctx, *cfg.Pipeline.Storage.S3)
-	}
-
-	return nil, fmt.Errorf("no remote object store backend is configured")
 }
diff --git a/internal/adapters/storage/factory_test.go b/internal/adapters/storage/factory_test.go
index 7c03cac..cc056f0 100644
--- a/internal/adapters/storage/factory_test.go
+++ b/internal/adapters/storage/factory_test.go
@@ -54,3 +54,35 @@ func TestNewObjectStoreFromConfigNoRemoteBackendConfigured(t *testing.T) {
 		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
 	}
 }
+
+func TestNewObjectStoreFromConfigDoesNotInferS3FromProviderFields(t *testing.T) {
+	called := false
+	original := newS3Client
+	t.Cleanup(func() { newS3Client = original })
+	newS3Client = func(_ context.Context, _ s3ClientOptions) (s3API, error) {
+		called = true
+		return &fakeS3API{}, nil
+	}
+
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{
+			Backend: config.StorageBackendLocal,
+			S3:      &config.StorageS3Config{Bucket: "my-archive"},
+		}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "no remote object store backend is configured") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
+	}
+	if called {
+		t.Fatal("NewObjectStoreFromConfig() constructed S3 from incidental provider fields")
+	}
+}
+
+func TestNewObjectStoreFromConfigRejectsUnknownBackend(t *testing.T) {
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{Backend: "s33"}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "unsupported pipeline.storage.backend") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want unsupported-backend error", err)
+	}
+}
diff --git a/internal/app/config_loader.go b/internal/app/config_loader.go
index b187b8d..ab516cb 100644
--- a/internal/app/config_loader.go
+++ b/internal/app/config_loader.go
@@ -41,7 +41,11 @@ func loadCommandConfig(ctx context.Context, pipelineFlag, campaignFlag, campaign
 		return nil, missingSessionConfigError(discoveredSession.Searched, "remote session loading requires a session_id")
 	}
 
-	sessionPrefix := artifacts.S3SessionPrefix(base.Pipeline.Storage.S3.RootPrefix, config.CampaignID(base.Campaign), sessionID)
+	rootPrefix := ""
+	if base.Pipeline.Storage.S3 != nil {
+		rootPrefix = base.Pipeline.Storage.S3.RootPrefix
+	}
+	sessionPrefix := artifacts.S3SessionPrefix(rootPrefix, config.CampaignID(base.Campaign), sessionID)
 	remoteKey := artifacts.S3SessionConfigKey(sessionPrefix)
 	partialCfg := &config.Config{
 		Pipeline:     base.Pipeline,
diff --git a/internal/app/object_store_test.go b/internal/app/object_store_test.go
index 5a845d9..baed396 100644
--- a/internal/app/object_store_test.go
+++ b/internal/app/object_store_test.go
@@ -142,24 +142,3 @@ func commandObjectStoreTestConfig(secretsDir string) *config.Config {
 	}
 	return cfg
 }
-
-func restoreEnvAfterTest(t *testing.T, names ...string) {
-	t.Helper()
-	originals := make(map[string]string, len(names))
-	present := make(map[string]bool, len(names))
-	for _, name := range names {
-		value, ok := os.LookupEnv(name)
-		originals[name] = value
-		present[name] = ok
-		_ = os.Unsetenv(name)
-	}
-	t.Cleanup(func() {
-		for _, name := range names {
-			if present[name] {
-				_ = os.Setenv(name, originals[name])
-			} else {
-				_ = os.Unsetenv(name)
-			}
-		}
-	})
-}
diff --git a/internal/app/operator_session_init_test.go b/internal/app/operator_session_init_test.go
new file mode 100644
index 0000000..645ab59
--- /dev/null
+++ b/internal/app/operator_session_init_test.go
@@ -0,0 +1,52 @@
+package app
+
+import (
+	"bytes"
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestSessionInitRejectsRenderedTemplateThatOmitsExpectedPreviousSession(t *testing.T) {
+	workspaceRoot := t.TempDir()
+	pipelinePath, campaignPath, _ := writeValidConfigFiles(t, workspaceRoot)
+	campaignDir := filepath.Dir(campaignPath)
+	templatePath := filepath.Join(campaignDir, "session.template.yml")
+	template := `session_id: {{ session_id }}
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+`
+	if err := os.WriteFile(templatePath, []byte(template), 0o644); err != nil {
+		t.Fatalf("write session template: %v", err)
+	}
+	campaign := `campaign_id: sample-campaign
+session_template_file: session.template.yml
+inputs:
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+  players_file: ./players.yml
+  party_file: ./party.yml
+`
+	if err := os.WriteFile(campaignPath, []byte(campaign), 0o644); err != nil {
+		t.Fatalf("write campaign config: %v", err)
+	}
+
+	var out bytes.Buffer
+	err := SessionInit(context.Background(), []string{
+		"2026-05-03",
+		"--config", pipelinePath,
+		"--campaign-file", campaignPath,
+		"--output", filepath.Join(t.TempDir(), "session.yml"),
+		"--previous-session-id", "2026-04-26",
+	}, &out)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "unused template variable value(s): previous_session_id") {
+		t.Fatalf("SessionInit() error = %q, want missing previous-session template error", err.Error())
+	}
+}
diff --git a/internal/app/secrets_env_test.go b/internal/app/secrets_env_test.go
index d6f0d96..6dfb36a 100644
--- a/internal/app/secrets_env_test.go
+++ b/internal/app/secrets_env_test.go
@@ -12,8 +12,7 @@ import (
 
 func TestLoadSecretsFromConfigLoadsValidFiles(t *testing.T) {
 	dir := t.TempDir()
-	unsetSecretEnvironment(t, "NARRATIO_TEST_SECRET_A")
-	unsetSecretEnvironment(t, "NARRATIO_TEST_SECRET_B")
+	restoreEnvAfterTest(t, "NARRATIO_TEST_SECRET_A", "NARRATIO_TEST_SECRET_B")
 	mustWriteSecretFile(t, filepath.Join(dir, "NARRATIO_TEST_SECRET_A"), "value-1\n")
 	mustWriteSecretFile(t, filepath.Join(dir, "NARRATIO_TEST_SECRET_B"), "value-2\r\n")
 	mustWriteSecretFile(t, filepath.Join(dir, "not-valid-name.txt"), "ignored")
@@ -162,7 +161,7 @@ func TestLoadSecretsFromConfigRejectsUnsafeModes(t *testing.T) {
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			dir := t.TempDir()
-			unsetSecretEnvironment(t, "OPENROUTER_API_KEY")
+			restoreEnvAfterTest(t, "OPENROUTER_API_KEY")
 			path := filepath.Join(dir, "OPENROUTER_API_KEY")
 			mustWriteSecretFile(t, path, tc.wantSecret)
 			if err := os.Chmod(dir, tc.directory); err != nil {
@@ -186,7 +185,7 @@ func TestLoadSecretsFromConfigRejectsUnsafeModes(t *testing.T) {
 func TestLoadSecretsFromConfigRejectsNonRegularAndOversizedEntries(t *testing.T) {
 	dir := t.TempDir()
 	ensureSecretDirectory(t, dir)
-	unsetSecretEnvironment(t, "OPENROUTER_API_KEY")
+	restoreEnvAfterTest(t, "OPENROUTER_API_KEY")
 	if err := os.Mkdir(filepath.Join(dir, "OPENROUTER_API_KEY"), secretDirectoryPrivateMode); err != nil {
 		t.Fatalf("Mkdir(non-regular entry) error = %v", err)
 	}
@@ -215,7 +214,7 @@ func TestLoadSecretsFromConfigRejectsSymlinkAndAncestorReplacement(t *testing.T)
 	dir := t.TempDir()
 	outside := t.TempDir()
 	ensureSecretDirectory(t, dir)
-	unsetSecretEnvironment(t, "OPENROUTER_API_KEY")
+	restoreEnvAfterTest(t, "OPENROUTER_API_KEY")
 	outsideValue := "outside-secret-value"
 	mustWriteSecretFile(t, filepath.Join(outside, "OPENROUTER_API_KEY"), outsideValue)
 	if err := os.Symlink(filepath.Join(outside, "OPENROUTER_API_KEY"), filepath.Join(dir, "OPENROUTER_API_KEY")); err != nil {
@@ -277,21 +276,6 @@ func secretConfig(dir string) *config.Config {
 	return &config.Config{Pipeline: &config.PipelineConfig{Secrets: &config.SecretsConfig{EnvDir: dir}}}
 }
 
-func unsetSecretEnvironment(t *testing.T, name string) {
-	t.Helper()
-	previous, existed := os.LookupEnv(name)
-	if err := os.Unsetenv(name); err != nil {
-		t.Fatalf("Unsetenv(%q): %v", name, err)
-	}
-	t.Cleanup(func() {
-		if existed {
-			_ = os.Setenv(name, previous)
-			return
-		}
-		_ = os.Unsetenv(name)
-	})
-}
-
 func ensureSecretDirectory(t *testing.T, directory string) {
 	t.Helper()
 	if err := os.Chmod(directory, secretDirectoryPrivateMode); err != nil {
diff --git a/internal/app/session_cli_test.go b/internal/app/session_cli_test.go
index 7a9db7f..6f5b06f 100644
--- a/internal/app/session_cli_test.go
+++ b/internal/app/session_cli_test.go
@@ -100,6 +100,26 @@ inputs:
 	}
 }
 
+func TestPlanRequiresExpectedPreviousSessionID(t *testing.T) {
+	workspaceRoot := t.TempDir()
+	pipelinePath, campaignPath, sessionPath := writeValidConfigFiles(t, workspaceRoot)
+
+	var out bytes.Buffer
+	err := Plan(context.Background(), []string{
+		"2026-05-03",
+		"--config", pipelinePath,
+		"--campaign-file", campaignPath,
+		"--session", sessionPath,
+		"--previous-session-id", "2026-04-26",
+	}, &out)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("error = %q, want missing previous-session expectation", err.Error())
+	}
+}
+
 func TestRunStageAcceptsPositionalSessionIDAndParsesStageName(t *testing.T) {
 	workspaceRoot := t.TempDir()
 	pipelinePath, campaignPath, sessionPath := writeValidConfigFiles(t, workspaceRoot)
diff --git a/internal/app/test_environment_test.go b/internal/app/test_environment_test.go
new file mode 100644
index 0000000..28094fc
--- /dev/null
+++ b/internal/app/test_environment_test.go
@@ -0,0 +1,31 @@
+package app
+
+import (
+	"os"
+	"testing"
+)
+
+// restoreEnvAfterTest clears names for a test and restores both their prior
+// values and their set/unset state during cleanup.
+func restoreEnvAfterTest(t *testing.T, names ...string) {
+	t.Helper()
+	originals := make(map[string]string, len(names))
+	present := make(map[string]bool, len(names))
+	for _, name := range names {
+		value, ok := os.LookupEnv(name)
+		originals[name] = value
+		present[name] = ok
+		if err := os.Unsetenv(name); err != nil {
+			t.Fatalf("Unsetenv(%q): %v", name, err)
+		}
+	}
+	t.Cleanup(func() {
+		for _, name := range names {
+			if present[name] {
+				_ = os.Setenv(name, originals[name])
+			} else {
+				_ = os.Unsetenv(name)
+			}
+		}
+	})
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index d277a6a..8005166 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -83,6 +83,13 @@ type StorageConfig struct {
 	S3      *StorageS3Config `yaml:"s3"`
 }
 
+const (
+	// StorageBackendLocal disables remote object-store operations.
+	StorageBackendLocal = "local"
+	// StorageBackendS3 enables the configured S3 object store.
+	StorageBackendS3 = "s3"
+)
+
 // StorageS3Config configures S3 storage coordinates.
 type StorageS3Config struct {
 	Bucket         string `yaml:"bucket"`
diff --git a/internal/config/load.go b/internal/config/load.go
index 3c11011..b03c349 100644
--- a/internal/config/load.go
+++ b/internal/config/load.go
@@ -74,15 +74,23 @@ func LoadSessionBytesWithOptions(label string, data []byte, opts SessionLoadOpti
 			strings.TrimSpace(cfg.SessionID),
 		)
 	}
-	if strings.TrimSpace(opts.PreviousSessionID) != "" &&
-		strings.TrimSpace(cfg.PreviousSessionID) != "" &&
-		strings.TrimSpace(cfg.PreviousSessionID) != strings.TrimSpace(opts.PreviousSessionID) {
-		return nil, fmt.Errorf(
-			"load session config: session file %q: previous_session_id mismatch: --previous-session-id %q does not match previous_session_id %q",
-			label,
-			strings.TrimSpace(opts.PreviousSessionID),
-			strings.TrimSpace(cfg.PreviousSessionID),
-		)
+	if expectedPreviousSessionID := strings.TrimSpace(opts.PreviousSessionID); expectedPreviousSessionID != "" {
+		actualPreviousSessionID := strings.TrimSpace(cfg.PreviousSessionID)
+		if actualPreviousSessionID == "" {
+			return nil, fmt.Errorf(
+				"load session config: session file %q: previous_session_id is required when --previous-session-id %q is provided",
+				label,
+				expectedPreviousSessionID,
+			)
+		}
+		if actualPreviousSessionID != expectedPreviousSessionID {
+			return nil, fmt.Errorf(
+				"load session config: session file %q: previous_session_id mismatch: --previous-session-id %q does not match previous_session_id %q",
+				label,
+				expectedPreviousSessionID,
+				actualPreviousSessionID,
+			)
+		}
 	}
 	return &cfg, nil
 }
@@ -296,8 +304,10 @@ func decodeStrictYAMLFromReader(kind, path string, r io.Reader, out any) error {
 		return fmt.Errorf("%s file %q: strict decode failed: %w", kind, path, err)
 	}
 
-	var extra any
-	if err := dec.Decode(&extra); err != nil && err != io.EOF {
+	var extra yaml.Node
+	if err := dec.Decode(&extra); err == nil {
+		return fmt.Errorf("%s file %q: must contain exactly one YAML document", kind, path)
+	} else if err != io.EOF {
 		return fmt.Errorf("%s file %q: trailing content decode failed: %w", kind, path, err)
 	}
 
@@ -392,8 +402,13 @@ func applyStorageDefaults(cfg *StorageConfig) {
 	if cfg == nil {
 		return
 	}
+	backend := strings.ToLower(strings.TrimSpace(cfg.Backend))
+	if backend == "" {
+		backend = StorageBackendLocal
+	}
+	cfg.Backend = backend
 	if cfg.S3 == nil {
-		cfg.S3 = &StorageS3Config{}
+		return
 	}
 	if cfg.S3.RootPrefix == "" {
 		cfg.S3.RootPrefix = DefaultStorageS3RootPrefix
diff --git a/internal/config/load_validate_test.go b/internal/config/load_validate_test.go
index 0190783..a4933bb 100644
--- a/internal/config/load_validate_test.go
+++ b/internal/config/load_validate_test.go
@@ -8,1013 +8,37 @@ import (
 	"testing"
 )
 
-func TestLoadAndValidate(t *testing.T) {
-	tests := []struct {
-		name         string
-		pipelineYAML string
-		sessionYAML  string
-		wantLoadErr  string
-		wantValidate string
-		checkDefault bool
-		wantRoot     string
-	}{
-		{
-			name: "valid minimal config",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-whisperx:
-  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
-seriatim:
-  binary: seriatim
-audita:
-  binary: audita
-notification:
-  timeout: 15s
-`,
-			sessionYAML: `session_id: 2026-05-03
+func TestRepresentativeLoadDefaultsAndValidation(t *testing.T) {
+	pipelinePath, sessionPath := writeConfigFiles(t, `whisperx:
+  transcribe_url: https://transcription.example.com/transcribe
+`, `session_id: 2026-05-03
 inputs:
   audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			checkDefault: true,
-			wantRoot:     "/tmp/narratio",
-		},
-		{
-			name: "seriatim and audita sections can be omitted",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-whisperx:
-  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
-notification:
-  timeout: 15s
-`,
-			sessionYAML: `session_id: 2026-05-03
-inputs:
-  audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			checkDefault: true,
-			wantRoot:     "/tmp/narratio",
-		},
-		{
-			name: "workspace root defaults when omitted",
-			pipelineYAML: `whisperx:
-  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
-notification:
-  timeout: 15s
-`,
-			sessionYAML: `session_id: 2026-05-03
-inputs:
-  audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			checkDefault: true,
-			wantRoot:     DefaultWorkspaceRoot,
-		},
-		{
-			name: "unknown pipeline field fails",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-bogus: true
-`,
-			sessionYAML: `session_id: 2026-05-03
-inputs:
-  audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			wantLoadErr: "pipeline file",
-		},
-		{
-			name: "legacy analyzer section fails strict decode",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-whisperx:
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-			pipelineYAML: `workspace:
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-			pipelineYAML: `workspace:
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-  party_file: ./party.yml
-`,
-			wantValidate: "pipeline config \"pipeline.yml\" invalid: pipeline.audita.validation_llm_concurrency must be > 0",
-		},
-		{
-			name: "invalid audita output_schema fails",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-whisperx:
-  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
-seriatim:
-  binary: seriatim
-audita:
-  binary: audita
-  output_schema: bad
-`,
-			sessionYAML: `session_id: 2026-05-03
-inputs:
-  audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			wantValidate: "pipeline config \"pipeline.yml\" invalid: pipeline.audita.output_schema must be one of: bare-segments, audita-v1",
-		},
-		{
-			name: "invalid audita work_dir_retention fails",
-			pipelineYAML: `workspace:
-  root: /tmp/narratio
-whisperx:
-  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
-seriatim:
-  binary: seriatim
-audita:
-  binary: audita
-  work_dir_retention: sometimes
-`,
-			sessionYAML: `session_id: 2026-05-03
-inputs:
-  audio_dir: ./audio
-  speakers_file: ./speakers.yml
-  autocorrect_file: ./autocorrect.yml
-  glossary_file: ./glossary.yml
-  players_file: ./players.yml
-  party_file: ./party.yml
-`,
-			wantValidate: "pipeline config \"pipeline.yml\" invalid: pipeline.audita.work_dir_retention must be one of: always, auto, never",
-		},
+`)
+
+	cfg, err := Load(pipelinePath, sessionPath)
+	if err != nil {
+		t.Fatalf("Load() error = %v", err)
 	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			pipelinePath, sessionPath := writeConfigFiles(t, tt.pipelineYAML, tt.sessionYAML)
-
-			cfg, err := Load(pipelinePath, sessionPath)
-			if tt.wantLoadErr != "" {
-				if err == nil {
-					t.Fatalf("expected load error containing %q, got nil", tt.wantLoadErr)
-				}
-				if !strings.Contains(err.Error(), tt.wantLoadErr) {
-					t.Fatalf("load error = %q, want to contain %q", err.Error(), tt.wantLoadErr)
-				}
-				if !strings.Contains(err.Error(), "strict decode failed") {
-					t.Fatalf("load error = %q, want strict decode context", err.Error())
-				}
-				return
-			}
-
-			if err != nil {
-				t.Fatalf("Load() error = %v", err)
-			}
-			if cfg.PipelinePath != pipelinePath {
-				t.Fatalf("PipelinePath = %q, want %q", cfg.PipelinePath, pipelinePath)
-			}
-			if cfg.SessionPath != sessionPath {
-				t.Fatalf("SessionPath = %q, want %q", cfg.SessionPath, sessionPath)
-			}
-			if tt.checkDefault {
-				if tt.wantRoot != "" && cfg.Pipeline.Workspace.Root != tt.wantRoot {
-					t.Fatalf("workspace.root = %q, want %q", cfg.Pipeline.Workspace.Root, tt.wantRoot)
-				}
-				if cfg.Pipeline.WhisperX.Language != "en" {
-					t.Fatalf("whisperx.language = %q, want %q", cfg.Pipeline.WhisperX.Language, "en")
-				}
-				if cfg.Pipeline.WhisperX.Timeout != "30m" {
-					t.Fatalf("whisperx.timeout = %q, want %q", cfg.Pipeline.WhisperX.Timeout, "30m")
-				}
-				if cfg.Pipeline.WhisperX.RetryDelay != "2s" {
-					t.Fatalf("whisperx.retry_delay = %q, want %q", cfg.Pipeline.WhisperX.RetryDelay, "2s")
-				}
-				if cfg.Pipeline.WhisperX.Retries == nil || *cfg.Pipeline.WhisperX.Retries != 3 {
-					t.Fatalf("whisperx.retries = %v, want 3", cfg.Pipeline.WhisperX.Retries)
-				}
-				if cfg.Pipeline.WhisperX.Concurrency == nil || *cfg.Pipeline.WhisperX.Concurrency != 2 {
-					t.Fatalf("whisperx.concurrency = %v, want 2", cfg.Pipeline.WhisperX.Concurrency)
-				}
-				if cfg.Pipeline.Seriatim.Timeout != "10m" {
-					t.Fatalf("seriatim.timeout = %q, want %q", cfg.Pipeline.Seriatim.Timeout, "10m")
-				}
-				if cfg.Pipeline.Seriatim.Binary != "seriatim" {
-					t.Fatalf("seriatim.binary = %q, want %q", cfg.Pipeline.Seriatim.Binary, "seriatim")
-				}
-				if cfg.Pipeline.Seriatim.OutputSchema != "seriatim-intermediate" {
-					t.Fatalf("seriatim.output_schema = %q, want %q", cfg.Pipeline.Seriatim.OutputSchema, "seriatim-intermediate")
-				}
-				if cfg.Pipeline.Seriatim.CoalesceGap == nil || *cfg.Pipeline.Seriatim.CoalesceGap != 3.0 {
-					t.Fatalf("seriatim.coalesce_gap = %v, want 3.0", cfg.Pipeline.Seriatim.CoalesceGap)
-				}
-				if cfg.Pipeline.Seriatim.Report == nil || *cfg.Pipeline.Seriatim.Report != true {
-					t.Fatalf("seriatim.report = %v, want true", cfg.Pipeline.Seriatim.Report)
-				}
-				if cfg.Pipeline.Audita.Timeout != "3h" {
-					t.Fatalf("audita.timeout = %q, want %q", cfg.Pipeline.Audita.Timeout, "3h")
-				}
-				if cfg.Pipeline.Audita.Binary != "audita" {
-					t.Fatalf("audita.binary = %q, want %q", cfg.Pipeline.Audita.Binary, "audita")
-				}
-				if cfg.Pipeline.Audita.LLMAPIKeyEnv != "" {
-					t.Fatalf("audita.llm_api_key_env = %q, want empty by default", cfg.Pipeline.Audita.LLMAPIKeyEnv)
-				}
-				if cfg.Pipeline.Audita.Modules != nil {
-					t.Fatalf("audita.modules = %#v, want nil default (optional override)", cfg.Pipeline.Audita.Modules)
-				}
-				if cfg.Pipeline.Audita.BaseURL != "" {
-					t.Fatalf("audita.base_url = %q, want empty default", cfg.Pipeline.Audita.BaseURL)
-				}
-				if cfg.Pipeline.Audita.Model != "" {
-					t.Fatalf("audita.model = %q, want empty default", cfg.Pipeline.Audita.Model)
-				}
-				if cfg.Pipeline.Audita.ValidationModel != "" {
-					t.Fatalf("audita.validation_model = %q, want empty default", cfg.Pipeline.Audita.ValidationModel)
-				}
-				if cfg.Pipeline.Audita.TotalLLMConcurrency != nil {
-					t.Fatalf("audita.total_llm_concurrency = %v, want nil default", cfg.Pipeline.Audita.TotalLLMConcurrency)
-				}
-				if cfg.Pipeline.Audita.ProposalLLMConcurrency != nil {
-					t.Fatalf("audita.proposal_llm_concurrency = %v, want nil default", cfg.Pipeline.Audita.ProposalLLMConcurrency)
-				}
-				if cfg.Pipeline.Audita.ValidationLLMConcurrency != nil {
-					t.Fatalf("audita.validation_llm_concurrency = %v, want nil default", cfg.Pipeline.Audita.ValidationLLMConcurrency)
-				}
-				if cfg.Pipeline.Audita.Report == nil || *cfg.Pipeline.Audita.Report != true {
-					t.Fatalf("audita.report = %v, want true", cfg.Pipeline.Audita.Report)
-				}
-				if cfg.Pipeline.Normalize == nil {
-					t.Fatal("normalize config should be present via defaults")
-				}
-				if cfg.Pipeline.Normalize.OutputPath != "transcripts/final.json" {
-					t.Fatalf("normalize.output_path = %q, want %q", cfg.Pipeline.Normalize.OutputPath, "transcripts/final.json")
-				}
-				if cfg.Pipeline.Normalize.OutputSchema != "seriatim-intermediate" {
-					t.Fatalf("normalize.output_schema = %q, want %q", cfg.Pipeline.Normalize.OutputSchema, "seriatim-intermediate")
-				}
-				if cfg.Pipeline.Normalize.Report == nil || *cfg.Pipeline.Normalize.Report != true {
-					t.Fatalf("normalize.report = %v, want true", cfg.Pipeline.Normalize.Report)
-				}
-			}
-
-			err = Validate(cfg)
-			if tt.wantValidate != "" {
-				if err == nil {
-					t.Fatalf("expected validation error containing %q, got nil", tt.wantValidate)
-				}
-				if !strings.Contains(err.Error(), tt.wantValidate) {
-					t.Fatalf("validation error = %q, want to contain %q", err.Error(), tt.wantValidate)
-				}
-				return
-			}
-			if err != nil {
-				t.Fatalf("Validate() error = %v", err)
-			}
-		})
+	if cfg.Pipeline.Workspace.Root != DefaultWorkspaceRoot {
+		t.Fatalf("workspace.root = %q, want %q", cfg.Pipeline.Workspace.Root, DefaultWorkspaceRoot)
+	}
+	if cfg.Pipeline.Storage.Backend != StorageBackendLocal || cfg.Pipeline.Storage.S3 != nil {
+		t.Fatalf("storage = %#v, want local backend without S3 configuration", cfg.Pipeline.Storage)
+	}
+	if cfg.Pipeline.WhisperX.Timeout != DefaultWhisperXTimeout || cfg.Pipeline.WhisperX.RetryDelay != DefaultWhisperXRetryDelay {
+		t.Fatalf("whisperx defaults = %#v, want timeout and retry-delay defaults", cfg.Pipeline.WhisperX)
+	}
+	if err := Validate(cfg); err != nil {
+		t.Fatalf("Validate() error = %v", err)
 	}
 }
 
 func TestValidateMissingAudioSource(t *testing.T) {
-	retries := 3
-	concurrency := 2
-	cfg := &Config{
-		Pipeline: &PipelineConfig{
-			Workspace: WorkspaceConfig{Root: "/tmp/narratio"},
-			WhisperX: WhisperXConfig{
-				TranscribeURL: "https://transcription.ai.rakestrawhome.com/transcribe",
-				Language:      "en",
-				Timeout:       "30m",
-				Retries:       &retries,
-				RetryDelay:    "2s",
-				Concurrency:   &concurrency,
-			},
-			Seriatim: SeriatimConfig{
-				Binary:       "seriatim",
-				Timeout:      "10m",
-				OutputSchema: "seriatim-intermediate",
-				CoalesceGap:  float64Ptr(3.0),
-				Report:       boolPtr(true),
-			},
-			Audita: AuditaConfig{
-				Binary:                   "audita",
-				Timeout:                  "3h",
-				LLMAPIKeyEnv:             "AUDITA_LLM_API_KEY",
-				Modules:                  []string{"glossary", "homophones"},
-				BaseURL:                  "https://openrouter.ai/api/v1",
-				Model:                    "openrouter/google/gemma-4-31b-it",
-				TotalLLMConcurrency:      intPtr(1),
-				ProposalLLMConcurrency:   intPtr(1),
-				ValidationModel:          "",
-				ValidationLLMConcurrency: intPtr(1),
-				Report:                   boolPtr(true),
-			},
-		},
-		Campaign: &CampaignConfig{
-			CampaignID: "sample-campaign",
-			Inputs: CampaignInputsConfig{
-				SpeakersFile:    "speakers.yml",
-				AutocorrectFile: "autocorrect.yml",
-				GlossaryFile:    "glossary.yml",
-				PlayersFile:     "players.yml",
-				PartyFile:       "party.yml",
-			},
-		},
-		Session: &SessionConfig{
-			SessionID: "2026-05-03",
-			Campaign:  "sample-campaign",
-			Inputs: SessionInputsConfig{
-				SpeakersFile:    "speakers.yml",
-				AutocorrectFile: "autocorrect.yml",
-				GlossaryFile:    "glossary.yml",
-				PlayersFile:     "players.yml",
-				PartyFile:       "party.yml",
-			},
-		},
-	}
+	cfg := loadedValidConfig(t)
+	cfg.Session.Inputs.AudioDir = ""
+	cfg.Session.Inputs.AudioFiles = nil
+	cfg.Session.Inputs.AudioS3 = nil
 
 	err := Validate(cfg)
 	if err == nil {
diff --git a/internal/config/loader_strict_test.go b/internal/config/loader_strict_test.go
new file mode 100644
index 0000000..3557e9d
--- /dev/null
+++ b/internal/config/loader_strict_test.go
@@ -0,0 +1,91 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestLoadersRejectTrailingYAMLDocuments(t *testing.T) {
+	dir := t.TempDir()
+	pipelinePath := filepath.Join(dir, "pipeline.yml")
+	campaignPath := filepath.Join(dir, "campaign.yml")
+	if err := os.WriteFile(pipelinePath, []byte("workspace:\n  root: /tmp/narratio\n---\nworkspace:\n  root: /other\n"), 0o644); err != nil {
+		t.Fatalf("write pipeline.yml: %v", err)
+	}
+	if err := os.WriteFile(campaignPath, []byte("campaign_id: sample-campaign\n---\nnull\n"), 0o644); err != nil {
+		t.Fatalf("write campaign.yml: %v", err)
+	}
+
+	tests := []struct {
+		name string
+		load func() error
+	}{
+		{
+			name: "pipeline",
+			load: func() error {
+				_, err := LoadPipeline(pipelinePath)
+				return err
+			},
+		},
+		{
+			name: "campaign",
+			load: func() error {
+				_, err := LoadCampaign(campaignPath)
+				return err
+			},
+		},
+		{
+			name: "remote session bytes",
+			load: func() error {
+				_, err := LoadSessionBytesWithOptions("s3://bucket/session.yml", []byte("session_id: 2026-05-03\n---\n# another document\nnull\n"), SessionLoadOptions{})
+				return err
+			},
+		},
+		{
+			name: "publish lock store",
+			load: func() error {
+				_, err := LoadPublishLockStoreBytes("s3://bucket/locks.yml", []byte("locks: []\n---\n{}\n"), nil, nil)
+				return err
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.load()
+			if err == nil || !strings.Contains(err.Error(), "exactly one YAML document") {
+				t.Fatalf("load error = %v, want exactly-one-document error", err)
+			}
+		})
+	}
+}
+
+func TestStrictYAMLRejectsParserExposedEmptyTrailingDocument(t *testing.T) {
+	var target struct {
+		Name string `yaml:"name"`
+	}
+	err := decodeStrictYAMLFromReader("test", "memory", strings.NewReader("name: one\n---\n"), &target)
+	if err == nil {
+		t.Fatal("decodeStrictYAMLFromReader() error = nil, want trailing-document error")
+	}
+	if !strings.Contains(err.Error(), "exactly one YAML document") {
+		t.Fatalf("decodeStrictYAMLFromReader() error = %v, want exactly-one-document error", err)
+	}
+}
+
+func TestStrictYAMLAcceptsSingleDocumentAndRejectsUnknownFields(t *testing.T) {
+	var target struct {
+		Name string `yaml:"name"`
+	}
+	if err := decodeStrictYAMLFromReader("test", "memory", strings.NewReader("name: one\n"), &target); err != nil {
+		t.Fatalf("decodeStrictYAMLFromReader(single document) error = %v", err)
+	}
+	if target.Name != "one" {
+		t.Fatalf("Name = %q, want one", target.Name)
+	}
+	if err := decodeStrictYAMLFromReader("test", "memory", strings.NewReader("unknown: one\n"), &target); err == nil || !strings.Contains(err.Error(), "strict decode failed") {
+		t.Fatalf("decodeStrictYAMLFromReader(unknown field) error = %v, want strict decode error", err)
+	}
+}
diff --git a/internal/config/session_template_test.go b/internal/config/session_template_test.go
index 9aaf434..7bf6bf3 100644
--- a/internal/config/session_template_test.go
+++ b/internal/config/session_template_test.go
@@ -121,6 +121,24 @@ inputs:
 	}
 }
 
+func TestLoadSessionWithOptionsRequiresExpectedPreviousSession(t *testing.T) {
+	dir := t.TempDir()
+	sessionPath := filepath.Join(dir, "session.yml")
+	sessionYAML := `session_id: 2026-05-03
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	_, err := LoadSessionWithOptions(sessionPath, SessionLoadOptions{PreviousSessionID: "2026-04-26"})
+	if err == nil || !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("LoadSessionWithOptions() error = %v, want missing previous-session expectation error", err)
+	}
+}
+
 func TestLoadSessionWithOptionsUnknownFieldStillRejected(t *testing.T) {
 	dir := t.TempDir()
 	sessionPath := filepath.Join(dir, "session.yml")
@@ -217,6 +235,17 @@ func TestLoadSessionBytesWithOptionsMismatchFails(t *testing.T) {
 	}
 }
 
+func TestLoadSessionBytesWithOptionsRequiresExpectedPreviousSession(t *testing.T) {
+	_, err := LoadSessionBytesWithOptions(
+		"s3://bucket/session.yml",
+		[]byte("session_id: 2026-05-03\n"),
+		SessionLoadOptions{PreviousSessionID: "2026-04-26"},
+	)
+	if err == nil || !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("LoadSessionBytesWithOptions() error = %v, want missing previous-session expectation error", err)
+	}
+}
+
 func assertConcreteSessionTemplateError(t *testing.T, err error, vars ...string) {
 	t.Helper()
 	if !strings.Contains(err.Error(), "session.yml must be concrete") {
diff --git a/internal/config/storage_publish_test.go b/internal/config/storage_publish_test.go
index 4e2d3cc..4e0aef7 100644
--- a/internal/config/storage_publish_test.go
+++ b/internal/config/storage_publish_test.go
@@ -153,6 +153,108 @@ storage:
 	}
 }
 
+func TestStorageBackendSelectionValidation(t *testing.T) {
+	tests := []struct {
+		name        string
+		storageYAML string
+		sessionYAML string
+		wantErr     string
+		wantBackend string
+	}{
+		{
+			name:        "omitted storage defaults local",
+			wantBackend: StorageBackendLocal,
+		},
+		{
+			name: "explicit local backend",
+			storageYAML: `storage:
+  backend: local
+`,
+			wantBackend: StorageBackendLocal,
+		},
+		{
+			name: "case insensitive s3 backend normalizes",
+			storageYAML: `storage:
+  backend: S3
+  s3:
+    bucket: my-dnd-archive
+`,
+			wantBackend: StorageBackendS3,
+		},
+		{
+			name: "unknown backend",
+			storageYAML: `storage:
+  backend: s33
+`,
+			wantErr: "pipeline.storage.backend must be one of: local, s3",
+		},
+		{
+			name: "s3 block requires s3 backend",
+			storageYAML: `storage:
+  backend: local
+  s3:
+    bucket: my-dnd-archive
+`,
+			wantErr: "pipeline.storage.s3 is only supported when pipeline.storage.backend is s3",
+		},
+		{
+			name: "s3 backend requires s3 block",
+			storageYAML: `storage:
+  backend: s3
+`,
+			wantErr: "pipeline.storage.s3 is required when pipeline.storage.backend is s3",
+		},
+		{
+			name: "s3 backend requires bucket for publish",
+			storageYAML: `storage:
+  backend: s3
+  s3: {}
+`,
+			wantErr: "pipeline.storage.s3.bucket is required when S3 session audio or publish upload is enabled",
+		},
+		{
+			name: "s3 audio requires s3 backend",
+			sessionYAML: `session_id: 2026-05-03
+inputs:
+  audio_s3:
+    prefix: audio/
+`,
+			wantErr: "pipeline.storage.backend must be s3 when session.inputs.audio_s3 is configured",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			pipelineYAML := testPipelineBaseYAML
+			if tt.storageYAML != "" {
+				pipelineYAML += "\n" + tt.storageYAML
+			}
+			sessionYAML := testSessionBaseYAML
+			if tt.sessionYAML != "" {
+				sessionYAML = tt.sessionYAML
+			}
+			pipelinePath, sessionPath := writeConfigFiles(t, pipelineYAML, sessionYAML)
+			cfg, err := Load(pipelinePath, sessionPath)
+			if err != nil {
+				t.Fatalf("Load() error = %v", err)
+			}
+			err = Validate(cfg)
+			if tt.wantErr != "" {
+				if err == nil || !strings.Contains(err.Error(), tt.wantErr) {
+					t.Fatalf("Validate() error = %v, want %q", err, tt.wantErr)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("Validate() error = %v", err)
+			}
+			if cfg.Pipeline.Storage.Backend != tt.wantBackend {
+				t.Fatalf("storage.backend = %q, want %q", cfg.Pipeline.Storage.Backend, tt.wantBackend)
+			}
+		})
+	}
+}
+
 func TestSpoolAndPublishDefaults(t *testing.T) {
 	pipelinePath, sessionPath := writeConfigFiles(t, testPipelineBaseYAML, testSessionBaseYAML)
 
@@ -681,6 +783,7 @@ func TestStorageS3BucketRequiredWhenS3DependentFeatureEnabled(t *testing.T) {
 	pipelineYAML := testPipelineBaseYAML + `
 storage:
   backend: s3
+  s3: {}
 `
 	sessionYAML := `session_id: 2026-05-03
 campaign: forsaken
diff --git a/internal/config/validate.go b/internal/config/validate.go
index 4f0c7d6..1f77341 100644
--- a/internal/config/validate.go
+++ b/internal/config/validate.go
@@ -130,8 +130,19 @@ func validatePipeline(cfg *PipelineConfig) error {
 }
 
 func validateStorage(cfg StorageConfig) error {
-	if cfg.S3 == nil {
+	backend := strings.ToLower(strings.TrimSpace(cfg.Backend))
+	switch backend {
+	case "", StorageBackendLocal:
+		if cfg.S3 != nil {
+			return fmt.Errorf("pipeline.storage.s3 is only supported when pipeline.storage.backend is s3")
+		}
 		return nil
+	case StorageBackendS3:
+		if cfg.S3 == nil {
+			return fmt.Errorf("pipeline.storage.s3 is required when pipeline.storage.backend is s3")
+		}
+	default:
+		return fmt.Errorf("pipeline.storage.backend must be one of: local, s3")
 	}
 	if strings.TrimSpace(cfg.S3.RootPrefix) == "" {
 		return fmt.Errorf("pipeline.storage.s3.root_prefix must be non-empty")
@@ -783,13 +794,16 @@ func validateCrossConfig(pipeline *PipelineConfig, session *SessionConfig) error
 	if pipeline == nil || session == nil {
 		return nil
 	}
-	if pipeline.Storage.S3 == nil {
-		return nil
-	}
 
 	audioS3Enabled := session.Inputs.AudioS3 != nil
 	publishUploadEnabled := publishUploadConfiguredForS3(pipeline)
-	if (audioS3Enabled || publishUploadEnabled) && strings.TrimSpace(pipeline.Storage.S3.Bucket) == "" {
+	if audioS3Enabled && !strings.EqualFold(strings.TrimSpace(pipeline.Storage.Backend), StorageBackendS3) {
+		return fmt.Errorf("pipeline.storage.backend must be s3 when session.inputs.audio_s3 is configured")
+	}
+	if !audioS3Enabled && !publishUploadEnabled {
+		return nil
+	}
+	if pipeline.Storage.S3 == nil || strings.TrimSpace(pipeline.Storage.S3.Bucket) == "" {
 		return fmt.Errorf("pipeline.storage.s3.bucket is required when S3 session audio or publish upload is enabled")
 	}
 	return nil
@@ -799,7 +813,7 @@ func publishUploadConfiguredForS3(pipeline *PipelineConfig) bool {
 	if pipeline == nil || pipeline.Publish == nil {
 		return false
 	}
-	if !strings.EqualFold(strings.TrimSpace(pipeline.Storage.Backend), "s3") {
+	if !strings.EqualFold(strings.TrimSpace(pipeline.Storage.Backend), StorageBackendS3) {
 		return false
 	}
 	enabled := true
@@ -975,9 +989,13 @@ func validateDuration(fieldName, value string) error {
 		return nil
 	}
 
-	if _, err := time.ParseDuration(trimmed); err != nil {
+	duration, err := time.ParseDuration(trimmed)
+	if err != nil {
 		return fmt.Errorf("%s must be a valid duration: %w", fieldName, err)
 	}
+	if duration <= 0 {
+		return fmt.Errorf("%s must be positive", fieldName)
+	}
 
 	return nil
 }
diff --git a/internal/config/validation_duration_test.go b/internal/config/validation_duration_test.go
new file mode 100644
index 0000000..a45a1b2
--- /dev/null
+++ b/internal/config/validation_duration_test.go
@@ -0,0 +1,114 @@
+package config
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestValidateDurationsRequirePositiveValues(t *testing.T) {
+	tests := []struct {
+		name string
+		set  func(*PipelineConfig, string)
+		want string
+	}{
+		{name: "whisperx timeout", set: func(p *PipelineConfig, value string) { p.WhisperX.Timeout = value }, want: "pipeline.whisperx.timeout"},
+		{name: "whisperx retry delay", set: func(p *PipelineConfig, value string) { p.WhisperX.RetryDelay = value }, want: "pipeline.whisperx.retry_delay"},
+		{name: "seriatim timeout", set: func(p *PipelineConfig, value string) { p.Seriatim.Timeout = value }, want: "pipeline.seriatim.timeout"},
+		{name: "audita timeout", set: func(p *PipelineConfig, value string) { p.Audita.Timeout = value }, want: "pipeline.audita.timeout"},
+		{name: "scriptorium timeout", set: func(p *PipelineConfig, value string) { p.Scriptorium.Timeout = value }, want: "pipeline.scriptorium.timeout"},
+		{name: "scriptorium artifact timeout", set: func(p *PipelineConfig, value string) {
+			if p.Scriptorium.Artifacts == nil {
+				p.Scriptorium.Artifacts = map[string]ScriptoriumArtifactConfig{}
+			}
+			p.Scriptorium.Artifacts["session_recap"] = ScriptoriumArtifactConfig{Timeout: value}
+		}, want: "pipeline.scriptorium.artifacts.session_recap.timeout"},
+		{name: "trim bounds timeout", set: func(p *PipelineConfig, value string) { p.Trim.Bounds.Timeout = value }, want: "pipeline.trim.bounds.timeout"},
+		{name: "notification timeout", set: func(p *PipelineConfig, value string) { p.Notification.Timeout = value }, want: "pipeline.notification.timeout"},
+	}
+
+	for _, value := range []string{"0s", "-1ms"} {
+		for _, tt := range tests {
+			t.Run(tt.name+"/"+value, func(t *testing.T) {
+				cfg := loadedValidConfig(t)
+				tt.set(cfg.Pipeline, value)
+				err := Validate(cfg)
+				if err == nil || !strings.Contains(err.Error(), tt.want+" must be positive") {
+					t.Fatalf("Validate() error = %v, want positive-value error for %s", err, tt.want)
+				}
+			})
+		}
+	}
+}
+
+func TestValidateNotariusTimeoutRequiresPositiveValue(t *testing.T) {
+	for _, value := range []string{"0s", "-1ms"} {
+		t.Run(value, func(t *testing.T) {
+			cfg := loadedValidConfig(t)
+			cfg.Pipeline.Notarius = &NotariusConfig{
+				Enabled:          true,
+				Binary:           "notarius",
+				ConfigPath:       "/tmp/notarius.yml",
+				PipelineID:       "session",
+				Timeout:          value,
+				WorkingDirectory: "/tmp",
+				Outputs: map[string]NotariusOutputConfig{
+					"npc_registry": {
+						LaneID:        "npc-registry",
+						MediaType:     "application/json",
+						SchemaID:      "notarius.dnd.npc_registry",
+						SchemaVersion: "v1",
+					},
+				},
+			}
+			err := Validate(cfg)
+			if err == nil || !strings.Contains(err.Error(), "pipeline.notarius.timeout must be positive") {
+				t.Fatalf("Validate() error = %v, want positive Notarius timeout error", err)
+			}
+		})
+	}
+}
+
+func TestValidateDurationRejectsOverflowAndAcceptsPositiveSubsecondValues(t *testing.T) {
+	cfg := loadedValidConfig(t)
+	cfg.Pipeline.WhisperX.Timeout = "999999999999999999999h"
+	if err := Validate(cfg); err == nil || !strings.Contains(err.Error(), "pipeline.whisperx.timeout must be a valid duration") {
+		t.Fatalf("Validate() overflow error = %v, want duration parse error", err)
+	}
+
+	cfg = loadedValidConfig(t)
+	cfg.Pipeline.WhisperX.Timeout = "1ms"
+	cfg.Pipeline.WhisperX.RetryDelay = "1ms"
+	cfg.Pipeline.Seriatim.Timeout = "1ms"
+	cfg.Pipeline.Audita.Timeout = "1ms"
+	cfg.Pipeline.Scriptorium.Timeout = "1ms"
+	cfg.Pipeline.Trim.Bounds.Timeout = "1ms"
+	cfg.Pipeline.Notification.Timeout = "1ms"
+	if cfg.Pipeline.Scriptorium.Artifacts == nil {
+		cfg.Pipeline.Scriptorium.Artifacts = map[string]ScriptoriumArtifactConfig{}
+	}
+	cfg.Pipeline.Scriptorium.Artifacts["session_recap"] = ScriptoriumArtifactConfig{Timeout: "1ms"}
+	if err := Validate(cfg); err != nil {
+		t.Fatalf("Validate() positive subsecond durations error = %v", err)
+	}
+}
+
+func TestValidateAllowsEmptyArtifactTimeoutFallback(t *testing.T) {
+	cfg := loadedValidConfig(t)
+	if cfg.Pipeline.Scriptorium.Artifacts == nil {
+		cfg.Pipeline.Scriptorium.Artifacts = map[string]ScriptoriumArtifactConfig{}
+	}
+	cfg.Pipeline.Scriptorium.Artifacts["session_recap"] = ScriptoriumArtifactConfig{Timeout: ""}
+	if err := Validate(cfg); err != nil {
+		t.Fatalf("Validate() empty artifact timeout error = %v", err)
+	}
+}
+
+func loadedValidConfig(t *testing.T) *Config {
+	t.Helper()
+	pipelinePath, sessionPath := writeConfigFiles(t, testPipelineBaseYAML, testSessionBaseYAML)
+	cfg, err := Load(pipelinePath, sessionPath)
+	if err != nil {
+		t.Fatalf("Load() error = %v", err)
+	}
+	return cfg
+}