Add remote session loading

internal/config/config.go

@@ -10,7 +10,8 @@ type Config struct {
 	CampaignPath string
 	SessionPath  string
 
-	StableInputs ResolvedStableInputs
+	StableInputs  ResolvedStableInputs
+	SessionSource SessionSource
 }
 
 // PipelineConfig contains durable pipeline-level settings.
@@ -262,3 +263,14 @@ type ResolvedInputFile struct {
 	ConfigPath string
 	Source     string
 }
+
+// SessionSource records where session.yml came from before materialization.
+type SessionSource struct {
+	Source    string
+	LocalPath string
+	S3Bucket  string
+	S3Key     string
+	S3Size    int64
+	S3ETag    string
+	SpoolPath string
+}

internal/config/load.go

@@ -49,20 +49,25 @@ func LoadSessionWithOptions(path string, opts SessionLoadOptions) (*SessionConfi
 	if err != nil {
 		return nil, fmt.Errorf("load session config: session file %q: open: %w", path, err)
 	}
+	return LoadSessionBytesWithOptions(path, sessionBytes, opts)
+}
 
-	rendered, err := renderSessionTemplate(string(sessionBytes), opts)
+// LoadSessionBytesWithOptions loads session configuration from YAML bytes with
+// strict field checking after template rendering.
+func LoadSessionBytesWithOptions(label string, data []byte, opts SessionLoadOptions) (*SessionConfig, error) {
+	rendered, err := renderSessionTemplate(string(data), opts)
 	if err != nil {
 		return nil, fmt.Errorf("load session config: %w", err)
 	}
 
 	var cfg SessionConfig
-	if err := decodeStrictYAMLFromReader("session", path, strings.NewReader(rendered), &cfg); err != nil {
+	if err := decodeStrictYAMLFromReader("session", label, strings.NewReader(rendered), &cfg); err != nil {
 		return nil, fmt.Errorf("load session config: %w", err)
 	}
 	if strings.TrimSpace(opts.SessionID) != "" && strings.TrimSpace(cfg.SessionID) != "" && strings.TrimSpace(cfg.SessionID) != strings.TrimSpace(opts.SessionID) {
 		return nil, fmt.Errorf(
 			"load session config: session file %q: session_id mismatch: --session-id %q does not match rendered session_id %q",
-			path,
+			label,
 			strings.TrimSpace(opts.SessionID),
 			strings.TrimSpace(cfg.SessionID),
 		)
@@ -72,7 +77,7 @@ func LoadSessionWithOptions(path string, opts SessionLoadOptions) (*SessionConfi
 		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 rendered previous_session_id %q",
-			path,
+			label,
 			strings.TrimSpace(opts.PreviousSessionID),
 			strings.TrimSpace(cfg.PreviousSessionID),
 		)
@@ -109,19 +114,35 @@ func LoadWithSessionOptions(pipelinePath, campaignPath, sessionPath string, sess
 		return nil, err
 	}
 
+	return Resolve(pipelinePath, pipelineCfg, campaignPath, campaignCfg, sessionPath, sessionCfg, SessionSource{
+		Source:    "session_config",
+		LocalPath: sessionPath,
+	})
+}
+
+// Resolve builds final stage-facing configuration from already loaded
+// pipeline, campaign, and session documents.
+func Resolve(pipelinePath string, pipelineCfg *PipelineConfig, campaignPath string, campaignCfg *CampaignConfig, sessionPath string, sessionCfg *SessionConfig, sessionSource SessionSource) (*Config, error) {
 	stableInputs, err := mergeCampaignSession(campaignCfg, sessionCfg, campaignPath, sessionPath)
 	if err != nil {
 		return nil, err
 	}
+	if strings.TrimSpace(sessionSource.Source) == "" {
+		sessionSource.Source = "session_config"
+	}
+	if strings.TrimSpace(sessionSource.LocalPath) == "" {
+		sessionSource.LocalPath = sessionPath
+	}
 
 	return &Config{
-		Pipeline:     pipelineCfg,
-		Campaign:     campaignCfg,
-		Session:      sessionCfg,
-		PipelinePath: pipelinePath,
-		CampaignPath: campaignPath,
-		SessionPath:  sessionPath,
-		StableInputs: stableInputs,
+		Pipeline:      pipelineCfg,
+		Campaign:      campaignCfg,
+		Session:       sessionCfg,
+		PipelinePath:  pipelinePath,
+		CampaignPath:  campaignPath,
+		SessionPath:   sessionPath,
+		StableInputs:  stableInputs,
+		SessionSource: sessionSource,
 	}, nil
 }
 

internal/config/session_template_test.go

@@ -241,3 +241,38 @@ inputs:
 		t.Fatalf("SessionID = %q, want 2026-05-03", cfg.SessionID)
 	}
 }
+
+func TestLoadSessionBytesWithOptionsUsesSameTemplateAndStrictDecode(t *testing.T) {
+	sessionYAML := []byte(`session_id: "{{ session_id }}"
+campaign: sample-campaign
+inputs:
+  audio_s3:
+    prefix: audio/
+`)
+
+	cfg, err := LoadSessionBytesWithOptions("s3://bucket/session.yml", sessionYAML, SessionLoadOptions{SessionID: "2026-05-03"})
+	if err != nil {
+		t.Fatalf("LoadSessionBytesWithOptions() error = %v", err)
+	}
+	if cfg.SessionID != "2026-05-03" {
+		t.Fatalf("SessionID = %q, want 2026-05-03", cfg.SessionID)
+	}
+
+	_, err = LoadSessionBytesWithOptions("s3://bucket/session.yml", []byte("session_id: 2026-05-03\nunknown: true\n"), SessionLoadOptions{})
+	if err == nil {
+		t.Fatal("expected strict decode error, got nil")
+	}
+	if !strings.Contains(err.Error(), "strict decode failed") {
+		t.Fatalf("error = %q, want strict decode context", err.Error())
+	}
+}
+
+func TestLoadSessionBytesWithOptionsMismatchFails(t *testing.T) {
+	_, err := LoadSessionBytesWithOptions("s3://bucket/session.yml", []byte("session_id: 2026-05-03\n"), SessionLoadOptions{SessionID: "2026-04-04"})
+	if err == nil {
+		t.Fatal("expected mismatch error, got nil")
+	}
+	if !strings.Contains(err.Error(), "session_id mismatch") {
+		t.Fatalf("error = %q, want mismatch context", err.Error())
+	}
+}