fix: always write model field to checkpoint metadata.json

cmd/entire/cli/agent/claudecode/lifecycle.go

@@ -133,6 +133,7 @@ func (c *ClaudeCodeAgent) parseTurnEnd(stdin io.Reader) (*agent.Event, error) {
 		Type:       agent.TurnEnd,
 		SessionID:  raw.SessionID,
 		SessionRef: raw.TranscriptPath,
+		Model:      raw.Model,
 		Timestamp:  time.Now(),
 	}, nil
 }
@@ -146,6 +147,7 @@ func (c *ClaudeCodeAgent) parseSessionEnd(stdin io.Reader) (*agent.Event, error)
 		Type:       agent.SessionEnd,
 		SessionID:  raw.SessionID,
 		SessionRef: raw.TranscriptPath,
+		Model:      raw.Model,
 		Timestamp:  time.Now(),
 	}, nil
 }

cmd/entire/cli/agent/claudecode/lifecycle_test.go

@@ -119,6 +119,23 @@ func TestParseHookEvent_TurnEnd(t *testing.T) {
 	}
 }
 
+func TestParseHookEvent_TurnEnd_IncludesModel(t *testing.T) {
+	t.Parallel()
+
+	ag := &ClaudeCodeAgent{}
+	input := `{"session_id": "sess-stop-model", "transcript_path": "/tmp/stop.jsonl", "model": "claude-opus-4-6"}`
+
+	event, err := ag.ParseHookEvent(context.Background(), HookNameStop, strings.NewReader(input))
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	require.NotNil(t, event, "expected event, got nil")
+	if event.Model != "claude-opus-4-6" {
+		t.Errorf("expected model 'claude-opus-4-6', got %q", event.Model)
+	}
+}
+
 func TestParseHookEvent_SessionEnd(t *testing.T) {
 	t.Parallel()
 
@@ -139,6 +156,23 @@ func TestParseHookEvent_SessionEnd(t *testing.T) {
 	}
 }
 
+func TestParseHookEvent_SessionEnd_IncludesModel(t *testing.T) {
+	t.Parallel()
+
+	ag := &ClaudeCodeAgent{}
+	input := `{"session_id": "end-model", "transcript_path": "/tmp/end.jsonl", "model": "claude-sonnet-4-20250514"}`
+
+	event, err := ag.ParseHookEvent(context.Background(), HookNameSessionEnd, strings.NewReader(input))
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	require.NotNil(t, event, "expected event, got nil")
+	if event.Model != "claude-sonnet-4-20250514" {
+		t.Errorf("expected model 'claude-sonnet-4-20250514', got %q", event.Model)
+	}
+}
+
 func TestParseHookEvent_SubagentStart(t *testing.T) {
 	t.Parallel()
 

cmd/entire/cli/agent/codex/lifecycle.go

@@ -89,6 +89,7 @@ func (c *CodexAgent) parseTurnStart(stdin io.Reader) (*agent.Event, error) {
 		SessionID:  raw.SessionID,
 		SessionRef: derefString(raw.TranscriptPath),
 		Prompt:     raw.Prompt,
+		Model:      raw.Model,
 		Timestamp:  time.Now(),
 	}, nil
 }
@@ -102,6 +103,7 @@ func (c *CodexAgent) parseTurnEnd(stdin io.Reader) (*agent.Event, error) {
 		Type:       agent.TurnEnd,
 		SessionID:  raw.SessionID,
 		SessionRef: derefString(raw.TranscriptPath),
+		Model:      raw.Model,
 		Timestamp:  time.Now(),
 	}, nil
 }

cmd/entire/cli/agent/codex/lifecycle_test.go

@@ -73,6 +73,7 @@ func TestParseHookEvent_UserPromptSubmit(t *testing.T) {
 	require.Equal(t, "test-uuid", event.SessionID)
 	require.Equal(t, "/tmp/rollout.jsonl", event.SessionRef)
 	require.Equal(t, "Create a hello.txt file", event.Prompt)
+	require.Equal(t, "gpt-4.1", event.Model)
 }
 
 func TestParseHookEvent_Stop(t *testing.T) {
@@ -96,6 +97,7 @@ func TestParseHookEvent_Stop(t *testing.T) {
 	require.Equal(t, agent.TurnEnd, event.Type)
 	require.Equal(t, "test-uuid", event.SessionID)
 	require.Equal(t, "/tmp/rollout.jsonl", event.SessionRef)
+	require.Equal(t, "gpt-4.1", event.Model)
 }
 
 func TestParseHookEvent_PreToolUse_ReturnsNil(t *testing.T) {

cmd/entire/cli/agent/cursor/lifecycle.go

@@ -88,6 +88,7 @@ func (c *CursorAgent) parseSessionStart(stdin io.Reader) (*agent.Event, error) {
 		Type:       agent.SessionStart,
 		SessionID:  raw.ConversationID,
 		SessionRef: raw.TranscriptPath,
+		Model:      raw.Model,
 		Timestamp:  time.Now(),
 	}, nil
 }
@@ -131,6 +132,7 @@ func (c *CursorAgent) parseSessionEnd(ctx context.Context, stdin io.Reader) (*ag
 		Type:       agent.SessionEnd,
 		SessionID:  raw.ConversationID,
 		SessionRef: c.resolveTranscriptRef(ctx, raw.ConversationID, raw.TranscriptPath),
+		Model:      raw.Model,
 		DurationMs: intFromJSON(raw.DurationMs),
 		Timestamp:  time.Now(),
 	}, nil

cmd/entire/cli/agent/cursor/lifecycle_test.go

@@ -14,6 +14,8 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+const testModel = "gpt-4o"
+
 func TestParseHookEvent_SessionStart(t *testing.T) {
 	t.Parallel()
 
@@ -40,6 +42,40 @@ func TestParseHookEvent_SessionStart(t *testing.T) {
 	}
 }
 
+func TestParseHookEvent_SessionStart_IncludesModel(t *testing.T) {
+	t.Parallel()
+
+	ag := &CursorAgent{}
+	input := `{"conversation_id": "sess-model", "transcript_path": "/tmp/t.jsonl", "model": "` + testModel + `"}`
+
+	event, err := ag.ParseHookEvent(context.Background(), HookNameSessionStart, strings.NewReader(input))
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	require.NotNil(t, event, "expected event, got nil")
+	if event.Model != testModel {
+		t.Errorf("expected model %q, got %q", testModel, event.Model)
+	}
+}
+
+func TestParseHookEvent_SessionStart_EmptyModel(t *testing.T) {
+	t.Parallel()
+
+	ag := &CursorAgent{}
+	input := `{"conversation_id": "sess-no-model", "transcript_path": "/tmp/t.jsonl"}`
+
+	event, err := ag.ParseHookEvent(context.Background(), HookNameSessionStart, strings.NewReader(input))
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	require.NotNil(t, event, "expected event, got nil")
+	if event.Model != "" {
+		t.Errorf("expected empty model, got %q", event.Model)
+	}
+}
+
 func TestParseHookEvent_TurnStart(t *testing.T) {
 	t.Parallel()
 
@@ -67,15 +103,15 @@ func TestParseHookEvent_TurnStart_IncludesModel(t *testing.T) {
 	t.Parallel()
 
 	ag := &CursorAgent{}
-	input := `{"conversation_id": "sess-m", "transcript_path": "/tmp/t.jsonl", "prompt": "hi", "model": "gpt-4o"}`
+	input := `{"conversation_id": "sess-m", "transcript_path": "/tmp/t.jsonl", "prompt": "hi", "model": "` + testModel + `"}`
 
 	event, err := ag.ParseHookEvent(context.Background(), HookNameBeforeSubmitPrompt, strings.NewReader(input))
 
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
-	if event.Model != "gpt-4o" {
-		t.Errorf("expected model 'gpt-4o', got %q", event.Model)
+	if event.Model != testModel {
+		t.Errorf("expected model %q, got %q", testModel, event.Model)
 	}
 }
 
@@ -166,6 +202,23 @@ func TestParseHookEvent_SessionEnd(t *testing.T) {
 	}
 }
 
+func TestParseHookEvent_SessionEnd_IncludesModel(t *testing.T) {
+	t.Parallel()
+
+	ag := &CursorAgent{}
+	input := `{"conversation_id": "end-model", "transcript_path": "/tmp/end.jsonl", "model": "` + testModel + `"}`
+
+	event, err := ag.ParseHookEvent(context.Background(), HookNameSessionEnd, strings.NewReader(input))
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	require.NotNil(t, event, "expected event, got nil")
+	if event.Model != testModel {
+		t.Errorf("expected model %q, got %q", testModel, event.Model)
+	}
+}
+
 func TestParseHookEvent_TurnEnd_CLINoTranscriptPath(t *testing.T) {
 	ag := &CursorAgent{}
 	// Set up a temp dir that simulates the Cursor project dir with a flat transcript

cmd/entire/cli/agent/vogon/hooks.go

@@ -45,6 +45,7 @@ func (v *Agent) ParseHookEvent(_ context.Context, hookName string, stdin io.Read
 			Type:       agent.SessionStart,
 			SessionID:  raw.SessionID,
 			SessionRef: raw.TranscriptPath,
+			Model:      raw.Model,
 			Timestamp:  time.Now(),
 		}, nil
 
@@ -58,6 +59,7 @@ func (v *Agent) ParseHookEvent(_ context.Context, hookName string, stdin io.Read
 			SessionID:  raw.SessionID,
 			SessionRef: raw.TranscriptPath,
 			Prompt:     raw.Prompt,
+			Model:      raw.Model,
 			Timestamp:  time.Now(),
 		}, nil
 
@@ -70,6 +72,7 @@ func (v *Agent) ParseHookEvent(_ context.Context, hookName string, stdin io.Read
 			Type:       agent.TurnEnd,
 			SessionID:  raw.SessionID,
 			SessionRef: raw.TranscriptPath,
+			Model:      raw.Model,
 			Timestamp:  time.Now(),
 		}, nil
 
@@ -82,6 +85,7 @@ func (v *Agent) ParseHookEvent(_ context.Context, hookName string, stdin io.Read
 			Type:       agent.SessionEnd,
 			SessionID:  raw.SessionID,
 			SessionRef: raw.TranscriptPath,
+			Model:      raw.Model,
 			Timestamp:  time.Now(),
 		}, nil
 
@@ -117,10 +121,12 @@ func (v *Agent) WriteHookResponse(message string) error {
 type sessionInfoRaw struct {
 	SessionID      string `json:"session_id"`
 	TranscriptPath string `json:"transcript_path"`
+	Model          string `json:"model,omitempty"`
 }
 
 type userPromptSubmitRaw struct {
 	SessionID      string `json:"session_id"`
 	TranscriptPath string `json:"transcript_path"`
 	Prompt         string `json:"prompt"`
+	Model          string `json:"model,omitempty"`
 }

cmd/entire/cli/checkpoint/checkpoint.go

@@ -387,8 +387,9 @@ type CommittedMetadata struct {
 	// Agent identifies the agent that created this checkpoint (e.g., "Claude Code", "Cursor")
 	Agent types.AgentType `json:"agent,omitempty"`
 
-	// Model is the LLM model used during the session (e.g., "claude-sonnet-4-20250514")
-	Model string `json:"model,omitempty"`
+	// Model is the LLM model used during the session (e.g., "claude-sonnet-4-20250514").
+	// Always written to metadata (empty string when unknown) so consumers can rely on the field's presence.
+	Model string `json:"model"`
 
 	// TurnID correlates checkpoints from the same agent turn.
 	// When a turn's work spans multiple commits, each gets its own checkpoint

cmd/entire/cli/checkpoint/checkpoint_test.go

@@ -2871,6 +2871,89 @@ func TestWriteCommitted_CLIVersionField(t *testing.T) {
 	}
 }
 
+func TestWriteCommitted_ModelFieldAlwaysPresent(t *testing.T) {
+	t.Parallel()
+
+	tempDir := t.TempDir()
+
+	repo, err := git.PlainInit(tempDir, false)
+	if err != nil {
+		t.Fatalf("failed to init git repo: %v", err)
+	}
+
+	worktree, err := repo.Worktree()
+	if err != nil {
+		t.Fatalf("failed to get worktree: %v", err)
+	}
+
+	readmeFile := filepath.Join(tempDir, "README.md")
+	if err := os.WriteFile(readmeFile, []byte("# Test"), 0o644); err != nil {
+		t.Fatalf("failed to write README: %v", err)
+	}
+	if _, err := worktree.Add("README.md"); err != nil {
+		t.Fatalf("failed to add README: %v", err)
+	}
+	if _, err := worktree.Commit("Initial commit", &git.CommitOptions{
+		Author: &object.Signature{Name: "Test", Email: "test@test.com"},
+	}); err != nil {
+		t.Fatalf("failed to commit: %v", err)
+	}
+
+	store := NewGitStore(repo)
+
+	checkpointID := id.MustCheckpointID("c1d2e3f4a5b6")
+	err = store.WriteCommitted(context.Background(), WriteCommittedOptions{
+		CheckpointID: checkpointID,
+		SessionID:    "test-session-model",
+		Strategy:     "manual-commit",
+		Agent:        agent.AgentTypeClaudeCode,
+		Transcript:   []byte("test transcript"),
+		AuthorName:   "Test Author",
+		AuthorEmail:  "test@example.com",
+	})
+	if err != nil {
+		t.Fatalf("WriteCommitted() error = %v", err)
+	}
+
+	ref, err := repo.Reference(plumbing.NewBranchReferenceName(paths.MetadataBranchName), true)
+	if err != nil {
+		t.Fatalf("failed to get metadata branch reference: %v", err)
+	}
+
+	commit, err := repo.CommitObject(ref.Hash())
+	if err != nil {
+		t.Fatalf("failed to get commit object: %v", err)
+	}
+
+	tree, err := commit.Tree()
+	if err != nil {
+		t.Fatalf("failed to get tree: %v", err)
+	}
+
+	sessionMetadataPath := checkpointID.Path() + "/0/" + paths.MetadataFileName
+	sessionMetadataFile, err := tree.File(sessionMetadataPath)
+	if err != nil {
+		t.Fatalf("failed to find session metadata.json at %s: %v", sessionMetadataPath, err)
+	}
+
+	sessionContent, err := sessionMetadataFile.Contents()
+	if err != nil {
+		t.Fatalf("failed to read session metadata.json: %v", err)
+	}
+
+	var sessionMetadata CommittedMetadata
+	if err := json.Unmarshal([]byte(sessionContent), &sessionMetadata); err != nil {
+		t.Fatalf("failed to parse session metadata.json: %v", err)
+	}
+
+	if sessionMetadata.Model != "" {
+		t.Errorf("CommittedMetadata.Model = %q, want empty string", sessionMetadata.Model)
+	}
+	if !strings.Contains(sessionContent, `"model": ""`) {
+		t.Errorf("session metadata.json should contain an explicit empty model field, got:\n%s", sessionContent)
+	}
+}
+
 func TestRedactSummary_Nil(t *testing.T) {
 	t.Parallel()
 	result := redactSummary(nil)

e2e/testutil/metadata.go

@@ -47,6 +47,7 @@ type SessionMetadata struct {
 	CreatedAt          time.Time   `json:"created_at"`
 	Branch             string      `json:"branch"`
 	Agent              string      `json:"agent"`
+	Model              string      `json:"model"`
 	CheckpointsCount   int         `json:"checkpoints_count"`
 	FilesTouched       []string    `json:"files_touched"`
 	TokenUsage         TokenUsage  `json:"token_usage"`

e2e/vogon/main.go

@@ -47,6 +47,7 @@ func main() {
 	fireHook(dir, "session-start", map[string]string{
 		"session_id":      sessionID,
 		"transcript_path": transcriptPath,
+		"model":           "vogon-llm-42",
 	})
 
 	if prompt != "" {
@@ -76,6 +77,7 @@ func main() {
 	fireHook(dir, "session-end", map[string]string{
 		"session_id":      sessionID,
 		"transcript_path": transcriptPath,
+		"model":           "vogon-llm-42",
 	})
 }
 
@@ -84,6 +86,7 @@ func runTurn(dir, sessionID, transcriptPath, prompt string) {
 		"session_id":      sessionID,
 		"transcript_path": transcriptPath,
 		"prompt":          prompt,
+		"model":           "vogon-llm-42",
 	})
 
 	appendTranscript(transcriptPath, "user", prompt)
@@ -97,6 +100,7 @@ func runTurn(dir, sessionID, transcriptPath, prompt string) {
 	fireHook(dir, "stop", map[string]string{
 		"session_id":      sessionID,
 		"transcript_path": transcriptPath,
+		"model":           "vogon-llm-42",
 	})
 }