diff --git a/internal/session/session.go b/internal/session/session.go
new file mode 100644
index 0000000..c14a1b4
--- /dev/null
+++ b/internal/session/session.go
@@ -0,0 +1,83 @@
+// internal/session/session.go
+package session
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// Entry is one skill invocation record, appended to the session JSONL log.
+type Entry struct {
+	SessionID   string          `json:"session_id"`
+	Timestamp   time.Time       `json:"timestamp"`
+	Skill       string          `json:"skill"`
+	Phase       string          `json:"phase,omitempty"`
+	ProjectRoot string          `json:"project_root,omitempty"`
+	Input       json.RawMessage `json:"input,omitempty"`
+	Attempts    []Attempt       `json:"attempts,omitempty"`
+	FinalStatus string          `json:"final_status"`
+	FilePath    string          `json:"file_path,omitempty"`
+	ModelUsed   string          `json:"model_used,omitempty"`
+	DurationMs  int64           `json:"duration_ms,omitempty"`
+}
+
+// Attempt represents one subprocess invocation within a skill call.
+type Attempt struct {
+	Attempt       int    `json:"attempt"`
+	Model         string `json:"model"`
+	OutputSummary string `json:"output_summary,omitempty"`
+	RunnerOutput  string `json:"runner_output,omitempty"`
+	Verified      bool   `json:"verified"`
+}
+
+// Append writes entry as a single JSON line to sessionsDir/{sessionID}.jsonl.
+func Append(sessionsDir, sessionID string, entry Entry) error {
+	if err := os.MkdirAll(sessionsDir, 0o755); err != nil {
+		return fmt.Errorf("create sessions dir: %w", err)
+	}
+	path := filepath.Join(sessionsDir, sessionID+".jsonl")
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return fmt.Errorf("open session log: %w", err)
+	}
+	defer f.Close()
+
+	line, err := json.Marshal(entry)
+	if err != nil {
+		return fmt.Errorf("marshal entry: %w", err)
+	}
+	_, err = fmt.Fprintf(f, "%s\n", line)
+	return err
+}
+
+// Read returns all entries for sessionID. Returns empty slice if no log exists.
+func Read(sessionsDir, sessionID string) ([]Entry, error) {
+	path := filepath.Join(sessionsDir, sessionID+".jsonl")
+	f, err := os.Open(path)
+	if os.IsNotExist(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("open session log: %w", err)
+	}
+	defer f.Close()
+
+	var entries []Entry
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		if len(line) == 0 {
+			continue
+		}
+		var e Entry
+		if err := json.Unmarshal(line, &e); err != nil {
+			return nil, fmt.Errorf("parse entry: %w", err)
+		}
+		entries = append(entries, e)
+	}
+	return entries, scanner.Err()
+}
diff --git a/internal/session/session_test.go b/internal/session/session_test.go
new file mode 100644
index 0000000..279654d
--- /dev/null
+++ b/internal/session/session_test.go
@@ -0,0 +1,63 @@
+// internal/session/session_test.go
+package session_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/mathiasbq/supervisor/internal/session"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAppend_WritesJSONLEntry(t *testing.T) {
+	dir := t.TempDir()
+	entry := session.Entry{
+		SessionID:   "test-session-1",
+		Timestamp:   time.Now().UTC(),
+		Skill:       "tdd_green",
+		Phase:       "green",
+		ProjectRoot: "/tmp/myproject",
+		FinalStatus: "pass",
+		ModelUsed:   "ollama/qwen3",
+		DurationMs:  5000,
+	}
+
+	require.NoError(t, session.Append(dir, "test-session-1", entry))
+
+	path := filepath.Join(dir, "test-session-1.jsonl")
+	data, err := os.ReadFile(path)
+	require.NoError(t, err)
+
+	var got session.Entry
+	require.NoError(t, json.Unmarshal(data, &got))
+	assert.Equal(t, "test-session-1", got.SessionID)
+	assert.Equal(t, "tdd_green", got.Skill)
+	assert.Equal(t, "pass", got.FinalStatus)
+}
+
+func TestAppend_AppendsMultipleEntries(t *testing.T) {
+	dir := t.TempDir()
+	for i := 0; i < 3; i++ {
+		require.NoError(t, session.Append(dir, "s1", session.Entry{
+			SessionID:   "s1",
+			Timestamp:   time.Now().UTC(),
+			Skill:       "tdd_red",
+			FinalStatus: "pass",
+		}))
+	}
+
+	entries, err := session.Read(dir, "s1")
+	require.NoError(t, err)
+	assert.Len(t, entries, 3)
+}
+
+func TestRead_EmptyWhenNoFile(t *testing.T) {
+	dir := t.TempDir()
+	entries, err := session.Read(dir, "missing")
+	require.NoError(t, err)
+	assert.Empty(t, entries)
+}