diff --git a/internal/agent/codex/codex.go b/internal/agent/codex/codex.go
new file mode 100644
index 0000000..ffa9611
--- /dev/null
+++ b/internal/agent/codex/codex.go
@@ -0,0 +1,14 @@
+package codex
+
+// Detector implements the agent.Detector interface for OpenAI Codex CLI.
+type Detector struct{}
+
+// New creates a new Codex CLI detector.
+func New() *Detector {
+	return &Detector{}
+}
+
+// Name returns the agent name.
+func (d *Detector) Name() string {
+	return "codex"
+}
diff --git a/internal/agent/codex/codex_test.go b/internal/agent/codex/codex_test.go
new file mode 100644
index 0000000..31bea19
--- /dev/null
+++ b/internal/agent/codex/codex_test.go
@@ -0,0 +1,18 @@
+package codex
+
+import (
+	"testing"
+
+	"github.com/partio-io/cli/internal/agent"
+)
+
+func TestDetectorImplementsInterface(t *testing.T) {
+	var _ agent.Detector = (*Detector)(nil)
+}
+
+func TestName(t *testing.T) {
+	d := New()
+	if got := d.Name(); got != "codex" {
+		t.Errorf("Name() = %q, want %q", got, "codex")
+	}
+}
diff --git a/internal/agent/codex/find_session_dir.go b/internal/agent/codex/find_session_dir.go
new file mode 100644
index 0000000..6d44e26
--- /dev/null
+++ b/internal/agent/codex/find_session_dir.go
@@ -0,0 +1,23 @@
+package codex
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// FindSessionDir returns the Codex CLI session directory for the given repo.
+// Codex stores sessions at ~/.codex/.
+func (d *Detector) FindSessionDir(repoRoot string) (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("getting home directory: %w", err)
+	}
+
+	sessionDir := filepath.Join(home, ".codex")
+	if _, err := os.Stat(sessionDir); err != nil {
+		return "", fmt.Errorf("no Codex session directory found: %w", err)
+	}
+
+	return sessionDir, nil
+}
diff --git a/internal/agent/codex/find_session_dir_test.go b/internal/agent/codex/find_session_dir_test.go
new file mode 100644
index 0000000..ad57667
--- /dev/null
+++ b/internal/agent/codex/find_session_dir_test.go
@@ -0,0 +1,59 @@
+package codex
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestFindSessionDir(t *testing.T) {
+	tests := []struct {
+		name      string
+		createDir bool
+		wantErr   bool
+	}{
+		{
+			name:      "returns directory when .codex exists",
+			createDir: true,
+			wantErr:   false,
+		},
+		{
+			name:      "returns error when .codex does not exist",
+			createDir: false,
+			wantErr:   true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			home := filepath.Join(tmpDir, "home")
+
+			if tt.createDir {
+				codexDir := filepath.Join(home, ".codex")
+				if err := os.MkdirAll(codexDir, 0o755); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			t.Setenv("HOME", home)
+
+			d := &Detector{}
+			got, err := d.FindSessionDir(filepath.Join(tmpDir, "repo"))
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			want := filepath.Join(home, ".codex")
+			if got != want {
+				t.Errorf("got %s, want %s", got, want)
+			}
+		})
+	}
+}
diff --git a/internal/agent/codex/process.go b/internal/agent/codex/process.go
new file mode 100644
index 0000000..17abd7d
--- /dev/null
+++ b/internal/agent/codex/process.go
@@ -0,0 +1,19 @@
+package codex
+
+import (
+	"os/exec"
+	"strings"
+)
+
+// IsRunning checks if a Codex CLI process is currently running.
+func (d *Detector) IsRunning() (bool, error) {
+	out, err := exec.Command("pgrep", "-f", "codex").Output()
+	if err != nil {
+		// pgrep returns exit code 1 if no processes found
+		if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() == 1 {
+			return false, nil
+		}
+		return false, err
+	}
+	return strings.TrimSpace(string(out)) != "", nil
+}
diff --git a/internal/agent/codex/process_test.go b/internal/agent/codex/process_test.go
new file mode 100644
index 0000000..05295ce
--- /dev/null
+++ b/internal/agent/codex/process_test.go
@@ -0,0 +1,44 @@
+package codex
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParsePgrepOutput(t *testing.T) {
+	tests := []struct {
+		name   string
+		output string
+		want   bool
+	}{
+		{
+			name:   "single PID",
+			output: "12345\n",
+			want:   true,
+		},
+		{
+			name:   "multiple PIDs",
+			output: "12345\n67890\n",
+			want:   true,
+		},
+		{
+			name:   "empty output",
+			output: "",
+			want:   false,
+		},
+		{
+			name:   "whitespace only",
+			output: "  \n  ",
+			want:   false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := strings.TrimSpace(tt.output) != ""
+			if got != tt.want {
+				t.Errorf("parsePgrepOutput(%q) = %v, want %v", tt.output, got, tt.want)
+			}
+		})
+	}
+}
diff --git a/internal/config/env.go b/internal/config/env.go
index 090b142..56b588a 100644
--- a/internal/config/env.go
+++ b/internal/config/env.go
@@ -15,4 +15,7 @@ func applyEnv(cfg *Config) {
 	if v := os.Getenv("PARTIO_LOG_LEVEL"); v != "" {
 		cfg.LogLevel = v
 	}
+	if v := os.Getenv("PARTIO_AGENT"); v != "" {
+		cfg.Agent = v
+	}
 }
diff --git a/internal/hooks/postcommit.go b/internal/hooks/postcommit.go
index a98a36f..72fc6f9 100644
--- a/internal/hooks/postcommit.go
+++ b/internal/hooks/postcommit.go
@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/partio-io/cli/internal/agent"
 	"github.com/partio-io/cli/internal/agent/claude"
 	"github.com/partio-io/cli/internal/attribution"
 	"github.com/partio-io/cli/internal/checkpoint"
@@ -66,10 +67,15 @@ func runPostCommit(repoRoot string, cfg config.Config) error {
 	}
 
 	// Parse agent session data
-	detector := claude.New()
-	sessionPath, sessionData, err := detector.FindLatestSession(repoRoot)
-	if err != nil {
-		slog.Warn("could not read agent session", "error", err)
+	detector := resolveDetector(cfg.Agent)
+	var sessionPath string
+	var sessionData *agent.SessionData
+	if cd, ok := detector.(*claude.Detector); ok {
+		var findErr error
+		sessionPath, sessionData, findErr = cd.FindLatestSession(repoRoot)
+		if findErr != nil {
+			slog.Warn("could not read agent session", "error", findErr)
+		}
 	}
 
 	// Skip if this session is already fully condensed and ended — re-processing
diff --git a/internal/hooks/precommit.go b/internal/hooks/precommit.go
index 06c2ec2..f203725 100644
--- a/internal/hooks/precommit.go
+++ b/internal/hooks/precommit.go
@@ -12,6 +12,12 @@ import (
 	"github.com/partio-io/cli/internal/session"
 )
 
+// claudeDetector is satisfied by *claude.Detector and allows
+// Claude-specific session operations in hooks.
+type claudeDetector interface {
+	FindLatestJSONLPath(repoRoot string) (string, error)
+}
+
 // preCommitState records the state captured during pre-commit for use by post-commit.
 type preCommitState struct {
 	AgentActive   bool   `json:"agent_active"`
@@ -27,7 +33,7 @@ func (r *Runner) PreCommit() error {
 }
 
 func runPreCommit(repoRoot string, cfg config.Config) error {
-	detector := claude.New()
+	detector := resolveDetector(cfg.Agent)
 
 	// Detect if agent is running
 	running, err := detector.IsRunning()
@@ -36,11 +42,11 @@ func runPreCommit(repoRoot string, cfg config.Config) error {
 		running = false
 	}
 
-	if running {
-		// Quick check: find the latest JSONL path without a full parse and see if
-		// we have already captured this session in a fully-condensed ended state.
-		// This avoids the expensive JSONL parse for stale sessions.
-		latestPath, pathErr := detector.FindLatestJSONLPath(repoRoot)
+	var sessionPath string
+
+	// Claude-specific: check for already-condensed sessions and find JSONL path.
+	if cd, ok := detector.(claudeDetector); running && ok {
+		latestPath, pathErr := cd.FindLatestJSONLPath(repoRoot)
 		if pathErr == nil {
 			sid := claude.PeekSessionID(latestPath)
 			if shouldSkipSession(filepath.Join(repoRoot, config.PartioDir), sid, latestPath) {
@@ -48,24 +54,31 @@ func runPreCommit(repoRoot string, cfg config.Config) error {
 				running = false
 			}
 		}
-	}
 
-	var sessionPath string
-	if running {
-		path, _, err := detector.FindLatestSession(repoRoot)
-		if err != nil {
-			slog.Debug("agent running but no session found", "error", err)
-		} else {
-			sessionPath = path
-			slog.Debug("agent session detected", "path", path)
+		if running {
+			if cdFull, ok := detector.(*claude.Detector); ok {
+				path, _, err := cdFull.FindLatestSession(repoRoot)
+				if err != nil {
+					slog.Debug("agent running but no session found", "error", err)
+				} else {
+					sessionPath = path
+					slog.Debug("agent session detected", "path", path)
+				}
+			}
 		}
 	}
 
 	branch, _ := git.CurrentBranch()
 	commitHash, _ := git.CurrentCommit()
 
+	// Claude requires a session path to be active; other agents are active when running.
+	agentActive := running
+	if _, ok := detector.(*claude.Detector); ok {
+		agentActive = running && sessionPath != ""
+	}
+
 	state := preCommitState{
-		AgentActive:   running && sessionPath != "",
+		AgentActive:   agentActive,
 		SessionPath:   sessionPath,
 		PreCommitHash: commitHash,
 		Branch:        branch,
diff --git a/internal/hooks/resolve.go b/internal/hooks/resolve.go
new file mode 100644
index 0000000..a612f69
--- /dev/null
+++ b/internal/hooks/resolve.go
@@ -0,0 +1,17 @@
+package hooks
+
+import (
+	"github.com/partio-io/cli/internal/agent"
+	"github.com/partio-io/cli/internal/agent/claude"
+	"github.com/partio-io/cli/internal/agent/codex"
+)
+
+// resolveDetector returns the appropriate agent detector for the given name.
+func resolveDetector(name string) agent.Detector {
+	switch name {
+	case "codex":
+		return codex.New()
+	default:
+		return claude.New()
+	}
+}