fix(specialist): cap max specialist nesting depth

internal/specialist/exec.go

@@ -25,6 +25,7 @@ import (
 const (
 	sessionTagSpecialist     = "specialist"
 	promptFileThresholdBytes = 4 * 1024
+	maxSpecialistDepth       = 8 // hard cap to prevent infinite recursion/resource exhaustion via Task
 )
 
 const SessionTagSpecialist = sessionTagSpecialist
@@ -218,6 +219,13 @@ func (executor Executor) Run(ctx context.Context, params TaskParameters, options
 	if options.CurrentDepth < 0 {
 		return ExecResult{}, fmt.Errorf("current depth cannot be negative")
 	}
+	// >= (not >): this Run call spawns a CHILD at options.CurrentDepth+1, so a
+	// parent already AT the cap must still be rejected here rather than being
+	// allowed to launch one more level before the child's own next call trips
+	// the guard.
+	if options.CurrentDepth >= maxSpecialistDepth {
+		return ExecResult{}, fmt.Errorf("spawning a specialist at depth %d would exceed maximum nesting depth %d", options.CurrentDepth+1, maxSpecialistDepth)
+	}
 	if strings.TrimSpace(params.Prompt) == "" {
 		return ExecResult{}, fmt.Errorf("specialist prompt is required")
 	}

internal/specialist/exec_test.go

@@ -1,6 +1,7 @@
 package specialist
 
 import (
+	"context"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -336,6 +337,39 @@ func TestBuildArgsRejectsInvalidInputs(t *testing.T) {
 	}
 }
 
+func TestRunRejectsDepthExceedingMax(t *testing.T) {
+	_, err := (Executor{}).Run(context.Background(), TaskParameters{
+		Prompt: "hi",
+	}, TaskRunOptions{CurrentDepth: maxSpecialistDepth + 1})
+	if err == nil || !strings.Contains(err.Error(), "depth") {
+		t.Fatalf("Run error = %v, want depth error", err)
+	}
+}
+
+// TestRunRejectsDepthAtMax covers the boundary: a parent already AT the cap
+// must be rejected too, since this Run call would launch a child one level
+// past it (--depth CurrentDepth+1). Only checking ">" here would let that
+// child start before the guard ever caught it. The guard sits before the
+// fresh/resume branch in Run, so both call shapes must be proven to reject
+// here rather than falling through to runFresh/runResume.
+func TestRunRejectsDepthAtMax(t *testing.T) {
+	tests := []struct {
+		name   string
+		params TaskParameters
+	}{
+		{name: "fresh", params: TaskParameters{Prompt: "hi"}},
+		{name: "resume", params: TaskParameters{Prompt: "hi", Resume: "child_session"}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := (Executor{}).Run(context.Background(), tc.params, TaskRunOptions{CurrentDepth: maxSpecialistDepth})
+			if err == nil || !strings.Contains(err.Error(), "depth") {
+				t.Fatalf("Run error = %v, want depth error", err)
+			}
+		})
+	}
+}
+
 func TestBuildArgsRejectsInvalidSessionIDs(t *testing.T) {
 	_, err := (Executor{NewSessionID: func() (string, error) { return "../escape", nil }}).BuildArgs(BuildArgsInput{
 		Prompt: "hi",