diff --git a/internal/agent/agent.go b/internal/agent/agent.go
new file mode 100644
index 0000000..dc9e402
--- /dev/null
+++ b/internal/agent/agent.go
@@ -0,0 +1,81 @@
+package agent
+
+import (
+	"fmt"
+
+	"github.com/warrenronsiek/ctask/internal/workspace"
+)
+
+// Profile describes a built-in agent. The Default command is what ctask
+// invokes when AgentSpec.Command is empty; Type is the canonical name.
+type Profile struct {
+	Type    string
+	Default string
+}
+
+// BuiltinProfiles enumerates the v0.6 built-in agents. "custom" is NOT
+// in this map — it is the escape hatch with no defaults. Keep this in
+// sync with workspace.knownAgentTypes AND workspace.IsBuiltinAgentType
+// (update all three together when a new built-in lands).
+var BuiltinProfiles = map[string]Profile{
+	"claude":   {Type: "claude", Default: "claude"},
+	"opencode": {Type: "opencode", Default: "opencode"},
+}
+
+// IsKnownType reports whether t is "claude", "opencode", or "custom".
+func IsKnownType(t string) bool {
+	if t == "custom" {
+		return true
+	}
+	_, ok := BuiltinProfiles[t]
+	return ok
+}
+
+// Resolved is the launch-ready agent value: a single executable name or
+// path, optional arguments, and an env-var map merged AFTER ctask's own
+// exported vars at launch time.
+type Resolved struct {
+	Type    string
+	Command string
+	Args    []string
+	Env     map[string]string
+}
+
+// Resolve combines a workspace's AgentSpec with the user-level
+// default_agent into a launch-ready Resolved. Resolution rules (per
+// v0.6 spec §5):
+//
+//  1. If spec.Type is empty, fall through to defaultAgent.
+//  2. If the resolved type is "custom", spec.Command is required.
+//  3. Otherwise, the resolved command is spec.Command if set,
+//     else BuiltinProfiles[type].Default.
+//  4. Args and Env are carried verbatim from the spec.
+//
+// Resolve does NOT call exec.LookPath. PATH validation is the launch
+// path's job (shell.ExecAgent fails fast with a diagnostic) and
+// `ctask agents check`'s job. Keeping Resolve I/O-free makes it
+// trivially testable and reusable.
+func Resolve(spec workspace.AgentSpec, defaultAgent string) (*Resolved, error) {
+	typ := spec.Type
+	if typ == "" {
+		typ = defaultAgent
+	}
+	if !IsKnownType(typ) {
+		return nil, fmt.Errorf("agent: unknown type %q (must be claude, opencode, or custom)", typ)
+	}
+
+	cmd := spec.Command
+	if cmd == "" {
+		if typ == "custom" {
+			return nil, fmt.Errorf("agent: type \"custom\" requires command field")
+		}
+		cmd = BuiltinProfiles[typ].Default
+	}
+
+	return &Resolved{
+		Type:    typ,
+		Command: cmd,
+		Args:    spec.Args,
+		Env:     spec.Env,
+	}, nil
+}
diff --git a/internal/agent/agent_test.go b/internal/agent/agent_test.go
new file mode 100644
index 0000000..7f44dce
--- /dev/null
+++ b/internal/agent/agent_test.go
@@ -0,0 +1,108 @@
+package agent
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/warrenronsiek/ctask/internal/workspace"
+)
+
+func TestResolveBuiltinClaude(t *testing.T) {
+	spec := workspace.AgentSpec{Type: "claude"}
+	got, err := Resolve(spec, "claude")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if got.Type != "claude" || got.Command != "claude" {
+		t.Errorf("got %+v, want type=claude command=claude", got)
+	}
+}
+
+func TestResolveBuiltinOpencode(t *testing.T) {
+	spec := workspace.AgentSpec{Type: "opencode"}
+	got, err := Resolve(spec, "claude") // default doesn't matter; spec wins
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if got.Command != "opencode" {
+		t.Errorf("Command = %q, want opencode", got.Command)
+	}
+}
+
+func TestResolveCustomRequiresCommand(t *testing.T) {
+	spec := workspace.AgentSpec{Type: "custom"}
+	_, err := Resolve(spec, "claude")
+	if err == nil || !strings.Contains(err.Error(), "requires command") {
+		t.Fatalf("got %v, want error mentioning 'requires command'", err)
+	}
+}
+
+func TestResolveCustomWithCommand(t *testing.T) {
+	spec := workspace.AgentSpec{Type: "custom", Command: "my-agent"}
+	got, err := Resolve(spec, "claude")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if got.Command != "my-agent" {
+		t.Errorf("Command = %q, want my-agent", got.Command)
+	}
+}
+
+func TestResolveFallbackToDefaultWhenTypeMissing(t *testing.T) {
+	spec := workspace.AgentSpec{} // legacy / new-without-type
+	got, err := Resolve(spec, "opencode")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if got.Type != "opencode" || got.Command != "opencode" {
+		t.Errorf("got %+v, want type=opencode command=opencode", got)
+	}
+}
+
+func TestResolveCommandOverride(t *testing.T) {
+	spec := workspace.AgentSpec{Type: "claude", Command: "/opt/claude"}
+	got, err := Resolve(spec, "claude")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if got.Command != "/opt/claude" {
+		t.Errorf("Command = %q, want /opt/claude", got.Command)
+	}
+}
+
+func TestResolveArgsAndEnvCarriedThrough(t *testing.T) {
+	spec := workspace.AgentSpec{
+		Type: "opencode",
+		Args: []string{"--model", "x"},
+		Env:  map[string]string{"K": "v"},
+	}
+	got, err := Resolve(spec, "claude")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if len(got.Args) != 2 || got.Args[0] != "--model" || got.Args[1] != "x" {
+		t.Errorf("Args = %v, want [--model x]", got.Args)
+	}
+	if got.Env["K"] != "v" {
+		t.Errorf("Env[K] = %q, want v", got.Env["K"])
+	}
+}
+
+func TestResolveUnknownDefaultRejected(t *testing.T) {
+	spec := workspace.AgentSpec{}
+	_, err := Resolve(spec, "gemini")
+	if err == nil || !strings.Contains(err.Error(), "unknown") {
+		t.Fatalf("got %v, want unknown-type error", err)
+	}
+}
+
+func TestIsKnownType(t *testing.T) {
+	for _, name := range []string{"claude", "opencode", "custom"} {
+		if !IsKnownType(name) {
+			t.Errorf("IsKnownType(%q) = false, want true", name)
+		}
+	}
+	if IsKnownType("gemini") {
+		t.Error("IsKnownType(gemini) = true, want false")
+	}
+}