diff --git a/internal/acp/handlers/client_initialize.go b/internal/acp/handlers/client_initialize.go
new file mode 100644
index 0000000..c02b542
--- /dev/null
+++ b/internal/acp/handlers/client_initialize.go
@@ -0,0 +1,44 @@
+package handlers
+
+// Server-initiated initialize call: types per spec §5.4. Supervisor sends this
+// immediately after spawn. clientCapabilities.terminal=false (MVP no terminal).
+
+type InitializeParams struct {
+	ProtocolVersion    int                `json:"protocolVersion"`
+	ClientCapabilities ClientCapabilities `json:"clientCapabilities"`
+	ClientInfo         ClientInfo         `json:"clientInfo"`
+}
+
+type ClientCapabilities struct {
+	Fs       FsCapability `json:"fs"`
+	Terminal bool         `json:"terminal"`
+}
+
+type FsCapability struct {
+	ReadTextFile  bool `json:"readTextFile"`
+	WriteTextFile bool `json:"writeTextFile"`
+}
+
+type ClientInfo struct {
+	Name    string `json:"name"`
+	Version string `json:"version"`
+}
+
+type InitializeResult struct {
+	ProtocolVersion   int            `json:"protocolVersion"`
+	AgentCapabilities map[string]any `json:"agentCapabilities,omitempty"`
+	AgentInfo         map[string]any `json:"agentInfo,omitempty"`
+	AuthMethods       []any          `json:"authMethods,omitempty"`
+}
+
+// BuildInitializeParams returns standard initialize params.
+func BuildInitializeParams(version string) InitializeParams {
+	return InitializeParams{
+		ProtocolVersion: 1,
+		ClientCapabilities: ClientCapabilities{
+			Fs:       FsCapability{ReadTextFile: true, WriteTextFile: true},
+			Terminal: false,
+		},
+		ClientInfo: ClientInfo{Name: "agent-coding-workflow", Version: version},
+	}
+}
diff --git a/internal/acp/handlers/client_session_new.go b/internal/acp/handlers/client_session_new.go
new file mode 100644
index 0000000..99f00dc
--- /dev/null
+++ b/internal/acp/handlers/client_session_new.go
@@ -0,0 +1,17 @@
+package handlers
+
+// session/new method types (spec §5.4).
+
+type SessionNewParams struct {
+	Cwd        string   `json:"cwd"`
+	McpServers []string `json:"mcpServers"` // MVP: no MCP, send empty slice
+}
+
+type SessionNewResult struct {
+	SessionID string `json:"sessionId"`
+}
+
+// BuildSessionNewParams returns standard session/new params.
+func BuildSessionNewParams(cwd string) SessionNewParams {
+	return SessionNewParams{Cwd: cwd, McpServers: []string{}}
+}