Slice 4: graceful project handoff (pending switch + ack)

Add pending-switch coordination to internal/activity (RequestSwitch/PendingSwitch/AckSwitch/CancelSwitch); AckSwitch atomically sets the active project and records switch-completed with Claude's summary. New MCP tools get_pending_switch and ack_switch (request is user-only via HTTP). HTTP GET/POST/DELETE /api/switch. New <handoff-bar> component (ask/waiting/cancel/completed) over SSE; activity-feed reflects switch-completed. Test covers request->ack->clear. Verified live: request/waiting/cancel over SSE.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/mcp/mcp.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"time"
 
 	mcpsdk "github.com/modelcontextprotocol/go-sdk/mcp"
 
@@ -44,6 +45,25 @@ type activityOutput struct {
 	Events []activity.Event `json:"events" jsonschema:"recent activity events, oldest first"`
 }
 
+// pendingSwitchOutput reports whether the user has asked Claude to switch projects.
+type pendingSwitchOutput struct {
+	Pending     bool      `json:"pending" jsonschema:"true if the user has requested a switch you should complete"`
+	Target      string    `json:"target,omitempty" jsonschema:"the repository path to switch to"`
+	Note        string    `json:"note,omitempty" jsonschema:"an optional note from the user"`
+	RequestedAt time.Time `json:"requestedAt,omitempty"`
+}
+
+// ackSwitchInput is the argument schema for ack_switch.
+type ackSwitchInput struct {
+	Summary string `json:"summary" jsonschema:"a short note on where you left the previous project (shown to the user)"`
+}
+
+// ackSwitchOutput reports the completed switch.
+type ackSwitchOutput struct {
+	Switched bool   `json:"switched"`
+	Target   string `json:"target,omitempty" jsonschema:"the repository now active"`
+}
+
 // NewServer builds the MCP server and registers the (currently read-only) tools.
 func NewServer(svc *service.Service, version string) *mcpsdk.Server {
 	s := mcpsdk.NewServer(&mcpsdk.Implementation{
@@ -100,6 +120,30 @@ func NewServer(svc *service.Service, version string) *mcpsdk.Server {
 		return nil, activityOutput{Events: svc.Activity(50)}, nil
 	})
 
+	// get_pending_switch — has the user asked you to switch projects? (§8.3)
+	mcpsdk.AddTool(s, &mcpsdk.Tool{
+		Name:        "get_pending_switch",
+		Description: "Check whether the user has asked you to switch to a different project. If pending is true, finish your current work to a SAFE stopping point (commit or stash so nothing is lost), then call ack_switch to complete the handoff.",
+	}, func(_ context.Context, _ *mcpsdk.CallToolRequest, _ struct{}) (*mcpsdk.CallToolResult, pendingSwitchOutput, error) {
+		p, ok := svc.PendingSwitch()
+		if !ok {
+			return nil, pendingSwitchOutput{Pending: false}, nil
+		}
+		return nil, pendingSwitchOutput{Pending: true, Target: p.Target, Note: p.Note, RequestedAt: p.RequestedAt}, nil
+	})
+
+	// ack_switch — complete a pending handoff and tell the user where you left off.
+	mcpsdk.AddTool(s, &mcpsdk.Tool{
+		Name:        "ack_switch",
+		Description: "Complete a pending project switch: makes the requested target the active project and clears the request. Call this only after reaching a safe stopping point in the current project. Pass a short summary of where you left it — the user is notified.",
+	}, func(_ context.Context, _ *mcpsdk.CallToolRequest, in ackSwitchInput) (*mcpsdk.CallToolResult, ackSwitchOutput, error) {
+		p, ok := svc.AckSwitch(in.Summary)
+		if !ok {
+			return nil, ackSwitchOutput{Switched: false}, fmt.Errorf("no pending switch to acknowledge")
+		}
+		return nil, ackSwitchOutput{Switched: true, Target: p.Target}, nil
+	})
+
 	return s
 }
 

internal/mcp/mcp_test.go

@@ -159,6 +159,48 @@ func TestMCPRoundTrip(t *testing.T) {
 	if !found {
 		t.Fatalf("expected an active-project-changed event by claude, got %+v", act.Events)
 	}
+
+	// --- graceful handoff (§8.3): user requests, Claude acks -----------------
+	svc.RequestSwitch(activity.ActorUser, states[0].Path, "fix a bug")
+
+	res, _ = cs.CallTool(ctx, &mcpsdk.CallToolParams{Name: "get_pending_switch"})
+	var ps struct {
+		Pending bool   `json:"pending"`
+		Target  string `json:"target"`
+	}
+	decodeResult(t, res, &ps)
+	if !ps.Pending || ps.Target != states[0].Path {
+		t.Fatalf("expected pending switch to %q, got %+v", states[0].Path, ps)
+	}
+
+	res, err = cs.CallTool(ctx, &mcpsdk.CallToolParams{
+		Name:      "ack_switch",
+		Arguments: map[string]any{"summary": "left tests green"},
+	})
+	if err != nil {
+		t.Fatalf("ack_switch: %v", err)
+	}
+	if res.IsError {
+		t.Fatalf("ack_switch tool error: %+v", res.Content)
+	}
+
+	res, _ = cs.CallTool(ctx, &mcpsdk.CallToolParams{Name: "get_pending_switch"})
+	decodeResult(t, res, &ps)
+	if ps.Pending {
+		t.Fatalf("expected no pending switch after ack, got %+v", ps)
+	}
+
+	// ack with nothing pending is a tool error.
+	res, err = cs.CallTool(ctx, &mcpsdk.CallToolParams{
+		Name:      "ack_switch",
+		Arguments: map[string]any{"summary": "nothing"},
+	})
+	if err != nil {
+		t.Fatalf("ack_switch(empty) protocol error: %v", err)
+	}
+	if !res.IsError {
+		t.Fatalf("expected IsError acking with no pending switch")
+	}
 }
 
 // decodeResult unmarshals the JSON text content of a tool result into v.