fix(chatd): handle soft-deleted workspaces in chattool start/create (#22997)

## Problem

Both `start_workspace` and `create_workspace` chattool tools failed to
handle soft-deleted workspaces correctly.

Coder uses soft-delete for workspaces (`deleted = true` on the row).
Both tools called `GetWorkspaceByID`, which queries
`workspaces_expanded` with **no** `deleted = false` filter — so it
returns the workspace row even when soft-deleted. The only deletion
check was for `sql.ErrNoRows`, which never fires because the row still
exists.

### `start_workspace` behavior (before fix)
1. Loads the soft-deleted workspace successfully
2. Finds the latest build (a delete transition)
3. Falls through to attempt to **start** the deleted workspace
4. Produces a confusing downstream error

### `create_workspace` behavior (before fix)
1. `checkExistingWorkspace` loads the soft-deleted workspace
2. If a delete build is **in-progress**: waits for it, then falsely
reports `already_exists` — blocks new workspace creation
3. If the delete build **succeeded**: accidentally allows creation
(because no agents are found), but via fragile logic rather than an
explicit check

## Fix

Add `ws.Deleted` checks immediately after `GetWorkspaceByID` succeeds in
both tools:

- **`startworkspace.go`**: Returns `"workspace was deleted; use
create_workspace to make a new one"`
- **`createworkspace.go`** (`checkExistingWorkspace`): Returns `(nil,
false, nil)` to allow new workspace creation

## Tests

- `TestStartWorkspace/DeletedWorkspace` — verifies `start_workspace`
returns deleted error and never calls `StartFn`
- `TestCheckExistingWorkspace_DeletedWorkspace` — verifies
`checkExistingWorkspace` allows creation for soft-deleted workspaces

coderd/chatd/chattool/createworkspace.go

@@ -2,7 +2,6 @@ package chattool
 
 import (
 	"context"
-	"database/sql"
 	"errors"
 	"fmt"
 	"strings"
@@ -241,15 +240,14 @@ func checkExistingWorkspace(
 		return nil, false, nil
 	}
 
-	// Check if workspace still exists.
 	ws, err := db.GetWorkspaceByID(ctx, chat.WorkspaceID.UUID)
 	if err != nil {
-		if xerrors.Is(err, sql.ErrNoRows) {
-			// Workspace was deleted — allow creation.
-			return nil, false, nil
-		}
 		return nil, false, xerrors.Errorf("load workspace: %w", err)
 	}
+	// Workspace was soft-deleted — allow creation.
+	if ws.Deleted {
+		return nil, false, nil
+	}
 
 	// Check the latest build status.
 	build, err := db.GetLatestWorkspaceBuildByWorkspaceID(ctx, ws.ID)

coderd/chatd/chattool/createworkspace_test.go

@@ -108,3 +108,35 @@ func TestWaitForAgentReady(t *testing.T) {
 		require.Empty(t, result)
 	})
 }
+
+func TestCheckExistingWorkspace_DeletedWorkspace(t *testing.T) {
+	t.Parallel()
+	ctrl := gomock.NewController(t)
+	db := dbmock.NewMockStore(ctrl)
+
+	chatID := uuid.New()
+	workspaceID := uuid.New()
+
+	// Mock GetChatByID returns a chat linked to a workspace.
+	db.EXPECT().
+		GetChatByID(gomock.Any(), chatID).
+		Return(database.Chat{
+			ID:          chatID,
+			WorkspaceID: uuid.NullUUID{UUID: workspaceID, Valid: true},
+		}, nil)
+
+	// Mock GetWorkspaceByID returns a soft-deleted workspace.
+	db.EXPECT().
+		GetWorkspaceByID(gomock.Any(), workspaceID).
+		Return(database.Workspace{
+			ID:      workspaceID,
+			Deleted: true,
+		}, nil)
+
+	result, done, err := checkExistingWorkspace(
+		context.Background(), db, chatID, nil,
+	)
+	require.NoError(t, err)
+	require.False(t, done, "should allow creation for deleted workspace")
+	require.Nil(t, result)
+}

coderd/chatd/chattool/startworkspace.go

@@ -2,7 +2,6 @@ package chattool
 
 import (
 	"context"
-	"database/sql"
 	"sync"
 
 	"charm.land/fantasy"
@@ -71,15 +70,15 @@ func StartWorkspace(options StartWorkspaceOptions) fantasy.AgentTool {
 
 			ws, err := options.DB.GetWorkspaceByID(ctx, chat.WorkspaceID.UUID)
 			if err != nil {
-				if xerrors.Is(err, sql.ErrNoRows) {
-					return fantasy.NewTextErrorResponse(
-						"workspace was deleted; use create_workspace to make a new one",
-					), nil
-				}
 				return fantasy.NewTextErrorResponse(
 					xerrors.Errorf("load workspace: %w", err).Error(),
 				), nil
 			}
+			if ws.Deleted {
+				return fantasy.NewTextErrorResponse(
+					"workspace was deleted; use create_workspace to make a new one",
+				), nil
+			}
 
 			build, err := options.DB.GetLatestWorkspaceBuildByWorkspaceID(ctx, ws.ID)
 			if err != nil {

coderd/chatd/chattool/startworkspace_test.go

@@ -174,6 +174,51 @@ func TestStartWorkspace(t *testing.T) {
 		require.True(t, ok)
 		require.True(t, started)
 	})
+
+	t.Run("DeletedWorkspace", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitLong)
+		db, _ := dbtestutil.NewDB(t)
+
+		user := dbgen.User(t, db, database.User{})
+		modelCfg := seedModelConfig(ctx, t, db, user.ID)
+		org := dbgen.Organization(t, db, database.Organization{})
+		_ = dbgen.OrganizationMember(t, db, database.OrganizationMember{
+			UserID:         user.ID,
+			OrganizationID: org.ID,
+		})
+		// Create a workspace that has been soft-deleted.
+		wsResp := dbfake.WorkspaceBuild(t, db, database.WorkspaceTable{
+			OwnerID:        user.ID,
+			OrganizationID: org.ID,
+			Deleted:        true,
+		}).Seed(database.WorkspaceBuild{
+			Transition: database.WorkspaceTransitionDelete,
+		}).Do()
+		ws := wsResp.Workspace
+
+		chat, err := db.InsertChat(ctx, database.InsertChatParams{
+			OwnerID:           user.ID,
+			WorkspaceID:       uuid.NullUUID{UUID: ws.ID, Valid: true},
+			LastModelConfigID: modelCfg.ID,
+			Title:             "test-deleted-workspace",
+		})
+		require.NoError(t, err)
+
+		tool := chattool.StartWorkspace(chattool.StartWorkspaceOptions{
+			DB:     db,
+			ChatID: chat.ID,
+			StartFn: func(_ context.Context, _ uuid.UUID, _ uuid.UUID, _ codersdk.CreateWorkspaceBuildRequest) (codersdk.WorkspaceBuild, error) {
+				t.Fatal("StartFn should not be called for deleted workspace")
+				return codersdk.WorkspaceBuild{}, nil
+			},
+			WorkspaceMu: &sync.Mutex{},
+		})
+
+		resp, err := tool.Run(ctx, fantasy.ToolCall{ID: "call-1", Name: "start_workspace", Input: "{}"})
+		require.NoError(t, err)
+		require.Contains(t, resp.Content, "workspace was deleted")
+	})
 }
 
 // seedModelConfig inserts a provider and model config for testing.