diff --git a/capture_tests/cbrain_cli_commands b/capture_tests/cbrain_cli_commands
index 09154a1..028629f 100755
--- a/capture_tests/cbrain_cli_commands
+++ b/capture_tests/cbrain_cli_commands
@@ -79,10 +79,10 @@ cbrain --json  project unswitch
 cbrain --jsonl project switch 10
 cbrain --jsonl project unswitch
 
-cbrain         project switch all  # 'all' not yet implemented as of Aug 2025
+cbrain         project switch all
 cbrain --json  project switch all
 cbrain --jsonl project switch all
-cbrain         project switch 3 # need to reset for rest of test to work
+cbrain         project switch 3 # reset project filter for task list below
 
 # Tags
 cbrain         tag list
@@ -103,10 +103,7 @@ cbrain --json  tag create --name NewTag2 --user-id 2 --group-id 3
 cbrain --jsonl tag create --name NewTag3 --user-id 2 --group-id 3
 
 # Tasks
-# NOTE: we needed to reset the current project to 3 above,
-# until 'group switch all' is implemented; this means only
-# tasks in group 3 are shown below, but that's OK, that's
-# what's in the test DB anyway
+# Reset above to project 3 so task list matches seeded DB view.
 cbrain         task list
 cbrain --json  task list
 cbrain --jsonl task list
@@ -115,8 +112,11 @@ cbrain         task show 2
 cbrain --json  task show 2
 cbrain --jsonl task show 2
 
-# Not yet implemented
-cbrain         task operation   # should provide error message
+# Task operations (hold on Completed task → skipped; deterministic, no BAC)
+cbrain         task operation hold --task-id 2
+cbrain --json  task operation hold --task-id 2
+cbrain --jsonl task operation hold --task-id 2
+cbrain         task operation hold  # missing --task-id / --batch-id
 
 # ToolConfigs, as admin user
 ./switch_session admin
diff --git a/capture_tests/expected_captures.txt b/capture_tests/expected_captures.txt
index 23ab5ee..da977c9 100644
--- a/capture_tests/expected_captures.txt
+++ b/capture_tests/expected_captures.txt
@@ -558,40 +558,43 @@ Stderr:
 (No output)
 
 ############################
-Command: cbrain         project switch all  # 'all' not yet implemented as of Aug 2025
-Status: 1
-Stdout: 74 bytes
+Command: cbrain         project switch all
+Status: 0
+Stdout: 32 bytes
 Stderr: 0 bytes
 
 Stdout:
-Error: Project switch 'all' not yet implemented as of Aug 2025 (group_id)
+Current project is "all" ID=all
 Stderr:
 (No output)
 
 ############################
 Command: cbrain --json  project switch all
-Status: 1
-Stdout: 74 bytes
+Status: 0
+Stdout: 35 bytes
 Stderr: 0 bytes
 
 Stdout:
-Error: Project switch 'all' not yet implemented as of Aug 2025 (group_id)
+{
+  "id": "all",
+  "name": "all"
+}
 Stderr:
 (No output)
 
 ############################
 Command: cbrain --jsonl project switch all
-Status: 1
-Stdout: 74 bytes
+Status: 0
+Stdout: 26 bytes
 Stderr: 0 bytes
 
 Stdout:
-Error: Project switch 'all' not yet implemented as of Aug 2025 (group_id)
+{"id":"all","name":"all"}
 Stderr:
 (No output)
 
 ############################
-Command: cbrain         project switch 3 # need to reset for rest of test to work
+Command: cbrain         project switch 3 # reset project filter for task list below
 Status: 0
 Stdout: 31 bytes
 Stderr: 0 bytes
@@ -998,18 +1001,67 @@ Stderr:
 (No output)
 
 ############################
-Command: cbrain         task operation   # should provide error message
+Command: cbrain         task operation hold --task-id 2
 Status: 0
-Stdout: 41 bytes
+Stdout: 182 bytes
+Stderr: 0 bytes
+
+Stdout:
+TASK OPERATION RESULT
+------------------------------
+Operation: hold
+Succeeded: 0
+Skipped:   1
+Failed:    0
+
+SKIPPED
+------------------------------
+Tasks have incompatible states: 2
+Stderr:
+(No output)
+
+############################
+Command: cbrain --json  task operation hold --task-id 2
+Status: 0
+Stdout: 142 bytes
 Stderr: 0 bytes
 
 Stdout:
 {
-  "message": "No operation selected"
+  "skipped_list": {
+    "Tasks have incompatible states": [
+      2
+    ]
+  },
+  "success_list": [],
+  "failed_list": {},
+  "bac_ids": []
 }
 Stderr:
 (No output)
 
+############################
+Command: cbrain --jsonl task operation hold --task-id 2
+Status: 0
+Stdout: 104 bytes
+Stderr: 0 bytes
+
+Stdout:
+{"skipped_list":{"Tasks have incompatible states":[2]},"success_list":[],"failed_list":{},"bac_ids":[]}
+Stderr:
+(No output)
+
+############################
+Command: cbrain         task operation hold  # missing --task-id / --batch-id
+Status: 1
+Stdout: 68 bytes
+Stderr: 0 bytes
+
+Stdout:
+Error: At least one --task-id or --batch-id is required (--task-id)
+Stderr:
+(No output)
+
 ############################
 Command: ./switch_session admin
 Status: 0
@@ -1187,36 +1239,36 @@ Stderr:
 ############################
 Command: cbrain         tool-config show 19 # visible to user admin
 Status: 1
-Stdout: 107 bytes
+Stdout: 117 bytes
 Stderr: 0 bytes
 
 Stdout:
 Authentication error (401): Unauthorized
-Error: Access denied. Please log in using authorized credentials.
+Error: Session expired or invalid. Run 'cbrain logout' then 'cbrain login'.
 Stderr:
 (No output)
 
 ############################
 Command: cbrain --json  tool-config show 19
 Status: 1
-Stdout: 107 bytes
+Stdout: 117 bytes
 Stderr: 0 bytes
 
 Stdout:
 Authentication error (401): Unauthorized
-Error: Access denied. Please log in using authorized credentials.
+Error: Session expired or invalid. Run 'cbrain logout' then 'cbrain login'.
 Stderr:
 (No output)
 
 ############################
 Command: cbrain --jsonl tool-config show 19
 Status: 1
-Stdout: 107 bytes
+Stdout: 117 bytes
 Stderr: 0 bytes
 
 Stdout:
 Authentication error (401): Unauthorized
-Error: Access denied. Please log in using authorized credentials.
+Error: Session expired or invalid. Run 'cbrain logout' then 'cbrain login'.
 Stderr:
 (No output)
 
diff --git a/cbrain_cli/cli_utils.py b/cbrain_cli/cli_utils.py
index 0594645..c8e616d 100644
--- a/cbrain_cli/cli_utils.py
+++ b/cbrain_cli/cli_utils.py
@@ -13,6 +13,20 @@
 from cbrain_cli import config as cbrain_config
 from cbrain_cli.config import DEFAULT_HEADERS, DEFAULT_TIMEOUT, auth_headers
 
+_debug = False
+
+
+def set_debug(flag: bool) -> None:
+    """Enable or disable debug output."""
+    global _debug
+    _debug = bool(flag)
+
+
+def debug_log(message: str) -> None:
+    """Print a debug line to stderr when debug mode is active."""
+    if _debug:
+        print(f"[DEBUG] {message}", file=sys.stderr)
+
 
 class CbrainClient:
     """
@@ -46,11 +60,23 @@ def _request(self, method, path, *, headers=None, body=None, params=None):
         if params:
             target = f"{target}?{urllib.parse.urlencode(params)}"
         hdrs = headers or auth_headers(self.token)
+        # strip host from full URL for debug display; preserve all query params
+        parsed = urllib.parse.urlsplit(target)
+        display_path = parsed.path
+        if display_path.startswith(self.base_url):
+            display_path = display_path[len(self.base_url) :]
+        query = urllib.parse.urlencode(params) if params else parsed.query
+        if query:
+            display_path = f"{display_path}?{query}"
+        debug_log(f"{method} {display_path}")
         req = urllib.request.Request(target, data=body, headers=hdrs, method=method)
         try:
             with urllib.request.urlopen(req, timeout=self.timeout) as r:
-                return r.read(), r.status
+                status = r.status
+                debug_log(f"→ HTTP {status}")
+                return r.read(), status
         except urllib.error.HTTPError as e:
+            debug_log(f"→ HTTP {e.code} ({e.reason})")
             raise CliApiError(e.reason or f"HTTP {e.code}", status=e.code) from e
 
     def get(self, path, params=None):
@@ -95,7 +121,7 @@ def post_multipart(self, path, body, content_type):
 
 PAGINATABLE_ACTIONS = {
     ("file", "list"),
-    ("dataprovider", "list"),
+    ("data-provider", "list"),
     ("tool", "list"),
     ("tool-config", "list"),
     ("tag", "list"),
@@ -204,7 +230,7 @@ def handle_connection_error(error):
 
         if error.code == 401:
             print(f"{status_description}: {error.reason}")
-            print("Error: Access denied. Please log in using authorized credentials.")
+            print("Error: Session expired or invalid. Run 'cbrain logout' then 'cbrain login'.")
         elif error.code in (400, 404, 422, 500):
             # Try to extract specific error message from response
             try:
diff --git a/cbrain_cli/data/files.py b/cbrain_cli/data/files.py
index d7e2548..059e728 100644
--- a/cbrain_cli/data/files.py
+++ b/cbrain_cli/data/files.py
@@ -92,7 +92,9 @@ def _change_provider(args, operation):
     if not file_ids:
         raise CliValidationError("File ID(s) are required", field="--file-id")
     if not dest_provider_id:
-        raise CliValidationError("Destination data provider ID is required", field="--dp-id")
+        raise CliValidationError(
+            "Destination data provider ID is required", field="--data-provider-id"
+        )
     payload = {
         "file_ids": file_ids,
         "data_provider_id_for_mv_cp": dest_provider_id,
diff --git a/cbrain_cli/data/projects.py b/cbrain_cli/data/projects.py
index 95a7b07..66b7fb4 100644
--- a/cbrain_cli/data/projects.py
+++ b/cbrain_cli/data/projects.py
@@ -25,23 +25,25 @@ def switch_project(args):
     if not group_id:
         raise CliValidationError("Group ID is required", field="group_id")
 
-    if group_id == "all":
-        raise CliValidationError(
-            "Project switch 'all' not yet implemented as of Aug 2025", field="group_id"
-        )
-
-    try:
-        group_id = int(group_id)
-    except ValueError:
-        raise CliValidationError(
-            f"Invalid group ID '{group_id}'. Must be a number or 'all'", field="group_id"
-        ) from None
+    # Server accepts numeric id or "all" (no single project filter).
+    if group_id != "all":
+        try:
+            group_id = int(group_id)
+        except ValueError:
+            raise CliValidationError(
+                f"Invalid group ID '{group_id}'. Must be a number or 'all'", field="group_id"
+            ) from None
 
     client = CbrainClient.from_credentials()
     _, switch_status = client.send("POST", f"/groups/switch?id={group_id}")
     if switch_status not in (200, 201, 204):
         raise CliApiError(f"Failed to switch project (HTTP {switch_status})")
-    group_data = client.get(f"/groups/{group_id}")
+
+    # "all" is session state only — no /groups/all resource.
+    if group_id == "all":
+        group_data = {"id": "all", "name": "all"}
+    else:
+        group_data = client.get(f"/groups/{group_id}")
 
     credentials = load_credentials()
     if credentials is not None:
@@ -124,6 +126,13 @@ def show_project(args):
     if not current_group_id:
         return None
 
+    # Session "all" has no Group row; mirror switch_project synthetic result.
+    if current_group_id == "all":
+        return {
+            "id": "all",
+            "name": credentials.get("current_group_name") or "all",
+        }
+
     try:
         return CbrainClient.from_credentials().get(f"/groups/{current_group_id}")
     except CliApiError as e:
diff --git a/cbrain_cli/data/tasks.py b/cbrain_cli/data/tasks.py
index 672cc3d..524238b 100644
--- a/cbrain_cli/data/tasks.py
+++ b/cbrain_cli/data/tasks.py
@@ -1,10 +1,28 @@
 from cbrain_cli.cli_utils import (
     CbrainClient,
     CliValidationError,
-    json_printer,
     pagination,
 )
 
+# Names accepted by CBRAIN POST /tasks/operation
+TASK_OPERATIONS = (
+    "terminate",
+    "archive",
+    "archive_file",
+    "unarchive",
+    "zap_wd",
+    "save_wd",
+    "hold",
+    "release",
+    "suspend",
+    "resume",
+    "duplicate",
+    "recover",
+    "restart_setup",
+    "restart_cluster",
+    "restart_postprocess",
+)
+
 
 def list_tasks(args):
     """
@@ -65,7 +83,39 @@ def show_task(args):
 
 def operation_task(args):
     """
-    Operation on a task.
+    Run a bulk operation on tasks.
     """
-    data, _ = CbrainClient.from_credentials().send("POST", "/tasks/operation")
-    json_printer(data)
+    operation = getattr(args, "operation", None)
+    if not operation:
+        raise CliValidationError("Operation is required", field="operation")
+    if operation not in TASK_OPERATIONS:
+        raise CliValidationError(
+            f"Unsupported operation: {operation}",
+            field="operation",
+        )
+
+    task_ids = getattr(args, "task_id", None) or []
+    batch_ids = getattr(args, "batch_id", None) or []
+    if not task_ids and not batch_ids:
+        raise CliValidationError(
+            "At least one --task-id or --batch-id is required",
+            field="--task-id",
+        )
+
+    payload = {"operation": operation}
+    if task_ids:
+        payload["tasklist"] = list(task_ids)
+    if batch_ids:
+        payload["batch_ids"] = list(batch_ids)
+
+    dup_bourreau_id = getattr(args, "dup_bourreau_id", None)
+    if dup_bourreau_id is not None:
+        payload["dup_bourreau_id"] = dup_bourreau_id
+    archive_dp_id = getattr(args, "archive_dp_id", None)
+    if archive_dp_id is not None:
+        payload["archive_dp_id"] = archive_dp_id
+    if getattr(args, "nozip", False):
+        payload["nozip"] = True
+
+    data, _ = CbrainClient.from_credentials().send("POST", "/tasks/operation", payload=payload)
+    return data
diff --git a/cbrain_cli/formatter/tasks_fmt.py b/cbrain_cli/formatter/tasks_fmt.py
index 972606b..14542e7 100644
--- a/cbrain_cli/formatter/tasks_fmt.py
+++ b/cbrain_cli/formatter/tasks_fmt.py
@@ -1,6 +1,10 @@
 import json
 
-from cbrain_cli.cli_utils import display_key_value_table, dynamic_table_print, output_json
+from cbrain_cli.cli_utils import (
+    display_key_value_table,
+    dynamic_table_print,
+    output_json,
+)
 
 
 def print_task_data(tasks_data, args):
@@ -117,3 +121,68 @@ def print_task_details(task_data, args):
         print("PARAMETERS")
         print("-" * 30)
         print(json.dumps(task_data.get("params"), indent=2))
+
+
+def print_task_operation_result(result, args):
+    """
+    Print the result of a bulk task operation.
+
+    Parameters
+    ----------
+    result : dict
+        Operation result payload from the API
+    args : argparse.Namespace
+        Command line arguments, including the --json flag
+    """
+    if output_json(args, result):
+        return
+
+    if not result:
+        print("No operation result.")
+        return
+
+    # Server sometimes returns a single message (e.g. no operation / no tasks).
+    if "message" in result and "success_list" not in result:
+        print(result["message"])
+        return
+
+    operation = getattr(args, "operation", None)
+    success = result.get("success_list") or []
+    failed = result.get("failed_list") or {}
+    skipped = result.get("skipped_list") or {}
+    bac_ids = result.get("bac_ids") or []
+
+    print("TASK OPERATION RESULT")
+    print("-" * 30)
+    if operation:
+        print(f"Operation: {operation}")
+    print(f"Succeeded: {len(success)}")
+    print(f"Skipped:   {sum(len(v) for v in skipped.values())}")
+    print(f"Failed:    {sum(len(v) for v in failed.values())}")
+
+    if success:
+        print()
+        print("SUCCEEDED TASKS")
+        print("-" * 30)
+        print(", ".join(str(task_id) for task_id in success))
+
+    if skipped:
+        print()
+        print("SKIPPED")
+        print("-" * 30)
+        for reason, ids in skipped.items():
+            print(f"{reason}: {', '.join(str(i) for i in ids)}")
+
+    if failed:
+        print()
+        print("FAILED")
+        print("-" * 30)
+        for reason, ids in failed.items():
+            print(f"{reason}: {', '.join(str(i) for i in ids)}")
+
+    if bac_ids:
+        print()
+        print("BACKGROUND ACTIVITIES")
+        print("-" * 30)
+        print(", ".join(str(bac_id) for bac_id in bac_ids))
+        print("Track with: cbrain background show <id>")
diff --git a/cbrain_cli/handlers.py b/cbrain_cli/handlers.py
index b64b032..201569d 100644
--- a/cbrain_cli/handlers.py
+++ b/cbrain_cli/handlers.py
@@ -315,6 +315,12 @@ def handle_task_show(args):
     tasks_fmt.print_task_details(result, args)
 
 
+def handle_task_operation(args):
+    """Run a task operation and display the result."""
+    result = tasks.operation_task(args)
+    tasks_fmt.print_task_operation_result(result, args)
+
+
 # Remote resource command handlers
 def handle_remote_resource_list(args):
     """Retrieve and display a list of remote computational resources available in CBRAIN."""
diff --git a/cbrain_cli/main.py b/cbrain_cli/main.py
index c610304..18ffbfc 100644
--- a/cbrain_cli/main.py
+++ b/cbrain_cli/main.py
@@ -11,9 +11,10 @@
     handle_errors,
     is_authenticated,
     pagination,
+    set_debug,
     version_info,
 )
-from cbrain_cli.data.tasks import operation_task
+from cbrain_cli.data.tasks import TASK_OPERATIONS
 from cbrain_cli.handlers import (
     handle_background_list,
     handle_background_show,
@@ -39,6 +40,7 @@
     handle_tag_show,
     handle_tag_update,
     handle_task_list,
+    handle_task_operation,
     handle_task_show,
     handle_tool_config_boutiques_descriptor,
     handle_tool_config_list,
@@ -68,6 +70,13 @@ def build_parser():
         action="store_true",
         help="Output in JSONL format (one JSON object per line)",
     )
+    parser.add_argument(
+        "--debug",
+        "--verbose",
+        dest="debug",
+        action="store_true",
+        help="Print sanitized request/response diagnostics to stderr",
+    )
 
     subparsers = parser.add_subparsers(dest="command", help="Available commands")
 
@@ -100,7 +109,12 @@ def build_parser():
         "--group-id", dest="group_id", type=int, help="Filter files by group ID"
     )
     file_list_parser.add_argument(
-        "--dp-id", dest="dp_id", type=int, help="Filter files by data provider ID"
+        "--data-provider-id",
+        "--data-provider",
+        "--dp-id",
+        dest="dp_id",
+        type=int,
+        help="Filter files by data provider ID",
     )
     file_list_parser.add_argument(
         "--user-id", dest="user_id", type=int, help="Filter files by user ID"
@@ -130,11 +144,13 @@ def build_parser():
     file_upload_parser = file_subparsers.add_parser("upload", help="Upload a file to CBRAIN")
     file_upload_parser.add_argument("file_path", help="Path to the file to upload")
     file_upload_parser.add_argument(
+        "--data-provider-id",
         "--data-provider",
+        "--dp-id",
         dest="data_provider",
         type=int,
         required=True,
-        help="Data provider ID",
+        help="Destination data provider ID",
     )
     file_upload_parser.add_argument("--group-id", dest="group_id", type=int, help="Group ID")
 
@@ -153,6 +169,8 @@ def build_parser():
         help="One or more file IDs to copy",
     )
     file_copy_parser.add_argument(
+        "--data-provider-id",
+        "--data-provider",
         "--dp-id",
         dest="dp_id",
         type=int,
@@ -174,6 +192,8 @@ def build_parser():
         help="One or more file IDs to move",
     )
     file_move_parser.add_argument(
+        "--data-provider-id",
+        "--data-provider",
         "--dp-id",
         dest="dp_id",
         type=int,
@@ -190,53 +210,57 @@ def build_parser():
     )
     file_delete_parser.set_defaults(func=handle_errors(handle_file_delete))
 
-    # Data provider commands
-    dataprovider_parser = subparsers.add_parser("dataprovider", help="Data provider operations")
-    dataprovider_subparsers = dataprovider_parser.add_subparsers(
+    # Data provider commands (alias: dataprovider)
+    data_provider_parser = subparsers.add_parser(
+        "data-provider",
+        aliases=["dataprovider"],
+        help="Data provider operations",
+    )
+    data_provider_subparsers = data_provider_parser.add_subparsers(
         dest="action", help="Data provider actions"
     )
 
-    # dataprovider list
-    dataprovider_list_parser = dataprovider_subparsers.add_parser(
+    # data-provider list
+    data_provider_list_parser = data_provider_subparsers.add_parser(
         "list", help="List data providers"
     )
-    dataprovider_list_parser.set_defaults(func=handle_errors(handle_dataprovider_list))
+    data_provider_list_parser.set_defaults(func=handle_errors(handle_dataprovider_list))
 
-    dataprovider_list_parser.add_argument(
+    data_provider_list_parser.add_argument(
         "--page", type=int, default=1, help="Page number (default: 1)"
     )
-    dataprovider_list_parser.add_argument(
+    data_provider_list_parser.add_argument(
         "--per-page",
         type=int,
         default=25,
         help="Number of data providers per page (5-1000, default: 25)",
     )
-    # dataprovider show
-    dataprovider_show_parser = dataprovider_subparsers.add_parser(
+    # data-provider show
+    data_provider_show_parser = data_provider_subparsers.add_parser(
         "show", help="Show data provider details"
     )
-    dataprovider_show_parser.add_argument("id", type=int, help="Data provider ID")
-    dataprovider_show_parser.set_defaults(func=handle_errors(handle_dataprovider_show))
+    data_provider_show_parser.add_argument("id", type=int, help="Data provider ID")
+    data_provider_show_parser.set_defaults(func=handle_errors(handle_dataprovider_show))
 
-    # dataprovider is_alive
-    dataprovider_is_alive_parser = dataprovider_subparsers.add_parser(
-        "is-alive", help="Check if a data provider is alive"
+    # data-provider is-alive
+    data_provider_is_alive_parser = data_provider_subparsers.add_parser(
+        "is-alive", help="Check if a data provider is reachable"
     )
-    dataprovider_is_alive_parser.add_argument("id", type=int, help="Data provider ID")
-    dataprovider_is_alive_parser.set_defaults(func=handle_errors(handle_dataprovider_is_alive))
+    data_provider_is_alive_parser.add_argument("id", type=int, help="Data provider ID")
+    data_provider_is_alive_parser.set_defaults(func=handle_errors(handle_dataprovider_is_alive))
 
-    # dataprovider delete-unregistered-files
-    dataprovider_delete_unregistered_files_parser = dataprovider_subparsers.add_parser(
+    # data-provider delete-unregistered-files
+    data_provider_delete_unregistered_files_parser = data_provider_subparsers.add_parser(
         "delete-unregistered-files",
         help="Delete unregistered files from a data provider",
     )
-    dataprovider_delete_unregistered_files_parser.add_argument(
+    data_provider_delete_unregistered_files_parser.add_argument(
         "id", type=int, help="Data provider ID"
     )
-    dataprovider_delete_unregistered_files_parser.add_argument(
+    data_provider_delete_unregistered_files_parser.add_argument(
         "-y", "--yes", action="store_true", help="Skip confirmation prompt"
     )
-    dataprovider_delete_unregistered_files_parser.set_defaults(
+    data_provider_delete_unregistered_files_parser.set_defaults(
         func=handle_errors(handle_dataprovider_delete_unregistered)
     )
 
@@ -411,7 +435,7 @@ def build_parser():
         type=lambda value: value.replace("-", "_"),
         choices=["bourreau_id"],
         metavar="bourreau-id",
-        help="Filter type (optional)",
+        help="Filter by bourreau (execution server) ID",
     )
     task_list_parser.add_argument("--page", type=int, default=1, help="Page number (default: 1)")
     task_list_parser.add_argument(
@@ -425,7 +449,7 @@ def build_parser():
         "bourreau_id",
         type=int,
         nargs="?",
-        help="Bourreau ID (required when filter is bourreau-id)",
+        help="Bourreau (execution server) ID; required with bourreau-id",
     )
     task_list_parser.set_defaults(func=handle_errors(handle_task_list))
 
@@ -435,12 +459,51 @@ def build_parser():
     task_show_parser.set_defaults(func=handle_errors(handle_task_show))
 
     # task operation
-    task_operation_parser = task_subparsers.add_parser("operation", help="operation on a task")
-    task_operation_parser.set_defaults(func=handle_errors(operation_task))
+    task_operation_parser = task_subparsers.add_parser(
+        "operation", help="Run a bulk operation on tasks"
+    )
+    task_operation_parser.add_argument(
+        "operation",
+        choices=TASK_OPERATIONS,
+        help="Operation to run (e.g. terminate, archive, hold, recover)",
+    )
+    task_operation_parser.add_argument(
+        "--task-id",
+        dest="task_id",
+        type=int,
+        nargs="+",
+        help="One or more task IDs",
+    )
+    task_operation_parser.add_argument(
+        "--batch-id",
+        dest="batch_id",
+        type=int,
+        nargs="+",
+        help="One or more batch IDs (expands to member tasks)",
+    )
+    task_operation_parser.add_argument(
+        "--dup-bourreau-id",
+        dest="dup_bourreau_id",
+        type=int,
+        help="Target bourreau ID for duplicate",
+    )
+    task_operation_parser.add_argument(
+        "--archive-dp-id",
+        dest="archive_dp_id",
+        type=int,
+        help="Data provider ID for archive_file",
+    )
+    task_operation_parser.add_argument(
+        "--nozip",
+        action="store_true",
+        help="Archive without compression (admin only)",
+    )
+    task_operation_parser.set_defaults(func=handle_errors(handle_task_operation))
 
-    # Remote resource commands
+    # Remote resource commands (CBRAIN bourreaux / execution servers)
     remote_resource_parser = subparsers.add_parser(
-        "remote-resource", help="Remote resource operations"
+        "remote-resource",
+        help="Remote resource (bourreau / execution server) operations",
     )
     remote_resource_subparsers = remote_resource_parser.add_subparsers(
         dest="action", help="Remote resource actions"
@@ -456,12 +519,16 @@ def build_parser():
     remote_resource_show_parser = remote_resource_subparsers.add_parser(
         "show", help="Show remote resource details"
     )
-    remote_resource_show_parser.add_argument("remote_resource", type=int, help="Remote resource ID")
+    remote_resource_show_parser.add_argument(
+        "remote_resource",
+        type=int,
+        help="Remote resource (bourreau) ID",
+    )
     remote_resource_show_parser.set_defaults(func=handle_errors(handle_remote_resource_show))
 
     command_parsers = {
         "file": file_parser,
-        "dataprovider": dataprovider_parser,
+        "data-provider": data_provider_parser,
         "project": project_parser,
         "tool": tool_parser,
         "tool-config": tool_configs_parser,
@@ -490,10 +557,15 @@ def main(argv=None):
     parser, command_parsers = build_parser()
     args = parser.parse_args(argv)
 
+    set_debug(getattr(args, "debug", False))
+
     if not args.command:
         parser.print_help()
         return
 
+    if args.command == "dataprovider":
+        args.command = "data-provider"
+
     if (args.command, getattr(args, "action", None)) in PAGINATABLE_ACTIONS:
         try:
             pagination(args, {})
@@ -518,7 +590,7 @@ def main(argv=None):
     # Handle authenticated commands.
     if args.command in [
         "file",
-        "dataprovider",
+        "data-provider",
         "project",
         "tool",
         "tool-config",
diff --git a/cbrain_cli/sessions.py b/cbrain_cli/sessions.py
index 76e44de..6d27cec 100644
--- a/cbrain_cli/sessions.py
+++ b/cbrain_cli/sessions.py
@@ -30,8 +30,27 @@ def create_session(args):
     if cbrain_config.CREDENTIALS_FILE.exists():
         creds = cbrain_config.load_credentials()
         if creds and creds.get("api_token") and creds.get("cbrain_url"):
-            print("Already logged in. Use 'cbrain logout' to logout.")
-            return 1
+            # File alone is not enough, probe server to detect expired tokens.
+            try:
+                CbrainClient.from_credentials().get("/session")
+            except CliApiError as e:
+                if e.status == 401:
+                    print("Saved session expired. Please log in again.")
+                elif e.status >= 500:
+                    print(f"Server returned HTTP {e.status} during session check.")
+                    print("The server may be temporarily unavailable. Try again later.")
+                    return 1
+                else:
+                    print(f"Server returned HTTP {e.status} during session check.")
+                    print("Use 'cbrain logout' to reset local credentials.")
+                    return 1
+            except urllib.error.URLError:
+                print(f"Cannot reach CBRAIN server at {creds['cbrain_url']}.")
+                print("Check your connection. Use 'cbrain logout' to reset local credentials.")
+                return 1
+            else:
+                print("Already logged in. Use 'cbrain logout' to logout.")
+                return 1
 
     # Get user input.
     cbrain_url = input("Enter CBRAIN server base URL [default: localhost:3000]: ").strip()
diff --git a/tests/test_cbrain_client.py b/tests/test_cbrain_client.py
index 14942c1..2b5a0dd 100644
--- a/tests/test_cbrain_client.py
+++ b/tests/test_cbrain_client.py
@@ -2,11 +2,20 @@
 
 import pytest
 
+import cbrain_cli.cli_utils as cu
 from cbrain_cli.cli_utils import CbrainClient, CliApiError
 from cbrain_cli.config import DEFAULT_TIMEOUT
 from tests.conftest import TOKEN, URL, install_auth
 
 
+@pytest.fixture
+def debug_mode():
+    """Enable debug mode for one test; always resets to False afterward."""
+    cu.set_debug(True)
+    yield
+    cu.set_debug(False)
+
+
 @pytest.fixture
 def client():
     return CbrainClient(URL, TOKEN, user_id=42)
@@ -108,3 +117,22 @@ def test_post_multipart(client, capture_urlopen):
     assert data == {"id": 1} and status == 201
     assert "multipart/form-data" in captured["headers"].get("Content-type", "")
     assert captured["headers"].get("Authorization") == f"Bearer {TOKEN}"
+
+
+def test_debug_off_by_default(capsys):
+    cu.debug_log("should not appear")
+    assert capsys.readouterr().err == ""
+
+
+def test_debug_lines_reach_stderr(debug_mode, capsys):
+    cu.debug_log("hello world")
+    assert "[DEBUG] hello world" in capsys.readouterr().err
+
+
+def test_debug_no_token_in_output(debug_mode, client, capture_urlopen, capsys):
+    configure, _ = capture_urlopen
+    configure({})
+    client.get("/tools")
+    err = capsys.readouterr().err
+    assert TOKEN not in err
+    assert "[DEBUG]" in err
diff --git a/tests/test_exit_codes.py b/tests/test_exit_codes.py
index 314b3a1..2d65df4 100644
--- a/tests/test_exit_codes.py
+++ b/tests/test_exit_codes.py
@@ -103,7 +103,8 @@ def test_handle_connection_error_401(capsys):
     handle_connection_error(HTTPError(URL, 401, "Unauthorized", {}, io.BytesIO(b"")))
     out = capsys.readouterr().out
     assert "Authentication error (401)" in out
-    assert "authorized credentials" in out
+    assert "cbrain logout" in out
+    assert "cbrain login" in out
 
 
 def test_handle_connection_error_json_body(capsys):
diff --git a/tests/test_formatters.py b/tests/test_formatters.py
index 8a70862..40a8b00 100644
--- a/tests/test_formatters.py
+++ b/tests/test_formatters.py
@@ -296,3 +296,36 @@ def test_print_task_details_normal(capsys):
     out = capsys.readouterr().out
     assert "Ready" in out
     assert "PARAMETERS" in out
+
+
+def test_print_task_operation_result_summary(capsys):
+    tasks_fmt.print_task_operation_result(
+        {
+            "success_list": [1, 2],
+            "skipped_list": {"Tasks have incompatible states": [3]},
+            "failed_list": {},
+            "bac_ids": [33],
+        },
+        make_args(operation="terminate"),
+    )
+    out = capsys.readouterr().out
+    assert "Operation: terminate" in out
+    assert "Succeeded: 2" in out
+    assert "Skipped:   1" in out
+    assert "3" in out
+    assert "33" in out
+    assert "cbrain background show" in out
+
+
+def test_print_task_operation_result_message(capsys):
+    tasks_fmt.print_task_operation_result(
+        {"message": "No tasks selected"},
+        make_args(),
+    )
+    assert capsys.readouterr().out.strip() == "No tasks selected"
+
+
+def test_print_task_operation_result_json(capsys):
+    data = {"success_list": [1], "bac_ids": [9]}
+    tasks_fmt.print_task_operation_result(data, make_args(json=True))
+    assert parse_json_output(capsys) == data
diff --git a/tests/test_handlers.py b/tests/test_handlers.py
index 56b7218..4538e6b 100644
--- a/tests/test_handlers.py
+++ b/tests/test_handlers.py
@@ -8,6 +8,7 @@
     handle_project_switch,
     handle_project_unswitch,
     handle_task_list,
+    handle_task_operation,
     handle_task_show,
 )
 from cbrain_cli.users import user_details, whoami_user
@@ -153,6 +154,48 @@ def test_list_handler_validation_error_returns_1(monkeypatch):
     assert handle_errors(handle_task_list)(make_args()) == 1
 
 
+def test_handle_task_operation_success(monkeypatch, capsys):
+    monkeypatch.setattr(
+        "cbrain_cli.handlers.tasks.operation_task",
+        lambda _: {
+            "success_list": [1],
+            "failed_list": {},
+            "skipped_list": {},
+            "bac_ids": [33],
+        },
+    )
+    assert handle_task_operation(make_args(operation="terminate")) is None
+    out = capsys.readouterr().out
+    assert "TASK OPERATION RESULT" in out
+    assert "Succeeded: 1" in out
+    assert "33" in out
+
+
+def test_handle_task_operation_empty_data(monkeypatch, capsys):
+    """Empty dict is domain data — formatter prints it, handler does not return 1."""
+    monkeypatch.setattr("cbrain_cli.handlers.tasks.operation_task", lambda _: {})
+    assert handle_task_operation(make_args()) is None
+    assert "No operation result." in capsys.readouterr().out
+
+
+def test_handle_task_operation_validation_error_returns_1(monkeypatch, capsys):
+    monkeypatch.setattr(
+        "cbrain_cli.handlers.tasks.operation_task",
+        MagicMock(side_effect=CliValidationError("bad operation")),
+    )
+    assert handle_errors(handle_task_operation)(make_args()) == 1
+    assert "bad operation" in capsys.readouterr().out
+
+
+def test_handle_task_operation_api_error_returns_1(monkeypatch, capsys):
+    monkeypatch.setattr(
+        "cbrain_cli.handlers.tasks.operation_task",
+        MagicMock(side_effect=CliApiError("Forbidden", status=403)),
+    )
+    assert handle_errors(handle_task_operation)(make_args()) == 1
+    assert "Forbidden" in capsys.readouterr().out
+
+
 def test_user_details_sends_current_token_in_header(monkeypatch, capture_urlopen, creds_file):
     """auth_headers(api_token) uses call-time credentials, not import-time globals."""
     from tests.conftest import write_auth_credentials
diff --git a/tests/test_main_dispatch.py b/tests/test_main_dispatch.py
index 79c81b2..6df1377 100644
--- a/tests/test_main_dispatch.py
+++ b/tests/test_main_dispatch.py
@@ -69,3 +69,21 @@ def test_main_no_command_prints_help(capsys):
 def test_main_missing_subcommand_action_returns_1(monkeypatch, fake_credentials, capsys):
     result = run_main(monkeypatch, ["cbrain", "file"])
     assert result == 1
+
+
+def test_main_dataprovider_alias_dispatches(monkeypatch, fake_credentials, capture_urlopen):
+    install_auth()
+    configure, captured = capture_urlopen
+    configure([])
+    result = run_main(monkeypatch, ["cbrain", "dataprovider", "list"])
+    assert result is None
+    assert "/data_providers" in captured["url"]
+
+
+def test_main_data_provider_canonical_dispatches(monkeypatch, fake_credentials, capture_urlopen):
+    install_auth()
+    configure, captured = capture_urlopen
+    configure([])
+    result = run_main(monkeypatch, ["cbrain", "data-provider", "list"])
+    assert result is None
+    assert "/data_providers" in captured["url"]
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 9a0ed01..b6c9dd0 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -18,6 +18,27 @@ def test_task_list_bourreau_id_args():
     assert args.bourreau_id == 7
 
 
+def test_task_operation_args():
+    parser, _command_parsers = build_parser()
+    args = parser.parse_args(["task", "operation", "terminate", "--task-id", "1", "2", "--nozip"])
+    assert args.action == "operation"
+    assert args.operation == "terminate"
+    assert args.task_id == [1, 2]
+    assert args.nozip is True
+
+
+def test_global_debug_flag():
+    parser, _command_parsers = build_parser()
+    args = parser.parse_args(["--debug", "task", "list"])
+    assert args.debug is True
+
+
+def test_global_verbose_alias():
+    parser, _command_parsers = build_parser()
+    args = parser.parse_args(["--verbose", "task", "list"])
+    assert args.debug is True
+
+
 def test_file_list_kebab_options_normalize_to_snake_case():
     parser, _command_parsers = build_parser()
     args = parser.parse_args(
@@ -26,7 +47,7 @@ def test_file_list_kebab_options_normalize_to_snake_case():
             "list",
             "--group-id",
             "5",
-            "--dp-id",
+            "--data-provider-id",
             "9",
             "--file-type",
             "TextFile",
@@ -40,6 +61,29 @@ def test_file_list_kebab_options_normalize_to_snake_case():
     assert args.per_page == 50
 
 
+def test_file_dp_id_aliases():
+    parser, _command_parsers = build_parser()
+    for flag in ("--data-provider-id", "--data-provider", "--dp-id"):
+        args = parser.parse_args(["file", "list", flag, "3"])
+        assert args.dp_id == 3
+
+
+def test_file_upload_data_provider_aliases():
+    parser, _command_parsers = build_parser()
+    for flag in ("--data-provider-id", "--data-provider", "--dp-id"):
+        args = parser.parse_args(["file", "upload", "/tmp/x", flag, "15"])
+        assert args.data_provider == 15
+
+
+def test_data_provider_command_and_alias():
+    parser, _command_parsers = build_parser()
+    canonical = parser.parse_args(["data-provider", "list"])
+    alias = parser.parse_args(["dataprovider", "list"])
+    assert canonical.command == "data-provider"
+    assert alias.command == "dataprovider"
+    assert canonical.action == alias.action == "list"
+
+
 def test_tag_create_kebab_options_normalize_to_snake_case():
     parser, _command_parsers = build_parser()
     args = parser.parse_args(
@@ -97,7 +141,7 @@ def test_command_parsers_include_model_commands():
     _parser, command_parsers = build_parser()
     for command in (
         "file",
-        "dataprovider",
+        "data-provider",
         "project",
         "tool",
         "tool-config",
diff --git a/tests/test_projects.py b/tests/test_projects.py
index 00181e5..f97605f 100644
--- a/tests/test_projects.py
+++ b/tests/test_projects.py
@@ -69,9 +69,29 @@ def test_switch_project_missing_group_id_raises():
         switch_project(make_args(group_id=None))
 
 
-def test_switch_project_all_raises():
-    with pytest.raises(CliValidationError):
-        switch_project(make_args(group_id="all"))
+def test_switch_project_all_saves_credentials(monkeypatch, creds_file):
+    from tests.conftest import write_auth_credentials
+
+    write_auth_credentials(creds_file)
+
+    switch_response = MagicMock()
+    switch_response.__enter__.return_value.read.return_value = b""
+    switch_response.__enter__.return_value.status = 200
+    monkeypatch.setattr("urllib.request.urlopen", MagicMock(side_effect=[switch_response]))
+
+    result = switch_project(make_args(group_id="all"))
+    assert result == {"id": "all", "name": "all"}
+    saved = json.loads(creds_file.read_text())
+    assert saved["current_group_id"] == "all"
+    assert saved["current_group_name"] == "all"
+
+
+def test_show_project_all_returns_synthetic(creds_file):
+    from tests.conftest import write_auth_credentials
+
+    write_auth_credentials(creds_file, current_group_id="all", current_group_name="all")
+    result = show_project(make_args(project_id=None))
+    assert result == {"id": "all", "name": "all"}
 
 
 def test_switch_project_invalid_string_raises():
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 85ac15c..e031c31 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -1,4 +1,5 @@
 import argparse
+import urllib.error
 
 import pytest
 
@@ -6,17 +7,44 @@
 from cbrain_cli.sessions import create_session, logout_session
 
 
-def test_create_session_already_logged_in(sessions_creds_file, capsys):
+def test_create_session_already_logged_in(sessions_creds_file, monkeypatch, capsys):
     import json
 
     sessions_creds_file.write_text(
-        json.dumps({"api_token": "tok", "cbrain_url": "http://localhost:3000"})
+        json.dumps({"api_token": "tok", "cbrain_url": "http://localhost:3000", "user_id": 1})
+    )
+    monkeypatch.setattr(
+        "cbrain_cli.cli_utils.CbrainClient.get",
+        lambda self, *_: {"user_id": 1, "cbrain_api_token": "tok"},
     )
     result = create_session(argparse.Namespace())
     assert result == 1
     assert "Already logged in" in capsys.readouterr().out
 
 
+def test_create_session_expired_token_allows_relogin(sessions_creds_file, monkeypatch, capsys):
+    import json
+
+    sessions_creds_file.write_text(
+        json.dumps({"api_token": "dead", "cbrain_url": "http://localhost:3000", "user_id": 1})
+    )
+
+    def _raise_401(self, *_):
+        raise CliApiError("Unauthorized", status=401)
+
+    monkeypatch.setattr("cbrain_cli.cli_utils.CbrainClient.get", _raise_401)
+    inputs = iter(["http://localhost:3000", "admin"])
+    monkeypatch.setattr("builtins.input", lambda _: next(inputs))
+    monkeypatch.setattr("getpass.getpass", lambda _: "secret")
+    monkeypatch.setattr(
+        "cbrain_cli.cli_utils.CbrainClient.post_form",
+        lambda self, *_a, **_k: {"cbrain_api_token": "newtok", "user_id": 1},
+    )
+    result = create_session(argparse.Namespace())
+    assert result == 0
+    assert "Saved session expired" in capsys.readouterr().out
+
+
 def test_create_session_empty_credentials_file_allows_login(
     sessions_creds_file, monkeypatch, capsys
 ):
@@ -116,6 +144,42 @@ def _raise_401(self, *_, **__):
     assert "Session already expired" in capsys.readouterr().out
 
 
+def test_create_session_unreachable_server_reports_connectivity(
+    sessions_creds_file, monkeypatch, capsys
+):
+    import json
+
+    sessions_creds_file.write_text(
+        json.dumps({"api_token": "tok", "cbrain_url": "http://localhost:3000", "user_id": 1})
+    )
+    monkeypatch.setattr(
+        "cbrain_cli.cli_utils.CbrainClient.get",
+        lambda self, *_: (_ for _ in ()).throw(urllib.error.URLError("Connection refused")),
+    )
+    result = create_session(argparse.Namespace())
+    assert result == 1
+    out = capsys.readouterr().out
+    assert "Cannot reach" in out
+    assert "Already logged in" not in out
+
+
+def test_create_session_server_error_reports_status(sessions_creds_file, monkeypatch, capsys):
+    import json
+
+    sessions_creds_file.write_text(
+        json.dumps({"api_token": "tok", "cbrain_url": "http://localhost:3000", "user_id": 1})
+    )
+    monkeypatch.setattr(
+        "cbrain_cli.cli_utils.CbrainClient.get",
+        lambda self, *_: (_ for _ in ()).throw(CliApiError("Server Error", status=500)),
+    )
+    result = create_session(argparse.Namespace())
+    assert result == 1
+    out = capsys.readouterr().out
+    assert "500" in out
+    assert "Already logged in" not in out
+
+
 def test_create_session_uses_default_url(monkeypatch, sessions_creds_file):
     inputs = iter(["", "admin"])
     monkeypatch.setattr("builtins.input", lambda _: next(inputs))
diff --git a/tests/test_tasks.py b/tests/test_tasks.py
index ae362c4..89ad54d 100644
--- a/tests/test_tasks.py
+++ b/tests/test_tasks.py
@@ -1,3 +1,5 @@
+import json
+
 import pytest
 
 from cbrain_cli.cli_utils import CliValidationError
@@ -71,12 +73,53 @@ def test_list_tasks_unsupported_filter_raises():
         list_tasks(make_task_args(filter_name="other", bourreau_id=1))
 
 
-def test_operation_task_prints_json(monkeypatch, capsys):
-    monkeypatch.setattr(
-        "cbrain_cli.cli_utils.CbrainClient.send",
-        lambda self, *_, **__: ({"status": "ok"}, 200),
-    )
+def test_operation_task_returns_data(capture_urlopen):
+    configure, captured = capture_urlopen
+    configure({"status": "ok"})
     from cbrain_cli.data.tasks import operation_task
 
-    operation_task(make_task_args())
-    assert '"status": "ok"' in capsys.readouterr().out
+    args = make_task_args(operation="terminate", task_id=[1, 2])
+    assert operation_task(args) == {"status": "ok"}
+    assert json.loads(captured["data"]) == {
+        "operation": "terminate",
+        "tasklist": [1, 2],
+    }
+
+
+def test_operation_task_requires_operation():
+    from cbrain_cli.data.tasks import operation_task
+
+    with pytest.raises(CliValidationError, match="Operation is required"):
+        operation_task(make_task_args(task_id=[1]))
+
+
+def test_operation_task_requires_task_or_batch():
+    from cbrain_cli.data.tasks import operation_task
+
+    with pytest.raises(CliValidationError, match="--task-id"):
+        operation_task(make_task_args(operation="hold"))
+
+
+def test_operation_task_optional_flags(capture_urlopen):
+    configure, captured = capture_urlopen
+    configure({"ok": True})
+    from cbrain_cli.data.tasks import operation_task
+
+    operation_task(
+        make_task_args(
+            operation="duplicate",
+            task_id=[5],
+            batch_id=[9],
+            dup_bourreau_id=3,
+            archive_dp_id=15,
+            nozip=True,
+        )
+    )
+    assert json.loads(captured["data"]) == {
+        "operation": "duplicate",
+        "tasklist": [5],
+        "batch_ids": [9],
+        "dup_bourreau_id": 3,
+        "archive_dp_id": 15,
+        "nozip": True,
+    }