diff --git a/CHANGES.md b/CHANGES.md
index dc68fe70..42abdb87 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -13,6 +13,16 @@ development source code and as such may not be routinely kept up to date.
 
 # __NEXT__
 
+## Development
+
+* A new debugging mode can be enabled by setting the `NEXTSTRAIN_DEBUG`
+  environment variable to `1` (or another truthy value).  Currently the only
+  effect is to print more information about handled (i.e. anticipated) errors.
+  For example, stack traces and parent exceptions in an exception chain are
+  normally omitted for handled errors, but setting this env var includes them.
+  Future debugging and troubleshooting features, like verbose operation
+  logging, will likely also condition on this new debugging mode.
+
 
 # 4.1.1 (18 July 2022)
 
diff --git a/nextstrain/cli/__init__.py b/nextstrain/cli/__init__.py
index 803017ee..674bff76 100644
--- a/nextstrain/cli/__init__.py
+++ b/nextstrain/cli/__init__.py
@@ -15,6 +15,7 @@
 
 from .argparse    import HelpFormatter, register_commands, register_default_command
 from .command     import build, view, deploy, remote, shell, update, check_setup, login, logout, whoami, version, debugger
+from .debug       import DEBUGGING
 from .errors      import NextstrainCliError
 from .util        import warn
 from .__version__ import __version__ # noqa: F401 (for re-export)
@@ -32,7 +33,10 @@ def run(args):
         return opts.__command__.run(opts)
 
     except NextstrainCliError as error:
-        warn(error)
+        if DEBUGGING:
+            traceback.print_exc()
+        else:
+            warn(error)
         return 1
 
     except AssertionError:
diff --git a/nextstrain/cli/debug.py b/nextstrain/cli/debug.py
new file mode 100644
index 00000000..56f13abb
--- /dev/null
+++ b/nextstrain/cli/debug.py
@@ -0,0 +1,12 @@
+"""
+Debug flags and utilities.
+
+.. envvar:: NEXTSTRAIN_DEBUG
+
+    Set to a truthy value (e.g. 1) to print more information about (handled)
+    errors.  For example, when this is not set or falsey, stack traces and
+    parent exceptions in an exception chain are omitted from handled errors.
+"""
+from os import environ
+
+DEBUGGING = bool(environ.get("NEXTSTRAIN_DEBUG"))