diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d17346..a350a28 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,12 @@ gem but are what an operator runs against their own image.
 
 ## next / unreleased
 
+### HotCell::Server
+
+#### Added
+
+* An operation can say why it couldn't decode an input: a subclass of `HotCell::UnreadableInput` that defines `self.cause` sends that cause with the `unreadable` failure. `HotCell::Codes::PROTECTED` names the first such cause, `protected`. See `docs/codes.md`.
+
 ### ActiveStorage::HotCell::Client
 
 #### Fixed
@@ -27,6 +33,12 @@ gem but are what an operator runs against their own image.
 
 * `hotcell-client-legacy` is a new gem that lets an application on a legacy Ruby version call a cell. It has no dependencies, and it speaks the same wire protocol as `hotcell-client` but leaves out Rails integration, Active Storage support and metrics. See [docs/client-legacy.md](docs/client-legacy.md). (#100)
 
+### ActiveStorage::HotCell::Server
+
+#### Added
+
+* The `mutool` and Poppler PDF previewers answer `unreadable` with cause `protected` for a password-protected PDF. An application that shows a lock placeholder for these PDFs can read the cause instead of matching the tool's error message.
+
 ## v1.0.0 / 2026-10-05
 
 ### Documentation
diff --git a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/mutool.rb b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/mutool.rb
index 5f72a47..c2d8bde 100644
--- a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/mutool.rb
+++ b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/mutool.rb
@@ -22,7 +22,8 @@ class Mutool < Pdf
               # separate change.
               def render(source, destination, page:, resolution:)
                 run! "mutool", "draw", "-F", "png", "-r", resolution.to_s, "-o", destination.path,
-                     source.fd_path, page.to_s, pass: [ source.to_io ]
+                     source.fd_path, page.to_s, pass: [ source.to_io ],
+                     causes: { /cannot authenticate password/ => ProtectedDocument }
               end
 
               def tool
diff --git a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/poppler.rb b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/poppler.rb
index 59b7489..4695344 100644
--- a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/poppler.rb
+++ b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/previewers/pdf/poppler.rb
@@ -22,7 +22,7 @@ class Poppler < Pdf
               def render(source, destination, page:, resolution:)
                 run! "pdftoppm", "-png", "-singlefile", "-cropbox", "-r", resolution.to_s,
                      "-f", page.to_s, "-l", page.to_s, source.fd_path, destination.path,
-                     pass: [ source.to_io ]
+                     pass: [ source.to_io ], causes: { /Incorrect password/ => ProtectedDocument }
 
                 destination.adopt destination.path(extension: "png")
               end
diff --git a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/tool_operation.rb b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/tool_operation.rb
index 986679b..08cabfa 100644
--- a/activestorage-hotcell-server/lib/active_storage/hot_cell/server/tool_operation.rb
+++ b/activestorage-hotcell-server/lib/active_storage/hot_cell/server/tool_operation.rb
@@ -20,16 +20,25 @@ class ToolOperation < Operation
         # UnreadableInput is always in the rescue list, and this descends from it.
         class UnreadableDocument < ::HotCell::UnreadableInput; end
 
+        # The document demands credentials the cell does not have.
+        class ProtectedDocument < UnreadableDocument
+          def self.cause
+            ::HotCell::Codes::PROTECTED
+          end
+        end
+
         private
           # Runs the tool with unsetenv_others and a written environment, and turns a non-zero exit into
           # `unreadable`. The tool's own stderr is the only useful diagnostic, and it is attacker-influenced,
           # so it is capped and scrubbed on its way onto the wire like any other error message.
-          def run!(*command, pass: [])
+          # `causes` maps stderr patterns to the UnreadableDocument subclass to raise instead.
+          def run!(*command, pass: [], causes: {})
             result = run_tool(*command, pass: pass)
             return result if result.ok?
 
-            raise UnreadableDocument, "#{command.first} exited #{result.status.exitstatus}: " \
-                                      "#{result.err.to_s.strip[0, 200]}"
+            error = causes.detect { |pattern, _| result.err.to_s.match?(pattern) }&.last || UnreadableDocument
+            raise error, "#{command.first} exited #{result.status.exitstatus}: " \
+                         "#{result.err.to_s.strip[0, 200]}"
           end
 
           # An empty output means the tool said it succeeded and produced nothing, which the client would
diff --git a/activestorage-hotcell-server/test/fixtures/protected.pdf b/activestorage-hotcell-server/test/fixtures/protected.pdf
new file mode 100644
index 0000000..0c23ffd
Binary files /dev/null and b/activestorage-hotcell-server/test/fixtures/protected.pdf differ
diff --git a/activestorage-hotcell-server/test/previewers_test.rb b/activestorage-hotcell-server/test/previewers_test.rb
index f85b584..eaf23ff 100644
--- a/activestorage-hotcell-server/test/previewers_test.rb
+++ b/activestorage-hotcell-server/test/previewers_test.rb
@@ -94,11 +94,25 @@ def test_something_that_is_not_a_pdf_is_unreadable
                                                         outputs: [ destination ])
 
         assert_predicate failure, :permanent?
+        assert_nil failure.cause
         assert_match "mutool", failure.message
       end
     end
   end
 
+  def test_a_password_protected_pdf_answers_the_protected_cause
+    Cell.boot do |cell|
+      with_output(".png") do |destination|
+        failure = assert_failed "unreadable", cell.call("active_storage.previewers.pdf.mutool",
+                                                        inputs: [ fixture("protected.pdf") ],
+                                                        outputs: [ destination ])
+
+        assert_equal HotCell::Codes::PROTECTED, failure.cause
+        assert_predicate failure, :permanent?
+      end
+    end
+  end
+
   # JPEG rather than PNG, because Rails' own video previewer yields image/jpeg and this drops into its place.
   def test_a_video_preview_comes_back_as_the_jpeg_rails_produces
     Cell.boot do |cell|
@@ -174,11 +188,25 @@ def test_poppler_on_something_that_is_not_a_pdf_is_unreadable
                                                         outputs: [ destination ])
 
         assert_predicate failure, :permanent?
+        assert_nil failure.cause
         assert_match "pdftoppm", failure.message
       end
     end
   end
 
+  def test_poppler_on_a_password_protected_pdf_answers_the_protected_cause
+    Cell.boot do |cell|
+      with_output(".png") do |destination|
+        failure = assert_failed "unreadable", cell.call("active_storage.previewers.pdf.poppler",
+                                                        inputs: [ fixture("protected.pdf") ],
+                                                        outputs: [ destination ])
+
+        assert_equal HotCell::Codes::PROTECTED, failure.cause
+        assert_predicate failure, :permanent?
+      end
+    end
+  end
+
   # Rails' default preview arguments choose the first keyframe or scene change rather than the literal first
   # frame, because so many videos open on black. A drop-in replacement has to keep that choice: fade_in.mp4
   # opens on half a second of black before a hard cut, and its preview must come from after the cut.
diff --git a/docs/active-storage.md b/docs/active-storage.md
index 0934313..731f3ce 100644
--- a/docs/active-storage.md
+++ b/docs/active-storage.md
@@ -85,6 +85,8 @@ The client raises the cell's `permanent:` or `transient:` class. See [Response c
   `ActiveStorage::CreateVariantsJob`, `ActiveStorage::PreviewImageJob`, and `ActiveStorage::TransformJob`,
   for each one that this Rails version has. Each registered cell that a shipped client names contributes
   its transient class. A cell that isn't registered yet contributes nothing.
+- **`Previewers::Pdf::Mutool`** and **`Previewers::Pdf::Poppler`** answer `unreadable` with cause `protected`
+  for a password-protected PDF. See [Causes of `unreadable`](codes.md#causes-of-unreadable).
 
 ## Limits
 
diff --git a/docs/codes.md b/docs/codes.md
index 3e94557..80d05c5 100644
--- a/docs/codes.md
+++ b/docs/codes.md
@@ -99,9 +99,21 @@ that the supervisor can't see: a cgroup out-of-memory kill chosen on aggregate p
 signalling another, which nothing prevents because workers share a uid. The worker that holds the request
 decides `memory` and `fsize` itself.
 
+## Causes of `unreadable`
+
+An `unreadable` failure is permanent whatever its cause. Most carry no cause. An operation that can tell
+why it couldn't decode the input sends one, so that a caller can act on the reason without parsing the
+message:
+
+- `protected`: the document demands a password that the cell doesn't have. The shipped `mutool` and Poppler PDF
+  previewers send it for a password-protected PDF.
+
+An operation sends a cause by raising a subclass of `HotCell::UnreadableInput` that defines `self.cause`.
+See [`unreadable(*classes)`](operation-api.md#unreadableclasses).
+
 ## Unknown codes and causes
 
-A code or cause that a client doesn't recognize isn't permanent:
+An unknown code, or an unknown cause of `killed`, isn't permanent:
 
 - The `permanent` flag travels on the wire, set by the side that knows. A client that's older than a code
   still disposes of that code correctly.
@@ -109,6 +121,8 @@ A code or cause that a client doesn't recognize isn't permanent:
 - An unknown `killed` cause is transient. Adding a kill reason to the supervisor without a row in
   `Codes::PERMANENT_BY_CAUSE` can't make it permanent.
 
+A cause of `unreadable` doesn't change its permanence, so an unknown one is still permanent.
+
 Retrying something permanent costs some work. Recording something transient is irreversible.
 
 **Caution:** A compromised cell can forge the split. The client believes a boolean `permanent` from the
diff --git a/docs/operation-api.md b/docs/operation-api.md
index c18e22f..f11d6fb 100644
--- a/docs/operation-api.md
+++ b/docs/operation-api.md
@@ -146,6 +146,10 @@ broke". When `perform` raises one of them, the cell answers `unreadable`, which
 [permanent](codes.md). `HotCell::UnreadableInput` is always included. A subclass adds to the classes that
 its ancestors declared.
 
+To tell the caller why the input can't be decoded, raise a subclass of `HotCell::UnreadableInput` that
+defines `self.cause`. The cell sends the cause with the `unreadable` failure. See
+[Causes of `unreadable`](codes.md#causes-of-unreadable).
+
 ### `abstract_operation`
 
 Marks a class that exists to be inherited from, not to be dispatched to.
diff --git a/hotcell-core/lib/hot_cell/codes.rb b/hotcell-core/lib/hot_cell/codes.rb
index 350576f..106c0c7 100644
--- a/hotcell-core/lib/hot_cell/codes.rb
+++ b/hotcell-core/lib/hot_cell/codes.rb
@@ -100,6 +100,10 @@ module Codes
 
     KILLED = "killed"
 
+    # A cause for `unreadable`, which is permanent whatever its cause: the document demands credentials the
+    # cell does not have. It lets a caller show a locked document differently from a broken one.
+    PROTECTED = "protected"
+
     class << self
       def permanent?(code, cause: nil)
         code = code.to_s
diff --git a/hotcell-server/lib/hot_cell/server/errors.rb b/hotcell-server/lib/hot_cell/server/errors.rb
index 71ae1d9..a79813d 100644
--- a/hotcell-server/lib/hot_cell/server/errors.rb
+++ b/hotcell-server/lib/hot_cell/server/errors.rb
@@ -7,8 +7,13 @@ class OperationError < StandardError; end
 
   # The input could not be decoded. Terminal: the same bytes fail the same way on an idle cell. Common
   # rather than exceptional — it covers truncated uploads, formats the build was not compiled with, and
-  # formats deliberately refused.
-  class UnreadableInput < OperationError; end
+  # formats deliberately refused. A subclass may name the failure's cause, for a caller that must act
+  # on the reason without parsing a message.
+  class UnreadableInput < OperationError
+    def self.cause
+      nil
+    end
+  end
 
   # Reported as `killed` with `cause: memory` rather than as an ordinary failure, because it is the
   # decompression-bomb case and a caller must be able to act on it without parsing a message.
diff --git a/hotcell-server/lib/hot_cell/worker.rb b/hotcell-server/lib/hot_cell/worker.rb
index fd188b4..732e48e 100644
--- a/hotcell-server/lib/hot_cell/worker.rb
+++ b/hotcell-server/lib/hot_cell/worker.rb
@@ -219,7 +219,8 @@ def perform(operation, inputs, outputs, payload, timing)
 
         response
       rescue *operation.unreadable => error
-        refuse "unreadable", error, timing
+        cause = error.class.respond_to?(:cause) ? error.class.cause : nil
+        refuse "unreadable", error, timing, cause: cause
       end
 
       # `post` returns what each output received and the worker used to throw all of it away, leaving the