diff --git a/crates/oa-desktop/src/app.rs b/crates/oa-desktop/src/app.rs
index 1e80ec40..107caacd 100644
--- a/crates/oa-desktop/src/app.rs
+++ b/crates/oa-desktop/src/app.rs
@@ -18,6 +18,8 @@ mod hosted;
 mod reference_authoring;
 #[cfg(test)]
 mod review_navigation_tests;
+#[cfg(test)]
+mod source_indentation_tests;
 pub(crate) mod source_rename;
 mod source_symbols;
 pub(crate) mod text_workspace;
@@ -427,6 +429,7 @@ pub enum Message {
     Pan(iced::Vector),
     Fit,
     SourceEdit(text_editor::Action),
+    SourceIndent(crate::source_navigation::SymbolContext, bool),
     ApplySource,
     RevertSource,
     SourceUndo,
@@ -3314,10 +3317,16 @@ impl App {
                 self.pan += delta;
             }
             Message::Fit => self.fit(),
-            Message::SourceEdit(action) => {
+            Message::SourceEdit(_) | Message::SourceIndent(..) => {
                 self.source_jump = None;
                 let before = self.source_editor.text();
-                self.source_editor.perform(action);
+                match message {
+                    Message::SourceEdit(action) => self.source_editor.perform(action),
+                    Message::SourceIndent(_, outdent) => {
+                        crate::source_indentation::apply(&mut self.source_editor, outdent);
+                    }
+                    _ => unreachable!(),
+                }
                 let after = self.source_editor.text();
                 if after != before {
                     self.source_navigation.refresh(&after);
diff --git a/crates/oa-desktop/src/app/source_indentation_tests.rs b/crates/oa-desktop/src/app/source_indentation_tests.rs
new file mode 100644
index 00000000..7477ab2f
--- /dev/null
+++ b/crates/oa-desktop/src/app/source_indentation_tests.rs
@@ -0,0 +1,77 @@
+//! Controller acceptance: one indentation gesture is one draft transaction.
+use super::*;
+
+fn fixture(source: &str) -> App {
+    let mut app = App::with_project(Project::empty());
+    app.page = Page::Text;
+    app.source_workspace = Workspace::new(
+        oa_core::source_project::SourceBundle {
+            entry: "project.arcadia".into(),
+            files: std::collections::BTreeMap::from([("project.arcadia".into(), source.into())]),
+        },
+        true,
+    );
+    app.show_source_file(false);
+    app
+}
+fn indent(app: &mut App, outdent: bool) {
+    let message = Message::SourceIndent(app.source_symbol_context(), outdent);
+    let _ = app.update(message);
+}
+
+#[test]
+fn indentation_updates_draft_snapshot_version_and_single_undo_redo_without_model_change() {
+    let source = "雨\r\n🚀\r\n";
+    let mut app = fixture(source);
+    app.source_editor.perform(text_editor::Action::SelectAll);
+    let version = app.source_version;
+    let revision = app.source_workspace.revision();
+    let model = app.document.revision();
+    indent(&mut app, false);
+    let after = "    雨\r\n    🚀\r\n";
+    assert_eq!(app.source_editor.text(), after);
+    assert_eq!(app.source_snapshot.as_ref(), after);
+    assert_eq!(app.source_workspace.files["project.arcadia"].text, after);
+    assert!(app.source_workspace.has_edits());
+    assert_eq!(app.source_version, version + 1);
+    assert_eq!(app.source_workspace.revision(), revision + 1);
+    assert_eq!(app.document.revision(), model);
+    let _ = app.update(Message::SourceUndo);
+    assert_eq!(app.source_editor.text(), source);
+    assert!(!app.source_workspace.has_edits());
+    let _ = app.update(Message::SourceRedo);
+    assert_eq!(app.source_editor.text(), after);
+    assert_eq!(app.document.revision(), model);
+}
+
+#[test]
+fn stale_hidden_and_modal_indentation_is_rejected_before_navigation_invalidation() {
+    for rejected in 0..7 {
+        let mut app = fixture("alpha");
+        let mut context = app.source_symbol_context();
+        match rejected {
+            0 => context.path = "elsewhere.arcadia".into(),
+            1 => context.session = Id::new_v4(),
+            2 => app.page = Page::Home,
+            3 => app.busy = true,
+            4 => app.delete_pending = true,
+            5 => app.hosted.visible = true,
+            _ => app.pending = Some(Pending::New),
+        }
+        let interaction = app.source_symbols.interaction;
+        let version = app.source_version;
+        let _ = app.update(Message::SourceIndent(context, false));
+        assert_eq!(app.source_editor.text(), "alpha");
+        assert_eq!(app.source_version, version);
+        assert_eq!(app.source_symbols.interaction, interaction);
+    }
+}
+
+#[test]
+fn no_op_outdent_does_not_add_history_or_increment_source_version() {
+    let mut app = fixture("alpha");
+    let version = app.source_version;
+    indent(&mut app, true);
+    assert_eq!(app.source_version, version);
+    assert!(!app.source_workspace.undo(false));
+}
diff --git a/crates/oa-desktop/src/app/source_symbols.rs b/crates/oa-desktop/src/app/source_symbols.rs
index 2b1d52de..e3e75caf 100644
--- a/crates/oa-desktop/src/app/source_symbols.rs
+++ b/crates/oa-desktop/src/app/source_symbols.rs
@@ -33,6 +33,9 @@ fn admission_notice(error: query::AdmissionError) -> String {
 impl App {
     pub(super) fn source_input_envelope(&self, message: &Message) -> bool {
         match message {
+            Message::SourceIndent(context, _) => {
+                context == &self.source_symbol_context() && self.source_navigation_available()
+            }
             Message::SourceSymbols(context, event) => {
                 context.session == self.source_workspace.session()
                     && context.path == self.source_workspace.active
diff --git a/crates/oa-desktop/src/main.rs b/crates/oa-desktop/src/main.rs
index 282766b3..02bf1b9e 100644
--- a/crates/oa-desktop/src/main.rs
+++ b/crates/oa-desktop/src/main.rs
@@ -28,6 +28,7 @@ mod recovery;
 mod route_edit;
 mod semantic_review;
 mod source_annotations;
+mod source_indentation;
 mod source_navigation;
 mod source_rename;
 mod source_tree;
diff --git a/crates/oa-desktop/src/source_indentation.rs b/crates/oa-desktop/src/source_indentation.rs
new file mode 100644
index 00000000..901de20a
--- /dev/null
+++ b/crates/oa-desktop/src/source_indentation.rs
@@ -0,0 +1,94 @@
+//! Source indentation is one workspace edit, with physical line endings intact.
+use iced::{
+    advanced::{graphics, text},
+    widget::text_editor::{Action, Content, Cursor, Edit, Position},
+};
+use std::sync::Arc;
+
+const INDENT: &str = "    ";
+
+/// Plain Tab inserts one canonical four-space level at a caret, or prefixes
+/// every selected line. Shift+Tab removes one leading tab or up to four spaces.
+/// A selection ending at column zero excludes that final line. All patches are
+/// local to lines, so CRLF, LF and Unicode bytes outside indentation survive.
+/// The caller records the resulting buffer once in workspace undo/history.
+pub fn apply<R>(content: &mut Content<R>, outdent: bool)
+where
+    R: text::Renderer<Editor = graphics::text::Editor>,
+{
+    let raw = content.cursor();
+    let selected = content.selection().is_some_and(|s| !s.is_empty());
+    if !selected && !outdent {
+        content.perform(Action::Edit(Edit::Paste(Arc::new(INDENT.into()))));
+        return;
+    }
+    // Determine affected lines before normalizing Word/Line mode. Those modes
+    // expand columns within their endpoint lines; the copied LF count reveals
+    // whether a zero-column endpoint includes the next line or only its edge.
+    let anchor = raw.selection.unwrap_or(raw.position);
+    let first = raw.position.line.min(anchor.line);
+    let mut last = raw.position.line.max(anchor.line);
+    if selected {
+        let selected_text = content.selection().unwrap();
+        let line_breaks = selected_text.bytes().filter(|b| *b == b'\n').count();
+        // Effective selection starts on the minimum raw endpoint line.
+        last = first + line_breaks;
+        if last > first && selected_text.ends_with('\n') {
+            last -= 1;
+        }
+    }
+    let patches: Vec<_> = (first..=last)
+        .filter_map(|line| {
+            let removed = if outdent {
+                let text = content.line(line)?;
+                if text.text.starts_with('\t') {
+                    1
+                } else {
+                    text.text.bytes().take(4).take_while(|b| *b == b' ').count()
+                }
+            } else {
+                0
+            };
+            (!outdent || removed > 0).then_some((line, removed))
+        })
+        .collect();
+    if patches.is_empty() {
+        return;
+    }
+    let cursor = crate::source_navigation::selection::capture_for_edit(content);
+    let replacement = Arc::new(if outdent {
+        String::new()
+    } else {
+        INDENT.into()
+    });
+    // Descending order leaves Iced's earliest changed line at the first patch.
+    for &(line, removed) in patches.iter().rev() {
+        content.move_to(Cursor {
+            position: Position { line, column: 0 },
+            selection: Some(Position {
+                line,
+                column: removed,
+            }),
+        });
+        content.perform(Action::Edit(Edit::Paste(replacement.clone())));
+    }
+    let transform = |mut position: Position| {
+        if let Some((_, removed)) = patches.iter().find(|(line, _)| *line == position.line) {
+            position.column = if outdent {
+                position.column.saturating_sub(*removed)
+            } else {
+                position.column + INDENT.len()
+            };
+        }
+        position
+    };
+    content.move_to(Cursor {
+        position: transform(cursor.position),
+        selection: cursor.selection.map(transform),
+    });
+}
+
+#[cfg(all(test, debug_assertions))]
+mod keyboard_tests;
+#[cfg(test)]
+mod tests;
diff --git a/crates/oa-desktop/src/source_indentation/keyboard_tests.rs b/crates/oa-desktop/src/source_indentation/keyboard_tests.rs
new file mode 100644
index 00000000..16121512
--- /dev/null
+++ b/crates/oa-desktop/src/source_indentation/keyboard_tests.rs
@@ -0,0 +1,105 @@
+//! Real mounted Iced keyboard dispatch, using the production binding function.
+use crate::{app::Message, source_navigation::SymbolContext, views::source_tests::FontLayout};
+use iced::{
+    Element, Font, Rectangle, Size,
+    advanced::{Layout, Shell, layout, text, widget as core_widget},
+    keyboard::{
+        self, Key, Modifiers,
+        key::{Code, Named, Physical},
+    },
+    widget::{
+        self,
+        text_editor::{Content, State},
+    },
+};
+
+#[test]
+fn mounted_tab_dispatches_only_for_focused_editable_editor_and_plain_shift_variants() {
+    for focused in [false, true] {
+        for editable in [false, true] {
+            for modifiers in [
+                Modifiers::empty(),
+                Modifiers::SHIFT,
+                Modifiers::CTRL,
+                Modifiers::ALT,
+                Modifiers::LOGO,
+            ] {
+                let context = SymbolContext {
+                    session: oa_core::Id::new_v4(),
+                    path: "project.arcadia".into(),
+                };
+                let content = Content::<FontLayout>::with_text("rain");
+                let mut editor = widget::text_editor(&content)
+                    .font(Font::MONOSPACE)
+                    .size(14)
+                    .padding(16)
+                    .height(iced::Fill)
+                    .key_binding(|press| crate::views::source_binding(press, &context, None));
+                if editable {
+                    editor = editor.on_action(Message::SourceEdit);
+                }
+                let mut view: Element<'_, Message, iced::Theme, FontLayout> = editor.into();
+                let mut tree = core_widget::Tree::new(view.as_widget());
+                let size = Size::new(600., 240.);
+                let node = view.as_widget_mut().layout(
+                    &mut tree,
+                    &FontLayout,
+                    &layout::Limits::new(Size::ZERO, size),
+                );
+                if focused {
+                    tree.state
+                        .downcast_mut::<State<text::highlighter::PlainText>>()
+                        .focus();
+                }
+                let event = iced::Event::Keyboard(keyboard::Event::KeyPressed {
+                    key: Key::Named(Named::Tab),
+                    modified_key: Key::Named(Named::Tab),
+                    physical_key: Physical::Code(Code::Tab),
+                    location: keyboard::Location::Standard,
+                    modifiers,
+                    text: None,
+                    repeat: false,
+                });
+                let mut messages = Vec::new();
+                let captured;
+                {
+                    let mut shell = Shell::new(&mut messages);
+                    view.as_widget_mut().update(
+                        &mut tree,
+                        &event,
+                        Layout::new(&node),
+                        iced::mouse::Cursor::Unavailable,
+                        &FontLayout,
+                        &mut iced::advanced::clipboard::Null,
+                        &mut shell,
+                        &Rectangle::with_size(size),
+                    );
+                    captured = shell.is_event_captured();
+                }
+                let plain = modifiers.is_empty() || modifiers == Modifiers::SHIFT;
+                let expected = focused && editable && plain;
+                let indentation: Vec<_> = messages
+                    .iter()
+                    .filter_map(|message| match message {
+                        Message::SourceIndent(emitted, outdent) => Some((emitted, outdent)),
+                        _ => None,
+                    })
+                    .collect();
+                assert_eq!(
+                    indentation.len(),
+                    usize::from(expected),
+                    "focused={focused}, editable={editable}, modifiers={modifiers:?}"
+                );
+                if expected {
+                    assert!(captured, "Tab must not escape to global focus traversal");
+                    assert_eq!(indentation[0], (&context, &modifiers.shift()));
+                } else if !focused || !editable {
+                    assert!(
+                        !captured,
+                        "non-editing Tab must remain available to focus traversal"
+                    );
+                }
+            }
+        }
+    }
+}
diff --git a/crates/oa-desktop/src/source_indentation/tests.rs b/crates/oa-desktop/src/source_indentation/tests.rs
new file mode 100644
index 00000000..70a17821
--- /dev/null
+++ b/crates/oa-desktop/src/source_indentation/tests.rs
@@ -0,0 +1,112 @@
+use super::*;
+use iced::widget::text_editor::Motion;
+
+type Buffer = Content<iced::Renderer>;
+fn at(line: usize, column: usize) -> Position {
+    Position { line, column }
+}
+fn range(start: Position, end: Position, reverse: bool) -> Cursor {
+    Cursor {
+        position: if reverse { start } else { end },
+        selection: Some(if reverse { end } else { start }),
+    }
+}
+
+#[test]
+fn caret_inserts_four_spaces_and_outdent_removes_only_leading_whitespace() {
+    let mut content = Buffer::with_text("  雨🚀\r\nnext");
+    content.move_to(Cursor {
+        position: at(0, "  雨".len()),
+        selection: None,
+    });
+    apply(&mut content, false);
+    assert_eq!(content.text(), "  雨    🚀\r\nnext");
+    assert_eq!(content.cursor().position, at(0, "  雨    ".len()));
+    apply(&mut content, true);
+    assert_eq!(content.text(), "雨    🚀\r\nnext");
+    assert_eq!(content.cursor().position, at(0, "雨    ".len()));
+    apply(&mut content, true);
+    assert_eq!(content.text(), "雨    🚀\r\nnext");
+}
+
+#[test]
+fn directional_blocks_preserve_unicode_endings_and_exclude_final_zero_column() {
+    let source = "雨\r\n🚀\nlast\r\n";
+    for reverse in [false, true] {
+        let mut content = Buffer::with_text(source);
+        let original = range(at(0, 0), at(2, 0), reverse);
+        content.move_to(original);
+        apply(&mut content, false);
+        assert_eq!(content.text(), "    雨\r\n    🚀\nlast\r\n");
+        assert_eq!(content.cursor(), range(at(0, 4), at(2, 0), reverse));
+        apply(&mut content, true);
+        assert_eq!(content.text(), source);
+        assert_eq!(content.cursor(), original);
+    }
+}
+
+#[test]
+fn outdent_handles_tabs_partial_spaces_and_clamps_endpoint_inside_removed_indent() {
+    for reverse in [false, true] {
+        let mut content = Buffer::with_text("\t雨\r\n  🚀\n    fin");
+        content.move_to(range(at(0, 0), at(2, 2), reverse));
+        apply(&mut content, true);
+        assert_eq!(content.text(), "雨\r\n🚀\nfin");
+        assert_eq!(content.cursor(), range(at(0, 0), at(2, 0), reverse));
+    }
+}
+
+#[test]
+fn word_and_line_selections_indent_effective_range_in_both_directions() {
+    for line_mode in [false, true] {
+        for reverse in [false, true] {
+            let mut content = Buffer::with_text("one alpha\r\ntwo beta\nthree gamma");
+            content.move_to(Cursor {
+                position: if reverse { at(2, 8) } else { at(0, 6) },
+                selection: None,
+            });
+            content.perform(if line_mode {
+                Action::SelectLine
+            } else {
+                Action::SelectWord
+            });
+            content.perform(Action::Select(if reverse {
+                Motion::DocumentStart
+            } else {
+                Motion::DocumentEnd
+            }));
+            let selected = content.selection().unwrap();
+            apply(&mut content, false);
+            assert_eq!(
+                content.text(),
+                "    one alpha\r\n    two beta\n    three gamma"
+            );
+            let cursor = content.cursor();
+            assert_eq!(
+                cursor.position.line < cursor.selection.unwrap().line,
+                reverse
+            );
+            apply(&mut content, true);
+            assert_eq!(content.text(), "one alpha\r\ntwo beta\nthree gamma");
+            assert_eq!(content.selection(), Some(selected));
+        }
+    }
+}
+
+#[test]
+fn no_op_outdent_preserves_semantic_word_selection_and_last_empty_line_is_excluded() {
+    let mut content = Buffer::with_text("alpha tail\r\n");
+    content.move_to(Cursor {
+        position: at(0, 2),
+        selection: None,
+    });
+    content.perform(Action::SelectWord);
+    let raw = content.cursor();
+    apply(&mut content, true);
+    assert_eq!(content.cursor(), raw);
+    assert_eq!(content.selection().as_deref(), Some("alpha"));
+    content.perform(Action::SelectAll);
+    apply(&mut content, false);
+    assert_eq!(content.text(), "    alpha tail\r\n");
+    assert_eq!(content.cursor().position, at(1, 0));
+}
diff --git a/crates/oa-desktop/src/source_navigation/selection.rs b/crates/oa-desktop/src/source_navigation/selection.rs
index 40f24b00..27944779 100644
--- a/crates/oa-desktop/src/source_navigation/selection.rs
+++ b/crates/oa-desktop/src/source_navigation/selection.rs
@@ -1,10 +1,12 @@
-//! Effective selected-byte ranges for a committed departure from a source view.
+//! Effective selected-byte ranges for committed source departures and edits.
 //!
 //! Iced's `Cursor` omits cosmic-text's Word/Line selection kind. Saving its raw
 //! endpoints can therefore save an empty range for a visibly selected word/line.
-//! Capture only after navigation admission and destination validation succeed,
-//! immediately before replacing/leaving the live editor. Never use this to inspect
-//! pending queries, failed/no-op navigation, or restore focus to the same editor.
+//! Departure capture requires navigation admission and destination validation,
+//! immediately before replacing/leaving the live editor. Edit capture requires
+//! an admitted edit with a nonempty patch plan. Neither is a passive query:
+//! never use these to inspect pending queries, failed/no-op navigation, or restore
+//! focus to the same editor.
 use iced::advanced::{graphics, text};
 use iced::widget::text_editor::{Action, Content, Cursor, Motion, Position};
 
@@ -21,6 +23,23 @@ use iced::widget::text_editor::{Action, Content, Cursor, Motion, Position};
 /// selection joins editor lines with LF regardless of physical CRLF/LF endings.
 /// Only the selected text is temporarily copied; retained state is one Cursor.
 pub fn capture<R>(content: &mut Content<R>) -> Cursor
+where
+    R: text::Renderer<Editor = graphics::text::Editor>,
+{
+    effective_range(content)
+}
+
+/// Capture immediately before a committed indentation edit. Like departure,
+/// this converts Word/Line selections to their effective directional byte range.
+/// The caller restores the transformed range after editing the same Content.
+pub fn capture_for_edit<R>(content: &mut Content<R>) -> Cursor
+where
+    R: text::Renderer<Editor = graphics::text::Editor>,
+{
+    effective_range(content)
+}
+
+fn effective_range<R>(content: &mut Content<R>) -> Cursor
 where
     R: text::Renderer<Editor = graphics::text::Editor>,
 {
diff --git a/crates/oa-desktop/src/views.rs b/crates/oa-desktop/src/views.rs
index a355c65c..5f0f88eb 100644
--- a/crates/oa-desktop/src/views.rs
+++ b/crates/oa-desktop/src/views.rs
@@ -1178,6 +1178,15 @@ pub(crate) fn source_binding(
     use crate::source_navigation::{SymbolEvent, query::Action};
     use iced::keyboard::{Key, key::Named};
     use widget::text_editor::{Binding, Status};
+    if matches!(press.status, Status::Focused { .. })
+        && press.key == Key::Named(Named::Tab)
+        && (press.modifiers.is_empty() || press.modifiers == iced::keyboard::Modifiers::SHIFT)
+    {
+        return Some(Binding::Custom(Message::SourceIndent(
+            context.clone(),
+            press.modifiers.shift(),
+        )));
+    }
     if matches!(press.status, Status::Focused { .. })
         && press.modifiers.is_empty()
         && press.key == Key::Named(Named::F2)