diff --git a/pmacs-gpu/src/main.rs b/pmacs-gpu/src/main.rs
index 2992310..ad17dc3 100644
--- a/pmacs-gpu/src/main.rs
+++ b/pmacs-gpu/src/main.rs
@@ -3071,7 +3071,15 @@ fn advance_minimap_col(col: usize, ch: char) -> usize {
 }
 
 fn minimap_style_color(style: CellStyle) -> [f32; 4] {
-    match style.fg {
+    // A set underline_color is the producer's diagnostic mark for the
+    // line (protocol v6, T M4.6 parity) — the minimap's gutter sign.
+    // It outranks the syntax-dominant fg so error/warning lines read
+    // at a glance.
+    let color = match style.underline_color {
+        CellColor::Default => style.fg,
+        marked => marked,
+    };
+    match color {
         CellColor::Default => MINIMAP_DEFAULT_LINE,
         CellColor::Rgb(r, g, b) => rgb_to_minimap_color(r, g, b),
         CellColor::Indexed(idx) => {
diff --git a/src/diag.rs b/src/diag.rs
index 994569b..97b30c7 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -81,6 +81,21 @@ impl DiagnosticSeverity {
         }
     }
 
+    /// Canonical severity color (T M4.6 / protocol v6): the
+    /// `underline_color` of this severity's squiggle, the column-0
+    /// marker background, and the minimap mark all share it. Indexed
+    /// 1/3/6/8 (red / yellow / cyan / gray) for 8/16-color terminal
+    /// portability.
+    #[must_use]
+    pub fn underline_color(self) -> Color {
+        match self {
+            Self::Error => Color::Indexed(1),
+            Self::Warning => Color::Indexed(3),
+            Self::Information => Color::Indexed(6),
+            Self::Hint => Color::Indexed(8),
+        }
+    }
+
     fn from_lsp_value(v: Option<&Value>) -> Self {
         match v.and_then(Value::as_i64) {
             Some(1) => Self::Error,
@@ -187,6 +202,12 @@ pub struct DiagnosticStore {
     /// here on the assumption that a fresh `publishDiagnostics`
     /// corresponds to the latest sent version.
     stale_uris: std::collections::HashSet<String>,
+    /// Per-URI change counter, bumped on every [`Self::set`] /
+    /// [`Self::clear`]. Diagnostics arrive without a CRDT generation
+    /// bump, so generation-keyed caches (the `FileStyleSummary`
+    /// producer's) additionally key on this to know a republish
+    /// happened (T M4.6 GPU parity).
+    epochs: HashMap<String, u64>,
 }
 
 impl DiagnosticStore {
@@ -207,6 +228,7 @@ impl DiagnosticStore {
         diags.sort_by(Diagnostic::compare_by_position);
         let uri = uri.into();
         self.stale_uris.remove(&uri);
+        *self.epochs.entry(uri.clone()).or_insert(0) += 1;
         if diags.is_empty() {
             self.by_uri.remove(&uri);
         } else {
@@ -219,6 +241,16 @@ impl DiagnosticStore {
     pub fn clear(&mut self, uri: &str) {
         self.by_uri.remove(uri);
         self.stale_uris.remove(uri);
+        *self.epochs.entry(uri.to_owned()).or_insert(0) += 1;
+    }
+
+    /// Monotonic per-URI change counter: how many times `set` /
+    /// `clear` ran for this URI. `0` for a URI never written.
+    /// Consumers cache against this to detect republishes that no
+    /// CRDT generation bump announces.
+    #[must_use]
+    pub fn epoch_for(&self, uri: &str) -> u64 {
+        self.epochs.get(uri).copied().unwrap_or(0)
     }
 
     /// Mark `uri`'s stored diagnostics as stale (T M11.8). Called
@@ -338,8 +370,7 @@ const TAB_WIDTH: u32 = 8;
 fn error_style() -> Style {
     Style {
         underline: UnderlineStyle::Curly,
-        // Indexed 1 (red) is portable across 8/16-color terminals.
-        underline_color: Color::Indexed(1),
+        underline_color: DiagnosticSeverity::Error.underline_color(),
         ..Style::default()
     }
 }
@@ -348,8 +379,7 @@ fn error_style() -> Style {
 fn warning_style() -> Style {
     Style {
         underline: UnderlineStyle::Curly,
-        // Indexed 3: yellow.
-        underline_color: Color::Indexed(3),
+        underline_color: DiagnosticSeverity::Warning.underline_color(),
         ..Style::default()
     }
 }
@@ -358,8 +388,7 @@ fn warning_style() -> Style {
 fn info_style() -> Style {
     Style {
         underline: UnderlineStyle::Single,
-        // Indexed 6: cyan.
-        underline_color: Color::Indexed(6),
+        underline_color: DiagnosticSeverity::Information.underline_color(),
         ..Style::default()
     }
 }
@@ -368,9 +397,7 @@ fn info_style() -> Style {
 fn hint_style() -> Style {
     Style {
         underline: UnderlineStyle::Dotted,
-        // Indexed 8: bright black ("gray") — present on 16-color
-        // terminals, subtle by design for hints.
-        underline_color: Color::Indexed(8),
+        underline_color: DiagnosticSeverity::Hint.underline_color(),
         ..Style::default()
     }
 }
@@ -391,14 +418,8 @@ fn style_for(severity: DiagnosticSeverity) -> Style {
 /// style-only), and zero-width diagnostics — invisible to the
 /// underline pass — still get a visible artifact.
 fn marker_style_for(severity: DiagnosticSeverity) -> Style {
-    let bg = match severity {
-        DiagnosticSeverity::Error => Color::Indexed(1),
-        DiagnosticSeverity::Warning => Color::Indexed(3),
-        DiagnosticSeverity::Information => Color::Indexed(6),
-        DiagnosticSeverity::Hint => Color::Indexed(8),
-    };
     Style {
-        bg,
+        bg: severity.underline_color(),
         ..Style::default()
     }
 }
@@ -881,6 +902,24 @@ mod tests {
         }
     }
 
+    #[test]
+    fn epoch_bumps_on_set_and_clear_per_uri() {
+        let mut store = DiagnosticStore::new();
+        assert_eq!(store.epoch_for("file:///a"), 0);
+        store.set("file:///a", vec![diag(0, DiagnosticSeverity::Error, "x")]);
+        assert_eq!(store.epoch_for("file:///a"), 1);
+        // An empty set (server reports clean) still counts — the
+        // consumer must refresh to drop its marks.
+        store.set("file:///a", vec![]);
+        assert_eq!(store.epoch_for("file:///a"), 2);
+        store.clear("file:///a");
+        assert_eq!(store.epoch_for("file:///a"), 3);
+        // mark_stale is not a content change; other URIs are isolated.
+        store.mark_stale("file:///a");
+        assert_eq!(store.epoch_for("file:///a"), 3);
+        assert_eq!(store.epoch_for("file:///b"), 0);
+    }
+
     #[test]
     fn view_advertises_diagnostic_kind() {
         // `pmacs.window._overlay_kinds()` introspection (task #23 wire-up,
diff --git a/src/semantic_render.rs b/src/semantic_render.rs
index aee239b..e86a257 100644
--- a/src/semantic_render.rs
+++ b/src/semantic_render.rs
@@ -113,7 +113,11 @@ pub struct SemanticRenderState {
     /// buffer at the same generation re-uses what the frontend
     /// already has and emits nothing. First emission happens on the
     /// first frame for a buffer; further emissions only after edits.
-    last_summary: HashMap<BufferId, u64>,
+    /// `(crdt_generation, diag_epoch)` the last emitted summary was
+    /// computed against. Diagnostics arrive without a generation
+    /// bump, so the epoch half catches republishes (minimap marks,
+    /// T M4.6 GPU parity).
+    last_summary: HashMap<BufferId, (u64, u64)>,
     /// `StyleSpans` recompute gate (perf). `scoped_style_spans` runs
     /// the tree-sitter highlights query over the *whole declared
     /// viewport* (which the GPU frontend sets to the entire buffer)
@@ -464,11 +468,17 @@ impl SemanticRenderState {
         if grammar_style_parse_not_ready(state, buffer_id) {
             return None;
         }
-        if self.last_summary.get(&buffer_id).copied() == Some(generation) {
+        // Diagnostics fold into the summary (minimap marks) but
+        // publish without a generation bump — key the cache on the
+        // diag store's per-URI epoch as well, so a republish
+        // refreshes the marks and anything else stays suppressed.
+        let diag_epoch = diagnostics_epoch(state, buffer_id);
+        if self.last_summary.get(&buffer_id).copied() == Some((generation, diag_epoch)) {
             return None;
         }
         let lines = scoped_file_summary(state, buffer_id);
-        self.last_summary.insert(buffer_id, generation);
+        self.last_summary
+            .insert(buffer_id, (generation, diag_epoch));
         Some(InstanceMessage::FileStyleSummary {
             buffer_id,
             generation,
@@ -838,6 +848,19 @@ fn diagnostics_store_stale(state: &EditorState, buffer_id: BufferId) -> bool {
     guard.is_stale(&uri)
 }
 
+/// The diag store's per-URI change epoch for `buffer_id`'s file, `0`
+/// for buffers with no file URI or no diagnostics history. Keys the
+/// `FileStyleSummary` cache (see [`SemanticRenderState::last_summary`]).
+fn diagnostics_epoch(state: &EditorState, buffer_id: BufferId) -> u64 {
+    let core = state.core.borrow();
+    let Some(uri) = buffer_file_uri(&core, buffer_id) else {
+        return 0;
+    };
+    let store = state.lsp_manager.borrow().diag_store();
+    let guard = store.lock().expect("diag store mutex poisoned");
+    guard.epoch_for(&uri)
+}
+
 /// Style-family staleness for the LSP-token authority. True only for
 /// a buffer with **no** tree-sitter view (policy A routes those
 /// through `lsp_scoped_style_spans`) whose semantic-token store entry
@@ -1182,6 +1205,9 @@ fn scoped_file_summary(state: &EditorState, buffer_id: BufferId) -> Vec<Style> {
     };
     let spans = scoped_style_spans(state, &vp_all);
     if spans.is_empty() {
+        // No styled runs — but diagnostic marks are independent of
+        // syntax styling (a plain-text buffer can still have lints).
+        overlay_diagnostic_marks(state, buffer_id, &mut out);
         return out;
     }
 
@@ -1214,9 +1240,49 @@ fn scoped_file_summary(state: &EditorState, buffer_id: BufferId) -> Vec<Style> {
             *line_dominant = winner.0;
         }
     }
+    overlay_diagnostic_marks(state, buffer_id, &mut out);
     out
 }
 
+/// Fold diagnostics into the file summary: each line a diagnostic
+/// touches gets the most severe severity's canonical color in
+/// `underline_color` (protocol v6) — the minimap's line marks, the
+/// GPU's equivalent of the TUI's column-0 gutter signs (T M4.6).
+/// Skipped while the URI's store entry is stale: the positions
+/// describe pre-edit text, same discipline as the decorations
+/// producer (the marks return on republish, which bumps the diag
+/// epoch and recomputes this summary).
+fn overlay_diagnostic_marks(state: &EditorState, buffer_id: BufferId, lines: &mut [Style]) {
+    let uri = {
+        let core = state.core.borrow();
+        let Some(uri) = buffer_file_uri(&core, buffer_id) else {
+            return;
+        };
+        uri
+    };
+    let store = state.lsp_manager.borrow().diag_store();
+    let guard = store.lock().expect("diag store mutex poisoned");
+    if guard.is_stale(&uri) {
+        return;
+    }
+    // Most severe per line wins; LSP numbering makes that the
+    // minimum severity value.
+    let mut best: Vec<Option<crate::diag::DiagnosticSeverity>> = vec![None; lines.len()];
+    for d in guard.for_uri(&uri) {
+        for li in d.start_line..=d.end_line {
+            let Some(slot) = best.get_mut(li as usize) else {
+                break;
+            };
+            *slot = Some(slot.map_or(d.severity, |s| s.min(d.severity)));
+        }
+    }
+    for (line, severity) in lines.iter_mut().zip(best) {
+        if let Some(s) = severity {
+            line.underline_color = s.underline_color();
+        }
+    }
+}
+
 /// The buffer's CRDT version projected to a monotonic scalar — the
 /// `generation` anchor for the semantic frame. `0` when the buffer is
 /// absent or not CRDT-backed (a `semantic_render` session always
@@ -2705,4 +2771,70 @@ mod tests {
         assert_eq!(lines[2], kw_style, "line 2 dominated by the LSP token");
         assert_eq!(lines[3], Style::default(), "trailing empty line → default");
     }
+
+    #[test]
+    fn file_style_summary_marks_diagnostic_lines_and_refreshes_on_republish() {
+        use crate::cell::Color;
+        use crate::diag::DiagnosticSeverity;
+
+        let state = empty_state();
+        let mut s = local();
+        let bid = active_buffer(&state);
+        // "abc\nde" with a Warning on line 1 (and a file path so the
+        // buffer has a URI).
+        seed_diagnostic(&state, bid);
+        s.set_viewport(bid, ByteRange { start: 0, end: 64 }, 0);
+
+        let first = s.render_frame(&state);
+        let (_, lines) = summary_of(&first).expect("first frame ships a summary");
+        assert_eq!(lines.len(), 2);
+        assert_eq!(
+            lines[0].underline_color,
+            Color::Default,
+            "clean line carries no mark"
+        );
+        assert_eq!(
+            lines[1].underline_color,
+            DiagnosticSeverity::Warning.underline_color(),
+            "diagnostic line carries the severity mark"
+        );
+
+        // Unchanged generation + diag epoch → suppressed.
+        assert!(summary_of(&s.render_frame(&state)).is_none());
+
+        // A republish moves the diagnostic to line 0 and escalates it.
+        // No CRDT edit happened — the diag epoch alone must re-emit
+        // the summary with refreshed marks.
+        let uri = crate::lsp::path_to_file_uri(std::path::Path::new("/tmp/m114.rs"));
+        state
+            .lsp_manager
+            .borrow()
+            .diag_store()
+            .lock()
+            .expect("diag store")
+            .set(
+                &uri,
+                vec![crate::diag::Diagnostic {
+                    start_line: 0,
+                    start_col: 0,
+                    end_line: 0,
+                    end_col: 3,
+                    severity: DiagnosticSeverity::Error,
+                    message: "boom".into(),
+                    source: None,
+                    code: None,
+                }],
+            );
+        let refreshed = s.render_frame(&state);
+        let (_, lines) = summary_of(&refreshed).expect("diag republish re-emits the summary");
+        assert_eq!(
+            lines[0].underline_color,
+            DiagnosticSeverity::Error.underline_color()
+        );
+        assert_eq!(
+            lines[1].underline_color,
+            Color::Default,
+            "the old warning mark is gone after the republish"
+        );
+    }
 }