diff --git a/AGENTS.md b/AGENTS.md
index a49d61b..2c0a552 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -61,6 +61,25 @@ renders as a blank canvas that looks exactly like a broken app.
 is missing" from "the data is off-screen" — which is how a viewport bug that
 three passing API checks had missed was finally found.
 
+## Diagnostic mode
+
+Chart geometry bugs live in the browser, which is usually on a different machine
+from whoever is debugging them. Rather than asking for console pastes:
+
+```
+open the chart with ?diag=1          # remembered until ?diag=0
+docker compose logs api | grep SNAPDBG
+```
+
+With it on, every snap the trendline tool computes is posted to
+`/api/debug/snap` and logged server-side — the cursor's time, price and x, the
+snapped time and price, how many bars were held, the first and last bar, and the
+chart's width. Throttled to about one a second. It reads the client's own
+numbers, which is exactly what "works in my headless run" cannot tell you.
+
+Extend it when the next geometry puzzle appears; the endpoint takes whatever
+fields `SnapReport` declares.
+
 ## Running tests
 
 ```
diff --git a/app/api/routes.py b/app/api/routes.py
index 2d524ba..530748a 100644
--- a/app/api/routes.py
+++ b/app/api/routes.py
@@ -1,3 +1,4 @@
+import logging
 import time
 import uuid
 
@@ -11,6 +12,8 @@ from app.api.deps import require_token
 
 # Everything here needs the token when CHART_AUTH_TOKEN is set. /health and
 # /version live in app.api.meta and stay open on purpose.
+logger = logging.getLogger(__name__)
+
 router = APIRouter(prefix="/api", dependencies=[Depends(require_token)])
 
 
@@ -198,6 +201,35 @@ def create_comment(request: Request, payload: CommentCreate):
     return line.to_dict()
 
 
+class SnapReport(BaseModel):
+    """What the browser computed for one snap, for diagnosing chart geometry."""
+    cursor_t: int | None = None
+    cursor_p: float | None = None
+    cursor_x: float | None = None
+    snapped_t: int | None = None
+    snapped_p: float | None = None
+    bars_held: int | None = None
+    first_bar_t: int | None = None
+    last_bar_t: int | None = None
+    tf: str | None = None
+    chart_w: float | None = None
+    note: str | None = None
+
+
+@router.post("/debug/snap", status_code=204)
+def debug_snap(payload: SnapReport):
+    """Record one snap sample from a browser running diagnostic mode.
+
+    Chart geometry bugs live in the client, and the browser is usually on a
+    different machine from whoever is debugging it — so its own numbers cannot
+    be read any other way. Off unless the page is opened with ?diag=1; see
+    AGENTS.md. Logged at warning level so it appears without reconfiguring
+    uvicorn's log levels.
+    """
+    logger.warning("SNAPDBG %s", payload.model_dump())
+    return Response(status_code=204)
+
+
 @router.get("/drawings")
 def drawings(request: Request):
     """Every drawing, comments included, for the sidebar list."""
diff --git a/screenshot2.png b/screenshot2.png
new file mode 100644
index 0000000..0218960
Binary files /dev/null and b/screenshot2.png differ
diff --git a/static/chart.js b/static/chart.js
index a389fd2..f0cdc6e 100644
--- a/static/chart.js
+++ b/static/chart.js
@@ -48,6 +48,20 @@ class ConfluenceChart {
   // rather than a click. Wide enough to survive a twitch on a deliberate click.
   static DRAG_THRESHOLD = 12;
 
+  /**
+   * Whether to report snap geometry to the server.
+   *
+   * `?diag=1` turns it on and remembers it; `?diag=0` turns it off. Kept out of
+   * the way rather than always on: it exists because the browser runs on a
+   * different machine from whoever is debugging it.
+   */
+  static diagnosticMode() {
+    const asked = new URLSearchParams(location.search).get('diag');
+    if (asked === '1') localStorage.setItem('chart-diag', '1');
+    if (asked === '0') localStorage.removeItem('chart-diag');
+    return localStorage.getItem('chart-diag') === '1';
+  }
+
   static snapToTick(price) {
     return Math.round(price / ConfluenceChart.TICK) * ConfluenceChart.TICK;
   }
@@ -581,7 +595,7 @@ class ConfluenceChart {
     if (!this.armedTool) return;
     const point = this.eventPoint(event);
     if (!point) return;
-    if (this.armedTool !== 'level') this.showSnapDot(this.snapPoint(point));
+    if (this.armedTool !== 'level') this.showSnapDot(this.snapPoint(point), point);
     if (this.gesture) {
       this.gesture.end = point;
       this.renderGesture();
@@ -591,7 +605,7 @@ class ConfluenceChart {
     if (this.pendingAnchor) this.renderPending(this.pendingAnchor, this.snapPoint(point));
   }
 
-  showSnapDot(snapped) {
+  showSnapDot(snapped, point = {}) {
     if (snapped.t == null) return this.hideSnapDot();
     const x = this.chart.timeScale().timeToCoordinate(snapped.t);
     const y = this.candles.priceToCoordinate(snapped.p);
@@ -602,6 +616,38 @@ class ConfluenceChart {
     // Says which it is before you commit, since the extreme decides the side.
     this.snapDot.dataset.side = snapped.snappedSide || '';
 
+    // Diagnostic mode: report what the client computed so it can be read on
+    // the server. Off unless asked for, because the browser is usually on a
+    // different machine and this is the only way to see its numbers.
+    //   turn on:  ?diag=1   (remembered)      turn off:  ?diag=0
+    if (ConfluenceChart.diagnosticMode() && snapped.t != null && this.bars.length) {
+      const step = this.bars.length > 1
+        ? this.bars[this.bars.length - 1].t - this.bars[this.bars.length - 2].t : 60;
+      const drift = point.t == null ? null : Math.abs(snapped.t - point.t);
+      {
+        const now = Date.now();
+        if (!this._lastSnapReport || now - this._lastSnapReport > 900) {
+          this._lastSnapReport = now;
+          const body = {
+            cursor_t: point.t, cursor_p: point.p, cursor_x: Math.round(point.x),
+            snapped_t: snapped.t, snapped_p: snapped.p,
+            bars_held: this.bars.length,
+            first_bar_t: this.bars[0].t, last_bar_t: this.bars[this.bars.length - 1].t,
+            chart_w: this.chartEl.clientWidth,
+            note: point.t == null
+              ? 'coordinateToTime returned null'
+              : `drift ${drift}s (${Math.round(drift / step)} bars), step ${step}s`,
+          };
+          fetch('/api/debug/snap', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json',
+                       ...(localStorage.getItem('chart-token') ? { 'X-Chart-Token': localStorage.getItem('chart-token') } : {}) },
+            body: JSON.stringify(body),
+          }).catch(() => {});
+        }
+      }
+    }
+
     const side = snapped.snappedSide === 'resistance' ? 'H' : snapped.snappedSide === 'support' ? 'L' : '';
     const clock = new Date(snapped.t * 1000)
       .toLocaleTimeString(undefined, { hour12: false, hour: '2-digit', minute: '2-digit' });