diff --git a/CHANGELOG.md b/CHANGELOG.md
index cd505abba..5ebb9e23c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -89,6 +89,24 @@ Open cycle — bug-fix / housekeeping. Entries land here as they merge.
   the per-measurement path). **Measured line metrics are unchanged.** No public API
   or behaviour change.
 
+- **Auto-size font fitting binary-searches the size grid.** A paragraph with
+  `autoSize(...)` resolved its font size by scanning every step from max down to
+  min, re-measuring the line at each candidate (up to ~50 measurements). Line width
+  is linear in font size, so the fit is monotonic — the search now binary-searches
+  the grid for the same boundary in ~log2(n) measurements instead of n. **Output is
+  byte-identical** — it returns the same grid size the linear scan did (covered by
+  the existing auto-size integration and snapshot tests). No public API or behaviour
+  change.
+
+### Deprecations
+
+- **`Font.adjustFontSizeToFit(...)` is deprecated.** The engine-internal
+  `Font#adjustFontSizeToFit` (and its `PdfFont` / `WordFont` implementations) is
+  unused and incorrect — the only real implementation re-measured with the
+  unchanged style, so it always returned the minimum size. Canonical auto-size is
+  resolved by the layout compiler. The method is kept for binary compatibility and
+  scheduled for removal in the next major.
+
 ### Tests / tooling
 
 - **Benchmark regression gate and measurement probe (benchmarks module, not part
diff --git a/src/main/java/com/demcha/compose/document/layout/TextFlowSupport.java b/src/main/java/com/demcha/compose/document/layout/TextFlowSupport.java
index 0da49991a..7155f88f7 100644
--- a/src/main/java/com/demcha/compose/document/layout/TextFlowSupport.java
+++ b/src/main/java/com/demcha/compose/document/layout/TextFlowSupport.java
@@ -585,15 +585,30 @@ private static DocumentTextStyle resolveAutoSizeTextStyle(ParagraphNode node,
         double minSize = autoSize.minSize();
         double step = Math.max(0.1, autoSize.step());
 
-        // Single-line text: shrink the font size until the longest logical line
-        // measures inside the available inner width, otherwise fall back to the
-        // smallest configured size.
-        for (double size = maxSize; size >= minSize - 1e-6; size -= step) {
-            DocumentTextStyle candidate = baseStyle.withSize(size);
+        // Single-line text: pick the largest grid size (maxSize, maxSize-step, …,
+        // down to >= minSize) whose longest logical line measures inside the
+        // available inner width, otherwise fall back to the smallest configured
+        // size. The fit predicate is monotonic in size (line width is linear in
+        // size), so binary-search the grid for the boundary instead of measuring
+        // at every step — the same size the linear scan returned, in ~log2(n)
+        // measurements rather than n.
+        int maxStepCount = (int) Math.floor((maxSize - minSize + 1e-6) / step);
+        int lo = 0;
+        int hi = maxStepCount;
+        int fitStep = -1;
+        while (lo <= hi) {
+            int mid = (lo + hi) >>> 1;
+            DocumentTextStyle candidate = baseStyle.withSize(maxSize - mid * step);
             if (paragraphFitsSingleLine(node, candidate, innerWidth, measurement)) {
-                return candidate;
+                fitStep = mid;      // fits — try a larger size (fewer steps down)
+                hi = mid - 1;
+            } else {
+                lo = mid + 1;       // too wide — need a smaller size (more steps)
             }
         }
+        if (fitStep >= 0) {
+            return baseStyle.withSize(maxSize - fitStep * step);
+        }
         return baseStyle.withSize(minSize);
     }
 
diff --git a/src/main/java/com/demcha/compose/engine/font/Font.java b/src/main/java/com/demcha/compose/engine/font/Font.java
index d6e87b8d3..a64a214df 100644
--- a/src/main/java/com/demcha/compose/engine/font/Font.java
+++ b/src/main/java/com/demcha/compose/engine/font/Font.java
@@ -46,7 +46,20 @@ default T fontType(TextDecoration textDecoration) {
 
     double scale(double size);
 
-    public TextStyle adjustFontSizeToFit(String text, TextStyle style, double availableWidth);
+    /**
+     * @param text           the text to fit
+     * @param style          the starting style
+     * @param availableWidth the width to fit within
+     * @return a re-sized style
+     * @deprecated Unused and incorrect: the only real implementation re-measures
+     *     with the unchanged {@code style}, so the loop never converges and the
+     *     result is always the minimum size. Canonical auto-size is resolved by the
+     *     layout compiler ({@code TextFlowSupport.resolveAutoSizeTextStyle}); this
+     *     method has no callers and is kept only for binary compatibility,
+     *     scheduled for removal in the next major.
+     */
+    @Deprecated
+    TextStyle adjustFontSizeToFit(String text, TextStyle style, double availableWidth);
 
     ContentSize getTightBounds(String text, TextStyle style);
 }
diff --git a/src/main/java/com/demcha/compose/engine/render/pdf/PdfFont.java b/src/main/java/com/demcha/compose/engine/render/pdf/PdfFont.java
index f84eb24b3..09386ce4d 100644
--- a/src/main/java/com/demcha/compose/engine/render/pdf/PdfFont.java
+++ b/src/main/java/com/demcha/compose/engine/render/pdf/PdfFont.java
@@ -29,7 +29,18 @@ public PdfFont(PDFont defaultFont, PDFont bold, PDFont italic, PDFont boldItalic
     }
 
 
-    // Adjust font size automatically based on the width of the text and available space
+    /**
+     * @param text           the text to fit
+     * @param style          the starting style
+     * @param availableWidth the width to fit within
+     * @return a re-sized style
+     * @deprecated Unused and incorrect — it re-measures with the unchanged
+     *     {@code style}, so {@code textWidth} never shrinks and the loop runs to the
+     *     minimum size regardless of fit. Canonical auto-size is handled by the
+     *     layout compiler; kept only for binary compatibility.
+     */
+    @Deprecated
+    @Override
     public TextStyle adjustFontSizeToFit(String text, TextStyle style, double availableWidth) {
         double textWidth = getTextWidth(style, text);
 
@@ -39,7 +50,6 @@ public TextStyle adjustFontSizeToFit(String text, TextStyle style, double availa
             newSize--;  // Reduce size
             textWidth = getTextWidth(style, text);  // Recalculate text width
         }
-        PDFont pdFont = fontType(style.decoration());
         return new TextStyle(style.fontName(), newSize, style.decoration(), style.color());
     }
 
diff --git a/src/main/java/com/demcha/compose/engine/render/word/WordFont.java b/src/main/java/com/demcha/compose/engine/render/word/WordFont.java
index db35a7b9f..2e3e93941 100644
--- a/src/main/java/com/demcha/compose/engine/render/word/WordFont.java
+++ b/src/main/java/com/demcha/compose/engine/render/word/WordFont.java
@@ -83,11 +83,14 @@ public double scale(double size) {
     }
 
     /**
-     * @param text
-     * @param style
-     * @param availableWidth
-     * @return
+     * @param text           the text to fit
+     * @param style          the starting style
+     * @param availableWidth the width to fit within
+     * @return {@code null} — never implemented
+     * @deprecated Unused; never implemented (returns {@code null}). Kept only for
+     *     binary compatibility. See {@code Font#adjustFontSizeToFit}.
      */
+    @Deprecated
     @Override
     public TextStyle adjustFontSizeToFit(String text, TextStyle style, double availableWidth) {
         return null;