diff --git a/src/styles/style.css b/src/styles/style.css
index 7a17170..56eee0f 100644
--- a/src/styles/style.css
+++ b/src/styles/style.css
@@ -814,10 +814,13 @@ a:hover {
 .btn-assistant {
   background: var(--agent-purple-dark);
   border-color: var(--agent-purple-dark);
-  color: #ffffff;
+  color: var(--control-fg-on-emphasis, #ffffff);
 }
 .btn-assistant:hover:not(:disabled) {
-  background: var(--agent-purple);
+  /* Dark mode's --agent-purple is a light lavender meant for text/accents,
+     not a button fill with light text, so hover darkens instead of
+     switching to it (fixes a ~1.9:1 contrast failure). */
+  filter: brightness(1.12);
 }
 .btn-assistant:disabled {
   background: var(--agent-purple-dark);
diff --git a/test/slm.test.js b/test/slm.test.js
index c512834..1a86dea 100644
--- a/test/slm.test.js
+++ b/test/slm.test.js
@@ -543,8 +543,11 @@ describe('assistant markup', () => {
 
     const hoverStart = css.indexOf('.btn-assistant:hover:not(:disabled) {');
     expect(hoverStart).toBeGreaterThan(-1);
+    // Hover brightens the base fill rather than swapping to --agent-purple,
+    // since that token is a light lavender in dark mode and would fail
+    // WCAG contrast against the button's white/dark text.
     const hoverRule = css.slice(hoverStart, css.indexOf('}', hoverStart));
-    expect(hoverRule).toContain('var(--agent-purple)');
+    expect(hoverRule).toContain('brightness');
   });
 
   it('announces status updates to assistive technology', () => {