diff --git a/static/boot.js b/static/boot.js
index 42de86d..2aa4b09 100644
--- a/static/boot.js
+++ b/static/boot.js
@@ -145,6 +145,7 @@ $('modelSelect').onchange=async()=>{
 };
 $('msg').addEventListener('input',()=>{
   autoResize();
+  updateSendBtn();
   const text=$('msg').value;
   if(text.startsWith('/')&&text.indexOf('\n')===-1){
     const prefix=text.slice(1);
diff --git a/static/index.html b/static/index.html
index 84d04d7..0a6ab94 100644
--- a/static/index.html
+++ b/static/index.html
@@ -231,9 +231,8 @@
             </button>
           </div>
           <div class="composer-right">
-            <button class="send-btn" id="btnSend">
-              <svg width="14" height="14" viewBox="0 0 24 24" fill="currentColor"><path d="M2.01 21L23 12 2.01 3 2 10l15 2-15 2z"/></svg>
-              Send
+            <button class="send-btn" id="btnSend" title="Send message" style="display:none">
+              <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"><line x1="12" y1="19" x2="12" y2="5"/><polyline points="5 12 12 5 19 12"/></svg>
             </button>
           </div>
         </div>
diff --git a/static/messages.js b/static/messages.js
index 6e7cbc0..fd37cb8 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -237,7 +237,7 @@ function transcript(){
   return lines.join('\n');
 }
 
-function autoResize(){const el=$('msg');el.style.height='auto';el.style.height=Math.min(el.scrollHeight,200)+'px';}
+function autoResize(){const el=$('msg');el.style.height='auto';el.style.height=Math.min(el.scrollHeight,200)+'px';updateSendBtn();}
 
 
 // ── Approval polling ──
diff --git a/static/style.css b/static/style.css
index 1bd8879..0269f41 100644
--- a/static/style.css
+++ b/static/style.css
@@ -193,10 +193,12 @@
   .mic-status{font-size:11px;color:#e94560;padding:4px 12px;display:flex;align-items:center;gap:6px;}
   .mic-dot{width:6px;height:6px;border-radius:50%;background:#e94560;animation:mic-pulse 1.2s ease-in-out infinite;flex-shrink:0;}
   .status-text{font-size:11px;color:var(--muted);padding-left:4px;}
-  .send-btn{padding:7px 18px;border-radius:10px;font-size:13px;font-weight:600;background:linear-gradient(135deg,#5ba8f5,#7cb9ff);border:none;color:#0a1628;cursor:pointer;display:flex;align-items:center;gap:6px;transition:all .15s;flex-shrink:0;letter-spacing:.01em;}
-  .send-btn:hover{background:linear-gradient(135deg,#7cb9ff,#a0d0ff);transform:translateY(-1px);}
-  .send-btn:active{transform:translateY(0);}
-  .send-btn:disabled{opacity:.4;cursor:not-allowed;}
+  .send-btn{width:34px;height:34px;border-radius:50%;background:#7cb9ff;border:none;color:#0a1628;cursor:pointer;display:flex;align-items:center;justify-content:center;flex-shrink:0;transition:background .15s,transform .15s,box-shadow .15s;box-shadow:0 2px 8px rgba(124,185,255,.35);}
+  .send-btn:hover{background:#a0d0ff;transform:scale(1.08);box-shadow:0 4px 14px rgba(124,185,255,.5);}
+  .send-btn:active{transform:scale(0.95);box-shadow:0 1px 4px rgba(124,185,255,.25);}
+  .send-btn:disabled{opacity:.35;cursor:not-allowed;transform:none;box-shadow:none;}
+  .send-btn.visible{animation:send-pop-in .18s cubic-bezier(.34,1.56,.64,1) forwards;}
+  @keyframes send-pop-in{from{opacity:0;transform:scale(.55);}to{opacity:1;transform:scale(1);}}
   .upload-bar-wrap{display:none;height:3px;background:rgba(255,255,255,.06);border-radius:0 0 16px 16px;overflow:hidden;}
   .upload-bar-wrap.active{display:block;}
   .upload-bar{height:100%;background:linear-gradient(90deg,var(--blue),#a0d0ff);width:0%;transition:width .3s ease;}
@@ -276,7 +278,7 @@
     .composer-wrap{padding:8px 10px 12px!important;}
     .composer-box{border-radius:12px;}
     .composer-box textarea{font-size:16px;min-height:40px;}
-    .send-btn{padding:6px 14px;font-size:13px;}
+    .send-btn{width:32px;height:32px;}
     /* Empty state */
     .empty-state h2{font-size:18px;}
     .empty-state p{font-size:13px;}
diff --git a/static/ui.js b/static/ui.js
index e71352f..f18c6d2 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -187,9 +187,25 @@ function setStatus(t){
     if(dismiss)dismiss.style.display=(!transient && !S.busy)?'inline':'none';
   }
 }
+function updateSendBtn(){
+  const btn=$('btnSend');
+  if(!btn) return;
+  const hasContent=$('msg').value.trim().length>0||S.pendingFiles.length>0;
+  const shouldShow=hasContent&&!S.busy;
+  if(shouldShow&&btn.style.display==='none'){
+    btn.style.display='';
+    // Remove then re-add class to retrigger animation each time
+    btn.classList.remove('visible');
+    requestAnimationFrame(()=>btn.classList.add('visible'));
+  } else if(!shouldShow&&btn.style.display!=='none'){
+    btn.style.display='none';
+    btn.classList.remove('visible');
+  }
+}
 function setBusy(v){
   S.busy=v;
   $('btnSend').disabled=v;
+  updateSendBtn();
   const dots=$('activityDots');
   if(dots) dots.style.display=v?'flex':'none';
   if(!v){
@@ -920,8 +936,9 @@ async function promptNewFolder(){
 
 function renderTray(){
   const tray=$('attachTray');tray.innerHTML='';
-  if(!S.pendingFiles.length){tray.classList.remove('has-files');return;}
+  if(!S.pendingFiles.length){tray.classList.remove('has-files');updateSendBtn();return;}
   tray.classList.add('has-files');
+  updateSendBtn();
   S.pendingFiles.forEach((f,i)=>{
     const chip=document.createElement('div');chip.className='attach-chip';
     chip.innerHTML=`&#128206; ${esc(f.name)} <button title="Remove">&#10005;</button>`;
diff --git a/tests/test_sprint21.py b/tests/test_sprint21.py
new file mode 100644
index 0000000..5aa70bf
--- /dev/null
+++ b/tests/test_sprint21.py
@@ -0,0 +1,349 @@
+"""
+Sprint 21 Tests: Send button polish — hidden until content, pop-in animation,
+icon-only circle design.
+"""
+import re
+import urllib.request
+
+BASE = "http://127.0.0.1:8788"
+
+
+def get_text(path):
+    with urllib.request.urlopen(BASE + path, timeout=10) as r:
+        return r.read().decode(), r.status
+
+
+# ── index.html ────────────────────────────────────────────────────────────
+
+
+def test_send_button_present():
+    """btnSend must still exist in the DOM."""
+    html, status = get_text("/")
+    assert status == 200
+    assert 'id="btnSend"' in html
+
+
+def test_send_button_hidden_by_default():
+    """btnSend must start hidden (display:none) — only shown when there is content."""
+    html, _ = get_text("/")
+    btn_match = re.search(r'id="btnSend"[^>]*>', html)
+    assert btn_match, "btnSend element not found"
+    assert 'display:none' in btn_match.group(0)
+
+
+def test_send_button_no_text_label():
+    """Send button must be icon-only — no visible 'Send' text label."""
+    html, _ = get_text("/")
+    # Find the full button element (from opening tag to closing tag)
+    btn_open_end = html.find('>', html.find('id="btnSend"')) + 1
+    btn_end = html.find('</button>', btn_open_end) + len('</button>')
+    btn_inner = html[btn_open_end:btn_end]
+    # Strip SVG content and any remaining tags; check visible text
+    no_svg = re.sub(r'<svg[^>]*>.*?</svg>', '', btn_inner, flags=re.DOTALL)
+    visible_text = re.sub(r'<[^>]+>', '', no_svg).strip()
+    assert visible_text == '', f"Send button has visible text: {visible_text!r}"
+
+
+def test_send_button_has_svg_icon():
+    """Send button must have an SVG icon."""
+    html, _ = get_text("/")
+    btn_start = html.find('id="btnSend"')
+    btn_end = html.find('</button>', btn_start) + len('</button>')
+    btn_html = html[btn_start:btn_end]
+    assert '<svg' in btn_html
+
+
+def test_send_button_has_title_attribute():
+    """btnSend must have a title attribute for accessibility (replaces text label)."""
+    html, _ = get_text("/")
+    btn_match = re.search(r'id="btnSend"[^>]*>', html)
+    assert btn_match
+    assert 'title=' in btn_match.group(0)
+
+
+def test_send_button_svg_arrow_up():
+    """Send button SVG should use an upward arrow (line + polyline or path)."""
+    html, _ = get_text("/")
+    btn_start = html.find('id="btnSend"')
+    btn_end = html.find('</button>', btn_start) + len('</button>')
+    btn_html = html[btn_start:btn_end]
+    # Must have some directional shape element
+    has_shape = ('<line' in btn_html or '<polyline' in btn_html or
+                 '<polygon' in btn_html or '<path' in btn_html)
+    assert has_shape, "Send button SVG missing directional shape"
+
+
+# ── style.css ────────────────────────────────────────────────────────────
+
+
+def test_send_btn_is_circle():
+    """send-btn must use border-radius:50% for the circle shape."""
+    css, status = get_text("/static/style.css")
+    assert status == 200
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'border-radius:50%' in rule or 'border-radius: 50%' in rule
+
+
+def test_send_btn_fixed_dimensions():
+    """send-btn must have explicit width and height (icon-circle, not text-padded)."""
+    css, _ = get_text("/static/style.css")
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'width:' in rule or 'width :' in rule
+    assert 'height:' in rule or 'height :' in rule
+
+
+def test_send_btn_no_old_padding():
+    """send-btn must not use text padding layout (old pill style removed)."""
+    css, _ = get_text("/static/style.css")
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    # Old style used padding:7px 18px — should be gone
+    assert 'padding:7px' not in rule and 'padding: 7px' not in rule
+
+
+def test_send_btn_blue_background():
+    """send-btn background must use the blue accent (#7cb9ff or similar)."""
+    css, _ = get_text("/static/style.css")
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert '7cb9ff' in rule or '5ba8f5' in rule or 'var(--blue)' in rule
+
+
+def test_send_btn_has_transition():
+    """send-btn must have transition for smooth hover/active states."""
+    css, _ = get_text("/static/style.css")
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'transition' in rule
+
+
+def test_send_btn_has_box_shadow():
+    """send-btn must have a box-shadow glow effect."""
+    css, _ = get_text("/static/style.css")
+    send_idx = css.find('.send-btn{')
+    brace_open = css.find('{', send_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'box-shadow' in rule
+
+
+def test_send_btn_hover_has_scale():
+    """send-btn:hover must use transform:scale for a satisfying hover effect."""
+    css, _ = get_text("/static/style.css")
+    hover_idx = css.find('.send-btn:hover{')
+    brace_open = css.find('{', hover_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'scale' in rule
+
+
+def test_send_btn_active_shrinks():
+    """send-btn:active must scale down slightly for tactile press feedback."""
+    css, _ = get_text("/static/style.css")
+    active_idx = css.find('.send-btn:active{')
+    brace_open = css.find('{', active_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'scale' in rule
+
+
+def test_send_btn_disabled_rule_exists():
+    """send-btn:disabled must still be styled."""
+    css, _ = get_text("/static/style.css")
+    assert '.send-btn:disabled' in css
+
+
+def test_send_btn_visible_class_defined():
+    """.send-btn.visible class must be defined for the pop-in animation."""
+    css, _ = get_text("/static/style.css")
+    assert '.send-btn.visible' in css
+
+
+def test_send_pop_in_keyframes_defined():
+    """@keyframes send-pop-in must be defined."""
+    css, _ = get_text("/static/style.css")
+    assert 'send-pop-in' in css
+    assert '@keyframes' in css
+
+
+def test_send_pop_in_uses_scale():
+    """send-pop-in keyframe must animate from a scaled-down state."""
+    css, _ = get_text("/static/style.css")
+    kf_idx = css.find('send-pop-in')
+    kf_start = css.rfind('@keyframes', 0, kf_idx)
+    # Find matching closing brace (simple scan)
+    depth = 0
+    kf_end = kf_start
+    for i, ch in enumerate(css[kf_start:], kf_start):
+        if ch == '{':
+            depth += 1
+        elif ch == '}':
+            depth -= 1
+            if depth == 0:
+                kf_end = i
+                break
+    kf_rule = css[kf_start:kf_end]
+    assert 'scale' in kf_rule
+
+
+def test_send_pop_in_uses_opacity():
+    """send-pop-in keyframe must fade in (opacity transition)."""
+    css, _ = get_text("/static/style.css")
+    kf_idx = css.find('send-pop-in')
+    kf_start = css.rfind('@keyframes', 0, kf_idx)
+    depth = 0
+    kf_end = kf_start
+    for i, ch in enumerate(css[kf_start:], kf_start):
+        if ch == '{':
+            depth += 1
+        elif ch == '}':
+            depth -= 1
+            if depth == 0:
+                kf_end = i
+                break
+    kf_rule = css[kf_start:kf_end]
+    assert 'opacity' in kf_rule
+
+
+def test_send_btn_mobile_override_no_padding():
+    """Mobile override for send-btn must not add text padding (keeps circle shape)."""
+    css, _ = get_text("/static/style.css")
+    # Find the @media block
+    media_idx = css.find('@media')
+    send_mobile_idx = css.find('.send-btn', media_idx)
+    if send_mobile_idx == -1:
+        return  # No mobile override, fine
+    brace_open = css.find('{', send_mobile_idx)
+    brace_close = css.find('}', brace_open)
+    rule = css[brace_open:brace_close]
+    assert 'padding:' not in rule and 'font-size' not in rule
+
+
+# ── ui.js ─────────────────────────────────────────────────────────────────
+
+
+def test_ui_js_update_send_btn_function():
+    """ui.js must define updateSendBtn() function."""
+    js, status = get_text("/static/ui.js")
+    assert status == 200
+    assert 'function updateSendBtn' in js
+
+
+def test_update_send_btn_checks_content():
+    """updateSendBtn must check textarea value length."""
+    js, _ = get_text("/static/ui.js")
+    fn_idx = js.find('function updateSendBtn')
+    fn_end = js.find('\n}', fn_idx) + 2
+    fn_body = js[fn_idx:fn_end]
+    assert 'msg' in fn_body
+    assert '.value' in fn_body
+    assert '.length' in fn_body or '.trim()' in fn_body
+
+
+def test_update_send_btn_checks_pending_files():
+    """updateSendBtn must also show send button when files are attached."""
+    js, _ = get_text("/static/ui.js")
+    fn_idx = js.find('function updateSendBtn')
+    fn_end = js.find('\n}', fn_idx) + 2
+    fn_body = js[fn_idx:fn_end]
+    assert 'pendingFiles' in fn_body
+
+
+def test_update_send_btn_uses_visible_class():
+    """updateSendBtn must add .visible class to trigger the pop-in animation."""
+    js, _ = get_text("/static/ui.js")
+    fn_idx = js.find('function updateSendBtn')
+    fn_end = js.find('\n}', fn_idx) + 2
+    fn_body = js[fn_idx:fn_end]
+    assert 'visible' in fn_body
+
+
+def test_update_send_btn_uses_display_none():
+    """updateSendBtn must hide the button with display:none when no content."""
+    js, _ = get_text("/static/ui.js")
+    fn_idx = js.find('function updateSendBtn')
+    fn_end = js.find('\n}', fn_idx) + 2
+    fn_body = js[fn_idx:fn_end]
+    assert 'display' in fn_body
+    assert 'none' in fn_body
+
+
+def test_set_busy_calls_update_send_btn():
+    """setBusy must call updateSendBtn() so button hides while agent is responding."""
+    js, _ = get_text("/static/ui.js")
+    busy_idx = js.find('function setBusy')
+    busy_end = js.find('\n}', busy_idx) + 2
+    busy_body = js[busy_idx:busy_end]
+    assert 'updateSendBtn' in busy_body
+
+
+def test_render_tray_calls_update_send_btn():
+    """renderTray must call updateSendBtn() so button appears when files are attached."""
+    js, _ = get_text("/static/ui.js")
+    tray_idx = js.find('function renderTray')
+    tray_end = js.find('\n}', tray_idx) + 2
+    tray_body = js[tray_idx:tray_end]
+    assert 'updateSendBtn' in tray_body
+
+
+# ── boot.js ──────────────────────────────────────────────────────────────
+
+
+def test_boot_js_input_calls_update_send_btn():
+    """boot.js input event listener must call updateSendBtn()."""
+    js, status = get_text("/static/boot.js")
+    assert status == 200
+    assert 'updateSendBtn' in js
+
+
+# ── messages.js ───────────────────────────────────────────────────────────
+
+
+def test_auto_resize_calls_update_send_btn():
+    """autoResize() must call updateSendBtn() so button hides after send clears textarea."""
+    js, status = get_text("/static/messages.js")
+    assert status == 200
+    assert 'updateSendBtn' in js
+
+
+# ── Regression: existing behaviour unchanged ──────────────────────────────
+
+
+def test_send_button_still_has_send_btn_class():
+    """btnSend must still carry class='send-btn' for CSS targeting."""
+    html, _ = get_text("/")
+    assert 'class="send-btn"' in html
+
+
+def test_ui_js_set_busy_still_disables_btn():
+    """setBusy must still set btnSend.disabled (not just hide it)."""
+    js, _ = get_text("/static/ui.js")
+    busy_idx = js.find('function setBusy')
+    busy_end = js.find('\n}', busy_idx) + 2
+    busy_body = js[busy_idx:busy_end]
+    assert "btnSend" in busy_body
+    assert 'disabled' in busy_body
+
+
+def test_index_html_attach_button_unchanged():
+    """btnAttach must still be present (no regression)."""
+    html, _ = get_text("/")
+    assert 'id="btnAttach"' in html
+
+
+def test_send_function_still_exists():
+    """send() function must still be defined in messages.js."""
+    js, _ = get_text("/static/messages.js")
+    assert 'async function send()' in js