diff --git a/.fork/customizations.yaml b/.fork/customizations.yaml
index 2bf7078900c..2f3d927f9e0 100644
--- a/.fork/customizations.yaml
+++ b/.fork/customizations.yaml
@@ -615,7 +615,12 @@
     the latch survive thread switches, and "only an empty prompt turns it off"
     is a rule about one composing session, not the component's lifetime.
     Which controls sit where is the whole point. Inside the box: the model
-    pill, the effort pill, and the send/stop button. Everything else is on the
+    pill, the effort pill, and the send/stop button. In dark mode a send
+    button without stage-channel art — any stage label that is not Dev/Nightly
+    — is flat pure white with a black icon
+    (data-fork-composer-send-tone="flat"); Dev and Nightly keep the
+    stage-channel fill, and light builds keep upstream's fill. Everything else
+    is on the
     control row below it — runtime mode ("Auto"), interaction mode ("Build"),
     the plan-sidebar toggle, the context meter, the preparing-worktree line,
     the no-provider badge, and, at the row's right end, the worktree/branch
@@ -711,7 +716,9 @@
     # sibling of the composer.
     - apps/web/src/components/ChatView.tsx
     # data-fork-composer-action on the send and stop buttons; the whole 24px
-    # square restyle hangs off it.
+    # square restyle hangs off it. Send also carries
+    # data-fork-composer-send-tone ("channel" with stage art, "flat"
+    # otherwise) for the dark-mode white/black fill.
     - apps/web/src/components/chat/ComposerPrimaryActions.tsx
     # The glass-shell pseudo-elements and .chat-composer-context-strip this
     # customization switches off. An upstream rework of that paint would leave
diff --git a/apps/web/src/__fork_guards__/forkComposerShell.test.ts b/apps/web/src/__fork_guards__/forkComposerShell.test.ts
index 540593c7396..5a73ce20bda 100644
--- a/apps/web/src/__fork_guards__/forkComposerShell.test.ts
+++ b/apps/web/src/__fork_guards__/forkComposerShell.test.ts
@@ -182,8 +182,21 @@ describe("fork guard: fork-composer-shell", () => {
     expect(theme).toMatch(/\[data-testid="composer-editor"\][\s\S]{0,160}min-height:\s*0/u);
   });
 
-  it("squares the send and stop buttons and reddens stop", () => {
+  it("squares the send and stop buttons, flattens release send, and reddens stop", () => {
     expect(theme).toMatch(/\[data-fork-composer-action\]\s*\{[\s\S]{0,200}border-radius:\s*6px/u);
+    // Flat (no stage art) is pure white / black icon in dark mode only; light
+    // and unmarked builds keep upstream's fill, so the rule must stay anchored
+    // on the dark-scoped marker prefix. Dev/Nightly keep tone=channel and the
+    // stage art; do not collapse them.
+    expect(primaryActions).toMatch(
+      /data-fork-composer-send-tone=\{\s*stageBackdropVariant\s*\?\s*"channel"\s*:\s*"flat"\s*\}/u,
+    );
+    expect(theme).toMatch(
+      /:root\[data-fork="noahhendrickson-t3code"\]\.dark\s+\[data-fork-composer-action="send"\]\[data-fork-composer-send-tone="flat"\][\s\S]{0,200}background:\s*#ffffff/u,
+    );
+    expect(theme).toMatch(
+      /:root\[data-fork="noahhendrickson-t3code"\]\.dark\s+\[data-fork-composer-action="send"\]\[data-fork-composer-send-tone="flat"\][\s\S]{0,200}color:\s*#000000/u,
+    );
     expect(theme).toMatch(
       /\[data-fork-composer-action="stop"\][\s\S]{0,120}background:\s*#ea3150/u,
     );
diff --git a/apps/web/src/components/chat/ComposerPrimaryActions.tsx b/apps/web/src/components/chat/ComposerPrimaryActions.tsx
index afeab513f60..c08098ba0a2 100644
--- a/apps/web/src/components/chat/ComposerPrimaryActions.tsx
+++ b/apps/web/src/components/chat/ComposerPrimaryActions.tsx
@@ -203,6 +203,7 @@ export const ComposerPrimaryActions = memo(function ComposerPrimaryActions({
       type="submit"
       /* fork:begin fork-composer-shell — see .fork/customizations.yaml#fork-composer-shell */
       data-fork-composer-action="send"
+      data-fork-composer-send-tone={stageBackdropVariant ? "channel" : "flat"}
       /* fork:end fork-composer-shell */
       className={cn(
         "relative isolate flex h-9 w-9 items-center justify-center overflow-hidden rounded-full text-primary-foreground shadow-xs transition-all duration-150 enabled:cursor-pointer enabled:inset-shadow-[0_1px_--theme(--color-white/16%)] hover:scale-105 active:inset-shadow-[0_1px_--theme(--color-black/8%)] active:shadow-none disabled:pointer-events-none disabled:opacity-30 disabled:shadow-none disabled:hover:scale-100 sm:h-8 sm:w-8",
diff --git a/apps/web/src/theme.custom.css b/apps/web/src/theme.custom.css
index 56f52cb4f6a..17e266087ff 100644
--- a/apps/web/src/theme.custom.css
+++ b/apps/web/src/theme.custom.css
@@ -434,6 +434,19 @@
   border-radius: 6px;
 }
 
+/* Flat send (tone="flat"): in dark mode, a send button without stage-channel
+   art — any stage label that is not Dev/Nightly — is flat pure white with a
+   black icon. Dev/Nightly keep the stage-channel fill (tone="channel").
+   Unlayered, so this beats upstream's Tailwind fill; the fenced attribute
+   survives upstream className rewrites. Unmarked or light builds never match
+   and fall through to upstream's primary fill. */
+:root[data-fork="noahhendrickson-t3code"].dark
+  [data-fork-composer-action="send"][data-fork-composer-send-tone="flat"] {
+  background: #ffffff;
+  color: #000000;
+  box-shadow: none;
+}
+
 :root[data-fork="noahhendrickson-t3code"].dark [data-fork-composer-action="stop"] {
   background: #ea3150;
 }