diff --git a/apps/desktop/src/styles/messages.css b/apps/desktop/src/styles/messages.css
index 68f78ca38d..237664b2a4 100644
--- a/apps/desktop/src/styles/messages.css
+++ b/apps/desktop/src/styles/messages.css
@@ -1646,7 +1646,8 @@
    (D319) and the nested `.subagent-run`. */
 
 .message-error {
-  width: min(100%, 680px);
+  /* The message column owns the dynamic chat width; do not add a second cap. */
+  width: 100%;
   border-radius: var(--radius-md);
   background: color-mix(in oklab, var(--ds-error) 7%, transparent);
   padding: 10px 12px;
diff --git a/apps/desktop/test/transcript-style.test.mjs b/apps/desktop/test/transcript-style.test.mjs
index effedd231e..e8c8f43736 100644
--- a/apps/desktop/test/transcript-style.test.mjs
+++ b/apps/desktop/test/transcript-style.test.mjs
@@ -194,6 +194,31 @@ test("assistant turns stay transparent full-width prose", () => {
     /\.tool-activity-group\.has-subagents\s*\{[^}]*background:\s*var\(--ds-tile\)/,
   );
 });
+test("assistant error cards follow the responsive transcript column", () => {
+  const errorCard = stylesSource.match(/\n\.message-error \{([^}]*)\}/)?.[1];
+  assert.ok(errorCard);
+  assert.match(errorCard, /width:\s*100%;/);
+  assert.doesNotMatch(errorCard, /max-width\s*:/);
+  assert.match(
+    stylesSource,
+    /\.message-row\.assistant \.message-col,\s*\.message-row\.system \.message-col,\s*\.message-row\.tool \.message-col \{\s*width:\s*min\(100%,\s*var\(--chat-prose-max-width,\s*720px\)\);/,
+  );
+});
+
+test("decision and outcome cards follow the responsive transcript band", () => {
+  const cardRules = [
+    stylesSource.match(/\n\.permission-card \{([^}]*)\}/)?.[1],
+    stylesSource.match(/\n\.asktool-card \{([^}]*)\}/)?.[1],
+    stylesSource.match(/\n\.turn-outcome-card \{([^}]*)\}/)?.[1],
+  ];
+  for (const rule of cardRules) {
+    assert.ok(rule);
+    assert.match(
+      rule,
+      /width:\s*min\(100%,\s*var\(--chat-prose-max-width,\s*720px\)\);/,
+    );
+  }
+});
 
 test("transcript density and hover actions are quiet", () => {
   assert.match(stylesSource, /\.message-row \{[\s\S]*?padding:\s*12px 0;/);
diff --git a/docs/spec/04-ux/08-component-spec.md b/docs/spec/04-ux/08-component-spec.md
index f915ee19e6..9c54367975 100644
--- a/docs/spec/04-ux/08-component-spec.md
+++ b/docs/spec/04-ux/08-component-spec.md
@@ -1627,8 +1627,9 @@ Single message render — either user (plaintext) or assistant (markdown streami
 ### 8.3 Layout
 
 - Max content band: 760px default, user-resizable via dual edge handles
-  (D439 / ADR 0277). Assistant, tool, and decision rows follow the band.
-  User plates stay `min(82%, 600px)`.
+  (D439 / ADR 0277). Assistant, tool, decision, and structured assistant-error
+  rows follow the band; error cards fill their message column without a second
+  fixed width cap. User plates stay `min(82%, 600px)`.
 - The live band is `min(available pane, preferred)`. Collapsing the sidebar
   no longer tightens a 640px ceiling; the outer pane stays fluid and the
   width transition follows the sidebar dock.
diff --git a/docs/spec/06-delivery/04-e2e-test-plan.md b/docs/spec/06-delivery/04-e2e-test-plan.md
index a04ae25419..44249df78f 100644
--- a/docs/spec/06-delivery/04-e2e-test-plan.md
+++ b/docs/spec/06-delivery/04-e2e-test-plan.md
@@ -9089,7 +9089,7 @@ This test plan spec is accepted when:
 ### US-UI-60c Compact assistant error card
 - Trigger a retriable provider/model failure in the transcript in light and dark themes.
 - Expect the assistant error to use a restrained inline surface with a thin error rail. The localized summary, stable code, and details disclosure share one compact header; the card does not render a second bottom action row.
-- Confirm the details remain expanded on first render, keep the redacted provider response and provider/model IDs, and expose an icon-only copy control with an accessible label/tooltip. On a narrow window, the header actions wrap without horizontal overflow.
+- Confirm the details remain expanded on first render, keep the redacted provider response and provider/model IDs, and expose an icon-only copy control with an accessible label/tooltip. On a narrow window, header actions wrap without horizontal overflow. At default and widened chat content widths, the card fills the assistant column; resizing the content band makes the card follow it, and a constrained pane does not overflow.
 - Expect the compact assistant error card itself to expose one localized **Continue** action beside the details disclosure. Click it and expect the app to append the localized continuation prompt (`Continue the current task` / `继续当前任务`) to the same session and start the next turn without truncating the failed turn.
 - For a terminal `PROVIDER_RATE_LIMITED` (including HTTP 429), expect the
   structured assistant error card to remain the only failure surface: it