diff --git a/wasm/dist-repl/repl.css b/wasm/dist-repl/repl.css
index 244bf2f..8935273 100644
--- a/wasm/dist-repl/repl.css
+++ b/wasm/dist-repl/repl.css
@@ -299,6 +299,9 @@ body.repl {
   opacity: 0.6;
 }
 .transcript .entry.error .tier-result { color: var(--err); }
+/* Error line appended after partial streamed output — keeps the
+ * lines that already showed and tints just the trailing error. */
+.transcript .tier-result .err-line { color: var(--err); }
 .transcript .entry + .entry { margin-top: 0.4rem; }
 
 /* ─── Prompt bar ───────────────────────────────────────────────── */
diff --git a/wasm/dist-repl/repl.js b/wasm/dist-repl/repl.js
index 0cf89d4..b7214d2 100644
--- a/wasm/dist-repl/repl.js
+++ b/wasm/dist-repl/repl.js
@@ -256,19 +256,40 @@ function attachStreaming(resultSpan, metaSpan, tier) {
                 .map((d) => d.textContent === " " ? "" : d.textContent)
                 .join("\n");
             const expected = text.replace(/\n$/, "");
-            // If the streamed text doesn't match the full eval output
-            // (e.g. an error fired without intermediate chunks), nuke
-            // and rebuild from the full text. Otherwise leave the
-            // streamed divs as-is — they already match.
-            if (streamedText !== expected || errText) {
-                resultSpan.textContent = "";
-                const combined = (text + (errText && text ? "\n" : "") + errText).replace(/\n$/, "");
-                const lines = combined.split("\n");
-                for (const ln of lines) {
-                    const d = makeLine();
-                    d.textContent = ln || " ";
-                    resultSpan.appendChild(d);
+            // Preserving partial output on cancel/error: a long-running
+            // (display ...) loop that fox cancels half-way through, or
+            // an asm tier that hits "index out of bounds" 5000 lines
+            // into a sieve, used to lose every line that already
+            // streamed. Now:
+            //   * success, streamed matches full output → keep DOM.
+            //   * success, no streaming happened       → rebuild from
+            //     the final text.
+            //   * error with prior streaming           → KEEP the
+            //     streamed divs, append a final divider + error line.
+            //   * error with no streaming              → show error.
+            if (!errText) {
+                if (streamedText !== expected) {
+                    resultSpan.textContent = "";
+                    const lines = expected.split("\n");
+                    for (const ln of lines) {
+                        const d = makeLine();
+                        d.textContent = ln || " ";
+                        resultSpan.appendChild(d);
+                    }
                 }
+            } else if (streamedText) {
+                // Append the error AFTER what's already on screen so
+                // the user keeps every line they were watching scroll.
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
+            } else {
+                resultSpan.textContent = "";
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
             }
             metaSpan.textContent = `${TIER_LABEL[tier] || tier} · ${(r.elapsed | 0)}ms`;
         },
diff --git a/wasm/repl/repl.css b/wasm/repl/repl.css
index 244bf2f..8935273 100644
--- a/wasm/repl/repl.css
+++ b/wasm/repl/repl.css
@@ -299,6 +299,9 @@ body.repl {
   opacity: 0.6;
 }
 .transcript .entry.error .tier-result { color: var(--err); }
+/* Error line appended after partial streamed output — keeps the
+ * lines that already showed and tints just the trailing error. */
+.transcript .tier-result .err-line { color: var(--err); }
 .transcript .entry + .entry { margin-top: 0.4rem; }
 
 /* ─── Prompt bar ───────────────────────────────────────────────── */
diff --git a/wasm/repl/repl.js b/wasm/repl/repl.js
index 0cf89d4..b7214d2 100644
--- a/wasm/repl/repl.js
+++ b/wasm/repl/repl.js
@@ -256,19 +256,40 @@ function attachStreaming(resultSpan, metaSpan, tier) {
                 .map((d) => d.textContent === " " ? "" : d.textContent)
                 .join("\n");
             const expected = text.replace(/\n$/, "");
-            // If the streamed text doesn't match the full eval output
-            // (e.g. an error fired without intermediate chunks), nuke
-            // and rebuild from the full text. Otherwise leave the
-            // streamed divs as-is — they already match.
-            if (streamedText !== expected || errText) {
-                resultSpan.textContent = "";
-                const combined = (text + (errText && text ? "\n" : "") + errText).replace(/\n$/, "");
-                const lines = combined.split("\n");
-                for (const ln of lines) {
-                    const d = makeLine();
-                    d.textContent = ln || " ";
-                    resultSpan.appendChild(d);
+            // Preserving partial output on cancel/error: a long-running
+            // (display ...) loop that fox cancels half-way through, or
+            // an asm tier that hits "index out of bounds" 5000 lines
+            // into a sieve, used to lose every line that already
+            // streamed. Now:
+            //   * success, streamed matches full output → keep DOM.
+            //   * success, no streaming happened       → rebuild from
+            //     the final text.
+            //   * error with prior streaming           → KEEP the
+            //     streamed divs, append a final divider + error line.
+            //   * error with no streaming              → show error.
+            if (!errText) {
+                if (streamedText !== expected) {
+                    resultSpan.textContent = "";
+                    const lines = expected.split("\n");
+                    for (const ln of lines) {
+                        const d = makeLine();
+                        d.textContent = ln || " ";
+                        resultSpan.appendChild(d);
+                    }
                 }
+            } else if (streamedText) {
+                // Append the error AFTER what's already on screen so
+                // the user keeps every line they were watching scroll.
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
+            } else {
+                resultSpan.textContent = "";
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
             }
             metaSpan.textContent = `${TIER_LABEL[tier] || tier} · ${(r.elapsed | 0)}ms`;
         },
diff --git a/www/repl/repl.css b/www/repl/repl.css
index 244bf2f..8935273 100644
--- a/www/repl/repl.css
+++ b/www/repl/repl.css
@@ -299,6 +299,9 @@ body.repl {
   opacity: 0.6;
 }
 .transcript .entry.error .tier-result { color: var(--err); }
+/* Error line appended after partial streamed output — keeps the
+ * lines that already showed and tints just the trailing error. */
+.transcript .tier-result .err-line { color: var(--err); }
 .transcript .entry + .entry { margin-top: 0.4rem; }
 
 /* ─── Prompt bar ───────────────────────────────────────────────── */
diff --git a/www/repl/repl.js b/www/repl/repl.js
index 0cf89d4..b7214d2 100644
--- a/www/repl/repl.js
+++ b/www/repl/repl.js
@@ -256,19 +256,40 @@ function attachStreaming(resultSpan, metaSpan, tier) {
                 .map((d) => d.textContent === " " ? "" : d.textContent)
                 .join("\n");
             const expected = text.replace(/\n$/, "");
-            // If the streamed text doesn't match the full eval output
-            // (e.g. an error fired without intermediate chunks), nuke
-            // and rebuild from the full text. Otherwise leave the
-            // streamed divs as-is — they already match.
-            if (streamedText !== expected || errText) {
-                resultSpan.textContent = "";
-                const combined = (text + (errText && text ? "\n" : "") + errText).replace(/\n$/, "");
-                const lines = combined.split("\n");
-                for (const ln of lines) {
-                    const d = makeLine();
-                    d.textContent = ln || " ";
-                    resultSpan.appendChild(d);
+            // Preserving partial output on cancel/error: a long-running
+            // (display ...) loop that fox cancels half-way through, or
+            // an asm tier that hits "index out of bounds" 5000 lines
+            // into a sieve, used to lose every line that already
+            // streamed. Now:
+            //   * success, streamed matches full output → keep DOM.
+            //   * success, no streaming happened       → rebuild from
+            //     the final text.
+            //   * error with prior streaming           → KEEP the
+            //     streamed divs, append a final divider + error line.
+            //   * error with no streaming              → show error.
+            if (!errText) {
+                if (streamedText !== expected) {
+                    resultSpan.textContent = "";
+                    const lines = expected.split("\n");
+                    for (const ln of lines) {
+                        const d = makeLine();
+                        d.textContent = ln || " ";
+                        resultSpan.appendChild(d);
+                    }
                 }
+            } else if (streamedText) {
+                // Append the error AFTER what's already on screen so
+                // the user keeps every line they were watching scroll.
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
+            } else {
+                resultSpan.textContent = "";
+                const d = makeLine();
+                d.textContent = errText;
+                d.classList.add("err-line");
+                resultSpan.appendChild(d);
             }
             metaSpan.textContent = `${TIER_LABEL[tier] || tier} · ${(r.elapsed | 0)}ms`;
         },