diff --git a/static/js/compare/sse.js b/static/js/compare/sse.js
new file mode 100644
index 000000000..7e60b0e12
--- /dev/null
+++ b/static/js/compare/sse.js
@@ -0,0 +1,65 @@
+function streamError(payload) {
+  const detail = payload?.data ?? payload;
+  const candidates = [detail?.message, detail?.text, detail?.error?.message,
+    detail?.error, payload?.message, payload?.text, payload?.error?.message,
+    payload?.error, typeof payload === 'string' ? payload : null];
+  const message = candidates.find(value => typeof value === 'string' && value.trim());
+  return new Error(message || 'Model request failed.');
+}
+
+/** Decode complete SSE frames, requiring the backend's terminal DONE event. */
+export async function consumeCompareEvents(response, onEvent, onActivity) {
+  const reader = response.body.getReader();
+  const decoder = new TextDecoder();
+  let line = '', event = 'message', data = [], skipLF = false;
+  try {
+    while (true) {
+      const { done, value } = await reader.read();
+      if (!done) onActivity?.();
+      let chunk = decoder.decode(value, { stream: !done });
+      if (skipLF && chunk.length) {
+        if (chunk.startsWith('\n')) chunk = chunk.slice(1);
+        skipLF = false;
+      }
+      const separators = /\r\n|\r|\n/g;
+      let start = 0, separator;
+      while ((separator = separators.exec(chunk))) {
+        line += chunk.slice(start, separator.index);
+        start = separator.index + separator[0].length;
+        skipLF = separator[0] === '\r' && start === chunk.length;
+        if (!line) {
+          if (data.length || event === 'error') {
+            const raw = data.join('\n');
+            if (event !== 'error' && raw === '[DONE]') return;
+            let payload;
+            try { payload = JSON.parse(raw); }
+            catch (_) {
+              if (event === 'error') throw streamError(raw);
+              throw new Error('Invalid model stream data.');
+            }
+            if (event === 'error' || payload?.type === 'error') throw streamError(payload);
+            if (!payload || typeof payload !== 'object' || Array.isArray(payload)) {
+              throw new Error('Invalid model stream data.');
+            }
+            await onEvent(payload);
+          }
+          event = 'message';
+          data = [];
+        } else if (!line.startsWith(':')) {
+          const colon = line.indexOf(':');
+          const field = colon < 0 ? line : line.slice(0, colon);
+          let value = colon < 0 ? '' : line.slice(colon + 1);
+          if (value.startsWith(' ')) value = value.slice(1);
+          if (field === 'data') data.push(value);
+          else if (field === 'event') event = value || 'message';
+        }
+        line = '';
+      }
+      line += chunk.slice(start);
+      if (done) throw new Error('Stream ended before completion.');
+    }
+  } finally {
+    try { await reader.cancel(); } catch (_) {}
+    reader.releaseLock();
+  }
+}
diff --git a/static/js/compare/stream.js b/static/js/compare/stream.js
index 4ea5c4e74..c2577cbed 100644
--- a/static/js/compare/stream.js
+++ b/static/js/compare/stream.js
@@ -1,5 +1,6 @@
 // compare/stream.js — SSE streaming to panes
 import state from './state.js';
+import { consumeCompareEvents } from './sse.js';
 import { addFinishBadge } from './vote.js?v=20260828resendcaldrag1';
 import { getModelCost, renderAskUserCard, safeDisplayImageSrc } from '../chatRenderer.js?v=20260914metricssummary1';
 import markdownModule from '../markdown.js';
@@ -472,25 +473,9 @@ async function streamToPane(paneIdx, sessionId, message, aiMsgEl, opts) {
     });
     if (!response.ok) throw new Error(`HTTP ${response.status}`);
 
-    const reader = response.body.getReader();
-    const decoder = new TextDecoder();
-    let buffer = '';
-
-    while (true) {
-      const { done, value } = await reader.read();
-      if (done) break;
-      _resetIdleTimeout();  // any chunk = stream is alive
-
-      buffer += decoder.decode(value, { stream: true });
-      const lines = buffer.split('\n');
-      buffer = lines.pop() || '';
-
-      for (const line of lines) {
-        if (!line.startsWith('data: ')) continue;
-        const data = line.slice(6);
-        if (data === '[DONE]') break;
+    await consumeCompareEvents(response,
+      (json) => {
         try {
-          const json = JSON.parse(data);
           if (json.type === 'metrics') {
             metrics = json.data;
 
@@ -672,7 +657,7 @@ async function streamToPane(paneIdx, sessionId, message, aiMsgEl, opts) {
             if (hist) hist.scrollTop = hist.scrollHeight;
           } else if (json.delta) {
             // Skip text deltas if we already rendered an image
-            if (aiMsgEl._imageData) continue;
+            if (aiMsgEl._imageData) return;
             // Capture TTFT on very first text delta
             if (!accumulated && !_ttft) _ttft = performance.now() - _timerStart;
             // On first delta, destroy spinner and prepare text area
@@ -701,8 +686,7 @@ async function streamToPane(paneIdx, sessionId, message, aiMsgEl, opts) {
             _scheduleLiveRender(target);
           }
         } catch (e) { console.warn('Compare stream render error:', e); }
-      }
-    }
+      }, _resetIdleTimeout);
 
     streamOk = true;
     // Destroy any remaining spinner
@@ -900,9 +884,15 @@ async function streamToPane(paneIdx, sessionId, message, aiMsgEl, opts) {
       }
     } else {
       console.error('Compare stream error:', error);
-      aiBody.innerHTML = '<span style="color:var(--color-error);">Error: ' + escapeHtml(error.message) + '</span>';
+      awaitingChoice = false;
+      const message = state._blindMode ? 'Model request failed. Try again.' : error.message;
+      aiBody.innerHTML = '<span style="color:var(--color-error);">Error: ' + escapeHtml(message) + '</span>';
     }
   } finally {
+    if (aiMsgEl._spinner) {
+      aiMsgEl._spinner.destroy();
+      aiMsgEl._spinner = null;
+    }
     clearTimeout(timeoutId);
     _timerDone = true;
     cancelAnimationFrame(_rafId);
diff --git a/static/sw.js b/static/sw.js
index 44cf2e2a1..768e293b7 100644
--- a/static/sw.js
+++ b/static/sw.js
@@ -7,7 +7,7 @@
 //   - Other static assets (images/fonts/libs): cache-first with bg refresh.
 //   - API / non-GET: never cached.
 // Bump CACHE_NAME whenever the precache list or SW logic changes.
-const CACHE_NAME = 'odysseus-v659-notes-reference';
+const CACHE_NAME = 'odysseus-v660-compare-stream-events';
 
 // KaTeX resolves these from its own stylesheet, so caching the CSS without them
 // gives offline math fallback glyphs instead of proper typesetting.
@@ -90,6 +90,7 @@ const PRECACHE = [
   '/static/js/search-chat.js',
   '/static/js/compare/index.js?v=20260909mobilepaneaddscroll1',
   '/static/js/compare/vote.js?v=20260828resendcaldrag1',
+  '/static/js/compare/sse.js',
   '/static/js/colorPicker.js?v=20260910eyedropper1',
   '/static/js/panels.js?v=20260909movepicklayer1',
   '/static/js/theme.js?v=20260911organsrain1',
diff --git a/tests/test_compare_stream_contract_browser.py b/tests/test_compare_stream_contract_browser.py
new file mode 100644
index 000000000..620a1768d
--- /dev/null
+++ b/tests/test_compare_stream_contract_browser.py
@@ -0,0 +1,70 @@
+"""Compare's actual stream consumer must preserve evidence and terminal state."""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parents[1]
+
+CASES = [
+    ("lf", 'data: {"delta":"Hello"}\n\ndata: [DONE]\n\n', False, True, "Hello"),
+    ("crlf", 'data:{"delta":"Héllö"}\r\n\r\ndata:[DONE]\r\n\r\n', False, True, "Héllö"),
+    ("multiline", 'data: {"delta":\ndata: "Hello"}\n\ndata: [DONE]\n\n', False, True, "Hello"),
+    ("eof", 'data: {"delta":"Partial"}\n\n', False, False, "ended before completion"),
+    ("named-error", 'event: error\ndata: {"text":"Provider Z private-model failed"}\n\n', False, False, "Provider Z private-model failed"),
+    ("typed-error", 'data: {"type":"error","data":{"message":"Provider Z <img src=x onerror=alert(1)>"}}\n\ndata: [DONE]\n\n', False, False, "Provider Z <img"),
+    ("blind-error", 'event:error\ndata:{"message":"Provider Z private-model failed"}\n\n', True, False, "request failed"),
+    ("empty-error", 'event:error\ndata:\n\n', False, False, "request failed"),
+    ("done-open", 'data: {"delta":"Hello"}\n\ndata: [DONE]\n\n', False, True, "Hello"),
+    ("heartbeat", 'data: {"delta":"Hello"}\n\ndata: [DONE]\n\n', False, True, "Hello"),
+    ("cr", 'data: {"delta":"Hello"}\r\rdata:[DONE]\r\r', False, True, "Hello"),
+    ("invalid", 'data: not-json\n\ndata: [DONE]\n\n', False, False, "Invalid model stream data"),
+]
+
+
+@pytest.mark.parametrize("kind,wire,blind,success,text", CASES)
+def test_compare_stream_contract(kind, wire, blind, success, text):
+    if not shutil.which("node"):
+        pytest.skip("node not installed")
+    script = r"""
+const assert=require('node:assert/strict');
+let chromium;try{({chromium}=require('playwright'));}catch(e){if(e.code==='MODULE_NOT_FOUND')process.exit(77);throw e;}
+(async()=>{
+ const browser=await chromium.launch({headless:true});
+ try{
+  const page=await browser.newPage();
+  await page.goto(process.env.ODYSSEUS_TEST_STATIC_ORIGIN+'/static/js/storage.js');
+  await page.setContent('<div class="compare-pane" data-pane="0"><div id="cmp-badge-0"></div><div id="cmp-history-0"><div class="msg msg-ai"><div class="body"></div></div></div></div>');
+  const result=await page.evaluate(async(data)=>{
+   const {default:state}=await import('/static/js/compare/state.js');
+   const {streamToPane}=await import('/static/js/compare/stream.js');
+   Object.assign(state,{isActive:true,_selectedModels:[{model:'fixture',name:'Fixture'}],_paneSessionIds:['session'],_compareMode:'chat',_parallel:true,_blindMode:data.blind,_timeout:data.kind==='heartbeat'?1:5});
+   const nativeFetch=window.fetch,bytes=new TextEncoder().encode(data.wire);let index=0,cancelled=false,context=null;
+   window.fetch=async(url,options)=>{
+    if(!String(url).endsWith('/api/chat_stream'))return nativeFetch(url,options);
+    context=options.body.get('search_context');
+    const heartbeats=[...Array(6).fill(': ping\n\n'),data.wire];
+    return new Response(new ReadableStream({start(controller){options.signal.addEventListener('abort',()=>controller.error(new DOMException('Stopped','AbortError')),{once:true});},async pull(controller){if(data.kind==='heartbeat'){if(index<heartbeats.length){await new Promise(r=>setTimeout(r,200));controller.enqueue(new TextEncoder().encode(heartbeats[index++]));}else controller.close();}else if(index<bytes.length)controller.enqueue(bytes.slice(index,index+=1));else if(data.kind!=='done-open')controller.close();},cancel(){cancelled=true;}}),{headers:{'Content-Type':'text/event-stream'}});
+   };
+   const message=document.querySelector('.msg-ai'),spinner=document.createElement('span');let spinnerDestroyed=false;spinner.textContent='Loading';message.querySelector('.body').appendChild(spinner);message._spinner={element:spinner,destroy(){spinnerDestroyed=true;spinner.remove();}};
+   await Promise.race([streamToPane(0,'session','Prompt',message,{skipBadge:true}),new Promise((_,reject)=>setTimeout(()=>reject(Error('Consumer did not finish at DONE')),data.kind==='heartbeat'?6000:1500))]);
+   const pane=document.querySelector('.compare-pane');
+   return {done:pane.classList.contains('is-done'),failed:pane.classList.contains('is-failed'),text:pane.innerText,images:pane.querySelectorAll('img').length,cancelled,spinnerDestroyed,spinnerOwned:!!message._spinner};
+  },JSON.parse(process.argv[1]));
+  const expected=JSON.parse(process.argv[1]);
+  assert.equal(result.done,expected.success);assert.equal(result.failed,!expected.success);assert(result.text.includes(expected.text),JSON.stringify(result));assert.equal(result.images,0);
+  assert(result.spinnerDestroyed);assert(!result.spinnerOwned);
+  if(expected.blind)assert(!/Provider Z|private-model/.test(result.text),JSON.stringify(result));
+  if(expected.kind==='done-open')assert(result.cancelled);
+  console.log(JSON.stringify(result));
+ }finally{await browser.close();}
+})().catch(e=>{console.error(e);process.exitCode=1;});
+"""
+    data = dict(kind=kind, wire=wire, blind=blind, success=success, text=text)
+    result = subprocess.run(["node", "-e", script, json.dumps(data)], cwd=ROOT,
+                            capture_output=True, text=True, timeout=30)
+    if result.returncode == 77:
+        pytest.skip("playwright not installed")
+    assert result.returncode == 0, result.stdout + result.stderr