diff --git a/docs/AGENT_TOOL_QUICKSTART.md b/docs/AGENT_TOOL_QUICKSTART.md
index e86ea1f0..c3bd0ee7 100644
--- a/docs/AGENT_TOOL_QUICKSTART.md
+++ b/docs/AGENT_TOOL_QUICKSTART.md
@@ -30,6 +30,8 @@ The [approval hook error case](https://aruintelligence.github.io/aml-core/agent-
 
 On the visual builder, **Copy fixture JSON** copies the selected case in the same shape as `--case`, including its project-authored provenance and scope. Include your checkout commit when sharing a reproduction; the copied fixture is not evidence of a real approval or delivery.
 
+Use **Compare with** on the visual builder to put two local outcomes, digest availability, claim counts, callback counts, and in-memory writes side by side. The address and **Copy case link** preserve the selected pair. The comparison only reads the checked-in fixture report; it does not run a tool or establish provider outcomes.
+
 The [declined approval case](https://aruintelligence.github.io/aml-core/agent-start.html?case=approval_declined#rehearsal) requests a fixture decision and receives a refusal. The receipt blocks before any claim or tool callback. A refusal is not a grant; a later attempt needs its own authenticated decision.
 
 ## Wrap your own function tool
diff --git a/docs/agent-start.html b/docs/agent-start.html
index db34697d..5b3b0b8c 100644
--- a/docs/agent-start.html
+++ b/docs/agent-start.html
@@ -17,7 +17,7 @@
     .flow{border:1px solid #517582;border-radius:25px;overflow:hidden;background:linear-gradient(130deg,#102536,#0a1525);box-shadow:0 26px 90px #0007}.bar{display:flex;justify-content:space-between;gap:12px;align-items:center;padding:17px 23px;border-bottom:1px solid var(--line);font:.74rem ui-monospace,monospace;letter-spacing:.13em;color:#c8dce5}.dots{display:flex;gap:7px}.dots i{width:8px;height:8px;background:#587586;border-radius:50%}.dots i:first-child{background:var(--cyan)}.stage{display:grid;grid-template-columns:94px minmax(0,1fr);border-bottom:1px solid var(--line)}.stage:last-child{border-bottom:0}.number{font:900 1.65rem ui-monospace,monospace;color:var(--cyan);padding:30px;border-right:1px solid var(--line);background:#061320}.content{padding:29px clamp(20px,4vw,46px)}.content h2{font-size:clamp(1.5rem,3vw,2.15rem);margin:0 0 11px}.content>p{color:var(--muted);max-width:780px;margin:0 0 18px}.code-row{display:flex;justify-content:space-between;align-items:center;gap:18px;flex-wrap:wrap}.copy{border:1px solid var(--cyan);background:#123745;color:var(--cyan);font-weight:850;padding:9px 14px;border-radius:9px}.copy:hover{background:#1d5464}.code{display:block;white-space:pre;overflow:auto;padding:19px;border:1px solid var(--line);border-radius:12px;background:#020b16;color:#dcf4f4;font:12px/1.7 ui-monospace,SFMono-Regular,monospace;margin:16px 0 11px}.small{color:var(--muted);font-size:.83rem}.small strong{color:var(--ink)}.outcomes{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin:20px 0 0}.outcome{border:1px solid var(--line);border-radius:12px;padding:16px;background:#ffffff08}.outcome b{display:block;font:.73rem ui-monospace,monospace;letter-spacing:.09em;color:var(--gold)}.outcome strong{display:block;margin:8px 0;color:var(--ink);font-size:1.13rem}.outcome p{margin:0;color:var(--muted);font-size:.82rem}.sketch{max-height:500px}.status{min-height:1.3em;color:var(--gold);font-size:.81rem;margin:8px 0 0}.links{display:flex;flex-wrap:wrap;gap:12px 20px;margin-top:20px}.links a{font-weight:800}.preflight{margin-top:18px;padding:14px 16px;border:1px solid #416779;border-radius:12px;background:#83edec0b;color:#d9e9ef;font-size:.82rem}.preflight strong{color:var(--cyan)}.notice{border-left:3px solid var(--gold);padding:14px 18px;background:#f5cc7e10;color:#e1d7bf;border-radius:0 11px 11px 0;max-width:780px}.next{display:grid;grid-template-columns:repeat(3,1fr);gap:15px;margin:50px 0 75px}.next a{padding:22px;border:1px solid var(--line);border-radius:16px;background:#ffffff08;text-decoration:none}.next a:hover{border-color:var(--cyan)}.next span{display:block;color:var(--gold);font:.72rem ui-monospace,monospace;letter-spacing:.1em}.next b{display:block;color:var(--ink);font-size:1.15rem;margin:11px 0 7px}.next small{color:var(--muted);font-size:.84rem}.foot{border-top:1px solid var(--line);color:var(--muted);padding:25px 0 50px;font-size:.84rem}.foot strong{color:var(--ink)}
     .rehearsal{margin-top:27px;border:1px solid #557988;border-radius:19px;overflow:hidden;background:#071522}.rehearsal-head{display:flex;justify-content:space-between;gap:14px;align-items:start;flex-wrap:wrap;padding:22px;border-bottom:1px solid var(--line);background:linear-gradient(100deg,#154050,#101d33)}.rehearsal-head span{font:800 .7rem ui-monospace,monospace;letter-spacing:.13em;color:var(--cyan)}.rehearsal-head h3{font-size:1.55rem;margin:7px 0}.rehearsal-head p{color:var(--muted);margin:0;max-width:630px;font-size:.87rem}.rehearsal-head a{font-size:.78rem;font-weight:800}.receipt-legend{display:flex;flex-wrap:wrap;gap:7px;list-style:none;margin:15px 0 0;padding:0}.receipt-legend li{border:1px solid var(--line);border-radius:999px;background:#081d2c;padding:6px 9px;color:var(--muted);font-size:.68rem}.receipt-legend b{font:800 .65rem ui-monospace,monospace;letter-spacing:.05em;margin-right:5px}.receipt-legend [data-tone="go"] b{color:#a9f1c0}.receipt-legend [data-tone="stop"] b{color:var(--rose)}.receipt-legend [data-tone="hold"] b{color:var(--gold)}.receipt-legend [data-tone="preflight"] b{color:#b7aeff}.case-search{display:flex;align-items:center;gap:12px;flex-wrap:wrap;padding:14px 17px;border-bottom:1px solid var(--line);background:#071c2a}.case-search label{font-weight:800;font-size:.78rem;color:var(--cyan)}.case-search input{flex:1 1 220px;min-width:0;border:1px solid #608697;border-radius:9px;background:#061320;color:var(--ink);padding:10px 12px;font:inherit;font-size:.84rem}.case-search input:focus-visible{outline:3px solid var(--gold);outline-offset:2px}.case-search p{flex-basis:100%;margin:0;color:var(--muted);font-size:.72rem}.case-picker button[hidden]{display:none}.case-picker{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:8px;padding:17px;border-bottom:1px solid var(--line)}.case-picker button{text-align:left;border:1px solid var(--line);border-radius:11px;padding:12px;background:#ffffff07;color:var(--ink);font-weight:800;font-size:.8rem;line-height:1.3}.case-picker button span{display:block;font:700 .62rem ui-monospace,monospace;letter-spacing:.1em;color:var(--gold);margin-bottom:5px}.case-picker button[data-tone="go"] span{color:#a9f1c0}.case-picker button[data-tone="stop"] span{color:var(--rose)}.case-picker button[data-tone="preflight"] span{color:#b7aeff}.case-picker button[aria-checked="true"]{border-color:var(--cyan);background:#83edec1c;box-shadow:inset 0 0 0 1px #83edec55}.case-picker button:hover{border-color:var(--cyan)}.case-detail{padding:22px}.case-detail h4{font-size:1.35rem;margin:5px 0}.case-detail>p{color:var(--muted);margin:0 0 18px}.case-detail .eyebrow{font-size:.64rem}.attempts{display:grid;grid-template-columns:repeat(auto-fit,minmax(210px,1fr));gap:11px}.attempt{border:1px solid var(--line);border-radius:12px;background:#ffffff06;padding:16px}.attempt[data-tone="stop"]{border-color:#a66275;background:#ff9eb20b}.attempt[data-tone="go"]{border-color:#70a98f;background:#a9f1c00b}.attempt[data-tone="hold"]{border-color:#b89d67;background:#f5cc7e0b}.attempt[data-tone="preflight"]{border-color:#9185cc;background:#b7aeff0b}.attempt small{display:block;color:var(--muted);font:700 .65rem ui-monospace,monospace;letter-spacing:.1em}.attempt strong{display:block;margin:8px 0;font-size:1.27rem}.attempt[data-tone="stop"] strong{color:var(--rose)}.attempt[data-tone="go"] strong{color:#a9f1c0}.attempt[data-tone="hold"] strong{color:var(--gold)}.attempt[data-tone="preflight"] strong{color:#b7aeff}.attempt .receipt-meaning{margin:0 0 9px;color:var(--muted);font-size:.75rem}.attempt code{display:block;color:#d4e4eb;font:.72rem/1.5 ui-monospace,monospace;overflow-wrap:anywhere}.case-metrics{display:flex;gap:9px;flex-wrap:wrap;margin-top:17px}.case-metrics span{border:1px solid var(--line);border-radius:999px;padding:7px 11px;color:#d8e9ef;font:.72rem ui-monospace,monospace}.case-detail .limit{margin:17px 0 0;color:var(--muted);font-size:.73rem}.host-move{margin-top:19px;padding:16px 18px;border-left:3px solid var(--gold);border-radius:0 10px 10px 0;background:#f5cc7e0d}.host-move span{display:block;color:var(--gold);font:800 .65rem ui-monospace,monospace;letter-spacing:.12em}.host-move p{margin:7px 0 0;color:#e4e1d9;font-size:.88rem}
     .attempt .trace-label{margin-top:17px}.hook-trace{list-style:none;margin:8px 0 0;padding:12px 0 0;border-top:1px solid var(--line)}.hook-trace li{position:relative;margin:0 0 8px;padding-left:17px;color:var(--muted);font-size:.73rem}.hook-trace li:before{content:"";position:absolute;left:0;top:.44em;width:6px;height:6px;border-radius:50%;background:var(--cyan)}.hook-trace li:last-child{margin:0}.hook-trace .none:before{background:#8297a5}
-    .case-command-row{display:flex;justify-content:space-between;align-items:center;gap:12px;flex-wrap:wrap;margin-top:21px}.case-command-row span{font:800 .67rem ui-monospace,monospace;letter-spacing:.12em;color:var(--cyan)}.case-actions{display:flex;gap:8px;flex-wrap:wrap}.case-command-row button{border:1px solid #608697;background:#113344;color:var(--cyan);border-radius:9px;padding:7px 11px;font-size:.77rem;font-weight:800}.case-command-row button:hover{background:#1a5264}.case-detail .case-command{margin:9px 0 0;padding:12px 14px}.case-detail .case-copy-status{min-height:1.2em;margin:5px 0 0;font-size:.73rem;color:var(--gold)}
+    .compare-control{display:flex;align-items:center;gap:12px;flex-wrap:wrap;margin-top:22px}.compare-control label{font-weight:800;font-size:.8rem;color:var(--cyan)}.compare-control select{min-width:0;max-width:100%;border:1px solid #608697;border-radius:9px;background:#061320;color:var(--ink);padding:9px 12px;font:inherit;font-size:.8rem}.compare-control select:focus-visible{outline:3px solid var(--gold);outline-offset:2px}.case-comparison{margin-top:13px;border:1px solid var(--line);border-radius:12px;overflow-x:auto;background:#071c2a}.case-comparison[hidden]{display:none}.case-comparison table{border-collapse:collapse;width:100%;min-width:470px;font-size:.77rem}.case-comparison th,.case-comparison td{text-align:left;vertical-align:top;padding:10px 12px;border-bottom:1px solid var(--line);overflow-wrap:anywhere}.case-comparison thead th{color:var(--gold);font-weight:800}.case-comparison tbody th{color:var(--muted);font-weight:700;width:28%}.case-comparison tr:last-child>*{border-bottom:0}.case-command-row{display:flex;justify-content:space-between;align-items:center;gap:12px;flex-wrap:wrap;margin-top:21px}.case-command-row span{font:800 .67rem ui-monospace,monospace;letter-spacing:.12em;color:var(--cyan)}.case-actions{display:flex;gap:8px;flex-wrap:wrap}.case-command-row button{border:1px solid #608697;background:#113344;color:var(--cyan);border-radius:9px;padding:7px 11px;font-size:.77rem;font-weight:800}.case-command-row button:hover{background:#1a5264}.case-detail .case-command{margin:9px 0 0;padding:12px 14px}.case-detail .case-copy-status{min-height:1.2em;margin:5px 0 0;font-size:.73rem;color:var(--gold)}
     @media(max-width:790px){.hero{grid-template-columns:1fr}.case{max-width:520px}.next{grid-template-columns:1fr}.case-picker{grid-template-columns:repeat(2,minmax(0,1fr))}}@media(max-width:530px){.top{flex-wrap:wrap}.top nav{gap:9px}.top nav a{font-size:.75rem}.stage{grid-template-columns:1fr}.number{padding:10px 19px;border-right:0;border-bottom:1px solid var(--line)}.bar{font-size:.63rem;flex-wrap:wrap;gap:8px 12px}.content{padding:20px}.hero h1{font-size:clamp(3rem,12vw,4.5rem)}.case-picker{grid-template-columns:1fr}.rehearsal-head,.case-detail{padding:18px}.attempts{grid-template-columns:minmax(0,1fr)}}@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}}
   </style>
 </head>
@@ -32,7 +32,7 @@
 git rev-parse HEAD
 node pilots/action-boundary/guarded-tool-demo.mjs
 node pilots/action-boundary/guarded-tool-rehearsal.mjs --check</code><p id="command-status" class="status" role="status" aria-live="polite"></p>
-          <div class="rehearsal" id="rehearsal" aria-labelledby="rehearsal-title"><div class="rehearsal-head"><div><span id="case-count-label">HOST REHEARSAL / CONTROLLED CASES</span><h3 id="rehearsal-title">Where does the call stop?</h3><p>Choose a condition. Use arrow keys, Home, or End after focusing a case. These are recorded results from the project adapter running local fixtures, with approval and callback counts visible.</p><ul class="receipt-legend" aria-label="Local rehearsal outcomes"><li data-tone="go"><b>DISPATCHED</b>fixture returned</li><li data-tone="stop"><b>BLOCKED</b>no tool callback</li><li data-tone="hold"><b>UNKNOWN</b>outcome uncertain</li><li data-tone="preflight"><b>HOST ERROR</b>no plan or receipt</li></ul></div><a href="#case-title">Jump to selected result ↓</a><a href="guarded-tool-rehearsal.json">Inspect the complete report ↗</a></div><div class="case-search"><label for="case-search">Find a case</label><input id="case-search" type="search" autocomplete="off" placeholder="Try claim, mapper, unknown…"><p id="case-search-status" role="status" aria-live="polite">Search by name, condition, or outcome.</p></div><div id="case-picker" class="case-picker" role="radiogroup" aria-label="Choose a host rehearsal case"></div><div class="case-detail" id="case-panel" role="region" aria-labelledby="case-title" tabindex="0"><span class="eyebrow">OBSERVED IN THE LOCAL REHEARSAL</span><h4 id="case-title" aria-live="polite">Loading the report…</h4><p id="case-change">Checking the project-authored snapshot.</p><div id="case-attempts" class="attempts"></div><div id="case-metrics" class="case-metrics"></div><div class="host-move"><span>HOST DESIGN MOVE</span><p id="case-host-move"></p></div><div class="case-command-row"><span>REPRODUCE OR SHARE THIS CASE</span><div class="case-actions"><button type="button" id="copy-case-command">Copy command</button><button type="button" id="copy-case-link">Copy case link</button><button type="button" id="copy-case-json">Copy fixture JSON</button></div></div><code class="code case-command" id="case-command">node pilots/action-boundary/guarded-tool-rehearsal.mjs --case changed_destination</code><p id="case-copy-status" class="case-copy-status" role="status" aria-live="polite"></p><p class="limit">The hook trace records which fixture hooks ran; it does not expose every internal policy check. A host mapper error occurs before a plan or receipt exists, but the mapper may already have changed host state. A <code>dispatched</code> receipt means the fixture callback returned. An <code>unknown</code> receipt does not mean no side effect occurred. A claim-hook error can leave a grant spent without entering the tool callback. An approval-hook error can leave host authorization state changed. In-memory fixture writes are local markers, not provider delivery. Neither proves real approval, service delivery, or production safety.</p></div></div>
+          <div class="rehearsal" id="rehearsal" aria-labelledby="rehearsal-title"><div class="rehearsal-head"><div><span id="case-count-label">HOST REHEARSAL / CONTROLLED CASES</span><h3 id="rehearsal-title">Where does the call stop?</h3><p>Choose a condition. Use arrow keys, Home, or End after focusing a case. These are recorded results from the project adapter running local fixtures, with approval and callback counts visible.</p><ul class="receipt-legend" aria-label="Local rehearsal outcomes"><li data-tone="go"><b>DISPATCHED</b>fixture returned</li><li data-tone="stop"><b>BLOCKED</b>no tool callback</li><li data-tone="hold"><b>UNKNOWN</b>outcome uncertain</li><li data-tone="preflight"><b>HOST ERROR</b>no plan or receipt</li></ul></div><a href="#case-title">Jump to selected result ↓</a><a href="guarded-tool-rehearsal.json">Inspect the complete report ↗</a></div><div class="case-search"><label for="case-search">Find a case</label><input id="case-search" type="search" autocomplete="off" placeholder="Try claim, mapper, unknown…"><p id="case-search-status" role="status" aria-live="polite">Search by name, condition, or outcome.</p></div><div id="case-picker" class="case-picker" role="radiogroup" aria-label="Choose a host rehearsal case"></div><div class="case-detail" id="case-panel" role="region" aria-labelledby="case-title" tabindex="0"><span class="eyebrow">OBSERVED IN THE LOCAL REHEARSAL</span><h4 id="case-title" aria-live="polite">Loading the report…</h4><p id="case-change">Checking the project-authored snapshot.</p><div id="case-attempts" class="attempts"></div><div id="case-metrics" class="case-metrics"></div><div class="host-move"><span>HOST DESIGN MOVE</span><p id="case-host-move"></p></div><div class="compare-control"><label for="case-compare">Compare with</label><select id="case-compare" aria-controls="case-comparison"><option value="">Choose another case…</option></select></div><div id="case-comparison" class="case-comparison" hidden><table><thead><tr><th scope="col">LOCAL OBSERVATION</th><th scope="col" id="compare-selected">Selected</th><th scope="col" id="compare-other">Compared</th></tr></thead><tbody id="compare-rows"></tbody></table></div><div class="case-command-row"><span>REPRODUCE OR SHARE THIS CASE</span><div class="case-actions"><button type="button" id="copy-case-command">Copy command</button><button type="button" id="copy-case-link">Copy case link</button><button type="button" id="copy-case-json">Copy fixture JSON</button></div></div><code class="code case-command" id="case-command">node pilots/action-boundary/guarded-tool-rehearsal.mjs --case changed_destination</code><p id="case-copy-status" class="case-copy-status" role="status" aria-live="polite"></p><p class="limit">The hook trace records which fixture hooks ran; it does not expose every internal policy check. A host mapper error occurs before a plan or receipt exists, but the mapper may already have changed host state. A <code>dispatched</code> receipt means the fixture callback returned. An <code>unknown</code> receipt does not mean no side effect occurred. A claim-hook error can leave a grant spent without entering the tool callback. An approval-hook error can leave host authorization state changed. In-memory fixture writes are local markers, not provider delivery. Neither proves real approval, service delivery, or production safety.</p></div></div>
         </div></div>
         <div class="stage"><div class="number">02</div><div class="content"><div class="code-row"><h2>Move the boundary into your host.</h2><button type="button" class="copy" data-copy="sketch">Copy host sketch</button></div><p>The real host owns the tool identity, effect, resource mapping, policy, authenticated approval, one-use claim, and service call. This sketch deliberately throws until you implement the three host functions. It imports the project preview from your checkout.</p><code id="sketch" class="code sketch">import { createGuardedToolExecute } from "./pilots/action-boundary/guarded-tool.mjs";
 
@@ -65,6 +65,6 @@
       <div class="next"><a href="action-boundary-studio.html"><span>DESIGN</span><b>Edit proposal and policy ↗</b><small>Export a calculation bundle and recheck it in Node.</small></a><a href="connected-mcp-lab.html"><span>TRACE</span><b>See a connected MCP call ↗</b><small>Compare a misleading tool hint with a host-owned rule.</small></a><a href="action-challenge.html?case=changed-destination#build"><span>CHALLENGE</span><b>Build your own adapter ↗</b><small>Run the public cases outside the reference implementation.</small></a></div>
     </main><footer class="foot"><strong>Scope:</strong> This is a project-authored, simulated function-tool path from a preview checkout. It does not intercept every agent tool, authenticate a human, send mail, prove delivery, or certify a production integration. ĀML™ / ĀRU Meaning Language™ is stewarded by ĀRU Intelligence Inc. · <a href="https://github.com/aruintelligence/aml-core/blob/main/CLAIMS.md">Claims and evidence</a>.</footer>
   </div>
-  <script type="module" src="agent-start.js?v=cf01361f476f"></script>
+  <script type="module" src="agent-start.js?v=9ba7ee4e1a53"></script>
 </body>
 </html>
diff --git a/docs/agent-start.js b/docs/agent-start.js
index 7fc76a37..de14a674 100644
--- a/docs/agent-start.js
+++ b/docs/agent-start.js
@@ -18,6 +18,31 @@ export function rehearsalCaseUrl(currentUrl, id, report) {
   return url.href;
 }
 
+export function rehearsalComparisonUrl(currentUrl, selectedId, comparedId, report) {
+  const url = new URL(rehearsalCaseUrl(currentUrl, selectedId, report));
+  if (comparedId && comparedId !== selectedId && report.cases.some(item => item.id === comparedId)) {
+    url.searchParams.set("compare", comparedId);
+  } else {
+    url.searchParams.delete("compare");
+  }
+  return url.href;
+}
+
+export function rehearsalComparisonRows(selected, compared) {
+  const outcomes = item => item.attempts.map(attempt =>
+    attempt.adapter_error ? "HOST ERROR (no receipt)" : attempt.execution_status.toUpperCase()).join(" → ");
+  const digest = item => item.attempts.every(attempt => attempt.adapter_error || attempt.proposal_sha256 === null) ?
+    "Unavailable" : "Present for valid proposals";
+  return [
+    ["Observed outcome", outcomes(selected), outcomes(compared)],
+    ["Proposal digest", digest(selected), digest(compared)],
+    ["Approval requests", selected.approval_requests, compared.approval_requests],
+    ["Claims consumed", selected.claims_consumed, compared.claims_consumed],
+    ["Fixture callbacks", selected.callback_count, compared.callback_count],
+    ["In-memory fixture writes", selected.fixture_state_changes, compared.fixture_state_changes]
+  ];
+}
+
 export function rehearsalCasePayload(report, id) {
   const item = report?.cases?.find(entry => entry.id === id);
   if (!item) throw new RangeError("Unknown rehearsal case");
@@ -158,6 +183,36 @@ if (typeof document !== "undefined" && document.getElementById("rehearsal")) {
     });
     $("case-picker").replaceChildren(...buttons);
     let selectedCase = initial;
+    let comparedCase = report.cases.some(item => item.id === params.get("compare") && item.id !== initial) ?
+      params.get("compare") : "";
+    $("case-compare").replaceChildren(new Option("Choose another case…", ""),
+      ...report.cases.map(item => new Option(item.title, item.id)));
+    $("case-compare").value = comparedCase;
+    function renderComparison() {
+      const selected = report.cases.find(item => item.id === selectedCase);
+      const compared = report.cases.find(item => item.id === comparedCase);
+      $("case-comparison").hidden = !compared;
+      if (!compared) return;
+      $("compare-selected").textContent = selected.title;
+      $("compare-other").textContent = compared.title;
+      $("compare-rows").replaceChildren(...rehearsalComparisonRows(selected, compared).map(([label, left, right]) => {
+        const row = document.createElement("tr");
+        const heading = document.createElement("th");
+        heading.scope = "row";
+        heading.textContent = label;
+        const first = document.createElement("td");
+        first.textContent = left;
+        const second = document.createElement("td");
+        second.textContent = right;
+        row.append(heading, first, second);
+        return row;
+      }));
+    }
+    $("case-compare").addEventListener("change", () => {
+      comparedCase = $("case-compare").value;
+      renderComparison();
+      history.replaceState(null, "", rehearsalComparisonUrl(location.href, selectedCase, comparedCase, report));
+    });
     function filterCases() {
       const visible = visibleRehearsalCaseIds(report, $("case-search").value, selectedCase);
       buttons.forEach(button => { button.hidden = !visible.includes(button.dataset.case); });
@@ -171,6 +226,11 @@ if (typeof document !== "undefined" && document.getElementById("rehearsal")) {
     function show(id, updateUrl) {
       const item = report.cases.find(entry => entry.id === id);
       selectedCase = id;
+      if (comparedCase === id) {
+        comparedCase = "";
+        $("case-compare").value = "";
+      }
+      for (const option of $("case-compare").options) option.disabled = option.value === id;
       buttons.forEach(button => {
         const selected = button.dataset.case === id;
         button.setAttribute("aria-checked", String(selected));
@@ -199,11 +259,12 @@ if (typeof document !== "undefined" && document.getElementById("rehearsal")) {
       );
       $("case-command").textContent = `node pilots/action-boundary/guarded-tool-rehearsal.mjs --case ${item.id}`;
       $("case-copy-status").textContent = "";
-      if (updateUrl) history.replaceState(null, "", rehearsalCaseUrl(location.href, id, report));
+      renderComparison();
+      if (updateUrl) history.replaceState(null, "", rehearsalComparisonUrl(location.href, id, comparedCase, report));
     }
     $("copy-case-link").addEventListener("click", async () => {
       try {
-        await navigator.clipboard.writeText(rehearsalCaseUrl(location.href, selectedCase, report));
+        await navigator.clipboard.writeText(rehearsalComparisonUrl(location.href, selectedCase, comparedCase, report));
         $("case-copy-status").textContent = "Case link copied.";
       } catch {
         $("case-copy-status").textContent = "Clipboard unavailable. Copy the address in your browser to share this case.";
diff --git a/test/agent-start.test.js b/test/agent-start.test.js
index 2be2fb4c..a415dd64 100644
--- a/test/agent-start.test.js
+++ b/test/agent-start.test.js
@@ -2,7 +2,7 @@ import assert from "node:assert/strict";
 import { createHash } from "node:crypto";
 import { readFileSync } from "node:fs";
 import test from "node:test";
-import { chooseRehearsalCase, nextRehearsalCaseIndex, rehearsalCasePayload, rehearsalCaseUrl, visibleRehearsalCaseIds } from "../docs/agent-start.js";
+import { chooseRehearsalCase, nextRehearsalCaseIndex, rehearsalCasePayload, rehearsalCaseUrl, rehearsalComparisonRows, rehearsalComparisonUrl, visibleRehearsalCaseIds } from "../docs/agent-start.js";
 import { selectGuardedToolCase } from "../pilots/action-boundary/guarded-tool-rehearsal.mjs";
 
 const report = JSON.parse(readFileSync(new URL("../docs/guarded-tool-rehearsal.json", import.meta.url), "utf8"));
@@ -25,6 +25,8 @@ test("the page labels the report as a fixture and offers the exact recheck comma
   assert.match(page, /id="case-picker"[^>]*role="radiogroup"/);
   assert.match(page, /<label for="case-search">Find a case<\/label>/);
   assert.match(page, /id="case-search-status" role="status" aria-live="polite"/);
+  assert.match(page, /<label for="case-compare">Compare with<\/label>/);
+  assert.match(page, /id="case-comparison"[^>]*hidden/);
   assert.match(page, /id="case-panel" role="region" aria-labelledby="case-title" tabindex="0"/);
   assert.match(page, /id="route-label"/);
   assert.match(page, /href="#case-title">Jump to selected result/);
@@ -79,6 +81,21 @@ test("case search keeps the selected choice visible and narrows keyboard order",
   assert.equal(claim[nextRehearsalCaseIndex("ArrowRight", position, claim.length)], "claim_revoked");
 });
 
+test("comparison distinguishes pre-plan errors, blocked receipts, and local writes", () => {
+  const byId = Object.fromEntries(report.cases.map(item => [item.id, item]));
+  const rows = rehearsalComparisonRows(byId.mapper_write_error, byId.invalid_arguments);
+  assert.deepEqual(rows[0], ["Observed outcome", "HOST ERROR (no receipt)", "BLOCKED"]);
+  assert.deepEqual(rows[1], ["Proposal digest", "Unavailable", "Unavailable"]);
+  assert.deepEqual(rows.at(-1), ["In-memory fixture writes", 1, 0]);
+  assert.deepEqual(rehearsalComparisonRows(byId.unknown_then_replay, byId.claim_revoked)[0],
+    ["Observed outcome", "UNKNOWN → BLOCKED", "BLOCKED"]);
+  const base = "https://example.test/agent-start.html?case=mapper_write_error#rehearsal";
+  assert.equal(rehearsalComparisonUrl(base, "mapper_write_error", "invalid_arguments", report),
+    "https://example.test/agent-start.html?case=mapper_write_error&compare=invalid_arguments#rehearsal");
+  assert.equal(rehearsalComparisonUrl(base + "", "mapper_write_error", "mapper_write_error", report), base);
+  assert.equal(rehearsalComparisonUrl(base, "mapper_write_error", "missing", report), base);
+});
+
 test("copied fixture payload matches the CLI case with its provenance and scope", () => {
   assert.deepEqual(rehearsalCasePayload(report, "unknown_then_replay"),
     selectGuardedToolCase(report, "unknown_then_replay"));