diff --git a/packages/mosaic/framework/runtime/pi/mosaic-extension.ts b/packages/mosaic/framework/runtime/pi/mosaic-extension.ts
index 10756ffe..5dc5c3c8 100644
--- a/packages/mosaic/framework/runtime/pi/mosaic-extension.ts
+++ b/packages/mosaic/framework/runtime/pi/mosaic-extension.ts
@@ -38,8 +38,27 @@ const RECEIPT_OBSERVER_CLIENT = join(
   'lease-broker',
   'receipt-observer-client.py',
 );
+const LEASE_PROMOTE = join(MOSAIC_HOME, 'tools', 'lease-broker', 'lease_promote.py');
 const RECOVERY_TOOL = 'mosaic_context_recover';
 
+// Lazy lease promotion: a lease is promoted on the FIRST DENIED MUTATOR, not at
+// session start. Two reasons that matter:
+//
+//  1. Promotion costs a whole model turn, because the receipt must be the entire
+//     message (hmac.compare_digest, "not a transcript substring"). Doing it at
+//     session start would collide with the Constitution's first-response mode
+//     declaration — the two cannot share a message. Deferring it means the mode
+//     declaration happens first and the receipt gets its own later turn, so no
+//     governance change is required.
+//  2. A read-only session never pays for it at all.
+//
+// Bounded so a model that will not emit the receipt verbatim degrades to the
+// current behaviour (denied mutators) rather than looping forever.
+const MAX_PROMOTION_ATTEMPTS = 3;
+let pendingReceiptChallenge: string | null = null;
+let pendingReceiptText: string | null = null;
+let promotionAttempts = 0;
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
@@ -126,6 +145,63 @@ function runPiLeaseRevoker(args: string[]): boolean {
   return result.status === 0;
 }
 
+/**
+ * Mint a promotion challenge and return the instruction the model must follow.
+ *
+ * The receipt has to be the model's ENTIRE next message: the broker compares it
+ * with `hmac.compare_digest`, so any surrounding prose fails. Returning it as
+ * the tool_call block `reason` is what puts it in front of the model — pi feeds
+ * that string back as the tool result.
+ *
+ * This never posts the receipt itself. `receipt-observer-client.py` would accept
+ * any string, so self-posting would satisfy the broker while proving nothing;
+ * the whole point of the exchange is that a live model echoes the challenge.
+ */
+function beginPiPromotion(): string | undefined {
+  if (promotionAttempts >= MAX_PROMOTION_ATTEMPTS) return undefined;
+  const result = spawnSync('python3', [LEASE_PROMOTE, '--begin'], {
+    encoding: 'utf8',
+    timeout: 10_000,
+    env: process.env,
+  });
+  if (result.status !== 0) return undefined;
+  try {
+    const reply = JSON.parse(String(result.stdout)) as {
+      ok?: boolean;
+      receipt?: string;
+      receipt_challenge?: string;
+    };
+    if (reply.ok !== true || !reply.receipt || !reply.receipt_challenge) return undefined;
+    pendingReceiptChallenge = reply.receipt_challenge;
+    pendingReceiptText = reply.receipt;
+    promotionAttempts += 1;
+    return reply.receipt;
+  } catch {
+    return undefined;
+  }
+}
+
+/** Complete promotion after the model emitted the receipt and message_end
+ * shipped it to the observer. Returns true when the lease reached VERIFIED. */
+function completePiPromotion(): boolean {
+  const challenge = pendingReceiptChallenge;
+  if (!challenge) return false;
+  pendingReceiptChallenge = null;
+  pendingReceiptText = null;
+  const result = spawnSync('python3', [LEASE_PROMOTE, '--complete', challenge], {
+    encoding: 'utf8',
+    timeout: 10_000,
+    env: process.env,
+  });
+  if (result.status !== 0) return false;
+  try {
+    const reply = JSON.parse(String(result.stdout)) as { state?: string };
+    return reply.state === 'VERIFIED';
+  } catch {
+    return false;
+  }
+}
+
 function checkPiMutatorGate(toolName: string): { block: true; reason: string } | undefined {
   const result = spawnSync('python3', [MUTATOR_GATE, '--runtime', 'pi'], {
     input: `${JSON.stringify({ tool_name: toolName })}\n`,
@@ -137,6 +213,29 @@ function checkPiMutatorGate(toolName: string): { block: true; reason: string } |
   const detail = String(result.stderr ?? '')
     .trim()
     .split('\n')[0];
+
+  // Only an UNVERIFIED lease is promotable. Any other denial (GATE_UNAVAILABLE,
+  // STALE_GENERATION, LEASE_EXPIRED, ANCESTRY_MISMATCH) means something is wrong
+  // that a receipt cannot fix — minting there would thrash the broker, since
+  // begin_verification revokes before it mints.
+  if (detail.includes('MUTATOR_UNVERIFIED') && pendingReceiptChallenge === null) {
+    const receipt = beginPiPromotion();
+    if (receipt) {
+      return {
+        block: true,
+        reason:
+          `${detail}\n\n` +
+          'This session holds an UNVERIFIED lease, so mutators are denied. To ' +
+          'promote it, reply with the following text and NOTHING ELSE — no ' +
+          'preamble, no explanation, no code fence, no trailing text. It is ' +
+          'compared byte-for-byte, so any extra character fails:\n\n' +
+          `${receipt}\n\n` +
+          'Emit exactly that as your entire next message. The lease will then be ' +
+          'VERIFIED and you can retry this tool.',
+      };
+    }
+  }
+
   return {
     block: true,
     reason: detail || 'BLOCKED: Mosaic mutator gate is unavailable or the lease is UNVERIFIED.',
@@ -370,7 +469,29 @@ export default function register(pi: ExtensionAPI) {
   // Pi records only a finalized assistant entry at message_end. It never uses
   // after_provider_response, which occurs before stream consumption.
   pi.on('message_end', async (event) => {
-    recordPiMessageEnd((event as unknown as { message?: unknown }).message);
+    const message = (event as unknown as { message?: unknown }).message;
+    recordPiMessageEnd(message);
+    // Complete ONLY when the message just observed IS the receipt.
+    //
+    // message_end also fires for the message that CONTAINED the blocked tool
+    // call — i.e. one turn BEFORE the model emits the receipt. Completing there
+    // makes observe_receipt compare against the wrong text, fail, and burn the
+    // challenge before the model ever answers. Gating on an exact text match
+    // mirrors the broker's own hmac.compare_digest semantics and waits for the
+    // right turn.
+    //
+    // Order still matters within this handler: recordPiMessageEnd must have
+    // shipped the message to the observer before observe_receipt asks about it.
+    if (pendingReceiptChallenge !== null && pendingReceiptText !== null) {
+      if (assistantMessageText(message) === pendingReceiptText) {
+        if (completePiPromotion()) {
+          promotionAttempts = 0;
+        }
+        // On failure the challenge is cleared, so the next denied mutator mints
+        // a fresh one — bounded by MAX_PROMOTION_ATTEMPTS, after which the
+        // session simply stays UNVERIFIED rather than looping.
+      }
+    }
   });
 
   // The recovery custom tool is the only Pi invocation that maps to the