diff --git a/packages/cultmesh-browser/src/index.ts b/packages/cultmesh-browser/src/index.ts
index 86d933d1..0398d313 100644
--- a/packages/cultmesh-browser/src/index.ts
+++ b/packages/cultmesh-browser/src/index.ts
@@ -19,6 +19,7 @@ import {
   base64ToBytes,
   bytesToBase64,
   isLoopbackEndpoint,
+  isUnsignedCertificate,
   verifyAuthorityRoute,
   verifyProviderSessionProof,
   type CultMeshAuthorityIdentity,
@@ -912,7 +913,11 @@ async function validateSessionAcceptance(
       `expected '${request.verseId}/${request.authorityRuntimeId}/${protocolId}/${route.generation}'.`,
     );
   }
-  if (trust.mode === "local-development" && isLoopbackEndpoint(route.endpoint) && !route.certificate) return undefined;
+  // `CultMeshVerifiedAuthorityRoute.IsLocalDevelopment` in C#: an unsigned
+  // loopback route under local-development needs no provider session proof.
+  if (trust.mode === "local-development" && isLoopbackEndpoint(route.endpoint) && isUnsignedCertificate(route.certificate)) {
+    return undefined;
+  }
   const providerKey = route.certificate?.providerKey;
   if (!providerKey || message.providerKeyId !== providerKey.keyId || !message.providerSignature) {
     return new Error("CultMesh authority did not prove possession of the Odin-certified provider key.");
diff --git a/packages/cultmesh-browser/test/cultmesh-browser.test.ts b/packages/cultmesh-browser/test/cultmesh-browser.test.ts
index 6c169d93..77237bb9 100644
--- a/packages/cultmesh-browser/test/cultmesh-browser.test.ts
+++ b/packages/cultmesh-browser/test/cultmesh-browser.test.ts
@@ -236,6 +236,37 @@ test("browser client rejects a credential-free authority impersonator", async ()
   );
 });
 
+test("browser client opens an unsigned-certificate loopback session under local-development without a provider proof, as in C#", async () => {
+  const connect = (route: CultMeshBrowserRoute, trust: { mode: "local-development" | "authenticated-remote"; odinRoots: CultMeshBrowserP256PublicKey[] }) =>
+    CultMeshBrowserClient.connect({
+      ...route,
+      runtimeId: "browser-unsigned-certificate-test",
+      rendezvous: { resolve: async () => route },
+      trust,
+      socketFactory: () => handshakeOnlySocket(route),
+    });
+  const blank = (route: CultMeshBrowserRoute) => ({ ...route, certificate: { ...route.certificate!, signature: "" } });
+
+  const loopback = await createSignedRoute("ws://127.0.0.1:4501/mesh");
+  const localTrust = { mode: "local-development" as const, odinRoots: [loopback.odinPublic] };
+  const client = await connect(blank(loopback.route), localTrust);
+  assert.equal(client.state, "connected");
+  await client.dispose();
+  // A signed loopback certificate is an authenticated route even under
+  // local-development; the provider must still prove its key.
+  await assert.rejects(connect(loopback.route, localTrust), /did not prove possession/);
+
+  const remote = await createSignedRoute("wss://provider.example/mesh");
+  await assert.rejects(
+    connect(blank(remote.route), { mode: "local-development", odinRoots: [remote.odinPublic] }),
+    /Odin-signed authority certificate/,
+  );
+  await assert.rejects(
+    connect(blank(remote.route), { mode: "authenticated-remote", odinRoots: [remote.odinPublic] }),
+    /Odin-signed authority certificate/,
+  );
+});
+
 test("browser client rejects a mutated or expired signed route before opening a provider socket", async () => {
   const fixture = await createSignedRoute("wss://provider.example/mesh");
   let opened = false;
diff --git a/packages/cultnet-ts/src/cultmesh-authority.ts b/packages/cultnet-ts/src/cultmesh-authority.ts
index fd0fcce6..40bcff1a 100644
--- a/packages/cultnet-ts/src/cultmesh-authority.ts
+++ b/packages/cultnet-ts/src/cultmesh-authority.ts
@@ -44,23 +44,30 @@ export interface CultMeshAuthorityTrustPolicy {
   now?: () => number;
 }
 
+// A missing certificate, or one whose signature is empty or whitespace, is an
+// unsigned route (`CultMeshRouteCertificate` trims the signature; `Verify`
+// treats a whitespace signature as no certificate). The verifier and the
+// browser's session-proof short-circuit share this one reading, as the C#
+// `Verify` and `IsLocalDevelopment` do.
+export function isUnsignedCertificate(certificate: CultMeshAuthorityRouteCertificate | undefined): boolean {
+  return !certificate || certificate.signature.trim() === "";
+}
+
 // Rule for rule, this is `CultMeshAuthorityTrustPolicy.Verify` in the C#
 // reference, in the same order: unsigned, channel protection, root lookup,
 // validity window, signature. The same input yields the same refusal on both
-// sides. A certificate whose signature is empty or whitespace is unsigned
-// (`CultMeshRouteCertificate` trims the signature; `Verify` treats a
-// whitespace signature as no certificate).
+// sides.
 export async function verifyAuthorityRoute(
   route: CultMeshAuthorityRouteView,
   trust: CultMeshAuthorityTrustPolicy,
 ): Promise<void> {
   const roots = trustedOdinRoots(trust);
   const certificate = route.certificate;
-  const signature = certificate?.signature.trim() ?? "";
-  if (!certificate || signature === "") {
+  if (!certificate || isUnsignedCertificate(certificate)) {
     if (trust.mode === "local-development" && isLoopbackEndpoint(route.endpoint)) return;
     throw new Error("Remote CultMesh routes require an Odin-signed authority certificate.");
   }
+  const signature = certificate.signature.trim();
   if (!isProtectedEndpoint(route.endpoint) && !(trust.mode === "local-development" && isLoopbackEndpoint(route.endpoint))) {
     throw new Error("Authenticated remote CultMesh routes require TLS or QUIC channel protection.");
   }
diff --git a/packages/cultnet-ts/src/index.ts b/packages/cultnet-ts/src/index.ts
index e9260122..4987b177 100644
--- a/packages/cultnet-ts/src/index.ts
+++ b/packages/cultnet-ts/src/index.ts
@@ -70,6 +70,7 @@ export {
   canonicalSession,
   isLoopbackEndpoint,
   isProtectedEndpoint,
+  isUnsignedCertificate,
   verifyAuthorityRoute,
   verifyP256,
   verifyProviderSessionProof,
diff --git a/scripts/mutate-cultmesh-authority.mjs b/scripts/mutate-cultmesh-authority.mjs
index cb4cae16..6053cb14 100644
--- a/scripts/mutate-cultmesh-authority.mjs
+++ b/scripts/mutate-cultmesh-authority.mjs
@@ -58,9 +58,14 @@ const mutations = [
     new: "const root = [...roots.values()][0];",
   },
   {
-    rule: "empty or whitespace signature is an unsigned route",
-    old: 'if (!certificate || signature === "") {',
-    new: "if (!certificate) {",
+    rule: "isUnsignedCertificate treats a whitespace signature as unsigned",
+    old: 'return !certificate || certificate.signature.trim() === "";',
+    new: 'return !certificate || certificate.signature === "";',
+  },
+  {
+    rule: "isUnsignedCertificate treats an empty signature as unsigned",
+    old: 'return !certificate || certificate.signature.trim() === "";',
+    new: "return !certificate;",
   },
   {
     rule: "root lookup precedes the validity window",