diff --git a/apps/api/src/templates/blob-template-store.security.spec.ts b/apps/api/src/templates/blob-template-store.security.spec.ts
index b8864b0..c9aa2b8 100644
--- a/apps/api/src/templates/blob-template-store.security.spec.ts
+++ b/apps/api/src/templates/blob-template-store.security.spec.ts
@@ -160,15 +160,20 @@ describe('BlobTemplateStore — environment isolation', () => {
   });
 });
 
-describe('BlobTemplateStore — tenant identity is interpolated into the blob name verbatim', () => {
-  // Documents CURRENT behaviour, not desired behaviour. Azure Blob Storage has a
-  // flat namespace, so these values are not path traversal — they are literal
-  // parts of the blob name. The point is that `tenantId` / `environment` are
-  // interpolated without the publisher's `assertSafeTenantId` /
-  // `assertSafeEnvironment` guards, so a value containing `/` silently produces a
-  // blob name outside the documented per-tenant prefix shape. Not reachable from
-  // a request today (identity comes from the credential map). Tracked in #59.
-  it('interpolates a slash-bearing tenantId verbatim, producing a blob name outside the documented prefix shape (known gap)', async () => {
+describe('BlobTemplateStore — tenant identity path validation', () => {
+  async function expectInvalidTemplate(
+    run: () => Promise<unknown>,
+    requestedPaths: string[],
+  ): Promise<void> {
+    await assert.rejects(run, (err: unknown) => {
+      assert.ok(err instanceof TemplateStoreError, 'expected TemplateStoreError');
+      assert.equal(err.code, PostKitErrorCode.INVALID_TEMPLATE);
+      return true;
+    });
+    assert.deepEqual(requestedPaths, [], 'no blob access may happen for an unsafe tenant path');
+  }
+
+  it('rejects a slash-bearing tenantId before storage access', async () => {
     const blobs = new Map<string, string>();
     const tenantId = 'tenant-a/production/templates/shared';
     const base = `templates/tenants/${tenantId}/production/templates/${TEMPLATE_KEY}`;
@@ -177,32 +182,52 @@ describe('BlobTemplateStore — tenant identity is interpolated into the blob na
     const { client, requestedPaths } = makeRecordingClient(blobs);
 
     const unvalidated = { tenantId, environment: 'production' } as unknown as TenantContext;
-    const loaded = await makeStore(client).load(unvalidated, TEMPLATE_KEY);
-
-    assert.equal(loaded.templateHtml, HTML);
-    for (const path of requestedPaths) {
-      assert.ok(
-        path.startsWith(`tenants/${tenantId}/production/templates/`),
-        'current behaviour: the tenantId is used as-is, extra segments included',
-      );
-    }
+    await expectInvalidTemplate(
+      () => makeStore(client).load(unvalidated, TEMPLATE_KEY),
+      requestedPaths,
+    );
   });
 
-  it('interpolates an environment value that is not a TenantEnvironment (known gap)', async () => {
+  it('rejects an environment value that is not a TenantEnvironment before storage access', async () => {
     const blobs = new Map<string, string>();
     const base = `templates/tenants/tenant-a/elsewhere/templates/${TEMPLATE_KEY}`;
     blobs.set(`${base}/template.html`, HTML);
     blobs.set(`${base}/metadata.json`, JSON.stringify(METADATA));
-    const { client } = makeRecordingClient(blobs);
+    const { client, requestedPaths } = makeRecordingClient(blobs);
 
     const unvalidated = {
       tenantId: 'tenant-a',
       environment: 'elsewhere',
     } as unknown as TenantContext;
 
-    const loaded = await makeStore(client).load(unvalidated, TEMPLATE_KEY);
-    assert.equal(loaded.templateHtml, HTML);
+    await expectInvalidTemplate(
+      () => makeStore(client).load(unvalidated, TEMPLATE_KEY),
+      requestedPaths,
+    );
   });
+
+  const unsafeTenantIds: Array<[label: string, tenantId: string]> = [
+    ['empty tenantId', ''],
+    ['parent traversal', '../tenant-b'],
+    ['nested slash', 'a/b'],
+    ['leading hyphen', '-acme'],
+    ['trailing hyphen', 'acme-'],
+    ['double dot segment', 'a..b'],
+  ];
+
+  for (const [label, tenantId] of unsafeTenantIds) {
+    it(`rejects ${label} with INVALID_TEMPLATE and makes no blob call`, async () => {
+      const { client, requestedPaths } = makeRecordingClient(new Map());
+      await expectInvalidTemplate(
+        () =>
+          makeStore(client).load(
+            { tenantId, environment: 'production' } as unknown as TenantContext,
+            TEMPLATE_KEY,
+          ),
+        requestedPaths,
+      );
+    });
+  }
 });
 
 describe('BlobTemplateStore — unsafe template keys are rejected before storage access', () => {
diff --git a/apps/api/src/templates/blob-template-store.ts b/apps/api/src/templates/blob-template-store.ts
index 84cf1fd..11ea788 100644
--- a/apps/api/src/templates/blob-template-store.ts
+++ b/apps/api/src/templates/blob-template-store.ts
@@ -4,6 +4,7 @@ import { DefaultAzureCredential } from '@azure/identity';
 import type {
   CompiledTemplate,
   TenantContext,
+  TenantEnvironment,
   TemplateSourceMetadata,
 } from '@singleton-sd/post-kit-types';
 import { PostKitErrorCode, TEMPLATE_SCHEMA_VERSION } from '@singleton-sd/post-kit-types';
@@ -12,6 +13,9 @@ import type { TemplateStore } from './template-store';
 
 /** Allowlist regex for safe template keys — alphanumeric, dots, hyphens, underscores only. */
 const SAFE_TEMPLATE_KEY = /^[a-zA-Z0-9._-]+$/;
+/** Matches publisher `assertSafeTenantId` — single path segment, no slashes or `..`. */
+const SAFE_TENANT_SEGMENT = /^[a-zA-Z0-9]([a-zA-Z0-9-]*[a-zA-Z0-9])?$/;
+const VALID_ENVIRONMENTS = new Set<TenantEnvironment>(['development', 'staging', 'production']);
 
 /**
  * Error thrown by BlobTemplateStore on load failures.
@@ -110,6 +114,8 @@ export class BlobTemplateStore implements TemplateStore {
    */
   async load(tenant: TenantContext, templateKey: string): Promise<CompiledTemplate> {
     validateTemplateKey(templateKey);
+    validateTenantId(tenant.tenantId);
+    validateEnvironment(tenant.environment);
 
     const { tenantId, environment } = tenant;
     const basePath = `tenants/${tenantId}/${environment}/templates/${templateKey}`;
@@ -158,6 +164,24 @@ function validateTemplateKey(templateKey: string): void {
   }
 }
 
+function validateTenantId(tenantId: string): void {
+  if (!tenantId || !SAFE_TENANT_SEGMENT.test(tenantId) || tenantId.includes('..')) {
+    throw new TemplateStoreError(
+      `Invalid tenantId: "${tenantId}". Use alphanumeric characters and hyphens only (no path segments).`,
+      PostKitErrorCode.INVALID_TEMPLATE,
+    );
+  }
+}
+
+function validateEnvironment(environment: string): void {
+  if (!VALID_ENVIRONMENTS.has(environment as TenantEnvironment)) {
+    throw new TemplateStoreError(
+      `Invalid environment: "${environment}". Must be one of: development, staging, production.`,
+      PostKitErrorCode.INVALID_TEMPLATE,
+    );
+  }
+}
+
 async function downloadBlob(
   containerClient: ReturnType<BlobServiceClient['getContainerClient']>,
   blobPath: string,