diff --git a/.github/workflows/preview-env.yml b/.github/workflows/preview-env.yml
index 38fe31d3..62958368 100644
--- a/.github/workflows/preview-env.yml
+++ b/.github/workflows/preview-env.yml
@@ -115,19 +115,29 @@ jobs:
             exit 1
           fi
 
-      # Build the preview lambdas' env from the ACTUAL prod config (single source
-      # of truth) + the live RDS endpoint. Reserved / credential keys are dropped;
-      # the module adds NODE_ENV. Only needed when creating the stack.
+      # Prod config is the single source of truth, DB_HOST included. Never re-derive
+      # it from `DBInstances[0]` -- this account hosts other C4C databases and that
+      # picked an unreachable one. Reserved / credential keys are dropped; the module
+      # adds NODE_ENV. Only needed when creating the stack.
       - name: Resolve preview lambda env
         if: steps.mode.outputs.create == 'true'
         run: |
-          DB_HOST=$(aws rds describe-db-instances --query "DBInstances[0].Endpoint.Address" --output text)
+          set -euo pipefail
           AUTH_ENV=$(aws lambda get-function-configuration --function-name branch-auth --query 'Environment.Variables' --output json)
           REPORTS_ENV=$(aws lambda get-function-configuration --function-name branch-reports --query 'Environment.Variables' --output json)
-          ENV_JSON=$(jq -n --argjson a "$AUTH_ENV" --argjson r "$REPORTS_ENV" --arg dbh "$DB_HOST" '
-            (($a // {}) + ($r // {}) + { "DB_HOST": $dbh })
+          ENV_JSON=$(jq -n --argjson a "$AUTH_ENV" --argjson r "$REPORTS_ENV" '
+            (($a // {}) + ($r // {}))
             | del(.NODE_ENV, .AWS_REGION, .AWS_DEFAULT_REGION, .AWS_ACCESS_KEY_ID, .AWS_SECRET_ACCESS_KEY, .AWS_SESSION_TOKEN)
           ')
+          # Fail loudly rather than shipping a preview that 401s on every DB call.
+          for key in DB_HOST DB_NAME DB_USER DB_PASSWORD COGNITO_USER_POOL_ID COGNITO_CLIENT_ID; do
+            value=$(jq -r --arg k "$key" '.[$k] // ""' <<<"$ENV_JSON")
+            if [ -z "$value" ]; then
+              echo "::error::branch-auth/branch-reports did not provide $key; preview lambdas would be misconfigured."
+              exit 1
+            fi
+          done
+          echo "Preview DB_HOST: $(jq -r '.DB_HOST' <<<"$ENV_JSON")"
           # Stash for the terraform step (multiline-safe).
           printf 'LAMBDA_ENV<<EOF\n%s\nEOF\n' "$ENV_JSON" >> "$GITHUB_ENV"
 
diff --git a/infrastructure/preview/variables.tf b/infrastructure/preview/variables.tf
index 3a0e3db1..5e0f900a 100644
--- a/infrastructure/preview/variables.tf
+++ b/infrastructure/preview/variables.tf
@@ -4,9 +4,9 @@ variable "pr_number" {
 }
 
 # Full runtime environment for the preview lambdas, resolved by the workflow
-# from the ACTUAL prod lambda config (branch-auth + branch-reports) plus the
-# shared RDS endpoint. Passing it in keeps prod as the single source of truth
-# for DB creds / Cognito ids / reports bucket rather than duplicating them here.
+# from the ACTUAL prod lambda config (branch-auth + branch-reports). Passing it in
+# keeps prod as the single source of truth for DB_HOST / DB creds / Cognito ids /
+# reports bucket rather than duplicating -- or re-deriving -- them here.
 # Preview envs deliberately reuse the shared RDS + Cognito pool (data risk is
 # accepted); migrations are NEVER run from this module -- the generated DB types
 # hardcode the `branch.` schema prefix, so a per-PR schema would require per-PR
diff --git a/shared/lambda-auth/src/authenticate.ts b/shared/lambda-auth/src/authenticate.ts
index f832ee8e..d0941119 100644
--- a/shared/lambda-auth/src/authenticate.ts
+++ b/shared/lambda-auth/src/authenticate.ts
@@ -49,39 +49,46 @@ export async function authenticateRequest(
   const token = extractToken(event);
   if (!token) return { isAuthenticated: false };
 
-  try {
-    const payload = await getVerifier().verify(token);
-
-    const dbUser = await db
-      .selectFrom('branch.users')
-      .where('cognito_sub', '=', payload.sub)
-      .selectAll()
-      .executeTakeFirst();
-
-    if (!dbUser) {
-      console.warn(
-        'User authenticated with Cognito but not found in database:',
-        payload.sub,
-      );
-      return { isAuthenticated: false };
-    }
-
-    const user: AuthenticatedUser = {
-      cognitoSub: payload.sub,
-      userId: dbUser.user_id,
-      email: payload.email as string | undefined,
-      isAdmin: dbUser.is_admin === true,
-      // Informational only. We deliberately do NOT promote on a Cognito
-      // "Admins" group: branch.users.is_admin is the single source of truth.
-      // A second source would make demotion via PATCH /users/{userId} silently
-      // ineffective, nothing in this codebase writes group membership, and no
-      // aws_cognito_user_group is defined in infrastructure/aws/cognito.tf.
-      cognitoGroups: payload['cognito:groups'] as string[] | undefined,
-    };
+  // Outside the try: missing config is a broken deployment, not a bad token.
+  const jwtVerifier = getVerifier();
 
-    return { user, isAuthenticated: true };
+  let payload: any;
+  try {
+    payload = await jwtVerifier.verify(token);
   } catch (error) {
+    // Only an unverifiable token is genuinely unauthenticated.
     console.error('Token verification failed:', error);
     return { isAuthenticated: false };
   }
+
+  // Uncaught on purpose: a DB outage is not a 401. Catching it hid an unreachable
+  // RDS behind "Authentication required" and logged users out. Handlers map to 500.
+  const dbUser = await db
+    .selectFrom('branch.users')
+    .where('cognito_sub', '=', payload.sub)
+    .selectAll()
+    .executeTakeFirst();
+
+  if (!dbUser) {
+    console.warn(
+      'User authenticated with Cognito but not found in database:',
+      payload.sub,
+    );
+    return { isAuthenticated: false };
+  }
+
+  const user: AuthenticatedUser = {
+    cognitoSub: payload.sub,
+    userId: dbUser.user_id,
+    email: payload.email as string | undefined,
+    isAdmin: dbUser.is_admin === true,
+    // Informational only. We deliberately do NOT promote on a Cognito
+    // "Admins" group: branch.users.is_admin is the single source of truth.
+    // A second source would make demotion via PATCH /users/{userId} silently
+    // ineffective, nothing in this codebase writes group membership, and no
+    // aws_cognito_user_group is defined in infrastructure/aws/cognito.tf.
+    cognitoGroups: payload['cognito:groups'] as string[] | undefined,
+  };
+
+  return { user, isAuthenticated: true };
 }
diff --git a/shared/lambda-auth/test/authenticate.test.ts b/shared/lambda-auth/test/authenticate.test.ts
index 2b9aa821..5de4b350 100644
--- a/shared/lambda-auth/test/authenticate.test.ts
+++ b/shared/lambda-auth/test/authenticate.test.ts
@@ -197,34 +197,36 @@ describe('authenticateRequest', () => {
     expect(mockCreate).toHaveBeenCalledWith(expect.objectContaining({ clientId: null }));
   });
 
-  it('degrades to unauthenticated (not a throw) when COGNITO_USER_POOL_ID is unset', async () => {
-    // This is why a missing env var manifests as blanket silent 401s across all
-    // six lambdas rather than a loud 500: getVerifier() throws inside the try.
+  it('throws (not a silent 401) when COGNITO_USER_POOL_ID is unset', async () => {
+    // Swallowing this gave blanket silent 401s across all six lambdas.
     delete process.env.COGNITO_USER_POOL_ID;
     const { authenticateRequest } = await loadModule();
     const { db } = makeDb({ user_id: 7, is_admin: true });
 
-    await expect(authenticateRequest(db, bearerEvent('good'))).resolves.toEqual({
-      isAuthenticated: false,
-    });
+    await expect(authenticateRequest(db, bearerEvent('good'))).rejects.toThrow(
+      'COGNITO_USER_POOL_ID',
+    );
     expect(mockCreate).not.toHaveBeenCalled();
   });
 
-  it('returns unauthenticated when the database query throws', async () => {
+  it('propagates a database failure instead of reporting it as unauthenticated', async () => {
+    // Regression guard for the preview-env outage (PR #316).
     mockVerify.mockResolvedValue({ sub: 'sub-1' });
     const { authenticateRequest } = await loadModule();
     const db = {
       selectFrom: () => ({
         where: () => ({
           selectAll: () => ({
-            executeTakeFirst: jest.fn().mockRejectedValue(new Error('db down')),
+            executeTakeFirst: jest
+              .fn()
+              .mockRejectedValue(new Error('timeout exceeded when trying to connect')),
           }),
         }),
       }),
     };
 
-    await expect(authenticateRequest(db, bearerEvent('good'))).resolves.toEqual({
-      isAuthenticated: false,
-    });
+    await expect(authenticateRequest(db, bearerEvent('good'))).rejects.toThrow(
+      'timeout exceeded when trying to connect',
+    );
   });
 });