feat: validate signers have signature fields before distribution (#2411)

API users were inadvertently sending documents without signature fields,
causing confusion for recipients and breaking their signing flows.

- Add getRecipientsWithMissingFields helper in recipients.ts
- Add server-side validation in sendDocument to block distribution
- Fix v1 API to return 400 instead of 500 for validation errors
- Consolidate UI signature field checks to use isSignatureFieldType
- Add E2E tests for both v1 and v2 APIs

.agents/plans/clear-amber-light-validate-signer-fields-on-distribute.md

@@ -0,0 +1,76 @@
+---
+date: 2026-01-26
+title: Validate Signer Fields On Distribute
+---
+
+## Summary
+
+Validate that signers have at least one signature field before allowing document/envelope distribution via API, matching the existing UI behavior.
+
+## Background
+
+The API originally allowed distributing documents/envelopes without validating that signers had signature fields assigned. This was intentional - we thought API users might have specific flows where this flexibility was needed.
+
+However, after running it this way for a while, we've observed that more often than not, API users inadvertently send documents without fields assigned. This causes confusion for their recipients (who receive a document with nothing to sign) and breaks their own systems expecting a completed signing flow.
+
+## Problem
+
+The API allowed distributing documents/envelopes even when signers had no signature fields assigned. This was inconsistent with the UI which validates this condition before allowing distribution.
+
+## Solution
+
+### 1. Create centralized validation helper
+
+**File**: `packages/lib/utils/recipients.ts`
+
+- Added `RECIPIENT_ROLES_THAT_REQUIRE_FIELDS` constant (currently only `SIGNER`)
+- Added `getRecipientsWithMissingFields()` function that returns recipients missing required fields
+- Uses existing `isSignatureFieldType` guard from `packages/prisma/guards/is-signature-field.ts`
+
+### 2. Add server-side validation
+
+**File**: `packages/lib/server-only/document/send-document.ts`
+
+- Added validation check that throws `AppError` with `INVALID_REQUEST` code when signers are missing signature fields
+- This blocks both v1 and v2 API distribution endpoints since they both use `sendDocument()`
+
+### 3. Fix v1 API error handling
+
+**File**: `packages/api/v1/implementation.ts`
+
+- Changed `sendDocument` endpoint to use `AppError.toRestAPIError(err)` instead of always returning 500
+- Now returns 400 for validation errors
+
+### 4. Update UI to use shared helper
+
+**Files**:
+
+- `apps/remix/app/components/dialogs/envelope-distribute-dialog.tsx`
+- `packages/ui/primitives/document-flow/add-fields.tsx`
+
+### 5. Consolidate `hasSignatureField` checks
+
+Updated to use `isSignatureFieldType` guard (checks both `SIGNATURE` and `FREE_SIGNATURE`):
+
+- `apps/remix/app/components/general/document-signing/document-signing-form.tsx`
+- `apps/remix/app/components/general/envelope-signing/envelope-signer-form.tsx`
+- `apps/remix/app/components/embed/multisign/multi-sign-document-signing-view.tsx`
+- `apps/remix/app/components/embed/embed-direct-template-client-page.tsx`
+- `apps/remix/app/components/embed/embed-document-signing-page-v1.tsx`
+
+### 6. Add E2E tests
+
+**Files**:
+
+- `packages/app-tests/e2e/api/v1/document-sending.spec.ts` - 5 new tests
+- `packages/app-tests/e2e/api/v2/distribute-validation.spec.ts` - 8 new tests
+
+## Test Coverage
+
+- Distribution fails when signer has no fields
+- Distribution fails when signer has only non-signature fields
+- Distribution succeeds with SIGNATURE field
+- Distribution succeeds with FREE_SIGNATURE field (v1 only via Prisma)
+- Distribution succeeds when VIEWER/CC/APPROVER have no fields
+- Distribution fails when one of multiple signers is missing signature field
+- Distribution succeeds when all signers have signature fields