diff --git a/src/schemas/registry.test.ts b/src/schemas/registry.test.ts
new file mode 100644
index 0000000..a46226c
--- /dev/null
+++ b/src/schemas/registry.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from 'vitest';
+import { BACKEND_REGISTRY, BackendType } from './registry';
+
+const EXPECTED_BACKENDS: BackendType[] = ['azureblob', 's3', 's3-compatible'];
+
+describe('Backend Schema Registry', () => {
+  it('exports all three required backend types', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      expect(BACKEND_REGISTRY[backend]).toBeDefined();
+    }
+  });
+
+  it('each backend has at least one field definition', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      expect(BACKEND_REGISTRY[backend].length).toBeGreaterThan(0);
+    }
+  });
+
+  it('each FieldDef has a non-empty key (snake_case, no camelCase)', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      for (const field of BACKEND_REGISTRY[backend]) {
+        expect(field.key).toBeTruthy();
+        // Reject camelCase — rclone keys are snake_case or lowercase
+        expect(field.key).not.toMatch(/[A-Z]/);
+      }
+    }
+  });
+
+  it('each FieldDef has a non-empty label', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      for (const field of BACKEND_REGISTRY[backend]) {
+        expect(field.label).toBeTruthy();
+      }
+    }
+  });
+
+  it('Azure Blob has account field (required)', () => {
+    const accountField = BACKEND_REGISTRY.azureblob.find(f => f.key === 'account');
+    expect(accountField).toBeDefined();
+    expect(accountField!.required).toBe(true);
+  });
+
+  it('S3 has access_key_id, secret_access_key, and region fields', () => {
+    const keys = BACKEND_REGISTRY.s3.map(f => f.key);
+    expect(keys).toContain('access_key_id');
+    expect(keys).toContain('secret_access_key');
+    expect(keys).toContain('region');
+  });
+
+  it('S3-compatible has endpoint field (required)', () => {
+    const endpointField = BACKEND_REGISTRY['s3-compatible'].find(f => f.key === 'endpoint');
+    expect(endpointField).toBeDefined();
+    expect(endpointField!.required).toBe(true);
+  });
+});
diff --git a/src/schemas/registry.ts b/src/schemas/registry.ts
new file mode 100644
index 0000000..f3309ff
--- /dev/null
+++ b/src/schemas/registry.ts
@@ -0,0 +1,112 @@
+// src/schemas/registry.ts
+// Backend Schema Registry — single source of truth for all rclone backend field definitions.
+// IMPORTANT: FieldDef.key values MUST match rclone config key names exactly.
+// These keys are used by Phase 2 generators to build rclone.conf INI content.
+// Verify against https://rclone.org/azureblob/ and https://rclone.org/s3/ before Phase 2.
+
+export type BackendType = 'azureblob' | 's3' | 's3-compatible';
+
+export interface FieldDef {
+  key: string;           // MUST match rclone config key exactly (snake_case)
+  label: string;
+  inputType: 'text' | 'password' | 'select' | 'toggle';
+  required: boolean;
+  placeholder?: string;
+  helpText?: string;
+  options?: { value: string; label: string }[]; // for inputType: 'select'
+}
+
+export const BACKEND_REGISTRY: Record<BackendType, FieldDef[]> = {
+  azureblob: [
+    {
+      key: 'account',
+      label: 'Storage Account Name',
+      inputType: 'text',
+      required: true,
+      placeholder: 'mystorageaccount',
+      helpText: 'The storage account name (not the full URL)',
+    },
+    {
+      key: 'key',
+      label: 'Access Key',
+      inputType: 'password',
+      required: false,
+      helpText: 'Base64-encoded storage account key. Provide either this or a SAS URL, not both.',
+    },
+    {
+      key: 'sas_url',
+      label: 'SAS URL',
+      inputType: 'password',
+      required: false,
+      placeholder: 'https://mystorageaccount.blob.core.windows.net/?sv=...',
+      helpText: 'Full SAS URL including account and container. Provide either this or an access key, not both.',
+    },
+  ],
+  s3: [
+    {
+      key: 'provider',
+      label: 'Provider',
+      inputType: 'select',
+      required: true,
+      options: [{ value: 'AWS', label: 'Amazon S3' }],
+    },
+    {
+      key: 'access_key_id',
+      label: 'Access Key ID',
+      inputType: 'text',
+      required: true,
+      placeholder: 'AKIAIOSFODNN7EXAMPLE',
+    },
+    {
+      key: 'secret_access_key',
+      label: 'Secret Access Key',
+      inputType: 'password',
+      required: true,
+    },
+    {
+      key: 'region',
+      label: 'Region',
+      inputType: 'text',
+      required: true,
+      placeholder: 'us-east-1',
+    },
+  ],
+  's3-compatible': [
+    {
+      key: 'provider',
+      label: 'Provider',
+      inputType: 'select',
+      required: true,
+      options: [{ value: 'Other', label: 'S3-Compatible' }],
+      helpText: 'Covers Wasabi, MinIO, Cloudflare R2, and any S3-compatible storage',
+    },
+    {
+      key: 'access_key_id',
+      label: 'Access Key ID',
+      inputType: 'text',
+      required: true,
+    },
+    {
+      key: 'secret_access_key',
+      label: 'Secret Access Key',
+      inputType: 'password',
+      required: true,
+    },
+    {
+      key: 'endpoint',
+      label: 'Endpoint URL',
+      inputType: 'text',
+      required: true,
+      placeholder: 'https://s3.wasabisys.com',
+      helpText: 'The S3-compatible endpoint URL for your storage provider',
+    },
+    {
+      key: 'region',
+      label: 'Region',
+      inputType: 'text',
+      required: false,
+      placeholder: 'us-east-1',
+      helpText: 'Optional for most S3-compatible providers',
+    },
+  ],
+};