diff --git a/apps/api/src/signatures/dto/create-envelope.dto.ts b/apps/api/src/signatures/dto/create-envelope.dto.ts
index 7b4a89d..2c9cea2 100644
--- a/apps/api/src/signatures/dto/create-envelope.dto.ts
+++ b/apps/api/src/signatures/dto/create-envelope.dto.ts
@@ -43,9 +43,17 @@ export class CreateEnvelopeDto {
   @IsUUID("4", { each: true })
   documentGenerationIds!: string[];
 
+  /**
+   * Omitted/empty → `contratante` signers are derived automatically from
+   * the envelope's origin (`clientRegistrationId`/`resellerId`): the
+   * legal representative (if authorized) plus every sócio marked
+   * `will_sign` — see `EnvelopeService.deriveContratanteSigners`. Provide
+   * this explicitly to override that derivation entirely (e.g. a signer
+   * not on file, or an origin-less envelope).
+   */
+  @IsOptional()
   @IsArray()
-  @ArrayMinSize(1)
   @ValidateNested({ each: true })
   @Type(() => SignerInputDto)
-  signers!: SignerInputDto[];
+  signers?: SignerInputDto[];
 }
diff --git a/apps/api/src/signatures/envelope.service.ts b/apps/api/src/signatures/envelope.service.ts
index 3af7d7f..d630b18 100644
--- a/apps/api/src/signatures/envelope.service.ts
+++ b/apps/api/src/signatures/envelope.service.ts
@@ -11,6 +11,10 @@ import {
   companiesRepo,
   documentsRepo,
   signaturesRepo,
+  clientRegistrationsRepo,
+  clientRegistrationPartnersRepo,
+  resellersRepo,
+  resellerPartnersRepo,
   type ActorType,
   type EnvelopeWithDetails,
   type SignatureEnvelope,
@@ -33,6 +37,18 @@ export interface RequestContext {
   userAgent: string | null;
 }
 
+interface ContratanteSignerDraft {
+  party: "contratante";
+  personType: "pf" | "pj" | null;
+  fullName: string;
+  cpf: string | null;
+  email: string;
+  phone: string | null;
+  companyName: string | null;
+  cnpj: string | null;
+  roleTitle: string | null;
+}
+
 function resolveActorType(roleKey: string): ActorType {
   if (roleKey === "super_admin") return "SUPER_ADMIN";
   if (roleKey === "admin") return "ADMIN";
@@ -119,6 +135,7 @@ export class EnvelopeService {
       });
     }
 
+    const contratanteSigners = await this.deriveContratanteSigners(dto);
     const signers = [
       {
         party: "contratada" as const,
@@ -127,17 +144,7 @@ export class EnvelopeService {
         email: company.legal_rep_email,
         roleTitle: company.legal_rep_role,
       },
-      ...dto.signers.map((s) => ({
-        party: "contratante" as const,
-        personType: s.personType ?? null,
-        fullName: s.fullName,
-        cpf: s.cpf ?? null,
-        email: s.email,
-        phone: s.phone ?? null,
-        companyName: s.companyName ?? null,
-        cnpj: s.cnpj ?? null,
-        roleTitle: s.roleTitle ?? null,
-      })),
+      ...contratanteSigners,
     ];
 
     const envelope = await signaturesRepo.createEnvelope(
@@ -161,6 +168,157 @@ export class EnvelopeService {
     return envelope;
   }
 
+  /**
+   * `dto.signers` explícito (não vazio) sempre vence — override manual
+   * completo, mesmo comportamento de antes desta função existir. Vazio/
+   * omitido: deriva automaticamente da origem do envelope (usuário pediu
+   * isso depois de completar o cadastro de sócios/representante legal em
+   * client_registrations e resellers) — representante legal autorizado
+   * (`*_rep_authorization_ack`) + cada sócio marcado `will_sign`, dedup
+   * por CPF (o representante pode também constar como sócio). PF de
+   * client_registrations não tem sócios/representante — o próprio titular
+   * é o único signatário possível.
+   */
+  private async deriveContratanteSigners(dto: CreateEnvelopeDto): Promise<ContratanteSignerDraft[]> {
+    if (dto.signers && dto.signers.length > 0) {
+      return dto.signers.map((s) => ({
+        party: "contratante" as const,
+        personType: s.personType ?? null,
+        fullName: s.fullName,
+        cpf: s.cpf ?? null,
+        email: s.email,
+        phone: s.phone ?? null,
+        companyName: s.companyName ?? null,
+        cnpj: s.cnpj ?? null,
+        roleTitle: s.roleTitle ?? null,
+      }));
+    }
+
+    if (dto.clientRegistrationId) {
+      const registration = await clientRegistrationsRepo.getClientRegistrationById(dto.clientRegistrationId);
+      if (!registration) throw new NotFoundException("Cadastro de cliente não encontrado");
+
+      if (registration.person_type === "pf") {
+        if (!registration.pf_full_name || !registration.pf_email) {
+          throw new BadRequestException(
+            "O cadastro de cliente não tem nome/e-mail completos para assinar — complete o cadastro ou informe os signatários manualmente.",
+          );
+        }
+        return [
+          {
+            party: "contratante" as const,
+            personType: "pf" as const,
+            fullName: registration.pf_full_name,
+            cpf: registration.pf_cpf,
+            email: registration.pf_email,
+            phone: registration.pf_mobile_phone,
+            companyName: null,
+            cnpj: null,
+            roleTitle: null,
+          },
+        ];
+      }
+
+      const partners = await clientRegistrationPartnersRepo.listPartnersForRegistration(dto.clientRegistrationId);
+      return this.deriveFromRepAndPartners({
+        companyName: registration.pj_company_name,
+        cnpj: registration.pj_cnpj,
+        repFullName: registration.pj_rep_full_name,
+        repCpf: registration.pj_rep_cpf,
+        repRole: registration.pj_rep_role,
+        repEmail: registration.pj_rep_email,
+        repMobilePhone: registration.pj_rep_mobile_phone,
+        repAuthorizationAck: registration.pj_rep_authorization_ack,
+        partners,
+        originLabel: "cadastro de cliente",
+      });
+    }
+
+    // `hasReseller` — a XOR de clientRegistrationId/resellerId já foi validada por quem chama.
+    const reseller = await resellersRepo.getResellerById(dto.resellerId!);
+    if (!reseller) throw new NotFoundException("Revenda não encontrada");
+    const partners = await resellerPartnersRepo.listPartnersForReseller(dto.resellerId!);
+    return this.deriveFromRepAndPartners({
+      companyName: reseller.name,
+      cnpj: reseller.document,
+      repFullName: reseller.rep_full_name,
+      repCpf: reseller.rep_cpf,
+      repRole: reseller.rep_role,
+      repEmail: reseller.rep_email,
+      repMobilePhone: reseller.rep_mobile_phone,
+      repAuthorizationAck: reseller.rep_authorization_ack,
+      partners,
+      originLabel: "revenda",
+    });
+  }
+
+  private deriveFromRepAndPartners(input: {
+    companyName: string | null;
+    cnpj: string | null;
+    repFullName: string | null;
+    repCpf: string | null;
+    repRole: string | null;
+    repEmail: string | null;
+    repMobilePhone: string | null;
+    repAuthorizationAck: boolean;
+    partners: Array<{
+      name: string;
+      cpf: string | null;
+      qualification: string | null;
+      will_sign: boolean;
+      signature_email: string | null;
+    }>;
+    originLabel: string;
+  }): ContratanteSignerDraft[] {
+    const signers: ContratanteSignerDraft[] = [];
+    const seenCpfs = new Set<string>();
+
+    if (input.repAuthorizationAck && input.repFullName && input.repEmail) {
+      signers.push({
+        party: "contratante",
+        personType: "pf",
+        fullName: input.repFullName,
+        cpf: input.repCpf,
+        email: input.repEmail,
+        phone: input.repMobilePhone,
+        companyName: input.companyName,
+        cnpj: input.cnpj,
+        roleTitle: input.repRole,
+      });
+      if (input.repCpf) seenCpfs.add(input.repCpf);
+    }
+
+    for (const partner of input.partners) {
+      if (!partner.will_sign) continue;
+      if (partner.cpf && seenCpfs.has(partner.cpf)) continue; // já coberto pelo representante
+      if (!partner.signature_email) {
+        throw new BadRequestException(
+          `O sócio "${partner.name}" está marcado para assinar mas não tem e-mail de assinatura cadastrado — complete o cadastro (${input.originLabel}) ou informe os signatários manualmente.`,
+        );
+      }
+      signers.push({
+        party: "contratante",
+        personType: "pf",
+        fullName: partner.name,
+        cpf: partner.cpf,
+        email: partner.signature_email,
+        phone: null,
+        companyName: input.companyName,
+        cnpj: input.cnpj,
+        roleTitle: partner.qualification,
+      });
+      if (partner.cpf) seenCpfs.add(partner.cpf);
+    }
+
+    if (signers.length === 0) {
+      throw new BadRequestException(
+        `Nenhum sócio marcado para assinar nem representante legal autorizado no cadastro (${input.originLabel}) — marque quem assina no cadastro ou informe os signatários manualmente nesta requisição.`,
+      );
+    }
+
+    return signers;
+  }
+
   /** eden.md §5.5: gera um token novo por signatário-alvo ainda não assinado (revoga implicitamente o anterior), 7 dias de validade, e-mail com o link. Envelope READY->SENT na primeira vez. */
   async sendInvite(id: string, dto: SendInviteDto, user: EdenAuthenticatedUser): Promise<EnvelopeWithDetails> {
     const envelope = await this.get(id, user);
diff --git a/apps/api/test/signature-envelope-auto-signers.spec.ts b/apps/api/test/signature-envelope-auto-signers.spec.ts
new file mode 100644
index 0000000..20c092c
--- /dev/null
+++ b/apps/api/test/signature-envelope-auto-signers.spec.ts
@@ -0,0 +1,302 @@
+import { INestApplication } from "@nestjs/common";
+import { Readable } from "node:stream";
+import { PDFDocument } from "pdf-lib";
+import { afterAll, beforeAll, describe, expect, it, vi } from "vitest";
+import { agent, bearer, closeTestApp, createTestApp, loginSuperAdmin, qaEmail, qaName, type AuthedTokens } from "./setup.js";
+
+/**
+ * `POST /v1/signature-envelopes` with `signers` omitted — contratante
+ * signers derived automatically from the envelope's origin
+ * (`clientRegistrationId`/`resellerId`): the legal representative (if
+ * `*_rep_authorization_ack`) plus every sócio marked `will_sign`, deduped
+ * by CPF. Mocks match `signature-envelope-flow.spec.ts` exactly (no real
+ * Chromium/MinIO/SMTP needed).
+ */
+const s3Store = new Map<string, Buffer>();
+
+vi.mock("@eden/integrations", async () => {
+  const actual = await vi.importActual<typeof import("@eden/integrations")>("@eden/integrations");
+  return {
+    ...actual,
+    renderPdf: vi.fn(async () => {
+      const doc = await PDFDocument.create();
+      doc.addPage();
+      return Buffer.from(await doc.save());
+    }),
+    uploadObject: vi.fn(async (key: string, buffer: Buffer) => {
+      s3Store.set(key, buffer);
+    }),
+    getObjectStream: vi.fn(async (key: string) => {
+      const buffer = s3Store.get(key);
+      if (!buffer) throw new Error(`qa mock S3: no object at ${key}`);
+      return Readable.from([buffer]);
+    }),
+    sendMail: vi.fn(async () => undefined),
+  };
+});
+
+describe("signature envelopes — automatic contratante signer derivation", () => {
+  let app: INestApplication;
+  let admin: AuthedTokens;
+  let companyId: string;
+  let generationId: string;
+
+  async function createGeneration(resellerId?: string, clientRegistrationId?: string) {
+    const template = await agent(app)
+      .post("/api/v1/documents/templates")
+      .set(...bearer(admin.accessToken))
+      .send({ key: `qa_test_autosig_${Date.now()}_${Math.random().toString(36).slice(2)}`, name: qaName("Template AutoSigners"), docGroup: "revenda" })
+      .expect(201);
+    const templateId = template.body.id;
+    await agent(app).post(`/api/v1/documents/templates/${templateId}/draft`).set(...bearer(admin.accessToken)).expect(201);
+    await agent(app)
+      .patch(`/api/v1/documents/templates/${templateId}/draft`)
+      .set(...bearer(admin.accessToken))
+      .send({ editorContent: { type: "doc", plainText: "Contrato de teste." } })
+      .expect(200);
+    await agent(app).post(`/api/v1/documents/templates/${templateId}/publish`).set(...bearer(admin.accessToken)).expect(201);
+    const generation = await agent(app)
+      .post("/api/v1/documents/generations")
+      .set(...bearer(admin.accessToken))
+      .send({ templateId, resellerId, clientRegistrationId, context: {} })
+      .expect(201);
+    return generation.body.id as string;
+  }
+
+  beforeAll(async () => {
+    app = await createTestApp();
+    admin = await loginSuperAdmin(app);
+
+    const company = await agent(app)
+      .post("/api/v1/companies")
+      .set(...bearer(admin.accessToken))
+      .send({
+        companyName: qaName("Empresa AutoSigners"),
+        legalRepName: "Representante Legal QA",
+        legalRepCpf: "11144477735",
+        legalRepEmail: qaEmail("autosig-contratada"),
+        legalRepRole: "Diretor",
+      })
+      .expect(201);
+    companyId = company.body.id;
+  });
+
+  afterAll(async () => {
+    await closeTestApp(app);
+  });
+
+  describe("origem client_registrations (PJ)", () => {
+    let registrationId: string;
+
+    beforeAll(async () => {
+      const reg = await agent(app)
+        .post("/api/v1/client-registrations/direct")
+        .set(...bearer(admin.accessToken))
+        .send({ personType: "pj", isPortability: false })
+        .expect(201);
+      registrationId = reg.body.id;
+      generationId = await createGeneration(undefined, registrationId);
+    });
+
+    it("400 — sem representante autorizado e sem sócio marcado pra assinar, nada pra derivar", async () => {
+      const res = await agent(app)
+        .post("/api/v1/signature-envelopes")
+        .set(...bearer(admin.accessToken))
+        .send({ companyId, clientRegistrationId: registrationId, documentGenerationIds: [generationId] })
+        .expect(400);
+      expect(res.body.message).toMatch(/Nenhum sócio marcado|representante legal autorizado/);
+    });
+
+    it("400 — sócio marcado pra assinar mas sem e-mail de assinatura", async () => {
+      await agent(app)
+        .post(`/api/v1/client-registrations/${registrationId}/partners`)
+        .set(...bearer(admin.accessToken))
+        .send({ name: qaName("Sócio Sem Email"), willSign: true })
+        .expect(201);
+
+      const res = await agent(app)
+        .post("/api/v1/signature-envelopes")
+        .set(...bearer(admin.accessToken))
+        .send({ companyId, clientRegistrationId: registrationId, documentGenerationIds: [generationId] })
+        .expect(400);
+      expect(res.body.message).toContain("e-mail de assinatura");
+    });
+
+    it("deriva o representante legal autorizado + o sócio marcado pra assinar, ignora o não-marcado", async () => {
+      // Cadastro próprio, dedicado — o `registrationId` do describe já
+      // acumulou, nos testes anteriores, um sócio `will_sign=true` sem
+      // e-mail (criado de propósito pra testar o 400) que continuaria
+      // presente e derrubaria este teste por um motivo alheio ao que ele
+      // verifica.
+      const reg = await agent(app)
+        .post("/api/v1/client-registrations/direct")
+        .set(...bearer(admin.accessToken))
+        .send({ personType: "pj", isPortability: false })
+        .expect(201);
+      const freshRegistrationId = reg.body.id;
+      const freshGenerationId = await createGeneration(undefined, freshRegistrationId);
+
+      const repEmail = qaEmail("autosig-pj-rep");
+      await agent(app)
+        .patch(`/api/v1/client-registrations/${freshRegistrationId}`)
+        .set(...bearer(admin.accessToken))
+        .send({
+          pjRepFullName: "Representante PJ QA",
+          pjRepRole: "Sócio-Administrador",
+          pjRepEmail: repEmail,
+          pjRepMobilePhone: "11988887777",
+          pjRepAuthorizationAck: true,
+        })
+        .expect(200);
+
+      const willSignEmail = qaEmail("autosig-pj-partner");
+      await agent(app)
+        .post(`/api/v1/client-registrations/${freshRegistrationId}/partners`)
+        .set(...bearer(admin.accessToken))
+        .send({ name: qaName("Sócio Assina"), willSign: true, signatureEmail: willSignEmail, qualification: "Sócio" })
+        .expect(201);
+      await agent(app)
+        .post(`/api/v1/client-registrations/${freshRegistrationId}/partners`)
+        .set(...bearer(admin.accessToken))
+        .send({ name: qaName("Sócio Não Assina") })
+        .expect(201);
+
+      const res = await agent(app)
+        .post("/api/v1/signature-envelopes")
+        .set(...bearer(admin.accessToken))
+        .send({ companyId, clientRegistrationId: freshRegistrationId, documentGenerationIds: [freshGenerationId] })
+        .expect(201);
+
+      const contratantes = res.body.signers.filter((s: { party: string }) => s.party === "contratante");
+      expect(contratantes).toHaveLength(2); // rep + o único sócio marcado pra assinar
+      const emails = contratantes.map((s: { email: string }) => s.email);
+      expect(emails).toContain(repEmail);
+      expect(emails).toContain(willSignEmail);
+    });
+  });
+
+  describe("origem client_registrations (PF)", () => {
+    it("usa o próprio titular como signatário único", async () => {
+      const reg = await agent(app)
+        .post("/api/v1/client-registrations/direct")
+        .set(...bearer(admin.accessToken))
+        .send({ personType: "pf", isPortability: false })
+        .expect(201);
+      const registrationId = reg.body.id;
+      const titularEmail = qaEmail("autosig-pf-titular");
+      await agent(app)
+        .patch(`/api/v1/client-registrations/${registrationId}`)
+        .set(...bearer(admin.accessToken))
+        .send({ pfFullName: "Titular PF QA", pfEmail: titularEmail, pfMobilePhone: "11977776666" })
+        .expect(200);
+
+      const genId = await createGeneration(undefined, registrationId);
+      const res = await agent(app)
+        .post("/api/v1/signature-envelopes")
+        .set(...bearer(admin.accessToken))
+        .send({ companyId, clientRegistrationId: registrationId, documentGenerationIds: [genId] })
+        .expect(201);
+
+      const contratantes = res.body.signers.filter((s: { party: string }) => s.party === "contratante");
+      expect(contratantes).toHaveLength(1);
+      expect(contratantes[0].email).toBe(titularEmail);
+      expect(contratantes[0].full_name).toBe("Titular PF QA");
+    });
+  });
+
+  describe("origem resellers", () => {
+    let resellerId: string;
+
+    beforeAll(async () => {
+      const reseller = await agent(app)
+        .post("/api/v1/resellers")
+        .set(...bearer(admin.accessToken))
+        .send({ name: qaName("Revenda AutoSigners") })
+        .expect(201);
+      resellerId = reseller.body.id;
+      generationId = await createGeneration(resellerId);
+    });
+
+    afterAll(async () => {
+      await agent(app)
+        .patch(`/api/v1/resellers/${resellerId}`)
+        .set(...bearer(admin.accessToken))
+        .send({ active: false })
+        .expect(200);
+    });
+
+    it("dedup por CPF — sócio com o mesmo CPF do representante aparece só uma vez", async () => {
+      const repCpf = "52998224725";
+      const repEmail = qaEmail("autosig-reseller-rep");
+      await agent(app)
+        .patch(`/api/v1/resellers/${resellerId}`)
+        .set(...bearer(admin.accessToken))
+        .send({
+          repFullName: "Representante Revenda QA",
+          repCpf,
+          repRole: "Diretor",
+          repEmail,
+          repAuthorizationAck: true,
+        })
+        .expect(200);
+
+      // Mesmo CPF do representante — não deve virar um segundo signatário.
+      await agent(app)
+        .post(`/api/v1/resellers/${resellerId}/partners`)
+        .set(...bearer(admin.accessToken))
+        .send({ name: "Representante Revenda QA", cpf: repCpf, willSign: true, signatureEmail: qaEmail("autosig-reseller-dup") })
+        .expect(201);
+      // (`toHaveLength(2)` abaixo já prova o dedup — rep + o único outro sócio, nunca 3.)
+
+      const otherEmail = qaEmail("autosig-reseller-other");
+      await agent(app)
+        .post(`/api/v1/resellers/${resellerId}/partners`)
+        .set(...bearer(admin.accessToken))
+        .send({ name: qaName("Outro Sócio Revenda"), cpf: "12345678909", willSign: true, signatureEmail: otherEmail })
+        .expect(201);
+
+      const res = await agent(app)
+        .post("/api/v1/signature-envelopes")
+        .set(...bearer(admin.accessToken))
+        .send({ companyId, resellerId, documentGenerationIds: [generationId] })
+        .expect(201);
+
+      const contratantes = res.body.signers.filter((s: { party: string }) => s.party === "contratante");
+      expect(contratantes).toHaveLength(2); // rep (dedup'd against the same-CPF partner) + the other partner
+      const emails = contratantes.map((s: { email: string }) => s.email);
+      expect(emails).toContain(repEmail);
+      expect(emails).toContain(otherEmail);
+    });
+  });
+
+  it("signers explícito continua funcionando como override manual completo, mesmo com rep/sócios disponíveis", async () => {
+    const reg = await agent(app)
+      .post("/api/v1/client-registrations/direct")
+      .set(...bearer(admin.accessToken))
+      .send({ personType: "pj", isPortability: false })
+      .expect(201);
+    const registrationId = reg.body.id;
+    await agent(app)
+      .patch(`/api/v1/client-registrations/${registrationId}`)
+      .set(...bearer(admin.accessToken))
+      .send({ pjRepFullName: "Rep Ignorado", pjRepEmail: qaEmail("autosig-override-ignored"), pjRepAuthorizationAck: true })
+      .expect(200);
+
+    const genId = await createGeneration(undefined, registrationId);
+    const manualEmail = qaEmail("autosig-override-manual");
+    const res = await agent(app)
+      .post("/api/v1/signature-envelopes")
+      .set(...bearer(admin.accessToken))
+      .send({
+        companyId,
+        clientRegistrationId: registrationId,
+        documentGenerationIds: [genId],
+        signers: [{ fullName: "Signatário Manual", email: manualEmail }],
+      })
+      .expect(201);
+
+    const contratantes = res.body.signers.filter((s: { party: string }) => s.party === "contratante");
+    expect(contratantes).toHaveLength(1);
+    expect(contratantes[0].email).toBe(manualEmail);
+  });
+});