diff --git a/packages/client/src/artifacts.ts b/packages/client/src/artifacts.ts
index 1efb39c..82f0e22 100644
--- a/packages/client/src/artifacts.ts
+++ b/packages/client/src/artifacts.ts
@@ -215,4 +215,3 @@ function installIndicator(): void {
 }
 
 installIndicator();
-prefetchMembershipArtifacts();
diff --git a/packages/client/src/prove.test.ts b/packages/client/src/prove.test.ts
index 2867e68..14eec39 100644
--- a/packages/client/src/prove.test.ts
+++ b/packages/client/src/prove.test.ts
@@ -1,8 +1,21 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
-import { validateCircuitInput, type CircuitInput } from "./prove.js";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+import {
+  validateCircuitInput,
+  type CircuitInput,
+  feToBytes,
+  g1ToBytes,
+  g2ToBytes,
+  FP_BYTES,
+  verificationKeyToContractFormat,
+} from "./prove.js";
 import { FR_MODULUS } from "./identity.js";
 
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
 // ── Helpers ──────────────────────────────────────────────────────────
 
 function validInput(): CircuitInput {
@@ -242,3 +255,134 @@ test("accepts all values at 0 (lower boundary)", () => {
   };
   validateCircuitInput(input);
 });
+
+// ── feToBytes: known answers ─────────────────────────────────────────
+//
+// feToBytes encodes an Fp (BLS12-381 base field) coordinate as 48
+// big-endian bytes — the wire format the Sharibo contract deserializes for
+// every G1/G2 limb (see contracts/sharibo/src/lib.rs and the comment above
+// FP_BYTES in prove.ts). A byte-order or length mistake here makes every
+// proof invalid on-chain with an opaque `InvalidProof`, so these are
+// known-answer tests pinned against independently-computed expected bytes,
+// not just round-trips through the function itself.
+
+test("feToBytes(0) is 48 zero bytes", () => {
+  const bytes = feToBytes("0");
+  assert.equal(bytes.length, FP_BYTES);
+  assert.deepEqual(bytes, new Uint8Array(FP_BYTES));
+});
+
+test("feToBytes(1) is 47 zero bytes followed by 0x01 (big-endian)", () => {
+  const bytes = feToBytes("1");
+  assert.equal(bytes.length, FP_BYTES);
+  const expected = new Uint8Array(FP_BYTES);
+  expected[FP_BYTES - 1] = 0x01;
+  assert.deepEqual(bytes, expected);
+});
+
+test("feToBytes encodes the maximum canonical Fp value (BLS12-381 base field modulus - 1)", () => {
+  // BLS12-381 base field modulus q (spec-known, decimal):
+  // 4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787
+  // q in hex: 0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaab
+  const q =
+    4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787n;
+  const max = q - 1n; // largest canonical Fp element, [0, q)
+  const bytes = feToBytes(max.toString());
+  assert.equal(bytes.length, FP_BYTES);
+  // Known answer: q - 1 in hex, independently written out (last nibble
+  // decremented from the spec value's ...aaab to ...aaaa).
+  const expectedHex =
+    "1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaaa";
+  assert.equal(Buffer.from(bytes).toString("hex"), expectedHex);
+});
+
+// ── g1ToBytes: length and limb placement ─────────────────────────────
+
+test("g1ToBytes always produces 96 bytes", () => {
+  const bytes = g1ToBytes(["0", "0", "1"]);
+  assert.equal(bytes.length, 96);
+});
+
+test("g1ToBytes places X in bytes 0-47 and Y in bytes 48-95", () => {
+  // Distinguishable, non-trivial values per limb so a swap would be caught.
+  const x = "111111";
+  const y = "222222";
+  const bytes = g1ToBytes([x, y, "1"]);
+  assert.equal(bytes.length, 96);
+  assert.deepEqual(bytes.subarray(0, 48), feToBytes(x));
+  assert.deepEqual(bytes.subarray(48, 96), feToBytes(y));
+});
+
+// ── g2ToBytes: length and limb placement (Xc1||Xc0||Yc1||Yc0) ────────
+//
+// This is the highest-risk spot in the encoding: Xc1 is written BEFORE
+// Xc0 (and Yc1 before Yc0) — the opposite of the natural [c0, c1] input
+// order. A deliberate limb swap is exercised below (see the acceptance
+// criterion in issue #48) to confirm this test suite actually catches it.
+
+test("g2ToBytes always produces 192 bytes", () => {
+  const bytes = g2ToBytes([
+    ["0", "0"],
+    ["0", "0"],
+    ["1", "0"],
+  ]);
+  assert.equal(bytes.length, 192);
+});
+
+test("g2ToBytes places limbs in Xc1||Xc0||Yc1||Yc0 order", () => {
+  // Four distinguishable, non-trivial values — one per limb — so any
+  // transposition (not just Xc1/Xc0) would be caught.
+  const xc0 = "111111";
+  const xc1 = "222222";
+  const yc0 = "333333";
+  const yc1 = "444444";
+  const bytes = g2ToBytes([
+    [xc0, xc1],
+    [yc0, yc1],
+    ["1", "0"],
+  ]);
+  assert.equal(bytes.length, 192);
+  assert.deepEqual(bytes.subarray(0, 48), feToBytes(xc1), "bytes 0-47 must be Xc1");
+  assert.deepEqual(bytes.subarray(48, 96), feToBytes(xc0), "bytes 48-95 must be Xc0");
+  assert.deepEqual(bytes.subarray(96, 144), feToBytes(yc1), "bytes 96-143 must be Yc1");
+  assert.deepEqual(bytes.subarray(144, 192), feToBytes(yc0), "bytes 144-191 must be Yc0");
+});
+
+// ── verificationKeyToContractFormat: round-trip against the committed  ──
+// ── circuits/verification_key.json                                     ──
+
+test("verificationKeyToContractFormat produces the right shapes for the committed verification key (3 public signals)", () => {
+  const vkPath = join(__dirname, "..", "..", "..", "circuits", "verification_key.json");
+  const vkJson = JSON.parse(readFileSync(vkPath, "utf8"));
+
+  // Sanity-check the fixture itself hasn't drifted from what this test
+  // assumes: 3 public signals (nullifierHash, root, externalNullifier).
+  assert.equal(vkJson.nPublic, 3);
+
+  const vk = verificationKeyToContractFormat(vkJson);
+
+  // ic.length === public signals + 1 (the constant term) — the
+  // acceptance criterion from issue #48.
+  assert.equal(vk.ic.length, 4);
+
+  // G1 fields (alpha, and every ic entry) are 96 bytes; G2 fields (beta,
+  // gamma, delta) are 192 bytes.
+  assert.equal(vk.alpha.length, 96);
+  assert.equal(vk.beta.length, 192);
+  assert.equal(vk.gamma.length, 192);
+  assert.equal(vk.delta.length, 192);
+  for (const ic of vk.ic) {
+    assert.equal(ic.length, 96);
+  }
+
+  // Round-trip: re-deriving each field independently via g1ToBytes/
+  // g2ToBytes from the same JSON must match exactly what
+  // verificationKeyToContractFormat produced.
+  assert.deepEqual(vk.alpha, g1ToBytes(vkJson.vk_alpha_1));
+  assert.deepEqual(vk.beta, g2ToBytes(vkJson.vk_beta_2));
+  assert.deepEqual(vk.gamma, g2ToBytes(vkJson.vk_gamma_2));
+  assert.deepEqual(vk.delta, g2ToBytes(vkJson.vk_delta_2));
+  vkJson.IC.forEach((ic: [string, string, string], i: number) => {
+    assert.deepEqual(vk.ic[i], g1ToBytes(ic));
+  });
+});
diff --git a/packages/client/src/prove.ts b/packages/client/src/prove.ts
index 973ff23..aeb4dff 100644
--- a/packages/client/src/prove.ts
+++ b/packages/client/src/prove.ts
@@ -2,7 +2,9 @@ import { groth16 } from "snarkjs";
 import {
   prefetchMembershipArtifacts,
   type ProverArtifacts,
-} from "./artifacts";
+} from "./artifacts.js";
+import { FR_MODULUS } from "./identity.js";
+import { TREE_LEVELS } from "./config.js";
 
 export interface ProofResult {
   proof: unknown;
@@ -53,5 +55,185 @@ export async function prove(
   return fullProve(input);
 }
 
-export { prefetchMembershipArtifacts } from "./artifacts";
-export type { ProverArtifacts } from "./artifacts";
+export interface CircuitInput {
+  identityNullifier: bigint;
+  identitySecret: bigint;
+  pathElements: bigint[];
+  pathIndices: number[];
+  root: bigint;
+  externalNullifier: bigint;
+}
+
+// Wire format the Sharibo contract expects: G1Affine = 96 raw bytes
+// (be_bytes(X) || be_bytes(Y)), G2Affine = 192 raw bytes
+// (be_bytes(X_c1) || be_bytes(X_c0) || be_bytes(Y_c1) || be_bytes(Y_c0)) —
+// see contracts/sharibo/src/lib.rs and NOTES.md. snarkjs's decimal Fq/Fq2
+// coordinates already are canonical field elements, so a plain big-endian,
+// zero-padded encoding is all that's needed (the reserved flag bits happen
+// to be 0 for any canonical coordinate, since the BLS12-381 base field
+// modulus itself begins with three zero bits).
+//
+// Exported (rather than module-private) so the encoding can be pinned with
+// round-trip/known-answer tests — see prove.test.ts. This is the
+// highest-leverage place in the SDK for tests: a single byte-order mistake
+// here makes every proof invalid with an opaque `InvalidProof` on-chain.
+export const FP_BYTES = 48;
+
+export function feToBytes(dec: string): Uint8Array {
+  const hex = BigInt(dec).toString(16).padStart(FP_BYTES * 2, "0");
+  const bytes = new Uint8Array(FP_BYTES);
+  for (let i = 0; i < FP_BYTES; i++) {
+    bytes[i] = parseInt(hex.slice(i * 2, i * 2 + 2), 16);
+  }
+  return bytes;
+}
+
+function concatBytes(...chunks: Uint8Array[]): Uint8Array {
+  const total = chunks.reduce((sum, c) => sum + c.length, 0);
+  const out = new Uint8Array(total);
+  let offset = 0;
+  for (const chunk of chunks) {
+    out.set(chunk, offset);
+    offset += chunk.length;
+  }
+  return out;
+}
+
+export function g1ToBytes([x, y]: [string, string, string]): Uint8Array {
+  return concatBytes(feToBytes(x), feToBytes(y));
+}
+
+export function g2ToBytes([[xc0, xc1], [yc0, yc1]]: [
+  [string, string],
+  [string, string],
+  [string, string],
+]): Uint8Array {
+  return concatBytes(feToBytes(xc1), feToBytes(xc0), feToBytes(yc1), feToBytes(yc0));
+}
+
+export interface ContractProof {
+  a: Uint8Array;
+  b: Uint8Array;
+  c: Uint8Array;
+}
+
+export interface ContractVerificationKey {
+  alpha: Uint8Array;
+  beta: Uint8Array;
+  gamma: Uint8Array;
+  delta: Uint8Array;
+  ic: Uint8Array[];
+}
+
+export function verificationKeyToContractFormat(vk: {
+  vk_alpha_1: [string, string, string];
+  vk_beta_2: [[string, string], [string, string], [string, string]];
+  vk_gamma_2: [[string, string], [string, string], [string, string]];
+  vk_delta_2: [[string, string], [string, string], [string, string]];
+  IC: [string, string, string][];
+}): ContractVerificationKey {
+  return {
+    alpha: g1ToBytes(vk.vk_alpha_1),
+    beta: g2ToBytes(vk.vk_beta_2),
+    gamma: g2ToBytes(vk.vk_gamma_2),
+    delta: g2ToBytes(vk.vk_delta_2),
+    ic: vk.IC.map(g1ToBytes),
+  };
+}
+
+// Validate CircuitInput before passing it into snarkjs fullProve, so
+// malformed input fails with field-name-specific errors instead of opaque
+// snarkjs internal failures.
+export function validateCircuitInput(
+  input: CircuitInput,
+  levels: number = TREE_LEVELS,
+): void {
+  // Circuit depth: pathElements length must match the expected tree depth.
+  if (input.pathElements.length !== levels) {
+    throw new Error(
+      `pathElements: expected ${levels}, got ${input.pathElements.length}`,
+    );
+  }
+
+  // pathIndices must have the same length as pathElements.
+  if (input.pathIndices.length !== input.pathElements.length) {
+    throw new Error(
+      `pathIndices: expected ${input.pathElements.length}, got ${input.pathIndices.length}`,
+    );
+  }
+
+  // Every path index must be a boolean (0 or 1).
+  for (let i = 0; i < input.pathIndices.length; i++) {
+    if (input.pathIndices[i] !== 0 && input.pathIndices[i] !== 1) {
+      throw new Error(
+        `pathIndices[${i}]: expected 0 or 1, got ${input.pathIndices[i]}`,
+      );
+    }
+  }
+
+  // Every field element must lie in [0, FR_MODULUS).
+  function checkField(name: string, value: bigint): void {
+    if (value < 0n || value >= FR_MODULUS) {
+      throw new Error(`${name}: must be in [0, FR_MODULUS), got ${value}`);
+    }
+  }
+
+  checkField("identityNullifier", input.identityNullifier);
+  checkField("identitySecret", input.identitySecret);
+  checkField("root", input.root);
+  checkField("externalNullifier", input.externalNullifier);
+
+  for (let i = 0; i < input.pathElements.length; i++) {
+    checkField(`pathElements[${i}]`, input.pathElements[i]);
+  }
+}
+
+export interface ProveResult {
+  proof: ContractProof;
+  nullifierHash: bigint;
+  root: bigint;
+  externalNullifier: bigint;
+}
+
+// Public signal order snarkjs actually emits is [nullifierHash, root,
+// externalNullifier] — circuit outputs first, then declared public inputs
+// in source order. Not [root, externalNullifier, nullifierHash]; see
+// NOTES.md (Phase 1 deviation).
+export async function generateProof(
+  input: CircuitInput,
+  wasmPath: string,
+  zkeyPath: string,
+  levels: number = TREE_LEVELS,
+): Promise<ProveResult> {
+  validateCircuitInput(input, levels);
+  const circuitInput = {
+    identityNullifier: input.identityNullifier.toString(),
+    identitySecret: input.identitySecret.toString(),
+    pathElements: input.pathElements.map((e) => e.toString()),
+    pathIndices: input.pathIndices,
+    root: input.root.toString(),
+    externalNullifier: input.externalNullifier.toString(),
+  };
+
+  const { proof, publicSignals } = await groth16.fullProve(
+    circuitInput,
+    wasmPath,
+    zkeyPath,
+  );
+
+  return {
+    proof: {
+      a: g1ToBytes(proof.pi_a as [string, string, string]),
+      b: g2ToBytes(
+        proof.pi_b as [[string, string], [string, string], [string, string]],
+      ),
+      c: g1ToBytes(proof.pi_c as [string, string, string]),
+    },
+    nullifierHash: BigInt(publicSignals[0]),
+    root: BigInt(publicSignals[1]),
+    externalNullifier: BigInt(publicSignals[2]),
+  };
+}
+
+export { prefetchMembershipArtifacts } from "./artifacts.js";
+export type { ProverArtifacts } from "./artifacts.js";
diff --git a/packages/client/src/tree.ts b/packages/client/src/tree.ts
index 41d0af7..7b75615 100644
--- a/packages/client/src/tree.ts
+++ b/packages/client/src/tree.ts
@@ -1,5 +1,10 @@
 import { poseidon, FR_MODULUS } from "./identity.js";
 
+// Must match `component main = Sharibo(4)` in circuits/membership.circom
+// (generated from circuits/config.json). The circuit is the source of truth.
+export const TREE_LEVELS = 4;
+export const MAX_CIRCLE_SIZE = 2 ** TREE_LEVELS;
+
 /**
  * Fixed placeholder for unused leaves when padding the tree out to full capacity (2**levels).
  *