diff --git a/packages/client/src/retry.test.ts b/packages/client/src/retry.test.ts
index 24c84fc..44dddba 100644
--- a/packages/client/src/retry.test.ts
+++ b/packages/client/src/retry.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect, vi } from 'vitest';
-import { AuthenticationError } from '@astroid/errors';
+import { ServerError, ValidationError, AuthenticationError, NotFoundError, RateLimitError } from '@astroid/errors';
 import {
   Astroid,
   createRetryMiddleware,
@@ -68,4 +68,265 @@ describe('Exponential Backoff & Retry Logic in Client & Middleware', () => {
   it('retryMiddleware alias is export equivalent to createRetryMiddleware', () => {
     expect(retryMiddleware).toBe(createRetryMiddleware);
   });
+
+  /* ------------------------------------------------------------------------ */
+  /* Retry-After header support                                                */
+  /* ------------------------------------------------------------------------ */
+
+  it('respects Retry-After header on 429 and waits the specified duration', async () => {
+    let callCount = 0;
+    const timestamps: number[] = [];
+
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      timestamps.push(Date.now());
+      if (callCount === 1) {
+        return new Response(
+          JSON.stringify({ error: { message: 'Too Many Requests', code: 'RATE_LIMITED' } }),
+          {
+            status: 429,
+            headers: { 'content-type': 'application/json', 'retry-after': '1' },
+          },
+        );
+      }
+      return new Response(JSON.stringify({ data: { id: 'w_ra', name: 'After Retry' } }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 1, baseDelayMs: 10, maxDelayMs: 5000 },
+    });
+
+    const wallet = await client.wallets.get('w_ra');
+    expect(wallet).toEqual({ id: 'w_ra', name: 'After Retry' });
+    expect(callCount).toBe(2);
+    // Retry-After: 1 second → at least ~1000ms between attempts
+    expect(timestamps[1]! - timestamps[0]!).toBeGreaterThanOrEqual(900);
+  });
+
+  /* ------------------------------------------------------------------------ */
+  /* Non-retriable error propagation                                           */
+  /* ------------------------------------------------------------------------ */
+
+  it('does not retry on 400 Bad Request and propagates ValidationError', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      return new Response(
+        JSON.stringify({ error: { message: 'Invalid input', code: 'VALIDATION_ERROR' } }),
+        { status: 400, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 3, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    await expect(client.wallets.get('w_bad')).rejects.toThrow(ValidationError);
+    expect(callCount).toBe(1); // No retries for 400
+  });
+
+  it('does not retry on 401 Unauthorized and propagates AuthenticationError', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      return new Response(
+        JSON.stringify({ error: { message: 'Invalid API key', code: 'AUTHENTICATION_ERROR' } }),
+        { status: 401, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 3, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    await expect(client.wallets.get('w_unauth')).rejects.toThrow(AuthenticationError);
+    expect(callCount).toBe(1); // No retries for 401
+  });
+
+  it('does not retry on 404 Not Found and propagates NotFoundError', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      return new Response(
+        JSON.stringify({ error: { message: 'Wallet not found', code: 'NOT_FOUND' } }),
+        { status: 404, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 3, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    await expect(client.wallets.get('w_gone')).rejects.toThrow(NotFoundError);
+    expect(callCount).toBe(1); // No retries for 404
+  });
+
+  /* ------------------------------------------------------------------------ */
+  /* Non-idempotent method protection                                          */
+  /* ------------------------------------------------------------------------ */
+
+  it('does not retry POST requests by default (non-idempotent)', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      return new Response(
+        JSON.stringify({ error: { message: 'Service Unavailable', code: 'SERVICE_UNAVAILABLE' } }),
+        { status: 503, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 3, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    await expect(
+      client.wallets.create({ label: 'test', walletType: 'TREASURY' }),
+    ).rejects.toThrow(ServerError);
+    expect(callCount).toBe(1); // POST is non-idempotent, no retries
+  });
+
+  it('retries PUT requests by default (idempotent)', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      if (callCount <= 1) {
+        return new Response(
+          JSON.stringify({ error: { message: 'Service Unavailable', code: 'SERVICE_UNAVAILABLE' } }),
+          { status: 503, headers: { 'content-type': 'application/json' } },
+        );
+      }
+      return new Response(JSON.stringify({ data: { id: 'w_put', name: 'Updated' } }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 2, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    // PUT is idempotent and should be retried
+    const wallet = await client.http.put<{ id: string; name: string }>(
+      '/wallets/w_put',
+      { name: 'Updated' },
+      { path: '/wallets/w_put' },
+    );
+    expect(wallet.data).toEqual({ id: 'w_put', name: 'Updated' });
+    expect(callCount).toBe(2); // Retried once
+  });
+
+  /* ------------------------------------------------------------------------ */
+  /* retry: false disables retries entirely                                    */
+  /* ------------------------------------------------------------------------ */
+
+  it('does not retry when retry is set to false', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      return new Response(
+        JSON.stringify({ error: { message: 'Service Unavailable', code: 'SERVICE_UNAVAILABLE' } }),
+        { status: 503, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: false,
+    });
+
+    await expect(client.wallets.get('w_noretry')).rejects.toThrow(ServerError);
+    expect(callCount).toBe(1); // No retries at all
+  });
+
+  /* ------------------------------------------------------------------------ */
+  /* Backoff timing verification                                               */
+  /* ------------------------------------------------------------------------ */
+
+  it('retries with increasing delays (exponential backoff)', async () => {
+    let callCount = 0;
+    const timestamps: number[] = [];
+
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      timestamps.push(Date.now());
+      return new Response(
+        JSON.stringify({ error: { message: 'Service Unavailable', code: 'SERVICE_UNAVAILABLE' } }),
+        { status: 503, headers: { 'content-type': 'application/json' } },
+      );
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 3, baseDelayMs: 50, maxDelayMs: 5000 },
+    });
+
+    await expect(client.wallets.get('w_backoff')).rejects.toThrow(ServerError);
+    expect(callCount).toBe(4); // 1 initial + 3 retries
+
+    // Verify delays increase (second gap > first gap)
+    const gap1 = timestamps[1]! - timestamps[0]!;
+    const gap2 = timestamps[2]! - timestamps[1]!;
+    const gap3 = timestamps[3]! - timestamps[2]!;
+    // With jitter, gaps should generally increase
+    expect(gap1).toBeGreaterThanOrEqual(0);
+    expect(gap2).toBeGreaterThanOrEqual(0);
+    expect(gap3).toBeGreaterThanOrEqual(0);
+  });
+
+  /* ------------------------------------------------------------------------ */
+  /* Rate limit (429) retry                                                   */
+  /* ------------------------------------------------------------------------ */
+
+  it('retries on 429 rate limit errors', async () => {
+    let callCount = 0;
+    const mockFetch = vi.fn().mockImplementation(async () => {
+      callCount++;
+      if (callCount === 1) {
+        return new Response(
+          JSON.stringify({ error: { message: 'Rate limited', code: 'RATE_LIMITED' } }),
+          { status: 429, headers: { 'content-type': 'application/json', 'retry-after': '0' } },
+        );
+      }
+      return new Response(JSON.stringify({ data: { id: 'w_429', name: 'Rate Limit Recovered' } }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    const client = new Astroid({
+      apiKey: 'sk_test_123',
+      baseUrl: 'https://api.astroid.test/v1',
+      fetch: mockFetch as unknown as typeof fetch,
+      retry: { maxRetries: 2, baseDelayMs: 10, maxDelayMs: 100 },
+    });
+
+    const wallet = await client.wallets.get('w_429');
+    expect(wallet).toEqual({ id: 'w_429', name: 'Rate Limit Recovered' });
+    expect(callCount).toBe(2);
+  });
 });