diff --git a/src/errors/index.ts b/src/errors/index.ts
index 45e89b7d..8d76762e 100644
--- a/src/errors/index.ts
+++ b/src/errors/index.ts
@@ -37,6 +37,7 @@ export {
   ProxyError,
   MigrationError,
   UserAbortError,
+  ValidationError,
   RetryableError,
   isCCSError,
   isRecoverableError,
diff --git a/src/utils/__tests__/retry-strategy.test.ts b/src/utils/__tests__/retry-strategy.test.ts
index 372babf5..58dd58e0 100644
--- a/src/utils/__tests__/retry-strategy.test.ts
+++ b/src/utils/__tests__/retry-strategy.test.ts
@@ -184,4 +184,31 @@ describe('withRetry', () => {
     expect(delay).toBeGreaterThanOrEqual(500);
     sleepSpy.mockRestore();
   });
+
+  it('swallows onRetry callback errors and continues retrying', async () => {
+    let attempt = 0;
+    const fn = mock(() => {
+      attempt++;
+      if (attempt < 3) {
+        return Promise.reject(new RetryableError('fail'));
+      }
+      return Promise.resolve('ok');
+    });
+    const onRetry = mock(() => {
+      throw new Error('callback blew up');
+    });
+
+    const result = await withRetry(fn, { maxRetries: 5, baseDelayMs: 1, onRetry });
+    expect(result).toBe('ok');
+    expect(fn).toHaveBeenCalledTimes(3);
+    expect(onRetry).toHaveBeenCalledTimes(2);
+  });
+
+  it('throws on negative baseDelayMs', async () => {
+    const fn = mock(() => Promise.resolve('ok'));
+    await expect(withRetry(fn, { maxRetries: 3, baseDelayMs: -1 })).rejects.toThrow(
+      'baseDelayMs must be >= 0'
+    );
+    expect(fn).not.toHaveBeenCalled();
+  });
 });
diff --git a/src/utils/retry-strategy.ts b/src/utils/retry-strategy.ts
index 6e778862..50489b6b 100644
--- a/src/utils/retry-strategy.ts
+++ b/src/utils/retry-strategy.ts
@@ -16,13 +16,13 @@ export interface RetryOptions {
   maxRetries: number;
   /** Base delay in ms for the first retry (default: 1000) */
   baseDelayMs: number;
-  /** Upper bound for the computed delay (default: 30000) */
+  /** Upper bound for the computed backoff delay. Note: server-provided `retryAfter` (from RetryableError) takes precedence and may exceed this cap. */
   maxDelayMs?: number;
-  /** Multiplier applied per attempt (default: 2) */
+  /** Multiplier applied per attempt (default: 2). Values <1 produce degrowth. */
   backoffMultiplier?: number;
   /** Override the default retryability check */
   retryableCheck?: (error: unknown) => boolean;
-  /** Callback fired before each retry (not fired on initial call) */
+  /** Callback fired before each retry. Errors thrown by this callback are swallowed to prevent aborting the retry loop. */
   onRetry?: (error: Error, attempt: number) => void;
 }
 
@@ -95,6 +95,9 @@ export async function withRetry<T>(fn: () => Promise<T>, options: RetryOptions):
   if (maxRetries < 0) {
     throw new Error('withRetry: maxRetries must be >= 0');
   }
+  if (baseDelayMs < 0) {
+    throw new Error('withRetry: baseDelayMs must be >= 0');
+  }
 
   const isRetryable = retryableCheck ?? defaultRetryableCheck;
   let lastError: unknown;
@@ -116,7 +119,11 @@ export async function withRetry<T>(fn: () => Promise<T>, options: RetryOptions):
       }
 
       const err = error instanceof Error ? error : new Error(String(error));
-      onRetry?.(err, attempt + 1);
+      try {
+        onRetry?.(err, attempt + 1);
+      } catch {
+        // Swallow callback errors — retry decision is already made
+      }
 
       const retryAfter = error instanceof RetryableError ? error.retryAfter : undefined;
       const delay = computeDelay(attempt, baseDelayMs, maxDelayMs, backoffMultiplier, retryAfter);