diff --git a/backend/src/controllers/stream.controller.ts b/backend/src/controllers/stream.controller.ts
index b7670e8..6d61a5f 100644
--- a/backend/src/controllers/stream.controller.ts
+++ b/backend/src/controllers/stream.controller.ts
@@ -61,6 +61,35 @@ function sumStringI128(values: string[]): string {
   return total.toString();
 }
 
+/**
+ * Thrown when a request body field fails presence/format validation. Kept
+ * distinct from generic errors so createStream can reliably map it to a 400
+ * response instead of falling through to the catch-all 500.
+ */
+class StreamValidationError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'StreamValidationError';
+  }
+}
+
+/**
+ * Validate presence and integer format of a required i128-style field, then
+ * coerce it to a BigInt. Any missing value or conversion failure (SyntaxError
+ * from a non-numeric string, TypeError from undefined/null/objects, etc.) is
+ * normalized into a StreamValidationError so the caller can map it to 400.
+ */
+function parseRequiredBigIntField(fieldName: string, value: unknown): bigint {
+  if (value === undefined || value === null || value === '') {
+    throw new StreamValidationError(`Missing required field: ${fieldName}`);
+  }
+  try {
+    return BigInt(value as bigint | number | string | boolean);
+  } catch {
+    throw new StreamValidationError(`Invalid ${fieldName}: must be a valid integer`);
+  }
+}
+
 /**
  * Create a new stream (stub for on-chain indexing)
  */
@@ -70,8 +99,6 @@ export const createStream = async (req: Request, res: Response) => {
 
     const parsedStreamId = Number.parseInt(streamId, 10);
     const parsedStartTime = Number.parseInt(startTime, 10);
-    const parsedRatePerSecond = BigInt(ratePerSecond);
-    const parsedDepositedAmount = BigInt(depositedAmount);
 
     if (!Number.isFinite(parsedStreamId)) {
       return res.status(400).json({ error: 'Invalid streamId: must be a valid integer' });
@@ -81,6 +108,21 @@ export const createStream = async (req: Request, res: Response) => {
       return res.status(400).json({ error: 'Invalid startTime: must be a non-negative integer' });
     }
 
+    // Presence/format validation happens here, before any BigInt coercion,
+    // so a malformed or missing numeric field always yields 400 rather than
+    // an uncaught SyntaxError/TypeError falling through to 500.
+    let parsedRatePerSecond: bigint;
+    let parsedDepositedAmount: bigint;
+    try {
+      parsedRatePerSecond = parseRequiredBigIntField('ratePerSecond', ratePerSecond);
+      parsedDepositedAmount = parseRequiredBigIntField('depositedAmount', depositedAmount);
+    } catch (validationError) {
+      if (validationError instanceof StreamValidationError) {
+        return res.status(400).json({ error: validationError.message });
+      }
+      throw validationError;
+    }
+
     if (parsedRatePerSecond <= 0n) {
       return res.status(400).json({ error: 'Invalid ratePerSecond: must be greater than zero' });
     }
@@ -774,4 +816,4 @@ export const resumeStream = async (req: Request, res: Response) => {
     logger.error('Error resuming stream:', error);
     return res.status(500).json({ error: 'Internal server error' });
   }
-};
\ No newline at end of file
+};
diff --git a/backend/tests/stream.controller.test.ts b/backend/tests/stream.controller.test.ts
index ef37fe1..f449536 100644
--- a/backend/tests/stream.controller.test.ts
+++ b/backend/tests/stream.controller.test.ts
@@ -89,6 +89,46 @@ describe('Stream Controller', () => {
       await createStream(req as Request, res as Response);
       expect(res.status).toHaveBeenCalledWith(400);
     });
+
+    it('should return 400 with a validation error for non-numeric ratePerSecond', async () => {
+      req.body.ratePerSecond = 'abc';
+      await createStream(req as Request, res as Response);
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.status).not.toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith(
+        expect.objectContaining({ error: expect.stringContaining('ratePerSecond') })
+      );
+    });
+
+    it('should return 400 with a validation error for non-numeric depositedAmount', async () => {
+      req.body.depositedAmount = 'xyz';
+      await createStream(req as Request, res as Response);
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.status).not.toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith(
+        expect.objectContaining({ error: expect.stringContaining('depositedAmount') })
+      );
+    });
+
+    it('should return 400, not 500, when ratePerSecond is missing', async () => {
+      delete req.body.ratePerSecond;
+      await createStream(req as Request, res as Response);
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.status).not.toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith(
+        expect.objectContaining({ error: expect.stringContaining('ratePerSecond') })
+      );
+    });
+
+    it('should return 400, not 500, when depositedAmount is missing', async () => {
+      delete req.body.depositedAmount;
+      await createStream(req as Request, res as Response);
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.status).not.toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith(
+        expect.objectContaining({ error: expect.stringContaining('depositedAmount') })
+      );
+    });
   });
 
   describe('listStreams', () => {