diff --git a/runtime/python_bridge.py b/runtime/python_bridge.py
index 1e53881..8bb8aa0 100644
--- a/runtime/python_bridge.py
+++ b/runtime/python_bridge.py
@@ -141,6 +141,65 @@ class ProtocolError(Exception):
 class InstanceHandleError(ValueError):
     """Raised when an instance handle is unknown or no longer valid."""
 
+_NO_DESERIALIZE = object()
+_ERR_BYTES_MISSING_B64 = 'Invalid bytes envelope: missing b64'
+_ERR_BYTES_MISSING_DATA = 'Invalid bytes envelope: missing data'
+_ERR_BYTES_INVALID_BASE64 = 'Invalid bytes envelope: invalid base64'
+
+
+def _deserialize_bytes_envelope(value) -> object:
+    """
+    Decode base64-encoded bytes envelopes from JS into Python bytes.
+
+    Supported shapes:
+    - { "__tywrap_bytes__": true, "b64": "..." }  (JS SafeCodec.encodeRequest)
+    - { "__type__": "bytes", "encoding": "base64", "data": "..." }  (legacy/compat)
+
+    Why: TS SafeCodec encodes Uint8Array/ArrayBuffer as base64 objects, but
+    Python handlers expect real bytes/bytearray to preserve behavior (e.g., len()).
+    """
+    if not isinstance(value, dict):
+        return _NO_DESERIALIZE
+
+    if value.get('__tywrap_bytes__') is True:
+        b64 = value.get('b64')
+        if not isinstance(b64, str):
+            raise ProtocolError(_ERR_BYTES_MISSING_B64)
+        try:
+            return base64.b64decode(b64, validate=True)
+        except Exception as exc:
+            raise ProtocolError(_ERR_BYTES_INVALID_BASE64) from exc
+
+    if value.get('__type__') == 'bytes' and value.get('encoding') == 'base64':
+        data = value.get('data')
+        if not isinstance(data, str):
+            raise ProtocolError(_ERR_BYTES_MISSING_DATA)
+        try:
+            return base64.b64decode(data, validate=True)
+        except Exception as exc:
+            raise ProtocolError(_ERR_BYTES_INVALID_BASE64) from exc
+
+    return _NO_DESERIALIZE
+
+
+def deserialize(value):
+    """
+    Recursively deserialize request values into Python-native types.
+
+    Why: requests are JSON-only; we need a small set of explicit decoders
+    (currently bytes) to restore Python semantics at the boundary.
+    """
+    decoded = _deserialize_bytes_envelope(value)
+    if decoded is not _NO_DESERIALIZE:
+        return decoded
+
+    if isinstance(value, list):
+        return [deserialize(item) for item in value]
+    if isinstance(value, dict):
+        # Preserve dict shape while decoding nested values.
+        return {k: deserialize(v) for k, v in value.items()}
+    return value
+
 
 _PROTOCOL_DIAGNOSTIC_MAX = 2048
 
@@ -648,8 +707,8 @@ def serialize_stdlib(obj):
 def handle_call(params):
     module_name = require_str(params, 'module')
     function_name = require_str(params, 'functionName')
-    args = coerce_list(params.get('args'), 'args')
-    kwargs = coerce_dict(params.get('kwargs'), 'kwargs')
+    args = deserialize(coerce_list(params.get('args'), 'args'))
+    kwargs = deserialize(coerce_dict(params.get('kwargs'), 'kwargs'))
     mod = importlib.import_module(module_name)
     func = getattr(mod, function_name)
     res = func(*args, **kwargs)
@@ -659,8 +718,8 @@ def handle_call(params):
 def handle_instantiate(params):
     module_name = require_str(params, 'module')
     class_name = require_str(params, 'className')
-    args = coerce_list(params.get('args'), 'args')
-    kwargs = coerce_dict(params.get('kwargs'), 'kwargs')
+    args = deserialize(coerce_list(params.get('args'), 'args'))
+    kwargs = deserialize(coerce_dict(params.get('kwargs'), 'kwargs'))
     mod = importlib.import_module(module_name)
     cls = getattr(mod, class_name)
     obj = cls(*args, **kwargs)
@@ -672,8 +731,8 @@ def handle_instantiate(params):
 def handle_call_method(params):
     handle_id = require_str(params, 'handle')
     method_name = require_str(params, 'methodName')
-    args = coerce_list(params.get('args'), 'args')
-    kwargs = coerce_dict(params.get('kwargs'), 'kwargs')
+    args = deserialize(coerce_list(params.get('args'), 'args'))
+    kwargs = deserialize(coerce_dict(params.get('kwargs'), 'kwargs'))
     if handle_id not in instances:
         raise InstanceHandleError(f'Unknown instance handle: {handle_id}')
     obj = instances[handle_id]
diff --git a/src/runtime/process-io.ts b/src/runtime/process-io.ts
index f3fa700..dc4b645 100644
--- a/src/runtime/process-io.ts
+++ b/src/runtime/process-io.ts
@@ -733,8 +733,10 @@ export class ProcessIO extends BoundedContext implements Transport {
       }
     }, this.writeQueueTimeoutMs);
 
-    // Unref the timer so it doesn't keep the process alive
-    entry.timeoutHandle.unref();
+    // Unref the timer so it doesn't keep the process alive (best-effort for non-Node runtimes)
+    if (typeof entry.timeoutHandle.unref === 'function') {
+      entry.timeoutHandle.unref();
+    }
 
     return entry;
   }
diff --git a/src/runtime/safe-codec.ts b/src/runtime/safe-codec.ts
index c28b115..2f28be4 100644
--- a/src/runtime/safe-codec.ts
+++ b/src/runtime/safe-codec.ts
@@ -319,12 +319,89 @@ export class SafeCodec {
   private readonly rejectNonStringKeys: boolean;
   private readonly maxPayloadBytes: number;
   private readonly bytesHandling: 'base64' | 'reject' | 'passthrough';
+  private readonly reviveValueBound: (key: string, value: unknown) => unknown;
+  private static readonly base64Pattern =
+    /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/;
 
   constructor(options: CodecOptions = {}) {
     this.rejectSpecialFloats = options.rejectSpecialFloats ?? true;
     this.rejectNonStringKeys = options.rejectNonStringKeys ?? true;
     this.maxPayloadBytes = options.maxPayloadBytes ?? DEFAULT_MAX_PAYLOAD_BYTES;
     this.bytesHandling = options.bytesHandling ?? 'base64';
+    this.reviveValueBound = this.reviveValue.bind(this);
+  }
+
+  private assertValidBase64(b64: string): void {
+    if (!SafeCodec.base64Pattern.test(b64)) {
+      throw new BridgeCodecError('Invalid base64 in bytes envelope', {
+        codecPhase: 'decode',
+        valueType: 'bytes',
+      });
+    }
+  }
+
+  /**
+   * Convert base64 string to Uint8Array.
+   *
+   * Why: Python bridge represents bytes/bytearray as base64 envelopes. Decoding them here
+   * restores ergonomic JS types at the boundary.
+   */
+  private fromBase64(b64: string): Uint8Array {
+    this.assertValidBase64(b64);
+
+    if (typeof Buffer !== 'undefined') {
+      const buf = Buffer.from(b64, 'base64');
+      return new Uint8Array(buf.buffer, buf.byteOffset, buf.length);
+    }
+    if (globalThis.atob) {
+      const bin = globalThis.atob(b64);
+      const arr = Array.from(bin, c => c.charCodeAt(0));
+      return new Uint8Array(arr);
+    }
+    throw new BridgeCodecError('Base64 decoding is not available in this runtime', {
+      codecPhase: 'decode',
+      valueType: 'bytes',
+    });
+  }
+
+  /**
+   * JSON.parse reviver that decodes bytes envelopes.
+   *
+   * Supported shapes:
+   * - { "__tywrap_bytes__": true, "b64": "..." } (JS SafeCodec.encodeRequest; also allowed in responses)
+   * - { "__type__": "bytes", "encoding": "base64", "data": "..." } (Python SafeCodec default encoder)
+   */
+  private reviveValue(_key: string, value: unknown): unknown {
+    if (value === null || typeof value !== 'object' || Array.isArray(value)) {
+      return value;
+    }
+    const obj = value as Record<string, unknown>;
+
+    if (obj.__tywrap_bytes__ === true && typeof obj.b64 === 'string') {
+      try {
+        return this.fromBase64(obj.b64);
+      } catch (err) {
+        const errorMessage = err instanceof Error ? err.message : String(err);
+        throw new BridgeCodecError(`Bytes envelope decode failed: ${errorMessage}`, {
+          codecPhase: 'decode',
+          valueType: 'bytes',
+        });
+      }
+    }
+
+    if (obj.__type__ === 'bytes' && obj.encoding === 'base64' && typeof obj.data === 'string') {
+      try {
+        return this.fromBase64(obj.data);
+      } catch (err) {
+        const errorMessage = err instanceof Error ? err.message : String(err);
+        throw new BridgeCodecError(`Bytes envelope decode failed: ${errorMessage}`, {
+          codecPhase: 'decode',
+          valueType: 'bytes',
+        });
+      }
+    }
+
+    return value;
   }
 
   private toBridgeExecutionError(error: NormalizedPythonError): BridgeExecutionError {
@@ -461,8 +538,11 @@ export class SafeCodec {
     // Parse JSON
     let parsed: unknown;
     try {
-      parsed = JSON.parse(payload);
+      parsed = JSON.parse(payload, this.reviveValueBound);
     } catch (err) {
+      if (err instanceof BridgeCodecError || err instanceof BridgeProtocolError) {
+        throw err;
+      }
       const errorMessage = err instanceof Error ? err.message : String(err);
       throw new BridgeCodecError(
         `JSON parse failed: ${errorMessage}. Payload snippet: ${summarizePayloadForError(payload)}`,
@@ -510,8 +590,11 @@ export class SafeCodec {
     // Parse JSON
     let parsed: unknown;
     try {
-      parsed = JSON.parse(payload);
+      parsed = JSON.parse(payload, this.reviveValueBound);
     } catch (err) {
+      if (err instanceof BridgeCodecError || err instanceof BridgeProtocolError) {
+        throw err;
+      }
       const errorMessage = err instanceof Error ? err.message : String(err);
       throw new BridgeCodecError(
         `JSON parse failed: ${errorMessage}. Payload snippet: ${summarizePayloadForError(payload)}`,
diff --git a/test/runtime_node.test.ts b/test/runtime_node.test.ts
index 238471d..c238f6b 100644
--- a/test/runtime_node.test.ts
+++ b/test/runtime_node.test.ts
@@ -112,6 +112,55 @@ describeNodeOnly('Node.js Runtime Bridge', () => {
       testTimeout
     );
 
+    it(
+      'should roundtrip Uint8Array as Python bytes',
+      async () => {
+        const pythonAvailable = await isPythonAvailable();
+        if (!pythonAvailable || !isBridgeScriptAvailable()) return;
+
+        const input = new Uint8Array([72, 101, 108, 108, 111]); // "Hello"
+
+        const length = await bridge.call<number>('builtins', 'len', [input]);
+        expect(length).toBe(5);
+
+        const output = await bridge.call<Uint8Array>('builtins', 'bytes', [input]);
+        expect(output).toBeInstanceOf(Uint8Array);
+        expect(Array.from(output)).toEqual(Array.from(input));
+      },
+      testTimeout
+    );
+
+    it(
+      'should support zero-length Uint8Array as Python bytes',
+      async () => {
+        const pythonAvailable = await isPythonAvailable();
+        if (!pythonAvailable || !isBridgeScriptAvailable()) return;
+
+        const input = new Uint8Array([]);
+
+        const length = await bridge.call<number>('builtins', 'len', [input]);
+        expect(length).toBe(0);
+
+        const output = await bridge.call<Uint8Array>('builtins', 'bytes', [input]);
+        expect(output).toBeInstanceOf(Uint8Array);
+        expect(output.length).toBe(0);
+      },
+      testTimeout
+    );
+
+    it(
+      'should reject malformed bytes envelopes with explicit protocol error',
+      async () => {
+        const pythonAvailable = await isPythonAvailable();
+        if (!pythonAvailable || !isBridgeScriptAvailable()) return;
+
+        await expect(
+          bridge.call('builtins', 'len', [{ __tywrap_bytes__: true, b64: '%%%' }])
+        ).rejects.toThrow(/Invalid bytes envelope: invalid base64/);
+      },
+      testTimeout
+    );
+
     it(
       'should handle function calls with kwargs',
       async () => {
diff --git a/test/safe-codec.test.ts b/test/safe-codec.test.ts
index 68aeec7..3b2531f 100644
--- a/test/safe-codec.test.ts
+++ b/test/safe-codec.test.ts
@@ -357,6 +357,47 @@ describe('decodeResponse - Basic', () => {
     expect(result).toEqual({ a: 1 });
   });
 
+  it('decodes Python bytes envelope (__type__: bytes) to Uint8Array', () => {
+    const payload = JSON.stringify({
+      id: 1,
+      result: { __type__: 'bytes', encoding: 'base64', data: 'SGVsbG8=' }, // "Hello"
+    });
+    const result = codec.decodeResponse<Uint8Array>(payload);
+    expect(result).toBeInstanceOf(Uint8Array);
+    expect(Array.from(result)).toEqual([72, 101, 108, 108, 111]);
+  });
+
+  it('decodes Tywrap bytes envelope (__tywrap_bytes__) to Uint8Array', () => {
+    const payload = JSON.stringify({
+      id: 1,
+      result: { __tywrap_bytes__: true, b64: 'SGVsbG8=' }, // "Hello"
+    });
+    const result = codec.decodeResponse<Uint8Array>(payload);
+    expect(result).toBeInstanceOf(Uint8Array);
+    expect(Array.from(result)).toEqual([72, 101, 108, 108, 111]);
+  });
+
+  it('decodes bytes envelope even when bytesHandling is reject', () => {
+    const rejectCodec = new SafeCodec({ bytesHandling: 'reject' });
+    const payload = JSON.stringify({
+      id: 1,
+      result: { __type__: 'bytes', encoding: 'base64', data: 'SGVsbG8=' }, // "Hello"
+    });
+    const result = rejectCodec.decodeResponse<Uint8Array>(payload);
+    expect(result).toBeInstanceOf(Uint8Array);
+    expect(Array.from(result)).toEqual([72, 101, 108, 108, 111]);
+  });
+
+  it('surfaces invalid bytes envelope base64 as a codec error', () => {
+    const payload = JSON.stringify({
+      id: 1,
+      result: { __tywrap_bytes__: true, b64: '%%%' },
+    });
+    expect(() => codec.decodeResponse(payload)).toThrow(BridgeCodecError);
+    expect(() => codec.decodeResponse(payload)).toThrow(/Bytes envelope decode failed/);
+    expect(() => codec.decodeResponse(payload)).not.toThrow(/JSON parse failed/);
+  });
+
   it('parses arrays', () => {
     const result = codec.decodeResponse<number[]>('[1, 2, 3]');
     expect(result).toEqual([1, 2, 3]);