diff --git a/src/a2a/client/service_parameters.py b/src/a2a/client/service_parameters.py
index cef250807..39fe79ce1 100644
--- a/src/a2a/client/service_parameters.py
+++ b/src/a2a/client/service_parameters.py
@@ -1,7 +1,10 @@
 from collections.abc import Callable
 from typing import TypeAlias
 
-from a2a.extensions.common import HTTP_EXTENSION_HEADER
+from a2a.extensions.common import (
+    HTTP_EXTENSION_HEADER,
+    get_requested_extensions,
+)
 
 
 ServiceParameters: TypeAlias = dict[str, str]
@@ -44,17 +47,18 @@ def create_from(
 
 
 def with_a2a_extensions(extensions: list[str]) -> ServiceParametersUpdate:
-    """Create a ServiceParametersUpdate that adds A2A extensions.
+    """Create a ServiceParametersUpdate that merges A2A extension URIs.
 
-    Args:
-        extensions: List of extension strings.
-
-    Returns:
-        A function that updates ServiceParameters with the extensions header.
+    Unions the supplied URIs with any already present in the A2A-Extensions
+    parameter, deduplicating and emitting them in sorted order. Repeated
+    calls accumulate rather than overwrite.
     """
 
     def update(parameters: ServiceParameters) -> None:
-        if extensions:
-            parameters[HTTP_EXTENSION_HEADER] = ','.join(extensions)
+        if not extensions:
+            return
+        existing = parameters.get(HTTP_EXTENSION_HEADER, '')
+        merged = sorted(get_requested_extensions([existing, *extensions]))
+        parameters[HTTP_EXTENSION_HEADER] = ','.join(merged)
 
     return update
diff --git a/tests/client/test_service_parameters.py b/tests/client/test_service_parameters.py
new file mode 100644
index 000000000..fbabd9719
--- /dev/null
+++ b/tests/client/test_service_parameters.py
@@ -0,0 +1,53 @@
+"""Tests for a2a.client.service_parameters module."""
+
+from a2a.client.service_parameters import (
+    ServiceParametersFactory,
+    with_a2a_extensions,
+)
+from a2a.extensions.common import HTTP_EXTENSION_HEADER
+
+
+def test_with_a2a_extensions_merges_dedupes_and_sorts():
+    """Repeated calls accumulate; duplicates collapse; output is sorted."""
+    parameters = ServiceParametersFactory.create(
+        [
+            with_a2a_extensions(['ext-c', 'ext-a']),
+            with_a2a_extensions(['ext-b', 'ext-a']),
+        ]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b,ext-c'
+
+
+def test_with_a2a_extensions_merges_existing_header_value():
+    """Pre-existing comma-separated header values are parsed and merged."""
+    parameters = ServiceParametersFactory.create_from(
+        {HTTP_EXTENSION_HEADER: 'ext-a, ext-b'},
+        [with_a2a_extensions(['ext-c'])],
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b,ext-c'
+
+
+def test_with_a2a_extensions_empty_is_noop():
+    """An empty extensions list leaves the header untouched / absent."""
+    parameters = ServiceParametersFactory.create(
+        [
+            with_a2a_extensions(['ext-a']),
+            with_a2a_extensions([]),
+        ]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a'
+    assert HTTP_EXTENSION_HEADER not in ServiceParametersFactory.create(
+        [with_a2a_extensions([])]
+    )
+
+
+def test_with_a2a_extensions_normalizes_input_strings():
+    """Input strings are split on commas and stripped, like header values."""
+    parameters = ServiceParametersFactory.create(
+        [with_a2a_extensions(['ext-a, ext-b', '  ext-c  '])]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b,ext-c'