diff --git a/python/packages/copilotstudio/agent_framework_copilotstudio/_agent.py b/python/packages/copilotstudio/agent_framework_copilotstudio/_agent.py
index 0bc9c484327..1f7c61a5874 100644
--- a/python/packages/copilotstudio/agent_framework_copilotstudio/_agent.py
+++ b/python/packages/copilotstudio/agent_framework_copilotstudio/_agent.py
@@ -5,6 +5,7 @@
 from collections.abc import AsyncIterable, Awaitable, Sequence
 from typing import Any, Literal, TypedDict, overload
 
+import aiohttp
 from agent_framework import (
     AgentMiddlewareTypes,
     AgentResponse,
@@ -20,11 +21,53 @@
 from agent_framework._settings import load_settings
 from agent_framework._types import AgentRunInputs
 from agent_framework.exceptions import AgentException
+from microsoft_agents.activity import Activity, ActivityTypes
 from microsoft_agents.copilotstudio.client import AgentType, ConnectionSettings, CopilotClient, PowerPlatformCloud
 
 from ._acquire_token import acquire_token
 
 
+async def _patched_post_request(
+    self: CopilotClient, url: str, data: dict[str, Any], headers: dict[str, Any]
+) -> AsyncIterable[Activity]:
+    async with aiohttp.ClientSession() as session, session.post(url, json=data, headers=headers) as response:
+        if response.status != 200:
+            detail = await response.text()
+            raise AgentException(f"Copilot Studio request to {url} failed ({response.status}): {detail}")
+        event_type = None
+        buffer = b""
+        async for chunk in response.content.iter_any():
+            buffer += chunk
+            while b"\n" in buffer:
+                line, buffer = buffer.split(b"\n", 1)
+                if line.startswith(b"event:"):
+                    event_type = line[6:].decode("utf-8").strip()
+                if line.startswith(b"data:") and event_type == "activity":
+                    activity_data = line[5:].decode("utf-8").strip()
+                    activity = Activity.model_validate_json(activity_data)
+
+                    if activity.type == ActivityTypes.message:
+                        self._current_conversation_id = activity.conversation.id  # pyright: ignore[reportPrivateUsage]
+
+                    yield activity
+        if buffer:
+            line = buffer
+            if line.startswith(b"event:"):
+                event_type = line[6:].decode("utf-8").strip()
+            if line.startswith(b"data:") and event_type == "activity":
+                activity_data = line[5:].decode("utf-8").strip()
+                activity = Activity.model_validate_json(activity_data)
+
+                if activity.type == ActivityTypes.message:
+                    self._current_conversation_id = activity.conversation.id  # pyright: ignore[reportPrivateUsage]
+
+                yield activity
+
+
+# Monkeypatch CopilotClient to handle data lines larger than 512KB without LineTooLong errors.
+CopilotClient.post_request = _patched_post_request
+
+
 class CopilotStudioSettings(TypedDict, total=False):
     """Copilot Studio model settings.
 
diff --git a/python/packages/copilotstudio/tests/test_line_too_long_bug.py b/python/packages/copilotstudio/tests/test_line_too_long_bug.py
new file mode 100644
index 00000000000..95af89359a9
--- /dev/null
+++ b/python/packages/copilotstudio/tests/test_line_too_long_bug.py
@@ -0,0 +1,65 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+from collections.abc import AsyncIterable
+
+import pytest
+from aiohttp import web
+from microsoft_agents.activity import Activity
+from microsoft_agents.copilotstudio.client import ConnectionSettings, CopilotClient
+
+import agent_framework_copilotstudio  # noqa: F401
+
+# Side-effect import: applies CopilotClient.post_request monkeypatch.
+
+
+async def mock_server_handler(request: web.Request) -> web.StreamResponse:
+    headers = {"Content-Type": "text/event-stream"}
+    response = web.StreamResponse(headers=headers)
+    await response.prepare(request)
+
+    # 600KB line (larger than aiohttp default limit of 524288 bytes)
+    long_data = "x" * 600000
+
+    # Write event type line
+    await response.write(b"event: activity\n")
+    # Write data line
+    data_line = f'data: {{"type":"message","text":"{long_data}","conversation":{{"id":"test"}}}}\n'.encode()
+    await response.write(data_line)
+    return response
+
+
+@pytest.fixture
+async def local_server() -> AsyncIterable[str]:
+    app = web.Application()
+    app.router.add_post("/test", mock_server_handler)
+    runner = web.AppRunner(app)
+    await runner.setup()
+    site = web.TCPSite(runner, "127.0.0.1", 0)
+    await site.start()
+
+    address = runner.addresses[0]
+    port = address[1]
+    # Yield the URL to the test
+    yield f"http://127.0.0.1:{port}/test"
+
+    await site.stop()
+    await runner.cleanup()
+
+
+@pytest.mark.asyncio
+async def test_copilot_client_post_request_long_line(local_server: str) -> None:
+    settings = ConnectionSettings(
+        environment_id="env",
+        agent_identifier="agent",
+        cloud=None,
+        copilot_agent_type=None,
+        custom_power_platform_cloud=None,
+    )
+    client = CopilotClient(settings=settings, token="token")
+
+    activities: list[Activity] = []
+    async for activity in client.post_request(local_server, {}, {}):
+        activities.append(activity)
+
+    assert len(activities) == 1
+    assert activities[0].text == "x" * 600000
diff --git a/python/packages/core/tests/core/test_function_invocation_logic.py b/python/packages/core/tests/core/test_function_invocation_logic.py
index ae01d82a226..b950aab0f06 100644
--- a/python/packages/core/tests/core/test_function_invocation_logic.py
+++ b/python/packages/core/tests/core/test_function_invocation_logic.py
@@ -2374,6 +2374,7 @@ def test_replace_approval_contents_with_results_allows_reused_call_id_after_comp
         ("call_reused", "second output"),
     ]
 
+
 def test_replace_approval_contents_with_results_uses_result_call_ids_for_placeholders() -> None:
     from agent_framework._tools import _collect_approval_responses, _replace_approval_contents_with_results