Python: fix LineTooLong error in CopilotStudioAgent

[Mahajan-Sachin]

Motivation & Context

This PR resolves Issue #7257 where calling CopilotStudioAgent fails with aiohttp.http_exceptions.LineTooLong: 400 when receiving streaming response data larger than 512KB (for example, when fetching a large payload from a Salesforce connector in Copilot Studio).

The root cause is that the underlying dependency microsoft-agents-copilotstudio-client implements CopilotClient.post_request using line-based iteration (async for line in response.content:). Under aiohttp, line-based reading triggers a hardcoded limit of 524288 bytes (512 KB) for any single line.

Proposed Changes

Since the client is an external dependency, we fix this in the framework layer by monkeypatching the client's HTTP request handler:

1. Monkeypatch CopilotClient.post_request:

   • Patched post_request inside agent_framework_copilotstudio/_agent.py to read the response content in raw chunks via response.content.iter_any().
   • Implemented custom chunk-based buffer splitting on newlines (\n) to parse SSE activities. This avoids line-limit exceptions entirely and handles lines of arbitrary length.
   • Suppressed Pyright reportPrivateUsage warning with an inline comment, since monkeypatching requires access to the client's protected _current_conversation_id field.

2. Added Regression Test:

   • Created packages/copilotstudio/tests/test_line_too_long_bug.py.
   • The test spins up a local asynchronous HTTP/SSE server that returns an event payload of 600 KB (exceeding the 512 KB limit).
   • Verifies that the patched client handles the stream correctly and parses the message without throwing exceptions.

Testing & Validation

1. Unit Tests:

   • Ran the new test and verified it passes:

     uv run pytest packages/copilotstudio/tests/test_line_too_long_bug.py
     # 1 passed in 0.17s

   • Ran the full copilotstudio package test suite:

     uv run poe test -P copilotstudio
     # 34 passed in 0.60s

2. Static Analysis:

   • Verified all linter, formatter, and type-checking checks pass with zero errors:

     uv run poe check -P copilotstudio
     # All formatting, lint, Pyright, MyPy, Ty, and Zuban tasks passed

Closes #7257

[Copilot]

Pull request overview

This PR addresses aiohttp’s LineTooLong failure mode when Copilot Studio returns very large SSE data: lines by replacing line-based streaming reads with chunk-based processing in the Copilot Studio Python integration, and adds a regression test that exercises a >512KB SSE line.

Changes:

• Monkeypatches microsoft_agents.copilotstudio.client.CopilotClient.post_request to consume SSE responses via raw chunks rather than line iteration.
• Adds an async aiohttp test server that emits a 600KB data: line and validates the client can parse it without raising LineTooLong.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/copilotstudio/agent_framework_copilotstudio/_agent.py	Monkeypatches CopilotClient SSE streaming read logic to avoid aiohttp’s line-length limit.
python/packages/copilotstudio/tests/test_line_too_long_bug.py	Adds regression test for >512KB SSE data: line handling.

[Mahajan-Sachin]

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[Mahajan-Sachin]

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[Mahajan-Sachin]

@copilot please check now

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/copilotstudio/agent_framework_copilotstudio
_agent.py	126	15	88%	35–36, 54–59, 61–62, 64, 217, 225, 295, 327
TOTAL	45615	4490	90%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9411	34 💤	0 ❌	0 🔥	2m 25s ⏱️

[giles17]

Hey @Mahajan-Sachin please can you address the failing checks?

[Mahajan-Sachin]

Hi @giles17,

I've addressed the failing check:

1. Synced the branch with the latest changes from upstream/main.
2. Fixed the pyupgrade syntax check failure in test_line_too_long_bug.py by changing .encode("utf-8") to .encode().
3. Ran all pre-commit formatting/linting checks locally, and they are now passing successfully.

Could you please approve the workflow runs so the CI checks can verify them on GitHub? Ready for review whenever you're free!