fix: send OpenAI tool results as string content

[MukundaKatta]

Fixes #25.

Summary

• Convert MCP tool results to string content for OpenAI tool messages instead of sending content part arrays
• Preserve text and text resources directly, joining multiple text parts with newlines
• Keep existing OpenAI content part conversion as the fallback for non-text content
• Add regression coverage for execute_tool_call and multi-part text tool results

Tests

• uv run ruff format src/mcp_agent/workflows/llm/augmented_llm_openai.py tests/workflows/llm/test_augmented_llm_openai.py
• uv run ruff check src/mcp_agent/workflows/llm/augmented_llm_openai.py tests/workflows/llm/test_augmented_llm_openai.py
• uv run pytest tests/workflows/llm/test_augmented_llm_openai.py -k "execute_tool_call_returns_string_content or tool_message_content_conversion_joins_text_parts" -q
• uv run pytest tests/workflows/llm/test_augmented_llm_openai.py -q

Summary by CodeRabbit

• Tests

  • Added test coverage for tool message content formatting and multi-part content concatenation

• Refactor

  • Improved the formatting of tool call results to use a streamlined content conversion approach

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 517e19b6-c261-49f6-ac5b-662938ae0994

📥 Commits

Reviewing files that changed from the base of the PR and between f62d849 and 7774750.

📒 Files selected for processing (2)

• src/mcp_agent/workflows/llm/augmented_llm_openai.py
• tests/workflows/llm/test_augmented_llm_openai.py

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📝 Walkthrough

Walkthrough

The PR addresses OpenAI API compatibility by converting tool message content from a list structure to a single string format. A new helper function flattens MCP content elements into newline-joined text, the tool call execution path integrates this helper, and tests validate the new behavior with appropriate test infrastructure.

Changes

Tool Message Content String Formatting

Layer / File(s)	Summary
Tool message content conversion helper src/mcp_agent/workflows/llm/augmented_llm_openai.py	New mcp_content_to_openai_tool_message_content function converts MCP content elements (TextContent, ImageContent, EmbeddedResource) into a single newline-joined string, extracting text directly and falling back to stringified content-part conversion for non-text types.
Execute tool call integration src/mcp_agent/workflows/llm/augmented_llm_openai.py	execute_tool_call now uses the new helper to set ChatCompletionToolMessageParam.content as a string instead of building a list of content part objects, ensuring OpenAI API compatibility.
Test infrastructure and validation tests/workflows/llm/test_augmented_llm_openai.py	Adds _OpenAITestLLM subclass with generate_stream override for test isolation, updates mock_llm fixture, and introduces two new tests validating string content return and newline-joined text concatenation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• lastmile-ai/mcp-agent#330: Both PRs modify OpenAIAugmentedLLM.execute_tool_call in src/mcp_agent/workflows/llm/augmented_llm_openai.py, changing how result.content is handled and converted into ChatCompletionToolMessageParam.
• lastmile-ai/mcp-agent#341: Both PRs modify the tool-call content conversion path in src/mcp_agent/workflows/llm/augmented_llm_openai.py—this PR changes how execute_tool_call formats content as a string, while the other adjusts mcp_content_to_openai_content_part used during content-part conversion.

Suggested reviewers

• rholinshead
• saqadri

Poem

🐰 A list became a string, the API cried with glee,
Tool messages now flow as text—OpenAI's harmony!
No more arrays confusing DeepSeek's parsing mind,
Just newlines joining wisdom, structured and kind. ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 54.55% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: converting OpenAI tool results to string content format instead of content arrays.
Linked Issues check	✅ Passed	The PR fully addresses issue #25 by converting MCP tool results to strings for OpenAI, preserving text directly, joining multiple parts with newlines, and falling back to existing content conversion for non-text types.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the linked issue: a new helper function, modified execute_tool_call behavior, test subclass for mocking, and two targeted test cases.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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