fix: preserve None content in assistant messages (#11906)

[Sanjays2402]

What does this PR do?

_build_assistant_message() normalized None content to "" via or "". This caused Anthropic-compatible proxies to reject responses containing empty text blocks — the proxy sees {"type": "text", "text": ""} and returns HTTP 400.

Fix: keep None as None instead of coercing to empty string. Only apply surrogate sanitization when content is truthy.

Before: _raw_content = assistant_message.content or ""
After: _raw_content = assistant_message.content + conditional sanitization

Related Issue

Fixes #11906

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• _build_assistant_message(): stop coercing None content to ""; sanitize surrogates only when content is truthy.
• Updated test_empty_content → test_none_content_preserved (asserts None not "").
• Added test_none_content_with_tool_calls for tool-call-only responses.

How to Test

1. Run Hermes against an Anthropic-compatible proxy that rejects empty text blocks.

2. Produce a tool-call-only assistant response (no text content).

3. Before: HTTP 400 from the proxy. After: request succeeds; assistant message carries content=None and tool calls only.

4. Run unit tests:

   pytest -q -k "none_content"
   

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix:)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes
• I've tested on my platform: macOS 26.5 (arm64)

Documentation & Housekeeping

• Documentation updates — N/A
• cli-config.yaml.example — N/A
• CONTRIBUTING.md / AGENTS.md — N/A
• Cross-platform impact considered — pure Python
• Tool descriptions/schemas — N/A

Screenshots / Logs

N/A — internal message normalization; covered by new unit tests.

[vominh1919]

LGTM! Clean minimal fix. Good: preserves None, conditional sanitization, clear comments, tests cover tool-call-only case. Minor suggestion: use is not None instead of truthy check for more explicit intent. Ready to merge.

[alt-glitch]

Related: PR #12737 addresses the same None-to-empty-string content normalization for Anthropic compatibility. Also see #11929.

[teknium1]

Thanks for the focused reproduction and regression coverage. The reported source behavior remains present in current main at agent/chat_completion_helpers.py:979-980, but an overlapping PR received an explicit maintainer decision against this fix direction: #11929 states that chat-completions content is intentionally byte-stable through history and that native Anthropic handling owns empty-content conversion (agent/anthropic_adapter.py:2031-2035).

Problems

• The proposed history rewrite conflicts with that established design decision.
• The active implementation has also moved: run_agent.py:5454-5457 is now a forwarder, and commit 4b25619bc4770396faf206429ddc180ad02231a9 placed the builder in agent/chat_completion_helpers.py.

Suggested changes

• Keep this open for maintainer direction rather than salvaging the patch unchanged; any renewed approach should address the downstream proxy contract without altering persisted chat-completions history.

Automated hermes-sweeper review.