Fix: Handle local file paths in audio content conversion

[sidonsoft]

Summary

This PR fixes a bug in message_to_agentscope_msg() where audio content with a local file path (e.g., /tmp/voice.ogg) would be incorrectly wrapped as Base64Source instead of being converted to a proper file:// URL.

Related PR: agentscope-ai/QwenPaw#1896

---

The Bug

In src/agentscope_runtime/adapters/agentscope/message.py, the audio handling logic has this branching:

elif cnt_type == "audio":
    if value and isinstance(value, str) and value.startswith("data:"):
        # Case 1: data: URI → Base64Source ✅
        ...
    else:
        parsed_url = urlparse(value)
        if parsed_url.scheme and parsed_url.netloc:
            # Case 2: Full URL → URLSource ✅
            ...
        else:
            # Case 3: Everything else → Base64Source ❌
            audio_extension = getattr(cnt, "format")
            base64_source = Base64Source(
                type="base64",
                media_type=f"audio/{audio_extension}",  # Could be "audio/None"
                data=value,  # Could be a file path!
            )

The problem: Case 3 assumes that non-URL strings are raw base64 data. But if value is a local file path like /tmp/voice.ogg:

1. It's not a data: URI → skipped Case 1
2. urlparse("/tmp/voice.ogg") gives scheme="", netloc="" → skipped Case 2
3. Falls through to Case 3, where it's treated as base64 data (it's not)

---

The Fix

Added a local file path check using os.path.isfile() before falling back to base64:

else:
    audio_extension = getattr(cnt, "format", None)

    if value and isinstance(value, str) and os.path.isfile(value):
        # Local file path → convert to file:// URL
        url_source = URLSource(
            type="url",
            url=Path(value).as_uri(),
            media_type=f"audio/{audio_extension}" if audio_extension else None,
        )
        msg_content.append(block_cls(type=cnt_type, source=url_source))
    else:
        # Fall back to base64 (existing behavior)
        base64_source = Base64Source(
            type="base64",
            media_type=f"audio/{audio_extension}" if audio_extension else None,
            data=value,
        )
        msg_content.append(block_cls(type=cnt_type, source=base64_source))

---

Why os.path.isfile() is Appropriate

The check is strict — it returns False if the file doesn't exist on disk. This is correct because:

1. The runner processes freshly downloaded Telegram/DingTalk audio files, so they should exist at conversion time
2. If the file doesn't exist, something else is broken upstream
3. Falling back to base64 for non-existent paths would produce file:// URLs that fail downstream with confusing errors

---

Additional Fixes

Issue	Before	After
getattr(cnt, "format")	Could raise AttributeError	getattr(cnt, "format", None) with default
media_type=f"audio/{audio_extension}"	Produces "audio/None" if format missing	None if extension missing
Style	-	Uses os.path.isfile() for consistency with CoPaw

---

Behavior Changes

Input	Before	After
/tmp/audio.ogg (existing file)	Base64Source(data="/tmp/audio.ogg", media_type="audio/None") ❌	URLSource(url="file:///tmp/audio.ogg") ✅
data:audio/ogg;base64,...	Base64Source(...) ✅	unchanged ✅
https://example.com/audio.ogg	URLSource(url="...") ✅	unchanged ✅
T2dnUw... (raw base64)	Base64Source(...) ✅	unchanged ✅

---

Coordination with CoPaw PR

This PR fixes the root cause in agentscope-runtime.

CoPaw PR #1896 adds downstream defensive handling in message_processing.py. That fix should remain as a safety net for:

1. Backwards compatibility with older runtime versions
2. Defense-in-depth against similar issues in other code paths

Both PRs are complementary:

• This PR → Fixes the source of the malformed blocks
• CoPaw PR → Defensive handling downstream

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