fix(anthropic): non-whitespace placeholder for synthetic leading user turn

[songxujiangyang]

Summary

_ensure_leading_user_turn (in agent/anthropic_adapter.py) prepends a synthetic user message when messages[0] is not role=user. This happens after a second context compaction on the auto path, where the summary is emitted as role=assistant with nothing in front of it (the system prompt lives outside messages[]), so messages[0] ends up assistant and the Messages API would 400.

The synthetic placeholder used a single space " " as its text.

The bug

Anthropic's own API tolerates a whitespace-only text block, but stricter Anthropic-compatible endpoints reject it. Observed against zenmux.ai (/api/anthropic):

HTTP 400 invalid_params: messages: text content blocks must contain non-whitespace text

This error is non-retryable, so once a conversation has compacted twice, every subsequent turn fails permanently. In our case a Feishu DM session silently stopped replying after auto-compaction — each inbound message hit the 400 and never recovered.

Fix

Use a minimal non-blank placeholder "." instead of " ". One-line change in the shared helper, so every caller (all provider paths that route through convert_messages_to_anthropic) is fixed at once.

Tests

The two existing tests froze the exact placeholder byte (== [{"type": "text", "text": " "}]) — change-detector shape. Updated them to assert the behavior contract instead: the prepended leading turn is role=user with non-whitespace text. This keeps them meaningful regardless of the exact placeholder character.

tests/agent/test_anthropic_adapter.py … 179 passed

(3 unrelated TestRunOauthSetupToken failures are pre-existing on main, verified by stashing this change.)

[alt-glitch]

This was generated by AI during triage.

Duplicate of #70081: both patches replace the whitespace-only synthesized Anthropic leading user turn with a printable placeholder to prevent the same strict-endpoint HTTP 400.

[teknium1]

Thanks for identifying the assembly-time gap. The premise is confirmed on current main: agent/anthropic_adapter.py:2612-2613 still synthesizes a leading user block containing a single space, after the per-message conversion and filtering chain (agent/anthropic_adapter.py:2672-2674).

Suggested changes

• During salvage, use the existing _EMPTY_TEXT_PLACEHOLDER rather than a new "." literal. Current main defines that canonical non-whitespace value at agent/anthropic_adapter.py:1931 and uses it in the existing whitespace-safety path at agent/anthropic_adapter.py:1950.
• Update the current leading-assistant regression at tests/agent/test_anthropic_adapter.py:871-884 to assert non-whitespace behavior; it still freezes the single-space value.

This is an automated hermes-sweeper review.

[teknium1]

Current main already has _EMPTY_TEXT_PLACEHOLDER for this exact non-whitespace invariant. Please use that canonical constant here rather than introducing a second placeholder literal.