test: regression coverage for header reapplication on model switch

Three tests for the #61099 salvage: OpenRouter attribution headers
present after switching to openrouter.ai, Kimi User-Agent sentinel
present after switching to api.kimi.com, and stale headers cleared
when switching to a provider with no URL-specific headers.
2/3 fail on unpatched main (DID NOT ATTACH), confirming the bug.

tests/run_agent/test_switch_model_reapplies_headers.py

@@ -0,0 +1,100 @@
+"""Regression tests for #61099: switch_model must reapply provider-specific
+default headers when it rebuilds _client_kwargs from scratch.
+
+Without _apply_client_headers_for_base_url() in the rebuild path, a /model
+switch drops OpenRouter attribution headers (HTTP-Referer / X-Title → logs
+show "Unknown") and, worse, functional headers like Kimi's User-Agent
+sentinel (403 without it).
+"""
+
+from unittest.mock import MagicMock, patch
+
+from run_agent import AIAgent
+from agent.context_compressor import ContextCompressor
+
+
+def _make_agent(provider="copilot", base_url="https://api.githubcopilot.com") -> AIAgent:
+    """Minimal AIAgent with a context_compressor, skipping __init__."""
+    agent = AIAgent.__new__(AIAgent)
+
+    agent.model = "claude-opus-4.8"
+    agent.provider = provider
+    agent.base_url = base_url
+    agent.api_key = "sk-primary"
+    agent.api_mode = "chat_completions"
+    agent.client = MagicMock()
+    agent.quiet_mode = True
+    agent._config_context_length = None
+    agent._client_kwargs = {"api_key": "sk-primary", "base_url": base_url}
+
+    compressor = ContextCompressor(
+        model=agent.model,
+        threshold_percent=0.50,
+        base_url=base_url,
+        api_key="sk-primary",
+        provider=provider,
+        quiet_mode=True,
+        config_context_length=None,
+    )
+    agent.context_compressor = compressor
+    agent._primary_runtime = {}
+
+    return agent
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=131_072)
+def test_switch_to_openrouter_reapplies_attribution_headers(mock_ctx_len):
+    """Switching to an openrouter.ai base_url must attach the OpenRouter
+    attribution headers (HTTP-Referer / X-Title) to the rebuilt client
+    kwargs — not ship a bare api_key+base_url client (#61099)."""
+    agent = _make_agent(provider="copilot", base_url="https://api.githubcopilot.com")
+
+    agent.switch_model(
+        "deepseek/deepseek-chat",
+        "openrouter",
+        api_key="sk-or-new",
+        base_url="https://openrouter.ai/api/v1",
+    )
+
+    headers = agent._client_kwargs.get("default_headers") or {}
+    assert "HTTP-Referer" in headers
+    assert headers.get("X-Title")
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=131_072)
+def test_switch_to_kimi_reapplies_user_agent_sentinel(mock_ctx_len):
+    """Kimi requires a User-Agent sentinel; a switch to api.kimi.com must
+    carry it or every request 403s."""
+    agent = _make_agent(provider="openrouter", base_url="https://openrouter.ai/api/v1")
+
+    agent.switch_model(
+        "kimi-k2",
+        "kimi",
+        api_key="sk-kimi",
+        base_url="https://api.kimi.com/v1",
+    )
+
+    headers = agent._client_kwargs.get("default_headers") or {}
+    assert headers.get("User-Agent", "").startswith("claude-code/")
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=131_072)
+def test_switch_away_from_headered_provider_clears_stale_headers(mock_ctx_len):
+    """Switching FROM a headered provider TO one with no URL-specific headers
+    must not carry the old provider's headers along."""
+    agent = _make_agent(provider="openrouter", base_url="https://openrouter.ai/api/v1")
+    agent._client_kwargs["default_headers"] = {
+        "HTTP-Referer": "https://hermes-agent.nousresearch.com",
+        "X-Title": "Hermes Agent",
+    }
+
+    agent.switch_model(
+        "MiniMax-M3",
+        "custom:minimax",
+        api_key="sk-minimax",
+        base_url="https://api.minimax.io/v1",
+    )
+
+    headers = agent._client_kwargs.get("default_headers") or {}
+    assert "HTTP-Referer" not in headers
+    assert "X-Title" not in headers