fix(opencode): strip /v1 from base_url on mid-session /model switch to Anthropic-routed models (#11286)

PR #4918 fixed the double-/v1 bug at fresh agent init by stripping the
trailing /v1 from OpenCode base URLs when api_mode is anthropic_messages
(so the Anthropic SDK's own /v1/messages doesn't land on /v1/v1/messages).
The same logic was missing from the /model mid-session switch path.

Repro: start a session on opencode-go with GLM-5 (or any chat_completions
model), then `/model minimax-m2.7`. switch_model() correctly sets
api_mode=anthropic_messages via opencode_model_api_mode(), but base_url
passes through as https://opencode.ai/zen/go/v1. The Anthropic SDK then
POSTs to https://opencode.ai/zen/go/v1/v1/messages, which returns the
OpenCode website 404 HTML page (title 'Not Found | opencode').

Same bug affects `/model claude-sonnet-4-6` on opencode-zen.

Verified upstream: POST /v1/messages returns clean JSON 401 with x-api-key
auth (route works), while POST /v1/v1/messages returns the exact HTML 404
users reported.

Fix mirrors runtime_provider.resolve_runtime_provider:
- hermes_cli/model_switch.py::switch_model() strips /v1 after the OpenCode
  api_mode override when the resolved mode is anthropic_messages.
- run_agent.py::AIAgent.switch_model() applies the same strip as
  defense-in-depth, so any direct caller can't reintroduce the double-/v1.

Tests: 9 new regression tests in tests/hermes_cli/test_model_switch_opencode_anthropic.py
covering minimax on opencode-go, claude on opencode-zen, chat_completions
(GLM/Kimi/Gemini) keeping /v1 intact, codex_responses (GPT) keeping /v1
intact, trailing-slash handling, and the agent-level defense-in-depth.

tests/hermes_cli/test_model_switch_opencode_anthropic.py

@@ -0,0 +1,252 @@
+"""Regression tests for OpenCode /v1 stripping during /model switch.
+
+When switching to an Anthropic-routed OpenCode model mid-session (e.g.
+``/model minimax-m2.7`` on opencode-go, or ``/model claude-sonnet-4-6``
+on opencode-zen), the resolved base_url must have its trailing ``/v1``
+stripped before being handed to the Anthropic SDK.
+
+Without the strip, the SDK prepends its own ``/v1/messages`` path and
+requests hit ``https://opencode.ai/zen/go/v1/v1/messages`` — a double
+``/v1`` that returns OpenCode's website 404 page with HTML body.
+
+``hermes_cli.runtime_provider.resolve_runtime_provider`` already strips
+``/v1`` at fresh agent init (PR #4918), but the ``/model`` mid-session
+switch path in ``hermes_cli.model_switch.switch_model`` was missing the
+same logic — these tests guard against that regression.
+"""
+
+from unittest.mock import patch
+
+import pytest
+
+from hermes_cli.model_switch import switch_model
+
+
+_MOCK_VALIDATION = {
+    "accepted": True,
+    "persist": True,
+    "recognized": True,
+    "message": None,
+}
+
+
+def _run_opencode_switch(
+    raw_input: str,
+    current_provider: str,
+    current_model: str,
+    current_base_url: str,
+    explicit_provider: str = "",
+    runtime_base_url: str = "",
+):
+    """Run switch_model with OpenCode mocks and return the result.
+
+    runtime_base_url defaults to current_base_url; tests can override it
+    to simulate the credential resolver returning a base_url different
+    from the session's current one.
+    """
+    effective_runtime_base = runtime_base_url or current_base_url
+    with (
+        patch("hermes_cli.model_switch.resolve_alias", return_value=None),
+        patch("hermes_cli.model_switch.list_provider_models", return_value=[]),
+        patch(
+            "hermes_cli.runtime_provider.resolve_runtime_provider",
+            return_value={
+                "api_key": "sk-opencode-fake",
+                "base_url": effective_runtime_base,
+                "api_mode": "chat_completions",
+            },
+        ),
+        patch(
+            "hermes_cli.models.validate_requested_model",
+            return_value=_MOCK_VALIDATION,
+        ),
+        patch("hermes_cli.model_switch.get_model_info", return_value=None),
+        patch("hermes_cli.model_switch.get_model_capabilities", return_value=None),
+        patch("hermes_cli.models.detect_provider_for_model", return_value=None),
+    ):
+        return switch_model(
+            raw_input=raw_input,
+            current_provider=current_provider,
+            current_model=current_model,
+            current_base_url=current_base_url,
+            current_api_key="sk-opencode-fake",
+            explicit_provider=explicit_provider,
+        )
+
+
+class TestOpenCodeGoV1Strip:
+    """OpenCode Go: ``/model minimax-*`` must strip /v1."""
+
+    def test_switch_to_minimax_m27_strips_v1(self):
+        """GLM-5 → MiniMax-M2.7: base_url loses trailing /v1."""
+        result = _run_opencode_switch(
+            raw_input="minimax-m2.7",
+            current_provider="opencode-go",
+            current_model="glm-5",
+            current_base_url="https://opencode.ai/zen/go/v1",
+        )
+
+        assert result.success, f"switch_model failed: {result.error_message}"
+        assert result.api_mode == "anthropic_messages"
+        assert result.base_url == "https://opencode.ai/zen/go", (
+            f"Expected /v1 stripped for anthropic_messages; got {result.base_url}"
+        )
+
+    def test_switch_to_minimax_m25_strips_v1(self):
+        """Same behavior for M2.5."""
+        result = _run_opencode_switch(
+            raw_input="minimax-m2.5",
+            current_provider="opencode-go",
+            current_model="kimi-k2.5",
+            current_base_url="https://opencode.ai/zen/go/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "anthropic_messages"
+        assert result.base_url == "https://opencode.ai/zen/go"
+
+    def test_switch_to_glm_leaves_v1_intact(self):
+        """OpenAI-compatible models (GLM, Kimi, MiMo) keep /v1."""
+        result = _run_opencode_switch(
+            raw_input="glm-5.1",
+            current_provider="opencode-go",
+            current_model="minimax-m2.7",
+            current_base_url="https://opencode.ai/zen/go",  # stripped from previous Anthropic model
+            runtime_base_url="https://opencode.ai/zen/go/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "chat_completions"
+        assert result.base_url == "https://opencode.ai/zen/go/v1", (
+            f"chat_completions must keep /v1; got {result.base_url}"
+        )
+
+    def test_switch_to_kimi_leaves_v1_intact(self):
+        result = _run_opencode_switch(
+            raw_input="kimi-k2.5",
+            current_provider="opencode-go",
+            current_model="glm-5",
+            current_base_url="https://opencode.ai/zen/go/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "chat_completions"
+        assert result.base_url == "https://opencode.ai/zen/go/v1"
+
+    def test_trailing_slash_also_stripped(self):
+        """``/v1/`` with trailing slash is also stripped cleanly."""
+        result = _run_opencode_switch(
+            raw_input="minimax-m2.7",
+            current_provider="opencode-go",
+            current_model="glm-5",
+            current_base_url="https://opencode.ai/zen/go/v1/",
+        )
+
+        assert result.success
+        assert result.api_mode == "anthropic_messages"
+        assert result.base_url == "https://opencode.ai/zen/go"
+
+
+class TestOpenCodeZenV1Strip:
+    """OpenCode Zen: ``/model claude-*`` must strip /v1."""
+
+    def test_switch_to_claude_sonnet_strips_v1(self):
+        """Gemini → Claude on opencode-zen: /v1 stripped."""
+        result = _run_opencode_switch(
+            raw_input="claude-sonnet-4-6",
+            current_provider="opencode-zen",
+            current_model="gemini-3-flash",
+            current_base_url="https://opencode.ai/zen/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "anthropic_messages"
+        assert result.base_url == "https://opencode.ai/zen"
+
+    def test_switch_to_gemini_leaves_v1_intact(self):
+        """Gemini on opencode-zen stays on chat_completions with /v1."""
+        result = _run_opencode_switch(
+            raw_input="gemini-3-flash",
+            current_provider="opencode-zen",
+            current_model="claude-sonnet-4-6",
+            current_base_url="https://opencode.ai/zen",  # stripped from previous Claude
+            runtime_base_url="https://opencode.ai/zen/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "chat_completions"
+        assert result.base_url == "https://opencode.ai/zen/v1"
+
+    def test_switch_to_gpt_uses_codex_responses_keeps_v1(self):
+        """GPT on opencode-zen uses codex_responses api_mode — /v1 kept."""
+        result = _run_opencode_switch(
+            raw_input="gpt-5.4",
+            current_provider="opencode-zen",
+            current_model="claude-sonnet-4-6",
+            current_base_url="https://opencode.ai/zen",
+            runtime_base_url="https://opencode.ai/zen/v1",
+        )
+
+        assert result.success
+        assert result.api_mode == "codex_responses"
+        assert result.base_url == "https://opencode.ai/zen/v1"
+
+
+class TestAgentSwitchModelDefenseInDepth:
+    """run_agent.AIAgent.switch_model() also strips /v1 as defense-in-depth."""
+
+    def test_agent_switch_model_strips_v1_for_anthropic_messages(self):
+        """Even if a caller hands in a /v1 URL, the agent strips it."""
+        from run_agent import AIAgent
+
+        # Build a bare agent instance without running __init__; we only want
+        # to exercise switch_model's base_url normalization logic.
+        agent = AIAgent.__new__(AIAgent)
+        agent.model = "glm-5"
+        agent.provider = "opencode-go"
+        agent.base_url = "https://opencode.ai/zen/go/v1"
+        agent.api_key = "sk-opencode-fake"
+        agent.api_mode = "chat_completions"
+        agent._client_kwargs = {}
+
+        # Intercept the expensive client rebuild — we only need to verify
+        # that base_url was normalized before it reached the Anthropic
+        # client factory.
+        captured = {}
+
+        def _fake_build_anthropic_client(api_key, base_url):
+            captured["api_key"] = api_key
+            captured["base_url"] = base_url
+            return object()  # placeholder client — no real calls expected
+
+        # The downstream cache/plumbing touches a bunch of private state
+        # that wasn't initialized above; we don't want to rebuild the full
+        # runtime for this single assertion, so short-circuit after the
+        # strip by raising inside the stubbed factory.
+        class _Sentinel(Exception):
+            pass
+
+        def _raise_after_capture(api_key, base_url):
+            captured["api_key"] = api_key
+            captured["base_url"] = base_url
+            raise _Sentinel("strip verified")
+
+        with patch(
+            "agent.anthropic_adapter.build_anthropic_client",
+            side_effect=_raise_after_capture,
+        ), patch("agent.anthropic_adapter.resolve_anthropic_token", return_value=""), patch(
+            "agent.anthropic_adapter._is_oauth_token", return_value=False
+        ):
+            with pytest.raises(_Sentinel):
+                agent.switch_model(
+                    new_model="minimax-m2.7",
+                    new_provider="opencode-go",
+                    api_key="sk-opencode-fake",
+                    base_url="https://opencode.ai/zen/go/v1",
+                    api_mode="anthropic_messages",
+                )
+
+        assert captured.get("base_url") == "https://opencode.ai/zen/go", (
+            f"agent.switch_model did not strip /v1; passed {captured.get('base_url')} "
+            "to build_anthropic_client"
+        )