fix(anthropic): reactive recovery for OAuth 1M-context beta rejection (#17752)

Keep context-1m-2025-08-07 in OAuth requests by default so 1M-capable
subscriptions retain full context. When Anthropic rejects a request with
400 'long context beta is not yet available for this subscription',
disable the beta for the rest of the session, rebuild the client, and
retry once.

Addresses #17680 (thanks @JayGwod for the clean reproduction) without
forcing every OAuth user off the 1M context window.

Changes:
- agent/error_classifier.py: new FailoverReason.oauth_long_context_beta_forbidden;
  pattern matches 400 + 'long context beta' + 'not yet available'. Narrow
  enough that the existing 429 tier-gate pattern keeps its own reason.
- agent/anthropic_adapter.py: _common_betas_for_base_url,
  build_anthropic_client, build_anthropic_kwargs gain drop_context_1m_beta
  kwarg. Default=False (1M stays). OAuth OAUTH_ONLY_BETAS unchanged.
- agent/transports/anthropic.py: build_kwargs forwards the flag.
- run_agent.py: self._oauth_1m_beta_disabled flag, retry-once guard,
  recovery branch next to the image-shrink path. _rebuild_anthropic_client
  honors the flag. The main build_kwargs call site threads it through for
  fast-mode extra_headers.
- hermes_cli/doctor.py, hermes_cli/models.py: sibling OAuth /v1/models
  probes get the same reactive retry — previously they'd falsely report
  the Anthropic API as unreachable for affected subscriptions.

Tests: 2190 tests/agent/ + 94 adjacent integration tests pass. New unit
tests cover the classifier pattern (including the collision guard against
the 429 tier-gate) and the drop_context_1m_beta adapter behavior (default
keeps 1M, flag strips only 1M while preserving every other beta).

agent/anthropic_adapter.py

@@ -461,7 +461,11 @@ def _requires_bearer_auth(base_url: str | None) -> bool:
     return normalized.startswith(("https://api.minimax.io/anthropic", "https://api.minimaxi.com/anthropic"))
 
 
-def _common_betas_for_base_url(base_url: str | None) -> list[str]:
+def _common_betas_for_base_url(
+    base_url: str | None,
+    *,
+    drop_context_1m_beta: bool = False,
+) -> list[str]:
     """Return the beta headers that are safe for the configured endpoint.
 
     MiniMax's Anthropic-compatible endpoints (Bearer-auth) reject requests
@@ -472,14 +476,30 @@ def _common_betas_for_base_url(base_url: str | None) -> list[str]:
     The ``context-1m-2025-08-07`` beta is also stripped for Bearer-auth
     endpoints — MiniMax hosts its own models, not Claude, so the header is
     irrelevant at best and risks request rejection at worst.
+
+    ``drop_context_1m_beta=True`` additionally strips the 1M-context beta on
+    otherwise-unrelated endpoints. The OAuth retry path flips this flag after
+    a subscription rejects the beta with
+    "The long context beta is not yet available for this subscription" so
+    subsequent requests in the same session don't repeat the probe. See the
+    reactive recovery loop in ``run_agent.py`` and issue-comment history on
+    PR #17680 for the full rationale.
     """
     if _requires_bearer_auth(base_url):
         _stripped = {_TOOL_STREAMING_BETA, _CONTEXT_1M_BETA}
         return [b for b in _COMMON_BETAS if b not in _stripped]
+    if drop_context_1m_beta:
+        return [b for b in _COMMON_BETAS if b != _CONTEXT_1M_BETA]
     return _COMMON_BETAS
 
 
-def build_anthropic_client(api_key: str, base_url: str = None, timeout: float = None):
+def build_anthropic_client(
+    api_key: str,
+    base_url: str = None,
+    timeout: float = None,
+    *,
+    drop_context_1m_beta: bool = False,
+):
     """Create an Anthropic client, auto-detecting setup-tokens vs API keys.
 
     If *timeout* is provided it overrides the default 900s read timeout.  The
@@ -488,6 +508,12 @@ def build_anthropic_client(api_key: str, base_url: str = None, timeout: float =
     Anthropic-compatible providers respect the same knob as OpenAI-wire
     providers.
 
+    ``drop_context_1m_beta=True`` strips ``context-1m-2025-08-07`` from the
+    client-level ``anthropic-beta`` header. Used by the reactive OAuth retry
+    path in ``run_agent.py`` when a subscription rejects the beta; leave at
+    its default on fresh clients so 1M-capable subscriptions keep the
+    capability.
+
     Returns an anthropic.Anthropic instance.
     """
     _anthropic_sdk = _get_anthropic_sdk()
@@ -517,7 +543,10 @@ def build_anthropic_client(api_key: str, base_url: str = None, timeout: float =
             kwargs["default_query"] = {"api-version": "2025-04-15"}
         else:
             kwargs["base_url"] = normalized_base_url
-    common_betas = _common_betas_for_base_url(normalized_base_url)
+    common_betas = _common_betas_for_base_url(
+        normalized_base_url,
+        drop_context_1m_beta=drop_context_1m_beta,
+    )
 
     if _is_kimi_coding_endpoint(base_url):
         # Kimi's /coding endpoint requires User-Agent: claude-code/0.1.0
@@ -1689,6 +1718,7 @@ def build_anthropic_kwargs(
     context_length: Optional[int] = None,
     base_url: str | None = None,
     fast_mode: bool = False,
+    drop_context_1m_beta: bool = False,
 ) -> Dict[str, Any]:
     """Build kwargs for anthropic.messages.create().
 
@@ -1877,7 +1907,10 @@ def build_anthropic_kwargs(
         kwargs.setdefault("extra_body", {})["speed"] = "fast"
         # Build extra_headers with ALL applicable betas (the per-request
         # extra_headers override the client-level anthropic-beta header).
-        betas = list(_common_betas_for_base_url(base_url))
+        betas = list(_common_betas_for_base_url(
+            base_url,
+            drop_context_1m_beta=drop_context_1m_beta,
+        ))
         if is_oauth:
             betas.extend(_OAUTH_ONLY_BETAS)
         betas.append(_FAST_MODE_BETA)

agent/error_classifier.py

@@ -54,6 +54,7 @@ class FailoverReason(enum.Enum):
     # Provider-specific
     thinking_signature = "thinking_signature"  # Anthropic thinking block sig invalid
     long_context_tier = "long_context_tier"    # Anthropic "extra usage" tier gate
+    oauth_long_context_beta_forbidden = "oauth_long_context_beta_forbidden"  # Anthropic OAuth subscription rejects 1M context beta — disable beta and retry
 
     # Catch-all
     unknown = "unknown"                  # Unclassifiable — retry with backoff
@@ -450,6 +451,25 @@ def classify_api_error(
             should_compress=True,
         )
 
+    # Anthropic OAuth subscription rejects the 1M-context beta header.
+    # Observed error body: "The long context beta is not yet available for
+    # this subscription." Returned as HTTP 400 from native Anthropic when
+    # the subscription doesn't include 1M context, even though the request
+    # carries ``anthropic-beta: context-1m-2025-08-07``. The recovery path
+    # in run_agent.py rebuilds the Anthropic client with the beta stripped
+    # and retries once. Pattern is narrow enough that it won't collide with
+    # the 429 tier-gate pattern above (different status, different phrase).
+    if (
+        status_code == 400
+        and "long context beta" in error_msg
+        and "not yet available" in error_msg
+    ):
+        return _result(
+            FailoverReason.oauth_long_context_beta_forbidden,
+            retryable=True,
+            should_compress=False,
+        )
+
     # ── 2. HTTP status code classification ──────────────────────────
 
     if status_code is not None:

agent/transports/anthropic.py

@@ -58,6 +58,7 @@ class AnthropicTransport(ProviderTransport):
             context_length: int | None
             base_url: str | None
             fast_mode: bool
+            drop_context_1m_beta: bool
         """
         from agent.anthropic_adapter import build_anthropic_kwargs
 
@@ -73,6 +74,7 @@ class AnthropicTransport(ProviderTransport):
             context_length=params.get("context_length"),
             base_url=params.get("base_url"),
             fast_mode=params.get("fast_mode", False),
+            drop_context_1m_beta=params.get("drop_context_1m_beta", False),
         )
 
     def normalize_response(self, response: Any, **kwargs) -> NormalizedResponse: