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fix(anthropic): restrict fast mode to Opus 4.6 (Anthropic API contract)

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Per https://platform.claude.com/docs/en/build-with-claude/fast-mode:
"Fast mode is currently supported on Opus 4.6 only. Sending speed: fast
with an unsupported model returns an error."

Pre-fix, _is_anthropic_fast_model() returned True for any claude-* model,
so /fast on Opus 4.7 (or Sonnet/Haiku) would persist agent.service_tier=fast
in config.yaml and the adapter would inject extra_body["speed"] = "fast"
on every subsequent request. Opus 4.7 returns:

  HTTP 400: 'claude-opus-4-7' does not support the `speed` parameter.

This wedged sessions across model upgrades (a user who ran /fast on Opus 4.6
and later switched the default model to 4.7 hit a hard 400 on every turn
until they manually edited config.yaml).

Changes:
- _is_anthropic_fast_model: gate on "opus-4-6" / "opus-4.6" only
- anthropic_adapter: add _supports_fast_mode predicate as defensive guard
  so stale request_overrides on an unsupported model are dropped silently
  instead of 400'ing
- Tests: flip the assertions that mirrored the bug (Sonnet/Haiku/Opus 4.7
  asserting fast-mode support) to match the documented API contract
This commit is contained in:
bobashopcashier 2026-05-03 21:02:16 -07:00 • committed by kshitij
parent a7417f8a4a
commit d89e7a3cd4
4 changed files with 138 additions and 24 deletions
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24
agent/anthropic_adapter.py
View file

@ -76,6 +76,7 @@ _ADAPTIVE_THINKING_SUBSTRINGS = ("4-6", "4.6", "4-7", "4.7")
# Models where temperature/top_p/top_k return 400 if set to non-default values.
# This is the Opus 4.7 contract; future 4.x+ models are expected to follow it.
_NO_SAMPLING_PARAMS_SUBSTRINGS = ("4-7", "4.7")
_FAST_MODE_SUPPORTED_SUBSTRINGS = ("opus-4-6", "opus-4.6")
# ── Max output token limits per Anthropic model ───────────────────────
# Source: Anthropic docs + Cline model catalog. Anthropic's API requires
@ -219,6 +220,17 @@ def _forbids_sampling_params(model: str) -> bool:
return any(v in model for v in _NO_SAMPLING_PARAMS_SUBSTRINGS)
def _supports_fast_mode(model: str) -> bool:
"""Return True for models that support Anthropic Fast Mode (speed=fast).
Per Anthropic docs, fast mode is currently supported on Opus 4.6 only.
Sending ``speed: "fast"`` to any other Claude model (including Opus 4.7)
returns HTTP 400. This guard prevents silently 400'ing when stale config
or older callers leave fast mode enabled across a model upgrade.
"""
return any(v in model for v in _FAST_MODE_SUPPORTED_SUBSTRINGS)
# Beta headers for enhanced features (sent with ALL auth types).
# As of Opus 4.7 (2026-04-16), the first two are GA on Claude 4.6+ — the
# beta headers are still accepted (harmless no-op) but not required. Kept
@ -1932,9 +1944,15 @@ def build_anthropic_kwargs(
# ── Fast mode (Opus 4.6 only) ────────────────────────────────────
# Adds extra_body.speed="fast" + the fast-mode beta header for ~2.5x
# output speed. Only for native Anthropic endpoints — third-party
# providers would reject the unknown beta header and speed parameter.
if fast_mode and not _is_third_party_anthropic_endpoint(base_url):
# output speed. Per Anthropic docs, fast mode is only supported on
# Opus 4.6 — Opus 4.7 and other models 400 on the speed parameter.
# Only for native Anthropic endpoints — third-party providers would
# reject the unknown beta header and speed parameter.
if (
fast_mode
and not _is_third_party_anthropic_endpoint(base_url)
and _supports_fast_mode(model)
):
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).

14
hermes_cli/models.py
View file

@ -1740,10 +1740,20 @@ def model_supports_fast_mode(model_id: Optional[str]) -> bool:
def _is_anthropic_fast_model(model_id: Optional[str]) -> bool:
"""Return True if the model is a Claude model eligible for Anthropic Fast Mode."""
"""Return True if the model is a Claude model eligible for Anthropic Fast Mode.
Fast mode is currently supported on Claude Opus 4.6 only. Per Anthropic's
docs (https://platform.claude.com/docs/en/build-with-claude/fast-mode):
"Fast mode is currently supported on Opus 4.6 only. Sending speed: fast
with an unsupported model returns an error." Opus 4.7 explicitly rejects
the ``speed`` parameter with HTTP 400.
"""
raw = _strip_vendor_prefix(str(model_id or ""))
base = raw.split(":")[0]
return base.startswith("claude-")
if not base.startswith("claude-"):
return False
# Only Opus 4.6 supports fast mode at present.
return "opus-4-6" in base or "opus-4.6" in base
def resolve_fast_mode_overrides(model_id: Optional[str]) -> dict[str, Any] | None:

39
tests/agent/test_anthropic_adapter.py
View file

@ -1113,6 +1113,45 @@ class TestBuildAnthropicKwargs:
assert _forbids_sampling_params("claude-opus-4-6") is False
assert _forbids_sampling_params("claude-sonnet-4-5") is False
def test_supports_fast_mode_predicate(self):
"""Fast mode is Opus 4.6 only — Opus 4.7 and others must be excluded."""
from agent.anthropic_adapter import _supports_fast_mode
assert _supports_fast_mode("claude-opus-4-6") is True
assert _supports_fast_mode("anthropic/claude-opus-4-6") is True
assert _supports_fast_mode("claude-opus-4-7") is False
assert _supports_fast_mode("claude-sonnet-4-6") is False
assert _supports_fast_mode("claude-haiku-4-5") is False
assert _supports_fast_mode("") is False
def test_fast_mode_omitted_for_unsupported_model(self):
"""fast_mode=True on Opus 4.7 must NOT inject speed=fast (API 400s)."""
kwargs = build_anthropic_kwargs(
model="claude-opus-4-7",
messages=[{"role": "user", "content": "hi"}],
tools=None,
max_tokens=1024,
reasoning_config=None,
fast_mode=True,
)
# extra_body either absent or doesn't carry "speed"
assert "speed" not in kwargs.get("extra_body", {})
# No fast-mode beta header should be added either
beta_header = (kwargs.get("extra_headers") or {}).get("anthropic-beta", "")
assert "fast-mode-2026-02-01" not in beta_header
def test_fast_mode_still_applied_on_opus_46(self):
"""Regression guard — fast mode must still work on Opus 4.6."""
kwargs = build_anthropic_kwargs(
model="claude-opus-4-6",
messages=[{"role": "user", "content": "hi"}],
tools=None,
max_tokens=1024,
reasoning_config=None,
fast_mode=True,
)
assert kwargs.get("extra_body", {}).get("speed") == "fast"
assert "fast-mode-2026-02-01" in kwargs["extra_headers"]["anthropic-beta"]
def test_reasoning_disabled(self):
kwargs = build_anthropic_kwargs(
model="claude-sonnet-4-20250514",

85
tests/cli/test_fast_command.py
View file

@ -128,17 +128,34 @@ class TestPriorityProcessingModels(unittest.TestCase):
assert model_supports_fast_mode(model), f"{model} should support fast mode"
def test_all_anthropic_models_supported(self):
"""Per Anthropic docs, fast mode is currently Opus 4.6 only.
Sending speed=fast to Opus 4.7, Sonnet, or Haiku returns HTTP 400.
Pre-fix this test asserted all Claude variants supported fast mode,
which mirrored the bug rather than the API contract.
"""
from hermes_cli.models import model_supports_fast_mode
# All Claude models support Anthropic Fast Mode — Opus, Sonnet, Haiku.
# Supported: Opus 4.6 in any form
supported = [
"claude-opus-4-7", "claude-opus-4-6", "claude-opus-4.6",
"claude-sonnet-4-6", "claude-sonnet-4.6", "claude-sonnet-4",
"claude-haiku-4-5", "claude-3-5-haiku",
"claude-opus-4-6", "claude-opus-4.6",
"anthropic/claude-opus-4-6", "anthropic/claude-opus-4.6",
]
for model in supported:
assert model_supports_fast_mode(model), f"{model} should support fast mode"
# Unsupported per Anthropic API: Opus 4.7, Sonnet, Haiku
unsupported = [
"claude-opus-4-7",
"claude-sonnet-4-6", "claude-sonnet-4.6", "claude-sonnet-4",
"claude-haiku-4-5", "claude-3-5-haiku",
]
for model in unsupported:
assert not model_supports_fast_mode(model), (
f"{model} should NOT support fast mode — Anthropic restricts "
f"speed=fast to Opus 4.6"
)
def test_codex_models_excluded(self):
"""Codex models route through Responses API and don't accept service_tier."""
from hermes_cli.models import model_supports_fast_mode
@ -257,18 +274,20 @@ class TestAnthropicFastMode(unittest.TestCase):
assert model_supports_fast_mode("anthropic/claude-opus-4-6") is True
assert model_supports_fast_mode("anthropic/claude-opus-4.6") is True
def test_anthropic_all_claude_models_supported(self):
def test_anthropic_non_opus46_models_excluded(self):
"""Anthropic restricts fast mode to Opus 4.6 — others must be excluded.
Per https://platform.claude.com/docs/en/build-with-claude/fast-mode,
sending speed=fast to Opus 4.7, Sonnet, or Haiku returns HTTP 400.
"""
from hermes_cli.models import model_supports_fast_mode
# All Claude models support fast mode — Opus, Sonnet, Haiku.
# The anthropic adapter gates speed=fast on native Anthropic
# endpoints only, so third-party proxies that reject the beta
# are protected downstream (see _is_third_party_anthropic_endpoint).
assert model_supports_fast_mode("claude-sonnet-4-6") is True
assert model_supports_fast_mode("claude-sonnet-4.6") is True
assert model_supports_fast_mode("claude-haiku-4-5") is True
assert model_supports_fast_mode("claude-opus-4-7") is True
assert model_supports_fast_mode("anthropic/claude-sonnet-4.6") is True
assert model_supports_fast_mode("claude-sonnet-4-6") is False
assert model_supports_fast_mode("claude-sonnet-4.6") is False
assert model_supports_fast_mode("claude-haiku-4-5") is False
assert model_supports_fast_mode("claude-opus-4-7") is False
assert model_supports_fast_mode("anthropic/claude-sonnet-4.6") is False
assert model_supports_fast_mode("anthropic/claude-opus-4-7") is False
def test_non_claude_models_not_anthropic_fast(self):
"""Non-Claude models should not be treated as Anthropic fast-mode."""
@ -294,6 +313,17 @@ class TestAnthropicFastMode(unittest.TestCase):
result = resolve_fast_mode_overrides("anthropic/claude-opus-4.6")
assert result == {"speed": "fast"}
def test_resolve_overrides_returns_none_for_unsupported_claude(self):
"""Opus 4.7 and other Claude models don't support fast mode (API 400s).
Per Anthropic docs, fast mode is currently Opus 4.6 only.
"""
from hermes_cli.models import resolve_fast_mode_overrides
assert resolve_fast_mode_overrides("claude-opus-4-7") is None
assert resolve_fast_mode_overrides("claude-sonnet-4-6") is None
assert resolve_fast_mode_overrides("claude-haiku-4-5") is None
def test_resolve_overrides_returns_service_tier_for_openai(self):
"""OpenAI models should still get service_tier, not speed."""
from hermes_cli.models import resolve_fast_mode_overrides
@ -302,13 +332,21 @@ class TestAnthropicFastMode(unittest.TestCase):
assert result == {"service_tier": "priority"}
def test_is_anthropic_fast_model(self):
"""Fast mode is currently Opus 4.6 only — other Claude variants must be excluded."""
from hermes_cli.models import _is_anthropic_fast_model
# Supported: Opus 4.6 in any form
assert _is_anthropic_fast_model("claude-opus-4-6") is True
assert _is_anthropic_fast_model("claude-opus-4.6") is True
assert _is_anthropic_fast_model("claude-sonnet-4-6") is True
assert _is_anthropic_fast_model("claude-haiku-4-5") is True
assert _is_anthropic_fast_model("anthropic/claude-opus-4-6") is True
assert _is_anthropic_fast_model("claude-opus-4.6:fast") is True
# Unsupported per Anthropic API contract — would 400 if we sent speed=fast
assert _is_anthropic_fast_model("claude-opus-4-7") is False
assert _is_anthropic_fast_model("claude-sonnet-4-6") is False
assert _is_anthropic_fast_model("claude-haiku-4-5") is False
# Non-Claude
assert _is_anthropic_fast_model("gpt-5.4") is False
assert _is_anthropic_fast_model("") is False
@ -320,14 +358,23 @@ class TestAnthropicFastMode(unittest.TestCase):
)
assert cli_mod.HermesCLI._fast_command_available(stub) is True
def test_fast_command_exposed_for_anthropic_sonnet(self):
"""Sonnet now supports Anthropic Fast Mode — the adapter gates on base_url."""
def test_fast_command_hidden_for_anthropic_sonnet(self):
"""Sonnet doesn't support fast mode (Opus 4.6 only) — /fast must be hidden."""
cli_mod = _import_cli()
stub = SimpleNamespace(
provider="anthropic", requested_provider="anthropic",
model="claude-sonnet-4-6", agent=None,
)
assert cli_mod.HermesCLI._fast_command_available(stub) is True
assert cli_mod.HermesCLI._fast_command_available(stub) is False
def test_fast_command_hidden_for_anthropic_opus_47(self):
"""Opus 4.7 doesn't support fast mode — /fast must be hidden."""
cli_mod = _import_cli()
stub = SimpleNamespace(
provider="anthropic", requested_provider="anthropic",
model="claude-opus-4-7", agent=None,
)
assert cli_mod.HermesCLI._fast_command_available(stub) is False
def test_fast_command_hidden_for_non_claude_non_openai(self):
"""Non-Claude, non-OpenAI models should not expose /fast."""