fix(anthropic): drop rejected sampling params and relax forced tool choice on Fable 5.1 - #856
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…hoice on Fable 5.1
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📝 WalkthroughWalkthroughThe Anthropic provider now detects model-specific request restrictions. It removes rejected sampling parameters and relaxes forced tool choices with system instructions. Adaptive-thinking model support, tests, and documentation are updated. ChangesAnthropic capability handling
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The current revision is not merge-ready because the Fable 5.1 and Mythos 5.1 compatibility behavior may not activate for the documented model names, leaving requests vulnerable to the upstream failures this change is meant to prevent, and the documentation still omits a supported reasoning field. Separately, forced tool selection becomes best-effort on affected models, so callers that depend on guaranteed tool invocation must validate responses. Sequence Diagram(s)sequenceDiagram
participant GoModel
participant convertToAnthropicRequest
participant AnthropicAPI
GoModel->>convertToAnthropicRequest: Provide model and request options
convertToAnthropicRequest->>convertToAnthropicRequest: Remove unsupported sampling parameters
convertToAnthropicRequest->>convertToAnthropicRequest: Relax forced tool choice and add instruction
convertToAnthropicRequest->>AnthropicAPI: Send converted request
Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 31.25% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 3 files. (1 skipped: 1 unsupported.) ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
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Inline comments:
In `@docs/providers/anthropic.mdx`:
- Around line 115-118: Update the Anthropic model guidance around the
adaptive-thinking description to explicitly exempt Fable 5.1 and Mythos 5.1,
documenting that thinking remains enabled even when reasoning is omitted. Ensure
the model list and surrounding note consistently describe both models and their
reasoning-token-only response behavior without contradicting the existing
adaptive-model guidance.
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docs/providers/anthropic.mdxinternal/providers/anthropic/anthropic.gointernal/providers/anthropic/anthropic_test.gointernal/providers/anthropic/request_translation.go
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Confidence Score: 4/5Not ready to merge until Mythos 5.1 reasoning is translated using adaptive thinking. A focused executable harness directly observed the incorrect Mythos 5.1 payload and exercised the related Fable tool-choice and sampling paths successfully. Files Needing Attention: internal/providers/anthropic/anthropic.go needs the Mythos 5 model family added to adaptive-thinking recognition, with regression coverage in internal/providers/anthropic/anthropic_test.go.
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Reviews (1): Last reviewed commit: "fix(anthropic): drop rejected sampling p..." | Re-trigger Greptile |
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Actionable comments posted: 2
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Inline comments:
In `@docs/providers/anthropic.mdx`:
- Line 97: Update the Anthropic documentation sentence around the adaptive
thinking configuration to state that `thinking: {type: "adaptive"}` is used when
either `reasoning.effort` or `reasoning_effort` is provided, matching the two
supported reasoning forms.
- Line 99: Update the adaptive-thinking model classification used by
applyReasoning and its associated tests so isAdaptiveThinkingModel recognizes
claude-mythos-5 and claude-mythos-5-1, matching the documented adaptive-thinking
behavior. If those models are intentionally unsupported, instead revise the
documentation to describe their legacy enabled-thinking behavior.
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docs/providers/anthropic.mdx
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Inline comments:
In `@internal/providers/anthropic/anthropic.go`:
- Line 265: Update the adaptive-thinking and sampling-rejection prefix lists
near matches for claude-fable-5 and claude-mythos-5 to include claude-fable-5-1
and claude-mythos-5-1 explicitly. Ensure these prefixes match their exact IDs
and dated snapshots through matchesModelPrefix while preserving the existing
negative boundary behavior for IDs such as claude-opus-4-65.
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docs/providers/anthropic.mdxinternal/providers/anthropic/anthropic.gointernal/providers/anthropic/anthropic_test.go
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Verified live against
claude-fable-5-1: chat, streaming, Responses, native/v1/messages, passthrough, tool calling and cost metering all worked already, but three requests that OpenAI clients send routinely failed with an upstream 400.User-visible impact
temperatureandtop_poutright (`temperature` is deprecated for this model). GoModel now drops both for those models and logs the discarded values. Older models still receive them unchanged.tool_choiceany/tool. GoModel downgradesrequiredand{"type":"function",...}toautoand appends Anthropic's documented replacement, an instruction naming the tool, to the system prompt.parallel_tool_calls: falseis preserved. Fable 5 and every other model keep forced tool use as before.Tests
Table-driven tests cover the model prefix matchers, the sampling drop (Fable 5.1, Opus 4.7, Sonnet 4.6 control), and the tool-choice downgrade (required, named function, existing system prompt, parallel off, auto untouched, Fable 5 control).
go test ./internal/providers/..., contract tests andmake lintpass. Live check on Fable 5.1 after the change: all four previously failing requests return 200 and the model calls the tool.Summary by CodeRabbit
New Features
Bug Fixes
Documentation