[aheritier]

Closes #4652. Part of #4583 and epic #4580.

Stacked on #4657 (base branch feat/claude5x-model-table). This PR's diff shows only its own three commits; GitHub retargets it to main when #4657 merges.

What

Refuse two Claude 5.x configurations that docker-agent cannot serve correctly yet, at client construction and before any inference call:

1. Server-side fallbacks involving Claude 5.x are rejected when the primary or any fallback is one of the nine 5.x models (gateway and Vertex spellings included), until feat(Claude): end-to-end server fallback for Claude 5.x #4584 handles a mid-response model switch. The error points to feat(Claude): end-to-end server fallback for Claude 5.x #4584. Absent or empty lists and chains without a 5.x model are unchanged. Malformed lists stay ignored, as before.
2. claude-mythos-5 on Bedrock is refused. AWS serves it only through Bedrock Mantle, not the Converse API used here. claude-mythos-5-1 still works. The model table gains a BedrockMantleOnly field.

Both checks run where every construction path already validates options: validateThinkingOptions for Anthropic Direct, Vertex and Model Garden, and bedrock.NewClient for Bedrock. Clones, routing and gateway branches all pass through them.

⚠️ Breaking change

Fable 5/5.1 with fallbacks (the headline refusal-fallback use case) now fails at startup. This follows the rollout policy approved in #4580: the request is refused rather than half-supported until #4584. examples/anthropic-fallbacks.yaml and the docs example now use claude-opus-4-8 → claude-opus-4-7, and the docs carry a warning callout.

Not in this PR

Making thinking_prefix_mismatch: error the 5.x default moved to #4653. The runtime edits the conversation prefix (num_history_items trimming, hooks, compaction), so error would break those sessions until signed history can be replayed exactly. Both issues are updated.

Validation

• task build passes.
• golangci-lint v2.14.0 reports 0 issues, and go run ./lint . reports no offenses.
• go test ./e2e -run '^TestWasmProviderDependencies$|^TestDependencies$' passes.
• task test fails only the 10 pre-existing pkg/teamloader TestLoadExamples Gemini cases (GOOGLE_API_KEY or GEMINI_API_KEY environment variable is required under .env.test). They fail identically on main.
• An independent review found no critical or important issues; its three minor findings are fixed in 782564e.

[Copilot]

🟢 Approval recommended

The scoped safeguards use existing construction validation paths, preserve permitted configurations, and have focused regression coverage with no unresolved blocking findings.

No open issues 🎉

What changed in this PR

Adds startup safeguards for unsupported Claude 5.x configurations, building on the request-policy changes in #4655.

Changes:

• Rejects non-empty fallback chains involving known Claude 5.x models until #4584.
• Refuses Mythos 5 on Bedrock Converse while allowing Mythos 5.1.
• Adds regression tests and updates documentation, schema descriptions, and fallback examples.

File	Description
pkg/​runtime/​structured_output.go	Stops unsupported Claude 5.x reminder retries.
pkg/​runtime/​structured_output_test.go	Tests failure after one plain-text response.
pkg/​runtime/​loop.go	Passes model identity to structured-output handling.
pkg/​modelinfo/​thinking_levels.go	Enables Claude 5.x effort levels.
pkg/​modelinfo/​modelinfo.go	Uses explicit Claude 5.x capabilities.
pkg/​modelinfo/​claude5.go	Defines model policies and Mantle-only status.
pkg/​modelinfo/​claude5_test.go	Tests model recognition and policies.
pkg/​model/​provider/​providerutil/​prefill.go	Detects final assistant prefills.
pkg/​model/​provider/​providerutil/​prefill_test.go	Tests prefill detection.
pkg/​model/​provider/​providers/​claude5_requests_test.go	Checks requests through provider factories.
pkg/​model/​provider/​factory.go	Warns when thinking cannot be disabled.
pkg/​model/​provider/​factory_test.go	Tests warning conditions.
pkg/​model/​provider/​bedrock/​client.go	Enforces Claude policies and refuses Mythos 5.
pkg/​model/​provider/​bedrock/​client_test.go	Tests request policies and Mythos restrictions.
pkg/​model/​provider/​anthropic/​vertex/​vertex_test.go	Tests fallback rejection on Vertex.
pkg/​model/​provider/​anthropic/​thinking.go	Preserves thinking intent and default effort.
pkg/​model/​provider/​anthropic/​thinking_binding.go	Adds fallback validation during construction.
pkg/​model/​provider/​anthropic/​thinking_binding_test.go	Updates binding expectations and warning coverage.
pkg/​model/​provider/​anthropic/​system_context_test.go	Uses a legacy model for prefill coverage.
pkg/​model/​provider/​anthropic/​features_integration_test.go	Tests thinking, prefills, and fallback restrictions.
pkg/​model/​provider/​anthropic/​fallbacks.go	Rejects fallback chains involving Claude 5.x.
pkg/​model/​provider/​anthropic/​fallbacks_test.go	Covers rejected and permitted fallback configurations.
pkg/​model/​provider/​anthropic/​client.go	Rejects Claude 5.x assistant prefills locally.
pkg/​model/​provider/​anthropic/​capabilities.go	Applies explicit model capabilities.
pkg/​model/​provider/​anthropic/​capabilities_test.go	Tests capability predicates.
examples/​anthropic-fallbacks.yaml	Switches to a permitted legacy fallback chain.
docs/​providers/​bedrock/​index.md	Documents policies and Mantle restrictions.
docs/​providers/​anthropic/​index.md	Documents Claude 5.x behavior and fallback refusal.
agent-schema.json	Updates provider-option descriptions.

_{Review effort: Balanced}

[aheritier]

Superseded by #4658: same commits, moved to a docker/docker-agent branch because GitHub stacks cannot include fork PRs.