[Bug]: Support Anthropic for Claude Opus 5.5 #414

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1. airlsyn

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   bugSomething isn't workingSomething isn't working

   needs-triageAwaiting maintainer assessmentAwaiting maintainer assessment

   on Oct 1, 2026
2. github-actions

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   desktopMiniMax Code desktop appMiniMax Code desktop app

   on Oct 1, 2026
3. hetaoBackend commented on Oct 1, 2026

   

   hetaoBackend

   on Oct 1, 2026

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   This was generated by AI during triage.

   @airlsyn — this is an excellent, precise report, and I verified your capability matrix row by row against current main. Your diagnosis is correct: on main, compaction breaks prefix-bound thinking on Opus 5.5, and the missing capability is real.

   Verification against main

   I checked each capability you listed, in third_party/pi-mono/packages/ai/src/providers/anthropic.ts on main:

   Capability	Your matrix	Verified on main
   Claude Opus 5.5 support	✅	✅ present
   Adaptive thinking	✅	✅ present (ThinkingLevel → effort mapping for adaptive models)
   Thinking signature replay	✅	✅ present (signature_delta handling)
   Signature-only thinking blocks	✅	✅ present
   thinking.block_binding	❌	❌ confirmed missing
   prefix_mismatch_behavior	❌	❌ confirmed missing
   thinking-binding-controls-2026-08-01	❌	❌ confirmed missing (no such beta header)
   input_transformations handling	❌	❌ confirmed missing
   Prefix-binding mismatch recovery	❌	❌ confirmed missing

   So your matrix is accurate, and your causal chain is right: preserved-thinking blocks are bound to the exact request prefix (system prompt, tools, earlier messages); a history rewrite like context compaction changes that prefix; without block_binding.prefix_mismatch_behavior: "drop_block" the API rejects the request instead of dropping the stale signed thinking — producing prefix_binding_mismatch, and surfacing as the error you're seeing. The fact that it works until the first "Context compacted" is exactly the signature of this bug.

   One clarification worth adding

   It's worth separating the two error strings you captured, because they have different causes:

   • prefix_binding_mismatch — the real, fixable bug on our side. It's a compatibility gap: the vendored pi is behind the thinking-binding-controls beta. This is the one that breaks you specifically after compaction, and it's what the fix below addresses.
   • "flagged that it may violate Anthropic's Terms of Service restrictions…" — that one is Anthropic-side policy enforcement on the request, separate from the prefix binding, and not something a client change can resolve.

   So the compaction failure you diagnosed is ours to fix; if you also hit the ToS string independently of compaction, that's a different (and non-actionable-on-our-side) condition.

   Status: a fix appears to be in progress locally

   Checking the working tree, there's an uncommitted, in-progress fix on a local branch fix/anthropic-thinking-block-binding that implements exactly this:

   • adds THINKING_BINDING_CONTROLS_BETA = "thinking-binding-controls-2026-08-01"
   • adds THINKING_BLOCK_BINDING_MODEL_ID matching claude-opus-5.5 / claude-sonnet-5.5 (and fable/mythos 5.1)
   • supportsThinkingBlockBinding() — on by default for those model IDs, overridable via a new compat.supportsThinkingBlockBinding
   • withThinkingBlockBinding() — injects block_binding: { prefix_mismatch_behavior: "drop_block" } on adaptive thinking, deliberately applied after host payload hooks and preserving a host-chosen block_binding

   There's no open upstream PR for it yet. Since external PRs are collaborator-restricted, the same path as #149 / #364 applies: land it, run pnpm verify, and post the commit here (or open it if you have collaborator access) so it can be brought in with attribution. If you'd rather file it as your own contribution, say so and we'll coordinate.

   Classification

   • Category: bug — a compatibility gap in the vendored Anthropic transport, reproducible and already diagnosed. (Note this is distinct from [Bug]: can't request Opus 5.5 succeed #387, where the request was simply "support Opus 5.5" and the failure was an upstream client-authenticity gate; here the compaction-specific failure is ours.)
   • State: needs-triage — fix is understood and in progress; needs a maintainer to land/merge it. (This tracker has no ready-for-agent label, so needs-triage + this note is the routing.)

   If after landing the fix you still see a failure immediately after compaction on Opus 5.5, please follow up with the new client logs — that would mean the binding fix isn't taking effect on the wire, and it's worth knowing. Your comparison against latest Pi and OpenCode is exactly the right reference; thanks for the matrix — it made verification fast.

   No credentials or sensitive data echoed.
4. Christian-Sidak commented on Oct 1, 2026

   

   Christian-Sidak

   on Oct 1, 2026

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   Here is the fix for the prefix_binding_mismatch / context compaction failure on Opus 5.5 and Sonnet 5.5.

   Branch: https://github.com/Christian-Sidak/minimax-code/tree/fix/issue-414
   Commit: Christian-Sidak@a0b7beb

   What the fix does (4 files changed):

   third_party/pi-mono/packages/ai/src/types.ts

   • Adds supportsThinkingBlockBinding?: boolean to AnthropicMessagesCompat

   third_party/pi-mono/packages/ai/src/providers/anthropic.ts

   • Adds THINKING_BINDING_CONTROLS_BETA = "thinking-binding-controls-2026-08-01" constant
   • Adds THINKING_BLOCK_BINDING_MODEL_IDS = ["claude-opus-5-5", "claude-sonnet-5-5", "claude-fable-5-1", "claude-mythos-5-1"]
   • Adds supportsThinkingBlockBinding(model) helper (auto-detects by model ID, overridable via compat.supportsThinkingBlockBinding)
   • Sends the thinking-binding-controls-2026-08-01 beta header for those models (in createClient)
   • Injects thinking.block_binding.prefix_mismatch_behavior: "drop_block" on adaptive thinking payloads for those models (in buildParams)
   • Adds the 5.5/5.1 model IDs to DEFAULT_THINKING_MODEL_IDS

   third_party/pi-mono/packages/ai/src/index.ts

   • Exports THINKING_BLOCK_BINDING_MODEL_IDS and supportsThinkingBlockBinding

   third_party/pi-mono/packages/ai/test/anthropic-thinking-block-binding.test.ts

   • Unit tests covering: model ID detection, compat overrides, beta header wiring, block_binding payload injection, and negative cases

   The fix matches the capability matrix from the original report.

   👍React with 👍1Reacted by airlsyn