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🤖 fix: send drop_block on the Coder AI Gateway route to anthropic-typed instances - #6765
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…ed instances Opus/Sonnet/Haiku 5.5 and Fable 5.1 bind replayed thinking to the system prompt and tool list. On the Coder gateway route Xum did not send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"`, so every prefix change (an app restart that changes the MCP tool list, a plugin consent upgrade, an MCP schema change) cost each conversation one 400 and a full strip-and-retry request. The block-binding check now also accepts the `coder` route when the model reaches an instance whose type is exactly "anthropic" (the same instance check between_tools uses, moved into `anthropicTypedCoderRoute`). The request beta opt-out still turns it off. Bedrock-typed and other instances stay excluded. Live probe through dogfood `claude-aws-us-east-2` (2026-10-15): all four models return the 400 without drop_block and 200 with it plus the beta header (`input_transformations: thinking_dropped`). Adding block_binding to an unchanged conversation keeps the whole prompt cache. --- _Generated with `xum` • Model: `anthropic:claude-opus-5-5` • Thinking: `high`_ <!-- mux-attribution: model=anthropic:claude-opus-5-5 thinking=high -->
--- _Generated with `xum` • Model: `anthropic:claude-opus-5-5` • Thinking: `high`_ <!-- mux-attribution: model=anthropic:claude-opus-5-5 thinking=high -->
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…outes The Coder route's block-binding check read only the request-level `disableBetaFeatures`. When `providers.anthropic.disableBetaFeatures` is true but persisted request options carry `false`, the factory keeps the `false` (`?? true`), so Xum sent block_binding and its beta header despite the backend-authoritative ZDR opt-out. The check now consults both sources, as the direct route does. The coder provider has no opt-out of its own (providerService surfaces disableBetaFeatures for anthropic only). --- _Generated with `xum` • Model: `anthropic:claude-opus-5-5` • Thinking: `high`_ <!-- mux-attribution: model=anthropic:claude-opus-5-5 thinking=high -->
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Summary
On the Coder AI Gateway route, Xum now sends Anthropic's
drop_blockthinking-binding control to anthropic-typed instances. Anthropic then drops a replayed thinking block whose prefix changed and answers 200, instead of rejecting the whole request with a signature 400 that Xum must repair and retry. Before this PR, only the direct Anthropic route sent it.Background
Opus, Sonnet and Haiku 5.5 and Fable 5.1 bind replayed thinking to the system prompt and tool list before it. When either changes, a replayed block fails its signature check. On one host the 400s ran at about 0.128 per Anthropic turn, and they clustered in the 20 minutes after each app start (31, 20, 28, 17 and 14 per start on Oct 14).
Comparison of each workspace's last request before an app start with its first request after it (8 starts, 281 workspaces, from
turn-enveloperows indurable-events.jsonl): the system prompt or tool list changed in 95 of 281 (34%), against 195 of 2,590 (7.5%) between two turns in one process.mcp_prompt_getmissing or added on the first turnThis PR does not stop these prefix changes. It removes the rejected request and the retry that each of them cost.
Implementation
anthropicBlockBindingRouteAvailablekeeps the direct-route check unchanged. On thecoderroute it allowsdrop_blockwhen the model reaches an instance whose type is exactlyanthropic, and the request-level beta opt-out still turns it off. Bedrock-typed and other instances stay excluded.anthropicTypedCoderRoute, whichbetween_toolseligibility already used, so both decisions share it.thinking-binding-controls-2026-08-01beta header together withthinking.blockBinding.Validation
providerOptions.test.ts:coder:anthropic/…ids andanthropic:ids routed onto Coder throughcanonicalRoutesgetdrop_block. Bedrock-typed instances and the beta opt-out do not. The test fails with the source change reverted.claude-aws-us-east-2(raw Messages API, about $0.10), for Opus 5.5, Sonnet 5.5, Haiku 5.5 and Fable 5.1:drop_blockreturns the same 400 as in the app log.drop_blockand the beta header returns 200 and reportsinput_transformations: [{type: thinking_dropped, reason: prefix_binding_mismatch}].drop_blockreturns 200 and drops nothing.block_bindingto an unchanged conversation read the whole cache (Haiku: 21,710 of 21,710 tokens), so turning it on costs no cache miss.Risks
Low to medium. The change affects every Claude 5.5 and Fable 5.1 request with thinking on the
coderroute to an anthropic-typed instance.block_bindingwith "Extra inputs are not permitted". The gateway passes the header through for Anthropic upstreams, and its Bedrock path converts or strips the header and the field together (fix: retry Anthropic thinking binding errors with drop_block coder#29792). Bedrock-typed instances are excluded anyway. An older gateway was not tested live.Generated with
xum• Model:anthropic:claude-opus-5-5• Thinking:high• Cost:$437.19