[BUG] Claude Fable 5 (Vertex Anthropic) streams a response but no text is rendered — empty thinking block with signature-only delta #31738

Description

When using claude-fable-5 through the google-vertex-anthropic provider, the request succeeds, the model streams for ~13s, the agent loop completes cleanly (step=0 → step=1 → exiting loop), no error is logged, but no assistant text is rendered in the UI. Other Vertex Anthropic models with the same adaptive-reasoning format (e.g. claude-opus-4-8) work perfectly.

Root cause analysis

claude-fable-5 only supports the new adaptive-thinking API. Direct API testing confirms:

thinking.type.enabled → rejected: "thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
thinking.type.disabled → rejected: thinking cannot be disabled ("Thinking defaults to adaptive mode when not specified").
Valid output_config.effort values: low | medium | high | xhigh | max.
The model always returns a thinking content block first, and in the common case that block is empty with a signature only (no thinking_delta). Verified via streamRawPredict:

content_block_start index 0 -> { "type": "thinking", "thinking": "", "signature": "" }
content_block_delta index 0 -> { "type": "signature_delta", "signature": "..." } # no thinking_delta
content_block_stop index 0
content_block_start index 1 -> { "type": "text", "text": "" }
content_block_delta index 1 -> { "type": "text_delta", "text": "¡Hola! ..." }
...
message_delta -> stop_reason: "end_turn"
The text part (index 1) is present and well-formed. OpenCode’s handling appears to swallow the response when the leading thinking block is empty/signature-only for this model, so the text block is never surfaced. Behavior differs by config:

Manual model entry with "reasoning": true → on 1.16.2 the request 400s (thinking.type.enabled sent); on 1.17.1 no error but blank output.
Manual model entry with "reasoning": false → blank output, no error.
Why catalog models work but a manual entry doesn’t

claude-fable-5 is not yet in the Models.dev catalog, so it has to be added as a manual provider model. Adding it manually makes OpenCode treat it as a generic Anthropic model and it doesn’t get the Fable/Opus-4.7+ adaptive-reasoning handling introduced in v1.17.0 (“Added Claude Fable reasoning support”) and v1.15.12/v1.16.0 (“adaptive reasoning controls for Anthropic Opus 4.7+”). Catalog-backed models like claude-opus-4-8 get the correct handling and render fine; a user-defined claude-fable-5 does not.

Relevant log (1.17.1)

stream providerID=google-vertex-anthropic modelID=claude-fable-5 ... step=0
("failed to add snapshot files" warnings unrelated)
loop ... step=1
exiting loop # ~13s of streaming, no error, no rendered text

Plugins

No response

OpenCode version

OpenCode Desktop v1.17.1 (also reproduced on v1.16.2)

Steps to reproduce

1. Configure the google-vertex-anthropic provider and add a manual model:
   {
   "provider": {
   "google-vertex-anthropic": {
   "options": { "project": "", "location": "global" },
   "models": {
   "claude-fable-5": { "name": "Claude Fable 5", "reasoning": true, "tool_call": true }
   }
   }
   }
   }
   (Vertex requires enabling Anthropic data sharing once via setPublisherModelConfig → dataSharingEnabledProvider: "anthropic".)
2. Select Claude Fable 5, send any prompt.
3. The stream runs and completes with no error, but no assistant text appears.

Screenshot and/or share link

No response

Operating System

macOS 26.3.1 (Apple Silicon)

Terminal

No response