[Bug]: Reasoning tokens are not passed through for Gemini

[skavans]

Prerequisites

• I have searched existing issues and discussions to avoid duplicates
• I am using the latest version (or have tested against main/nightly)

Description

When streaming the inference, the bifrost gateway seems to not properly pass the delta.reasoning tokens through.

Steps to reproduce

If I request the provider directly, I see the following:

~ % curl -X POST https://provider/v1/chat/completions \
-H "Content-Type: application/json" \
-H "Authorization: Bearer token-here" \
-d '{
  "model": "gemini-3-pro-preview",
  "stream": true,
  "messages": [
    {"role": "user", "content": "tell me fun 10 word story"}
  ]
}'
data: {"id":"chatcmpl-gemini-1764531905290","object":"chat.completion.chunk","created":1764531905,"model":"gemini-3-pro-preview","choices":[{"index":0,"delta":{"content":"","reasoning":"**Exploring Word Count Limits**\n\nI'm currently dwelling on the constraint of a strict 10-word limit for this \"fun\" story. My mind is cycling through potential themes, primarily animals like cats, dogs, and penguins, and also considering food as a possible subject.\n\n\n","reasoning_details":[]},"finish_reason":null}]}

data: {"id":"chatcmpl-gemini-1764531908469","object":"chat.completion.chunk","created":1764531908,"model":"gemini-3-pro-preview","choices":[{"index":0,"delta":{"content":"","reasoning":"**Analyzing Story Elements**\n\nI've been analyzing the recent drafts, and I'm leaning toward the \"Taco Mars\" story for its quirky, amusing concept. Its whimsical nature and simplicity truly resonate. Comparing the \"hat-wearing cat\" with the \"Mars tacos\" narrative, the latter wins.\n\n\n","reasoning_details":[]},"finish_reason":null}]}

data: {"id":"chatcmpl-gemini-1764531911101","object":"chat.completion.chunk","created":1764531911,"model":"gemini-3-pro-preview","choices":[{"index":0,"delta":{"content":"","reasoning":"**Crafting the Narrative**\n\nI've decided to refine the \"Mars Tacos\" concept, and it feels quite solid now. I've successfully met the 10-word constraint with \"We went to Mars and found they only eat tacos.\" I'm also content with the word count and the whimsical nature. I'm satisfied now, and ready to finalize the details and declare this effort finished.\n\n\n","reasoning_details":[]},"finish_reason":null}]}

data: {"id":"chatcmpl-gemini-1764531911122","object":"chat.completion.chunk","created":1764531911,"model":"gemini-3-pro-preview","choices":[{"index":0,"delta":{"content":"We went to Mars and found they only eat tacos."},"finish_reason":null}]}

data: {"id":"chatcmpl-gemini-1764531911131","object":"chat.completion.chunk","created":1764531911,"model":"gemini-3-pro-preview","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: [DONE]

You can see the delta.reasoning tokens here. But when I request the same provider via the bifrost, I see empty reasoning:

~ % curl -X POST http://localhost:8080/v1/chat/completions \
-H "Content-Type: application/json" \
-d '{
  "model": "navy/gemini-3-pro-preview",
  "stream": true,
  "messages": [
    {"role": "user", "content": "tell me fun 10 word story"}
  ]
}'
data: {"id":"chatcmpl-gemini-1764531872043","choices":[{"index":0,"delta":{"content":""}}],"created":1764531872,"model":"gemini-3-pro-preview","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":0,"chunk_index":0}}

data: {"id":"chatcmpl-gemini-1764531876106","choices":[{"index":0,"delta":{"content":""}}],"created":1764531876,"model":"gemini-3-pro-preview","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":4059,"chunk_index":1}}

data: {"id":"chatcmpl-gemini-1764531880027","choices":[{"index":0,"delta":{"content":""}}],"created":1764531880,"model":"gemini-3-pro-preview","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":3945,"chunk_index":2}}

data: {"id":"chatcmpl-gemini-1764531882551","choices":[{"index":0,"delta":{"content":""}}],"created":1764531882,"model":"gemini-3-pro-preview","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":2499,"chunk_index":3}}

data: {"id":"chatcmpl-gemini-1764531882570","choices":[{"index":0,"delta":{"content":"My cat learned to type; he just bought a boat."}}],"created":1764531882,"model":"gemini-3-pro-preview","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":18,"chunk_index":4}}

data: {"id":"chatcmpl-gemini-1764531872043","choices":[{"index":0,"finish_reason":"stop","delta":{}}],"created":0,"model":"","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":{"total_tokens":0},"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"gemini-3-pro-preview","latency":10548,"chunk_index":5}}

data: [DONE]

Expected behavior

I expect to see the reasoning content passed through the bifrost gateway

Actual behavior

The reasoning content is empty

Affected area(s)

Core (Go)

Version

v1.3.37

Environment

Relevant logs/output

Regression?

No response

Severity

High (major functionality broken)

[coderabbitai]

Duplicate Detection

ℹ️ This issue may be similar to #438.

Related Issues

• [Bug]: Gemini ChatCompletionStream last chunk nil content #438: [Bug]: Gemini ChatCompletionStream last chunk nil content

Suggested Issue Assignees

• Sherrbethead

Actions

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[Pratham-Mishra04]

Hey @skavans this issue stems from #609. We follow gemini's openai's compatible APIs internally which doesnt fully support reasoning yet. We actually got a community PR for this but it was later closed but we are picking this migration up internally on priority this week which is resolve this issue🙇🏼‍♂️

[skavans]

Hey @Pratham-Mishra04,

It's not only the Gemini. It behaves the same for all the models (at least on the provider I use). For instance:

~ % curl -X POST http://localhost:8080/v1/chat/completions \
-H "Content-Type: application/json" \
-d '{
  "model": "navy/qwen3-235b-a22b-thinking-2507",
  "stream": true,
  "messages": [
    {"role": "user", "content": "tell me fun 10 word story"}
  ]
}'
data: {"id":"gen-1764534751-PWjbdbN5wcKCtCB8s5fk","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764534751,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"qwen3-235b-a22b-thinking-2507","latency":0,"chunk_index":0}}

data: {"id":"gen-1764534751-PWjbdbN5wcKCtCB8s5fk","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764534751,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"qwen3-235b-a22b-thinking-2507","latency":0,"chunk_index":1}}

data: {"id":"gen-1764534751-PWjbdbN5wcKCtCB8s5fk","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764534751,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"qwen3-235b-a22b-thinking-2507","latency":50,"chunk_index":2}}

data: {"id":"gen-1764534751-PWjbdbN5wcKCtCB8s5fk","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764534751,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"navy","model_requested":"qwen3-235b-a22b-thinking-2507","latency":56,"chunk_index":3}}

[Pratham-Mishra04]

Oh okay got it, I'll check and update this thread

[skavans]

Works the same even for Openrouter

~ % curl -X POST http://localhost:8080/v1/chat/completions \
-H "Content-Type: application/json" \
-d '{
  "model": "openrouter/qwen/qwen3-235b-a22b-thinking-2507",
  "stream": true,
  "messages": [
    {"role": "user", "content": "tell me fun 10 word story"}
  ]
}'
data: {"id":"gen-1764535157-WRdoGvKDs0TEBs3ZiD09","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764535157,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"openrouter","model_requested":"qwen/qwen3-235b-a22b-thinking-2507","latency":0,"chunk_index":0}}

data: {"id":"gen-1764535157-WRdoGvKDs0TEBs3ZiD09","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764535157,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"openrouter","model_requested":"qwen/qwen3-235b-a22b-thinking-2507","latency":0,"chunk_index":1}}

data: {"id":"gen-1764535157-WRdoGvKDs0TEBs3ZiD09","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764535157,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"openrouter","model_requested":"qwen/qwen3-235b-a22b-thinking-2507","latency":22,"chunk_index":2}}

data: {"id":"gen-1764535157-WRdoGvKDs0TEBs3ZiD09","choices":[{"index":0,"delta":{"role":"assistant","content":""}}],"created":1764535157,"model":"qwen/qwen3-235b-a22b-thinking-2507","object":"chat.completion.chunk","service_tier":"","system_fingerprint":"","usage":null,"extra_fields":{"request_type":"chat_completion_stream","provider":"openrouter","model_requested":"qwen/qwen3-235b-a22b-thinking-2507","latency":26,"chunk_index":3}}

Thanks!