Fix SSE streaming format to comply with specification

[Copilot]

Fix SSE streaming format to comply with specification

• Understand the current streaming implementation and identify issues
• Fix SSE format in chat.go to explicitly use proper formatting
• Fix SSE format in completion.go to explicitly use proper formatting
• Ensure consistency across all streaming responses
• Verify changes with format validation tests
• Run code quality checks (go vet, gofmt)
• Address code review feedback for error handling
• Fix finish_reason to use pointer for proper null handling
• Remove empty content from initial streaming chunks
• Ensure proper stream termination with finish_reason
• Improve code consistency
• Move common finish reasons to constants

Complete Solution

This PR fixes all streaming response issues causing OpenAI API client failures:

SSE Format Fixes ✅

1. Explicit SSE formatting: data: {json}\n\n with proper %s specifier
2. Error message streaming: Proper JSON responses with correct SSE format
3. Robust error handling: Explicit marshal error checking with fallbacks

OpenAI Compliance Fixes ✅

4. Empty finish_reason: Changed to *string with omitempty

   • Streaming chunks: field omitted (null)
   • Final chunk: "finish_reason": "stop"

5. Empty initial chunks: Initial message now only sends role

   • Before: {"delta": {"role": "assistant", "content": ""}}
   • After: {"delta": {"role": "assistant"}}

6. Proper stream termination: Final chunk has empty delta with finish_reason

   • Before: {"delta": {"content": ""}, "finish_reason": "stop"}
   • After: {"delta": {}, "finish_reason": "stop"} + data: [DONE]

Code Quality Improvements ✅

7. Constants for finish reasons: Centralized constants prevent typos and improve maintainability
   • FinishReasonStop = "stop"
   • FinishReasonToolCalls = "tool_calls"
   • FinishReasonFunctionCall = "function_call"

Streaming Flow (OpenAI Compliant)

// 1. Initial chunk - only role
data: {"choices": [{"index": 0, "delta": {"role": "assistant"}}]}

// 2. Content chunks - no finish_reason field
data: {"choices": [{"index": 0, "delta": {"content": "Hello"}}]}
data: {"choices": [{"index": 0, "delta": {"content": " world"}}]}

// 3. Final chunk - empty delta with finish_reason
data: {"choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}]}
data: [DONE]

Files Modified

• core/http/endpoints/openai/constants.go: New file with finish reason constants
• core/http/endpoints/openai/chat.go: Streaming fixes + use constants
• core/http/endpoints/openai/completion.go: Streaming fixes + use constants
• core/http/endpoints/openai/realtime.go: Consistency updates + use constants
• core/schema/openai.go: FinishReason schema change

Validation

✅ Passes go vet and gofmt
✅ No security vulnerabilities (CodeQL)
✅ OpenAI API specification compliant
✅ Backward compatible for non-streaming responses

Fixes #7180

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Address streaming responses incongruences</issue_title>
<issue_description>LocalAI version:
3.7.0

Environment, CPU architecture, OS, and Version:

Describe the bug
It seems that the way streaming is implemented in LocalAI breaks some clients, such as sst/opencode#3994 , and see also: #1194

To Reproduce
N/A yet

Expected behavior
Both opencode and other software using streaming token should work

Logs

Additional context

• A list of things that doesn't seem to work are noted in task update but no model output with LocalAI / Qwen3 8B sst/opencode#3994 (comment)
• Streaming implementation is in https://github.com/mudler/LocalAI/blob/master/core/http/endpoints/openai/chat.go for OpenAI
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Address streaming responses incongruences #7180

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[netlify]

✅ Deploy Preview for localai ready!

Name	Link
🔨 Latest commit	9c0c1ae
🔍 Latest deploy log	https://app.netlify.com/projects/localai/deploys/690e5fdc1892a8000879ceab
😎 Deploy Preview	https://deploy-preview-7182--localai.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[mudler]

@copilot try to address additionally also these points:

• Empty finish_reason: LocalAI sends "finish_reason": "" instead of "stop" or null
• Empty initial chunks: Sends empty content chunks that cause streaming clients to terminate early
• Missing proper stream termination: Doesn't properly signal completion to OpenCode's processor

[Copilot]

@copilot try to address additionally also these points:

• Empty finish_reason: LocalAI sends "finish_reason": "" instead of "stop" or null
  ...

All three issues have been addressed in commit deefcb0:

1. Empty finish_reason: Changed schema to use *string with omitempty - streaming chunks now omit the field (null) while final chunks properly set it to "stop"

2. Empty initial chunks: Initial message now only sends role without content field - removed Content: &textContentToReturn which was empty

3. Missing proper stream termination: Final chunk now sends empty delta {} with finish_reason, followed by data: [DONE] per OpenAI spec

The streaming format now matches OpenAI's specification:

• Initial: {"delta": {"role": "assistant"}}
• Chunks: {"delta": {"content": "text"}} (no finish_reason)
• Final: {"delta": {}, "finish_reason": "stop"} + data: [DONE]

[mudler]

@copilot move common finish reasons to constants

[Copilot]

Done in commit 9c0c1ae. Created constants.go with:

• FinishReasonStop = "stop"
• FinishReasonToolCalls = "tool_calls"
• FinishReasonFunctionCall = "function_call"

All string literals replaced with these constants across chat.go, completion.go, and realtime.go.