[Bug]: Pattern system prompt merges user input into single system message, causing empty responses on OpenAI-compatible providers that require a user turn

What happened?

When using a pattern with an OpenAI-compatible provider (e.g. Ollama Cloud via OPENAI_API_BASE_URL), Fabric sends only a single message with role=system containing both the pattern content and the user input appended at the # INPUT: section. No role=user message is sent. Several OpenAI-compatible providers require at least one user turn to generate a response and return an empty response when only a system message is present.

To reproduce

Configure Fabric with an OpenAI-compatible provider via OPENAI_API_BASE_URL (e.g. Ollama Cloud at https://ollama.com/v1) Run any pattern: echo "test input" | fabric -p summarize Observe empty response

Enabling --debug=4 confirms only one message is sent:

DEBUG: FABRIC->LLM request messages (1) DEBUG: FABRIC->LLM [0] role=system content="# IDENTITY and PURPOSE\n...\n# INPUT:\n\nINPUT:\ntest input" DEBUG: LLM->FABRIC response content=""

The same provider works correctly when called directly with separate system and user messages.

Root cause

In internal/core/chatter.go, BuildSession sets inputUsed = true whenever a pattern is loaded, then uses this flag to suppress appending the user message:

if len(request.Message.MultiContent) > 0 || (request.Message != nil && !inputUsed) { session.Append(request.Message) }

When inputUsed is true the user message is never appended, resulting in a single system message.

What I did to fix it for me but that might not be done because other providers

Remove the !inputUsed guard so the user message is always appended as a separate turn:

go if len(request.Message.MultiContent) > 0 || request.Message != nil { session.Append(request.Message) }

And remove the now-unused inputUsed variable and assignment.

Operating System

Linux - amd64

OS Version

Debian 13

How did you install Fabric?

Release Binary - Linux (amd64)

Version

Fabric version: v1.4.473

Relevant log output

Relevant screenshots (optional)

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