fix(core): allow generating responses in empty threads

[roaminro]

Fixes #13045

ensureGeminiCompatibleMessages was throwing a MastraError when there were no user/assistant messages (only system messages). This blocked generating responses in empty threads (e.g. generate welcome messages via CopilotKit's useAgent hook).

The fix replaces the throw with a warning via the Mastra logger. Providers that handle system-only prompts natively (Anthropic, OpenAI) now work without issues. Providers that require a user message (Gemini) will return their own error, and users will see a helpful warning in their logs pointing them in the right direction.

Summary by CodeRabbit

• Bug Fixes
  • Fixed an error that occurred when generating responses in threads containing only system messages. Providers supporting system-only prompts (Anthropic, OpenAI) now work correctly. Providers requiring user messages (Gemini) log a warning instead of throwing an error.

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Project	Deployment	Actions	Updated (UTC)
mastra-docs	Ready	Preview, Comment	Feb 17, 2026 11:34pm
mastra-docs-1.x	Building	Preview, Comment	Feb 17, 2026 11:34pm

[changeset-bot]

🦋 Changeset detected

Latest commit: 81d3559

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 20 packages

Name	Type
@mastra/core	Patch
@mastra/mcp-docs-server	Patch
@internal/playground	Patch
@mastra/client-js	Patch
@mastra/react	Patch
@mastra/opencode	Patch
@mastra/longmemeval	Patch
mastra	Patch
@mastra/deployer-cloud	Patch
@mastra/playground-ui	Patch
@mastra/server	Patch
@mastra/deployer	Patch
create-mastra	Patch
@mastra/express	Patch
@mastra/fastify	Patch
@mastra/hono	Patch
@mastra/koa	Patch
@mastra/deployer-cloudflare	Patch
@mastra/deployer-netlify	Patch
@mastra/deployer-vercel	Patch

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

The PR addresses issue #13045 by replacing error-throwing behavior with optional logging when processing empty or system-only message lists. This allows providers supporting system-only prompts to function while providers requiring user messages log warnings instead of failing.

Changes

Cohort / File(s)	Summary
Changelog Entry .changeset/blue-times-shine.md	Documents patch release fixing empty thread response generation error for system-only prompts.
Core Message Handling packages/core/src/agent/message-list/message-list.ts, packages/core/src/agent/message-list/utils/provider-compat.ts	Added optional logger parameter to MessageList constructor and ensureGeminiCompatibleMessages function. Replaced error-throwing on empty/system-only messages with optional warning-based logging.
Workflow Integration packages/core/src/agent/workflows/prepare-stream/prepare-memory-step.ts	Updated MessageList instantiation to pass logger from capabilities context.
Test Updates packages/core/src/agent/message-list/tests/message-list.test.ts, packages/core/src/agent/message-list/tests/message-list-gemini.test.ts, packages/core/src/agent/message-list/tests/message-list-v5.test.ts	Replaced error assertions with empty array and system-message array expectations. Added mock logger utility for warning assertions. Added test group for issue #13045 covering empty thread response generation across SDK versions.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise, descriptive, follows imperative mood, and clearly summarizes the main change of fixing empty thread response generation.
Linked Issues check	✅ Passed	The PR fully addresses issue #13045 by replacing the hard error in ensureGeminiCompatibleMessages with optional logger-based warnings, allowing system-only prompts to work for compatible providers.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the empty thread generation issue: updating message-list handling, adding logging support, adjusting provider compatibility logic, and updating comprehensive tests.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch roamin/issue-13045

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