Whitespace content causes API validation errors in provider transformations

[jerome-benoit]

Problem

Provider APIs (Anthropic, OpenAI) reject requests containing empty or whitespace-only text content, causing validation failures during message transformation.

Current Behavior:

• Untrimmed whitespace in message content passes through transformation functions
• Empty strings after whitespace removal are sent to provider APIs
• APIs respond with validation errors rejecting empty content blocks

Root Cause:
No consistent whitespace trimming or empty content filtering across provider transformation functions (anthropic.ts, openai.ts, openai-compatible.ts, transform.ts).

Impact

• User messages with accidental whitespace cause request failures
• Poor UX when legitimate content is rejected due to formatting
• Inconsistent behavior across different providers

Proposed Solution

Implement consistent whitespace trimming pattern across all transformations:

const t = content.trim()
if (t.length > 0) {
  parts.push({ type: "text", text: t })
}

Scope:

• Apply trimming to all text content before API submission
• Filter empty/whitespace-only messages and parts
• Preserve tool results and streaming deltas unchanged
• Add comprehensive test coverage

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• TUI can auto-submit whitespace prompt, creating “empty user message” and breaking Claude/OpenAI-compatible proxies #8679: TUI can auto-submit whitespace prompt, creating "empty user message" and breaking Claude/OpenAI-compatible proxies
• when (mistakenly) sending empty message the session stop with the error: text cannot be empty. #7493: when (mistakely) sending empty message the session stop with the error: text cannot be empty
• Empty message content causes Anthropic API errors during compaction #6446: Empty message content causes Anthropic API errors during compaction (CLOSED)
• Call Missing content in messages: text content blocks must be non-empty #5028: Call Missing content in messages: text content blocks must be non-empty (CLOSED)

Feel free to ignore if none of these address your specific case.