Skip empty text blocks when building Anthropic messages

[PratikDhanave (PratikDhanave)]

Anthropic rejects empty text blocks. The system-prompt path in buildMessageParams already guards against forwarding a TextContent with empty text (if c.Text != ""), but the message path in buildMessageParam did not — so an assistant or user turn carrying an empty TextContent alongside real content produced an invalid empty text block in the outbound request.

This mirrors the Python client (_prepare_message_for_anthropic), which skips empty text blocks for the same reason.

Change

• Guard the message-path *message.TextContent case with if c.Text != "", matching the existing system-path guard.

Test

• TestEmptyTextContentIsSkipped: an assistant turn with an empty TextContent followed by a real one now serializes to a single text block. Fails before the change (2 blocks, one with "text":""), passes after.

[Copilot]

🟢 Approval recommended

No unresolved review issues remain.

Pull request overview

Prevents invalid empty Anthropic text blocks when building messages.

Changes:

• Skips empty TextContent values.
• Adds regression coverage for mixed empty and non-empty text.

File summaries

File	Description
provider/anthropicprovider/agent.go	Filters empty text blocks.
provider/anthropicprovider/agent_test.go	Tests empty text block omission.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

Scope: internal-only (bug fix in unexported buildMessageParam helper; no exported API change)

Changed Go contract: None. provider/anthropicprovider/agent.go's unexported buildMessageParam now skips *message.TextContent with Text == "" when building Anthropic message blocks, mirroring the existing guard already present in the system-prompt path (buildMessageParams). No exported types, functions, or signatures changed.

Upstream evidence reviewed: python/packages/anthropic/agent_framework_anthropic/_chat_client.py, method _prepare_message_for_anthropic (around line 944-957): case "text": # Skip empty text content blocks - Anthropic API rejects them \n if content.text: a_content.append(...). This confirms Python already skips empty text blocks in the message-conversion path, exactly matching the fix applied here.

Result: aligned. This is a straightforward bug fix that brings the Go message-path behavior in line with both the existing Go system-path guard and the upstream Python client's documented behavior (Anthropic API rejects empty text blocks). No divergence, no new public surface, no feature gating concerns. Test coverage (TestEmptyTextContentIsSkipped) is appropriate and mirrors the semantic being fixed.

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