[dotnet-port-fixes] fix(a2a): reject outbound messages without sendable content

[microsoft-agent-framework-for-go]

Summary

Port the pinned .NET A2A validation fix to the Go outbound A2A provider. Requests whose messages contain no convertible A2A parts now return an error before any transport call, including empty message lists, empty text, empty contents, and unsupported contents.

Ported .NET PRs

Pinned upstream revision: ebcf4db7f9462ef6c377761e0b3159c4c5a73027.
Inspected recent groups: A2A validation commit 44fe223da7ea6853eefb7053e25c2c213a2e5c15 (source) and AG-UI resolver ordering commit 501cc6f6c702d444caabe8e1aba1732f9bc3bc3f (source). The selected A2A change is fix-only under the existing Go API; the AG-UI change was excluded because Go delegates serialization to the external AG-UI SDK rather than configuring ASP.NET resolver chains.

Duplicate checks used the fixed issue query repo:microsoft/agent-framework-go A2A empty message parts no content send request validation and PR query repo:microsoft/agent-framework-go A2A messagesToA2AMessage empty parts validation, before and after editing. The issue search surfaced filtered microsoft/agent-framework-go#540; its direct read showed only an already-merged helper extraction and no overlap with this validation fix. The PR query returned no results.

Breaking Changes

No. This corrects invalid outbound requests by returning an error before the remote A2A service is called.

Tests and Examples

• go test ./provider/a2aprovider
• git diff --check
• Added black-box coverage for empty messages, empty contents, empty text, unsupported contents, and both streaming and non-streaming runs; assertions verify no transport call occurs.

Notes

Go changes are limited to provider/a2aprovider/a2a.go and provider/a2aprovider/a2a_test.go. No public API, configuration, default, generated file, binary, or build artifact was added.

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[github-actions]

API consistency review

Result	Scope
aligned	user-visible behavior

Changed Go contract

provider/a2aprovider run/messagesToA2AMessage: previously, calling Run with zero messages silently returned no updates, while non-empty messages that produced no sendable Parts (e.g. empty text, unsupported content types) were sent to the remote agent anyway. The PR removes the early len(messages) == 0 short-circuit and instead rejects any request whose combined content parts are empty — for both empty and non-empty message slices — with errors.New("a2aprovider: the provided messages contain no content that can be sent to the remote agent"), and verifies no A2A transport call occurs. No new exported symbols are introduced.

Upstream evidence reviewed

Implementation	Source	Contract checked
.NET	CreateA2AMessage in A2AAgent.cs	Throws ArgumentException("The provided messages contain no content that can be sent to the remote agent.", nameof(messages)) whenever a2aMessage.Parts.Count == 0, regardless of whether messages was empty or merely devoid of convertible content.
Python	_prepare_message_for_a2a in _agent.py and run in _agent.py	Raises ValueError("Message.contents is empty; cannot convert to A2AMessage.") for empty contents, and AgentInvalidRequestException when normalized_messages is empty and no continuation token is supplied.

Assessment

Both upstream implementations already reject requests that resolve to no sendable content, using wording functionally identical to the new Go error. This PR brings Go into parity by replacing the previous silent no-op on empty messages with an explicit error covering both the empty-slice and empty-content-after-conversion cases, matching .NET's Parts.Count == 0 check and Python's content-emptiness validation. No divergence found; behavior, trigger condition, and surfaced error semantics are aligned across all three implementations.

Warning

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[Copilot]

🟢 Approval recommended

The focused validation covers both send modes, preserves continuation handling, and has no identified blocking issues.

No open issues 🎉

What changed in this PR

Adds outbound A2A validation so messages without sendable content fail before contacting the remote agent.

Changes:

• Rejects empty message lists and messages with no convertible A2A parts.
• Adds regression coverage for streaming and non-streaming requests, verifying no transport calls occur.

File	Description
provider/​a2aprovider/​a2a.go	Rejects requests without sendable content.
provider/​a2aprovider/​a2a_test.go	Tests rejection cases and absence of transport calls.

_{Review effort: Balanced}