Fix Response.Update: keep the earliest valid CreatedAt

[PratikDhanave (PratikDhanave)]

Summary

(*Response).Update overwrote an already-set CreatedAt whenever a later update carried a strictly-later timestamp (the guard used update.CreatedAt.After(...)), in both the message-level and response-level branches. As a result a message/response collected from a stream ended up stamped with the last streamed chunk's time instead of the first valid one.

.NET parity

.NET ChatResponseExtensions.ProcessUpdate keeps the first valid timestamp, not the latest:

if (message.CreatedAt is null && IsValidCreatedAt(update.CreatedAt))
{
    message.CreatedAt = update.CreatedAt;
}
...
if (response.CreatedAt is null && IsValidCreatedAt(update.CreatedAt))
{
    response.CreatedAt = update.CreatedAt;
}

This PR mirrors that: CreatedAt is set only when it is currently zero and the update's value is valid. It also adds isValidCreatedAt, mirroring the .NET IsValidCreatedAt helper (t.After(time.Unix(0, 0))), so a zero or epoch-zero timestamp is treated as unset — it cannot populate CreatedAt, and a later valid value can still take hold.

Tests

agent/response_test.go:

• Updated TestResponse_Update_CreatedAt and TestResponse_CreatedAt, which previously asserted latest-wins, to assert the first valid timestamp is retained.
• Added TestResponse_Update_CreatedAt_FirstValidWins (a strictly-later second update keeps the first timestamp on both message and response) and TestResponse_Update_CreatedAt_EpochZeroIgnored (an epoch-zero timestamp is treated as unset and a later valid value takes hold).

Both new tests fail against the previous latest-wins logic and pass with the fix.

Verified with go build ./..., go vet ./agent/..., and go test ./... (full suite green).

Note

The originally-scoped item also proposed scoping AdditionalProperties in Update by MessageID (matching the same .NET ProcessUpdate). That change is intentionally left out of this PR: the Go providers currently surface response-level metadata (e.g. foundry ServedModel, openai EndUserId/Error) by stamping it onto message-bearing updates and relying on Update merging those properties into the response. Scoping by MessageID in isolation would misroute that metadata onto messages and break provider behavior, so it needs a coordinated provider change and is better handled separately.

[Copilot]

Pull request overview

This PR adjusts (*Response).Update so CreatedAt is populated only once using the earliest valid timestamp from a stream of updates, matching the .NET behavior and preventing later chunks from overwriting the initial creation time.

Changes:

• Update message-level and response-level CreatedAt merge logic to “first valid wins”.
• Add isValidCreatedAt helper to treat zero / epoch-zero timestamps as unset.
• Update and extend unit tests to assert earliest-valid timestamp behavior and epoch-zero handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
agent/response.go	Changes CreatedAt merge semantics and introduces isValidCreatedAt to ignore default/uninitialized timestamps.
agent/response_test.go	Updates existing assertions and adds new tests for first-valid-wins and epoch-zero ignored behavior.

Comments suppressed due to low confidence (1)

agent/response.go:219

• Same issue as message CreatedAt: resp.CreatedAt being epoch-zero (or otherwise invalid) will not be replaced because the guard only checks IsZero(). To fully treat epoch-zero as unset, the "already set" check should use isValidCreatedAt on the current value as well.

	if resp.CreatedAt.IsZero() && isValidCreatedAt(update.CreatedAt) {
		resp.CreatedAt = update.CreatedAt
	}

---

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[PratikDhanave (PratikDhanave)]

Good catch — fixed in bb29447: the existing-value guard now uses !isValidCreatedAt(...) (both lines 189 and 217) so a pre-existing epoch-zero CreatedAt is treated as unset and a later valid timestamp can replace it. Added a test covering a Response/Message pre-seeded with epoch-zero.

[github-actions]

Parity Review: Approved

This PR corrects a behavioral divergence in Response.Update by keeping the first valid CreatedAt timestamp rather than the latest, matching the .NET ChatResponseExtensions.ProcessUpdate behavior.

Scope: Only agent/response.go and agent/response_test.go are changed. No exported identifiers were added or removed; only the behavior of the existing (*Response).Update method changed. The isValidCreatedAt helper is unexported.

Cross-repo consistency: Aligned with .NET (ProcessUpdate keeps first valid timestamp). No equivalent Python stream-merging implementation found for CreatedAt in python/packages/core/agent_framework/, so no Python parity concern.

No public API surface change detected — the public-api-change label is not needed. Parity review is green.

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