fix(server): send PushNotificationConfig.authentication as an Authorization header

[JakubWorek]

What

Spec 4.3.3 gives the webhook request an Authorization: {authentication_scheme} {credentials} header, built from PushNotificationConfig.authentication.

BasePushNotificationSender never read that field - the only header it has ever sent is X-A2A-Notification-Token, from the sibling token. A receiver therefore had no credential to check and no way to tell a genuine push from a forged one, which is the whole point of registering authentication in the first place.

Nothing suggests this was left to the application. The constructor docstring explicitly justifies delegating URL screening, on the grounds that the spec makes it a SHOULD, and says nothing about auth, which the spec makes a MUST. The code already reads the field next to it. This reads as an omission.

Only when both halves are present

AuthenticationInfo is a message, so a config can set scheme without credentials. Sending on that would put Bearer with nothing after it on the wire - a header that looks like a credential and is not. Both must be non-empty.

Note the singular scheme. Only the v0.3 compat type has repeated schemes, and conversions.py:252-261 collapses it to schemes[0] on the way in, so the sender only ever deals with one.

Heads-up on the ACTS side

SEC-PUSH-001, SEC-PUSH-002 and PUSH-DELIV-001 currently pass in nightly CI for every SDK, so this may look like a fix to something that was not broken. It is not: the assertion that inspects the delivered request is new, added in a2aproject/A2A@82c277f. The corpus CI runs predates it, and the older tests only checked that the config API accepted the config - which, as that commit's own note puts it, "a server that stores it and pushes anonymously, or never pushes, satisfies".

A consequence worth stating: no other SDK has been measured against the real assertion yet, so unlike the rest of this stack there is no cross-SDK corroboration here. The code is the evidence - push_info.authentication appears nowhere under src/a2a/server/ before this change.

Result

Clears SEC-PUSH-001, SEC-PUSH-002 (must) and PUSH-DELIV-001 (may) on all three bindings, and with them the last failing MUST:

binding	must	verdict
jsonrpc	59/59	CONFORMANT
grpc	47/47	CONFORMANT
rest	49/49	CONFORMANT

The three PRs after this one are SHOULD-level and change the verdict for nobody; they take the remaining non-MUST failures to zero.

Closes #585

[github-actions]

🧪 Code Coverage (vs jakubworek/acts-context-id-mismatch)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/tasks/base_push_notification_sender.py	95.12%	94.34%	🔴 -0.78%
Total	93.12%	93.12%	⚪️ -0.00%

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[ConnorMoss02]

This also fixes #585, open since December, so adding Closes #585 would close it. #598 targets the same issue but reads authentication.schemes, which the v1 AuthenticationInfo no longer has.