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ADR-057: Trust A2A Plugin Header Output

  • Status: Accepted
  • Date: 2026-10-06
  • Deciders: Platform Team

Note

Returning an empty value for a received header is equivalent to omitting it.

Context

A2A plugins supply per-call downstream credentials. The existing A2A paths filter returned plugin headers as caller input. This drops plugin credentials and can produce duplicate logical headers when casing differs.

Decision

Trust headers returned by the A2A pre-invoke hook. Keep caller filtering and sanitized hook input unchanged.

Resolve caller, configured, plugin, and protocol headers in the shared A2A protocol layer. Compare names case-insensitively. Plugin values override configured authentication. An empty Authorization removes authorization from every lower-priority source. Omission removes only caller headers that the plugin received. The gateway owns required protocol headers and removes X-Vault-Tokens before egress.

Consequences

Trusted plugins can inject custom credentials without an A2A caller-header allowlist entry. Operators must restrict plugin installation and bindings to trusted plugins.