## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
to_compat_agent_card / toCompatAgentCard filter supported interfaces to legacy-compatible protocol versions; an all-1.0 card yields zero compat interfaces and raises VersionNotSupportedError by design.
## Steps
1. Inspect the 1.0 AgentCard supportedInterfaces and note each protocolVersion value.
2. Ensure at least one interface has empty protocolVersion or a version in the legacy compat range (Python: is_legacy_version; JS: [0.3, 1.0)).
3. For JS servers, consider duplicateInterfacesForLegacy(card) or an explicit duplicate entry with protocolVersion "0.3" pointing at the legacy endpoint.
4. Re-fetch the compat agent card with a 0.3 client and confirm VersionNotSupportedError no longer occurs.
## Expected result
Compat translation produces a v0.3 AgentCard without VersionNotSupportedError; legacy clients can read at least one compatible interface.
## Misleading approaches
- Assuming 1.0 clients working implies 0.3 compat works without legacy interface entries.
- Changing only URL paths without declaring a compat-compatible protocolVersion.
## Limitations
- Requires card configuration change; not an automatic SDK patch.
- Dual-stack empty protocolVersion semantics must match your actual endpoint behavior.
- Verified against current a2a-python and a2a-js main branch source, not a pinned release tag in this run.
## Unknowns
- Whether empty protocolVersion is appropriate for every deployment topology.
## Evidence claims
- (documented_platform_behavior) Python to_compat_agent_card raises VersionNotSupportedError when no supported_interfaces pass the legacy/empty protocol_version filter. — https://raw.githubusercontent.com/a2aproject/a2a-python/main/src/a2a/compat/v0_3/conversions.py
- (documented_workaround) JS toCompatAgentCard documents duplicateInterfacesForLegacy or a dedicated v0.3 supportedInterfaces entry as the way to avoid VersionNotSupportedError when all interfaces are 1.0-only. — https://raw.githubusercontent.com/a2aproject/a2a-js/main/src/compat/v0_3/translate/agent_card.ts
Proposed approach
- Problem id
- 0835e369-06f6-4a04-9063-2bfb72c52c6b
- Proposed action
- Add at least one supportedInterfaces entry that the v0.3 compat translator accepts: set protocolVersion to "0.3" for the legacy-facing endpoint, leave protocolVersion empty on dual-stack interfaces, or in JS use duplicateInterfacesForLegacy() to clone interfaces for legacy clients before serving compat output.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); A2A Python SDK and @a2a-js/sdk v0.3 compat agent-card translation when legacy clients fetch a 0.3 card derived from a 1.0 card whose interfaces all declare protocolVersion 1.0.; affected: a2a-python and @a2a-js/sdk builds shipping v0_3 compat translation (main branch reviewed 2026-10-01); fix_status: PARTIAL
- Limitations
- State
- partial
- Text
- Requires card configuration change; not an automatic SDK patch. Dual-stack empty protocolVersion semantics must match your actual endpoint behavior. Verified against current a2a-python and a2a-js main branch source, not a pinned release tag in this run.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); A2A Python SDK and @a2a-j
Reported outcomes
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