Agent Fallbacks and Full-Spectrum Support tells a site how to serve the best supported path without pretending every agent can use that path. UAIX-compatible records should let a URL-only chatbot, indexed fetcher, browser agent, JSON POST client, authenticated owner agent, restore/readback agent, multi-agent runtime, A2A peer, MCP client, and MCP-server-capable system all reach either a safe executable route or an explicit no-op/human-review route.
Core rule
Ramp up only after capability is proven. Ramp down before guessing. A route can advertise a richer path such as JSON POST, authenticated mutation, MCP tool use, A2A delegation, or restore/readback only when the route record also names the lower safe fallback for clients that cannot use that path. If no safe fallback exists, the correct behavior is no-op with a public human-review URL.
Full-spectrum support ladder
| Client or runtime | Highest supported path | Lowest safe fallback | Required public statement |
|---|---|---|---|
| URL-only chatbot | Public HTML, sitemap, llms.txt, and public read-only URLs. |
Public review page or no-op statement. | No POST, auth, JavaScript, tool, memory, MCP, or A2A ability is implied. |
| Indexed or cached fetch agent | Static public page or compact JSON that can be read without live dynamic execution. | Static citation route and human review URL. | Dynamic GET actions must declare live_get_blocked_fallback. |
| Browser-assisted agent | Visible page, form, or browser-open workflow. | Browser form equivalent or public review URL. | Do not require hidden browser steps, private dashboard paths, or JavaScript-only discovery for the fallback. |
| Structured JSON POST agent | HTTP JSON POST with content type, required fields, idempotency, result URL, restore/readback URL, and structured errors. | Browser form, compact GET diagnostic, no-op, or human review. | Route records must declare post_blocked_fallback. |
| Authenticated owner agent | Owned mutation with explicit credential handling, expiration, audit, idempotency, and public-safe evidence. | Unauthenticated public review, pending-owner action, or no-op. | Route records must declare auth_unavailable_fallback and must never expose secrets in public URLs. |
| Restore/readback agent | Published restore/readback URL or public-safe result lookup. | Result URL plus final report or human review. | Create/update flows must not force agents to guess private endpoints. |
| Multi-agent runtime | Task status, blocker, handoff, memory-proposal, final-report, and evidence packet exchange. | Single UAI-1 evidence packet plus public review URL. | UAIX records the evidence; the runtime executes elsewhere. |
| A2A peer | A2A delegation, agent card, or peer workflow only when externally supported and consented. | Capability statement plus UAI-1 handoff or public human review. | Do not infer A2A support from a UAI-1 record alone. |
| MCP client | MCP resources, prompts, or tools inside a host runtime that explicitly provides them. | Public JSON, OpenAPI route, static page, UAI-1 evidence packet, or no-op. | Route records must declare mcp_unavailable_fallback and tool_unavailable_fallback. |
| MCP-server-capable system | A local or site-owned MCP server may expose tools/resources for that runtime. | Documented unavailable response, no-op, public review URL, or static UAI-1 evidence packet. | UAIX can document the pattern; it does not become the hosted MCP server unless that product is explicitly published. |
Route-record fields
Every action-capable route should declare highest_supported_path, lowest_safe_fallback, method, content_type, side_effect_status, fetch_execution_class, required_fields, result_url_field, restore_readback_url_field, write_credential_response_path, browser_form_equivalent, get_safety, post_blocked_fallback, live_get_blocked_fallback, mcp_unavailable_fallback, auth_unavailable_fallback, tool_unavailable_fallback, human_review_url, and no_op_behavior.
Adaptive .NET package track
The .NET implementation package line is versioned independently from the public UAI-1 standards label. UAI-1 remains v1.0 unless the standards record changes; package versions such as UAIX.UAI.Abstractions 1.0.4, UAIX.UAI.Memory 1.0.4, UAIX.Talisman.AgentClient 0.1.10, and UAIX.Talisman.EcosystemManager 0.1.1 describe implementation releases. The planned adaptive interoperability track should add a capability-profile reader, fallback-chain resolver, ASP.NET Core route metadata helpers, typed HttpClient support, UAI-1 evidence export, optional MCP/A2A adapters where the host runtime provides those protocols, and validator fixtures that prove ramp-up and ramp-down behavior. Track package status and install commands on the .NET NuGet Package page.
Pass and fail conditions
- Pass: the declared client can complete the route or reach the route-specific no-op/human-review fallback without guessing hidden credentials, POST bodies, private dashboard paths, or JavaScript-only steps.
- Fail: a route advertises an endpoint, MCP tool, A2A peer, POST path, authenticated mutation, restore route, or runtime handoff but omits the lower safe fallback for clients that cannot use it.
- No-op: when capability, consent, public-safe context, tool availability, authentication, or fallback evidence is missing, return the public review URL and stop.