Go declarative runtime silently ignores spec.declarative.context.compaction while being the CRD default #2728

Summary

As of 0.10.0 the Agent CRD defaults spec.declarative.runtime to go, so any agent that omits the field runs on the Go ADK. The Go runtime does not implement spec.declarative.context.compaction — for long-running tool-using agents that field is what bounds session history growth. An operator upgrading to 0.10.0 without an explicit runtime therefore loses compaction, and the only signal is a status condition that no UI or printer column surfaces.

Evidence (all at tag v0.10.0, commit ae008d1e)

1. go is the CRD default — kagent.dev_agents.yaml:13236 default: go (same at kagent.dev_sandboxagents.yaml:10893). Note go/api/v1alpha2/agent_types.go:325-336 (EffectiveDeclarativeRuntime) still documents and implements a Python fallback, which is unreachable for objects created through the API.

2. Compaction is unimplemented on Go — go/core/internal/controller/reconciler/reconciler.go:946-948:

   // Context compression: Not yet implemented in Go runtime
   if decl.Context != nil && decl.Context.Compaction != nil {
       unsupported = append(unsupported, "context compression/compaction (not implemented in Go runtime)")
   }

   grep -rn Compaction go/adk returns zero functional hits, while the controller compiles the config regardless (translator/agent/compiler.go:265-302) — the field is delivered to the runtime and dropped.

3. The gap is soft-failed — reconciler.go:341-350 sets UnsupportedFeatures=True; the agent still reconciles Accepted and serves traffic, just without compaction. Good that a signal exists; it is invisible in the UI and in kubectl get agents.

Scope

1. Implement context.compaction in the Go runtime. The upstream primitives are not in place yet — in google/adk-go, #298 "ADR-010: Native SDK Session History Compaction" (design) and #1001 "Add EventCompaction support to EventActions and compaction filtering in contents_processor.go" (implementation) are both open as of 2026-09-07; see also discussion #589. Either consume that work when it lands, or implement session-history compaction kagent-side for the Go runtime.

2. Publish an authoritative runtime parity matrix in the docs, generated from or cross-checked against validateRuntimeFeatures, and replace "most features supported" in the CRD description with a link to it.

3. Make degradation harder to miss — surface UnsupportedFeatures on the agent card / UI and in kubectl get agents printer columns, and consider an opt-in strict mode that fails reconciliation rather than ignoring the field. Given go is now the default, this is the difference between a visible upgrade note and a silent regression.

4. Close the smaller deltas or drop the fields — file upload is Go-only ("Python ADK agents do not yet support file attachments"), and executeCodeBlocks is inert on both runtimes pending google/adk-python#3921 while still being an accepted API field.

Acceptance criteria

• An agent with runtime: go and context.compaction set demonstrably compacts session history (test or trace evidence), and UnsupportedFeatures is not raised for it.
• Docs carry a parity matrix that matches validateRuntimeFeatures.
• Until item 1 ships, the compaction-on-Go gap is stated in the docs where runtime is described, not only in the reconciler.