fix(go-adk): add per-call session isolation for Agent tools

[yashrajshuklaaa]

Summary

Fixes #2137

The bug : every call a parent agent makes to a sub-agent (Agent tool) reuses the same A2A context_id since it's minted once when the tool is built. worker just uses that context_id as its session id directly ( executor.go: sessionID := reqCtx.ContextID ) so if a coordinator fires off N parallel calls to the same sub-agent in one turn , they all pile into a single shared session instead of getting their own.

Fix is an opt-in IsolateSessions flag on Agent-type tools :

spec:
  declarative:
    tools:
      - type: Agent
        agent:
          name: worker
        isolateSessions: true   # each call to worker gets its own session

Default behavior (flag unset/false) doesn't change
one context_id for the tool's whole lifetime so stateful sub-agents keep their session continuity. with the flag on we mint a fresh context_id on every call so each invocation is isolated.

Doesn't touch the x-kagent-root-context-id header stuff. that's still what carries cross-turn continuity and it stays stable either way.

What changed

• agent_types.go: added Tool.IsolateSessions *bool plus a CEL rule so it can only be set when type: Agent
• adk/types.go: RemoteAgentConfig.IsolateSessions bool
• compiler.go: passes the flag through into RemoteAgentConfig
• agent.go: forwards the flag to NewKAgentRemoteA2ATool. also skips adding an entry to subagentSessionIDs when isolated since there's no single session id to pre-stamp function_call parts with anymore
• remote_a2a_tool.go: added isolateSessions plus a small nextContextID() helper that either returns the stable id or mints a new one. This gets reported back as subagent_session_id
• Python types.py: added isolate_sessions for schema parity only, it doesn't do anything on that side yet (matches what the issue scoped out)
• Regenerated CRDs/deepcopy with make controller-manifests
• Added a test for the new context-id logic and a golden fixture (agent_with_isolated_session_tool) to check it flows through the whole translation pipeline

About the UI

Isolated tools don't have one fixed session id, so the executor can't pre-populate the stamp map for them at startup. Instead the UI grabs the session id per call from subagent_session_id in the function_response. AgentCallDisplay already reads that field, so nothing new needed there.

Not doing in this PR

• Not a concurrency limiter, that's separate (max-concurrency.md)
• HITL resume is unaffected, it still uses the context_id from the confirmation payload
• Only the Go runtime respects this flag right now. Python accepts it for config parity but the low-level tool doesn't use it yet

Tests

Ran go test ./adk/pkg/tools/... ./adk/pkg/agent/... ./api/... and the golden translator tests. Everything passes.
Confirmed isolate_sessions: true shows up correctly in the generated config.json for the new fixture.

One small thing I noticed

In handleResume, processResult now sets subagent_session_id from the contextID I pass in, but there's older code right after it that sets the same key again with a fallback to lastContextID. Not wrong, just a bit redundant now since it writes the same value twice in the normal case. Left it as is since it's harmless but flagging it in case someone wants it cleaned up.

[Copilot]

Pull request overview

Adds an opt-in isolateSessions flag for Agent-type tools so each sub-agent invocation can use a fresh A2A context_id (and therefore a distinct sub-agent session), fixing the “parallel fan-out collapses into one shared worker session” bug described in #2137.

Changes:

• Introduces Tool.IsolateSessions (CRD + CEL validation) and threads it through translation into RemoteAgentConfig.IsolateSessions.
• Updates the Go remote A2A tool to mint per-call context_id when isolation is enabled and to avoid pre-stamping a single subagent session id for isolated tools.
• Adds translation fixture coverage and a unit test for the new context-id selection logic; adds Python schema parity field.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
python/packages/kagent-adk/src/kagent/adk/types.py	Adds isolate_sessions field for config/schema parity (no runtime behavior change in Python).
helm/kagent-crds/templates/kagent.dev_sandboxagents.yaml	Regenerates CRD template to include isolateSessions and CEL rule.
helm/kagent-crds/templates/kagent.dev_agents.yaml	Regenerates CRD template to include isolateSessions and CEL rule.
go/core/internal/controller/translator/agent/testdata/outputs/agent_with_isolated_session_tool.json	Adds golden output asserting isolate_sessions: true is emitted into config.json.
go/core/internal/controller/translator/agent/testdata/inputs/agent_with_isolated_session_tool.yaml	Adds translator input fixture exercising isolateSessions: true.
go/core/internal/controller/translator/agent/compiler.go	Threads tool.IsolateSessions into RemoteAgentConfig.IsolateSessions.
go/api/v1alpha2/zz_generated.deepcopy.go	Regenerates deepcopy to include Tool.IsolateSessions.
go/api/v1alpha2/agent_types.go	Adds Tool.IsolateSessions *bool and CEL validation restricting it to Agent tools.
go/api/config/crd/bases/kagent.dev_sandboxagents.yaml	Regenerates CRD base to include isolateSessions schema + validation.
go/api/config/crd/bases/kagent.dev_agents.yaml	Regenerates CRD base to include isolateSessions schema + validation.
go/api/adk/types.go	Adds RemoteAgentConfig.IsolateSessions (isolate_sessions) to runtime config.
go/adk/pkg/tools/remote_a2a_tool.go	Implements per-call context id minting for isolated tools and reports back per-call session id.
go/adk/pkg/tools/remote_a2a_tool_test.go	Adds unit test for nextContextID() isolation semantics.
go/adk/pkg/agent/agent.go	Passes isolation flag into tool creation and skips pre-stamp map entry when isolated.

Files not reviewed (1)

• go/api/v1alpha2/zz_generated.deepcopy.go: Generated file

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[yashrajshuklaaa]

PTAL @EItanya