runtime: stop implicit SessionID search attribute injection

docs/runtime.md

@@ -1400,7 +1400,7 @@ client.Run(ctx, "session-1", msgs,
     runtime.WithMetadata(map[string]any{"request_id": "abc"}),
     runtime.WithTaskQueue("custom-queue"),
     runtime.WithMemo(map[string]any{"workflow_name": "Chat"}),
-    runtime.WithSearchAttributes(map[string]any{"SessionID": "s1"}),
+    runtime.WithSearchAttributes(map[string]any{"tenant": "acme"}),
     runtime.WithTiming(runtime.Timing{
         Budget: 2 * time.Minute,
         Plan:   30 * time.Second,
@@ -1409,6 +1409,10 @@ client.Run(ctx, "session-1", msgs,
 )
 ```
 
+Search attributes are passed through to the workflow engine as caller-owned
+index metadata. The runtime does not mirror `SessionID` into engine search
+attributes automatically.
+
 `Timing.Plan` and `Timing.Tools` are semantic attempt budgets. They bound how
 long a healthy planner or tool attempt may run once execution starts. Queue-wait
 timeouts and heartbeat-based liveness detection are engine-specific concerns and
@@ -1860,8 +1864,8 @@ var ErrRateLimited = errors.New("model: rate limited")
 3. **Choose one streaming path.** Either use the decorated model client OR
    `planner.ConsumeStream`, never both.
 
-4. **Set SessionID.** Every run requires a session ID for proper grouping and
-   memory association.
+4. **Set SessionID for sessionful runs.** `Run` and `Start` require a session ID
+   for grouping and memory association. `OneShotRun` is explicitly sessionless.
 
 5. **Trust the contracts.** Don't add defensive checks for values guaranteed by
    Goa validation or construction. Let violations fail fast.

runtime/agent/runtime/runtime.go

@@ -1392,13 +1392,9 @@ func (r *Runtime) startRunOn(ctx context.Context, input *RunInput, workflowName,
 		}
 	}
 	if requireSession {
-		if req.SearchAttributes == nil {
-			req.SearchAttributes = make(map[string]any, 1)
-		}
 		if v, ok := req.SearchAttributes["SessionID"]; ok && v != input.SessionID {
 			return nil, fmt.Errorf("workflow search attribute SessionID=%v does not match session id %q", v, input.SessionID)
 		}
-		req.SearchAttributes["SessionID"] = input.SessionID
 		now := time.Now().UTC()
 		if err := r.SessionStore.UpsertRun(ctx, session.RunMeta{
 			AgentID:   string(input.AgentID),

runtime/agent/runtime/runtime_test.go

@@ -103,6 +103,26 @@ func TestStartRunRequiresSessionID(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestStartRunDoesNotInjectSessionSearchAttribute(t *testing.T) {
+	eng := &stubEngine{}
+	rt := &Runtime{
+		Engine:       eng,
+		logger:       telemetry.NoopLogger{},
+		metrics:      telemetry.NoopMetrics{},
+		tracer:       telemetry.NoopTracer{},
+		SessionStore: sessioninmem.New(),
+		agents: map[agent.Ident]AgentRegistration{
+			"service.agent": {ID: "service.agent", Workflow: engine.WorkflowDefinition{Name: "service.workflow", TaskQueue: "q"}},
+		},
+	}
+	client := rt.MustClient(agent.Ident("service.agent"))
+	_, err := rt.CreateSession(context.Background(), "sess-1")
+	require.NoError(t, err)
+	_, err = client.Start(context.Background(), "sess-1", nil)
+	require.NoError(t, err)
+	require.Nil(t, eng.last.SearchAttributes)
+}
+
 func TestFinishWithoutToolCalls_UsesPlannerFinalToolResult(t *testing.T) {
 	rt := &Runtime{
 		logger:        telemetry.NoopLogger{},
@@ -524,8 +544,7 @@ func TestStartRunForwardsWorkflowOptions(t *testing.T) {
 	require.Equal(t, in.RunID, eng.last.ID)
 	require.Equal(t, in.WorkflowOptions.Memo, eng.last.Memo)
 	require.Equal(t, map[string]any{
-		"SessionID": in.SessionID,
-		"sa":        "x",
+		"sa": "x",
 	}, eng.last.SearchAttributes)
 	require.Equal(t, 5, eng.last.RetryPolicy.MaxAttempts)
 	require.Equal(t, 5*time.Second, eng.last.RetryPolicy.InitialInterval)