state: expose estimateTokens/stripCold on interface; remove type assertion

pkg/agent/loop.go

@@ -75,10 +75,8 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 
 	for {
 		// Proactive context gate: strip cold material before calling the backend.
-		if s, ok := state.(*Session); ok {
-			if budget := contextBudget(ctx, cfg.Backend); budget > 0 && s.estimateTokens() > budget {
-				s.stripColdResults()
-			}
+		if budget := contextBudget(ctx, cfg.Backend); budget > 0 && state.estimateTokens() > budget {
+			state.stripCold()
 		}
 
 		sc := StepContext{

pkg/agent/loop_maxsteps_helpers_test.go

@@ -69,3 +69,13 @@ func (s *simpleState) update(msg backend.Message, results []toolResult) {
 		})
 	}
 }
+
+func (s *simpleState) estimateTokens() int {
+	chars := 0
+	for _, m := range s.msgs {
+		chars += len(m.Content)
+	}
+	return chars / 4
+}
+
+func (s *simpleState) stripCold() {}

pkg/agent/session.go

@@ -531,6 +531,9 @@ func (s *Session) stripColdResults() {
 	}
 }
 
+// stripCold implements the state interface.
+func (s *Session) stripCold() { s.stripColdResults() }
+
 // contextDebug returns a multi-line breakdown of the history.
 func (s *Session) contextDebug() string {
 	var sb strings.Builder

pkg/agent/state.go

@@ -40,4 +40,9 @@ type state interface {
 	// Called once per loop iteration after Act completes.
 	update(assistant backend.Message, results []toolResult)
 
+	// estimateTokens returns a rough token count of the current history.
+	estimateTokens() int
+
+	// stripCold collapses large tool-result messages in-place to reduce context size.
+	stripCold()
 }