diff --git a/pkg/agent/instance.go b/pkg/agent/instance.go
index dfbef9fbc..df339676e 100644
--- a/pkg/agent/instance.go
+++ b/pkg/agent/instance.go
@@ -83,6 +83,11 @@ func NewAgentInstance(
 		maxTokens = 8192
 	}
 
+	contextWindow := defaults.ContextWindow
+	if contextWindow == 0 {
+		contextWindow = 128000
+	}
+
 	temperature := 0.7
 	if defaults.Temperature != nil {
 		temperature = *defaults.Temperature
@@ -104,7 +109,7 @@ func NewAgentInstance(
 		MaxIterations:  maxIter,
 		MaxTokens:      maxTokens,
 		Temperature:    temperature,
-		ContextWindow:  maxTokens,
+		ContextWindow:  contextWindow,
 		Provider:       provider,
 		Sessions:       sessionsManager,
 		ContextBuilder: contextBuilder,
diff --git a/pkg/agent/instance_test.go b/pkg/agent/instance_test.go
index fcc8e9bea..16c5c01aa 100644
--- a/pkg/agent/instance_test.go
+++ b/pkg/agent/instance_test.go
@@ -93,3 +93,56 @@ func TestNewAgentInstance_DefaultsTemperatureWhenUnset(t *testing.T) {
 		t.Fatalf("Temperature = %f, want %f", agent.Temperature, 0.7)
 	}
 }
+
+func TestNewAgentInstance_UsesConfiguredContextWindow(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "agent-instance-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	cfg := &config.Config{
+		Agents: config.AgentsConfig{
+			Defaults: config.AgentDefaults{
+				Workspace:         tmpDir,
+				Model:             "test-model",
+				MaxTokens:         1234,
+				MaxToolIterations: 5,
+				ContextWindow:     64000,
+			},
+		},
+	}
+
+	provider := &mockProvider{}
+	agent := NewAgentInstance(nil, &cfg.Agents.Defaults, cfg, provider)
+
+	if agent.ContextWindow != 64000 {
+		t.Fatalf("ContextWindow = %d, want %d", agent.ContextWindow, 64000)
+	}
+}
+
+func TestNewAgentInstance_DefaultsContextWindowWhenZero(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "agent-instance-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	cfg := &config.Config{
+		Agents: config.AgentsConfig{
+			Defaults: config.AgentDefaults{
+				Workspace:         tmpDir,
+				Model:             "test-model",
+				MaxTokens:         1234,
+				MaxToolIterations: 5,
+			},
+		},
+	}
+
+	provider := &mockProvider{}
+	agent := NewAgentInstance(nil, &cfg.Agents.Defaults, cfg, provider)
+
+	if agent.ContextWindow != 128000 {
+		t.Fatalf("ContextWindow = %d, want %d", agent.ContextWindow, 128000)
+	}
+}
diff --git a/pkg/config/config.go b/pkg/config/config.go
index 20556011a..87a8a3cf3 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -175,6 +175,7 @@ type AgentDefaults struct {
 	ImageModel          string   `json:"image_model,omitempty"           env:"PICOCLAW_AGENTS_DEFAULTS_IMAGE_MODEL"`
 	ImageModelFallbacks []string `json:"image_model_fallbacks,omitempty"`
 	MaxTokens           int      `json:"max_tokens"                      env:"PICOCLAW_AGENTS_DEFAULTS_MAX_TOKENS"`
+	ContextWindow       int      `json:"context_window"                  env:"PICOCLAW_AGENTS_DEFAULTS_CONTEXT_WINDOW"`
 	Temperature         *float64 `json:"temperature,omitempty"           env:"PICOCLAW_AGENTS_DEFAULTS_TEMPERATURE"`
 	MaxToolIterations   int      `json:"max_tool_iterations"             env:"PICOCLAW_AGENTS_DEFAULTS_MAX_TOOL_ITERATIONS"`
 }