diff --git a/pkg/agent/context_budget.go b/pkg/agent/context_budget.go
new file mode 100644
index 000000000..2eec9c267
--- /dev/null
+++ b/pkg/agent/context_budget.go
@@ -0,0 +1,133 @@
+// PicoClaw - Ultra-lightweight personal AI agent
+// License: MIT
+//
+// Copyright (c) 2026 PicoClaw contributors
+
+package agent
+
+import (
+	"encoding/json"
+	"unicode/utf8"
+
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
+
+// isSafeBoundary reports whether index is a valid position to split a message
+// history for truncation or compression. Splitting at index means:
+//   - history[:index] is dropped or summarized
+//   - history[index:] is kept
+//
+// A boundary is safe when the kept portion begins at a "user" message,
+// ensuring no tool-call sequence (assistant+ToolCalls → tool results)
+// is torn apart across the split.
+func isSafeBoundary(history []providers.Message, index int) bool {
+	if index <= 0 || index >= len(history) {
+		return true
+	}
+	return history[index].Role == "user"
+}
+
+// findSafeBoundary locates the nearest safe split point to targetIndex.
+// It scans backward first (preserving more context), then forward.
+// Returns targetIndex unchanged only when no safe boundary exists.
+func findSafeBoundary(history []providers.Message, targetIndex int) int {
+	if len(history) == 0 {
+		return 0
+	}
+	if targetIndex <= 0 {
+		return 0
+	}
+	if targetIndex >= len(history) {
+		return len(history)
+	}
+
+	if isSafeBoundary(history, targetIndex) {
+		return targetIndex
+	}
+
+	// Backward scan: prefer keeping more messages.
+	for i := targetIndex - 1; i > 0; i-- {
+		if isSafeBoundary(history, i) {
+			return i
+		}
+	}
+
+	// Forward scan: fall back to keeping fewer messages.
+	for i := targetIndex + 1; i < len(history); i++ {
+		if isSafeBoundary(history, i) {
+			return i
+		}
+	}
+
+	return targetIndex
+}
+
+// estimateMessageTokens estimates the token count for a single message,
+// including Content, ToolCalls arguments, and ToolCallID metadata.
+// Uses a heuristic of 2.5 characters per token.
+func estimateMessageTokens(msg providers.Message) int {
+	chars := utf8.RuneCountInString(msg.Content)
+
+	for _, tc := range msg.ToolCalls {
+		// Count tool call metadata: ID, type, function name
+		chars += len(tc.ID) + len(tc.Type) + len(tc.Name)
+		if tc.Function != nil {
+			chars += len(tc.Function.Name) + len(tc.Function.Arguments)
+		}
+	}
+
+	if msg.ToolCallID != "" {
+		chars += len(msg.ToolCallID)
+	}
+
+	// Per-message overhead for role label, JSON structure, separators.
+	const messageOverhead = 12
+	chars += messageOverhead
+
+	return chars * 2 / 5
+}
+
+// estimateToolDefsTokens estimates the total token cost of tool definitions
+// as they appear in the LLM request. Each tool's name, description, and
+// JSON schema parameters contribute to the context window budget.
+func estimateToolDefsTokens(defs []providers.ToolDefinition) int {
+	if len(defs) == 0 {
+		return 0
+	}
+
+	totalChars := 0
+	for _, d := range defs {
+		totalChars += len(d.Function.Name) + len(d.Function.Description)
+
+		if d.Function.Parameters != nil {
+			if paramJSON, err := json.Marshal(d.Function.Parameters); err == nil {
+				totalChars += len(paramJSON)
+			}
+		}
+
+		// Per-tool overhead: type field, JSON structure, separators.
+		totalChars += 20
+	}
+
+	return totalChars * 2 / 5
+}
+
+// isOverContextBudget checks whether the assembled messages plus tool definitions
+// and output reserve would exceed the model's context window. This enables
+// proactive compression before calling the LLM, rather than reacting to 400 errors.
+func isOverContextBudget(
+	contextWindow int,
+	messages []providers.Message,
+	toolDefs []providers.ToolDefinition,
+	maxTokens int,
+) bool {
+	msgTokens := 0
+	for _, m := range messages {
+		msgTokens += estimateMessageTokens(m)
+	}
+
+	toolTokens := estimateToolDefsTokens(toolDefs)
+	total := msgTokens + toolTokens + maxTokens
+
+	return total > contextWindow
+}
diff --git a/pkg/agent/context_budget_test.go b/pkg/agent/context_budget_test.go
new file mode 100644
index 000000000..c8a6b19c5
--- /dev/null
+++ b/pkg/agent/context_budget_test.go
@@ -0,0 +1,545 @@
+package agent
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
+
+// msgUser creates a user message.
+func msgUser(content string) providers.Message {
+	return providers.Message{Role: "user", Content: content}
+}
+
+// msgAssistant creates a plain assistant message (no tool calls).
+func msgAssistant(content string) providers.Message {
+	return providers.Message{Role: "assistant", Content: content}
+}
+
+// msgAssistantTC creates an assistant message with tool calls.
+func msgAssistantTC(toolIDs ...string) providers.Message {
+	tcs := make([]providers.ToolCall, len(toolIDs))
+	for i, id := range toolIDs {
+		tcs[i] = providers.ToolCall{
+			ID:   id,
+			Type: "function",
+			Name: "tool_" + id,
+			Function: &providers.FunctionCall{
+				Name:      "tool_" + id,
+				Arguments: `{"key":"value"}`,
+			},
+		}
+	}
+	return providers.Message{Role: "assistant", ToolCalls: tcs}
+}
+
+// msgTool creates a tool result message.
+func msgTool(callID, content string) providers.Message {
+	return providers.Message{Role: "tool", ToolCallID: callID, Content: content}
+}
+
+func TestIsSafeBoundary(t *testing.T) {
+	tests := []struct {
+		name    string
+		history []providers.Message
+		index   int
+		want    bool
+	}{
+		{
+			name:    "empty history, index 0",
+			history: nil,
+			index:   0,
+			want:    true,
+		},
+		{
+			name:    "single user message, index 0",
+			history: []providers.Message{msgUser("hi")},
+			index:   0,
+			want:    true,
+		},
+		{
+			name:    "single user message, index 1 (end)",
+			history: []providers.Message{msgUser("hi")},
+			index:   1,
+			want:    true,
+		},
+		{
+			name: "at user message",
+			history: []providers.Message{
+				msgAssistant("hello"),
+				msgUser("how are you"),
+				msgAssistant("fine"),
+			},
+			index: 1,
+			want:  true,
+		},
+		{
+			name: "at assistant without tool calls",
+			history: []providers.Message{
+				msgUser("hello"),
+				msgAssistant("response"),
+				msgUser("follow up"),
+			},
+			index: 1,
+			want:  false,
+		},
+		{
+			name: "at assistant with tool calls",
+			history: []providers.Message{
+				msgUser("search something"),
+				msgAssistantTC("tc1"),
+				msgTool("tc1", "result"),
+				msgAssistant("here is what I found"),
+			},
+			index: 1,
+			want:  false,
+		},
+		{
+			name: "at tool result",
+			history: []providers.Message{
+				msgUser("do something"),
+				msgAssistantTC("tc1"),
+				msgTool("tc1", "done"),
+				msgAssistant("completed"),
+			},
+			index: 2,
+			want:  false,
+		},
+		{
+			name: "negative index",
+			history: []providers.Message{
+				msgUser("hello"),
+			},
+			index: -1,
+			want:  true,
+		},
+		{
+			name: "index beyond length",
+			history: []providers.Message{
+				msgUser("hello"),
+			},
+			index: 5,
+			want:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := isSafeBoundary(tt.history, tt.index)
+			if got != tt.want {
+				t.Errorf("isSafeBoundary(history, %d) = %v, want %v", tt.index, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestFindSafeBoundary(t *testing.T) {
+	tests := []struct {
+		name        string
+		history     []providers.Message
+		targetIndex int
+		want        int
+	}{
+		{
+			name:        "empty history",
+			history:     nil,
+			targetIndex: 0,
+			want:        0,
+		},
+		{
+			name:        "target at 0",
+			history:     []providers.Message{msgUser("hi")},
+			targetIndex: 0,
+			want:        0,
+		},
+		{
+			name:        "target beyond length",
+			history:     []providers.Message{msgUser("hi")},
+			targetIndex: 5,
+			want:        1,
+		},
+		{
+			name: "target already at user message",
+			history: []providers.Message{
+				msgUser("q1"),
+				msgAssistant("a1"),
+				msgUser("q2"),
+				msgAssistant("a2"),
+			},
+			targetIndex: 2,
+			want:        2,
+		},
+		{
+			name: "target at assistant, scan backward finds user",
+			history: []providers.Message{
+				msgUser("q1"),
+				msgAssistant("a1"),
+				msgUser("q2"),
+				msgAssistant("a2"),
+				msgUser("q3"),
+			},
+			targetIndex: 3, // assistant "a2"
+			want:        2, // backward to user "q2"
+		},
+		{
+			name: "target inside tool sequence, scan backward finds user",
+			history: []providers.Message{
+				msgUser("q1"),
+				msgAssistant("a1"),
+				msgUser("q2"),
+				msgAssistantTC("tc1", "tc2"),
+				msgTool("tc1", "r1"),
+				msgTool("tc2", "r2"),
+				msgAssistant("summary"),
+				msgUser("q3"),
+			},
+			targetIndex: 4, // tool result "r1"
+			want:        2, // backward: 3=assistant+TC (not safe), 2=user → safe
+		},
+		{
+			name: "target inside tool sequence, backward finds user before chain",
+			history: []providers.Message{
+				msgUser("q1"),
+				msgAssistant("a1"),
+				msgUser("q2"),
+				msgAssistantTC("tc1", "tc2"),
+				msgTool("tc1", "r1"),
+				msgTool("tc2", "r2"),
+				msgAssistant("summary"),
+				msgUser("q3"),
+			},
+			targetIndex: 5, // tool result "r2"
+			want:        2, // backward: 4=tool, 3=assistant+TC, 2=user → safe
+		},
+		{
+			name: "no backward user, scan forward finds one",
+			history: []providers.Message{
+				msgAssistantTC("tc1"),
+				msgTool("tc1", "r1"),
+				msgAssistant("a1"),
+				msgUser("q1"),
+			},
+			targetIndex: 1, // tool result
+			want:        3, // forward to user "q1"
+		},
+		{
+			name: "multi-step tool chain preserves atomicity",
+			history: []providers.Message{
+				msgUser("q1"),
+				msgAssistant("a1"),
+				msgUser("q2"),
+				msgAssistantTC("tc1"),
+				msgTool("tc1", "r1"),
+				msgAssistantTC("tc2"),
+				msgTool("tc2", "r2"),
+				msgAssistant("final"),
+				msgUser("q3"),
+				msgAssistant("a3"),
+			},
+			targetIndex: 5, // second assistant+TC
+			want:        2, // backward: 4=tool, 3=assistant+TC, 2=user → safe
+		},
+		{
+			name: "all non-user messages returns target unchanged",
+			history: []providers.Message{
+				msgAssistant("a1"),
+				msgAssistant("a2"),
+				msgAssistant("a3"),
+			},
+			targetIndex: 1,
+			want:        1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := findSafeBoundary(tt.history, tt.targetIndex)
+			if got != tt.want {
+				t.Errorf("findSafeBoundary(history, %d) = %d, want %d",
+					tt.targetIndex, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestFindSafeBoundary_BackwardScanSkipsToolSequence(t *testing.T) {
+	// A long tool-call chain: user → assistant+TC → tool → tool → ... → assistant → user
+	// Target is inside the chain; boundary should skip the entire chain backward.
+	history := []providers.Message{
+		msgUser("start"),                 // 0
+		msgAssistant("before chain"),     // 1
+		msgUser("trigger"),               // 2 ← expected safe boundary
+		msgAssistantTC("t1", "t2", "t3"), // 3
+		msgTool("t1", "r1"),              // 4
+		msgTool("t2", "r2"),              // 5
+		msgTool("t3", "r3"),              // 6
+		msgAssistantTC("t4"),             // 7
+		msgTool("t4", "r4"),              // 8
+		msgAssistant("chain done"),       // 9
+		msgUser("next"),                  // 10
+	}
+
+	// Target at index 6 (middle of tool results)
+	got := findSafeBoundary(history, 6)
+	if got != 2 {
+		t.Errorf("findSafeBoundary(history, 6) = %d, want 2 (user before chain)", got)
+	}
+}
+
+func TestEstimateMessageTokens(t *testing.T) {
+	tests := []struct {
+		name string
+		msg  providers.Message
+		want int // minimum expected tokens (exact value depends on overhead)
+	}{
+		{
+			name: "plain user message",
+			msg:  msgUser("Hello, world!"),
+			want: 1, // at least some tokens
+		},
+		{
+			name: "empty message still has overhead",
+			msg:  providers.Message{Role: "user"},
+			want: 1, // message overhead alone
+		},
+		{
+			name: "assistant with tool calls",
+			msg:  msgAssistantTC("tc_123"),
+			want: 1,
+		},
+		{
+			name: "tool result with ID",
+			msg:  msgTool("call_abc", "Here is the search result with lots of content"),
+			want: 1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := estimateMessageTokens(tt.msg)
+			if got < tt.want {
+				t.Errorf("estimateMessageTokens() = %d, want >= %d", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestEstimateMessageTokens_ToolCallsContribute(t *testing.T) {
+	plain := msgAssistant("thinking")
+	withTC := providers.Message{
+		Role:    "assistant",
+		Content: "thinking",
+		ToolCalls: []providers.ToolCall{
+			{
+				ID:   "call_1",
+				Type: "function",
+				Name: "web_search",
+				Function: &providers.FunctionCall{
+					Name:      "web_search",
+					Arguments: `{"query":"picoclaw agent framework","max_results":5}`,
+				},
+			},
+		},
+	}
+
+	plainTokens := estimateMessageTokens(plain)
+	withTCTokens := estimateMessageTokens(withTC)
+
+	if withTCTokens <= plainTokens {
+		t.Errorf("message with ToolCalls (%d tokens) should exceed plain message (%d tokens)",
+			withTCTokens, plainTokens)
+	}
+}
+
+func TestEstimateMessageTokens_MultibyteContent(t *testing.T) {
+	// Multi-byte characters (e.g. emoji, accented letters) are single runes
+	// but may map to different token counts. The heuristic should still produce
+	// reasonable estimates via RuneCountInString.
+	msg := msgUser("caf\u00e9 na\u00efve r\u00e9sum\u00e9 \u00fcber stra\u00dfe")
+	tokens := estimateMessageTokens(msg)
+	if tokens <= 0 {
+		t.Errorf("multibyte message should produce positive token count, got %d", tokens)
+	}
+}
+
+func TestEstimateMessageTokens_LargeArguments(t *testing.T) {
+	// Simulate a tool call with large JSON arguments.
+	largeArgs := fmt.Sprintf(`{"content":"%s"}`, strings.Repeat("x", 5000))
+	msg := providers.Message{
+		Role: "assistant",
+		ToolCalls: []providers.ToolCall{
+			{
+				ID:   "call_large",
+				Type: "function",
+				Name: "write_file",
+				Function: &providers.FunctionCall{
+					Name:      "write_file",
+					Arguments: largeArgs,
+				},
+			},
+		},
+	}
+
+	tokens := estimateMessageTokens(msg)
+	// 5000+ chars → at least 2000 tokens with the 2.5 char/token heuristic
+	if tokens < 2000 {
+		t.Errorf("large tool call arguments should produce significant token count, got %d", tokens)
+	}
+}
+
+// --- estimateToolDefsTokens tests ---
+
+func TestEstimateToolDefsTokens(t *testing.T) {
+	tests := []struct {
+		name string
+		defs []providers.ToolDefinition
+		want int // minimum expected tokens
+	}{
+		{
+			name: "empty tool list",
+			defs: nil,
+			want: 0,
+		},
+		{
+			name: "single tool with params",
+			defs: []providers.ToolDefinition{
+				{
+					Type: "function",
+					Function: providers.ToolFunctionDefinition{
+						Name:        "web_search",
+						Description: "Search the web for information",
+						Parameters: map[string]any{
+							"type": "object",
+							"properties": map[string]any{
+								"query": map[string]any{"type": "string"},
+							},
+							"required": []any{"query"},
+						},
+					},
+				},
+			},
+			want: 1,
+		},
+		{
+			name: "tool without params",
+			defs: []providers.ToolDefinition{
+				{
+					Type: "function",
+					Function: providers.ToolFunctionDefinition{
+						Name:        "list_dir",
+						Description: "List directory contents",
+					},
+				},
+			},
+			want: 1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := estimateToolDefsTokens(tt.defs)
+			if got < tt.want {
+				t.Errorf("estimateToolDefsTokens() = %d, want >= %d", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestEstimateToolDefsTokens_ScalesWithCount(t *testing.T) {
+	makeTool := func(name string) providers.ToolDefinition {
+		return providers.ToolDefinition{
+			Type: "function",
+			Function: providers.ToolFunctionDefinition{
+				Name:        name,
+				Description: "A test tool that does something useful",
+				Parameters: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"input": map[string]any{"type": "string", "description": "Input value"},
+					},
+				},
+			},
+		}
+	}
+
+	one := estimateToolDefsTokens([]providers.ToolDefinition{makeTool("tool_a")})
+	three := estimateToolDefsTokens([]providers.ToolDefinition{
+		makeTool("tool_a"), makeTool("tool_b"), makeTool("tool_c"),
+	})
+
+	if three <= one {
+		t.Errorf("3 tools (%d tokens) should exceed 1 tool (%d tokens)", three, one)
+	}
+}
+
+// --- isOverContextBudget tests ---
+
+func TestIsOverContextBudget(t *testing.T) {
+	systemMsg := providers.Message{Role: "system", Content: strings.Repeat("x", 1000)}
+	userMsg := msgUser("hello")
+	smallHistory := []providers.Message{systemMsg, msgUser("q1"), msgAssistant("a1"), userMsg}
+
+	tools := []providers.ToolDefinition{
+		{
+			Type: "function",
+			Function: providers.ToolFunctionDefinition{
+				Name:        "test_tool",
+				Description: "A test tool",
+				Parameters:  map[string]any{"type": "object"},
+			},
+		},
+	}
+
+	tests := []struct {
+		name          string
+		contextWindow int
+		messages      []providers.Message
+		toolDefs      []providers.ToolDefinition
+		maxTokens     int
+		want          bool
+	}{
+		{
+			name:          "within budget",
+			contextWindow: 100000,
+			messages:      smallHistory,
+			toolDefs:      tools,
+			maxTokens:     4096,
+			want:          false,
+		},
+		{
+			name:          "over budget with small window",
+			contextWindow: 100, // very small window
+			messages:      smallHistory,
+			toolDefs:      tools,
+			maxTokens:     4096,
+			want:          true,
+		},
+		{
+			name:          "large max_tokens eats budget",
+			contextWindow: 2000,
+			messages:      smallHistory,
+			toolDefs:      tools,
+			maxTokens:     1800, // leaves almost no room
+			want:          true,
+		},
+		{
+			name:          "empty messages within budget",
+			contextWindow: 10000,
+			messages:      nil,
+			toolDefs:      nil,
+			maxTokens:     4096,
+			want:          false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := isOverContextBudget(tt.contextWindow, tt.messages, tt.toolDefs, tt.maxTokens)
+			if got != tt.want {
+				t.Errorf("isOverContextBudget() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
diff --git a/pkg/agent/instance.go b/pkg/agent/instance.go
index 0c7baa1ee..c34f9b4a4 100644
--- a/pkg/agent/instance.go
+++ b/pkg/agent/instance.go
@@ -127,6 +127,17 @@ func NewAgentInstance(
 		maxTokens = 8192
 	}
 
+	contextWindow := defaults.ContextWindow
+	if contextWindow == 0 {
+		// Default heuristic: 4x the output token limit.
+		// Most models have context windows well above their output limits
+		// (e.g., GPT-4o 128k ctx / 16k out, Claude 200k ctx / 8k out).
+		// 4x is a conservative lower bound that avoids premature
+		// summarization while remaining safe — the reactive
+		// forceCompression handles any overshoot.
+		contextWindow = maxTokens * 4
+	}
+
 	temperature := 0.7
 	if defaults.Temperature != nil {
 		temperature = *defaults.Temperature
@@ -224,7 +235,7 @@ func NewAgentInstance(
 		MaxTokens:                 maxTokens,
 		Temperature:               temperature,
 		ThinkingLevel:             thinkingLevel,
-		ContextWindow:             maxTokens,
+		ContextWindow:             contextWindow,
 		SummarizeMessageThreshold: summarizeMessageThreshold,
 		SummarizeTokenPercent:     summarizeTokenPercent,
 		Provider:                  provider,
diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 21516e7de..f20f2c938 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -17,7 +17,6 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
-	"unicode/utf8"
 
 	"github.com/sipeed/picoclaw/pkg/bus"
 	"github.com/sipeed/picoclaw/pkg/channels"
@@ -931,6 +930,24 @@ func (al *AgentLoop) runAgentLoop(
 	maxMediaSize := cfg.Agents.Defaults.GetMaxMediaSize()
 	messages = resolveMediaRefs(messages, al.mediaStore, maxMediaSize)
 
+	// 1.5. Proactive context budget check: compress before LLM call
+	// rather than waiting for a 400 context-length error.
+	if !opts.NoHistory {
+		toolDefs := agent.Tools.ToProviderDefs()
+		if isOverContextBudget(agent.ContextWindow, messages, toolDefs, agent.MaxTokens) {
+			logger.WarnCF("agent", "Proactive compression: context budget exceeded before LLM call",
+				map[string]any{"session_key": opts.SessionKey})
+			al.forceCompression(agent, opts.SessionKey)
+			newHistory := agent.Sessions.GetHistory(opts.SessionKey)
+			newSummary := agent.Sessions.GetSummary(opts.SessionKey)
+			messages = agent.ContextBuilder.BuildMessages(
+				newHistory, newSummary, opts.UserMessage,
+				opts.Media, opts.Channel, opts.ChatID,
+			)
+			messages = resolveMediaRefs(messages, al.mediaStore, maxMediaSize)
+		}
+	}
+
 	// 2. Save user message to session
 	agent.Sessions.AddMessage(opts.SessionKey, "user", opts.UserMessage)
 
@@ -1539,7 +1556,8 @@ func (al *AgentLoop) maybeSummarize(agent *AgentInstance, sessionKey, channel, c
 }
 
 // forceCompression aggressively reduces context when the limit is hit.
-// It drops the oldest 50% of messages (keeping system prompt and last user message).
+// It drops the oldest ~50% of messages (keeping system prompt and last user message),
+// aligning the split to a safe boundary so tool-call sequences stay intact.
 func (al *AgentLoop) forceCompression(agent *AgentInstance, sessionKey string) {
 	history := agent.Sessions.GetHistory(sessionKey)
 	if len(history) <= 4 {
@@ -1554,8 +1572,8 @@ func (al *AgentLoop) forceCompression(agent *AgentInstance, sessionKey string) {
 		return
 	}
 
-	// Helper to find the mid-point of the conversation
-	mid := len(conversation) / 2
+	// Find a safe mid-point that does not split a tool-call sequence.
+	mid := findSafeBoundary(conversation, len(conversation)/2)
 
 	// New history structure:
 	// 1. System Prompt (with compression note appended)
@@ -1687,12 +1705,18 @@ func (al *AgentLoop) summarizeSession(agent *AgentInstance, sessionKey string) {
 	history := agent.Sessions.GetHistory(sessionKey)
 	summary := agent.Sessions.GetSummary(sessionKey)
 
-	// Keep last 4 messages for continuity
+	// Keep last few messages for continuity, aligned to a safe boundary
+	// so that no tool-call sequence is split.
 	if len(history) <= 4 {
 		return
 	}
 
-	toSummarize := history[:len(history)-4]
+	safeCut := findSafeBoundary(history, len(history)-4)
+	if safeCut <= 0 {
+		return
+	}
+	keepCount := len(history) - safeCut
+	toSummarize := history[:safeCut]
 
 	// Oversized Message Guard
 	maxMessageTokens := agent.ContextWindow / 2
@@ -1757,7 +1781,7 @@ func (al *AgentLoop) summarizeSession(agent *AgentInstance, sessionKey string) {
 
 	if finalSummary != "" {
 		agent.Sessions.SetSummary(sessionKey, finalSummary)
-		agent.Sessions.TruncateHistory(sessionKey, 4)
+		agent.Sessions.TruncateHistory(sessionKey, keepCount)
 		agent.Sessions.Save(sessionKey)
 	}
 }
@@ -1895,15 +1919,14 @@ func (al *AgentLoop) summarizeBatch(
 }
 
 // estimateTokens estimates the number of tokens in a message list.
-// Uses a safe heuristic of 2.5 characters per token to account for CJK and other
-// overheads better than the previous 3 chars/token.
+// Counts Content, ToolCalls arguments, and ToolCallID metadata so that
+// tool-heavy conversations are not systematically undercounted.
 func (al *AgentLoop) estimateTokens(messages []providers.Message) int {
-	totalChars := 0
+	total := 0
 	for _, m := range messages {
-		totalChars += utf8.RuneCountInString(m.Content)
+		total += estimateMessageTokens(m)
 	}
-	// 2.5 chars per token = totalChars * 2 / 5
-	return totalChars * 2 / 5
+	return total
 }
 
 func (al *AgentLoop) handleCommand(
diff --git a/pkg/config/config.go b/pkg/config/config.go
index a8b8f337f..a3720b656 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -228,6 +228,7 @@ 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,omitempty"        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"`
 	SummarizeMessageThreshold int            `json:"summarize_message_threshold"     env:"PICOCLAW_AGENTS_DEFAULTS_SUMMARIZE_MESSAGE_THRESHOLD"`