diff --git a/runtime/llm/client.go b/runtime/llm/client.go
index e07db59..08e1469 100644
--- a/runtime/llm/client.go
+++ b/runtime/llm/client.go
@@ -89,8 +89,19 @@ func (c *Client) run(ctx context.Context, model string, messages []Message, tool
 	defer resp.Body.Close()
 	if resp.StatusCode >= 400 {
 		slurp, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
-		retryable := resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode >= 500
-		emitError(out, "", retryable, fmt.Sprintf("provider status %d: %s", resp.StatusCode, strings.TrimSpace(string(slurp))))
+		msg := fmt.Sprintf("provider status %d: %s", resp.StatusCode, strings.TrimSpace(string(slurp)))
+		ev := Event{
+			Kind:         KindError,
+			Retryable:    StatusRetryable(resp.StatusCode) && !ContextOverflow(msg),
+			ErrorMessage: msg,
+			Err:          &StreamError{Message: msg, Retryable: StatusRetryable(resp.StatusCode) && !ContextOverflow(msg)},
+			Message:      Message{Role: RoleAssistant, StopReason: StopError},
+		}
+		if d, ok := ParseRetryAfter(resp.Header.Get("Retry-After"), time.Now()); ok {
+			ev.HasRetryAfter = true
+			ev.RetryAfter = d
+		}
+		out <- ev
 		return
 	}
 	for ev := range ParseSSE(ctx, resp.Body) {
diff --git a/runtime/llm/providers.go b/runtime/llm/providers.go
index 6d9a048..456f55a 100644
--- a/runtime/llm/providers.go
+++ b/runtime/llm/providers.go
@@ -4,7 +4,10 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"math/rand"
+	"net/http"
 	"os"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -382,31 +385,136 @@ func firstNonEmpty(a, b string) string {
 	return b
 }
 
+const (
+	retryBaseDelay = 200 * time.Millisecond
+	maxRetryAfter  = 30 * time.Second
+)
+
+// overflowNeedles are substrings that mean the prompt does not fit.
+// Matching is case-insensitive. A bare "context" is not enough.
+var overflowNeedles = []string{
+	"status 413",
+	"payload too large",
+	"context_length_exceeded",
+	"context length",
+	"maximum context",
+	"context window",
+	"prompt is too long",
+	"exceeds the maximum",
+	"exceeds max",
+	"too many tokens",
+	"input is too long",
+	"input length",
+	"tokens exceed",
+	"上下文",
+	"超出最大",
+	"输入过长",
+	"提示过长",
+	"请求过长",
+	"请求体过大",
+}
+
 // ContextOverflow reports provider errors that ask the caller to compact.
 func ContextOverflow(message string) bool {
 	m := strings.ToLower(message)
-	return strings.Contains(m, "context") && (strings.Contains(m, "overflow") || strings.Contains(m, "too long") || strings.Contains(m, "length"))
+	for _, needle := range overflowNeedles {
+		if strings.Contains(m, strings.ToLower(needle)) {
+			return true
+		}
+	}
+	return false
+}
+
+// CapacityLimited reports an in-stream overload that is worth retrying
+// before any delta has been forwarded.
+func CapacityLimited(message string) bool {
+	m := strings.ToLower(message)
+	return strings.Contains(m, "overloaded") || strings.Contains(m, "at capacity") || strings.Contains(m, "too many requests") || strings.Contains(m, "rate limit")
+}
+
+// StatusRetryable is true for timeouts, rate limits, server errors, and 529.
+func StatusRetryable(code int) bool {
+	return code == http.StatusRequestTimeout || code == http.StatusTooManyRequests || code == 529 || code >= 500
+}
+
+// ParseRetryAfter reads a delay in seconds or an HTTP date.
+// The returned duration is capped. ok is false when the header is missing or garbage.
+func ParseRetryAfter(raw string, now time.Time) (time.Duration, bool) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return 0, false
+	}
+	if secs, err := strconv.Atoi(raw); err == nil {
+		if secs < 0 {
+			return 0, false
+		}
+		return capRetryAfter(time.Duration(secs) * time.Second), true
+	}
+	when, err := http.ParseTime(raw)
+	if err != nil {
+		return 0, false
+	}
+	d := when.Sub(now)
+	if d < 0 {
+		d = 0
+	}
+	return capRetryAfter(d), true
+}
+
+func capRetryAfter(d time.Duration) time.Duration {
+	if d > maxRetryAfter {
+		return maxRetryAfter
+	}
+	return d
+}
+
+func retryBackoff(extraTry int) time.Duration {
+	if extraTry < 1 {
+		extraTry = 1
+	}
+	shift := extraTry - 1
+	if shift > 4 {
+		shift = 4
+	}
+	delay := retryBaseDelay * time.Duration(1<<uint(shift))
+	jitter := time.Duration(rand.Int63n(int64(delay/2) + 1))
+	return delay + jitter
+}
+
+func forwardedDelta(ev Event) bool {
+	switch ev.Kind {
+	case KindTextDelta, KindReasoningDelta, KindToolCallDelta:
+		return true
+	default:
+		return false
+	}
 }
 
 // WithRetry re-invokes base on a retryable terminal error, up to attempts
-// extra tries. onEvent receives auto_retry_start and auto_retry_end.
-// Context-overflow errors are returned immediately so the caller can compact.
+// extra tries. Events are forwarded as they arrive. A retry happens only when
+// no delta has been forwarded yet. onEvent receives auto_retry_start and
+// auto_retry_end. Context-overflow errors are returned immediately.
+// attempts <= 0 disables extra tries.
 func WithRetry(base StreamFunc, attempts int, onEvent func(kind, detail string)) StreamFunc {
-	if attempts < 1 {
-		attempts = 1
+	if attempts < 0 {
+		attempts = 0
 	}
 	return func(ctx context.Context, model string, messages []Message, tools []ToolDecl, opts StreamOptions) <-chan Event {
 		out := make(chan Event)
 		go func() {
 			defer close(out)
-			var last []Event
+			headerWait := time.Duration(0)
+			useHeader := false
 			for try := 0; try <= attempts; try++ {
 				if try > 0 {
+					delay := retryBackoff(try)
+					if useHeader {
+						delay = headerWait
+					}
 					detail := fmt.Sprintf("attempt %d", try+1)
 					if onEvent != nil {
 						onEvent("auto_retry_start", detail)
 					}
-					delay := time.Duration(200*(1<<uint(try-1))) * time.Millisecond
 					select {
 					case <-ctx.Done():
 						emitAbort(out, "", ctx.Err())
@@ -414,32 +522,52 @@ func WithRetry(base StreamFunc, attempts int, onEvent func(kind, detail string))
 					case <-time.After(delay):
 					}
 				}
-				last = nil
+				useHeader = false
+				forwarded := false
+				var terminal *Event
 				for ev := range base(ctx, model, messages, tools, opts) {
-					last = append(last, ev)
-				}
-				overflow, retryable := false, false
-				for _, ev := range last {
-					if ev.Kind == KindError && ContextOverflow(ev.ErrorMessage) {
-						overflow = true
+					if terminal != nil {
+						continue
 					}
-					if ev.Kind == KindError && ev.Retryable {
-						retryable = true
+					if ev.Kind == KindError {
+						cp := ev
+						terminal = &cp
+						continue
+					}
+					if forwardedDelta(ev) {
+						forwarded = true
+					}
+					select {
+					case <-ctx.Done():
+						return
+					case out <- ev:
+					}
+					if ev.Kind == KindMessageEnd {
+						if try > 0 && onEvent != nil {
+							onEvent("auto_retry_end", fmt.Sprintf("attempt %d", try+1))
+						}
+						return
 					}
 				}
-				if overflow || !retryable || try == attempts {
+				if terminal == nil {
+					return
+				}
+				overflow := ContextOverflow(terminal.ErrorMessage)
+				if forwarded || overflow || !terminal.Retryable || try == attempts {
 					if try > 0 && onEvent != nil {
 						onEvent("auto_retry_end", fmt.Sprintf("attempt %d", try+1))
 					}
-					for _, ev := range last {
-						select {
-						case <-ctx.Done():
-							return
-						case out <- ev:
-						}
+					select {
+					case <-ctx.Done():
+						return
+					case out <- *terminal:
 					}
 					return
 				}
+				if terminal.HasRetryAfter {
+					useHeader = true
+					headerWait = terminal.RetryAfter
+				}
 			}
 		}()
 		return out
diff --git a/runtime/llm/providers_test.go b/runtime/llm/providers_test.go
index 27a184d..eeb6c56 100644
--- a/runtime/llm/providers_test.go
+++ b/runtime/llm/providers_test.go
@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestParseModelRefAndThinking(t *testing.T) {
@@ -141,6 +142,75 @@ func TestRetryThenSuccess(t *testing.T) {
 	}
 }
 
+func TestContextOverflowPatterns(t *testing.T) {
+	hits := []string{
+		"provider status 413: payload too large",
+		"context_length_exceeded",
+		"prompt is too long",
+		"input length exceeds the maximum",
+		"maximum context length exceeded",
+		"context length overflow",
+		"too many tokens",
+		"输入过长，请缩短上下文",
+		"请求过长",
+		"超出最大长度",
+	}
+	for _, msg := range hits {
+		if !ContextOverflow(msg) {
+			t.Fatalf("want overflow: %s", msg)
+		}
+	}
+	if ContextOverflow("context cancelled") || ContextOverflow("temporary network error") {
+		t.Fatal("unrelated context text must not count as overflow")
+	}
+}
+
+func TestParseRetryAfter(t *testing.T) {
+	now := time.Date(2026, 10, 3, 12, 0, 0, 0, time.UTC)
+	if d, ok := ParseRetryAfter("2", now); !ok || d != 2*time.Second {
+		t.Fatalf("seconds: %v %v", d, ok)
+	}
+	if d, ok := ParseRetryAfter("not-a-date", now); ok || d != 0 {
+		t.Fatalf("garbage should fail: %v %v", d, ok)
+	}
+	if _, ok := ParseRetryAfter("", now); ok {
+		t.Fatal("empty header")
+	}
+	far := now.Add(2 * time.Hour).Format(http.TimeFormat)
+	if d, ok := ParseRetryAfter(far, now); !ok || d != maxRetryAfter {
+		t.Fatalf("date cap: %v %v", d, ok)
+	}
+	past := now.Add(-time.Minute).Format(http.TimeFormat)
+	if d, ok := ParseRetryAfter(past, now); !ok || d != 0 {
+		t.Fatalf("past date: %v %v", d, ok)
+	}
+}
+
+func TestRetryDoesNotRepeatDeltas(t *testing.T) {
+	n := 0
+	base := StreamFunc(func(ctx context.Context, model string, messages []Message, tools []ToolDecl, opts StreamOptions) <-chan Event {
+		out := make(chan Event, 2)
+		n++
+		out <- Event{Kind: KindTextDelta, Text: "a"}
+		out <- Event{Kind: KindError, Retryable: true, ErrorMessage: "overloaded"}
+		close(out)
+		return out
+	})
+	var texts []string
+	var errors int
+	for ev := range WithRetry(base, 2, nil)(context.Background(), "m", nil, nil, StreamOptions{}) {
+		if ev.Kind == KindTextDelta {
+			texts = append(texts, ev.Text)
+		}
+		if ev.Kind == KindError {
+			errors++
+		}
+	}
+	if n != 1 || len(texts) != 1 || texts[0] != "a" || errors != 1 {
+		t.Fatalf("calls=%d texts=%v errors=%d", n, texts, errors)
+	}
+}
+
 func TestContextOverflowIsNotRetried(t *testing.T) {
 	if !ContextOverflow("maximum context length exceeded") {
 		t.Fatal("expected overflow")
diff --git a/runtime/llm/sse.go b/runtime/llm/sse.go
index 0bd3f32..92a367b 100644
--- a/runtime/llm/sse.go
+++ b/runtime/llm/sse.go
@@ -56,6 +56,7 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 	order := []int{}
 	var usage Usage
 	finished := ""
+	sawDone := false
 	sc := bufio.NewScanner(r)
 	sc.Buffer(make([]byte, 0, 64*1024), 4*1024*1024)
 	reader := &sseEventReader{sc: sc}
@@ -69,6 +70,7 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 			return
 		}
 		if data == "[DONE]" {
+			sawDone = true
 			break
 		}
 		var chunk sseChunk
@@ -77,7 +79,9 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 			return
 		}
 		if chunk.Error != nil && chunk.Error.Message != "" {
-			emitError(out, content.String(), false, chunk.Error.Message)
+			msg := chunk.Error.Message
+			retryable := CapacityLimited(msg) && !ContextOverflow(msg)
+			emitError(out, content.String(), retryable, msg)
 			return
 		}
 		if chunk.Usage != nil {
@@ -157,6 +161,10 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 		emitAbort(out, content.String(), ctx.Err())
 		return
 	}
+	// length、空结束原因，或没有 [DONE] 的中断，都不把半截工具调用交给执行层。
+	if !toolFinishExecutable(finished, sawDone) {
+		order = nil
+	}
 	msg := Message{Role: RoleAssistant, Content: content.String(), StopReason: mapFinish(finished, len(order) > 0)}
 	for _, idx := range order {
 		msg.ToolCalls = append(msg.ToolCalls, *calls[idx])
@@ -222,13 +230,32 @@ func (r *sseEventReader) next() (string, bool) {
 	return "", false
 }
 
+// toolFinishExecutable 只在供应商明确以工具调用或正常 stop 结束时执行。
+// length、缺失的结束原因，以及没有 [DONE] 的断开，都视为未完成。
+func toolFinishExecutable(reason string, sawDone bool) bool {
+	switch reason {
+	case "tool_calls", "function_call":
+		return true
+	case "stop":
+		return true
+	case "length":
+		return false
+	case "":
+		return false
+	default:
+		return sawDone && reason != "error" && reason != "content_filter"
+	}
+}
+
 func mapFinish(reason string, hasTools bool) StopReason {
 	switch reason {
 	case "tool_calls", "function_call":
 		return StopToolCalls
 	case "content_filter", "error":
 		return StopError
-	case "stop", "length", "":
+	case "length":
+		return StopEnd
+	case "stop", "":
 		if hasTools {
 			return StopToolCalls
 		}
diff --git a/runtime/llm/sse_test.go b/runtime/llm/sse_test.go
index 3a4f801..5b943bc 100644
--- a/runtime/llm/sse_test.go
+++ b/runtime/llm/sse_test.go
@@ -93,6 +93,51 @@ func TestParseSSEToolCallIncrements(t *testing.T) {
 	}
 }
 
+func TestParseSSELengthDropsUnfinishedToolCall(t *testing.T) {
+	body := strings.Join([]string{
+		`data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_1","function":{"name":"read_file","arguments":"{\"path\":"}}]}}]}`,
+		`data: {"choices":[{"delta":{},"finish_reason":"length"}]}`,
+		`data: [DONE]`,
+		"",
+	}, "\n")
+	var end Event
+	for ev := range ParseSSE(context.Background(), strings.NewReader(body)) {
+		if ev.Kind == KindMessageEnd {
+			end = ev
+		}
+	}
+	if end.Message.StopReason != StopEnd || len(end.Message.ToolCalls) != 0 {
+		t.Fatalf("truncated tool call must not be executable: %#v", end.Message)
+	}
+	if end.FinishReason != "length" {
+		t.Fatalf("finish %q", end.FinishReason)
+	}
+}
+
+func TestParseSSEUncleanEndDropsToolCall(t *testing.T) {
+	body := `data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_1","function":{"name":"read_file","arguments":"{\"path\":"}}]}}]}`
+	var end Event
+	for ev := range ParseSSE(context.Background(), strings.NewReader(body)) {
+		if ev.Kind == KindMessageEnd {
+			end = ev
+		}
+	}
+	if end.Kind != KindMessageEnd || end.Message.StopReason != StopEnd || len(end.Message.ToolCalls) != 0 {
+		t.Fatalf("unclean stream must not execute tools: %#v", end)
+	}
+}
+
+func TestParseSSECapacityErrorIsRetryable(t *testing.T) {
+	body := "data: {\"error\":{\"message\":\"server is at capacity\"}}\n\n"
+	var got Event
+	for ev := range ParseSSE(context.Background(), strings.NewReader(body)) {
+		got = ev
+	}
+	if got.Kind != KindError || !got.Retryable || !CapacityLimited(got.ErrorMessage) {
+		t.Fatalf("event %#v", got)
+	}
+}
+
 func TestParseSSEMalformedIsError(t *testing.T) {
 	body := "data: {not-json}\n\n"
 	var got Event
diff --git a/runtime/llm/types.go b/runtime/llm/types.go
index abf7035..b000cac 100644
--- a/runtime/llm/types.go
+++ b/runtime/llm/types.go
@@ -3,6 +3,7 @@ package llm
 import (
 	"context"
 	"encoding/json"
+	"time"
 )
 
 // Role is a transcript role carried into the provider request.
@@ -69,13 +70,19 @@ type Image struct {
 	Data string `json:"data,omitempty"`
 }
 
-// Usage counts tokens reported by the provider. Zero means the provider omitted usage.
+// Usage counts tokens. On one provider response, PromptTokens, CompletionTokens
+// and TotalTokens are that request. On a loop Result they are the billing sum
+// across requests; Last* is only the latest request, for context observation.
+// Zero means the provider omitted usage.
 type Usage struct {
-	PromptTokens     int `json:"prompt_tokens"`
-	CompletionTokens int `json:"completion_tokens"`
-	TotalTokens      int `json:"total_tokens"`
-	CacheReadTokens  int `json:"cache_read_tokens,omitempty"`
-	CacheWriteTokens int `json:"cache_write_tokens,omitempty"`
+	PromptTokens         int `json:"prompt_tokens"`
+	CompletionTokens     int `json:"completion_tokens"`
+	TotalTokens          int `json:"total_tokens"`
+	CacheReadTokens      int `json:"cache_read_tokens,omitempty"`
+	CacheWriteTokens     int `json:"cache_write_tokens,omitempty"`
+	LastPromptTokens     int `json:"last_prompt_tokens,omitempty"`
+	LastCompletionTokens int `json:"last_completion_tokens,omitempty"`
+	LastTotalTokens      int `json:"last_total_tokens,omitempty"`
 }
 
 // Event is one streamed update. Failures are KindError plus a final assistant
@@ -96,6 +103,10 @@ type Event struct {
 	Err          error
 	Retryable    bool
 	ErrorMessage string
+	// RetryAfter is the provider's requested wait. HasRetryAfter distinguishes
+	// a real zero from a missing or unparseable header.
+	RetryAfter    time.Duration
+	HasRetryAfter bool
 }
 
 // StreamOptions tunes one provider request.
diff --git a/runtime/loop.go b/runtime/loop.go
index cd9c9e4..74f9530 100644
--- a/runtime/loop.go
+++ b/runtime/loop.go
@@ -192,6 +192,9 @@ func Run(ctx context.Context, cfg Config, user llm.Message) (Result, error) {
 		usage.PromptTokens += end.Usage.PromptTokens
 		usage.CompletionTokens += end.Usage.CompletionTokens
 		usage.TotalTokens += end.Usage.TotalTokens
+		usage.LastPromptTokens = end.Usage.PromptTokens
+		usage.LastCompletionTokens = end.Usage.CompletionTokens
+		usage.LastTotalTokens = end.Usage.TotalTokens
 		if err := persist(cfg, last); err != nil {
 			return Result{}, err
 		}
@@ -464,10 +467,30 @@ func cancelledToolMessage(call llm.ToolCall, err error) llm.Message {
 	}
 }
 
+func rejectIncompleteToolCall(call llm.ToolCall) string {
+	if strings.TrimSpace(call.Name) == "" {
+		return "工具调用未执行：名称不完整"
+	}
+	args := strings.TrimSpace(call.Arguments)
+	if !json.Valid([]byte(args)) || !strings.HasPrefix(args, "{") {
+		return "工具调用未执行：参数不是完整的 JSON 对象"
+	}
+	return ""
+}
+
 func runOne(ctx context.Context, cfg Config, call llm.ToolCall) llm.Message {
 	if err := ctx.Err(); err != nil {
 		return cancelledToolMessage(call, err)
 	}
+	if why := rejectIncompleteToolCall(call); why != "" {
+		msg := llm.Message{Role: llm.RoleTool, Name: call.Name, ToolCallID: call.ID, Content: why, IsError: true}
+		emit(cfg, Event{Type: "tool_execution_start", Tool: call.Name, Detail: call.Arguments})
+		emit(cfg, Event{Type: "tool_execution_end", Tool: call.Name, Text: why})
+		if cfg.AfterTool != nil {
+			cfg.AfterTool(call, msg)
+		}
+		return msg
+	}
 	emit(cfg, Event{Type: "tool_execution_start", Tool: call.Name, Detail: call.Arguments})
 	if cfg.BeforeTool != nil {
 		if err := cfg.BeforeTool(call); err != nil {
@@ -616,8 +639,8 @@ func budgetExceeded(b *Budget, usage llm.Usage, toolCalls int, now time.Time) st
 	if !b.Deadline.IsZero() && now.After(b.Deadline) {
 		return fmt.Sprintf("budget_exceeded: deadline %s passed", b.Deadline.Format(time.RFC3339))
 	}
-	if b.MaxPromptTokens > 0 && usage.PromptTokens > b.MaxPromptTokens {
-		return fmt.Sprintf("budget_exceeded: prompt tokens %d > max %d", usage.PromptTokens, b.MaxPromptTokens)
+	if b.MaxPromptTokens > 0 && usage.LastPromptTokens > b.MaxPromptTokens {
+		return fmt.Sprintf("budget_exceeded: prompt tokens %d > max %d", usage.LastPromptTokens, b.MaxPromptTokens)
 	}
 	if b.MaxCompletionTokens > 0 && usage.CompletionTokens > b.MaxCompletionTokens {
 		return fmt.Sprintf("budget_exceeded: completion tokens %d > max %d", usage.CompletionTokens, b.MaxCompletionTokens)
diff --git a/runtime/loop_budget_test.go b/runtime/loop_budget_test.go
index 5da9702..f2a09a7 100644
--- a/runtime/loop_budget_test.go
+++ b/runtime/loop_budget_test.go
@@ -194,6 +194,57 @@ func TestLoopBudgetPromptTokens(t *testing.T) {
 	}
 }
 
+func TestLoopUsageSplitsLastRequestFromCumulative(t *testing.T) {
+	res, err := Run(context.Background(), Config{
+		Model:  "fake",
+		Tools:  tools.WorkspaceTools(t.TempDir()),
+		Budget: &Budget{MaxPromptTokens: 10},
+		Stream: usageScript(llm.Usage{PromptTokens: 6, CompletionTokens: 1, TotalTokens: 7},
+			llm.Message{Role: llm.RoleAssistant, StopReason: llm.StopToolCalls, ToolCalls: []llm.ToolCall{{
+				ID: "c1", Name: "list_dir", Arguments: `{}`,
+			}}},
+			llm.Message{Role: llm.RoleAssistant, Content: "ok", StopReason: llm.StopEnd},
+		),
+	}, llm.Message{Role: llm.RoleUser, Content: "go"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Err != nil || res.Text != "ok" {
+		t.Fatalf("last request stays under the window: err=%v text=%q", res.Err, res.Text)
+	}
+	if res.Usage.PromptTokens != 12 || res.Usage.LastPromptTokens != 6 {
+		t.Fatalf("billing sum %d last %d", res.Usage.PromptTokens, res.Usage.LastPromptTokens)
+	}
+}
+
+func TestLoopRejectsIncompleteToolCall(t *testing.T) {
+	tool := &countingTool{name: "list_dir"}
+	reg := tools.NewRegistry(t.TempDir())
+	reg.Register(tool)
+	res, err := Run(context.Background(), Config{
+		Model: "fake",
+		Tools: reg,
+		Stream: script(llm.Message{Role: llm.RoleAssistant, StopReason: llm.StopToolCalls, ToolCalls: []llm.ToolCall{{
+			ID: "c1", Name: "list_dir", Arguments: `{"path":`,
+		}}}, llm.Message{Role: llm.RoleAssistant, Content: "stopped", StopReason: llm.StopEnd}),
+	}, llm.Message{Role: llm.RoleUser, Content: "go"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if tool.calls != 0 {
+		t.Fatalf("incomplete call executed %d times", tool.calls)
+	}
+	var paired bool
+	for _, m := range res.Messages {
+		if m.Role == llm.RoleTool && m.ToolCallID == "c1" && m.IsError {
+			paired = true
+		}
+	}
+	if !paired {
+		t.Fatal("expected a paired error result")
+	}
+}
+
 func TestLoopBudgetDeadline(t *testing.T) {
 	res, err := Run(context.Background(), Config{
 		Model:  "fake",