diff --git a/pkg/runtime/client.go b/pkg/runtime/client.go
index 195e83c9b..947b4e664 100644
--- a/pkg/runtime/client.go
+++ b/pkg/runtime/client.go
@@ -400,6 +400,10 @@ func (c *Client) runAgentWithAgentName(ctx context.Context, sessionID, agent, ag
 		defer resp.Body.Close()
 
 		scanner := bufio.NewScanner(resp.Body)
+		// A single SSE line can carry a large tool response; raise the cap
+		// above bufio's 64 KiB default so an oversized line does not silently
+		// truncate the stream (bufio.ErrTooLong).
+		scanner.Buffer(make([]byte, 0, bufio.MaxScanTokenSize), maxSSELineBytes)
 		for scanner.Scan() {
 			line := scanner.Bytes()
 			if len(line) == 0 || line[0] == ':' {
@@ -438,7 +442,12 @@ func (c *Client) runAgentWithAgentName(ctx context.Context, sessionID, agent, ag
 			eventChan <- e
 		}
 
+		// Surface a read failure (e.g. an over-long line) instead of ending
+		// the stream silently — otherwise the run appears to stop with no
+		// error after the last event that fit.
 		if err := scanner.Err(); err != nil {
+			slog.DebugContext(ctx, "event", "scanner_error", err)
+			eventChan <- Error(fmt.Sprintf("reading event stream: %v", err))
 			return
 		}
 	}()
@@ -541,6 +550,10 @@ func (c *Client) StreamSessionEvents(ctx context.Context, sessionID string) (<-c
 		defer resp.Body.Close()
 
 		scanner := bufio.NewScanner(resp.Body)
+		// A single SSE line can carry a large tool response; raise the cap
+		// above bufio's 64 KiB default so an oversized line does not silently
+		// truncate the stream (bufio.ErrTooLong).
+		scanner.Buffer(make([]byte, 0, bufio.MaxScanTokenSize), maxSSELineBytes)
 		for scanner.Scan() {
 			line := scanner.Bytes()
 			if len(line) == 0 || line[0] == ':' {
@@ -579,8 +592,12 @@ func (c *Client) StreamSessionEvents(ctx context.Context, sessionID string) (<-c
 			eventChan <- e
 		}
 
+		// Surface a read failure (e.g. an over-long line) instead of ending
+		// the stream silently — otherwise the run appears to stop with no
+		// error after the last event that fit.
 		if err := scanner.Err(); err != nil {
 			slog.DebugContext(ctx, "scanner error", "error", err)
+			eventChan <- Error(fmt.Sprintf("reading event stream: %v", err))
 		}
 	}()
 
diff --git a/pkg/runtime/defaults.go b/pkg/runtime/defaults.go
index 36983ecf7..13ecfc371 100644
--- a/pkg/runtime/defaults.go
+++ b/pkg/runtime/defaults.go
@@ -12,6 +12,17 @@ import "time"
 // so the buffer size is set in exactly one place.
 const defaultEventChannelCapacity = 128
 
+// maxSSELineBytes bounds a single Server-Sent-Events line the client SSE
+// reader will accept. Each event is delivered as one `data: {json}` line, and
+// a tool result can be large (e.g. a big tool-response payload), so the default
+// bufio.Scanner cap of 64 KiB (bufio.MaxScanTokenSize) truncates the stream:
+// the oversized line trips bufio.ErrTooLong, the scan ends, and the run appears
+// to stop silently after the preceding event. The reader raises its buffer to
+// this ceiling and surfaces any remaining scanner error as an Error event
+// rather than closing the stream without a trace. Sized to comfortably hold a
+// large tool response while still bounding per-line memory.
+const maxSSELineBytes = 16 * 1024 * 1024
+
 // defaultMaxOverflowCompactions caps the number of consecutive
 // context-overflow auto-compactions that the run loop will attempt before
 // giving up and surfacing the error to the caller. The runtime's