diff --git a/go/client.go b/go/client.go
index c88a68ac3..d522b3bce 100644
--- a/go/client.go
+++ b/go/client.go
@@ -34,6 +34,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"os"
 	"os/exec"
@@ -91,6 +92,9 @@ type Client struct {
 	processDone            chan struct{}
 	processErrorPtr        *error
 	osProcess              atomic.Pointer[os.Process]
+	stderrBuf              []byte
+	stderrBufMux           sync.Mutex
+	stderrDone             chan struct{} // closed when the current process's stderr drain goroutine finishes
 
 	// RPC provides typed server-scoped RPC methods.
 	// This field is nil until the client is connected via Start().
@@ -275,15 +279,17 @@ func (c *Client) Start(ctx context.Context) error {
 	// Connect to the server
 	if err := c.connectToServer(ctx); err != nil {
 		killErr := c.killProcess()
+		stderrErr := c.stderrError()
 		c.state = StateError
-		return errors.Join(err, killErr)
+		return errors.Join(err, killErr, stderrErr)
 	}
 
 	// Verify protocol version compatibility
 	if err := c.verifyProtocolVersion(ctx); err != nil {
 		killErr := c.killProcess()
+		stderrErr := c.stderrError()
 		c.state = StateError
-		return errors.Join(err, killErr)
+		return errors.Join(err, killErr, stderrErr)
 	}
 
 	c.state = StateConnected
@@ -335,6 +341,9 @@ func (c *Client) Stop() error {
 		}
 	}
 	c.process = nil
+	c.stderrBufMux.Lock()
+	c.stderrBuf = nil
+	c.stderrBufMux.Unlock()
 
 	// Close external TCP connection if exists
 	if c.isExternalServer && c.conn != nil {
@@ -408,6 +417,9 @@ func (c *Client) ForceStop() {
 		_ = c.killProcess() // Ignore errors since we're force stopping
 	}
 	c.process = nil
+	c.stderrBufMux.Lock()
+	c.stderrBuf = nil
+	c.stderrBufMux.Unlock()
 
 	// Close external TCP connection if exists
 	if c.isExternalServer && c.conn != nil {
@@ -434,6 +446,42 @@ func (c *Client) ForceStop() {
 	c.RPC = nil
 }
 
+func (c *Client) getStderrOutput() string {
+	c.stderrBufMux.Lock()
+	defer c.stderrBufMux.Unlock()
+	return string(c.stderrBuf)
+}
+
+func (c *Client) stderrError() error {
+	if output := c.getStderrOutput(); output != "" {
+		return errors.New("stderr: " + output)
+	}
+	return nil
+}
+
+func (c *Client) startStderrDrain(stderr io.ReadCloser, done chan struct{}) {
+	go func() {
+		defer close(done)
+		buf := make([]byte, 1024)
+		for {
+			n, err := stderr.Read(buf)
+			if n > 0 {
+				c.stderrBufMux.Lock()
+				// Append to buffer, keep tail if > 64KB
+				c.stderrBuf = append(c.stderrBuf, buf[:n]...)
+				if len(c.stderrBuf) > 64*1024 {
+					n := copy(c.stderrBuf, c.stderrBuf[len(c.stderrBuf)-64*1024:])
+					c.stderrBuf = c.stderrBuf[:n]
+				}
+				c.stderrBufMux.Unlock()
+			}
+			if err != nil {
+				return
+			}
+		}
+	}()
+}
+
 func (c *Client) ensureConnected() error {
 	if c.client != nil {
 		return nil
@@ -1105,6 +1153,11 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 		c.process.Env = append(c.process.Env, "COPILOT_SDK_AUTH_TOKEN="+c.options.GitHubToken)
 	}
 
+	// Clear previous stderr buffer
+	c.stderrBufMux.Lock()
+	c.stderrBuf = nil
+	c.stderrBufMux.Unlock()
+
 	if c.useStdio {
 		// For stdio mode, we need stdin/stdout pipes
 		stdin, err := c.process.StdinPipe()
@@ -1117,11 +1170,17 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			return fmt.Errorf("failed to create stdout pipe: %w", err)
 		}
 
+		stderr, err := c.process.StderrPipe()
+		if err != nil {
+			return fmt.Errorf("failed to create stderr pipe: %w", err)
+		}
+
 		if err := c.process.Start(); err != nil {
-			return fmt.Errorf("failed to start CLI server: %w", err)
+			closeErr := stderr.Close()
+			return errors.Join(fmt.Errorf("failed to start CLI server: %w", err), closeErr)
 		}
 
-		c.monitorProcess()
+		c.monitorProcess(stderr)
 
 		// Create JSON-RPC client immediately
 		c.client = jsonrpc2.NewClient(stdin, stdout)
@@ -1138,11 +1197,17 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			return fmt.Errorf("failed to create stdout pipe: %w", err)
 		}
 
+		stderr, err := c.process.StderrPipe()
+		if err != nil {
+			return fmt.Errorf("failed to create stderr pipe: %w", err)
+		}
+
 		if err := c.process.Start(); err != nil {
-			return fmt.Errorf("failed to start CLI server: %w", err)
+			closeErr := stderr.Close()
+			return errors.Join(fmt.Errorf("failed to start CLI server: %w", err), closeErr)
 		}
 
-		c.monitorProcess()
+		c.monitorProcess(stderr)
 
 		scanner := bufio.NewScanner(stdout)
 		timeout := time.After(10 * time.Second)
@@ -1152,10 +1217,12 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			select {
 			case <-timeout:
 				killErr := c.killProcess()
-				return errors.Join(errors.New("timeout waiting for CLI server to start"), killErr)
+				stderrErr := c.stderrError()
+				return errors.Join(errors.New("timeout waiting for CLI server to start"), killErr, stderrErr)
 			case <-c.processDone:
 				killErr := c.killProcess()
-				return errors.Join(errors.New("CLI server process exited before reporting port"), killErr)
+				stderrErr := c.stderrError()
+				return errors.Join(errors.New("CLI server process exited before reporting port"), killErr, stderrErr)
 			default:
 				if scanner.Scan() {
 					line := scanner.Text()
@@ -1163,7 +1230,8 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 						port, err := strconv.Atoi(matches[1])
 						if err != nil {
 							killErr := c.killProcess()
-							return errors.Join(fmt.Errorf("failed to parse port: %w", err), killErr)
+							stderrErr := c.stderrError()
+							return errors.Join(fmt.Errorf("failed to parse port: %w", err), killErr, stderrErr)
 						}
 						c.actualPort = port
 						return nil
@@ -1175,13 +1243,17 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 }
 
 func (c *Client) killProcess() error {
+	var err error
 	if p := c.osProcess.Swap(nil); p != nil {
-		if err := p.Kill(); err != nil {
-			return fmt.Errorf("failed to kill CLI process: %w", err)
+		if err = p.Kill(); err != nil {
+			err = fmt.Errorf("failed to kill CLI process: %w", err)
 		}
 	}
+	if c.stderrDone != nil {
+		<-c.stderrDone
+	}
 	c.process = nil
-	return nil
+	return err
 }
 
 // monitorProcess signals when the CLI process exits and captures any exit error.
@@ -1189,7 +1261,10 @@ func (c *Client) killProcess() error {
 // error value, so goroutines from previous processes can't overwrite the
 // current one. Closing the channel synchronizes with readers, guaranteeing
 // they see the final processError value.
-func (c *Client) monitorProcess() {
+func (c *Client) monitorProcess(stderr io.ReadCloser) {
+	stderrDone := make(chan struct{})
+	c.stderrDone = stderrDone
+	c.startStderrDrain(stderr, stderrDone)
 	done := make(chan struct{})
 	c.processDone = done
 	proc := c.process
@@ -1198,6 +1273,7 @@ func (c *Client) monitorProcess() {
 	c.processErrorPtr = &processError
 	go func() {
 		waitErr := proc.Wait()
+		<-stderrDone
 		if waitErr != nil {
 			processError = fmt.Errorf("CLI process exited: %w", waitErr)
 		} else {
diff --git a/go/client_test.go b/go/client_test.go
index 752bdc758..91e42b147 100644
--- a/go/client_test.go
+++ b/go/client_test.go
@@ -3,9 +3,11 @@ package copilot
 import (
 	"encoding/json"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"strings"
 	"sync"
 	"testing"
 )
@@ -528,3 +530,96 @@ func TestClient_StartStopRace(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestClient_StderrCapture(t *testing.T) {
+	buildStderrFixtureCLI := func(t *testing.T) string {
+		t.Helper()
+
+		tmpDir := t.TempDir()
+		mainPath := filepath.Join(tmpDir, "main.go")
+		binaryName := "stderr-fixture.exe"
+		binaryPath := filepath.Join(tmpDir, binaryName)
+
+		source := `package main
+
+		import (
+			"os"
+			"strconv"
+			"strings"
+		)
+
+		func main() {
+			if sizeString := os.Getenv("TEST_STDERR_SIZE"); sizeString != "" {
+				if size, _ := strconv.Atoi(sizeString); size > 0 {
+					_, _ = os.Stderr.WriteString(strings.Repeat("x", size))
+				}
+			}
+			if message := os.Getenv("TEST_STDERR"); message != "" {
+				_, _ = os.Stderr.WriteString(message)
+			}
+			os.Exit(1)
+		}`
+
+		if err := os.WriteFile(mainPath, []byte(source), 0600); err != nil {
+			t.Fatal(err)
+		}
+
+		cmd := exec.Command("go", "build", "-o", binaryPath, mainPath)
+		if output, err := cmd.CombinedOutput(); err != nil {
+			t.Fatal(err, "\n", string(output))
+		}
+
+		return binaryPath
+	}
+
+	fixturePath := buildStderrFixtureCLI(t)
+
+	t.Run("captures stderr on startup failure", func(t *testing.T) {
+		client := NewClient(&ClientOptions{
+			CLIPath: fixturePath,
+			Env:     append(os.Environ(), "TEST_STDERR=something went wrong"),
+		})
+
+		err := client.Start(t.Context())
+		if err == nil {
+			t.Fatal("Expected error, got nil")
+		}
+
+		if !strings.Contains(err.Error(), "something went wrong") {
+			t.Errorf("Expected error to contain stderr output, got: %v", err)
+		}
+	})
+
+	t.Run("caps stderr buffer", func(t *testing.T) {
+		client := NewClient(&ClientOptions{
+			CLIPath: fixturePath,
+			Env:     append(os.Environ(), "TEST_STDERR_SIZE=70000"),
+		})
+
+		if err := client.Start(t.Context()); err == nil {
+			t.Fatal("Expected error, got nil")
+		}
+
+		output := client.getStderrOutput()
+		if len(output) > 64*1024+100 { // Allow some slack but it should be close to 64KB
+			t.Errorf("Expected buffer to be capped around 64KB, got %d bytes", len(output))
+		}
+	})
+
+	t.Run("clears buffer on stop", func(t *testing.T) {
+		client := NewClient(&ClientOptions{})
+		client.stderrBufMux.Lock()
+		client.stderrBuf = []byte("dirty buffer")
+		client.stderrBufMux.Unlock()
+
+		if err := client.Stop(); err != nil {
+			t.Fatal(err)
+		}
+
+		client.stderrBufMux.Lock()
+		if client.stderrBuf != nil {
+			t.Error("Expected stderrBuf to be nil after Stop")
+		}
+		client.stderrBufMux.Unlock()
+	})
+}
diff --git a/go/internal/jsonrpc2/jsonrpc2.go b/go/internal/jsonrpc2/jsonrpc2.go
index 09505c06d..997828d51 100644
--- a/go/internal/jsonrpc2/jsonrpc2.go
+++ b/go/internal/jsonrpc2/jsonrpc2.go
@@ -59,8 +59,7 @@ type Client struct {
 	stopChan        chan struct{}
 	wg              sync.WaitGroup
 	processDone     chan struct{} // closed when the underlying process exits
-	processError    error         // set before processDone is closed
-	processErrorMu  sync.RWMutex  // protects processError
+	processErrorPtr *error        // points to error set before processDone is closed
 }
 
 // NewClient creates a new JSON-RPC client
@@ -78,22 +77,17 @@ func NewClient(stdin io.WriteCloser, stdout io.ReadCloser) *Client {
 // and stores the error that should be returned to pending/future requests.
 func (c *Client) SetProcessDone(done chan struct{}, errPtr *error) {
 	c.processDone = done
-	// Monitor the channel and copy the error when it closes
-	go func() {
-		<-done
-		if errPtr != nil {
-			c.processErrorMu.Lock()
-			c.processError = *errPtr
-			c.processErrorMu.Unlock()
-		}
-	}()
+	c.processErrorPtr = errPtr
 }
 
-// getProcessError returns the process exit error if the process has exited
+// getProcessError returns the process exit error if the process has exited.
+// Must only be called after <-c.processDone to ensure visibility of the error
+// written before close(done) in the monitor goroutine.
 func (c *Client) getProcessError() error {
-	c.processErrorMu.RLock()
-	defer c.processErrorMu.RUnlock()
-	return c.processError
+	if c.processErrorPtr != nil {
+		return *c.processErrorPtr
+	}
+	return nil
 }
 
 // Start begins listening for messages in a background goroutine