fix(mcp): repair stdio startup diagnostics

internal/agent/tools/mcp/channel.go

@@ -285,6 +285,9 @@ type channelTransport struct {
 }
 
 // Connect implements mcp.Transport.
+// unwrapTransport implements [transportWrapper].
+func (t *channelTransport) unwrapTransport() mcp.Transport { return t.inner }
+
 func (t *channelTransport) Connect(ctx context.Context) (mcp.Connection, error) {
 	conn, err := t.inner.Connect(ctx)
 	if err != nil {

internal/agent/tools/mcp/init.go

@@ -967,6 +967,25 @@ func createSession(ctx context.Context, cfg *config.ConfigStore, name string, m
 	}, nil
 }
 
+// transportWrapper is implemented by every transport decorator crush layers
+// around a base transport, so diagnostics that need the innermost transport
+// can reach it without knowing which decorators are in play.
+type transportWrapper interface {
+	unwrapTransport() mcp.Transport
+}
+
+// unwrapTransport peels every decorator off a transport and returns the
+// innermost one.
+func unwrapTransport(transport mcp.Transport) mcp.Transport {
+	for {
+		w, ok := transport.(transportWrapper)
+		if !ok {
+			return transport
+		}
+		transport = w.unwrapTransport()
+	}
+}
+
 // maybeStdioErr if a stdio mcp prints an error in non-json format, it'll fail
 // to parse, and the cli will then close it, causing the EOF error.
 // so, if we got an EOF err, and the transport is STDIO, we try to exec it
@@ -978,6 +997,13 @@ func maybeStdioErr(err error, transport mcp.Transport) error {
 	if !errors.Is(err, io.EOF) {
 		return err
 	}
+	// The transport is wrapped in one or more decorators before Connect (the
+	// channel gate today); the stdio transport we're probing for is the
+	// innermost one. Unwrap all of them — without this the assertion below
+	// never matches and stdio startup failures report a bare EOF instead of
+	// the child's actual output. Every wrapper must implement
+	// unwrapTransport or it will hide this diagnostic again.
+	transport = unwrapTransport(transport)
 	ct, ok := transport.(*mcp.CommandTransport)
 	if !ok {
 		return err
@@ -1277,7 +1303,14 @@ func clearMCPData(name string) {
 func stdioCheck(old *exec.Cmd) error {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
 	defer cancel()
-	cmd := exec.CommandContext(ctx, old.Path, old.Args...)
+	// old.Args includes argv0 as the first element; exec.CommandContext
+	// prepends old.Path as argv0, so we must skip it to avoid duplication
+	// (e.g. "npx npx -y pkg" instead of "npx -y pkg").
+	args := old.Args
+	if len(args) > 0 {
+		args = args[1:]
+	}
+	cmd := exec.CommandContext(ctx, old.Path, args...)
 	cmd.Env = old.Env
 	out, err := cmd.CombinedOutput()
 	if err == nil || errors.Is(ctx.Err(), context.DeadlineExceeded) {

internal/agent/tools/mcp/lifecycle_test.go

@@ -3,6 +3,8 @@ package mcp
 import (
 	"context"
 	"errors"
+	"io"
+	"os/exec"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -393,3 +395,63 @@ func TestGetOrRenewClient_RestoresPromptsAndResources(t *testing.T) {
 	require.Equal(t, Counts{Tools: 1, Prompts: 1, Resources: 1}, info.Counts,
 		"reported counts must match the restored registries")
 }
+
+// testTransportWrapper is a second, test-local decorator. maybeStdioErr must
+// see through an arbitrary stack of them, not just the one wrapper that
+// happens to exist in createSession today.
+type testTransportWrapper struct {
+	mcp.Transport
+	inner mcp.Transport
+}
+
+func (t *testTransportWrapper) unwrapTransport() mcp.Transport { return t.inner }
+
+// TestMaybeStdioErr_UnwrapsChannelTransport pins that maybeStdioErr sees
+// through the channelTransport wrapper to the inner CommandTransport.
+//
+// Every transport is wrapped in a channelTransport before Connect, so the
+// *mcp.CommandTransport assertion never matched and a failed stdio server (a
+// missing npx, node not on PATH) reported a bare EOF with the child's stderr
+// thrown away — the exact diagnostic stdioCheck exists to provide. We assert
+// both that the unwrap reaches the command (the error is no longer bare EOF)
+// and that the re-executed child's output surfaces in the joined error.
+func TestMaybeStdioErr_UnwrapsChannelTransport(t *testing.T) {
+	cmd := exec.CommandContext(t.Context(), "sh", "-c", "echo 'startup failed: bad config'; exit 3")
+	inner := &mcp.CommandTransport{Command: cmd}
+	wrapped := &channelTransport{inner: inner, name: "t", gate: newChannelGate()}
+
+	got := maybeStdioErr(io.EOF, wrapped)
+	require.Error(t, got)
+	require.NotEqual(t, io.EOF, got, "the unwrap must reach the command transport")
+	require.ErrorContains(t, got, "startup failed: bad config",
+		"the re-executed child's output must surface in the error")
+}
+
+// TestMaybeStdioErr_UnwrapsEveryWrapper pins the unwrap against future
+// decorators: it must peel the whole stack, not a fixed number of layers.
+func TestMaybeStdioErr_UnwrapsEveryWrapper(t *testing.T) {
+	cmd := exec.CommandContext(t.Context(), "sh", "-c", "echo boom-diagnostic >&2; exit 3")
+	var transport mcp.Transport = &mcp.CommandTransport{Command: cmd}
+	transport = &channelTransport{inner: transport, name: "t", gate: newChannelGate()}
+	transport = &testTransportWrapper{inner: transport}
+
+	got := maybeStdioErr(io.EOF, transport)
+	require.ErrorContains(t, got, "boom-diagnostic",
+		"stdio diagnostics must survive every transport decorator")
+}
+
+// TestStdioCheck_DoesNotDuplicateArgv0 pins the argv0 handling in the
+// diagnostic re-run. exec.Cmd.Args carries argv0 as its first element and
+// exec.CommandContext prepends Path as argv0 itself, so passing Args through
+// whole re-ran "sh sh -c ..." — and the error reported that malformed
+// command's failure instead of the child's real startup output.
+func TestStdioCheck_DoesNotDuplicateArgv0(t *testing.T) {
+	cmd := exec.CommandContext(t.Context(), "sh", "-c", "echo 'real startup error'; exit 3")
+
+	err := stdioCheck(cmd)
+	require.Error(t, err)
+	require.ErrorContains(t, err, "real startup error",
+		"the re-run must execute the original command, not a duplicated argv0")
+	require.NotContains(t, err.Error(), "cannot execute binary file",
+		"a duplicated argv0 makes the shell try to exec itself as a script")
+}