Fix sidecar launch failure: resolve script path to absolute at startup

Root cause: sidecar command used relative path "sidecar/claude/src/index.ts"
but subprocess CWD was the workspace directory, not the Symphony project dir.
tsx couldn't find the file and exited silently (stderr at debug level).

Fixes:
1. Resolve sidecar script path to absolute using os.Getwd() at startup.
   Store in closure variable captured by adapter factory.
2. Call tsx directly instead of wrapping in bash -lc.
3. Validate sidecar script exists at startup with clear error message.
4. Add sidecar path check to --doctor output.
5. Log subprocess stderr at WARN (was DEBUG) so crash reasons are visible.
6. Remove empty default for SidecarCommand — resolved dynamically in main.

End-to-end verified: sidecar now initializes, creates sessions, completes
prompt turns with stop_reason=completed against real GitHub project items.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shivamstaq

cmd/symphony/main.go

@@ -173,6 +173,24 @@ func main() {
 		},
 	})
 
+	// Resolve sidecar script path to absolute (relative to where symphony was invoked)
+	symphonyDir, _ := os.Getwd()
+	var sidecarScript string
+	if cfg.Agent.Kind == "claude_code" {
+		sidecarScript = cfg.Claude.SidecarCommand
+		if sidecarScript == "" {
+			sidecarScript = filepath.Join(symphonyDir, "sidecar", "claude", "src", "index.ts")
+		} else if !filepath.IsAbs(sidecarScript) {
+			sidecarScript = filepath.Join(symphonyDir, sidecarScript)
+		}
+		// Validate sidecar script exists
+		if _, err := os.Stat(sidecarScript); err != nil {
+			logger.Error("sidecar script not found", "path", sidecarScript, "error", err)
+			os.Exit(1)
+		}
+		logger.Info("sidecar script resolved", "path", sidecarScript)
+	}
+
 	// Create worker runner
 	runner := orchestrator.NewRunner(orchestrator.WorkerDeps{
 		WorkspaceManager: wsMgr,
@@ -181,15 +199,10 @@ func main() {
 				Kind: cfg.Agent.Kind,
 				Cwd:  cwd,
 			}
-			// Set command based on agent kind
 			switch cfg.Agent.Kind {
 			case "claude_code":
-				acfg.Command = "bash"
-				cmd := cfg.Claude.SidecarCommand
-				if cmd == "" {
-					cmd = "tsx sidecar/claude/src/index.ts"
-				}
-				acfg.Args = []string{"-lc", cmd}
+				acfg.Command = "tsx"
+				acfg.Args = []string{sidecarScript}
 			case "opencode":
 				acfg.Command = "opencode"
 				acfg.Args = []string{"acp"}
@@ -392,6 +405,17 @@ func runDoctor(cfg *config.ServiceConfig, wsRoot, stateDir string) {
 			os.Exit(1)
 		}
 		fmt.Println("PASS: tsx found on PATH")
+		// Check sidecar script
+		sidecar := cfg.Claude.SidecarCommand
+		if sidecar == "" {
+			cwd, _ := os.Getwd()
+			sidecar = filepath.Join(cwd, "sidecar", "claude", "src", "index.ts")
+		}
+		if _, err := os.Stat(sidecar); err != nil {
+			fmt.Printf("FAIL: sidecar script not found: %s\n", sidecar)
+			os.Exit(1)
+		}
+		fmt.Printf("PASS: sidecar script found: %s\n", sidecar)
 	case "opencode":
 		if err := checkBinaryExists("opencode"); err != nil {
 			fmt.Printf("FAIL: opencode not found on PATH: %v\n", err)

internal/adapter/subprocess.go

@@ -214,14 +214,9 @@ func (a *SubprocessAdapter) handleCallbackRequest(msg *Message) {
 }
 
 func (a *SubprocessAdapter) drainStderr() {
-	buf := make([]byte, 4096)
-	for {
-		n, err := a.stderr.Read(buf)
-		if n > 0 {
-			slog.Debug("adapter stderr", "output", string(buf[:n]))
-		}
-		if err != nil {
-			return
-		}
+	data, err := io.ReadAll(a.stderr)
+	if len(data) > 0 {
+		slog.Warn("adapter stderr", "output", string(data))
 	}
+	_ = err
 }

internal/config/service_config.go

@@ -261,7 +261,7 @@ func (c *ServiceConfig) applyDefaults() {
 	c.Claude = ClaudeConfig{
 		AdapterMode:     "sdk_sidecar",
 		SDKLanguage:     "typescript",
-		SidecarCommand:  "tsx sidecar/claude/src/index.ts",
+		SidecarCommand:  "", // resolved to absolute path at startup in main.go
 		ContinueOnPause: true,
 	}
 	c.OpenCode = OpenCodeConfig{