feat(daemon): add opencode as supported agent provider (#341)

* feat(daemon): add opencode as supported agent provider

Add opencode backend alongside claude and codex. The backend spawns
`opencode run --format json`, parses streaming JSON events (text,
tool_use, error, step_start/finish), and supports --prompt for system
prompts. Includes CLI detection, AGENTS.md runtime config, native skill
discovery via .config/opencode/skills/, and 21 tests covering handlers,
JSON parsing, and integration-level processEvents scenarios.

* chore: add .tool-versions to gitignore

.gitignore

@@ -36,6 +36,7 @@ apps/web/test-results/
 
 # local settings
 .claude/
+.tool-versions
 
 # feature tracking
 _features/

server/internal/daemon/config.go

@@ -11,15 +11,15 @@ import (
 )
 
 const (
-	DefaultServerURL            = "ws://localhost:8080/ws"
+	DefaultServerURL             = "ws://localhost:8080/ws"
-	DefaultPollInterval         = 3 * time.Second
+	DefaultPollInterval          = 3 * time.Second
-	DefaultHeartbeatInterval    = 15 * time.Second
+	DefaultHeartbeatInterval     = 15 * time.Second
-	DefaultAgentTimeout         = 2 * time.Hour
+	DefaultAgentTimeout          = 2 * time.Hour
-	DefaultRuntimeName          = "Local Agent"
+	DefaultRuntimeName           = "Local Agent"
 	DefaultConfigReloadInterval  = 5 * time.Second
 	DefaultWorkspaceSyncInterval = 30 * time.Second
 	DefaultHealthPort            = 19514
-	DefaultMaxConcurrentTasks   = 20
+	DefaultMaxConcurrentTasks    = 20
 )
 
 // Config holds all daemon configuration.
@@ -30,7 +30,7 @@ type Config struct {
 	RuntimeName        string
 	CLIVersion         string                // multica CLI version (e.g. "0.1.13")
 	Profile            string                // profile name (empty = default)
-	Agents             map[string]AgentEntry // "claude" -> entry, "codex" -> entry
+	Agents             map[string]AgentEntry // "claude" -> entry, "codex" -> entry, "opencode" -> entry
 	WorkspacesRoot     string                // base path for execution envs (default: ~/multica_workspaces)
 	KeepEnvAfterTask   bool                  // preserve env after task for debugging
 	HealthPort         int                   // local HTTP port for health checks (default: 19514)
@@ -85,8 +85,15 @@ func LoadConfig(overrides Overrides) (Config, error) {
 			Model: strings.TrimSpace(os.Getenv("MULTICA_CODEX_MODEL")),
 		}
 	}
+	opencodePath := envOrDefault("MULTICA_OPENCODE_PATH", "opencode")
+	if _, err := exec.LookPath(opencodePath); err == nil {
+		agents["opencode"] = AgentEntry{
+			Path:  opencodePath,
+			Model: strings.TrimSpace(os.Getenv("MULTICA_OPENCODE_MODEL")),
+		}
+	}
 	if len(agents) == 0 {
-		return Config{}, fmt.Errorf("no agent CLI found: install claude or codex and ensure it is on PATH")
+		return Config{}, fmt.Errorf("no agent CLI found: install claude, codex, or opencode and ensure it is on PATH")
 	}
 
 	// Host info

server/internal/daemon/execenv/context.go

@@ -11,9 +11,10 @@ import (
 // writeContextFiles renders and writes .agent_context/issue_context.md and
 // skills into the appropriate provider-native location.
 //
-// Claude:  skills → {workDir}/.claude/skills/{name}/SKILL.md  (native discovery)
+// Claude:   skills → {workDir}/.claude/skills/{name}/SKILL.md  (native discovery)
-// Codex:   skills → handled separately in Prepare via codex-home
+// Codex:    skills → handled separately in Prepare via codex-home
-// Default: skills → {workDir}/.agent_context/skills/{name}/SKILL.md
+// OpenCode: skills → {workDir}/.config/opencode/skills/{name}/SKILL.md  (native discovery)
+// Default:  skills → {workDir}/.agent_context/skills/{name}/SKILL.md
 func writeContextFiles(workDir, provider string, ctx TaskContextForEnv) error {
 	contextDir := filepath.Join(workDir, ".agent_context")
 	if err := os.MkdirAll(contextDir, 0o755); err != nil {
@@ -50,6 +51,9 @@ func resolveSkillsDir(workDir, provider string) (string, error) {
 	case "claude":
 		// Claude Code natively discovers skills from .claude/skills/ in the workdir.
 		skillsDir = filepath.Join(workDir, ".claude", "skills")
+	case "opencode":
+		// OpenCode natively discovers skills from .config/opencode/skills/ in the workdir.
+		skillsDir = filepath.Join(workDir, ".config", "opencode", "skills")
 	default:
 		// Fallback: write to .agent_context/skills/ (referenced by meta config).
 		skillsDir = filepath.Join(workDir, ".agent_context", "skills")

server/internal/daemon/execenv/execenv_test.go

@@ -441,6 +441,148 @@ func TestInjectRuntimeConfigNoSkills(t *testing.T) {
 	}
 }
 
+func TestWriteContextFilesOpencodeNativeSkills(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+
+	ctx := TaskContextForEnv{
+		IssueID: "opencode-skill-test",
+		AgentSkills: []SkillContextForEnv{
+			{
+				Name:    "Go Conventions",
+				Content: "Follow Go conventions.",
+				Files: []SkillFileContextForEnv{
+					{Path: "templates/example.go", Content: "package main"},
+				},
+			},
+		},
+	}
+
+	if err := writeContextFiles(dir, "opencode", ctx); err != nil {
+		t.Fatalf("writeContextFiles failed: %v", err)
+	}
+
+	// Skills should be in .config/opencode/skills/ (native discovery).
+	skillMd, err := os.ReadFile(filepath.Join(dir, ".config", "opencode", "skills", "go-conventions", "SKILL.md"))
+	if err != nil {
+		t.Fatalf("failed to read .config/opencode/skills/go-conventions/SKILL.md: %v", err)
+	}
+	if !strings.Contains(string(skillMd), "Follow Go conventions.") {
+		t.Error("SKILL.md missing content")
+	}
+
+	// Supporting files should also be under .config/opencode/skills/.
+	supportFile, err := os.ReadFile(filepath.Join(dir, ".config", "opencode", "skills", "go-conventions", "templates", "example.go"))
+	if err != nil {
+		t.Fatalf("failed to read supporting file: %v", err)
+	}
+	if string(supportFile) != "package main" {
+		t.Errorf("supporting file content = %q, want %q", string(supportFile), "package main")
+	}
+
+	// .agent_context/skills/ should NOT exist for OpenCode.
+	if _, err := os.Stat(filepath.Join(dir, ".agent_context", "skills")); !os.IsNotExist(err) {
+		t.Error("expected .agent_context/skills/ to NOT exist for OpenCode provider")
+	}
+
+	// issue_context.md should still be in .agent_context/.
+	if _, err := os.Stat(filepath.Join(dir, ".agent_context", "issue_context.md")); os.IsNotExist(err) {
+		t.Error("expected .agent_context/issue_context.md to exist")
+	}
+}
+
+func TestInjectRuntimeConfigOpencode(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+
+	ctx := TaskContextForEnv{
+		IssueID:     "test-issue-id",
+		AgentSkills: []SkillContextForEnv{{Name: "Coding", Content: "Write good code."}},
+	}
+
+	if err := InjectRuntimeConfig(dir, "opencode", ctx); err != nil {
+		t.Fatalf("InjectRuntimeConfig failed: %v", err)
+	}
+
+	// OpenCode uses AGENTS.md (same as codex).
+	content, err := os.ReadFile(filepath.Join(dir, "AGENTS.md"))
+	if err != nil {
+		t.Fatalf("failed to read AGENTS.md: %v", err)
+	}
+
+	s := string(content)
+	if !strings.Contains(s, "Multica Agent Runtime") {
+		t.Error("AGENTS.md missing meta skill header")
+	}
+	if !strings.Contains(s, "Coding") {
+		t.Error("AGENTS.md missing skill name")
+	}
+	if !strings.Contains(s, "discovered automatically") {
+		t.Error("AGENTS.md missing native skill discovery hint")
+	}
+
+	// CLAUDE.md should NOT exist.
+	if _, err := os.Stat(filepath.Join(dir, "CLAUDE.md")); !os.IsNotExist(err) {
+		t.Error("expected CLAUDE.md to NOT exist for OpenCode provider")
+	}
+}
+
+func TestPrepareWithRepoContextOpencode(t *testing.T) {
+	t.Parallel()
+	workspacesRoot := t.TempDir()
+
+	taskCtx := TaskContextForEnv{
+		IssueID: "c3d4e5f6-a7b8-9012-cdef-123456789012",
+		Repos: []RepoContextForEnv{
+			{URL: "https://github.com/org/backend", Description: "Go backend"},
+		},
+	}
+	env, err := Prepare(PrepareParams{
+		WorkspacesRoot: workspacesRoot,
+		WorkspaceID:    "ws-test-oc",
+		TaskID:         "c3d4e5f6-a7b8-9012-cdef-123456789012",
+		AgentName:      "OpenCode Agent",
+		Provider:       "opencode",
+		Task:           taskCtx,
+	}, testLogger())
+	if err != nil {
+		t.Fatalf("Prepare failed: %v", err)
+	}
+	defer env.Cleanup(true)
+
+	if err := InjectRuntimeConfig(env.WorkDir, "opencode", taskCtx); err != nil {
+		t.Fatalf("InjectRuntimeConfig failed: %v", err)
+	}
+
+	// Workdir should only contain expected entries.
+	entries, err := os.ReadDir(env.WorkDir)
+	if err != nil {
+		t.Fatalf("failed to read workdir: %v", err)
+	}
+	for _, e := range entries {
+		name := e.Name()
+		if name != ".agent_context" && name != "AGENTS.md" {
+			t.Errorf("unexpected entry in workdir: %s", name)
+		}
+	}
+
+	// AGENTS.md should contain repo info.
+	content, err := os.ReadFile(filepath.Join(env.WorkDir, "AGENTS.md"))
+	if err != nil {
+		t.Fatalf("failed to read AGENTS.md: %v", err)
+	}
+	s := string(content)
+	for _, want := range []string{
+		"multica repo checkout",
+		"https://github.com/org/backend",
+		"Go backend",
+	} {
+		if !strings.Contains(s, want) {
+			t.Errorf("AGENTS.md missing %q", want)
+		}
+	}
+}
+
 func TestInjectRuntimeConfigUnknownProvider(t *testing.T) {
 	t.Parallel()
 	dir := t.TempDir()

server/internal/daemon/execenv/runtime_config.go

@@ -10,15 +10,16 @@ import (
 // InjectRuntimeConfig writes the meta skill content into the runtime-specific
 // config file so the agent discovers its environment through its native mechanism.
 //
-// For Claude: writes {workDir}/CLAUDE.md  (skills discovered natively from .claude/skills/)
+// For Claude:   writes {workDir}/CLAUDE.md  (skills discovered natively from .claude/skills/)
-// For Codex:  writes {workDir}/AGENTS.md  (skills discovered natively via CODEX_HOME)
+// For Codex:    writes {workDir}/AGENTS.md  (skills discovered natively via CODEX_HOME)
+// For OpenCode: writes {workDir}/AGENTS.md  (skills discovered natively from .config/opencode/skills/)
 func InjectRuntimeConfig(workDir, provider string, ctx TaskContextForEnv) error {
 	content := buildMetaSkillContent(provider, ctx)
 
 	switch provider {
 	case "claude":
 		return os.WriteFile(filepath.Join(workDir, "CLAUDE.md"), []byte(content), 0o644)
-	case "codex":
+	case "codex", "opencode":
 		return os.WriteFile(filepath.Join(workDir, "AGENTS.md"), []byte(content), 0o644)
 	default:
 		// Unknown provider — skip config injection, prompt-only mode.
@@ -114,8 +115,8 @@ func buildMetaSkillContent(provider string, ctx TaskContextForEnv) string {
 		case "claude":
 			// Claude discovers skills natively from .claude/skills/ — just list names.
 			b.WriteString("You have the following skills installed (discovered automatically):\n\n")
-		case "codex":
+		case "codex", "opencode":
-			// Codex discovers skills natively via CODEX_HOME/skills/ — just list names.
+			// Codex and OpenCode discover skills natively from their respective paths — just list names.
 			b.WriteString("You have the following skills installed (discovered automatically):\n\n")
 		default:
 			b.WriteString("Detailed skill instructions are in `.agent_context/skills/`. Each subdirectory contains a `SKILL.md`.\n\n")

server/internal/daemon/repocache/cache.go

@@ -29,7 +29,7 @@ type CachedRepo struct {
 
 // Cache manages bare git clones for workspace repositories.
 type Cache struct {
-	root   string       // base directory for all caches (e.g. ~/multica_workspaces/.repos)
+	root   string // base directory for all caches (e.g. ~/multica_workspaces/.repos)
 	logger *slog.Logger
 	mu     sync.Mutex
 }
@@ -195,7 +195,7 @@ func (c *Cache) CreateWorktree(params WorktreeParams) (*WorktreeResult, error) {
 	}
 
 	// Exclude agent context files from git tracking.
-	for _, pattern := range []string{".agent_context", "CLAUDE.md", "AGENTS.md", ".claude"} {
+	for _, pattern := range []string{".agent_context", "CLAUDE.md", "AGENTS.md", ".claude", ".config/opencode"} {
 		_ = excludeFromGit(worktreePath, pattern)
 	}
 

server/pkg/agent/agent.go

@@ -1,5 +1,5 @@
 // Package agent provides a unified interface for executing prompts via
-// coding agents (Claude Code, Codex). It mirrors the happy-cli AgentBackend
+// coding agents (Claude Code, Codex, OpenCode). It mirrors the happy-cli AgentBackend
 // pattern, translated to idiomatic Go.
 package agent
 
@@ -25,7 +25,7 @@ type ExecOptions struct {
 	SystemPrompt    string
 	MaxTurns        int
 	Timeout         time.Duration
-	ResumeSessionID string // if non-empty, resume a previous Claude Code session
+	ResumeSessionID string // if non-empty, resume a previous agent session
 }
 
 // Session represents a running agent execution.
@@ -73,13 +73,13 @@ type Result struct {
 
 // Config configures a Backend instance.
 type Config struct {
-	ExecutablePath string            // path to CLI binary (claude or codex)
+	ExecutablePath string            // path to CLI binary (claude, codex, or opencode)
 	Env            map[string]string // extra environment variables
 	Logger         *slog.Logger
 }
 
 // New creates a Backend for the given agent type.
-// Supported types: "claude", "codex".
+// Supported types: "claude", "codex", "opencode".
 func New(agentType string, cfg Config) (Backend, error) {
 	if cfg.Logger == nil {
 		cfg.Logger = slog.Default()
@@ -90,8 +90,10 @@ func New(agentType string, cfg Config) (Backend, error) {
 		return &claudeBackend{cfg: cfg}, nil
 	case "codex":
 		return &codexBackend{cfg: cfg}, nil
+	case "opencode":
+		return &opencodeBackend{cfg: cfg}, nil
 	default:
-		return nil, fmt.Errorf("unknown agent type: %q (supported: claude, codex)", agentType)
+		return nil, fmt.Errorf("unknown agent type: %q (supported: claude, codex, opencode)", agentType)
 	}
 }
 

server/pkg/agent/opencode.go

@@ -0,0 +1,312 @@
+package agent
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+// opencodeBackend implements Backend by spawning `opencode run --format json`
+// and reading streaming JSON events from stdout — the same pattern as Claude.
+type opencodeBackend struct {
+	cfg Config
+}
+
+func (b *opencodeBackend) Execute(ctx context.Context, prompt string, opts ExecOptions) (*Session, error) {
+	execPath := b.cfg.ExecutablePath
+	if execPath == "" {
+		execPath = "opencode"
+	}
+	if _, err := exec.LookPath(execPath); err != nil {
+		return nil, fmt.Errorf("opencode executable not found at %q: %w", execPath, err)
+	}
+
+	timeout := opts.Timeout
+	if timeout == 0 {
+		timeout = 20 * time.Minute
+	}
+	runCtx, cancel := context.WithTimeout(ctx, timeout)
+
+	args := []string{"run", "--format", "json"}
+	if opts.Model != "" {
+		args = append(args, "--model", opts.Model)
+	}
+	if opts.SystemPrompt != "" {
+		args = append(args, "--prompt", opts.SystemPrompt)
+	}
+	if opts.MaxTurns > 0 {
+		b.cfg.Logger.Warn("opencode does not support --max-turns; ignoring", "maxTurns", opts.MaxTurns)
+	}
+	if opts.ResumeSessionID != "" {
+		args = append(args, "--session", opts.ResumeSessionID)
+	}
+	args = append(args, prompt)
+
+	cmd := exec.CommandContext(runCtx, execPath, args...)
+	if opts.Cwd != "" {
+		cmd.Dir = opts.Cwd
+	}
+
+	env := buildEnv(b.cfg.Env)
+	// Auto-approve all tool use in daemon mode.
+	env = append(env, `OPENCODE_PERMISSION={"*":"allow"}`)
+	cmd.Env = env
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		cancel()
+		return nil, fmt.Errorf("opencode stdout pipe: %w", err)
+	}
+	cmd.Stderr = newLogWriter(b.cfg.Logger, "[opencode:stderr] ")
+
+	if err := cmd.Start(); err != nil {
+		cancel()
+		return nil, fmt.Errorf("start opencode: %w", err)
+	}
+
+	b.cfg.Logger.Info("opencode started", "pid", cmd.Process.Pid, "cwd", opts.Cwd, "model", opts.Model)
+
+	msgCh := make(chan Message, 256)
+	resCh := make(chan Result, 1)
+
+	go func() {
+		defer cancel()
+		defer close(msgCh)
+		defer close(resCh)
+
+		startTime := time.Now()
+		scanResult := b.processEvents(stdout, msgCh)
+
+		// Wait for process exit.
+		exitErr := cmd.Wait()
+		duration := time.Since(startTime)
+
+		if runCtx.Err() == context.DeadlineExceeded {
+			scanResult.status = "timeout"
+			scanResult.errMsg = fmt.Sprintf("opencode timed out after %s", timeout)
+		} else if runCtx.Err() == context.Canceled {
+			scanResult.status = "aborted"
+			scanResult.errMsg = "execution cancelled"
+		} else if exitErr != nil && scanResult.status == "completed" {
+			scanResult.status = "failed"
+			scanResult.errMsg = fmt.Sprintf("opencode exited with error: %v", exitErr)
+		}
+
+		b.cfg.Logger.Info("opencode finished", "pid", cmd.Process.Pid, "status", scanResult.status, "duration", duration.Round(time.Millisecond).String())
+
+		resCh <- Result{
+			Status:     scanResult.status,
+			Output:     scanResult.output,
+			Error:      scanResult.errMsg,
+			DurationMs: duration.Milliseconds(),
+			SessionID:  scanResult.sessionID,
+		}
+	}()
+
+	return &Session{Messages: msgCh, Result: resCh}, nil
+}
+
+// ── Event handlers ──
+
+// eventResult holds the accumulated state from processing the event stream.
+type eventResult struct {
+	status    string
+	errMsg    string
+	output    string
+	sessionID string
+}
+
+// processEvents reads JSON lines from r, dispatches events to ch, and returns
+// the accumulated result. This is the core scanner loop, extracted for testability.
+func (b *opencodeBackend) processEvents(r io.Reader, ch chan<- Message) eventResult {
+	var output strings.Builder
+	var sessionID string
+	finalStatus := "completed"
+	var finalError string
+
+	scanner := bufio.NewScanner(r)
+	scanner.Buffer(make([]byte, 0, 1024*1024), 10*1024*1024)
+
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+
+		var event opencodeEvent
+		if err := json.Unmarshal([]byte(line), &event); err != nil {
+			continue
+		}
+
+		if event.SessionID != "" {
+			sessionID = event.SessionID
+		}
+
+		switch event.Type {
+		case "text":
+			b.handleTextEvent(event, ch, &output)
+		case "tool_use":
+			b.handleToolUseEvent(event, ch)
+		case "error":
+			b.handleErrorEvent(event, ch, &finalStatus, &finalError)
+		case "step_start":
+			trySend(ch, Message{Type: MessageStatus, Status: "running"})
+		case "step_finish":
+			// Captures final session ID from step_finish if present.
+		}
+	}
+
+	// Check for scanner errors (e.g. broken pipe, read errors).
+	if scanErr := scanner.Err(); scanErr != nil {
+		b.cfg.Logger.Warn("opencode stdout scanner error", "error", scanErr)
+		if finalStatus == "completed" {
+			finalStatus = "failed"
+			finalError = fmt.Sprintf("stdout read error: %v", scanErr)
+		}
+	}
+
+	return eventResult{
+		status:    finalStatus,
+		errMsg:    finalError,
+		output:    output.String(),
+		sessionID: sessionID,
+	}
+}
+
+func (b *opencodeBackend) handleTextEvent(event opencodeEvent, ch chan<- Message, output *strings.Builder) {
+	text := event.Part.Text
+	if text != "" {
+		output.WriteString(text)
+		trySend(ch, Message{Type: MessageText, Content: text})
+	}
+}
+
+// handleToolUseEvent processes "tool_use" events from opencode. A single
+// tool_use event contains both the call and result in part.state when the
+// tool has completed (state.status == "completed").
+func (b *opencodeBackend) handleToolUseEvent(event opencodeEvent, ch chan<- Message) {
+	// Extract input from state.input (the tool invocation parameters).
+	var input map[string]any
+	if event.Part.State != nil && event.Part.State.Input != nil {
+		_ = json.Unmarshal(event.Part.State.Input, &input)
+	}
+
+	// Emit the tool-use message.
+	trySend(ch, Message{
+		Type:   MessageToolUse,
+		Tool:   event.Part.Tool,
+		CallID: event.Part.CallID,
+		Input:  input,
+	})
+
+	// If the tool has completed, also emit a tool-result message.
+	if event.Part.State != nil && event.Part.State.Status == "completed" {
+		outputStr := extractToolOutput(event.Part.State.Output)
+		trySend(ch, Message{
+			Type:   MessageToolResult,
+			Tool:   event.Part.Tool,
+			CallID: event.Part.CallID,
+			Output: outputStr,
+		})
+	}
+}
+
+// handleErrorEvent processes "error" events from opencode. OpenCode can exit
+// with RC=0 even on errors (e.g. invalid model), so error events are the
+// reliable signal for failures.
+func (b *opencodeBackend) handleErrorEvent(event opencodeEvent, ch chan<- Message, finalStatus, finalError *string) {
+	errMsg := ""
+	if event.Error != nil {
+		errMsg = event.Error.Message()
+	}
+	if errMsg == "" {
+		errMsg = "unknown opencode error"
+	}
+
+	b.cfg.Logger.Warn("opencode error event", "error", errMsg)
+	trySend(ch, Message{Type: MessageError, Content: errMsg})
+
+	*finalStatus = "failed"
+	*finalError = errMsg
+}
+
+// extractToolOutput converts the tool state output (which may be a string or
+// structured object) into a string.
+func extractToolOutput(output any) string {
+	if output == nil {
+		return ""
+	}
+	if s, ok := output.(string); ok {
+		return s
+	}
+	data, _ := json.Marshal(output)
+	return string(data)
+}
+
+// ── JSON types for `opencode run --format json` stdout events ──
+
+// opencodeEvent represents a single JSON line from `opencode run --format json`.
+//
+// Event types observed in real output:
+//
+//	"step_start"  — agent step begins
+//	"text"        — text output from agent (part.text)
+//	"tool_use"    — tool invocation with call and result (part.tool, part.callID, part.state)
+//	"error"       — error from opencode (error.name, error.data.message)
+//	"step_finish" — agent step completes (includes token usage)
+type opencodeEvent struct {
+	Type      string            `json:"type"`
+	Timestamp int64             `json:"timestamp,omitempty"`
+	SessionID string            `json:"sessionID,omitempty"`
+	Part      opencodeEventPart `json:"part"`
+	Error     *opencodeError    `json:"error,omitempty"`
+}
+
+// opencodeEventPart represents the part field in an opencode event.
+type opencodeEventPart struct {
+	ID        string `json:"id,omitempty"`
+	MessageID string `json:"messageID,omitempty"`
+	SessionID string `json:"sessionID,omitempty"`
+	Type      string `json:"type,omitempty"`
+
+	// Text events
+	Text string `json:"text,omitempty"`
+
+	// Tool use events
+	Tool   string             `json:"tool,omitempty"`
+	CallID string             `json:"callID,omitempty"`
+	State  *opencodeToolState `json:"state,omitempty"`
+}
+
+// opencodeToolState represents the state of a tool invocation.
+type opencodeToolState struct {
+	Status string          `json:"status,omitempty"`
+	Input  json.RawMessage `json:"input,omitempty"`
+	Output any             `json:"output,omitempty"`
+}
+
+// opencodeError represents an error event from opencode.
+type opencodeError struct {
+	Name string           `json:"name,omitempty"`
+	Data *opencodeErrData `json:"data,omitempty"`
+}
+
+// Message returns the human-readable error message.
+func (e *opencodeError) Message() string {
+	if e.Data != nil && e.Data.Message != "" {
+		return e.Data.Message
+	}
+	if e.Name != "" {
+		return e.Name
+	}
+	return ""
+}
+
+type opencodeErrData struct {
+	Message string `json:"message,omitempty"`
+}

server/pkg/agent/opencode_test.go

@@ -0,0 +1,711 @@
+package agent
+
+import (
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"strings"
+	"testing"
+)
+
+func TestNewReturnsOpencodeBackend(t *testing.T) {
+	t.Parallel()
+	b, err := New("opencode", Config{ExecutablePath: "/nonexistent/opencode"})
+	if err != nil {
+		t.Fatalf("New(opencode) error: %v", err)
+	}
+	if _, ok := b.(*opencodeBackend); !ok {
+		t.Fatalf("expected *opencodeBackend, got %T", b)
+	}
+}
+
+// ── Text event tests ──
+
+func TestOpencodeHandleTextEvent(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+	var output strings.Builder
+
+	event := opencodeEvent{
+		Type:      "text",
+		SessionID: "ses_abc",
+		Part: opencodeEventPart{
+			Type: "text",
+			Text: "Hello from opencode",
+		},
+	}
+
+	b.handleTextEvent(event, ch, &output)
+
+	if output.String() != "Hello from opencode" {
+		t.Errorf("output: got %q, want %q", output.String(), "Hello from opencode")
+	}
+	msg := <-ch
+	if msg.Type != MessageText {
+		t.Errorf("type: got %v, want MessageText", msg.Type)
+	}
+	if msg.Content != "Hello from opencode" {
+		t.Errorf("content: got %q, want %q", msg.Content, "Hello from opencode")
+	}
+}
+
+func TestOpencodeHandleTextEventEmpty(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+	var output strings.Builder
+
+	event := opencodeEvent{
+		Type: "text",
+		Part: opencodeEventPart{Type: "text", Text: ""},
+	}
+
+	b.handleTextEvent(event, ch, &output)
+
+	if output.String() != "" {
+		t.Errorf("expected empty output, got %q", output.String())
+	}
+	if len(ch) != 0 {
+		t.Errorf("expected no messages, got %d", len(ch))
+	}
+}
+
+// ── Tool use event tests (real opencode schema) ──
+
+func TestOpencodeHandleToolUseEventCompleted(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+
+	// Real opencode tool_use event: single event with state containing both
+	// call parameters and result.
+	event := opencodeEvent{
+		Type: "tool_use",
+		Part: opencodeEventPart{
+			Tool:   "bash",
+			CallID: "call_BHA1",
+			State: &opencodeToolState{
+				Status: "completed",
+				Input:  json.RawMessage(`{"command":"pwd","description":"Prints current working directory path"}`),
+				Output: "/tmp/multica\n",
+			},
+		},
+	}
+
+	b.handleToolUseEvent(event, ch)
+
+	// Should emit both a tool-use and a tool-result message.
+	if len(ch) != 2 {
+		t.Fatalf("expected 2 messages, got %d", len(ch))
+	}
+
+	// First: tool-use
+	msg := <-ch
+	if msg.Type != MessageToolUse {
+		t.Errorf("type: got %v, want MessageToolUse", msg.Type)
+	}
+	if msg.Tool != "bash" {
+		t.Errorf("tool: got %q, want %q", msg.Tool, "bash")
+	}
+	if msg.CallID != "call_BHA1" {
+		t.Errorf("callID: got %q, want %q", msg.CallID, "call_BHA1")
+	}
+	if cmd, ok := msg.Input["command"].(string); !ok || cmd != "pwd" {
+		t.Errorf("input.command: got %v", msg.Input["command"])
+	}
+
+	// Second: tool-result
+	msg = <-ch
+	if msg.Type != MessageToolResult {
+		t.Errorf("type: got %v, want MessageToolResult", msg.Type)
+	}
+	if msg.CallID != "call_BHA1" {
+		t.Errorf("callID: got %q, want %q", msg.CallID, "call_BHA1")
+	}
+	if msg.Output != "/tmp/multica\n" {
+		t.Errorf("output: got %q", msg.Output)
+	}
+}
+
+func TestOpencodeHandleToolUseEventPending(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+
+	// Tool use with pending status — only emit tool-use, no result.
+	event := opencodeEvent{
+		Type: "tool_use",
+		Part: opencodeEventPart{
+			Tool:   "read",
+			CallID: "call_ABC",
+			State: &opencodeToolState{
+				Status: "pending",
+				Input:  json.RawMessage(`{"filePath":"/tmp/test.go"}`),
+			},
+		},
+	}
+
+	b.handleToolUseEvent(event, ch)
+
+	if len(ch) != 1 {
+		t.Fatalf("expected 1 message for pending tool, got %d", len(ch))
+	}
+	msg := <-ch
+	if msg.Type != MessageToolUse {
+		t.Errorf("type: got %v, want MessageToolUse", msg.Type)
+	}
+	if msg.Tool != "read" {
+		t.Errorf("tool: got %q, want %q", msg.Tool, "read")
+	}
+}
+
+func TestOpencodeHandleToolUseEventStructuredOutput(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+
+	// Tool with structured (non-string) output.
+	event := opencodeEvent{
+		Type: "tool_use",
+		Part: opencodeEventPart{
+			Tool:   "glob",
+			CallID: "call_XYZ",
+			State: &opencodeToolState{
+				Status: "completed",
+				Input:  json.RawMessage(`{"pattern":"*.go"}`),
+				Output: map[string]any{"files": []any{"main.go", "main_test.go"}},
+			},
+		},
+	}
+
+	b.handleToolUseEvent(event, ch)
+
+	// tool-use + tool-result
+	if len(ch) != 2 {
+		t.Fatalf("expected 2 messages, got %d", len(ch))
+	}
+	<-ch // skip tool-use
+	msg := <-ch
+	if msg.Type != MessageToolResult {
+		t.Errorf("type: got %v, want MessageToolResult", msg.Type)
+	}
+	if !strings.Contains(msg.Output, "main.go") {
+		t.Errorf("output should contain 'main.go', got %q", msg.Output)
+	}
+}
+
+func TestOpencodeHandleToolUseEventNilState(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{}
+	ch := make(chan Message, 10)
+
+	// Tool use with no state at all — should emit tool-use with no crash.
+	event := opencodeEvent{
+		Type: "tool_use",
+		Part: opencodeEventPart{
+			Tool:   "write",
+			CallID: "call_NUL",
+		},
+	}
+
+	b.handleToolUseEvent(event, ch)
+
+	if len(ch) != 1 {
+		t.Fatalf("expected 1 message, got %d", len(ch))
+	}
+	msg := <-ch
+	if msg.Type != MessageToolUse {
+		t.Errorf("type: got %v, want MessageToolUse", msg.Type)
+	}
+}
+
+// ── Error event tests ──
+
+func TestOpencodeHandleErrorEvent(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 10)
+	status := "completed"
+	errMsg := ""
+
+	event := opencodeEvent{
+		Type:      "error",
+		SessionID: "ses_abc",
+		Error: &opencodeError{
+			Name: "UnknownError",
+			Data: &opencodeErrData{
+				Message: "Model not found: definitely/not-a-model.",
+			},
+		},
+	}
+
+	b.handleErrorEvent(event, ch, &status, &errMsg)
+
+	if status != "failed" {
+		t.Errorf("status: got %q, want %q", status, "failed")
+	}
+	if errMsg != "Model not found: definitely/not-a-model." {
+		t.Errorf("error: got %q", errMsg)
+	}
+	msg := <-ch
+	if msg.Type != MessageError {
+		t.Errorf("type: got %v, want MessageError", msg.Type)
+	}
+	if msg.Content != "Model not found: definitely/not-a-model." {
+		t.Errorf("content: got %q", msg.Content)
+	}
+}
+
+func TestOpencodeHandleErrorEventNameOnly(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 10)
+	status := "completed"
+	errMsg := ""
+
+	// Error with name but no data.message — should fall back to name.
+	event := opencodeEvent{
+		Type: "error",
+		Error: &opencodeError{
+			Name: "RateLimitError",
+		},
+	}
+
+	b.handleErrorEvent(event, ch, &status, &errMsg)
+
+	if errMsg != "RateLimitError" {
+		t.Errorf("error: got %q, want %q", errMsg, "RateLimitError")
+	}
+}
+
+func TestOpencodeHandleErrorEventNilError(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 10)
+	status := "completed"
+	errMsg := ""
+
+	event := opencodeEvent{Type: "error"}
+
+	b.handleErrorEvent(event, ch, &status, &errMsg)
+
+	if errMsg != "unknown opencode error" {
+		t.Errorf("error: got %q, want %q", errMsg, "unknown opencode error")
+	}
+}
+
+// ── JSON parsing tests with real fixtures ──
+
+func TestOpencodeEventParsingTextFixture(t *testing.T) {
+	t.Parallel()
+
+	line := `{"type":"text","timestamp":1775116675833,"sessionID":"ses_abc","part":{"id":"prt_123","messageID":"msg_456","sessionID":"ses_abc","type":"text","text":"pong","time":{"start":1775116675833,"end":1775116675833}}}`
+
+	var event opencodeEvent
+	if err := json.Unmarshal([]byte(line), &event); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if event.Type != "text" {
+		t.Errorf("type: got %q, want %q", event.Type, "text")
+	}
+	if event.SessionID != "ses_abc" {
+		t.Errorf("sessionID: got %q, want %q", event.SessionID, "ses_abc")
+	}
+	if event.Part.Text != "pong" {
+		t.Errorf("part.text: got %q, want %q", event.Part.Text, "pong")
+	}
+}
+
+func TestOpencodeEventParsingToolUseFixture(t *testing.T) {
+	t.Parallel()
+
+	// Real `tool_use` JSON from live `opencode run --format json` output.
+	line := `{"type":"tool_use","timestamp":1775117187163,"sessionID":"ses_abc","part":{"id":"prt_123","messageID":"msg_456","sessionID":"ses_abc","type":"tool","tool":"bash","callID":"call_BHA1","state":{"status":"completed","input":{"command":"pwd","description":"Prints current working directory path"},"output":"/tmp/multica\n","metadata":{"exit":0},"time":{"start":1775117187092,"end":1775117187162}}}}`
+
+	var event opencodeEvent
+	if err := json.Unmarshal([]byte(line), &event); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if event.Type != "tool_use" {
+		t.Errorf("type: got %q, want %q", event.Type, "tool_use")
+	}
+	if event.Part.Tool != "bash" {
+		t.Errorf("part.tool: got %q, want %q", event.Part.Tool, "bash")
+	}
+	if event.Part.CallID != "call_BHA1" {
+		t.Errorf("part.callID: got %q, want %q", event.Part.CallID, "call_BHA1")
+	}
+	if event.Part.State == nil {
+		t.Fatal("part.state is nil")
+	}
+	if event.Part.State.Status != "completed" {
+		t.Errorf("state.status: got %q, want %q", event.Part.State.Status, "completed")
+	}
+
+	// Parse state.input
+	var input map[string]any
+	if err := json.Unmarshal(event.Part.State.Input, &input); err != nil {
+		t.Fatalf("unmarshal state.input: %v", err)
+	}
+	if input["command"] != "pwd" {
+		t.Errorf("state.input.command: got %v, want %q", input["command"], "pwd")
+	}
+
+	// state.output should be a string
+	if output, ok := event.Part.State.Output.(string); !ok || output != "/tmp/multica\n" {
+		t.Errorf("state.output: got %v (%T)", event.Part.State.Output, event.Part.State.Output)
+	}
+}
+
+func TestOpencodeEventParsingErrorFixture(t *testing.T) {
+	t.Parallel()
+
+	line := `{"type":"error","timestamp":1775117233612,"sessionID":"ses_abc","error":{"name":"UnknownError","data":{"message":"Model not found: definitely/not-a-model."}}}`
+
+	var event opencodeEvent
+	if err := json.Unmarshal([]byte(line), &event); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if event.Type != "error" {
+		t.Errorf("type: got %q, want %q", event.Type, "error")
+	}
+	if event.Error == nil {
+		t.Fatal("error field is nil")
+	}
+	if event.Error.Name != "UnknownError" {
+		t.Errorf("error.name: got %q", event.Error.Name)
+	}
+	if got := event.Error.Message(); got != "Model not found: definitely/not-a-model." {
+		t.Errorf("error.Message(): got %q", got)
+	}
+}
+
+func TestOpencodeEventParsingStepStartFixture(t *testing.T) {
+	t.Parallel()
+
+	line := `{"type":"step_start","timestamp":1775116675819,"sessionID":"ses_abc","part":{"id":"prt_123","messageID":"msg_456","sessionID":"ses_abc","snapshot":"abc123","type":"step-start"}}`
+
+	var event opencodeEvent
+	if err := json.Unmarshal([]byte(line), &event); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if event.Type != "step_start" {
+		t.Errorf("type: got %q, want %q", event.Type, "step_start")
+	}
+	if event.SessionID != "ses_abc" {
+		t.Errorf("sessionID: got %q", event.SessionID)
+	}
+}
+
+func TestOpencodeStepFinishParsing(t *testing.T) {
+	t.Parallel()
+
+	line := `{"type":"step_finish","timestamp":1775116676180,"sessionID":"ses_abc","part":{"id":"prt_789","reason":"stop","snapshot":"abc123","messageID":"msg_456","sessionID":"ses_abc","type":"step-finish","tokens":{"total":14674,"input":14585,"output":89,"reasoning":82,"cache":{"write":0,"read":0}},"cost":0}}`
+
+	var event opencodeEvent
+	if err := json.Unmarshal([]byte(line), &event); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if event.Type != "step_finish" {
+		t.Errorf("type: got %q, want %q", event.Type, "step_finish")
+	}
+	if event.SessionID != "ses_abc" {
+		t.Errorf("sessionID: got %q", event.SessionID)
+	}
+}
+
+// ── extractToolOutput tests ──
+
+func TestExtractToolOutputString(t *testing.T) {
+	t.Parallel()
+	if got := extractToolOutput("hello\n"); got != "hello\n" {
+		t.Errorf("got %q, want %q", got, "hello\n")
+	}
+}
+
+func TestExtractToolOutputNil(t *testing.T) {
+	t.Parallel()
+	if got := extractToolOutput(nil); got != "" {
+		t.Errorf("got %q, want empty", got)
+	}
+}
+
+func TestExtractToolOutputStructured(t *testing.T) {
+	t.Parallel()
+	obj := map[string]any{"key": "value"}
+	got := extractToolOutput(obj)
+	if !strings.Contains(got, `"key"`) || !strings.Contains(got, `"value"`) {
+		t.Errorf("got %q, expected JSON containing key/value", got)
+	}
+}
+
+// ── opencodeError.Message() tests ──
+
+func TestOpencodeErrorMessage(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		err  *opencodeError
+		want string
+	}{
+		{
+			name: "data message",
+			err:  &opencodeError{Name: "Err", Data: &opencodeErrData{Message: "details"}},
+			want: "details",
+		},
+		{
+			name: "name only",
+			err:  &opencodeError{Name: "RateLimitError"},
+			want: "RateLimitError",
+		},
+		{
+			name: "empty",
+			err:  &opencodeError{},
+			want: "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.err.Message(); got != tt.want {
+				t.Errorf("Message() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// ── Integration-level tests: processEvents ──
+//
+// These feed multiple JSON lines through processEvents and verify the
+// accumulated result (status, output, sessionID, error) and emitted messages.
+
+func TestOpencodeProcessEventsHappyPath(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Simulate a successful run: step_start → text → tool_use → text → step_finish
+	lines := strings.Join([]string{
+		`{"type":"step_start","timestamp":1000,"sessionID":"ses_happy","part":{"type":"step-start"}}`,
+		`{"type":"text","timestamp":1001,"sessionID":"ses_happy","part":{"type":"text","text":"Analyzing the issue..."}}`,
+		`{"type":"tool_use","timestamp":1002,"sessionID":"ses_happy","part":{"tool":"bash","callID":"call_1","state":{"status":"completed","input":{"command":"ls"},"output":"file1.go\nfile2.go\n"}}}`,
+		`{"type":"text","timestamp":1003,"sessionID":"ses_happy","part":{"type":"text","text":" Done."}}`,
+		`{"type":"step_finish","timestamp":1004,"sessionID":"ses_happy","part":{"type":"step-finish"}}`,
+	}, "\n")
+
+	result := b.processEvents(strings.NewReader(lines), ch)
+
+	// Verify result.
+	if result.status != "completed" {
+		t.Errorf("status: got %q, want %q", result.status, "completed")
+	}
+	if result.sessionID != "ses_happy" {
+		t.Errorf("sessionID: got %q, want %q", result.sessionID, "ses_happy")
+	}
+	if result.output != "Analyzing the issue... Done." {
+		t.Errorf("output: got %q, want %q", result.output, "Analyzing the issue... Done.")
+	}
+	if result.errMsg != "" {
+		t.Errorf("errMsg: got %q, want empty", result.errMsg)
+	}
+
+	// Drain and verify messages.
+	close(ch)
+	var msgs []Message
+	for m := range ch {
+		msgs = append(msgs, m)
+	}
+
+	// Expected: status(running), text, tool-use, tool-result, text, = 5 messages
+	if len(msgs) != 5 {
+		t.Fatalf("expected 5 messages, got %d: %+v", len(msgs), msgs)
+	}
+	if msgs[0].Type != MessageStatus || msgs[0].Status != "running" {
+		t.Errorf("msg[0]: got %+v, want status=running", msgs[0])
+	}
+	if msgs[1].Type != MessageText || msgs[1].Content != "Analyzing the issue..." {
+		t.Errorf("msg[1]: got %+v", msgs[1])
+	}
+	if msgs[2].Type != MessageToolUse || msgs[2].Tool != "bash" {
+		t.Errorf("msg[2]: got %+v, want tool-use(bash)", msgs[2])
+	}
+	if msgs[3].Type != MessageToolResult || msgs[3].Output != "file1.go\nfile2.go\n" {
+		t.Errorf("msg[3]: got %+v, want tool-result", msgs[3])
+	}
+	if msgs[4].Type != MessageText || msgs[4].Content != " Done." {
+		t.Errorf("msg[4]: got %+v", msgs[4])
+	}
+}
+
+func TestOpencodeProcessEventsErrorCausesFailedStatus(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Simulate: step_start → error (model not found) → step_finish.
+	// OpenCode exits RC=0 on error events, so the error event is the only
+	// signal that something went wrong.
+	lines := strings.Join([]string{
+		`{"type":"step_start","timestamp":1000,"sessionID":"ses_err","part":{"type":"step-start"}}`,
+		`{"type":"error","timestamp":1001,"sessionID":"ses_err","error":{"name":"UnknownError","data":{"message":"Model not found: bad/model"}}}`,
+		`{"type":"step_finish","timestamp":1002,"sessionID":"ses_err","part":{"type":"step-finish"}}`,
+	}, "\n")
+
+	result := b.processEvents(strings.NewReader(lines), ch)
+
+	if result.status != "failed" {
+		t.Errorf("status: got %q, want %q", result.status, "failed")
+	}
+	if result.errMsg != "Model not found: bad/model" {
+		t.Errorf("errMsg: got %q", result.errMsg)
+	}
+	if result.sessionID != "ses_err" {
+		t.Errorf("sessionID: got %q, want %q", result.sessionID, "ses_err")
+	}
+
+	close(ch)
+	var errorMsgs int
+	for m := range ch {
+		if m.Type == MessageError {
+			errorMsgs++
+		}
+	}
+	if errorMsgs != 1 {
+		t.Errorf("expected 1 error message, got %d", errorMsgs)
+	}
+}
+
+func TestOpencodeProcessEventsSessionIDExtracted(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Session ID should be captured from the last event that has one.
+	lines := strings.Join([]string{
+		`{"type":"step_start","timestamp":1000,"sessionID":"ses_first","part":{"type":"step-start"}}`,
+		`{"type":"text","timestamp":1001,"sessionID":"ses_updated","part":{"type":"text","text":"hi"}}`,
+	}, "\n")
+
+	result := b.processEvents(strings.NewReader(lines), ch)
+
+	if result.sessionID != "ses_updated" {
+		t.Errorf("sessionID: got %q, want %q (should use last seen)", result.sessionID, "ses_updated")
+	}
+
+	close(ch)
+}
+
+func TestOpencodeProcessEventsScannerError(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Use an ioErrReader that returns valid data then an I/O error, which
+	// triggers scanner.Err() and should set status to "failed".
+	result := b.processEvents(&ioErrReader{
+		data: `{"type":"text","sessionID":"ses_scan","part":{"text":"before error"}}` + "\n",
+	}, ch)
+
+	if result.status != "failed" {
+		t.Errorf("status: got %q, want %q", result.status, "failed")
+	}
+	if !strings.Contains(result.errMsg, "stdout read error") {
+		t.Errorf("errMsg: got %q, want it to contain 'stdout read error'", result.errMsg)
+	}
+	// The text event before the error should still be captured.
+	if result.output != "before error" {
+		t.Errorf("output: got %q, want %q", result.output, "before error")
+	}
+
+	close(ch)
+}
+
+// ioErrReader delivers data on the first Read, then returns an error on the second.
+type ioErrReader struct {
+	data string
+	read bool
+}
+
+func (r *ioErrReader) Read(p []byte) (int, error) {
+	if !r.read {
+		r.read = true
+		n := copy(p, r.data)
+		return n, nil
+	}
+	return 0, fmt.Errorf("simulated I/O error")
+}
+
+func TestOpencodeProcessEventsEmptyLines(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Empty lines and invalid JSON should be skipped without error.
+	lines := strings.Join([]string{
+		"",
+		"   ",
+		"not json at all",
+		`{"type":"text","sessionID":"ses_ok","part":{"text":"valid"}}`,
+		"",
+	}, "\n")
+
+	result := b.processEvents(strings.NewReader(lines), ch)
+
+	if result.status != "completed" {
+		t.Errorf("status: got %q, want %q", result.status, "completed")
+	}
+	if result.output != "valid" {
+		t.Errorf("output: got %q, want %q", result.output, "valid")
+	}
+	if result.sessionID != "ses_ok" {
+		t.Errorf("sessionID: got %q, want %q", result.sessionID, "ses_ok")
+	}
+
+	close(ch)
+	var msgs []Message
+	for m := range ch {
+		msgs = append(msgs, m)
+	}
+	if len(msgs) != 1 || msgs[0].Type != MessageText {
+		t.Errorf("expected 1 text message, got %d: %+v", len(msgs), msgs)
+	}
+}
+
+func TestOpencodeProcessEventsErrorDoesNotRevertToCompleted(t *testing.T) {
+	t.Parallel()
+
+	b := &opencodeBackend{cfg: Config{Logger: slog.Default()}}
+	ch := make(chan Message, 256)
+
+	// Error event followed by more text — status should remain "failed".
+	lines := strings.Join([]string{
+		`{"type":"error","sessionID":"ses_x","error":{"name":"RateLimitError"}}`,
+		`{"type":"text","sessionID":"ses_x","part":{"text":"recovered?"}}`,
+	}, "\n")
+
+	result := b.processEvents(strings.NewReader(lines), ch)
+
+	if result.status != "failed" {
+		t.Errorf("status: got %q, want %q (error should stick)", result.status, "failed")
+	}
+	if result.errMsg != "RateLimitError" {
+		t.Errorf("errMsg: got %q, want %q", result.errMsg, "RateLimitError")
+	}
+
+	close(ch)
+}