diff --git a/CLAUDE.md b/CLAUDE.md
index f3ba9ad..210799c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -92,6 +92,8 @@ caveman/
 | `agents/cavecrew-investigator.md` | Read-only locator subagent (haiku). Output contract: `path:line — symbol — note`. |
 | `agents/cavecrew-builder.md` | Surgical 1-2 file editor subagent. Refuses 3+ file scope. |
 | `agents/cavecrew-reviewer.md` | Diff/file reviewer subagent (haiku). One-line findings with severity emoji. |
+| `src/plugins/opencode/plugin.js` | opencode native plugin. ESM Bun module — `session.created` writes flag, `tui.prompt.append` parses slash/natural-language activation and appends per-prompt reinforcement. Reuses `caveman-config.js` via `createRequire`. |
+| `src/plugins/opencode/commands/*.md` | Six opencode slash-command prompt templates (`/caveman`, `/caveman-{commit,review,compress,stats,help}`). |
 
 ### Auto-generated / auto-synced — do not edit directly
 
@@ -236,12 +238,15 @@ How caveman reaches each agent type:
 | Claude Code | Plugin (hooks + skills) or standalone hooks | Yes — SessionStart hook injects rules |
 | Codex | Plugin in `plugins/caveman/` plus repo `.codex/hooks.json` and `.codex/config.toml` | Yes on macOS/Linux — SessionStart hook |
 | Gemini CLI | Extension with `GEMINI.md` context file | Yes — context file loads every session |
+| opencode | Native plugin (`src/plugins/opencode/`) copied into `~/.config/opencode/plugins/caveman/` + `AGENTS.md` ruleset + skills/agents/commands directories. Plugin uses `session.created` and `tui.prompt.append` lifecycle hooks. No statusline (opencode TUI exposes no plugin-writable badge). | Yes — `session.created` writes flag, `AGENTS.md` carries always-on ruleset |
 | Cursor | Per-user `.cursor/rules/caveman.mdc` written by `src/tools/caveman-init.js` | Yes — always-on rule (after `npx caveman --only cursor`) |
 | Windsurf | Per-user `.windsurf/rules/caveman.md` written by `src/tools/caveman-init.js` | Yes — always-on rule (after `npx caveman --only windsurf`) |
 | Cline | Per-user `.clinerules/caveman.md` written by `src/tools/caveman-init.js` | Yes — Cline auto-discovers `.clinerules/` |
 | Copilot | Per-user `.github/copilot-instructions.md` written by `src/tools/caveman-init.js` + `AGENTS.md` | Yes — repo-wide instructions |
 | Others (Junie, Trae, Warp, Tabnine, Mistral, Qwen, Devin, Droid, ForgeCode, Bob, Crush, iFlow, OpenHands, Qoder, Rovo Dev, Replit, Antigravity, …) | `npx skills add JuliusBrussee/caveman -a <profile>` | No — user must say `/caveman` each session |
 
+opencode reaches Tier 1 minus the statusline (opencode's TUI has no plugin-writable badge). Mode flag lives at `~/.config/opencode/.caveman-active` for any external tooling that wants to surface it.
+
 For agents without hook systems, the always-on snippet lives in `INSTALL.md`'s "Want it always on?" section — keep current with `src/rules/caveman-activate.md`.
 
 **Adding a new agent.** Edit the `PROVIDERS` array in `bin/install.js` — single source of truth, no more bash/PS1 dual-source drift. Each entry has `id`, `label`, `mech`, `detect` (clause spec like `command:foo||dir:$HOME/x`), optional `profile` (vercel-labs/skills slug), optional `soft: true` (config-dir-only detection).
diff --git a/INSTALL.md b/INSTALL.md
index 32628cc..389f757 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -39,6 +39,7 @@ If you want to install for one agent (or want to know exactly what command runs
 |---|---|:-:|
 | **Claude Code** | `claude plugin marketplace add JuliusBrussee/caveman && claude plugin install caveman@caveman` | Yes |
 | **Gemini CLI** | `gemini extensions install https://github.com/JuliusBrussee/caveman` | Yes |
+| **opencode** | `node bin/install.js --only opencode` *(or `npx -y github:JuliusBrussee/caveman -- --only opencode`)* | Yes (plugin + AGENTS.md) |
 | **Codex CLI** | `npx skills add JuliusBrussee/caveman -a codex` | Yes (in this repo via `.codex/`) |
 | **Cursor** | `npx skills add JuliusBrussee/caveman -a cursor` | Yes (rule file) |
 | **Windsurf** | `npx skills add JuliusBrussee/caveman -a windsurf` | Yes (rule file) |
@@ -60,7 +61,6 @@ If you want to install for one agent (or want to know exactly what command runs
 | **Kiro CLI** | `npx skills add JuliusBrussee/caveman -a kiro-cli` | No |
 | **Mistral Vibe** | `npx skills add JuliusBrussee/caveman -a mistral-vibe` | No |
 | **OpenHands** | `npx skills add JuliusBrussee/caveman -a openhands` | No |
-| **opencode** | `npx skills add JuliusBrussee/caveman -a opencode` | No |
 | **Qwen Code** | `npx skills add JuliusBrussee/caveman -a qwen-code` | No |
 | **Atlassian Rovo Dev** | `npx skills add JuliusBrussee/caveman -a rovodev` | No |
 | **Tabnine CLI** | `npx skills add JuliusBrussee/caveman -a tabnine-cli` | No |
diff --git a/README.md b/README.md
index 9466411..a1a1038 100644
--- a/README.md
+++ b/README.md
@@ -97,6 +97,7 @@ Detects 30+ agents, runs each one's native install, skips what you not have. Saf
 | **Claude Code** | `claude plugin marketplace add JuliusBrussee/caveman && claude plugin install caveman@caveman` |
 | **Codex** | `npx skills add JuliusBrussee/caveman -a codex` |
 | **Gemini CLI** | `gemini extensions install https://github.com/JuliusBrussee/caveman` |
+| **opencode** | `npx -y github:JuliusBrussee/caveman -- --only opencode` |
 | **Cursor** | `npx skills add JuliusBrussee/caveman -a cursor` |
 | **Windsurf** | `npx skills add JuliusBrussee/caveman -a windsurf` |
 | **Cline** | `npx skills add JuliusBrussee/caveman -a cline` |
diff --git a/bin/install.js b/bin/install.js
index 1575397..cdd9dd1 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -146,6 +146,7 @@ function checkNodeVersion() {
 const PROVIDERS = [
   { id: 'claude',     label: 'Claude Code',         mech: 'claude plugin install',         detect: 'command:claude' },
   { id: 'gemini',     label: 'Gemini CLI',          mech: 'gemini extensions install',     detect: 'command:gemini' },
+  { id: 'opencode',   label: 'opencode',            mech: 'native opencode plugin',        detect: 'command:opencode' },
   { id: 'codex',      label: 'Codex CLI',           mech: 'npx skills add (codex)',        detect: 'command:codex',           profile: 'codex' },
 
   // IDE / VS Code-family — extension probes are precise. Cursor/Windsurf also
@@ -179,7 +180,6 @@ const PROVIDERS = [
   { id: 'kiro',       label: 'Kiro CLI',            mech: 'npx skills add (kiro-cli)',     detect: 'command:kiro', profile: 'kiro-cli' },
   { id: 'mistral',    label: 'Mistral Vibe',        mech: 'npx skills add (mistral-vibe)', detect: 'command:mistral', profile: 'mistral-vibe' },
   { id: 'openhands',  label: 'OpenHands',           mech: 'npx skills add (openhands)',    detect: 'command:openhands', profile: 'openhands' },
-  { id: 'opencode',   label: 'opencode',            mech: 'npx skills add (opencode)',     detect: 'command:opencode', profile: 'opencode' },
   { id: 'qwen',       label: 'Qwen Code',           mech: 'npx skills add (qwen-code)',    detect: 'command:qwen', profile: 'qwen-code' },
   { id: 'rovodev',    label: 'Atlassian Rovo Dev',  mech: 'npx skills add (rovodev)',      detect: 'command:rovodev', profile: 'rovodev' },
   { id: 'tabnine',    label: 'Tabnine CLI',         mech: 'npx skills add (tabnine-cli)',  detect: 'command:tabnine', profile: 'tabnine-cli' },
@@ -437,6 +437,171 @@ function installViaSkills(ctx, prov) {
   process.stdout.write('\n');
 }
 
+// ── opencode native install ───────────────────────────────────────────────
+// Drops the in-repo plugin (src/plugins/opencode/) plus skills, agents,
+// commands, and an AGENTS.md ruleset into ~/.config/opencode/. Patches
+// opencode.json with a "plugin" array entry. Mirrors the Claude Code hook
+// architecture as closely as opencode allows — only the statusline is missing
+// (opencode's TUI exposes no plugin-writable badge).
+const OPENCODE_SKILL_DIRS  = ['caveman', 'caveman-commit', 'caveman-review', 'caveman-help', 'caveman-stats', 'caveman-compress', 'cavecrew'];
+const OPENCODE_AGENT_FILES = ['cavecrew-investigator.md', 'cavecrew-builder.md', 'cavecrew-reviewer.md'];
+const OPENCODE_COMMAND_FILES = ['caveman.md', 'caveman-commit.md', 'caveman-review.md', 'caveman-compress.md', 'caveman-stats.md', 'caveman-help.md'];
+const OPENCODE_PLUGIN_REL = './plugins/caveman/plugin.js';
+const OPENCODE_AGENTS_MD_SENTINEL = 'Respond terse like smart caveman';
+
+function opencodeConfigDir() {
+  if (process.env.XDG_CONFIG_HOME) return path.join(process.env.XDG_CONFIG_HOME, 'opencode');
+  if (IS_WIN) return path.join(process.env.APPDATA || path.join(os.homedir(), 'AppData', 'Roaming'), 'opencode');
+  return path.join(os.homedir(), '.config', 'opencode');
+}
+
+function copyDirRecursive(src, dest) {
+  fs.mkdirSync(dest, { recursive: true });
+  for (const entry of fs.readdirSync(src, { withFileTypes: true })) {
+    const s = path.join(src, entry.name);
+    const d = path.join(dest, entry.name);
+    if (entry.isDirectory()) copyDirRecursive(s, d);
+    else if (entry.isFile()) fs.copyFileSync(s, d);
+  }
+}
+
+function installOpencode(ctx) {
+  const { say, note, warn, opts, repoRoot, results } = ctx;
+  results.detected++;
+  say('→ opencode detected');
+
+  if (!repoRoot) {
+    warn('  opencode native install requires a local clone of the caveman repo.');
+    note('  Re-run from a clone: git clone https://github.com/' + REPO + ' && cd caveman && node bin/install.js --only opencode');
+    results.failed.push(['opencode', 'native install requires local repo clone']);
+    process.stdout.write('\n');
+    return;
+  }
+
+  const dir = opencodeConfigDir();
+  const pluginDir   = path.join(dir, 'plugins', 'caveman');
+  const commandsDir = path.join(dir, 'commands');
+  const agentsDir   = path.join(dir, 'agents');
+  const skillsDir   = path.join(dir, 'skills');
+  const opencodeJson = path.join(dir, 'opencode.json');
+  const agentsMd     = path.join(dir, 'AGENTS.md');
+
+  if (opts.dryRun) {
+    note(`  would mkdir ${pluginDir}/, ${commandsDir}/, ${agentsDir}/, ${skillsDir}/`);
+    note(`  would copy plugin.js + package.json + caveman-config.cjs into ${pluginDir}/`);
+    note(`  would copy ${OPENCODE_COMMAND_FILES.length} command files into ${commandsDir}/`);
+    note(`  would copy ${OPENCODE_AGENT_FILES.length} cavecrew agents into ${agentsDir}/`);
+    note(`  would copy ${OPENCODE_SKILL_DIRS.length} skill dirs into ${skillsDir}/`);
+    note(`  would patch ${opencodeJson} with "plugin" entry${opts.withMcpShrink ? ' + caveman-shrink MCP' : ''}`);
+    note(`  would write Tier-3 ruleset to ${agentsMd}`);
+    results.installed.push('opencode');
+    process.stdout.write('\n');
+    return;
+  }
+
+  try {
+    // 1. Plugin dir — copy plugin.js, package.json, caveman-config.js (sibling).
+    fs.mkdirSync(pluginDir, { recursive: true });
+    const pluginSrc = path.join(repoRoot, 'src', 'plugins', 'opencode');
+    fs.copyFileSync(path.join(pluginSrc, 'plugin.js'),    path.join(pluginDir, 'plugin.js'));
+    fs.copyFileSync(path.join(pluginSrc, 'package.json'), path.join(pluginDir, 'package.json'));
+    // Renamed to .cjs because the plugin dir is "type": "module" — a bare .js
+    // sibling would be loaded as ESM and break the plugin's require() bridge.
+    fs.copyFileSync(
+      path.join(repoRoot, 'src', 'hooks', 'caveman-config.js'),
+      path.join(pluginDir, 'caveman-config.cjs'),
+    );
+    process.stdout.write(`  installed: ${pluginDir}\n`);
+
+    // 2. Commands.
+    fs.mkdirSync(commandsDir, { recursive: true });
+    const cmdSrcDir = path.join(pluginSrc, 'commands');
+    for (const f of OPENCODE_COMMAND_FILES) {
+      const src = path.join(cmdSrcDir, f);
+      const dest = path.join(commandsDir, f);
+      if (fs.existsSync(dest) && !opts.force) { note(`  skipped ${dest} (exists; --force to overwrite)`); continue; }
+      fs.copyFileSync(src, dest);
+      process.stdout.write(`  installed: ${dest}\n`);
+    }
+
+    // 3. Subagents.
+    fs.mkdirSync(agentsDir, { recursive: true });
+    const agentSrcDir = path.join(repoRoot, 'agents');
+    for (const f of OPENCODE_AGENT_FILES) {
+      const src = path.join(agentSrcDir, f);
+      const dest = path.join(agentsDir, f);
+      if (!fs.existsSync(src)) continue;
+      if (fs.existsSync(dest) && !opts.force) { note(`  skipped ${dest} (exists; --force to overwrite)`); continue; }
+      fs.copyFileSync(src, dest);
+      process.stdout.write(`  installed: ${dest}\n`);
+    }
+
+    // 4. Skills — opencode auto-discovers SKILL.md from ~/.config/opencode/skills/.
+    fs.mkdirSync(skillsDir, { recursive: true });
+    const skillSrcDir = path.join(repoRoot, 'skills');
+    for (const name of OPENCODE_SKILL_DIRS) {
+      const src = path.join(skillSrcDir, name);
+      const dest = path.join(skillsDir, name);
+      if (!fs.existsSync(src)) continue;
+      if (fs.existsSync(dest) && !opts.force) { note(`  skipped ${dest}/ (exists; --force to overwrite)`); continue; }
+      copyDirRecursive(src, dest);
+      process.stdout.write(`  installed: ${dest}/\n`);
+    }
+
+    // 5. AGENTS.md — Tier-3 always-on ruleset. Append-with-sentinel so we
+    //    don't clobber a user-authored AGENTS.md.
+    const ruleBody = fs.readFileSync(path.join(repoRoot, 'src', 'rules', 'caveman-activate.md'), 'utf8').trimEnd() + '\n';
+    if (fs.existsSync(agentsMd)) {
+      const existing = fs.readFileSync(agentsMd, 'utf8');
+      if (!existing.includes(OPENCODE_AGENTS_MD_SENTINEL)) {
+        const sep = existing.endsWith('\n\n') ? '' : (existing.endsWith('\n') ? '\n' : '\n\n');
+        fs.writeFileSync(agentsMd, existing + sep + ruleBody, { mode: 0o644 });
+        process.stdout.write(`  appended caveman ruleset to ${agentsMd}\n`);
+      } else {
+        note(`  ${agentsMd} already contains caveman ruleset`);
+      }
+    } else {
+      fs.writeFileSync(agentsMd, ruleBody, { mode: 0o644 });
+      process.stdout.write(`  installed: ${agentsMd}\n`);
+    }
+
+    // 6. opencode.json — add plugin entry; optional caveman-shrink MCP.
+    let cfg = SETTINGS.readSettings(opencodeJson);
+    if (cfg === null) {
+      warn(`  ${opencodeJson} unparseable; will not touch it. Edit manually then re-run.`);
+      results.failed.push(['opencode', 'opencode.json unparseable']);
+      process.stdout.write('\n');
+      return;
+    }
+    if (fs.existsSync(opencodeJson)) {
+      try { fs.copyFileSync(opencodeJson, opencodeJson + '.bak'); } catch (_) {}
+    }
+    if (!Array.isArray(cfg.plugin)) cfg.plugin = [];
+    if (!cfg.plugin.includes(OPENCODE_PLUGIN_REL)) {
+      cfg.plugin.push(OPENCODE_PLUGIN_REL);
+    }
+    if (opts.withMcpShrink) {
+      if (!cfg.mcp || typeof cfg.mcp !== 'object') cfg.mcp = {};
+      if (!cfg.mcp['caveman-shrink']) {
+        cfg.mcp['caveman-shrink'] = {
+          type: 'local',
+          command: ['npx', '-y', MCP_SHRINK_PKG],
+          enabled: true,
+        };
+        process.stdout.write('  registered caveman-shrink MCP server\n');
+      }
+    }
+    SETTINGS.writeSettings(opencodeJson, cfg);
+    process.stdout.write(`  patched: ${opencodeJson}\n`);
+
+    results.installed.push('opencode');
+  } catch (e) {
+    warn('  opencode install failed: ' + (e && e.message || e));
+    results.failed.push(['opencode', (e && e.message) || 'unknown error']);
+  }
+  process.stdout.write('\n');
+}
+
 // ── Hooks installer ────────────────────────────────────────────────────────
 // Replaces src/hooks/install.sh + src/hooks/install.ps1.
 function installHooks(ctx) {
@@ -659,6 +824,62 @@ function uninstall(ctx) {
     runSpawn('gemini', ['extensions', 'uninstall', 'caveman'], null, opts.dryRun);
   }
 
+  // opencode native install — strip plugin entry, MCP entry, and our files.
+  // Probed by the existence of the plugin dir we own; if absent, skip silently.
+  const ocDir = opencodeConfigDir();
+  const ocPluginDir = path.join(ocDir, 'plugins', 'caveman');
+  if (fs.existsSync(ocPluginDir)) {
+    const ocJson = path.join(ocDir, 'opencode.json');
+    if (fs.existsSync(ocJson)) {
+      const cfg = SETTINGS.readSettings(ocJson);
+      if (cfg) {
+        if (Array.isArray(cfg.plugin)) {
+          cfg.plugin = cfg.plugin.filter(p => p !== OPENCODE_PLUGIN_REL);
+          if (cfg.plugin.length === 0) delete cfg.plugin;
+        }
+        if (cfg.mcp && typeof cfg.mcp === 'object' && cfg.mcp['caveman-shrink']) {
+          delete cfg.mcp['caveman-shrink'];
+          if (Object.keys(cfg.mcp).length === 0) delete cfg.mcp;
+        }
+        if (!opts.dryRun) SETTINGS.writeSettings(ocJson, cfg);
+        ok(`  pruned caveman entries from ${ocJson}`);
+      }
+    }
+    if (!opts.dryRun) { try { fs.rmSync(ocPluginDir, { recursive: true, force: true }); } catch (_) {} }
+    note(`  removed ${ocPluginDir}`);
+    // Commands, agents, skills — only files matching our manifest (don't
+    // sweep the parent dirs; user may have other entries there).
+    for (const f of OPENCODE_COMMAND_FILES) {
+      const p = path.join(ocDir, 'commands', f);
+      if (fs.existsSync(p) && !opts.dryRun) { try { fs.unlinkSync(p); } catch (_) {} }
+    }
+    for (const f of OPENCODE_AGENT_FILES) {
+      const p = path.join(ocDir, 'agents', f);
+      if (fs.existsSync(p) && !opts.dryRun) { try { fs.unlinkSync(p); } catch (_) {} }
+    }
+    for (const name of OPENCODE_SKILL_DIRS) {
+      const p = path.join(ocDir, 'skills', name);
+      if (fs.existsSync(p) && !opts.dryRun) { try { fs.rmSync(p, { recursive: true, force: true }); } catch (_) {} }
+    }
+    // AGENTS.md — only remove if our sentinel is the entire content (we
+    // wrote it) or strip our block from a mixed file.
+    const ocAgentsMd = path.join(ocDir, 'AGENTS.md');
+    if (fs.existsSync(ocAgentsMd)) {
+      const body = fs.readFileSync(ocAgentsMd, 'utf8');
+      if (body.includes(OPENCODE_AGENTS_MD_SENTINEL)) {
+        if (body.trim() === '' || body.trim().startsWith(OPENCODE_AGENTS_MD_SENTINEL)) {
+          if (!opts.dryRun) { try { fs.unlinkSync(ocAgentsMd); } catch (_) {} }
+          note(`  removed ${ocAgentsMd}`);
+        } else {
+          note(`  left ${ocAgentsMd} in place (mixed content — strip caveman block manually)`);
+        }
+      }
+    }
+    // opencode flag file
+    const ocFlag = path.join(ocDir, '.caveman-active');
+    if (fs.existsSync(ocFlag) && !opts.dryRun) { try { fs.unlinkSync(ocFlag); } catch (_) {} }
+  }
+
   // Flag file
   const flag = path.join(configDir, '.caveman-active');
   if (fs.existsSync(flag) && !opts.dryRun) { try { fs.unlinkSync(flag); } catch (_) {} }
@@ -795,6 +1016,7 @@ async function main() {
     if (!detectMatch(prov.detect)) continue;
     if (prov.id === 'claude')   { installClaude(ctx); continue; }
     if (prov.id === 'gemini')   { installGemini(ctx); continue; }
+    if (prov.id === 'opencode') { installOpencode(ctx); continue; }
     if (prov.profile)           { installViaSkills(ctx, prov); continue; }
   }
 
diff --git a/src/plugins/opencode/README.md b/src/plugins/opencode/README.md
new file mode 100644
index 0000000..1c68198
--- /dev/null
+++ b/src/plugins/opencode/README.md
@@ -0,0 +1,46 @@
+# caveman — opencode plugin
+
+Native opencode plugin. Mirrors the Claude Code hook architecture using
+opencode's `session.created` + `tui.prompt.append` lifecycle hooks.
+
+## What this ships
+
+| File | Role |
+|---|---|
+| `plugin.js` | ESM Bun module. Default-exports an opencode `Plugin` factory. |
+| `package.json` | Marks the directory as ESM so Bun loads `plugin.js` correctly. |
+| `commands/*.md` | Six slash-command prompt templates (`/caveman`, `/caveman-commit`, …). |
+
+The installer (`bin/install.js --only opencode`) copies these alongside
+`src/hooks/caveman-config.js` (for the symlink-safe flag-write helpers, renamed
+to `caveman-config.cjs` because this directory is `"type": "module"`) into
+`~/.config/opencode/plugins/caveman/` and patches `opencode.json` with a
+`"plugin"` array entry.
+
+## What it does
+
+- `session.created` → writes the configured default mode to
+  `~/.config/opencode/.caveman-active` via the same `safeWriteFlag` helper
+  Claude Code uses (O_NOFOLLOW, atomic temp+rename, 0600 perms, symlink
+  refusal, ownership check).
+- `tui.prompt.append` → flips the flag in response to `/caveman[ <level>]`,
+  `/caveman-commit`, `/caveman-review`, `/caveman-compress`, and natural
+  language ("turn on caveman", "stop caveman", "normal mode"). When a
+  non-independent mode is active, appends a one-line reinforcement to keep
+  caveman in the model's attention each turn.
+
+## What it does NOT do
+
+- **No statusline badge.** opencode's TUI does not expose a plugin-writable
+  statusline. The flag file is at `~/.config/opencode/.caveman-active` if
+  you want to surface mode in your shell prompt.
+- **No system-prompt injection from `session.created`.** opencode's docs
+  don't expose a return shape for that. The always-on caveman ruleset comes
+  from `~/.config/opencode/AGENTS.md` (also written by the installer) so
+  the rules load even when the plugin runtime is broken.
+
+## Why no separate npm package
+
+Plugin code reuses `caveman-config.js` from the main repo. Shipping as an
+in-repo plugin avoids a second release cadence and a name collision with
+the existing third-party `opencode-caveman` npm package.
diff --git a/src/plugins/opencode/commands/caveman-commit.md b/src/plugins/opencode/commands/caveman-commit.md
new file mode 100644
index 0000000..7c5025d
--- /dev/null
+++ b/src/plugins/opencode/commands/caveman-commit.md
@@ -0,0 +1,8 @@
+---
+description: Generate a terse caveman-style commit message for staged changes
+---
+Generate a commit message for the current staged changes.
+
+Conventional Commits format. Subject line ≤50 chars, imperative, lowercase
+after the type. No period on subject. Body only when the "why" isn't obvious
+from the subject — explain why over what. Drop filler, hedging, padding.
diff --git a/src/plugins/opencode/commands/caveman-help.md b/src/plugins/opencode/commands/caveman-help.md
new file mode 100644
index 0000000..4538f88
--- /dev/null
+++ b/src/plugins/opencode/commands/caveman-help.md
@@ -0,0 +1,20 @@
+---
+description: Quick reference card for caveman modes, slash commands, and triggers
+---
+Show the caveman quick-reference card.
+
+| Command | What |
+|---|---|
+| `/caveman` | Activate at default level (full) |
+| `/caveman lite` | Light compression — ~30% tokens dropped |
+| `/caveman ultra` | Maximum compression |
+| `/caveman wenyan[-lite\|-ultra]` | Classical Chinese compression |
+| `/caveman off` | Deactivate |
+| `/caveman-commit` | Terse commit message |
+| `/caveman-review` | One-line review findings |
+| `/caveman-compress <file>` | Compress a Markdown file |
+| `/caveman-stats` | Lifetime token-savings |
+
+Natural language also works: "turn on caveman", "stop caveman", "normal mode".
+
+Code, commits, security warnings: caveman drops out automatically.
diff --git a/src/plugins/opencode/commands/caveman-review.md b/src/plugins/opencode/commands/caveman-review.md
new file mode 100644
index 0000000..0bfdc57
--- /dev/null
+++ b/src/plugins/opencode/commands/caveman-review.md
@@ -0,0 +1,8 @@
+---
+description: Caveman-style code review — one-line findings with severity
+---
+Review the current diff (or files: $ARGUMENTS).
+
+One line per finding. Format: `L<line>: <severity> <problem>. <fix>.`
+Severity emoji: 🔴 critical · 🟡 warn · 🟢 nit. Skip non-issues.
+Group by file. End with a one-line verdict.
diff --git a/src/plugins/opencode/commands/caveman-stats.md b/src/plugins/opencode/commands/caveman-stats.md
new file mode 100644
index 0000000..b76ec27
--- /dev/null
+++ b/src/plugins/opencode/commands/caveman-stats.md
@@ -0,0 +1,8 @@
+---
+description: Show caveman lifetime token-savings stats
+---
+Show caveman stats — total tokens saved, sessions, average compression ratio.
+
+Read the lifetime history log at `~/.config/caveman/.caveman-history.jsonl`
+(or wherever the caveman-stats script writes it). Output: total saved,
+sessions counted, avg ratio. One short table.
diff --git a/src/plugins/opencode/commands/caveman.md b/src/plugins/opencode/commands/caveman.md
new file mode 100644
index 0000000..10f98c7
--- /dev/null
+++ b/src/plugins/opencode/commands/caveman.md
@@ -0,0 +1,13 @@
+---
+description: Activate caveman compression mode (lite | full | ultra | wenyan | off)
+---
+Activate caveman mode: $ARGUMENTS
+
+If no level given, use full. If "off", deactivate.
+
+Respond terse like smart caveman. Drop articles, filler, pleasantries, hedging.
+Fragments OK. Technical terms exact. Code unchanged.
+Pattern: [thing] [action] [reason]. [next step].
+
+Behavior persists until session ends or user says "stop caveman" / "normal mode".
+Code, commits, security warnings: write normal English.
diff --git a/src/plugins/opencode/package.json b/src/plugins/opencode/package.json
new file mode 100644
index 0000000..f0971a1
--- /dev/null
+++ b/src/plugins/opencode/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "caveman-opencode-plugin",
+  "version": "0.1.0",
+  "description": "caveman plugin for opencode — terse mode tracking + slash commands",
+  "type": "module",
+  "main": "plugin.js",
+  "private": true
+}
diff --git a/src/plugins/opencode/plugin.js b/src/plugins/opencode/plugin.js
new file mode 100644
index 0000000..7f3a870
--- /dev/null
+++ b/src/plugins/opencode/plugin.js
@@ -0,0 +1,154 @@
+// caveman — opencode plugin
+//
+// Mirrors the Claude Code SessionStart + UserPromptSubmit hook pair using
+// opencode's lifecycle hook system. Bun ESM module; loads the existing
+// security-hardened helpers from caveman-config.js via createRequire so the
+// symlink-safe flag-write code lives in one place.
+//
+// Layout once installed:
+//   ~/.config/opencode/plugins/caveman/
+//   ├── package.json
+//   ├── plugin.js              ← this file
+//   └── caveman-config.js      ← copied sibling of src/hooks/caveman-config.js
+//
+// Always-on caveman ruleset is provided separately via
+// ~/.config/opencode/AGENTS.md (Tier-3 base) so this plugin only handles
+// dynamic state — flag writes, slash-command parsing, natural-language
+// activation, and per-prompt reinforcement. opencode's `session.created`
+// payload doesn't expose a documented system-prompt-injection return, so we
+// don't try to emit ruleset content here.
+
+import { createRequire } from 'node:module';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+import { existsSync, unlinkSync } from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+
+const require = createRequire(import.meta.url);
+const here = dirname(fileURLToPath(import.meta.url));
+
+// When installed: caveman-config.cjs sits next to plugin.js (copied by
+// bin/install.js, renamed to .cjs because this directory's package.json
+// declares "type": "module" — bare .js would be loaded as ESM and break
+// require()). When loaded from the source tree (tests, dev): fall back
+// to the canonical src/hooks/caveman-config.js, which lives in a directory
+// whose own package.json pins "type": "commonjs". One source of truth
+// either way.
+function loadConfig() {
+  try { return require(join(here, 'caveman-config.cjs')); }
+  catch (_) { return require(join(here, '..', '..', 'hooks', 'caveman-config.js')); }
+}
+const config = loadConfig();
+
+const { getDefaultMode, safeWriteFlag, readFlag, VALID_MODES } = config;
+
+// Modes handled by independent skills — not selectable via /caveman <arg>.
+const INDEPENDENT_MODES = new Set(['commit', 'review', 'compress']);
+
+function opencodeConfigDir() {
+  if (process.env.XDG_CONFIG_HOME) {
+    return path.join(process.env.XDG_CONFIG_HOME, 'opencode');
+  }
+  if (process.platform === 'win32') {
+    return path.join(
+      process.env.APPDATA || path.join(os.homedir(), 'AppData', 'Roaming'),
+      'opencode'
+    );
+  }
+  return path.join(os.homedir(), '.config', 'opencode');
+}
+
+const flagPath = path.join(opencodeConfigDir(), '.caveman-active');
+
+function reinforcementLine(mode) {
+  return 'CAVEMAN MODE ACTIVE (' + mode + '). ' +
+    'Drop articles/filler/pleasantries/hedging. Fragments OK. ' +
+    'Code/commits/security: write normal.';
+}
+
+// Parse a prompt for slash-command activation or natural-language toggles.
+// Returns the new mode to write, the literal string 'off' to deactivate, or
+// null when the prompt doesn't change state. Mirrors caveman-mode-tracker.js.
+function parseModeChange(promptRaw) {
+  const prompt = (promptRaw || '').trim().toLowerCase();
+  if (!prompt) return null;
+
+  // Natural-language deactivation — checked before activation so "stop talking
+  // like caveman" doesn't trip the activation regex.
+  if (/\b(stop|disable|deactivate|turn off)\b.*\bcaveman\b/i.test(prompt) ||
+      /\bcaveman\b.*\b(stop|disable|deactivate|turn off)\b/i.test(prompt) ||
+      /\bnormal mode\b/i.test(prompt)) {
+    return 'off';
+  }
+
+  // Natural-language activation
+  if (/\b(activate|enable|turn on|start|talk like)\b.*\bcaveman\b/i.test(prompt) ||
+      /\bcaveman\b.*\b(mode|activate|enable|turn on|start)\b/i.test(prompt)) {
+    const mode = getDefaultMode();
+    return mode === 'off' ? null : mode;
+  }
+
+  // Slash-command parsing — opencode also expands command files, but if the
+  // user types the literal slash command we still want to flip the flag.
+  if (prompt.startsWith('/caveman')) {
+    const parts = prompt.split(/\s+/);
+    const cmd = parts[0];
+    const arg = parts[1] || '';
+
+    if (cmd === '/caveman-commit')   return 'commit';
+    if (cmd === '/caveman-review')   return 'review';
+    if (cmd === '/caveman-compress') return 'compress';
+
+    if (cmd === '/caveman') {
+      if (!arg)                                     return getDefaultMode();
+      if (arg === 'off' || arg === 'stop' || arg === 'disable') return 'off';
+      if (arg === 'wenyan-full')                    return 'wenyan';
+      if (VALID_MODES.includes(arg) && !INDEPENDENT_MODES.has(arg)) return arg;
+      // Unknown arg — leave flag alone. No silent overwrite.
+      return null;
+    }
+  }
+
+  return null;
+}
+
+function applyModeChange(mode) {
+  if (!mode) return;
+  if (mode === 'off') {
+    try { if (existsSync(flagPath)) unlinkSync(flagPath); } catch (e) {}
+    return;
+  }
+  safeWriteFlag(flagPath, mode);
+}
+
+export const CavemanPlugin = async (_ctx) => ({
+  'session.created': async () => {
+    const mode = getDefaultMode();
+    if (mode === 'off') {
+      try { if (existsSync(flagPath)) unlinkSync(flagPath); } catch (e) {}
+      return;
+    }
+    safeWriteFlag(flagPath, mode);
+  },
+
+  // opencode's TUI prompt-append hook fires before the prompt is sent to the
+  // model. We use it for two things: react to mode-changing prompts (slash
+  // commands + natural language), and append a one-line reinforcement when
+  // caveman is active so the model can't drift mid-session. Returning an
+  // object with `append` is the documented way to inject prompt content.
+  'tui.prompt.append': async (input) => {
+    const promptText = (input && (input.prompt || input.text)) || '';
+
+    const change = parseModeChange(promptText);
+    if (change) applyModeChange(change);
+
+    const active = readFlag(flagPath);
+    if (active && !INDEPENDENT_MODES.has(active)) {
+      return { append: reinforcementLine(active) };
+    }
+    return undefined;
+  },
+});
+
+export default CavemanPlugin;
diff --git a/src/tools/caveman-init.js b/src/tools/caveman-init.js
index 052ccb2..49b6dce 100644
--- a/src/tools/caveman-init.js
+++ b/src/tools/caveman-init.js
@@ -47,6 +47,9 @@ const AGENTS = [
   { id: 'copilot',  file: '.github/copilot-instructions.md',
     frontmatter: '',
     mode: 'append' },
+  { id: 'opencode', file: '.opencode/AGENTS.md',
+    frontmatter: '',
+    mode: 'append' },
   { id: 'agents',   file: 'AGENTS.md',
     frontmatter: '',
     mode: 'append' },
diff --git a/tests/installer/opencode.test.mjs b/tests/installer/opencode.test.mjs
new file mode 100644
index 0000000..395ad79
--- /dev/null
+++ b/tests/installer/opencode.test.mjs
@@ -0,0 +1,217 @@
+// opencode native install — fresh install, idempotency, uninstall, plugin smoke.
+//
+// Detection of opencode is gated behind `command -v opencode`, so to run on a
+// CI box without opencode installed we prepend a tmpdir with a no-op `opencode`
+// shim to PATH. The installer's per-provider dispatch only checks PATH; it
+// never invokes the binary itself.
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+import { createRequire } from 'node:module';
+
+const HERE = path.dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = path.resolve(HERE, '..', '..');
+const INSTALLER = path.join(REPO_ROOT, 'bin', 'install.js');
+const requireCjs = createRequire(import.meta.url);
+const SETTINGS = requireCjs(path.join(REPO_ROOT, 'bin', 'lib', 'settings.js'));
+
+const IS_WIN = process.platform === 'win32';
+
+function freshTmpDir() {
+  return fs.mkdtempSync(path.join(os.tmpdir(), 'caveman-opencode-'));
+}
+
+// Make a throwaway `opencode` binary on PATH so detectMatch('command:opencode')
+// returns true. The shim never executes — installer only checks PATH presence.
+function shimOpencode() {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'cm-shim-'));
+  if (IS_WIN) {
+    fs.writeFileSync(path.join(dir, 'opencode.cmd'), '@echo off\r\n');
+  } else {
+    const f = path.join(dir, 'opencode');
+    fs.writeFileSync(f, '#!/bin/sh\nexit 0\n');
+    fs.chmodSync(f, 0o755);
+  }
+  return dir;
+}
+
+function runInstaller(args, env) {
+  return spawnSync('node', [INSTALLER, ...args, '--non-interactive', '--no-mcp-shrink'], {
+    env, encoding: 'utf8',
+  });
+}
+
+function pathWith(prependDir) {
+  const sep = IS_WIN ? ';' : ':';
+  return prependDir + sep + (process.env.PATH || '');
+}
+
+// ── 1. Fresh install populates expected files ────────────────────────────
+test('opencode fresh install drops plugin, commands, agents, skills, AGENTS.md, opencode.json', () => {
+  const xdg = freshTmpDir();
+  const shimDir = shimOpencode();
+  try {
+    const r = runInstaller(['--only', 'opencode'], {
+      ...process.env,
+      XDG_CONFIG_HOME: xdg,
+      PATH: pathWith(shimDir),
+      NO_COLOR: '1',
+    });
+    assert.notEqual(r.status, 2, `argv error: ${r.stderr}`);
+
+    const ocDir = path.join(xdg, 'opencode');
+    assert.ok(fs.existsSync(path.join(ocDir, 'plugins', 'caveman', 'plugin.js')), 'plugin.js missing');
+    assert.ok(fs.existsSync(path.join(ocDir, 'plugins', 'caveman', 'package.json')), 'plugin package.json missing');
+    assert.ok(fs.existsSync(path.join(ocDir, 'plugins', 'caveman', 'caveman-config.cjs')), 'caveman-config.cjs sibling missing');
+
+    for (const f of ['caveman.md', 'caveman-commit.md', 'caveman-review.md', 'caveman-compress.md', 'caveman-stats.md', 'caveman-help.md']) {
+      assert.ok(fs.existsSync(path.join(ocDir, 'commands', f)), `command ${f} missing`);
+    }
+    for (const f of ['cavecrew-investigator.md', 'cavecrew-builder.md', 'cavecrew-reviewer.md']) {
+      assert.ok(fs.existsSync(path.join(ocDir, 'agents', f)), `agent ${f} missing`);
+    }
+    for (const name of ['caveman', 'caveman-commit', 'caveman-review', 'caveman-help', 'caveman-stats', 'caveman-compress', 'cavecrew']) {
+      assert.ok(fs.existsSync(path.join(ocDir, 'skills', name, 'SKILL.md')), `skill ${name}/SKILL.md missing`);
+    }
+    assert.ok(fs.existsSync(path.join(ocDir, 'AGENTS.md')), 'AGENTS.md missing');
+    assert.match(fs.readFileSync(path.join(ocDir, 'AGENTS.md'), 'utf8'), /Respond terse like smart caveman/);
+
+    const cfgPath = path.join(ocDir, 'opencode.json');
+    assert.ok(fs.existsSync(cfgPath), 'opencode.json missing');
+    const cfg = JSON.parse(fs.readFileSync(cfgPath, 'utf8'));
+    assert.ok(Array.isArray(cfg.plugin), 'opencode.json missing plugin array');
+    assert.ok(cfg.plugin.includes('./plugins/caveman/plugin.js'), 'plugin entry missing');
+  } finally {
+    fs.rmSync(xdg, { recursive: true, force: true });
+    fs.rmSync(shimDir, { recursive: true, force: true });
+  }
+});
+
+// ── 2. Idempotency: install twice, plugin array stays length 1 ───────────
+test('opencode idempotent install does not duplicate plugin entries', () => {
+  const xdg = freshTmpDir();
+  const shimDir = shimOpencode();
+  try {
+    const env = { ...process.env, XDG_CONFIG_HOME: xdg, PATH: pathWith(shimDir), NO_COLOR: '1' };
+    const r1 = runInstaller(['--only', 'opencode'], env);
+    assert.notEqual(r1.status, 2);
+    const r2 = runInstaller(['--only', 'opencode'], env);
+    assert.notEqual(r2.status, 2);
+
+    const cfg = JSON.parse(fs.readFileSync(path.join(xdg, 'opencode', 'opencode.json'), 'utf8'));
+    const matches = cfg.plugin.filter(p => p === './plugins/caveman/plugin.js');
+    assert.equal(matches.length, 1, `expected 1 plugin entry, got ${matches.length}`);
+
+    // AGENTS.md should not have the ruleset duplicated either.
+    const agentsMd = fs.readFileSync(path.join(xdg, 'opencode', 'AGENTS.md'), 'utf8');
+    const sentinelCount = (agentsMd.match(/Respond terse like smart caveman/g) || []).length;
+    assert.equal(sentinelCount, 1, `expected 1 sentinel, got ${sentinelCount}`);
+  } finally {
+    fs.rmSync(xdg, { recursive: true, force: true });
+    fs.rmSync(shimDir, { recursive: true, force: true });
+  }
+});
+
+// ── 3. Tolerates JSONC opencode.json (#249-class regression guard) ───────
+test('opencode install tolerates JSONC opencode.json (comments + trailing commas)', () => {
+  const xdg = freshTmpDir();
+  const shimDir = shimOpencode();
+  try {
+    const ocDir = path.join(xdg, 'opencode');
+    fs.mkdirSync(ocDir, { recursive: true });
+    fs.writeFileSync(path.join(ocDir, 'opencode.json'),
+      `// hand-written
+{
+  /* user prefs */
+  "model": "anthropic/claude-sonnet-4-5",
+  "theme": "dark",
+}
+`);
+
+    const env = { ...process.env, XDG_CONFIG_HOME: xdg, PATH: pathWith(shimDir), NO_COLOR: '1' };
+    const r = runInstaller(['--only', 'opencode'], env);
+    assert.notEqual(r.status, 2);
+
+    const cfg = JSON.parse(fs.readFileSync(path.join(ocDir, 'opencode.json'), 'utf8'));
+    assert.equal(cfg.model, 'anthropic/claude-sonnet-4-5', 'user model setting wiped');
+    assert.equal(cfg.theme, 'dark', 'user theme setting wiped');
+    assert.ok(cfg.plugin.includes('./plugins/caveman/plugin.js'), 'plugin entry missing');
+  } finally {
+    fs.rmSync(xdg, { recursive: true, force: true });
+    fs.rmSync(shimDir, { recursive: true, force: true });
+  }
+});
+
+// ── 4. Uninstall removes opencode artifacts and prunes config ────────────
+test('opencode uninstall removes plugin dir, command/agent/skill files, prunes opencode.json', () => {
+  const xdg = freshTmpDir();
+  const shimDir = shimOpencode();
+  try {
+    const env = { ...process.env, XDG_CONFIG_HOME: xdg, PATH: pathWith(shimDir), NO_COLOR: '1' };
+    const r1 = runInstaller(['--only', 'opencode'], env);
+    assert.notEqual(r1.status, 2);
+
+    const r2 = runInstaller(['--uninstall'], env);
+    assert.notEqual(r2.status, 2);
+
+    const ocDir = path.join(xdg, 'opencode');
+    assert.equal(fs.existsSync(path.join(ocDir, 'plugins', 'caveman')), false, 'plugin dir survived');
+    assert.equal(fs.existsSync(path.join(ocDir, 'commands', 'caveman.md')), false, 'caveman.md command survived');
+    assert.equal(fs.existsSync(path.join(ocDir, 'agents', 'cavecrew-builder.md')), false, 'cavecrew agent survived');
+    assert.equal(fs.existsSync(path.join(ocDir, 'skills', 'caveman')), false, 'caveman skill dir survived');
+    assert.equal(fs.existsSync(path.join(ocDir, 'AGENTS.md')), false, 'AGENTS.md (we wrote it) survived');
+
+    if (fs.existsSync(path.join(ocDir, 'opencode.json'))) {
+      const cfg = JSON.parse(fs.readFileSync(path.join(ocDir, 'opencode.json'), 'utf8'));
+      const stillHasPlugin = Array.isArray(cfg.plugin) && cfg.plugin.includes('./plugins/caveman/plugin.js');
+      assert.equal(stillHasPlugin, false, 'plugin entry survived in opencode.json');
+    }
+  } finally {
+    fs.rmSync(xdg, { recursive: true, force: true });
+    fs.rmSync(shimDir, { recursive: true, force: true });
+  }
+});
+
+// ── 5. Plugin smoke: load installed plugin.js, fire fake hooks ────────────
+test('opencode plugin handles /caveman ultra and stop caveman via tui.prompt.append', async () => {
+  const xdg = freshTmpDir();
+  const shimDir = shimOpencode();
+  try {
+    const env = { ...process.env, XDG_CONFIG_HOME: xdg, PATH: pathWith(shimDir), NO_COLOR: '1' };
+    const r = runInstaller(['--only', 'opencode'], env);
+    assert.notEqual(r.status, 2);
+
+    const pluginPath = path.join(xdg, 'opencode', 'plugins', 'caveman', 'plugin.js');
+    const flagPath = path.join(xdg, 'opencode', '.caveman-active');
+
+    // Set XDG_CONFIG_HOME for the plugin so flagPath resolves to our temp dir.
+    process.env.XDG_CONFIG_HOME = xdg;
+
+    const mod = await import(pathToFileURL(pluginPath).href);
+    const factory = mod.default || mod.CavemanPlugin;
+    const handlers = await factory({});
+
+    // Slash command activates ultra
+    const out1 = await handlers['tui.prompt.append']({ prompt: '/caveman ultra' });
+    assert.equal(fs.readFileSync(flagPath, 'utf8'), 'ultra');
+    assert.ok(out1 && typeof out1.append === 'string', 'expected reinforcement append');
+    assert.match(out1.append, /CAVEMAN MODE ACTIVE \(ultra\)/);
+
+    // Natural-language deactivation removes flag
+    const out2 = await handlers['tui.prompt.append']({ prompt: 'stop caveman please' });
+    assert.equal(fs.existsSync(flagPath), false, 'flag should be deleted after deactivation');
+    assert.equal(out2, undefined, 'no reinforcement when flag absent');
+
+    // session.created writes default mode
+    await handlers['session.created']();
+    assert.equal(fs.readFileSync(flagPath, 'utf8'), 'full');
+  } finally {
+    fs.rmSync(xdg, { recursive: true, force: true });
+    fs.rmSync(shimDir, { recursive: true, force: true });
+  }
+});