diff --git a/packages/web/server/lib/opencode/DOCUMENTATION.md b/packages/web/server/lib/opencode/DOCUMENTATION.md
index 8576cde3..937113d0 100644
--- a/packages/web/server/lib/opencode/DOCUMENTATION.md
+++ b/packages/web/server/lib/opencode/DOCUMENTATION.md
@@ -68,7 +68,7 @@ This module provides OpenCode server integration utilities for the web server ru
 - `removeProviderConfig(providerId, workingDirectory, scope?)`: Removes a provider block from the selected config layer.
 
 ## Public exports (shared.js)
-- `OPENCODE_CONFIG_DIR`, `AGENT_DIR`, `COMMAND_DIR`, `SKILL_DIR`, `CONFIG_FILE`: Path constants. `OPENCODE_CONFIG` is resolved at call time for the custom config layer path.
+- `OPENCODE_CONFIG_DIR`, `AGENT_DIR`, `COMMAND_DIR`, `SKILL_DIR`, `CONFIG_FILE`: Path constants rooted at `$XDG_CONFIG_HOME/opencode` when `XDG_CONFIG_HOME` is non-empty, otherwise `~/.config/opencode`. These constants are evaluated when the module loads; no files are migrated. `OPENCODE_CONFIG` remains a separate explicit config-file path and is resolved at call time for the custom config layer; it does not replace the global config directory.
 - `AGENT_SCOPE`, `COMMAND_SCOPE`, `SKILL_SCOPE`: Scope constants with USER and PROJECT values.
 - `ensureDirs()`: Creates required OpenCode directories.
 - `parseMdFile(filePath)`, `writeMdFile(filePath, frontmatter, body)`: Markdown file operations with YAML frontmatter.
diff --git a/packages/web/server/lib/opencode/config-paths.test.js b/packages/web/server/lib/opencode/config-paths.test.js
new file mode 100644
index 00000000..93957d90
--- /dev/null
+++ b/packages/web/server/lib/opencode/config-paths.test.js
@@ -0,0 +1,108 @@
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+const originalXdgConfigHome = process.env.XDG_CONFIG_HOME;
+const originalOpenCodeConfig = process.env.OPENCODE_CONFIG;
+
+afterEach(() => {
+  if (originalXdgConfigHome === undefined) delete process.env.XDG_CONFIG_HOME;
+  else process.env.XDG_CONFIG_HOME = originalXdgConfigHome;
+  if (originalOpenCodeConfig === undefined) delete process.env.OPENCODE_CONFIG;
+  else process.env.OPENCODE_CONFIG = originalOpenCodeConfig;
+  vi.resetModules();
+});
+
+async function loadOpenCodeModules() {
+  vi.resetModules();
+  return Promise.all([
+    import('./shared.js'),
+    import('./agents.js'),
+    import('./commands.js'),
+    import('./skills.js'),
+    import('./snippets.js'),
+    import('./plugins.js'),
+    import('./routes.js'),
+  ]);
+}
+
+describe('OpenCode global config paths', () => {
+  it('derives all shared constants from a non-empty XDG_CONFIG_HOME', async () => {
+    const xdgConfigHome = fs.mkdtempSync(path.join(os.tmpdir(), 'openchamber-xdg-'));
+    process.env.XDG_CONFIG_HOME = xdgConfigHome;
+
+    const [{ OPENCODE_CONFIG_DIR, AGENT_DIR, COMMAND_DIR, SKILL_DIR, CONFIG_FILE }] = await loadOpenCodeModules();
+    const configDir = path.join(xdgConfigHome, 'opencode');
+    expect(OPENCODE_CONFIG_DIR).toBe(configDir);
+    expect(AGENT_DIR).toBe(path.join(configDir, 'agents'));
+    expect(COMMAND_DIR).toBe(path.join(configDir, 'commands'));
+    expect(SKILL_DIR).toBe(path.join(configDir, 'skills'));
+    expect(CONFIG_FILE).toBe(path.join(configDir, 'config.json'));
+
+    fs.rmSync(xdgConfigHome, { recursive: true, force: true });
+  });
+
+  it.each([undefined, '  '])('falls back to ~/.config/opencode when XDG_CONFIG_HOME is %s', async (xdgConfigHome) => {
+    if (xdgConfigHome === undefined) delete process.env.XDG_CONFIG_HOME;
+    else process.env.XDG_CONFIG_HOME = xdgConfigHome;
+
+    const [{ OPENCODE_CONFIG_DIR }] = await loadOpenCodeModules();
+    expect(OPENCODE_CONFIG_DIR).toBe(path.join(os.homedir(), '.config', 'opencode'));
+  });
+
+  it('keeps global CRUD below XDG_CONFIG_HOME while project files stay in the project', async () => {
+    const root = fs.mkdtempSync(path.join(os.tmpdir(), 'openchamber-config-paths-'));
+    const xdgConfigHome = path.join(root, 'xdg');
+    const projectDir = path.join(root, 'project');
+    process.env.XDG_CONFIG_HOME = xdgConfigHome;
+    delete process.env.OPENCODE_CONFIG;
+
+    const [, agents, commands, skills, snippets, plugins] = await loadOpenCodeModules();
+    agents.createAgent('global-agent', { description: 'Global', prompt: 'Global prompt' }, projectDir, 'user');
+    commands.createCommand('global-command', { description: 'Global', template: 'Global template' }, projectDir, 'user');
+    skills.createSkill('global-skill', { description: 'Global', instructions: 'Global instructions' }, projectDir, 'user');
+    snippets.createSnippet('global-snippet', { content: 'Global snippet' }, projectDir, 'global');
+    plugins.createPluginEntry({ spec: 'global-plugin', scope: 'user' }, projectDir);
+
+    const configDir = path.join(xdgConfigHome, 'opencode');
+    expect(fs.existsSync(path.join(configDir, 'agents', 'global-agent.md'))).toBe(true);
+    expect(fs.existsSync(path.join(configDir, 'commands', 'global-command.md'))).toBe(true);
+    expect(fs.existsSync(path.join(configDir, 'skills', 'global-skill', 'SKILL.md'))).toBe(true);
+    expect(fs.existsSync(path.join(configDir, 'snippet', 'global-snippet.md'))).toBe(true);
+    expect(JSON.parse(fs.readFileSync(path.join(configDir, 'config.json'), 'utf8')).plugin).toEqual(['global-plugin']);
+
+    agents.createAgent('project-agent', { description: 'Project', prompt: 'Project prompt' }, projectDir, 'project');
+    commands.createCommand('project-command', { description: 'Project', template: 'Project template' }, projectDir, 'project');
+    skills.createSkill('project-skill', { description: 'Project', instructions: 'Project instructions' }, projectDir, 'project');
+
+    expect(fs.existsSync(path.join(projectDir, '.opencode', 'agents', 'project-agent.md'))).toBe(true);
+    expect(fs.existsSync(path.join(projectDir, '.opencode', 'commands', 'project-command.md'))).toBe(true);
+    expect(fs.existsSync(path.join(projectDir, '.opencode', 'skills', 'project-skill', 'SKILL.md'))).toBe(true);
+
+    fs.rmSync(root, { recursive: true, force: true });
+  });
+
+  it('writes global AGENTS.md below XDG_CONFIG_HOME', async () => {
+    const root = fs.mkdtempSync(path.join(os.tmpdir(), 'openchamber-agents-md-'));
+    process.env.XDG_CONFIG_HOME = path.join(root, 'xdg');
+    delete process.env.OPENCODE_CONFIG;
+
+    const [, , , , , , routes] = await loadOpenCodeModules();
+    const handlers = new Map();
+    const app = {
+      get(route, ...callbacks) { handlers.set(`GET ${route}`, callbacks.at(-1)); },
+      put(route, ...callbacks) { handlers.set(`PUT ${route}`, callbacks.at(-1)); },
+      post() {},
+      delete() {},
+    };
+    routes.registerOpenCodeRoutes(app, {});
+
+    const response = { json: vi.fn(), status: vi.fn(() => response) };
+    await handlers.get('PUT /api/behavior/agents-md')({ body: { content: 'Global behavior' } }, response);
+
+    expect(fs.readFileSync(path.join(process.env.XDG_CONFIG_HOME, 'opencode', 'AGENTS.md'), 'utf8')).toBe('Global behavior');
+    expect(response.json).toHaveBeenCalled();
+    fs.rmSync(root, { recursive: true, force: true });
+  });
+});
diff --git a/packages/web/server/lib/opencode/plugins.js b/packages/web/server/lib/opencode/plugins.js
index 143298fd..b429458b 100644
--- a/packages/web/server/lib/opencode/plugins.js
+++ b/packages/web/server/lib/opencode/plugins.js
@@ -1,8 +1,8 @@
 import fs from 'fs';
-import os from 'os';
 import path from 'path';
 import {
   AGENT_SCOPE,
+  OPENCODE_CONFIG_DIR,
   readConfigFile,
   readConfigLayer,
   writeConfig,
@@ -71,7 +71,7 @@ function getActiveOpencodeConfigDir() {
   if (customConfigPath) {
     return path.dirname(path.resolve(customConfigPath));
   }
-  return path.join(os.homedir(), '.config', 'opencode');
+  return OPENCODE_CONFIG_DIR;
 }
 
 function getActiveUserConfigPaths() {
diff --git a/packages/web/server/lib/opencode/providers.test.js b/packages/web/server/lib/opencode/providers.test.js
index 35078f05..ea53f501 100644
--- a/packages/web/server/lib/opencode/providers.test.js
+++ b/packages/web/server/lib/opencode/providers.test.js
@@ -9,6 +9,7 @@ import {
   getProviderSources,
   removeProviderConfig,
 } from './providers.js';
+import { OPENCODE_CONFIG_DIR } from './shared.js';
 
 let projectDir;
 
@@ -334,8 +335,8 @@ describe('custom provider config persistence', () => {
     expect(sources.sources.custom.exists).toBe(false);
 
     for (const userPath of [
-      path.join(os.homedir(), '.config', 'opencode', 'opencode.json'),
-      path.join(os.homedir(), '.config', 'opencode', 'config.json'),
+      path.join(OPENCODE_CONFIG_DIR, 'opencode.json'),
+      path.join(OPENCODE_CONFIG_DIR, 'config.json'),
     ]) {
       if (!fs.existsSync(userPath)) continue;
       const userConfig = readJson(userPath);
@@ -373,8 +374,8 @@ describe('custom provider config persistence', () => {
       expect(sources.sources.project.exists).toBe(false);
 
       for (const userPath of [
-        path.join(os.homedir(), '.config', 'opencode', 'opencode.json'),
-        path.join(os.homedir(), '.config', 'opencode', 'config.json'),
+        path.join(OPENCODE_CONFIG_DIR, 'opencode.json'),
+        path.join(OPENCODE_CONFIG_DIR, 'config.json'),
       ]) {
         if (!fs.existsSync(userPath)) continue;
         const userConfig = readJson(userPath);
diff --git a/packages/web/server/lib/opencode/routes.js b/packages/web/server/lib/opencode/routes.js
index fdb7d95c..5c5f21ca 100644
--- a/packages/web/server/lib/opencode/routes.js
+++ b/packages/web/server/lib/opencode/routes.js
@@ -1,12 +1,12 @@
 import express from 'express';
 import { createProjectIdFromPath } from '../projects/project-id.js';
 import fs from 'fs';
-import os from 'os';
 import path from 'path';
 import {
   buildDeferredRestartResponse,
 } from './config-mutation-response.js';
 import { getClaudeCliAuthStatus } from './claude-cli-auth.js';
+import { OPENCODE_CONFIG_DIR } from './shared.js';
 
 export const registerOpenCodeRoutes = (app, dependencies) => {
   const {
@@ -807,7 +807,7 @@ ${desktopReturn ? `<a class="return" href="openchamber://focus/mcp-auth">Return
   });
 
   // Behavior / Global AGENTS.md endpoints
-  const AGENTS_MD_PATH = path.join(os.homedir(), '.config', 'opencode', 'AGENTS.md');
+  const AGENTS_MD_PATH = path.join(OPENCODE_CONFIG_DIR, 'AGENTS.md');
   const MAX_BEHAVIOR_PROMPT_SIZE = 1024 * 1024; // 1 MB
 
   app.get('/api/behavior/agents-md', async (_req, res) => {
diff --git a/packages/web/server/lib/opencode/shared.js b/packages/web/server/lib/opencode/shared.js
index d01168f3..d154cc29 100644
--- a/packages/web/server/lib/opencode/shared.js
+++ b/packages/web/server/lib/opencode/shared.js
@@ -6,7 +6,10 @@ import { parse as parseJsonc, printParseErrorCode } from 'jsonc-parser';
 
 // ============== PATH CONSTANTS ==============
 
-const OPENCODE_CONFIG_DIR = path.join(os.homedir(), '.config', 'opencode');
+const XDG_CONFIG_HOME = typeof process.env.XDG_CONFIG_HOME === 'string' && process.env.XDG_CONFIG_HOME.trim()
+  ? process.env.XDG_CONFIG_HOME.trim()
+  : path.join(os.homedir(), '.config');
+const OPENCODE_CONFIG_DIR = path.join(XDG_CONFIG_HOME, 'opencode');
 const AGENT_DIR = path.join(OPENCODE_CONFIG_DIR, 'agents');
 const COMMAND_DIR = path.join(OPENCODE_CONFIG_DIR, 'commands');
 const SKILL_DIR = path.join(OPENCODE_CONFIG_DIR, 'skills');
diff --git a/packages/web/server/lib/opencode/skill-routes.js b/packages/web/server/lib/opencode/skill-routes.js
index 0abe7f37..703aa382 100644
--- a/packages/web/server/lib/opencode/skill-routes.js
+++ b/packages/web/server/lib/opencode/skill-routes.js
@@ -1,5 +1,6 @@
 import { createOpencodeClient } from '@opencode-ai/sdk/v2';
 import { buildDeferredRestartResponse } from './config-mutation-response.js';
+import { OPENCODE_CONFIG_DIR } from './shared.js';
 
 /**
  * Matches how OpenCode reads its own boolean env flags: any value other than
@@ -110,7 +111,7 @@ export const registerSkillRoutes = (app, dependencies) => {
     }
 
     const userRoots = [
-      path.join(home, '.config', 'opencode'),
+      OPENCODE_CONFIG_DIR,
       path.join(home, '.opencode'),
       path.join(home, '.claude', 'skills'),
       path.join(home, '.agents', 'skills'),
diff --git a/packages/web/server/lib/opencode/snippets.js b/packages/web/server/lib/opencode/snippets.js
index 08f1dcd2..46fff970 100644
--- a/packages/web/server/lib/opencode/snippets.js
+++ b/packages/web/server/lib/opencode/snippets.js
@@ -1,9 +1,8 @@
 import fs from 'fs';
 import path from 'path';
-import os from 'os';
 import yaml from 'yaml';
+import { OPENCODE_CONFIG_DIR } from './shared.js';
 
-const OPENCODE_CONFIG_DIR = path.join(os.homedir(), '.config', 'opencode');
 const GLOBAL_SNIPPET_DIR = path.join(OPENCODE_CONFIG_DIR, 'snippet');
 const GLOBAL_SNIPPET_DIR_ALT = path.join(OPENCODE_CONFIG_DIR, 'snippets');
 const SNIPPET_EXTENSION = '.md';
diff --git a/packages/web/server/lib/quota/utils/auth.js b/packages/web/server/lib/quota/utils/auth.js
index 5325dfdb..6d88b2b8 100644
--- a/packages/web/server/lib/quota/utils/auth.js
+++ b/packages/web/server/lib/quota/utils/auth.js
@@ -1,8 +1,8 @@
 import fs from 'fs';
 import path from 'path';
 import os from 'os';
+import { OPENCODE_CONFIG_DIR } from '../../opencode/shared.js';
 
-const OPENCODE_CONFIG_DIR = path.join(os.homedir(), '.config', 'opencode');
 const OPENCODE_DATA_DIR = path.join(os.homedir(), '.local', 'share', 'opencode');
 
 export const ANTIGRAVITY_ACCOUNTS_PATHS = [
diff --git a/packages/web/server/lib/skills-catalog/DOCUMENTATION.md b/packages/web/server/lib/skills-catalog/DOCUMENTATION.md
index 5c0b020e..4289a59b 100644
--- a/packages/web/server/lib/skills-catalog/DOCUMENTATION.md
+++ b/packages/web/server/lib/skills-catalog/DOCUMENTATION.md
@@ -58,7 +58,7 @@ The following functions are internal helpers used by exported functions:
 - `safeRm(dir)`: Safely remove directory recursively (ignores errors).
 - `ensureDir(dirPath)`: Ensure directory exists with recursive creation.
 - `copyDirectoryNoSymlinks(srcDir, dstDir)`: Copy directory contents without symlinks, with path traversal protection.
-- `normalizeUserSkillDir(userSkillDir)`: Normalize user skill directory path (handles legacy `~/.config/opencode/skill` → `~/.config/opencode/skills` migration).
+- `normalizeUserSkillDir(userSkillDir)`: Normalize the user skill directory path (handles the legacy `skill` directory in the XDG config location, or `~/.config/opencode/skill` when XDG is unset, by selecting the plural `skills` directory when appropriate).
 
 ### Git Clone Helpers (`install.js`, `scan.js`)
 - `cloneRepo({ cloneUrl, identity, tempDir })`: Clone git repository with preferred partial clone (`--filter=blob:none`) and fallback. Uses non-interactive mode.
diff --git a/packages/web/server/lib/skills-catalog/install.js b/packages/web/server/lib/skills-catalog/install.js
index d72be52f..ac556ce0 100644
--- a/packages/web/server/lib/skills-catalog/install.js
+++ b/packages/web/server/lib/skills-catalog/install.js
@@ -4,13 +4,14 @@ import path from 'path';
 
 import { assertGitAvailable, looksLikeAuthError, runGit } from './git.js';
 import { parseSkillRepoSource } from './source.js';
+import { OPENCODE_CONFIG_DIR } from '../opencode/shared.js';
 
 const SKILL_NAME_PATTERN = /^[a-z0-9][a-z0-9-]*[a-z0-9]$|^[a-z0-9]$/;
 
 function normalizeUserSkillDir(userSkillDir) {
   if (!userSkillDir) return null;
-  const legacySkillDir = path.join(os.homedir(), '.config', 'opencode', 'skill');
-  const pluralSkillDir = path.join(os.homedir(), '.config', 'opencode', 'skills');
+  const legacySkillDir = path.join(OPENCODE_CONFIG_DIR, 'skill');
+  const pluralSkillDir = path.join(OPENCODE_CONFIG_DIR, 'skills');
   if (userSkillDir === legacySkillDir) {
     if (fs.existsSync(legacySkillDir) && !fs.existsSync(pluralSkillDir)) return legacySkillDir;
     return pluralSkillDir;