diff --git a/src/agent-cli-provider/adapters/opencode.ts b/src/agent-cli-provider/adapters/opencode.ts
index 804bafd0..ffc5e35f 100644
--- a/src/agent-cli-provider/adapters/opencode.ts
+++ b/src/agent-cli-provider/adapters/opencode.ts
@@ -85,6 +85,7 @@ function detectCliFeatures(helpText?: string | null): OpencodeCliFeatures {
     supportsModel: unknown ? true : /--model\b/.test(help),
     supportsVariant: unknown ? true : /--variant\b/.test(help),
     supportsCwd: unknown ? false : /--cwd\b/.test(help),
+    supportsDir: unknown ? false : /--dir\b/.test(help),
     supportsAutoApprove: false,
     unknown,
   };
@@ -109,6 +110,8 @@ function addOpencodeOptionalArgs(args: string[], options: BuildProviderCommandOp
 
   if (options.cwd && features.supportsCwd) {
     args.push('--cwd', options.cwd);
+  } else if (options.cwd && features.supportsDir) {
+    args.push('--dir', options.cwd);
   }
 }
 
diff --git a/src/agent-cli-provider/types.ts b/src/agent-cli-provider/types.ts
index ef2ca433..2f08eaf2 100644
--- a/src/agent-cli-provider/types.ts
+++ b/src/agent-cli-provider/types.ts
@@ -77,6 +77,7 @@ export interface OpencodeCliFeatures extends BaseCliFeatures {
   readonly supportsModel: boolean;
   readonly supportsVariant: boolean;
   readonly supportsCwd: boolean;
+  readonly supportsDir: boolean;
   readonly supportsAutoApprove: false;
 }
 
@@ -97,6 +98,7 @@ export interface CliFeatureOverrides {
   readonly supportsJson?: boolean;
   readonly supportsOutputSchema?: boolean;
   readonly supportsCwd?: boolean;
+  readonly supportsDir?: boolean;
   readonly supportsConfigOverride?: boolean;
   readonly supportsSkipGitRepoCheck?: boolean;
   readonly supportsVariant?: boolean;
diff --git a/tests/provider-cli-builder.test.js b/tests/provider-cli-builder.test.js
index b01ed216..4c02c174 100644
--- a/tests/provider-cli-builder.test.js
+++ b/tests/provider-cli-builder.test.js
@@ -226,6 +226,83 @@ describe('Opencode provider helper builder', function () {
     assert.ok(result.args.includes('--variant'));
     assert.ok(result.args.includes('high'));
   });
+
+  describe('worktree isolation via --dir flag', function () {
+    it('passes --dir <cwd> when supportsDir is true', function () {
+      const result = buildCommand('opencode', 'test prompt', {
+        cwd: '/worktrees/spinning-cosmos-71',
+        cliFeatures: { supportsDir: true, supportsCwd: false },
+      });
+
+      assert.ok(result.args.includes('--dir'), 'expected --dir flag in args');
+      assert.ok(
+        result.args.includes('/worktrees/spinning-cosmos-71'),
+        'expected worktree path in args'
+      );
+      assert.ok(!result.args.includes('--cwd'), '--cwd must not be used when supportsDir is true');
+    });
+
+    it('falls back to --cwd when supportsCwd is true and supportsDir is false', function () {
+      const result = buildCommand('opencode', 'test prompt', {
+        cwd: '/worktrees/spinning-cosmos-71',
+        cliFeatures: { supportsDir: false, supportsCwd: true },
+      });
+
+      assert.ok(result.args.includes('--cwd'), 'expected --cwd flag in args');
+      assert.ok(!result.args.includes('--dir'), '--dir must not be used when supportsDir is false');
+    });
+
+    it('sets commandSpec.cwd even when neither --dir nor --cwd flag is supported', function () {
+      const result = buildCommand('opencode', 'test prompt', {
+        cwd: '/worktrees/spinning-cosmos-71',
+        cliFeatures: { supportsDir: false, supportsCwd: false },
+      });
+
+      assert.strictEqual(
+        result.cwd,
+        '/worktrees/spinning-cosmos-71',
+        'commandSpec.cwd must be set so the process spawns in the worktree'
+      );
+      assert.ok(!result.args.includes('--dir'));
+      assert.ok(!result.args.includes('--cwd'));
+    });
+
+    it('detects supportsDir from help text containing --dir', function () {
+      const { opencodeAdapter } = require('../lib/agent-cli-provider/adapters/opencode');
+      const features = opencodeAdapter.detectCliFeatures(
+        'Usage: opencode run [options]\n  --dir  Working directory\n  --model  Model to use\n'
+      );
+
+      assert.strictEqual(
+        features.supportsDir,
+        true,
+        'supportsDir must be true when --dir appears in help text'
+      );
+      assert.strictEqual(
+        features.supportsCwd,
+        false,
+        'supportsCwd must be false when --cwd is absent from help text'
+      );
+    });
+
+    it('detects supportsDir as false from help text without --dir', function () {
+      const { opencodeAdapter } = require('../lib/agent-cli-provider/adapters/opencode');
+      const features = opencodeAdapter.detectCliFeatures(
+        'Usage: opencode run [options]\n  --cwd  Working directory\n'
+      );
+
+      assert.strictEqual(
+        features.supportsDir,
+        false,
+        'supportsDir must be false when --dir is absent from help text'
+      );
+      assert.strictEqual(
+        features.supportsCwd,
+        true,
+        'supportsCwd must be true when --cwd appears in help text'
+      );
+    });
+  });
 });
 
 describe('Claude provider helper builder', function () {