diff --git a/src/adapters/codex.ts b/src/adapters/codex.ts
index 27e7ba8..2623ef0 100644
--- a/src/adapters/codex.ts
+++ b/src/adapters/codex.ts
@@ -269,7 +269,9 @@ export class CodexAdapter implements AgentAdapter {
       promptFilePath = await writePromptFile(opts.prompt);
       promptFd = await openPromptFd(promptFilePath);
     } else {
-      args.push(opts.prompt);
+      // Use -- separator to prevent prompts starting with dashes (e.g. YAML
+      // frontmatter "---") from being interpreted as CLI options by codex.
+      args.push("--", opts.prompt);
     }
 
     const env = buildSpawnEnv(undefined, opts.env);
diff --git a/src/adapters/opencode-launch.test.ts b/src/adapters/opencode-launch.test.ts
index d917eaa..af44da2 100644
--- a/src/adapters/opencode-launch.test.ts
+++ b/src/adapters/opencode-launch.test.ts
@@ -74,7 +74,13 @@ describe("OpenCodeAdapter launch", () => {
     const modelIdx = args.indexOf("--model");
     const promptIdx = args.indexOf("fix the bug");
     expect(modelIdx).toBeLessThan(promptIdx);
-    expect(args).toEqual(["run", "--model", "deepseek-r1", "fix the bug"]);
+    expect(args).toEqual([
+      "run",
+      "--model",
+      "deepseek-r1",
+      "--",
+      "fix the bug",
+    ]);
   });
 
   it("omits --model flag when opts.model is not set", async () => {
@@ -87,6 +93,22 @@ describe("OpenCodeAdapter launch", () => {
     expect(spawnCalls).toHaveLength(1);
     const args = spawnCalls[0].args;
     expect(args).not.toContain("--model");
-    expect(args).toEqual(["run", "fix the bug"]);
+    expect(args).toEqual(["run", "--", "fix the bug"]);
+  });
+
+  it("inserts -- before prompts that start with dashes (e.g. YAML frontmatter)", async () => {
+    const dashPrompt = "---\ntitle: My Spec\n---\nBuild this.";
+    await adapter.launch({
+      adapter: "opencode",
+      prompt: dashPrompt,
+      cwd: tmpDir,
+    });
+
+    expect(spawnCalls).toHaveLength(1);
+    const args = spawnCalls[0].args;
+    // -- must appear immediately before the prompt positional
+    const separatorIdx = args.indexOf("--");
+    expect(separatorIdx).toBeGreaterThan(-1);
+    expect(args[separatorIdx + 1]).toBe(dashPrompt);
   });
 });
diff --git a/src/adapters/opencode.ts b/src/adapters/opencode.ts
index 3fca28f..9f2e6d7 100644
--- a/src/adapters/opencode.ts
+++ b/src/adapters/opencode.ts
@@ -357,7 +357,9 @@ export class OpenCodeAdapter implements AgentAdapter {
       promptFilePath = await writePromptFile(opts.prompt);
       promptFd = await openPromptFd(promptFilePath);
     } else {
-      args.push(opts.prompt);
+      // Use -- separator to prevent prompts starting with dashes (e.g. YAML
+      // frontmatter "---") from being interpreted as CLI options by opencode.
+      args.push("--", opts.prompt);
     }
 
     const env = buildSpawnEnv(undefined, opts.env);
diff --git a/src/adapters/pi-rust.ts b/src/adapters/pi-rust.ts
index 3e0877f..0178ff4 100644
--- a/src/adapters/pi-rust.ts
+++ b/src/adapters/pi-rust.ts
@@ -342,7 +342,9 @@ export class PiRustAdapter implements AgentAdapter {
 
     const args = useTempFile
       ? ["--print", "--mode", "json"]
-      : ["--print", "--mode", "json", opts.prompt];
+      : // Use -- separator to prevent prompts starting with dashes from being
+        // interpreted as CLI options by pi-rust.
+        ["--print", "--mode", "json", "--", opts.prompt];
 
     if (useTempFile) {
       promptFilePath = await writePromptFile(opts.prompt);