fix(opencode-local): pad prompts below the opencode 1.16.2 silent-exit floor (ORA-284)

opencode 1.16.2 has a non-deterministic silent-exit mode that fires on
short stdin prompts (observed 0-byte / <200-byte run logs, ~32% stall
rate on FoundingEngineer wake-payload-only runs). The root cause lives
in the opencode binary, not the adapter. As a server-side workaround,
ensure every prompt is at least 256 bytes by appending a deterministic
Paperclip Runtime Context block when the joined sections fall short.

- New exported helper padShortPrompt(input) in execute.ts
- New exported constant OPENCODE_PROMPT_MIN_LENGTH_CHARS = 256
- New promptMetrics.promptPadded (0/1) so the server can observe how
  often the workaround fires
- Four new unit tests cover the long-prompt, short-prompt, empty-cwd,
  and trailing-newline paths

Refs: ORA-284, ORA-275, ORA-276

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/adapters/opencode-local/src/server/execute.test.ts

@@ -1,6 +1,10 @@
 import { afterEach, describe, expect, it } from "vitest";
 
 import { ensureRemoteOpenCodeModelConfiguredAndAvailable } from "./execute.js";
+import {
+  OPENCODE_PROMPT_MIN_LENGTH_CHARS,
+  padShortPrompt,
+} from "./execute.js";
 
 describe("ensureRemoteOpenCodeModelConfiguredAndAvailable", () => {
   afterEach(() => {
@@ -60,3 +64,56 @@ describe("ensureRemoteOpenCodeModelConfiguredAndAvailable", () => {
     ).rejects.toThrow();
   });
 });
+
+describe("padShortPrompt (ORA-284)", () => {
+  // opencode 1.16.2 has a non-deterministic silent-exit mode that fires on
+  // short stdin prompts. The pad forces every prompt above the threshold so
+  // the binary boots into a normal session instead of exiting cleanly with
+  // 0 bytes of output.
+  it("returns the input untouched when it is already at or above the floor", () => {
+    const longPrompt = "x".repeat(OPENCODE_PROMPT_MIN_LENGTH_CHARS);
+    const out = padShortPrompt({
+      prompt: longPrompt,
+      agentId: "agent-1",
+      runId: "run-1",
+      executionCwd: "/tmp",
+    });
+    expect(out).toBe(longPrompt);
+  });
+
+  it("pads prompts that are below the floor and embeds run / agent context", () => {
+    const out = padShortPrompt({
+      prompt: "wake payload only",
+      agentId: "agent-1",
+      runId: "run-1",
+      executionCwd: "/tmp/oracle",
+    });
+    expect(out.length).toBeGreaterThanOrEqual(OPENCODE_PROMPT_MIN_LENGTH_CHARS);
+    expect(out).toContain("run-1");
+    expect(out).toContain("agent-1");
+    expect(out).toContain("/tmp/oracle");
+    expect(out).toContain("ORA-284");
+  });
+
+  it("uses a default cwd marker when executionCwd is empty", () => {
+    const out = padShortPrompt({
+      prompt: "tiny",
+      agentId: "agent-2",
+      runId: "run-2",
+      executionCwd: "",
+    });
+    expect(out).toContain("(default)");
+  });
+
+  it("does not insert a duplicate blank line when the prompt already ends with newline", () => {
+    const out = padShortPrompt({
+      prompt: "tiny\n",
+      agentId: "agent-3",
+      runId: "run-3",
+      executionCwd: "/tmp",
+    });
+    // Pad block already begins with a leading blank line, so we tolerate one
+    // extra newline rather than concatenating two of them.
+    expect(out.startsWith("tiny\n\n## Paperclip Runtime Context")).toBe(true);
+  });
+});

packages/adapters/opencode-local/src/server/execute.ts

@@ -66,6 +66,39 @@ function firstNonEmptyLine(text: string): string {
   );
 }
 
+// ORA-284: opencode 1.16.2 has a non-deterministic silent-exit mode that fires
+// on short stdin prompts (observed 0-byte / <200-byte run logs in production,
+// ~32% stall rate on FoundingEngineer wake-payload-only runs). The root cause
+// lives in the opencode binary, not in the adapter. As a server-side workaround
+// we pad any prompt shorter than this floor with a deterministic, content-
+// meaningful block drawn from the run context so the prompt is well above the
+// observed failure threshold (~50 bytes, with a 4x safety margin).
+export const OPENCODE_PROMPT_MIN_LENGTH_CHARS = 256;
+
+export function padShortPrompt(input: {
+  prompt: string;
+  agentId: string;
+  runId: string;
+  executionCwd: string;
+}): string {
+  const { prompt, agentId, runId, executionCwd } = input;
+  if (prompt.length >= OPENCODE_PROMPT_MIN_LENGTH_CHARS) return prompt;
+  const padBlock = [
+    "",
+    "## Paperclip Runtime Context",
+    "",
+    `Run: \`${runId}\``,
+    `Agent: \`${agentId}\``,
+    `Cwd: \`${executionCwd || "(default)"}\``,
+    "",
+    "You are running as a Paperclip-managed agent. The previous prompt was",
+    `shorter than the ${OPENCODE_PROMPT_MIN_LENGTH_CHARS}-byte opencode 1.16.2`,
+    "stability floor (ORA-284), so this metadata block was appended to avoid the",
+    "binary's silent-exit mode. Continue with the task described above.",
+  ].join("\n");
+  return prompt.endsWith("\n") ? prompt + padBlock : prompt + "\n" + padBlock;
+}
+
 function parseModelProvider(model: string | null): string | null {
   if (!model) return null;
   const trimmed = model.trim();
@@ -546,20 +579,29 @@ export async function execute(ctx: AdapterExecutionContext): Promise<AdapterExec
     const shouldUseResumeDeltaPrompt = Boolean(sessionId) && wakePrompt.length > 0;
     const renderedPrompt = shouldUseResumeDeltaPrompt ? "" : renderTemplate(promptTemplate, templateData);
     const sessionHandoffNote = asString(context.paperclipSessionHandoffMarkdown, "").trim();
-    const prompt = joinPromptSections([
+    const joinedSections = joinPromptSections([
       instructionsPrefix,
       renderedBootstrapPrompt,
       wakePrompt,
       sessionHandoffNote,
       renderedPrompt,
     ]);
-    const promptMetrics = {
+    const promptWasPadded = joinedSections.length < OPENCODE_PROMPT_MIN_LENGTH_CHARS;
+    const prompt = padShortPrompt({
+      prompt: joinedSections,
+      agentId: agent.id,
+      runId,
+      executionCwd: effectiveExecutionCwd,
+    });
+    const promptMetrics: Record<string, number> = {
       promptChars: prompt.length,
       instructionsChars: instructionsPrefix.length,
       bootstrapPromptChars: renderedBootstrapPrompt.length,
       wakePromptChars: wakePrompt.length,
       sessionHandoffChars: sessionHandoffNote.length,
       heartbeatPromptChars: renderedPrompt.length,
+      // ORA-284: 1 when the short-prompt pad block was appended, 0 otherwise.
+      promptPadded: promptWasPadded ? 1 : 0,
     };
 
     // Optional diagnostic: surface OpenCode's own logs on stderr (captured into the