fix(opencode): actionable error when a session outgrows the model window#23908

@@ -0,0 +1,33 @@
+/**
+ * Why an OpenCode prompt was refused as oversized.
+ *
+ * OpenCode reports a session that outgrew the model's context window as a
+ * bare InvalidRequest ("The request contains invalid parameters"), which
+ * reads as a bug in the request. Retrying the same run then fails
+ * identically, because the history that broke the request is still there.
+ * This maps that case to an actionable message: fork from an earlier
+ * completed run or start a fresh thread.
+ *
+ * Only fires with positive size evidence — a known window and a turn whose
+ * observed input already meets it — so genuine malformed requests keep
+ * their original error. A refused first prompt (no steps yet) carries no
+ * size signal and is left alone for the same reason.
+ */
+export const oversizedSessionMessage = (input: {
+  readonly causeTag: string | undefined;
+  readonly steps: number;
+  readonly inputTokens: number;
+  readonly windowTokens: number | undefined;
+}): string | null => {
+  if (input.causeTag !== "InvalidRequestError") return null;
+  if (input.windowTokens === undefined || input.steps === 0) return null;
+  if (input.inputTokens < input.windowTokens) return null;

@@ -3708,9 +3709,29 @@ export const make = Effect.fn("OpenCode2Adapter.make")(function* (instanceId: Pr
               state.active === turn
                 ? Effect.gen(function* () {
                     if (!CLEAR_PROMPT_REJECTIONS.has(cause._tag)) markUnsettled(state);
+                    // A prompt the session outgrew reads as a bare
+                    // InvalidRequest; say what actually happened instead of
+                    // the provider's generic text. Anything without size
+                    // evidence keeps the original error.
+                    const oversized = oversizedSessionMessage({
+                      causeTag: cause._tag,
+                      steps: turn.steps,
+                      inputTokens: turn.usage.input,