fix(pi): fail closed on unrepresentable compat spread

scripts/pi-transport-capture.mjs

@@ -91,6 +91,7 @@ writeFileSync(
     `export { streamSimple as openaiResponses } from "${PI}/packages/ai/src/api/openai-responses.ts";`,
     `export { streamSimple as openaiCompletions } from "${PI}/packages/ai/src/api/openai-completions.ts";`,
     `export { composeModelProvider } from "${PI}/packages/coding-agent/src/core/provider-composer.ts";`,
+    `export { resolveConfigValueOrThrow } from "${PI}/packages/coding-agent/src/core/resolve-config-value.ts";`,
   ].join("\n"),
 );
 const bundlePath = join(harnessDirectory, "bundle.mjs");
@@ -242,6 +243,115 @@ const compatProvider = adapters.composeModelProvider(
 );
 const compatSpread = compatProvider.getModels()[0].compat;
 
+function jsonSafeJavaScriptValue(value) {
+  if (typeof value === "string") {
+    const codeUnits = Array.from({ length: value.length }, (_, index) =>
+      value.charCodeAt(index),
+    );
+    const hasLoneSurrogate = codeUnits.some((unit, index) => {
+      if (unit >= 0xd800 && unit <= 0xdbff) {
+        return !(
+          index + 1 < codeUnits.length &&
+          codeUnits[index + 1] >= 0xdc00 &&
+          codeUnits[index + 1] <= 0xdfff
+        );
+      }
+      if (unit >= 0xdc00 && unit <= 0xdfff) {
+        return !(
+          index > 0 &&
+          codeUnits[index - 1] >= 0xd800 &&
+          codeUnits[index - 1] <= 0xdbff
+        );
+      }
+      return false;
+    });
+    return hasLoneSurrogate
+      ? {
+          $javascriptStringUtf16: codeUnits.map((unit) =>
+            unit.toString(16).padStart(4, "0"),
+          ),
+        }
+      : value;
+  }
+  if (Array.isArray(value)) {
+    return value.map(jsonSafeJavaScriptValue);
+  }
+  if (value && typeof value === "object") {
+    return Object.fromEntries(
+      Object.entries(value).map(([key, child]) => [
+        key,
+        jsonSafeJavaScriptValue(child),
+      ]),
+    );
+  }
+  return value;
+}
+
+function captureCompatSpread(label, baseValue, overlayValue) {
+  const providerId = `compat-${label}`;
+  const provider = adapters.composeModelProvider(
+    providerId,
+    undefined,
+    {
+      getProvider(candidate) {
+        if (candidate !== providerId) return undefined;
+        return {
+          api: "openai-responses",
+          baseUrl: "https://compat.example/v1",
+          apiKey: "literal",
+          compat: { chatTemplateKwargs: baseValue },
+          models: [{ id: "m" }],
+          modelOverrides: {
+            m: { compat: { chatTemplateKwargs: overlayValue } },
+          },
+        };
+      },
+    },
+    undefined,
+  );
+  return {
+    label,
+    baseValue,
+    overlayValue,
+    result: jsonSafeJavaScriptValue(
+      provider.getModels()[0].compat.chatTemplateKwargs,
+    ),
+  };
+}
+
+const compatEdgeCases = [
+  captureCompatSpread("ascii-string-to-string", "ab", "cd"),
+  captureCompatSpread("astral-string-to-object", "😀", { named: true }),
+  captureCompatSpread("string-to-array", "ab", ["first", "second"]),
+];
+
+const resolverInputs = [
+  "literal-secret",
+  "cash$money",
+  "café$literal",
+  "$$literal-$!bang",
+  "prefix-${PI_CAPTURE_MISSING}-suffix",
+];
+const resolverCases = resolverInputs.map((input) => {
+  try {
+    return {
+      input,
+      status: "success",
+      result: adapters.resolveConfigValueOrThrow(
+        input,
+        "transport capture",
+        {},
+      ),
+    };
+  } catch (error) {
+    return {
+      input,
+      status: "error",
+      error: error instanceof Error ? error.message : String(error),
+    };
+  }
+});
+
 server.close();
 console.log(
   JSON.stringify(
@@ -252,6 +362,8 @@ console.log(
       baseUrl,
       results,
       compatSpread,
+      compatEdgeCases,
+      resolverCases,
     },
     null,
     2,