fix(bridge): allow explicit 0 to disable the describe cap

updateSettingsSchema previously rejected modalityBridgeVisionMaxChars: 0
because the field's range was min(100).max(50000), so a dashboard PATCH
sending the explicit "unlimited" sentinel would 400. Widen the schema to
z.union([z.literal(0), z.number().int().min(100).max(50000)]) so 0
validates as its own valid value, not just an implicit default.

src/shared/validation/settingsSchemas.ts

@@ -349,7 +349,9 @@ export const updateSettingsSchema = z.object({
   modalityBridgeVisionPrompt: z.string().max(5000).optional(),
   modalityBridgeVisionTimeout: z.number().int().min(1000).max(300000).optional(),
   modalityBridgeVisionMaxImages: z.number().int().min(1).max(20).optional(),
-  modalityBridgeVisionMaxChars: z.number().int().min(100).max(50000).optional(),
+  modalityBridgeVisionMaxChars: z
+    .union([z.literal(0), z.number().int().min(100).max(50000)])
+    .optional(),
   modalityBridgeAudioEnabled: z.boolean().optional(),
   modalityBridgeAudioModel: z.string().max(200).optional(),
   modalityBridgeAudioTimeout: z.number().int().min(1000).max(300000).optional(),

tests/unit/vision-bridge-maxchars.test.ts

@@ -14,6 +14,7 @@ import assert from "node:assert/strict";
 const { VisionBridgeGuardrail } = await import("../../src/lib/guardrails/visionBridge.ts");
 const { resolveVisionBridgeRuntimeSettings } =
   await import("../../src/shared/constants/modalityBridgeDefaults.ts");
+const { updateSettingsSchema } = await import("../../src/shared/validation/settingsSchemas.ts");
 import type { GuardrailContext } from "../../src/lib/guardrails/base.ts";
 import type { VisionModelConfig } from "../../src/lib/guardrails/visionBridgeHelpers.ts";
 
@@ -108,3 +109,34 @@ test("resolveVisionBridgeRuntimeSettings({}) defaults maxChars to 0 (no cap)", (
   const runtime = resolveVisionBridgeRuntimeSettings({});
   assert.equal(runtime.maxChars, 0);
 });
+
+test("updateSettingsSchema accepts an explicit modalityBridgeVisionMaxChars: 0 to disable the cap", async () => {
+  const parsed = updateSettingsSchema.safeParse({ modalityBridgeVisionMaxChars: 0 });
+  assert.equal(parsed.success, true, "explicit 0 must validate (disables the cap)");
+
+  const guardrail = new VisionBridgeGuardrail({
+    deps: {
+      getSettings: async () => ({
+        visionBridgeEnabled: true,
+        modalityBridgeVisionMaxChars: 0,
+      }),
+      callVisionModel: async (_imageDataUri: string, _config: VisionModelConfig) =>
+        LONG_DESCRIPTION,
+    },
+  });
+
+  const body = imageBody("maxchars-explicit-zero-test");
+  const context: GuardrailContext = { model: TEXT_ONLY_MODEL, log: console };
+  const result = await guardrail.preCall(body, context);
+
+  const text = findImageDescriptionText(result.modifiedPayload);
+  assert.ok(text, "expected a [Image 1]: text part in the modified payload");
+  const description = text!.slice("[Image 1]: ".length);
+  assert.equal(description, LONG_DESCRIPTION, "explicit 0 must disable truncation entirely");
+  assert.ok(!description.endsWith("…"), "explicit 0 must never add an ellipsis");
+});
+
+test("updateSettingsSchema rejects a value below the 100 floor that is not the explicit-0 sentinel", () => {
+  const parsed = updateSettingsSchema.safeParse({ modalityBridgeVisionMaxChars: 50 });
+  assert.equal(parsed.success, false, "50 is neither the 0 sentinel nor >= 100");
+});