diff --git a/apps/server/src/provider/agentContextBudget.json b/apps/server/src/provider/agentContextBudget.json
index 43b48e60b6314..d714a45324d72 100644
--- a/apps/server/src/provider/agentContextBudget.json
+++ b/apps/server/src/provider/agentContextBudget.json
@@ -1,5 +1,5 @@
 {
   "claudeSystemAppendBytes": 5930,
   "codexAdditionalContextBytes": 7324,
-  "mcpToolListJsonBytes": 49792
+  "mcpToolListJsonBytes": 50736
 }
diff --git a/packages/contracts/src/modelSelection.test.ts b/packages/contracts/src/modelSelection.test.ts
index f719d82c511f3..85c5f59cf656d 100644
--- a/packages/contracts/src/modelSelection.test.ts
+++ b/packages/contracts/src/modelSelection.test.ts
@@ -70,3 +70,13 @@ it.effect(
       assert.deepStrictEqual(encoded.options, decoded.options);
     }),
 );
+
+it("ModelSelection gives every field a JSON Schema type, which Vertex AI requires of tool parameters", () => {
+  const { properties } = Schema.toJsonSchemaDocument(ModelSelection).schema as {
+    readonly properties: Record<string, { type?: string; anyOf?: Array<{ type?: string }> }>;
+  };
+  for (const [field, schema] of Object.entries(properties)) {
+    const types = schema.anyOf?.map((branch) => branch.type) ?? [schema.type];
+    assert.notInclude(types, undefined, field);
+  }
+});
diff --git a/packages/contracts/src/modelSelection.ts b/packages/contracts/src/modelSelection.ts
index e1ee73cfcbafa..ef10e20a56692 100644
--- a/packages/contracts/src/modelSelection.ts
+++ b/packages/contracts/src/modelSelection.ts
@@ -13,10 +13,10 @@ const ModelSelectionWire = Schema.Struct({
 });
 
 const ModelSelectionSource = Schema.Struct({
-  provider: Schema.optional(Schema.Unknown),
-  instanceId: Schema.optional(Schema.Unknown),
-  model: Schema.Unknown,
-  options: Schema.optional(Schema.Unknown),
+  provider: Schema.optional(Schema.NullOr(Schema.String)),
+  instanceId: Schema.optional(Schema.String),
+  model: Schema.String,
+  options: Schema.optional(Schema.toEncoded(ProviderOptionSelections)),
 });
 
 export const ModelSelection = ModelSelectionSource.pipe(