@@ -1613,9 +1613,16 @@ export namespace ProviderTransform {
 
         const result: any = {}
         for (const [key, value] of Object.entries(obj)) {
+          // altimate_change start — Gemini rejects empty-string enum/const values ("cannot be empty");
+          // drop them and keep the branch's type. Tool arguments are still validated by Zod at runtime.
+          if (key === "const" && value === "") continue
+          if (key === "enum" && Array.isArray(value) && value.every((v) => String(v) === "")) continue
+          // altimate_change end
           if (key === "enum" && Array.isArray(value)) {
             // Convert all enum values to strings
-            result[key] = value.map((v) => String(v))
+            // altimate_change start — skip empty-string values Gemini rejects
+            result[key] = value.map((v) => String(v)).filter((v) => v !== "")
+            // altimate_change end
             // If we have integer type with enum, change type to string
             if (result.type === "integer" || result.type === "number") {
               result.type = "string"
 
@@ -0,0 +1,107 @@
+import { afterEach, describe, expect, test } from "bun:test"
+import { Effect } from "effect"
+import { LayerNode } from "@opencode-ai/core/effect/layer-node"
+import { Agent } from "@/agent/agent"
+import { Config } from "@/config/config"
+import { RuntimeFlags } from "@/effect/runtime-flags"
+import type { Provider } from "@/provider/provider"
+import { ModelID, ProviderID } from "@/provider/schema"
+import { ProviderTransform } from "@/provider/transform"
+import { ToolJsonSchema } from "@/tool/json-schema"
+import { ToolRegistry } from "@/tool/registry"
+import { TestConfig } from "../../fixture/config"
+import { disposeAllInstances } from "../../fixture/fixture"
+import { testEffect } from "../../lib/effect"
+
+const it = testEffect(
+  LayerNode.buildLayer(LayerNode.group([ToolRegistry.node, Agent.node]), {
+    replacements: [
+      LayerNode.replace(Config.node, TestConfig.layer()),
+      LayerNode.replace(RuntimeFlags.node, RuntimeFlags.layer({ enableParallel: false })),
+    ],
+  }),
+)
+
+afterEach(disposeAllInstances)
+
+function isObject(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value)
+}
+
+// Gemini rejects a function declaration whose enum (or const) contains an empty string:
+// "function_declarations[i].parameters...enum[0]: cannot be empty".
+function emptyEnumValues(schema: unknown, location = "#"): string[] {
+  if (Array.isArray(schema)) return schema.flatMap((child, index) => emptyEnumValues(child, `${location}/${index}`))
+  if (!isObject(schema)) return []
+  const found: string[] = []
+  if (Array.isArray(schema.enum) && schema.enum.some((value) => String(value) === "")) found.push(`${location}/enum`)
+  if (schema.const === "") found.push(`${location}/const`)
+  for (const [key, child] of Object.entries(schema)) {
+    if (key === "enum" || key === "const" || key === "default") continue
+    found.push(...emptyEnumValues(child, `${location}/${key}`))
+  }
+  return found
+}
+
+function model(providerID: ProviderID): Provider.Model {
+  const id = "gemini-4-argon"
+  return {
+    id: ModelID.make(id),
+    providerID,
+    api: { id, npm: providerID === ProviderID.google ? "@ai-sdk/google" : "@ai-sdk/google-vertex", url: "https://example.invalid" },
+    name: id,
+    capabilities: {
+      temperature: true,
+      reasoning: true,
+      attachment: false,
+      toolcall: true,
+      input: { text: true, audio: false, image: false, video: false, pdf: false },
+      output: { text: true, audio: false, image: false, video: false, pdf: false },
+      interleaved: false,
+    },
+    cost: { input: 0, output: 0, cache: { read: 0, write: 0 } },
+    limit: { context: 1_000_000, output: 64_000 },
+    status: "active",
+    release_date: "2026-01-01",
+    options: {},
+    headers: {},
+  }
+}
+
+describe("Gemini tool JSON schemas", () => {
+  for (const providerID of [ProviderID.google, ProviderID.googleVertex]) {
+    for (const name of ["analyst", "builder"]) {
+      it.instance(`${providerID}: no registered ${name} tool declares an empty-string enum or const`, () =>
+        Effect.gen(function* () {
+          const registry = yield* ToolRegistry.Service
+          const agents = yield* Agent.Service
+          const agent = yield* agents.get(name)
+          if (!agent) throw new Error(`${name} agent not found`)
+          const target = model(providerID)
+          const tools = yield* registry.tools({ providerID, modelID: target.id, agent })
+          expect(tools.some((tool) => tool.id === "answer_commit")).toBe(name === "analyst")
+          const errors = tools.flatMap((tool) =>
+            emptyEnumValues(ProviderTransform.schema(target, ToolJsonSchema.fromTool(tool))).map(
+              (location) => `${tool.id}: ${location}`,
+            ),
+          )
+          expect(errors).toEqual([])
+        }),
+      )
+    }
+  }
+
+  test("drops empty-string enum and const values but keeps the remaining enum", () => {
+    const result = ProviderTransform.schema(model(ProviderID.googleVertex), {
+      type: "object",
+      properties: {
+        unresolved: { anyOf: [{ type: "object", properties: {} }, { type: "string", enum: [""] }] },
+        blank: { type: "string", const: "" },
+        mode: { type: "string", enum: ["", "fast", "slow"] },
+      },
+    } as any) as any
+    expect(result.properties.unresolved.anyOf[1]).toEqual({ type: "string" })
+    expect(result.properties.blank).toEqual({ type: "string" })
+    expect(result.properties.mode.enum).toEqual(["fast", "slow"])
+  })
+})