diff --git a/packages/opencode/src/mcp/catalog.ts b/packages/opencode/src/mcp/catalog.ts
index 0cd2238edffe..1a5ef4e2b3a9 100644
--- a/packages/opencode/src/mcp/catalog.ts
+++ b/packages/opencode/src/mcp/catalog.ts
@@ -54,7 +54,7 @@ export function convertTool(mcpTool: MCPToolDef, client: Client, timeout?: numbe
       const result = await client.callTool(
         {
           name: mcpTool.name,
-          arguments: (args || {}) as Record<string, unknown>,
+          arguments: cleanToolArguments(args, inputSchema),
         },
         CallToolResultSchema,
         {
@@ -81,6 +81,26 @@ export function convertTool(mcpTool: MCPToolDef, client: Client, timeout?: numbe
   })
 }
 
+// Some providers (notably OpenAI/GPT) fill omitted optional string parameters
+// with empty strings. MCP servers treat "" as an explicit value rather than
+// "unset", which breaks tools whose optionals carry distinct semantics
+// (e.g. Slack search cursor/after/before returning no results). Drop the
+// empty-string values for optional declared properties so the field is omitted
+// from the tools/call request, matching the Anthropic path. See #33341.
+export function cleanToolArguments(args: unknown, schema: JSONSchema7): Record<string, unknown> {
+  if (!args || typeof args !== "object" || Array.isArray(args)) return {}
+  const required = new Set(
+    Array.isArray(schema.required) ? schema.required.filter((item): item is string => typeof item === "string") : [],
+  )
+  const properties = schema.properties ?? {}
+  const cleaned: Record<string, unknown> = {}
+  for (const [key, value] of Object.entries(args as Record<string, unknown>)) {
+    if (value === "" && !required.has(key) && key in properties) continue
+    cleaned[key] = value
+  }
+  return cleaned
+}
+
 export function fetch<T extends { name: string }>(
   clientName: string,
   client: Client,
diff --git a/packages/opencode/test/mcp/catalog.test.ts b/packages/opencode/test/mcp/catalog.test.ts
new file mode 100644
index 000000000000..b74caf253806
--- /dev/null
+++ b/packages/opencode/test/mcp/catalog.test.ts
@@ -0,0 +1,68 @@
+import { expect, test } from "bun:test"
+import type { JSONSchema7 } from "ai"
+import { McpCatalog } from "../../src/mcp/catalog"
+
+// Mirrors Slack's official MCP search tools (see #33341): `query` is required,
+// the rest are optional string fields. The OpenAI/GPT path fills the omitted
+// optionals with "", which Slack treats as an explicit value.
+const schema: JSONSchema7 = {
+  type: "object",
+  properties: {
+    query: { type: "string" },
+    cursor: { type: "string" },
+    after: { type: "string" },
+    before: { type: "string" },
+    context_channel_id: { type: "string" },
+  },
+  required: ["query"],
+}
+
+const clean = McpCatalog.cleanToolArguments
+
+test("drops empty-string optional string args, keeps required", () => {
+  expect(clean({ query: "match", cursor: "" }, schema)).toEqual({ query: "match" })
+})
+
+test("drops every omitted optional that the OpenAI path fills as empty string", () => {
+  expect(
+    clean(
+      { query: "match", cursor: "", after: "", before: "", context_channel_id: "" },
+      schema,
+    ),
+  ).toEqual({ query: "match" })
+})
+
+test("keeps non-empty optional string args", () => {
+  expect(clean({ query: "match", cursor: "page-2" }, schema)).toEqual({
+    query: "match",
+    cursor: "page-2",
+  })
+})
+
+test("keeps empty string for required fields", () => {
+  expect(clean({ query: "", cursor: "" }, schema)).toEqual({ query: "" })
+})
+
+test("does not drop falsy non-string values for optionals", () => {
+  expect(clean({ query: "match", cursor: 0 }, schema)).toEqual({ query: "match", cursor: 0 })
+})
+
+test("leaves unknown keys untouched", () => {
+  expect(clean({ query: "match", unknown: "" }, schema)).toEqual({ query: "match", unknown: "" })
+})
+
+test("normalizes non-object args to an empty object", () => {
+  expect(clean(null, schema)).toEqual({})
+  expect(clean(undefined, schema)).toEqual({})
+  expect(clean("query", schema)).toEqual({})
+  expect(clean([], schema)).toEqual({})
+})
+
+test("treats args with no required list as all-optional", () => {
+  const allOptional: JSONSchema7 = {
+    type: "object",
+    properties: { cursor: { type: "string" } },
+  }
+  expect(clean({ cursor: "" }, allOptional)).toEqual({})
+  expect(clean({ cursor: "x" }, allOptional)).toEqual({ cursor: "x" })
+})