diff --git a/main.ts b/main.ts
index fc5c77d..c6e3c64 100644
--- a/main.ts
+++ b/main.ts
@@ -343,6 +343,7 @@ export async function main(): Promise<MainResult> {
       jobId: runInfo.jobId,
       mcpServerUrl: "",
       tmpdir,
+      resolvedModel,
     };
     await using mcpHttpServer = await startMcpHttpServer(toolContext, { outputSchema });
     toolContext.mcpServerUrl = mcpHttpServer.url;
diff --git a/mcp/geminiSanitizer.ts b/mcp/geminiSanitizer.ts
new file mode 100644
index 0000000..3349c25
--- /dev/null
+++ b/mcp/geminiSanitizer.ts
@@ -0,0 +1,126 @@
+import type { StandardSchemaV1 } from "@standard-schema/spec";
+import type { Tool } from "fastmcp";
+import type { ToolContext } from "./server.ts";
+
+// ── gemini schema sanitizer ────────────────────────────────────────────────────
+//
+// gemini's generateContent API expects an OpenAPI 3.0 Schema subset, not full
+// JSON Schema. arktype emits constructs that gemini rejects with errors like:
+//   - "any_of[0].enum: only allowed for STRING type"
+//   - "parameters.type schema didn't specify the schema type field"
+//   - "anyOf must be the only field in a schema node"
+//
+// specific transforms applied here:
+//   1. collapse `{anyOf: [{enum:["a"]}, {enum:["b"]}]}` (arktype's string-enum
+//      encoding) into the direct form gemini wants: `{type:"string", enum:[...]}`.
+//      also handles `{const:"a"}` variants defensively (arktype 2.x may emit these).
+//   2. when `anyOf` / `oneOf` can't be collapsed but has sibling fields
+//      (e.g. `type`, `description`, `items`), strip those siblings — gemini
+//      rejects `anyOf` alongside any peer keywords. see opencode #14659.
+//   3. drop `$schema` metadata and rename `$defs` -> `definitions` (draft-07
+//      compatibility; gemini doesn't understand `$defs`).
+//
+// gated to gemini-routed traffic via `isGeminiRouted()` so other providers
+// continue to see the original (untransformed) schema.
+
+function parseStringEnumBranch(item: unknown): { values: string[] } | null {
+  if (!item || typeof item !== "object") return null;
+  const record = item as Record<string, unknown>;
+  if (Array.isArray(record.enum)) {
+    const strings = record.enum.filter((v): v is string => typeof v === "string");
+    return strings.length === record.enum.length && strings.length > 0 ? { values: strings } : null;
+  }
+  if (typeof record.const === "string") {
+    return { values: [record.const] };
+  }
+  return null;
+}
+
+function collapseStringUnion(branches: unknown[]): { type: "string"; enum: string[] } | null {
+  const values: string[] = [];
+  for (const item of branches) {
+    const parsed = parseStringEnumBranch(item);
+    if (!parsed) return null;
+    values.push(...parsed.values);
+  }
+  if (values.length === 0) return null;
+  return { type: "string", enum: [...new Set(values)] };
+}
+
+/**
+ * Recursively transform a JSON schema to gemini's stricter subset.
+ * See module header for the exact transforms applied.
+ */
+export function sanitizeForGemini(schema: unknown): unknown {
+  if (!schema || typeof schema !== "object") return schema;
+  if (Array.isArray(schema)) return schema.map(sanitizeForGemini);
+
+  const source = schema as Record<string, unknown>;
+
+  // case 1: collapsible string-enum union → `{type:"string", enum:[...]}`
+  for (const unionKey of ["anyOf", "oneOf"] as const) {
+    const branches = source[unionKey];
+    if (Array.isArray(branches) && branches.length > 0) {
+      const collapsed = collapseStringUnion(branches);
+      if (collapsed) {
+        const result: Record<string, unknown> = { ...collapsed };
+        if (typeof source.description === "string") result.description = source.description;
+        return result;
+      }
+    }
+  }
+
+  // case 2: non-collapsible anyOf/oneOf → strip sibling fields (gemini rule)
+  if (Array.isArray(source.anyOf) || Array.isArray(source.oneOf)) {
+    const result: Record<string, unknown> = {};
+    if (Array.isArray(source.anyOf)) result.anyOf = source.anyOf.map(sanitizeForGemini);
+    if (Array.isArray(source.oneOf)) result.oneOf = source.oneOf.map(sanitizeForGemini);
+    return result;
+  }
+
+  // case 3: generic pass — drop $schema, rename $defs, recurse
+  const sanitized: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(source)) {
+    if (key === "$schema") continue;
+    if (key === "$defs") {
+      sanitized.definitions = sanitizeForGemini(value);
+      continue;
+    }
+    sanitized[key] = sanitizeForGemini(value);
+  }
+  return sanitized;
+}
+
+/**
+ * Wraps a StandardSchemaV1 so its `toJsonSchema()` output is sanitized
+ * for gemini. other methods on the schema are passed through unchanged.
+ */
+export function wrapSchemaForGemini(schema: StandardSchemaV1<any>): StandardSchemaV1<any> {
+  const originalToJsonSchema = (schema as any).toJsonSchema?.bind(schema);
+  if (!originalToJsonSchema) return schema;
+  return new Proxy(schema, {
+    get(target, prop) {
+      if (prop === "toJsonSchema") {
+        return () => sanitizeForGemini(originalToJsonSchema());
+      }
+      return (target as any)[prop];
+    },
+  }) as StandardSchemaV1<any>;
+}
+
+export function sanitizeToolForGemini<T extends Tool<any, any>>(tool: T): T {
+  if (!tool.parameters) return tool;
+  return { ...tool, parameters: wrapSchemaForGemini(tool.parameters) } as T;
+}
+
+/**
+ * true when the effective upstream model is served by google's generative
+ * language API — directly (`google/*`), via opencode (`opencode/gemini-*`),
+ * or via openrouter (`openrouter/google/gemini-*`). slug-substring match
+ * works because every gemini route's model id contains "gemini".
+ */
+export function isGeminiRouted(ctx: ToolContext): boolean {
+  const effective = ctx.payload.proxyModel ?? ctx.resolvedModel ?? ctx.payload.model;
+  if (!effective) return false;
+  return effective.toLowerCase().includes("gemini");
+}
diff --git a/mcp/server.ts b/mcp/server.ts
index 5010ded..e8004ef 100644
--- a/mcp/server.ts
+++ b/mcp/server.ts
@@ -149,6 +149,10 @@ export interface ToolContext {
   jobId: string | undefined;
   mcpServerUrl: string;
   tmpdir: string;
+  // resolved upstream model specifier (e.g. "google/gemini-3.1-pro-preview").
+  // undefined when payload.proxyModel is set or when the alias is unresolvable.
+  // used by the schema sanitizer to detect Gemini-routed traffic.
+  resolvedModel: string | undefined;
 }
 
 const mcpPortStart = 3764;
diff --git a/mcp/shared.ts b/mcp/shared.ts
index cce2b14..01a02fd 100644
--- a/mcp/shared.ts
+++ b/mcp/shared.ts
@@ -2,6 +2,7 @@ import type { StandardSchemaV1 } from "@standard-schema/spec";
 import { encode as toonEncode } from "@toon-format/toon";
 import type { FastMCP, Tool } from "fastmcp";
 import { formatJsonValue, log } from "../utils/cli.ts";
+import { isGeminiRouted, sanitizeToolForGemini } from "./geminiSanitizer.ts";
 import type { ToolContext } from "./server.ts";
 
 export const tool = <const params>(
@@ -61,9 +62,10 @@ export const execute = <T, R extends Record<string, any> | string>(
   return _fn;
 };
 
-export const addTools = (_ctx: ToolContext, server: FastMCP<any>, tools: Tool<any, any>[]) => {
+export const addTools = (ctx: ToolContext, server: FastMCP<any>, tools: Tool<any, any>[]) => {
+  const shouldSanitize = isGeminiRouted(ctx);
   for (const tool of tools) {
-    server.addTool(tool);
+    server.addTool(shouldSanitize ? sanitizeToolForGemini(tool) : tool);
   }
   return server;
 };
diff --git a/models.ts b/models.ts
index 539cb52..d9688c4 100644
--- a/models.ts
+++ b/models.ts
@@ -86,7 +86,7 @@ export const providers = {
       },
       "gpt-codex-mini": {
         displayName: "GPT Codex Mini",
-        resolve: "openai/codex-mini-latest",
+        resolve: "openai/gpt-5.1-codex-mini",
         openRouterResolve: "openrouter/openai/gpt-5.1-codex-mini",
       },
       o3: {
diff --git a/test/list-aliases.ts b/test/list-aliases.ts
index a82152c..0f4062e 100644
--- a/test/list-aliases.ts
+++ b/test/list-aliases.ts
@@ -3,7 +3,12 @@
  * matrix job. `agent` is auto-derived from the alias provider and matches the
  * harness the runtime would pick in production.
  *
- * usage: `node action/test/list-aliases.ts`
+ * set MATRIX_FILTER to a substring to restrict the matrix to matching aliases
+ * — useful for iterating on a single provider without paying for every model.
+ *
+ * usage:
+ *   node action/test/list-aliases.ts
+ *   MATRIX_FILTER=gemini node action/test/list-aliases.ts
  */
 import { modelAliases } from "../models.ts";
 
@@ -11,11 +16,15 @@ function agentForSlug(slug: string): "claude" | "opencode" {
   return slug.startsWith("anthropic/") ? "claude" : "opencode";
 }
 
-const matrix = modelAliases.map((alias) => ({
-  slug: alias.slug,
-  agent: agentForSlug(alias.slug),
-  // readable display name (GHA renders slashes awkwardly in matrix job titles)
-  name: alias.slug.replace("/", "-"),
-}));
+const filter = process.env.MATRIX_FILTER?.trim() ?? "";
+
+const matrix = modelAliases
+  .filter((alias) => (filter ? alias.slug.toLowerCase().includes(filter.toLowerCase()) : true))
+  .map((alias) => ({
+    slug: alias.slug,
+    agent: agentForSlug(alias.slug),
+    // readable display name (GHA renders slashes awkwardly in matrix job titles)
+    name: alias.slug.replace("/", "-"),
+  }));
 
 process.stdout.write(JSON.stringify(matrix));