fix(mcp): sanitize for gemini when model is unresolved (#697)

* fix(mcp): sanitize for gemini when model is unresolved

isGeminiRouted() previously required the effective model string to
contain "gemini" — but when payload.model="auto" (or any unresolved
slug) reaches addTools(), `effective` is the literal "auto", which
doesn't match. opencode then auto-selects gemini *after* the MCP
server has registered raw arktype schemas, and every tool turn dies
on `function_declarations[*].properties[*].any_of[*].enum: only
allowed for STRING type`.

widen the gate: any unresolved specifier (undefined / "auto" / a
slug without a `provider/` prefix) is treated as gemini-routed and
sanitized. the transforms are universally compatible normalizations
so the false-positive cost is negligible. tighten case 3 to preserve
`description` so the only lossy path no longer drops operator-facing
context.

fixes #676.

* revert case-3 description preservation

per pullfrog review on #697: keeping `description` as a peer of
`anyOf`/`oneOf` directly contradicts the file's own header (lines
19-21) and the upstream opencode #14659 rationale that gates this
sanitizer — gemini requires anyOf to be the ONLY field on a schema
node, sibling keywords trigger
`anyOf must be the only field in a schema node`. the change was
speculative scope creep with no evidence, and would silently
re-introduce a different gemini failure for any future schema using
`.describe().or(...)`. the bug fix for #676 doesn't need it (arktype
doesn't emit non-collapsible anyOf for current tool schemas).

mcp/geminiSanitizer.ts

@@ -176,13 +176,32 @@ export function sanitizeToolForGemini<T extends Tool<any, any>>(tool: T): 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".
+ * true when the effective upstream model is — or might become — google
+ * generative language API traffic. matches:
+ *   - direct `google/*`, opencode `opencode/gemini-*`, openrouter
+ *     `openrouter/google/gemini-*` (slug substring "gemini" wins).
+ *   - any unresolved specifier: `undefined`, `"auto"`, or a slug that
+ *     didn't map through the alias registry (no `provider/` prefix).
+ *     these flow through the agent's own auto-select, which may land
+ *     on gemini *after* the MCP server has already registered tools —
+ *     at which point sanitization is too late to apply. erring on the
+ *     side of sanitizing is safe: cases 1 + 2 are universally
+ *     compatible JSON-Schema normalizations (enum-only → typed string,
+ *     collapsible const-unions → string enum); case 3 is gemini-
+ *     specific but only fires on non-collapsible unions, which arktype
+ *     does not emit for our current tool schemas. see issue #676 for
+ *     the prod failure that motivated this widening.
  */
 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");
+  if (!effective) return true;
+  const normalized = effective.toLowerCase();
+  if (normalized.includes("gemini")) return true;
+  // every concrete model resolved through the registry carries a
+  // `provider/` prefix (e.g. "anthropic/claude-opus-4-7"). anything
+  // without a slash is either the literal `"auto"` alias or an
+  // unrecognized slug that resolveModel logged a warning for — both
+  // route through the agent's late auto-select, which may pick gemini.
+  if (!normalized.includes("/")) return true;
+  return false;
 }