diff --git a/src/app/api/v1/resources/providers/handlers.ts b/src/app/api/v1/resources/providers/handlers.ts
index 93398344c..96c609e87 100644
--- a/src/app/api/v1/resources/providers/handlers.ts
+++ b/src/app/api/v1/resources/providers/handlers.ts
@@ -16,7 +16,7 @@ import {
   publicActionErrorDetail,
 } from "@/lib/api/v1/_shared/error-envelope";
 import { redactHeaderRecord, redactUrlCredentials } from "@/lib/api/v1/_shared/redaction";
-import { parseHonoJsonBody } from "@/lib/api/v1/_shared/request-body";
+import { parseHonoJsonBody, type SchemaOutput } from "@/lib/api/v1/_shared/request-body";
 import {
   createdResponse,
   jsonResponse,
@@ -773,11 +773,16 @@ function providerNotFound(c: Context): Response {
   });
 }
 
-type JsonBodySchema<T> = {
-  safeParse: (value: unknown) => { success: true; data: T } | { success: false; error: ZodError };
+type JsonBodySchema = {
+  safeParse: (
+    value: unknown
+  ) => { success: true; data: unknown } | { success: false; error: ZodError };
 };
 
-async function parseJson<T>(c: Context, schema: JsonBodySchema<T>): Promise<T | Response> {
+async function parseJson<S extends JsonBodySchema>(
+  c: Context,
+  schema: S
+): Promise<SchemaOutput<S> | Response> {
   const body = await parseHonoJsonBody(c, schema);
   if (!body.ok) return body.response;
   return body.data;
diff --git a/src/lib/api/v1/_shared/request-body.ts b/src/lib/api/v1/_shared/request-body.ts
index 60094b359..0b6aa2705 100644
--- a/src/lib/api/v1/_shared/request-body.ts
+++ b/src/lib/api/v1/_shared/request-body.ts
@@ -3,10 +3,17 @@ import { createProblemResponse, normalizeZodPath } from "./error-envelope";
 
 export type ParsedBodyResult<T> = { ok: true; data: T } | { ok: false; response: Response };
 
-type JsonBodySchema<T> = {
-  safeParse: (value: unknown) => { success: true; data: T } | { success: false; error: z.ZodError };
+type JsonBodySchema = {
+  safeParse: (
+    value: unknown
+  ) => { success: true; data: unknown } | { success: false; error: z.ZodError };
 };
 
+// zod 4 marks the schema output type parameter as `out` (covariant), so the
+// compiler cannot infer it from a value parameter. Extract it from the
+// `_output` phantom property instead.
+export type SchemaOutput<S> = S extends { _output: infer O } ? O : unknown;
+
 type ParseJsonBodyOptions = {
   validationErrorCode?: (error: z.ZodError) => string | undefined;
 };
@@ -20,10 +27,10 @@ type HonoJsonRequest = {
   };
 };
 
-export async function parseJsonBody<T>(
+export async function parseJsonBody<S extends JsonBodySchema>(
   request: Request,
-  schema: JsonBodySchema<T>
-): Promise<ParsedBodyResult<T>> {
+  schema: S
+): Promise<ParsedBodyResult<SchemaOutput<S>>> {
   const contentType = request.headers.get("content-type") ?? "";
   if (!contentType.toLowerCase().includes("application/json")) {
     return {
@@ -70,14 +77,14 @@ export async function parseJsonBody<T>(
     };
   }
 
-  return { ok: true, data: parsed.data };
+  return { ok: true, data: parsed.data as SchemaOutput<S> };
 }
 
-export async function parseHonoJsonBody<T>(
+export async function parseHonoJsonBody<S extends JsonBodySchema>(
   c: HonoJsonRequest,
-  schema: JsonBodySchema<T>,
+  schema: S,
   options?: ParseJsonBodyOptions
-): Promise<ParsedBodyResult<T>> {
+): Promise<ParsedBodyResult<SchemaOutput<S>>> {
   const contentType =
     c.req.header("content-type") ??
     c.req.header("Content-Type") ??
@@ -128,5 +135,5 @@ export async function parseHonoJsonBody<T>(
     };
   }
 
-  return { ok: true, data: parsed.data };
+  return { ok: true, data: parsed.data as SchemaOutput<S> };
 }
diff --git a/src/lib/api/v1/schemas/audit-logs.ts b/src/lib/api/v1/schemas/audit-logs.ts
index 5814d7386..14635b1a9 100644
--- a/src/lib/api/v1/schemas/audit-logs.ts
+++ b/src/lib/api/v1/schemas/audit-logs.ts
@@ -23,7 +23,9 @@ export const AuditLogListQuerySchema = z.object({
   success: z
     .enum(["true", "false"])
     .optional()
-    .transform((val) => (val === undefined ? undefined : val === "true"))
+    .transform((val: "true" | "false" | undefined) =>
+      val === undefined ? undefined : val === "true"
+    )
     .describe("Optional success filter."),
   from: IsoDateTimeStringSchema.optional().describe("Optional inclusive start time."),
   to: IsoDateTimeStringSchema.optional().describe("Optional inclusive end time."),
diff --git a/src/lib/api/v1/schemas/me.ts b/src/lib/api/v1/schemas/me.ts
index 4880d1240..7b084c105 100644
--- a/src/lib/api/v1/schemas/me.ts
+++ b/src/lib/api/v1/schemas/me.ts
@@ -3,7 +3,7 @@ import { z } from "@hono/zod-openapi";
 const NumberQuerySchema = z.coerce.number().optional();
 const BooleanQuerySchema = z
   .union([z.literal("true"), z.literal("false"), z.boolean()])
-  .transform((value) => value === true || value === "true")
+  .transform((value: boolean | "true" | "false") => value === true || value === "true")
   .optional();
 
 export const MeUsageLogsQuerySchema = z.object({
diff --git a/src/lib/api/v1/schemas/system-config.ts b/src/lib/api/v1/schemas/system-config.ts
index 5dd0253a6..944e4cab2 100644
--- a/src/lib/api/v1/schemas/system-config.ts
+++ b/src/lib/api/v1/schemas/system-config.ts
@@ -28,7 +28,7 @@ const CodexPriorityBillingSourceSchema = z
 const TimeZoneSchema = z
   .string()
   .refine(
-    (value) => {
+    (value: string) => {
       try {
         new Intl.DateTimeFormat("en-US", { timeZone: value });
         return true;
diff --git a/src/lib/api/v1/schemas/usage-logs.ts b/src/lib/api/v1/schemas/usage-logs.ts
index 2b01bc88a..2bca6378e 100644
--- a/src/lib/api/v1/schemas/usage-logs.ts
+++ b/src/lib/api/v1/schemas/usage-logs.ts
@@ -3,7 +3,7 @@ import { z } from "@hono/zod-openapi";
 const NumberQuerySchema = z.coerce.number().optional();
 const BooleanQuerySchema = z
   .union([z.literal("true"), z.literal("false"), z.boolean()])
-  .transform((value) => value === true || value === "true")
+  .transform((value: boolean | "true" | "false") => value === true || value === "true")
   .optional();
 
 export const UsageLogsQuerySchema = z.object({