diff --git a/.agents/plans/validate-sponsored-micros-inputs.plan.md b/.agents/plans/validate-sponsored-micros-inputs.plan.md
new file mode 100644
index 00000000..1ddc4270
--- /dev/null
+++ b/.agents/plans/validate-sponsored-micros-inputs.plan.md
@@ -0,0 +1,87 @@
+---
+name: validate-sponsored-micros-inputs
+overview: "Reject non-finite and non-integer JSON number inputs for sponsored purchase micro-USDC fields with client-error responses before transaction preparation."
+todos:
+  - id: reproduce-invalid-micros-response
+    content: Add a focused failing regression showing an overflowing JSON numeric micro-USDC value reaches BigInt conversion and throws a generic error
+    status: completed
+  - id: validate-numeric-micros
+    content: Validate numeric inputs before BigInt conversion and preserve the sponsored route's client-error classification
+    status: completed
+  - id: cover-helper-and-route-contract
+    content: Add behavioral helper coverage and route-level no-downstream-side-effect coverage for invalid numeric micro-USDC input
+    status: completed
+  - id: verify-focused-change
+    content: Run focused tests, format, lint, typecheck, full web tests, webpack build, and whitespace checks
+    status: completed
+isProject: false
+---
+
+# Validate Sponsored Micro-USDC Inputs
+
+## Goal
+
+Make `POST /api/transactions/sponsored/purchase/prepare` reject invalid JSON numeric values for micro-USDC fields with a `400` response before sponsored transaction preparation, rather than leaking a `BigInt` conversion failure as a `500`.
+
+## Scope
+
+- In scope: sponsored purchase preparation's `expectedPriceUsdcMicros` and `maxSetupFeeUsdcMicros` numeric parsing, its shared `parseNonNegativeBigInt` helper, and focused regression coverage.
+- Out of scope: sponsored checkout economics, feature flags, rate-limit policy, transaction-account construction, database changes, wallet behavior, and Base mainnet enablement.
+
+## Verified Gap (2026-09-02)
+
+- `web/app/api/transactions/sponsored/purchase/prepare/route.ts:21-26` accepts every JavaScript `number` from JSON and forwards it to `prepareSponsoredPurchase` at lines 64-71.
+- `web/lib/sponsoredPurchase.ts:117-124` passes numeric input directly to `BigInt(value)`. Valid JSON `1e309` parses as `Infinity`; `BigInt(Infinity)` throws a `RangeError`.
+- The route's handler maps only expected validation messages to `400` (`route.ts:73-83`), so that conversion error becomes a generic `500`.
+- Existing `web/__tests__/api/sponsored-transaction-routes.test.ts` covers literal `null` bodies but not invalid numeric micro-USDC values.
+
+## Files To Change
+
+- `web/lib/sponsoredPurchase.ts`: reject non-finite, fractional, and unsafe numeric inputs with a stable validation error before invoking `BigInt`.
+- `web/app/api/transactions/sponsored/purchase/prepare/route.ts`: classify that stable input-validation error as a `400`.
+- `web/__tests__/lib/sponsoredPurchase.test.ts`: add behavioral regressions for invalid numbers at the pure parsing boundary.
+- `web/__tests__/api/sponsored-transaction-routes.test.ts`: assert an overflowing numeric request returns `400` and never calls `prepareSponsoredPurchase`.
+
+## Implementation Steps
+
+1. Add a route-level RED regression with a valid buyer/listing payload and `expectedPriceUsdcMicros: 1e309`; assert a client error and no transaction helper invocation.
+2. Add pure helper regressions for `Infinity`, fractional values, and unsafe numeric integers, while preserving valid integer/string input behavior.
+3. Validate numbers with `Number.isSafeInteger` before `BigInt` conversion. Use an error message whose existing route classification consistently returns `400`.
+4. Keep strings as the canonical representation for arbitrarily large micro-USDC values, so exact high-value client input remains supported.
+5. Re-run focused tests and repository quality gates.
+
+## Verification
+
+Run with Node 24 as required by `AGENTS.md`:
+
+```bash
+. "$HOME/.nvm/nvm.sh" --no-use && { nvm use --silent || nvm install; }
+npm test --workspace @agentvouch/web -- __tests__/api/sponsored-transaction-routes.test.ts __tests__/lib/sponsoredPurchase.test.ts --maxWorkers=1 --no-fileParallelism
+npm run format:check
+npm run lint:web
+npm run typecheck
+npm test --workspace @agentvouch/web -- --maxWorkers=1 --no-fileParallelism
+npm exec --workspace @agentvouch/web -- next build --webpack
+git diff --check
+```
+
+Acceptance criteria: invalid numeric JSON values receive a `400` without invoking sponsored transaction preparation; the shared helper fails with a stable validation error before `BigInt`; valid exact string values remain supported.
+
+## Rollout
+
+Ship as a narrow API input-hardening PR. No data migration, deployment configuration, wallet transaction, or on-chain state change is required.
+
+## Execution Note (2026-09-02)
+
+- RED: `npm test --workspace @agentvouch/web -- __tests__/lib/sponsoredPurchase.test.ts --maxWorkers=1 --no-fileParallelism` failed before the fix because `BigInt(Infinity)` threw `The number Infinity cannot be converted to a BigInt because it is not an integer` instead of a stable validation error.
+- The implementation rejects unsafe numeric inputs at the route boundary before `prepareSponsoredPurchase` runs, and the shared parser independently rejects non-finite, fractional, and unsafe numeric values. Exact high-value values remain supported as decimal strings.
+- Passed under Node `v24.10.0`: focused sponsored tests (2 files / 27 tests), `npm run format:check`, `npm run lint:web`, `npm run typecheck`, full web Vitest (128 files / 930 tests), `npm exec --workspace @agentvouch/web -- next build --webpack`, and `git diff --check`.
+- The build retained the repository's existing `ox`/`viem` dynamic-import warning and expected static-generation `DATABASE_URL` fallback logs. No live sponsored transaction, database, wallet, or deployment flow was run.
+
+## Rollback
+
+Revert the focused commit. The patch changes only invalid request handling and associated tests.
+
+## Blockers
+
+- None known. Do not broaden the shared parser's accepted types or alter sponsored fee economics.
diff --git a/web/__tests__/api/sponsored-transaction-routes.test.ts b/web/__tests__/api/sponsored-transaction-routes.test.ts
index 4e597263..af72ec6f 100644
--- a/web/__tests__/api/sponsored-transaction-routes.test.ts
+++ b/web/__tests__/api/sponsored-transaction-routes.test.ts
@@ -53,6 +53,31 @@ describe("sponsored transaction routes", () => {
     expect(transactionMocks.prepareSponsoredPurchase).not.toHaveBeenCalled();
   });
 
+  it("rejects overflowing numeric micro-USDC fields before preparing a transaction", async () => {
+    for (const [field, error] of [
+      ["expectedPriceUsdcMicros", "Invalid expectedPriceUsdcMicros"],
+      ["maxSetupFeeUsdcMicros", "Invalid maxSetupFeeUsdcMicros"],
+    ]) {
+      const response = await preparePurchase(
+        new NextRequest(
+          "http://localhost/api/transactions/sponsored/purchase/prepare",
+          {
+            method: "POST",
+            headers: {
+              "Content-Type": "application/json",
+              "X-Forwarded-For": "198.51.100.5",
+            },
+            body: `{"buyerPubkey":"buyer","listingAddress":"listing","${field}":1e309}`,
+          }
+        )
+      );
+
+      expect(response.status).toBe(400);
+      await expect(response.json()).resolves.toEqual({ error });
+      expect(transactionMocks.prepareSponsoredPurchase).not.toHaveBeenCalled();
+    }
+  });
+
   it("rejects a null purchase-submit body before submitting a transaction", async () => {
     const response = await submitPurchase(
       nullJsonRequest(
diff --git a/web/__tests__/lib/sponsoredPurchase.test.ts b/web/__tests__/lib/sponsoredPurchase.test.ts
index bdf86b7a..08e57d68 100644
--- a/web/__tests__/lib/sponsoredPurchase.test.ts
+++ b/web/__tests__/lib/sponsoredPurchase.test.ts
@@ -10,6 +10,7 @@ import {
   assertSponsoredTransactionSignatures,
   getSponsoredTransactionDebug,
   getSponsoredCoreInstructions,
+  parseNonNegativeBigInt,
 } from "@/lib/sponsoredPurchase";
 import { bufferKoraTokenFee, getSponsoredSponsorMode } from "@/lib/koraSponsor";
 import {
@@ -47,6 +48,22 @@ describe("assertBuyerIsNotSponsor", () => {
   });
 });
 
+describe("parseNonNegativeBigInt", () => {
+  it("rejects unsafe JSON numbers before BigInt conversion", () => {
+    for (const value of [Infinity, 1.5, Number.MAX_SAFE_INTEGER + 1]) {
+      expect(() =>
+        parseNonNegativeBigInt(value, "expectedPriceUsdcMicros")
+      ).toThrow("expectedPriceUsdcMicros must be a non-negative safe integer");
+    }
+  });
+
+  it("keeps exact string micro-USDC values available", () => {
+    expect(
+      parseNonNegativeBigInt("9007199254740993", "expectedPriceUsdcMicros")
+    ).toBe(9007199254740993n);
+  });
+});
+
 describe("Kora sponsor mode helpers", () => {
   it("defaults to the bespoke server sponsor and accepts explicit Kora mode", () => {
     delete process.env.AGENTVOUCH_SPONSOR_MODE;
diff --git a/web/app/api/transactions/sponsored/purchase/prepare/route.ts b/web/app/api/transactions/sponsored/purchase/prepare/route.ts
index 3352cf22..fc186a67 100644
--- a/web/app/api/transactions/sponsored/purchase/prepare/route.ts
+++ b/web/app/api/transactions/sponsored/purchase/prepare/route.ts
@@ -18,12 +18,15 @@ function stringOrNull(value: unknown): string | null {
   return typeof value === "string" && value.trim() ? value.trim() : null;
 }
 
-function bigintishOrNull(value: unknown): string | number | bigint | null {
-  return typeof value === "string" ||
-    typeof value === "number" ||
-    typeof value === "bigint"
-    ? value
-    : null;
+function bigintishOrNull(
+  value: unknown,
+  label: string
+): string | number | bigint | null {
+  if (typeof value === "number") {
+    if (Number.isSafeInteger(value)) return value;
+    throw new Error(`Invalid ${label}`);
+  }
+  return typeof value === "string" || typeof value === "bigint" ? value : null;
 }
 
 export async function POST(request: NextRequest) {
@@ -65,9 +68,15 @@ export async function POST(request: NextRequest) {
       buyerPubkey,
       listingAddress,
       skillDbId: stringOrNull(body.skillDbId),
-      expectedPriceUsdcMicros: bigintishOrNull(body.expectedPriceUsdcMicros),
+      expectedPriceUsdcMicros: bigintishOrNull(
+        body.expectedPriceUsdcMicros,
+        "expectedPriceUsdcMicros"
+      ),
       expectedUsdcMint: stringOrNull(body.expectedUsdcMint),
-      maxSetupFeeUsdcMicros: bigintishOrNull(body.maxSetupFeeUsdcMicros),
+      maxSetupFeeUsdcMicros: bigintishOrNull(
+        body.maxSetupFeeUsdcMicros,
+        "maxSetupFeeUsdcMicros"
+      ),
     });
     return NextResponse.json(result);
   } catch (error: unknown) {
diff --git a/web/lib/sponsoredPurchase.ts b/web/lib/sponsoredPurchase.ts
index 03c40669..3ca21977 100644
--- a/web/lib/sponsoredPurchase.ts
+++ b/web/lib/sponsoredPurchase.ts
@@ -119,6 +119,9 @@ export function parseNonNegativeBigInt(
   label: string
 ) {
   if (value === null || value === undefined || value === "") return null;
+  if (typeof value === "number" && !Number.isSafeInteger(value)) {
+    throw new Error(`${label} must be a non-negative safe integer`);
+  }
   const parsed = BigInt(value);
   if (parsed < 0n) throw new Error(`${label} must be non-negative`);
   return parsed;