diff --git a/src/services/bridge/client.ts b/src/services/bridge/client.ts
index db4a23aa9..4e68f66c0 100644
--- a/src/services/bridge/client.ts
+++ b/src/services/bridge/client.ts
@@ -369,7 +369,8 @@ export class BridgeClient {
     method: string,
     path: string,
     body?: unknown,
-    idempotencyKey?: string,
+    // undefined → auto-generate; null → endpoint does not accept the header
+    idempotencyKey?: string | null,
   ): Promise<T> {
     const url = `${this.baseUrl}${path}`
     const headers: Record<string, string> = {
@@ -377,8 +378,10 @@ export class BridgeClient {
       "Content-Type": "application/json",
     }
 
-    // Bridge rejects Idempotency-Key on GET and DELETE endpoints.
-    if (!["GET", "DELETE"].includes(method.toUpperCase())) {
+    // Bridge rejects Idempotency-Key on GET and DELETE endpoints, and on a
+    // handful of POST resources that disable it explicitly (callers pass
+    // null for those — see exchangePlaidPublicToken).
+    if (!["GET", "DELETE"].includes(method.toUpperCase()) && idempotencyKey !== null) {
       if (idempotencyKey) {
         headers["Idempotency-Key"] = idempotencyKey
       } else {
@@ -555,10 +558,14 @@ export class BridgeClient {
     linkToken: string,
     publicToken: string,
   ): Promise<PlaidExchangePublicTokenResponse> {
+    // Bridge explicitly disables Idempotency-Key on this resource (the
+    // exchange is already idempotent per link token) and 400s when the
+    // header is present: "Cannot set Idempotency-Key on this request".
     return this.request<PlaidExchangePublicTokenResponse>(
       "POST",
       `/plaid_exchange_public_token/${encodeURIComponent(linkToken)}`,
       { public_token: publicToken },
+      null,
     )
   }
 
diff --git a/test/flash/unit/services/bridge/client.spec.ts b/test/flash/unit/services/bridge/client.spec.ts
index d32b4e8dd..a788a85de 100644
--- a/test/flash/unit/services/bridge/client.spec.ts
+++ b/test/flash/unit/services/bridge/client.spec.ts
@@ -216,3 +216,47 @@ describe("BridgeClient transfer deletion", () => {
     expect(init.headers["Idempotency-Key"]).toBeUndefined()
   })
 })
+
+describe("BridgeClient Plaid exchange idempotency", () => {
+  const originalFetch = global.fetch
+
+  beforeEach(() => {
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true,
+      json: async () => ({ message: "ok" }),
+    } as Response)
+  })
+
+  afterEach(() => {
+    global.fetch = originalFetch
+  })
+
+  it("does not send an Idempotency-Key on the plaid public_token exchange", async () => {
+    // Bridge explicitly disables the header on this resource and 400s when it
+    // is present ("Cannot set Idempotency-Key on this request") — found live
+    // on the first production Plaid link (flash-mobile#668 device test).
+    const client = new BridgeClient()
+
+    await client.exchangePlaidPublicToken("lt_abc", "public-token-1")
+
+    const [url, init] = (global.fetch as jest.Mock).mock.calls[0]
+    expect(url).toContain("/plaid_exchange_public_token/lt_abc")
+    expect(init.method).toBe("POST")
+    expect(init.headers["Idempotency-Key"]).toBeUndefined()
+  })
+
+  it("still auto-generates an Idempotency-Key for plaid link-token requests", async () => {
+    ;(global.fetch as jest.Mock).mockResolvedValue({
+      ok: true,
+      json: async () => ({ link_token: "lt_abc", expires_at: "later" }),
+    } as Response)
+    const client = new BridgeClient()
+
+    await client.createPlaidLinkRequest("cust_1" as never)
+
+    const [, init] = (global.fetch as jest.Mock).mock.calls[0]
+    expect(init.method).toBe("POST")
+    expect(init.headers["Idempotency-Key"]).toEqual(expect.any(String))
+    expect(init.headers["Idempotency-Key"]).not.toHaveLength(0)
+  })
+})