diff --git a/crates/yaak-templates/pkg/yaak_templates_bg.wasm b/crates/yaak-templates/pkg/yaak_templates_bg.wasm
index e02abbca..ef7c337b 100644
Binary files a/crates/yaak-templates/pkg/yaak_templates_bg.wasm and b/crates/yaak-templates/pkg/yaak_templates_bg.wasm differ
diff --git a/crates/yaak-wasm/pkg/yaak_wasm_bg.js b/crates/yaak-wasm/pkg/yaak_wasm_bg.js
index 8450af8d..60789744 100644
--- a/crates/yaak-wasm/pkg/yaak_wasm_bg.js
+++ b/crates/yaak-wasm/pkg/yaak_wasm_bg.js
@@ -697,22 +697,22 @@ export function __wbg_warn_b6f36cac66fc96a4(arg0, arg1) {
     console.warn(arg0, arg1);
 }
 export function __wbindgen_cast_0000000000000001(arg0, arg1) {
-    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [Externref], shim_idx: 1117, ret: Result(Unit), inner_ret: Some(Result(Unit)) }, mutable: true }) -> Externref`.
+    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [Externref], shim_idx: 1123, ret: Result(Unit), inner_ret: Some(Result(Unit)) }, mutable: true }) -> Externref`.
     const ret = makeMutClosure(arg0, arg1, wasm_bindgen__convert__closures_____invoke__ha1c2fa93df0107f3);
     return ret;
 }
 export function __wbindgen_cast_0000000000000002(arg0, arg1) {
-    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [NamedExternref("Event")], shim_idx: 212, ret: Unit, inner_ret: Some(Unit) }, mutable: true }) -> Externref`.
+    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [NamedExternref("Event")], shim_idx: 214, ret: Unit, inner_ret: Some(Unit) }, mutable: true }) -> Externref`.
     const ret = makeMutClosure(arg0, arg1, wasm_bindgen__convert__closures_____invoke__ha7903b6e296dd8f4);
     return ret;
 }
 export function __wbindgen_cast_0000000000000003(arg0, arg1) {
-    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [NamedExternref("IDBVersionChangeEvent")], shim_idx: 83, ret: Result(Unit), inner_ret: Some(Result(Unit)) }, mutable: true }) -> Externref`.
+    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [NamedExternref("IDBVersionChangeEvent")], shim_idx: 164, ret: Result(Unit), inner_ret: Some(Result(Unit)) }, mutable: true }) -> Externref`.
     const ret = makeMutClosure(arg0, arg1, wasm_bindgen__convert__closures_____invoke__h4381d8e749fe46cf);
     return ret;
 }
 export function __wbindgen_cast_0000000000000004(arg0, arg1) {
-    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [], shim_idx: 210, ret: Unit, inner_ret: Some(Unit) }, mutable: true }) -> Externref`.
+    // Cast intrinsic for `Closure(Closure { owned: true, function: Function { arguments: [], shim_idx: 212, ret: Unit, inner_ret: Some(Unit) }, mutable: true }) -> Externref`.
     const ret = makeMutClosure(arg0, arg1, wasm_bindgen__convert__closures_____invoke__ha1b480b83daa641f);
     return ret;
 }
diff --git a/crates/yaak-wasm/pkg/yaak_wasm_bg.wasm b/crates/yaak-wasm/pkg/yaak_wasm_bg.wasm
index 4828e9e8..778ee83a 100644
Binary files a/crates/yaak-wasm/pkg/yaak_wasm_bg.wasm and b/crates/yaak-wasm/pkg/yaak_wasm_bg.wasm differ
diff --git a/plugins/auth-oauth2/README.md b/plugins/auth-oauth2/README.md
index 773a949c..8f0e30be 100644
--- a/plugins/auth-oauth2/README.md
+++ b/plugins/auth-oauth2/README.md
@@ -51,6 +51,8 @@ Direct username/password authentication.
 - **Token Persistence**: Stores tokens between sessions
 - **Flexible Configuration**: Supports custom authorization and token endpoints
 - **Scope Management**: Configure required OAuth scopes for your API
+- **Custom Parameters**: Add headers and params to the authorization, token, and refresh
+  requests, replacing generated entries of the same name
 - **Error Handling**: Comprehensive error handling and user feedback
 
 ## Usage
diff --git a/plugins/auth-oauth2/src/customParams.ts b/plugins/auth-oauth2/src/customParams.ts
new file mode 100644
index 00000000..e1a9c3ba
--- /dev/null
+++ b/plugins/auth-oauth2/src/customParams.ts
@@ -0,0 +1,121 @@
+import type { JsonPrimitive } from "@yaakapp/api";
+
+export interface NameValue {
+  name: string;
+  value: string;
+}
+
+/** Custom entries for a single outgoing request */
+export interface CustomRequestParams {
+  headers: NameValue[];
+  body: NameValue[];
+}
+
+export interface CustomParams {
+  /** Query parameters appended to the authorization URL */
+  authorizationQuery: NameValue[];
+  token: CustomRequestParams;
+  /** Already merged over `token`, so the refresh request only reads this */
+  refresh: CustomRequestParams;
+}
+
+export const NO_CUSTOM_PARAMS: CustomParams = {
+  authorizationQuery: [],
+  token: { headers: [], body: [] },
+  refresh: { headers: [], body: [] },
+};
+
+/**
+ * A key_value form input stores its rows as a JSON-encoded array of pairs. The
+ * whole string goes through the template engine before it reaches the plugin,
+ * so a row's value can be a template.
+ */
+export function parsePairs(value: JsonPrimitive | undefined): NameValue[] {
+  if (value == null || value === "") return [];
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(String(value));
+  } catch {
+    console.log("[oauth2] Ignoring custom parameters that failed to parse");
+    return [];
+  }
+
+  if (!Array.isArray(parsed)) return [];
+
+  const pairs: NameValue[] = [];
+  for (const row of parsed) {
+    if (row == null || typeof row !== "object") continue;
+    const { name, value, enabled } = row as { name?: unknown; value?: unknown; enabled?: unknown };
+    if (enabled === false) continue;
+    const trimmedName = typeof name === "string" ? name.trim() : "";
+    if (trimmedName === "") continue;
+    pairs.push({ name: trimmedName, value: rowValue(value) });
+  }
+
+  return pairs;
+}
+
+/** The pair editor writes strings, but a hand-edited file may hold any primitive */
+function rowValue(value: unknown): string {
+  if (typeof value === "string") return value;
+  if (typeof value === "number" || typeof value === "boolean") return String(value);
+  return "";
+}
+
+/**
+ * Read every custom parameter list off the auth form.
+ *
+ * Token entries also apply to the refresh request, so the refresh lists are the
+ * token lists with the refresh-specific rows merged over them.
+ */
+export function readCustomParams(values: Record<string, JsonPrimitive | undefined>): CustomParams {
+  const tokenHeaders = parsePairs(values.tokenHeaders);
+  const tokenBody = parsePairs(values.tokenBodyParams);
+
+  return {
+    authorizationQuery: parsePairs(values.authorizationParams),
+    token: { headers: tokenHeaders, body: tokenBody },
+    refresh: {
+      headers: mergeHeaders(tokenHeaders, parsePairs(values.refreshHeaders)),
+      body: mergeFormParams(tokenBody, parsePairs(values.refreshBodyParams)),
+    },
+  };
+}
+
+/**
+ * Custom entries win over generated ones of the same name: every generated
+ * entry the custom list names is dropped, then the custom list is appended.
+ * Dropping by name rather than replacing in place means two custom rows sharing
+ * a name both survive, which repeatable headers and params rely on.
+ */
+function mergeByName<T extends NameValue>(
+  generated: T[],
+  custom: NameValue[],
+  { caseInsensitive }: { caseInsensitive: boolean },
+): (T | NameValue)[] {
+  if (custom.length === 0) return generated;
+
+  const normalize = (name: string) => (caseInsensitive ? name.toLowerCase() : name);
+  const overridden = new Set(custom.map((c) => normalize(c.name)));
+
+  return [...generated.filter((g) => !overridden.has(normalize(g.name))), ...custom];
+}
+
+export function mergeHeaders<T extends NameValue>(generated: T[], custom: NameValue[]) {
+  return mergeByName(generated, custom, { caseInsensitive: true });
+}
+
+export function mergeFormParams<T extends NameValue>(generated: T[], custom: NameValue[]) {
+  return mergeByName(generated, custom, { caseInsensitive: false });
+}
+
+/** Apply custom query parameters to an authorization URL, in place */
+export function applyQueryParams(url: URL, custom: NameValue[]) {
+  for (const name of new Set(custom.map((c) => c.name))) {
+    url.searchParams.delete(name);
+  }
+  for (const { name, value } of custom) {
+    url.searchParams.append(name, value);
+  }
+}
diff --git a/plugins/auth-oauth2/src/fetchAccessToken.ts b/plugins/auth-oauth2/src/fetchAccessToken.ts
index 32e43b9c..9b7372dc 100644
--- a/plugins/auth-oauth2/src/fetchAccessToken.ts
+++ b/plugins/auth-oauth2/src/fetchAccessToken.ts
@@ -1,4 +1,6 @@
 import type { Context, HttpRequest, HttpUrlParameter } from "@yaakapp/api";
+import type { CustomRequestParams } from "./customParams";
+import { mergeFormParams, mergeHeaders } from "./customParams";
 import type { AccessTokenRawResponse } from "./store";
 
 export async function fetchAccessToken(
@@ -10,9 +12,10 @@ export async function fetchAccessToken(
     scope: string | null;
     audience: string | null;
     params: HttpUrlParameter[];
+    custom?: CustomRequestParams;
   } & ({ clientAssertion: string } | { clientSecret: string; credentialsInBody: boolean }),
 ): Promise<AccessTokenRawResponse> {
-  const { clientId, grantType, accessTokenUrl, scope, audience, params } = args;
+  const { clientId, grantType, accessTokenUrl, scope, audience, params, custom } = args;
   console.log("[oauth2] Getting access token", accessTokenUrl);
   const httpRequest: Partial<HttpRequest> = {
     method: "POST",
@@ -61,6 +64,12 @@ export async function fetchAccessToken(
     httpRequest.headers?.push({ name: "Authorization", value });
   }
 
+  // Merged last so custom entries override the credential headers and params above
+  if (custom) {
+    httpRequest.headers = mergeHeaders(httpRequest.headers ?? [], custom.headers);
+    httpRequest.body = { form: mergeFormParams(httpRequest.body?.form ?? [], custom.body) };
+  }
+
   httpRequest.authenticationType = "none"; // Don't inherit workspace auth
   const { httpResponse: resp, body: responseBody } = await ctx.httpRequest.send({ httpRequest });
 
diff --git a/plugins/auth-oauth2/src/getOrRefreshAccessToken.ts b/plugins/auth-oauth2/src/getOrRefreshAccessToken.ts
index 86aee3e9..abc38f24 100644
--- a/plugins/auth-oauth2/src/getOrRefreshAccessToken.ts
+++ b/plugins/auth-oauth2/src/getOrRefreshAccessToken.ts
@@ -1,4 +1,6 @@
 import type { Context, HttpRequest } from "@yaakapp/api";
+import type { CustomRequestParams } from "./customParams";
+import { mergeFormParams, mergeHeaders } from "./customParams";
 import type { AccessToken, AccessTokenRawResponse, TokenStoreArgs } from "./store";
 import { deleteToken, getToken, storeToken } from "./store";
 import { isTokenExpired } from "./util";
@@ -14,6 +16,7 @@ export async function getOrRefreshAccessToken(
     clientSecret,
     tokenName,
     forceRefresh,
+    custom,
   }: {
     scope: string | null;
     accessTokenUrl: string;
@@ -22,6 +25,7 @@ export async function getOrRefreshAccessToken(
     clientSecret: string;
     tokenName?: "access_token" | "id_token";
     forceRefresh?: boolean;
+    custom?: CustomRequestParams;
   },
 ): Promise<AccessToken | null> {
   const token = await getToken(ctx, tokenArgs);
@@ -69,6 +73,12 @@ export async function getOrRefreshAccessToken(
     httpRequest.headers?.push({ name: "Authorization", value });
   }
 
+  // Merged last so custom entries override the credential headers and params above
+  if (custom) {
+    httpRequest.headers = mergeHeaders(httpRequest.headers ?? [], custom.headers);
+    httpRequest.body = { form: mergeFormParams(httpRequest.body?.form ?? [], custom.body) };
+  }
+
   httpRequest.authenticationType = "none"; // Don't inherit workspace auth
   const { httpResponse: resp, body: responseBody } = await ctx.httpRequest.send({ httpRequest });
 
diff --git a/plugins/auth-oauth2/src/grants/authorizationCode.ts b/plugins/auth-oauth2/src/grants/authorizationCode.ts
index a015b01b..54f8ace7 100644
--- a/plugins/auth-oauth2/src/grants/authorizationCode.ts
+++ b/plugins/auth-oauth2/src/grants/authorizationCode.ts
@@ -1,6 +1,8 @@
 import { createHash, randomBytes } from "node:crypto";
 import type { Context } from "@yaakapp/api";
 import { getRedirectUrlViaExternalBrowser } from "../callbackServer";
+import type { CustomParams } from "../customParams";
+import { applyQueryParams, NO_CUSTOM_PARAMS } from "../customParams";
 import { fetchAccessToken } from "../fetchAccessToken";
 import { getOrRefreshAccessToken } from "../getOrRefreshAccessToken";
 import type { AccessToken, TokenStoreArgs } from "../store";
@@ -36,6 +38,7 @@ export async function getAuthorizationCode(
     pkce,
     tokenName,
     externalBrowser,
+    customParams = NO_CUSTOM_PARAMS,
   }: {
     authorizationUrl: string;
     accessTokenUrl: string;
@@ -52,6 +55,7 @@ export async function getAuthorizationCode(
     } | null;
     tokenName: "access_token" | "id_token";
     externalBrowser?: ExternalBrowserOptions;
+    customParams?: CustomParams;
   },
 ): Promise<AccessToken> {
   const tokenArgs: TokenStoreArgs = {
@@ -68,6 +72,7 @@ export async function getAuthorizationCode(
     clientSecret,
     credentialsInBody,
     tokenName,
+    custom: customParams.refresh,
   });
   if (token != null) {
     return token;
@@ -92,6 +97,10 @@ export async function getAuthorizationCode(
     authorizationUrl.searchParams.set("code_challenge_method", pkce.challengeMethod);
   }
 
+  // Applied before redirect_uri, which belongs to the callback flow rather than
+  // to the user: overriding it would send the code somewhere nothing listens
+  applyQueryParams(authorizationUrl, customParams.authorizationQuery);
+
   let code: string;
   let actualRedirectUri: string | null = redirectUri;
 
@@ -130,6 +139,7 @@ export async function getAuthorizationCode(
       ...(pkce ? [{ name: "code_verifier", value: pkce.codeVerifier }] : []),
       ...(actualRedirectUri ? [{ name: "redirect_uri", value: actualRedirectUri }] : []),
     ],
+    custom: customParams.token,
   });
 
   return storeToken(ctx, tokenArgs, response, tokenName);
diff --git a/plugins/auth-oauth2/src/grants/clientCredentials.ts b/plugins/auth-oauth2/src/grants/clientCredentials.ts
index d8b34fa8..222e479b 100644
--- a/plugins/auth-oauth2/src/grants/clientCredentials.ts
+++ b/plugins/auth-oauth2/src/grants/clientCredentials.ts
@@ -1,6 +1,8 @@
 import { createPrivateKey, randomUUID } from "node:crypto";
 import type { Context } from "@yaakapp/api";
 import jwt, { type Algorithm } from "jsonwebtoken";
+import type { CustomParams } from "../customParams";
+import { NO_CUSTOM_PARAMS } from "../customParams";
 import { fetchAccessToken } from "../fetchAccessToken";
 import type { TokenStoreArgs } from "../store";
 import { getToken, storeToken } from "../store";
@@ -109,6 +111,7 @@ export async function getClientCredentials(
     clientAssertionSecretBase64,
     clientCredentialsMethod,
     clientAssertionAlgorithm,
+    customParams = NO_CUSTOM_PARAMS,
   }: {
     accessTokenUrl: string;
     clientId: string;
@@ -120,6 +123,7 @@ export async function getClientCredentials(
     clientAssertionSecretBase64: boolean;
     clientCredentialsMethod: string;
     clientAssertionAlgorithm: string;
+    customParams?: CustomParams;
   },
 ) {
   const tokenArgs: TokenStoreArgs = {
@@ -143,6 +147,7 @@ export async function getClientCredentials(
     clientId,
     scope,
     params: [],
+    custom: customParams.token,
   };
 
   const fetchParams: Parameters<typeof fetchAccessToken>[1] =
diff --git a/plugins/auth-oauth2/src/grants/implicit.ts b/plugins/auth-oauth2/src/grants/implicit.ts
index d9b69766..ebbf2106 100644
--- a/plugins/auth-oauth2/src/grants/implicit.ts
+++ b/plugins/auth-oauth2/src/grants/implicit.ts
@@ -1,5 +1,7 @@
 import type { Context } from "@yaakapp/api";
 import { getRedirectUrlViaExternalBrowser } from "../callbackServer";
+import type { CustomParams } from "../customParams";
+import { applyQueryParams, NO_CUSTOM_PARAMS } from "../customParams";
 import type { AccessToken, AccessTokenRawResponse } from "../store";
 import { getDataDirKey, getToken, storeToken } from "../store";
 import { isTokenExpired } from "../util";
@@ -18,6 +20,7 @@ export async function getImplicit(
     audience,
     tokenName,
     externalBrowser,
+    customParams = NO_CUSTOM_PARAMS,
   }: {
     authorizationUrl: string;
     responseType: string;
@@ -28,6 +31,7 @@ export async function getImplicit(
     audience: string | null;
     tokenName: "access_token" | "id_token";
     externalBrowser?: ExternalBrowserOptions;
+    customParams?: CustomParams;
   },
 ): Promise<AccessToken> {
   const tokenArgs = {
@@ -59,6 +63,10 @@ export async function getImplicit(
     );
   }
 
+  // Applied before redirect_uri, which belongs to the callback flow rather than
+  // to the user: overriding it would send the token somewhere nothing listens
+  applyQueryParams(authorizationUrl, customParams.authorizationQuery);
+
   let newToken: AccessToken;
 
   // Use external browser flow if enabled
diff --git a/plugins/auth-oauth2/src/grants/password.ts b/plugins/auth-oauth2/src/grants/password.ts
index dbf55b31..0be38dc9 100644
--- a/plugins/auth-oauth2/src/grants/password.ts
+++ b/plugins/auth-oauth2/src/grants/password.ts
@@ -1,4 +1,6 @@
 import type { Context } from "@yaakapp/api";
+import type { CustomParams } from "../customParams";
+import { NO_CUSTOM_PARAMS } from "../customParams";
 import { fetchAccessToken } from "../fetchAccessToken";
 import { getOrRefreshAccessToken } from "../getOrRefreshAccessToken";
 import type { AccessToken, TokenStoreArgs } from "../store";
@@ -16,6 +18,7 @@ export async function getPassword(
     credentialsInBody,
     audience,
     scope,
+    customParams = NO_CUSTOM_PARAMS,
   }: {
     accessTokenUrl: string;
     clientId: string;
@@ -25,6 +28,7 @@ export async function getPassword(
     scope: string | null;
     audience: string | null;
     credentialsInBody: boolean;
+    customParams?: CustomParams;
   },
 ): Promise<AccessToken> {
   const tokenArgs: TokenStoreArgs = {
@@ -40,6 +44,7 @@ export async function getPassword(
     clientId,
     clientSecret,
     credentialsInBody,
+    custom: customParams.refresh,
   });
   if (token != null) {
     return token;
@@ -57,6 +62,7 @@ export async function getPassword(
       { name: "username", value: username },
       { name: "password", value: password },
     ],
+    custom: customParams.token,
   });
 
   return storeToken(ctx, tokenArgs, response);
diff --git a/plugins/auth-oauth2/src/index.ts b/plugins/auth-oauth2/src/index.ts
index 02113a18..5531e1fa 100644
--- a/plugins/auth-oauth2/src/index.ts
+++ b/plugins/auth-oauth2/src/index.ts
@@ -11,6 +11,7 @@ import {
   DEFAULT_LOCALHOST_PORT,
   stopActiveServer,
 } from "./callbackServer";
+import { readCustomParams } from "./customParams";
 import {
   type CallbackType,
   DEFAULT_PKCE_METHOD,
@@ -468,6 +469,48 @@ export const plugin: PluginDefinition = {
                 values.clientCredentialsMethod === "client_assertion",
             }),
           },
+          {
+            type: "key_value",
+            name: "authorizationParams",
+            label: "Authorization Params",
+            description: "Query params appended to the authorization URL.",
+            optional: true,
+            dynamic: hiddenIfNot(["authorization_code", "implicit"]),
+          },
+          {
+            type: "key_value",
+            name: "tokenHeaders",
+            label: "Token Headers",
+            description:
+              "Headers sent with the token request, and when refreshing unless overridden.",
+            optional: true,
+            dynamic: hiddenIfNot(["authorization_code", "password", "client_credentials"]),
+          },
+          {
+            type: "key_value",
+            name: "tokenBodyParams",
+            label: "Token Body Params",
+            description:
+              "Form params sent with the token request, and when refreshing unless overridden.",
+            optional: true,
+            dynamic: hiddenIfNot(["authorization_code", "password", "client_credentials"]),
+          },
+          {
+            type: "key_value",
+            name: "refreshHeaders",
+            label: "Refresh Headers",
+            description: "Headers sent only when refreshing the token.",
+            optional: true,
+            dynamic: hiddenIfNot(["authorization_code", "password"]),
+          },
+          {
+            type: "key_value",
+            name: "refreshBodyParams",
+            label: "Refresh Body Params",
+            description: "Form params sent only when refreshing the token.",
+            optional: true,
+            dynamic: hiddenIfNot(["authorization_code", "password"]),
+          },
         ],
       },
       {
@@ -506,6 +549,7 @@ export const plugin: PluginDefinition = {
       const headerPrefix = stringArg(values, "headerPrefix");
       const grantType = stringArg(values, "grantType") as GrantType;
       const credentialsInBody = values.credentials === "body";
+      const customParams = readCustomParams(values);
       const tokenName = values.tokenName === "id_token" ? "id_token" : "access_token";
 
       // Build external browser options if enabled
@@ -546,6 +590,7 @@ export const plugin: PluginDefinition = {
             : null,
           tokenName: tokenName,
           externalBrowser: externalBrowserOptions,
+          customParams,
         });
       } else if (grantType === "implicit") {
         const authorizationUrl = stringArg(values, "authorizationUrl");
@@ -561,6 +606,7 @@ export const plugin: PluginDefinition = {
           state: stringArgOrNull(values, "state"),
           tokenName: tokenName,
           externalBrowser: externalBrowserOptions,
+          customParams,
         });
       } else if (grantType === "client_credentials") {
         const accessTokenUrl = stringArg(values, "accessTokenUrl");
@@ -577,6 +623,7 @@ export const plugin: PluginDefinition = {
           scope: stringArgOrNull(values, "scope"),
           audience: stringArgOrNull(values, "audience"),
           credentialsInBody,
+          customParams,
         });
       } else if (grantType === "password") {
         const accessTokenUrl = stringArg(values, "accessTokenUrl");
@@ -591,6 +638,7 @@ export const plugin: PluginDefinition = {
           scope: stringArgOrNull(values, "scope"),
           audience: stringArgOrNull(values, "audience"),
           credentialsInBody,
+          customParams,
         });
       } else {
         throw new Error(`Invalid grant type ${String(grantType)}`);
diff --git a/plugins/auth-oauth2/tests/authorizationCode.test.ts b/plugins/auth-oauth2/tests/authorizationCode.test.ts
new file mode 100644
index 00000000..f167066b
--- /dev/null
+++ b/plugins/auth-oauth2/tests/authorizationCode.test.ts
@@ -0,0 +1,137 @@
+import type { HttpRequest } from "@yaakapp/api";
+import { describe, expect, test } from "vite-plus/test";
+import { readCustomParams } from "../src/customParams";
+import { getAuthorizationCode } from "../src/grants/authorizationCode";
+
+const REDIRECT_URI = "https://app.example.com/callback";
+
+/**
+ * Drives the embedded-browser flow: records the authorization URL the plugin
+ * opens, then navigates straight to the redirect with a code.
+ */
+function createMockContext() {
+  const opened: string[] = [];
+  const sent: Partial<HttpRequest>[] = [];
+  const values = new Map<string, unknown>();
+
+  const ctx = {
+    store: {
+      async set<T>(key: string, value: T) {
+        values.set(key, value);
+      },
+      async get<T>(key: string) {
+        return values.get(key) as T | undefined;
+      },
+    },
+    window: {
+      async openUrl({
+        url,
+        onNavigate,
+      }: {
+        url: string;
+        onNavigate: (e: { url: string }) => Promise<void>;
+      }) {
+        opened.push(url);
+        // Deferred because onNavigate closes the window it is handed back from
+        setTimeout(() => onNavigate({ url: `${REDIRECT_URI}?code=code-123` }), 0);
+        return { close() {} };
+      },
+    },
+    httpRequest: {
+      async send({ httpRequest }: { httpRequest: Partial<HttpRequest> }) {
+        sent.push(httpRequest);
+        return {
+          httpResponse: { status: 200, error: null },
+          body: {
+            async text() {
+              return JSON.stringify({ access_token: "token-123" });
+            },
+          },
+        };
+      },
+    },
+  } as never;
+
+  return { ctx, opened, sent };
+}
+
+const baseArgs = {
+  authorizationUrl: "https://auth.example.com/authorize",
+  accessTokenUrl: "https://auth.example.com/token",
+  clientId: "client-123",
+  clientSecret: "secret",
+  redirectUri: REDIRECT_URI,
+  scope: "openid",
+  state: null,
+  audience: null,
+  credentialsInBody: true,
+  pkce: null,
+  tokenName: "access_token" as const,
+};
+
+function pairs(...rows: { name: string; value: string; enabled?: boolean }[]) {
+  return JSON.stringify(rows.map((r) => ({ enabled: true, ...r })));
+}
+
+describe("authorization code custom parameters", () => {
+  test("puts custom authorization params on the authorize URL", async () => {
+    const { ctx, opened } = createMockContext();
+
+    await getAuthorizationCode(ctx, "request-1", {
+      ...baseArgs,
+      customParams: readCustomParams({
+        authorizationParams: pairs(
+          { name: "prompt", value: "consent" },
+          { name: "realm", value: "employees" },
+          { name: "skipped", value: "nope", enabled: false },
+        ),
+      }),
+    });
+
+    const url = new URL(opened[0]!);
+    expect(url.searchParams.get("prompt")).toBe("consent");
+    expect(url.searchParams.get("realm")).toBe("employees");
+    expect(url.searchParams.has("skipped")).toBe(false);
+    // Generated params are still there
+    expect(url.searchParams.get("response_type")).toBe("code");
+    expect(url.searchParams.get("client_id")).toBe("client-123");
+    expect(url.searchParams.get("scope")).toBe("openid");
+    expect(url.searchParams.get("redirect_uri")).toBe(REDIRECT_URI);
+  });
+
+  test("keeps the flow's redirect_uri when a custom param tries to change it", async () => {
+    const { ctx, opened } = createMockContext();
+
+    await getAuthorizationCode(ctx, "request-1", {
+      ...baseArgs,
+      customParams: readCustomParams({
+        authorizationParams: pairs({ name: "redirect_uri", value: "https://evil.example.com" }),
+      }),
+    });
+
+    expect(new URL(opened[0]!).searchParams.get("redirect_uri")).toBe(REDIRECT_URI);
+  });
+
+  test("sends custom token entries with the exchange that follows", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await getAuthorizationCode(ctx, "request-1", {
+      ...baseArgs,
+      customParams: readCustomParams({
+        tokenHeaders: pairs(
+          { name: "Origin", value: "https://app.example.com" },
+          { name: "X-Skipped", value: "nope", enabled: false },
+        ),
+        tokenBodyParams: pairs({ name: "realm", value: "employees" }),
+      }),
+    });
+
+    const headers = sent[0]!.headers ?? [];
+    expect(headers).toContainEqual({ name: "Origin", value: "https://app.example.com" });
+    expect(headers.map((h: { name: string }) => h.name)).not.toContain("X-Skipped");
+
+    const form = sent[0]!.body?.form ?? [];
+    expect(form).toContainEqual({ name: "realm", value: "employees" });
+    expect(form).toContainEqual({ name: "code", value: "code-123" });
+  });
+});
diff --git a/plugins/auth-oauth2/tests/customParams.test.ts b/plugins/auth-oauth2/tests/customParams.test.ts
new file mode 100644
index 00000000..ff0fbf36
--- /dev/null
+++ b/plugins/auth-oauth2/tests/customParams.test.ts
@@ -0,0 +1,164 @@
+import { describe, expect, test } from "vite-plus/test";
+import {
+  applyQueryParams,
+  mergeFormParams,
+  mergeHeaders,
+  parsePairs,
+  readCustomParams,
+} from "../src/customParams";
+
+function pairs(...rows: { name: string; value: string; enabled?: boolean }[]) {
+  return JSON.stringify(rows.map((r) => ({ enabled: true, ...r })));
+}
+
+describe("parsePairs", () => {
+  test("reads enabled rows", () => {
+    expect(parsePairs(pairs({ name: "Origin", value: "https://app.example.com" }))).toEqual([
+      { name: "Origin", value: "https://app.example.com" },
+    ]);
+  });
+
+  test("skips disabled rows", () => {
+    const value = pairs(
+      { name: "Origin", value: "https://app.example.com" },
+      { name: "X-Debug", value: "1", enabled: false },
+    );
+
+    expect(parsePairs(value).map((p) => p.name)).toEqual(["Origin"]);
+  });
+
+  test("skips rows without a name", () => {
+    expect(parsePairs(pairs({ name: "  ", value: "ignored" }))).toEqual([]);
+  });
+
+  test("trims names and keeps empty values", () => {
+    expect(parsePairs(pairs({ name: " realm ", value: "" }))).toEqual([
+      { name: "realm", value: "" },
+    ]);
+  });
+
+  test("returns nothing for missing or malformed input", () => {
+    expect(parsePairs(undefined)).toEqual([]);
+    expect(parsePairs("")).toEqual([]);
+    expect(parsePairs("not json")).toEqual([]);
+    expect(parsePairs('{"name":"Origin"}')).toEqual([]);
+  });
+});
+
+describe("merge precedence", () => {
+  const generated = [
+    { name: "User-Agent", value: "yaak" },
+    { name: "Content-Type", value: "application/x-www-form-urlencoded" },
+  ];
+
+  test("keeps generated entries that are not overridden", () => {
+    const merged = mergeHeaders(generated, [{ name: "Origin", value: "https://app.example.com" }]);
+
+    expect(merged).toEqual([...generated, { name: "Origin", value: "https://app.example.com" }]);
+  });
+
+  test("replaces generated headers by name, ignoring case", () => {
+    const merged = mergeHeaders(generated, [{ name: "content-type", value: "application/json" }]);
+
+    expect(merged).toEqual([
+      { name: "User-Agent", value: "yaak" },
+      { name: "content-type", value: "application/json" },
+    ]);
+  });
+
+  test("keeps repeated custom entries of the same name", () => {
+    const merged = mergeHeaders(generated, [
+      { name: "X-Trace", value: "a" },
+      { name: "X-Trace", value: "b" },
+    ]);
+
+    expect(merged.filter((h) => h.name === "X-Trace")).toHaveLength(2);
+  });
+
+  test("matches form params case-sensitively", () => {
+    const merged = mergeFormParams(
+      [{ name: "scope", value: "openid" }],
+      [{ name: "Scope", value: "other" }],
+    );
+
+    expect(merged).toEqual([
+      { name: "scope", value: "openid" },
+      { name: "Scope", value: "other" },
+    ]);
+  });
+});
+
+describe("applyQueryParams", () => {
+  test("appends custom params and overrides generated ones", () => {
+    const url = new URL("https://auth.example.com/authorize?client_id=abc&scope=openid");
+
+    applyQueryParams(url, [
+      { name: "scope", value: "openid email" },
+      { name: "realm", value: "employees" },
+    ]);
+
+    expect(url.searchParams.get("client_id")).toBe("abc");
+    expect(url.searchParams.getAll("scope")).toEqual(["openid email"]);
+    expect(url.searchParams.get("realm")).toBe("employees");
+  });
+
+  test("keeps repeated custom params", () => {
+    const url = new URL("https://auth.example.com/authorize");
+
+    applyQueryParams(url, [
+      { name: "resource", value: "one" },
+      { name: "resource", value: "two" },
+    ]);
+
+    expect(url.searchParams.getAll("resource")).toEqual(["one", "two"]);
+  });
+});
+
+describe("readCustomParams", () => {
+  test("sends token entries on the refresh request too", () => {
+    const custom = readCustomParams({
+      tokenHeaders: pairs({ name: "Origin", value: "https://app.example.com" }),
+      tokenBodyParams: pairs({ name: "realm", value: "employees" }),
+    });
+
+    expect(custom.refresh.headers).toEqual([{ name: "Origin", value: "https://app.example.com" }]);
+    expect(custom.refresh.body).toEqual([{ name: "realm", value: "employees" }]);
+  });
+
+  test("lets a refresh entry override the token entry of the same name", () => {
+    const custom = readCustomParams({
+      tokenHeaders: pairs({ name: "Origin", value: "https://app.example.com" }),
+      refreshHeaders: pairs({ name: "origin", value: "https://refresh.example.com" }),
+      tokenBodyParams: pairs({ name: "realm", value: "employees" }),
+      refreshBodyParams: pairs({ name: "realm", value: "service" }),
+    });
+
+    // The token request is unaffected by the refresh-only entries
+    expect(custom.token.headers).toEqual([{ name: "Origin", value: "https://app.example.com" }]);
+    expect(custom.token.body).toEqual([{ name: "realm", value: "employees" }]);
+
+    expect(custom.refresh.headers).toEqual([
+      { name: "origin", value: "https://refresh.example.com" },
+    ]);
+    expect(custom.refresh.body).toEqual([{ name: "realm", value: "service" }]);
+  });
+
+  test("reads authorization params separately", () => {
+    const custom = readCustomParams({
+      authorizationParams: pairs({ name: "prompt", value: "consent" }),
+    });
+
+    expect(custom.authorizationQuery).toEqual([{ name: "prompt", value: "consent" }]);
+    expect(custom.token.headers).toEqual([]);
+  });
+
+  test("is empty when nothing is configured", () => {
+    const custom = readCustomParams({ clientId: "abc" });
+
+    expect(custom).toEqual({
+      authorizationQuery: [],
+      token: { headers: [], body: [] },
+      refresh: { headers: [], body: [] },
+    });
+  });
+});
diff --git a/plugins/auth-oauth2/tests/fetchAccessToken.test.ts b/plugins/auth-oauth2/tests/fetchAccessToken.test.ts
index 5ee0206d..8f02d2b1 100644
--- a/plugins/auth-oauth2/tests/fetchAccessToken.test.ts
+++ b/plugins/auth-oauth2/tests/fetchAccessToken.test.ts
@@ -36,6 +36,12 @@ function formValue(httpRequest: Partial<HttpRequest>, name: string) {
   return (httpRequest.body?.form ?? []).find((p: { name: string }) => p.name === name)?.value;
 }
 
+function headerValue(httpRequest: Partial<HttpRequest>, name: string) {
+  return (httpRequest.headers ?? []).find(
+    (h: { name: string }) => h.name.toLowerCase() === name.toLowerCase(),
+  )?.value;
+}
+
 const baseArgs = {
   clientId: "client-123",
   accessTokenUrl: "https://auth.example.com/token",
@@ -91,3 +97,102 @@ describe("fetchAccessToken scope handling", () => {
     expect(formNames(sent[0]!)).not.toContain("scope");
   });
 });
+
+describe("fetchAccessToken custom parameters", () => {
+  test("sends a custom header with the token request", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await fetchAccessToken(ctx, {
+      ...baseArgs,
+      grantType: "authorization_code",
+      custom: {
+        headers: [{ name: "Origin", value: "https://app.example.com" }],
+        body: [],
+      },
+    });
+
+    expect(headerValue(sent[0]!, "Origin")).toBe("https://app.example.com");
+  });
+
+  test("keeps the generated headers that are not overridden", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await fetchAccessToken(ctx, {
+      ...baseArgs,
+      grantType: "client_credentials",
+      credentialsInBody: false,
+      custom: {
+        headers: [{ name: "Origin", value: "https://app.example.com" }],
+        body: [],
+      },
+    });
+
+    expect(headerValue(sent[0]!, "User-Agent")).toBe("yaak");
+    expect(headerValue(sent[0]!, "Content-Type")).toBe("application/x-www-form-urlencoded");
+    expect(headerValue(sent[0]!, "Accept")).toBe(
+      "application/x-www-form-urlencoded, application/json",
+    );
+    // Basic credentials still go out untouched
+    expect(headerValue(sent[0]!, "Authorization")).toMatch(/^Basic /);
+  });
+
+  test("replaces a generated header of the same name", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await fetchAccessToken(ctx, {
+      ...baseArgs,
+      grantType: "client_credentials",
+      credentialsInBody: false,
+      custom: {
+        headers: [
+          { name: "content-type", value: "application/json" },
+          { name: "Authorization", value: "Custom abc123" },
+        ],
+        body: [],
+      },
+    });
+
+    const contentTypes = (sent[0]!.headers ?? []).filter(
+      (h: { name: string }) => h.name.toLowerCase() === "content-type",
+    );
+    expect(contentTypes).toEqual([{ name: "content-type", value: "application/json" }]);
+    expect(headerValue(sent[0]!, "Authorization")).toBe("Custom abc123");
+  });
+
+  test("sends custom body params in the form-encoded body", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await fetchAccessToken(ctx, {
+      ...baseArgs,
+      grantType: "password",
+      params: [{ name: "username", value: "alice" }],
+      custom: {
+        headers: [],
+        body: [{ name: "realm", value: "employees" }],
+      },
+    });
+
+    expect(sent[0]!.bodyType).toBe("application/x-www-form-urlencoded");
+    expect(formValue(sent[0]!, "realm")).toBe("employees");
+    // Generated params survive alongside it
+    expect(formValue(sent[0]!, "grant_type")).toBe("password");
+    expect(formValue(sent[0]!, "username")).toBe("alice");
+    expect(formValue(sent[0]!, "client_id")).toBe("client-123");
+  });
+
+  test("replaces a generated body param of the same name", async () => {
+    const { ctx, sent } = createMockContext();
+
+    await fetchAccessToken(ctx, {
+      ...baseArgs,
+      grantType: "client_credentials",
+      custom: {
+        headers: [],
+        body: [{ name: "scope", value: "custom-scope" }],
+      },
+    });
+
+    expect(formNames(sent[0]!).filter((n: string) => n === "scope")).toHaveLength(1);
+    expect(formValue(sent[0]!, "scope")).toBe("custom-scope");
+  });
+});
diff --git a/plugins/auth-oauth2/tests/getOrRefreshAccessToken.test.ts b/plugins/auth-oauth2/tests/getOrRefreshAccessToken.test.ts
new file mode 100644
index 00000000..4ab82515
--- /dev/null
+++ b/plugins/auth-oauth2/tests/getOrRefreshAccessToken.test.ts
@@ -0,0 +1,141 @@
+import type { HttpRequest } from "@yaakapp/api";
+import { describe, expect, test } from "vite-plus/test";
+import { getOrRefreshAccessToken } from "../src/getOrRefreshAccessToken";
+import type { TokenStoreArgs } from "../src/store";
+import { storeToken } from "../src/store";
+
+/**
+ * Captures the refresh request handed to ctx.httpRequest.send and replies with
+ * a minimal successful token response.
+ */
+function createMockContext() {
+  const sent: Partial<HttpRequest>[] = [];
+  const values = new Map<string, unknown>();
+
+  const ctx = {
+    store: {
+      async set<T>(key: string, value: T) {
+        values.set(key, value);
+      },
+      async get<T>(key: string) {
+        return values.get(key) as T | undefined;
+      },
+      async delete(key: string) {
+        return values.delete(key);
+      },
+    },
+    httpRequest: {
+      async send({ httpRequest }: { httpRequest: Partial<HttpRequest> }) {
+        sent.push(httpRequest);
+        return {
+          httpResponse: { status: 200, error: null },
+          body: {
+            async text() {
+              return JSON.stringify({ access_token: "refreshed-token" });
+            },
+          },
+        };
+      },
+    },
+  } as never;
+
+  return { ctx, sent };
+}
+
+const tokenArgs: TokenStoreArgs = {
+  contextId: "request-1",
+  clientId: "client-123",
+  accessTokenUrl: "https://auth.example.com/token",
+  authorizationUrl: null,
+};
+
+const refreshArgs = {
+  accessTokenUrl: "https://auth.example.com/token",
+  scope: "openid",
+  clientId: "client-123",
+  clientSecret: "secret",
+  credentialsInBody: true,
+  forceRefresh: true,
+};
+
+function headerValue(httpRequest: Partial<HttpRequest>, name: string) {
+  return (httpRequest.headers ?? []).find(
+    (h: { name: string }) => h.name.toLowerCase() === name.toLowerCase(),
+  )?.value;
+}
+
+function formValue(httpRequest: Partial<HttpRequest>, name: string) {
+  return (httpRequest.body?.form ?? []).find((p: { name: string }) => p.name === name)?.value;
+}
+
+async function seedToken(ctx: never) {
+  await storeToken(ctx, tokenArgs, {
+    access_token: "old-token",
+    refresh_token: "refresh-123",
+  });
+}
+
+describe("getOrRefreshAccessToken custom parameters", () => {
+  test("sends custom headers and body params with the refresh request", async () => {
+    const { ctx, sent } = createMockContext();
+    await seedToken(ctx);
+
+    const token = await getOrRefreshAccessToken(ctx, tokenArgs, {
+      ...refreshArgs,
+      custom: {
+        headers: [{ name: "Origin", value: "https://app.example.com" }],
+        body: [{ name: "realm", value: "employees" }],
+      },
+    });
+
+    expect(token?.response.access_token).toBe("refreshed-token");
+    expect(headerValue(sent[0]!, "Origin")).toBe("https://app.example.com");
+    expect(formValue(sent[0]!, "realm")).toBe("employees");
+    // Generated entries survive
+    expect(headerValue(sent[0]!, "User-Agent")).toBe("yaak");
+    expect(formValue(sent[0]!, "grant_type")).toBe("refresh_token");
+    expect(formValue(sent[0]!, "refresh_token")).toBe("refresh-123");
+    expect(formValue(sent[0]!, "client_secret")).toBe("secret");
+  });
+
+  test("replaces generated entries of the same name", async () => {
+    const { ctx, sent } = createMockContext();
+    await seedToken(ctx);
+
+    await getOrRefreshAccessToken(ctx, tokenArgs, {
+      ...refreshArgs,
+      custom: {
+        headers: [{ name: "user-agent", value: "custom-agent" }],
+        body: [{ name: "scope", value: "custom-scope" }],
+      },
+    });
+
+    const userAgents = (sent[0]!.headers ?? []).filter(
+      (h: { name: string }) => h.name.toLowerCase() === "user-agent",
+    );
+    expect(userAgents).toEqual([{ name: "user-agent", value: "custom-agent" }]);
+
+    const scopes = (sent[0]!.body?.form ?? []).filter((p: { name: string }) => p.name === "scope");
+    expect(scopes).toEqual([{ name: "scope", value: "custom-scope" }]);
+  });
+
+  test("sends the generated request untouched with no custom parameters", async () => {
+    const { ctx, sent } = createMockContext();
+    await seedToken(ctx);
+
+    await getOrRefreshAccessToken(ctx, tokenArgs, refreshArgs);
+
+    expect((sent[0]!.headers ?? []).map((h: { name: string }) => h.name)).toEqual([
+      "User-Agent",
+      "Accept",
+      "Content-Type",
+    ]);
+    expect((sent[0]!.body?.form ?? []).map((p: { name: string }) => p.name)).toEqual([
+      "grant_type",
+      "refresh_token",
+      "scope",
+      "client_id",
+      "client_secret",
+    ]);
+  });
+});