diff --git a/src/errors.ts b/src/errors.ts
index 0e2f32d..b67cc54 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -21,7 +21,10 @@ export class HaloPsaError extends Error {
 }
 
 /**
- * Authentication error (400 bad credentials, 401 unauthorized)
+ * Authentication error (401 unauthorized; also 400 from the OAuth token
+ * endpoint itself, thrown directly by AuthManager — never by HttpClient,
+ * since resource requests never carry skipAuth and so never reach
+ * HttpClient's own 400 branch, see HaloPsaBadRequestError)
  */
 export class HaloPsaAuthenticationError extends HaloPsaError {
   constructor(message: string, statusCode: number = 401, response?: unknown) {
@@ -68,6 +71,22 @@ export class HaloPsaValidationError extends HaloPsaError {
   }
 }
 
+/**
+ * Bad request error (400 from a resource endpoint that isn't a recognized
+ * validation-error shape — a malformed or incomplete request payload, not
+ * a credentials problem. Resource requests are always authenticated via a
+ * Bearer token by the time they reach here, so a plain 400 here can't be
+ * "bad credentials" the way it legitimately can be on the OAuth token
+ * endpoint itself; see HaloPsaAuthenticationError)
+ */
+export class HaloPsaBadRequestError extends HaloPsaError {
+  constructor(message: string, response?: unknown) {
+    super(message, 400, response);
+    this.name = 'HaloPsaBadRequestError';
+    Object.setPrototypeOf(this, HaloPsaBadRequestError.prototype);
+  }
+}
+
 /**
  * Rate limit exceeded error (429)
  */
diff --git a/src/http.ts b/src/http.ts
index b078481..13a0b97 100644
--- a/src/http.ts
+++ b/src/http.ts
@@ -8,6 +8,7 @@ import type { RateLimiter } from './rate-limiter.js';
 import {
   HaloPsaError,
   HaloPsaAuthenticationError,
+  HaloPsaBadRequestError,
   HaloPsaForbiddenError,
   HaloPsaNotFoundError,
   HaloPsaValidationError,
@@ -163,14 +164,20 @@ export class HttpClient {
 
     switch (response.status) {
       case 400:
-        // Could be bad credentials on token request or validation error
+        // Every request that reaches here is a resource call (skipAuth is
+        // never true outside this file — the OAuth token endpoint is
+        // fetched directly by AuthManager, not through HttpClient), so a
+        // 400 here is never a credentials problem: the Bearer token, if
+        // wrong, fails as a 401 below, not a 400. This is the request body
+        // itself being rejected — a missing/invalid field HaloPSA's
+        // server-side validation requires but this SDK doesn't mark
+        // `required` (e.g. Actions' `outcome`), or similar.
         if (this.isValidationError(responseBody)) {
           const errors = this.parseValidationErrors(responseBody);
           throw new HaloPsaValidationError('Validation error', errors, responseBody);
         }
-        throw new HaloPsaAuthenticationError(
-          'Bad request - invalid credentials or parameters',
-          400,
+        throw new HaloPsaBadRequestError(
+          `Bad request (400): ${method} ${url} rejected the request parameters`,
           responseBody
         );
 
diff --git a/src/index.ts b/src/index.ts
index 30f1a6f..324aa7c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -14,6 +14,7 @@ export { DEFAULT_RATE_LIMIT_CONFIG } from './config.js';
 export {
   HaloPsaError,
   HaloPsaAuthenticationError,
+  HaloPsaBadRequestError,
   HaloPsaForbiddenError,
   HaloPsaNotFoundError,
   HaloPsaValidationError,
diff --git a/tests/unit/http.test.ts b/tests/unit/http.test.ts
index 856d5e1..fd9dfa7 100644
--- a/tests/unit/http.test.ts
+++ b/tests/unit/http.test.ts
@@ -14,8 +14,11 @@ import { AuthManager } from '../../src/auth.js';
 import { RateLimiter } from '../../src/rate-limiter.js';
 import {
   HaloPsaError,
+  HaloPsaAuthenticationError,
+  HaloPsaBadRequestError,
   HaloPsaNotFoundError,
   HaloPsaServerError,
+  HaloPsaValidationError,
 } from '../../src/errors.js';
 import type { ResolvedConfig } from '../../src/config.js';
 
@@ -124,6 +127,37 @@ describe('HttpClient response handling', () => {
     expect((err as HaloPsaServerError).response).toEqual({ message: 'boom' });
   }, 15000);
 
+  it('a non-validation-shaped 400 raises HaloPsaBadRequestError, not an auth error', async () => {
+    // Regression: node-halopsa#78 — a 400 from a resource endpoint (e.g. a
+    // required field like Actions' `outcome` missing) used to throw
+    // HaloPsaAuthenticationError with a "invalid credentials or parameters"
+    // message, which read as a permissions/credentials problem to callers
+    // even though the Bearer token was never in question — a bad token
+    // fails as 401, not 400. This body has neither `errors` nor
+    // `validation_errors`, so it isn't the recognized validation shape.
+    vi.mocked(fetch).mockResolvedValue(
+      realResponse('{"message":"outcome is required"}', { status: 400 })
+    );
+    const err = await makeClient()
+      .request('/Actions', { method: 'POST', body: [{ ticket_id: 1, note: 'hi' }] })
+      .catch((e: unknown) => e);
+    expect(err).toBeInstanceOf(HaloPsaBadRequestError);
+    expect(err).not.toBeInstanceOf(HaloPsaAuthenticationError);
+    expect((err as HaloPsaBadRequestError).message).not.toMatch(/credentials/i);
+    expect((err as HaloPsaBadRequestError).response).toEqual({ message: 'outcome is required' });
+  });
+
+  it('a validation-shaped 400 still raises HaloPsaValidationError', async () => {
+    vi.mocked(fetch).mockResolvedValue(
+      realResponse('{"errors":[{"field":"outcome","message":"is required"}]}', { status: 400 })
+    );
+    const err = await makeClient()
+      .request('/Actions', { method: 'POST', body: [{ ticket_id: 1, note: 'hi' }] })
+      .catch((e: unknown) => e);
+    expect(err).toBeInstanceOf(HaloPsaValidationError);
+    expect((err as HaloPsaValidationError).errors).toEqual([{ field: 'outcome', message: 'is required' }]);
+  });
+
   it('generic non-2xx statuses raise HaloPsaError with the raw body', async () => {
     vi.mocked(fetch).mockResolvedValue(
       realResponse('teapot', { status: 418, headers: { 'content-type': 'text/plain' } })