@@ -43,7 +43,7 @@ import {
   record,
   string,
 } from "valibot";
-import { ApiError } from "../errors.js";
+import { ApiError, projectsNotFoundError } from "../errors.js";
 import { logger } from "../logger.js";
 import { getSdkConfig } from "../sentry-client.js";
 import { getOrgSdkConfig, unwrapValidatedResult } from "./infrastructure.js";
@@ -656,11 +656,32 @@ export async function assembleArtifactBundle(
     path: { organization_id_or_slug: org },
     body,
   });
-  return unwrapValidatedResult(
-    result,
-    "Failed to assemble artifact bundle",
-    AssembleResponseSchema,
-    `organizations/${org}/artifactbundle/assemble/`,
+  try {
+    return unwrapValidatedResult(
+      result,
+      "Failed to assemble artifact bundle",
+      AssembleResponseSchema,
+      `organizations/${org}/artifactbundle/assemble/`,
+    );
+  } catch (error) {
+    const projects = body.projects ?? [];
+    if (projects.length > 0 && isInvalidProjectsError(error)) {
+      throw projectsNotFoundError(org, projects);
+    }
+    throw error;
+  }
+}
+
+/**
+ * The assemble endpoint's 400 when a project slug or ID is not an active
+ * project of the org (`OrganizationArtifactBundleAssembleEndpoint` in Sentry,
+ * CLI-344).
+ */
+function isInvalidProjectsError(error: unknown): boolean {
+  return (
+    error instanceof ApiError &&
+    error.status === 400 &&
+    (error.detail?.includes("One or more projects are invalid") ?? false)
   );
 }
 
 
@@ -28,6 +28,7 @@ import { getEnv } from "../env.js";
 import {
   ApiError,
   buildValidationMessage,
+  projectsNotFoundError,
   ValidationError,
   validationError,
 } from "../errors.js";
@@ -268,7 +269,68 @@ export async function createRelease(
   if (result.data) {
     return result.data as SentryRelease;
   }
-  return unwrapResult<SentryRelease>(result, "Failed to create release");
+  try {
+    return unwrapResult<SentryRelease>(result, "Failed to create release");
+  } catch (error) {
+    throw explainCreateReleaseError(orgSlug, body, error);
+  }
+}
+
+/**
+ * 400 details from `OrganizationReleasesEndpoint.post` in Sentry:
+ * `{"projects":["Invalid project ids or slugs"]}` when a project does not
+ * exist or the auth token cannot access it, and
+ * `{"projects":["You do not have permission to one of the projects: a, b"]}`
+ * when the release already exists for projects the token cannot access.
+ */
+const INVALID_RELEASE_PROJECTS = "Invalid project ids or slugs";
+const RELEASE_PROJECT_PERMISSION_RE =
+  /You do not have permission to one of the projects: ([^"\]]*)/;
+
+/**
+ * The release serializer's 400 for a version it rejects, such as `latest` or
+ * a name with a `/` (`Release.is_valid_version`) or one over 200 characters.
+ */
+const RELEASE_VERSION_ERROR_RE = /"version":\[/;
+
+/**
+ * Turn the server's 400s for a project or version the user passed to
+ * {@link createRelease} into errors that name the value and how to fix it
+ * (CLI-2G3). Any other error, including other 400s, is returned unchanged.
+ */
+function explainCreateReleaseError(
+  orgSlug: string,
+  body: { version: string; projects?: string[] },
+  error: unknown,
+): unknown {
+  if (!(error instanceof ApiError && error.status === 400 && error.detail)) {
+    return error;
+  }
+  const { detail } = error;
+  const projects = body.projects ?? [];
+  if (detail.includes(INVALID_RELEASE_PROJECTS) && projects.length > 0) {
+    return projectsNotFoundError(orgSlug, projects, [
+      "Check that your auth token can access the project",
+    ]);
+  }
+  const denied = detail.match(RELEASE_PROJECT_PERMISSION_RE)?.[1];
+  if (denied !== undefined) {
+    return validationError(
+      `Release '${body.version}' already exists in organization '${orgSlug}' for projects your auth token cannot access: ${denied}.`,
+      [],
+      "release.projects",
+      "Use an auth token that can access those projects, or a different release version.",
+    );
+  }
+  if (RELEASE_VERSION_ERROR_RE.test(detail)) {
+    return validationError(
+      `Sentry does not allow the release version '${body.version}'.`,
+      [sentry`release propose-version`],
+      "release.version",
+      "A release version cannot be 'latest', '.' or '..', contain a slash, backslash, tab or newline, or be longer than 200 characters.",
+    );
+  }
+  return error;
 }
 
 /**
 
@@ -149,6 +149,17 @@ export function classifySilenced(error: unknown): SilenceReason | null {
   ) {
     return "user_validation";
   }
+  // A ValidationError with field "release.version" or "release.projects"
+  // means Sentry rejected the version the user passed to `release create`
+  // (`latest`, a name with a `/`), or the release already exists for projects
+  // the auth token cannot access (CLI-2G3). The user sees which value to
+  // change; the metric keeps the volume, split by `validation_field`.
+  if (
+    error instanceof ValidationError &&
+    (error.field === "release.version" || error.field === "release.projects")
+  ) {
+    return "user_validation";
+  }
   // A ValidationError with field "input" means the --input file path the user
   // supplied does not exist on disk. Pure user-input noise, not a CLI bug —
   // the user sees a clear "File not found" message (CLI-1JY).
@@ -188,10 +199,11 @@ export function classifySilenced(error: unknown): SilenceReason | null {
     return "process_exit";
   }
   // A 400 (Bad Request) signals a malformed request the CLI built — a code
-  // defect — so it is always captured. A user's unparseable `--query` is NOT a
-  // 400 here: it is converted to a ValidationError at the command boundary
-  // (toSearchQueryError) so the user gets an actionable message and the bug
-  // signal for CLI-authored bad queries is preserved (CLI-FA).
+  // defect — so it is always captured. 400s caused by user input are
+  // converted to CLI errors where the request is made, so the user gets an
+  // actionable message and the bug signal for the rest is preserved: an
+  // unparseable `--query` (toSearchQueryError, CLI-FA), or a project slug or
+  // release version Sentry rejects (CLI-2G3, CLI-344).
   return null;
 }
 
 
@@ -575,6 +575,33 @@ export class ResolutionError extends CliError {
   }
 }
 
+/**
+ * {@link ResolutionError} for project slugs that Sentry rejected for an org
+ * without saying which one (release create, artifact bundle assemble).
+ *
+ * @param org - Organization slug
+ * @param projects - The project slugs that were sent
+ * @param extraSuggestions - More bullets shown under "Or:"
+ */
+export function projectsNotFoundError(
+  org: string,
+  projects: string[],
+  extraSuggestions: string[] = [],
+): ResolutionError {
+  const quoted = projects.map((p) => `'${p}'`).join(", ");
+  return new ResolutionError(
+    projects.length === 1
+      ? `Project ${quoted}`
+      : `One of the projects ${quoted}`,
+    `was not found in organization '${org}'`,
+    sentry`project list ${org}/`,
+    [
+      "Check the slug passed with --project (or the bundler plugin's `project` option)",
+      ...extraSuggestions,
+    ],
+  );
+}
+
 /**
  * Build a {@link ValidationError} message with Try: examples and optional Note: section.
  *
 
@@ -20,7 +20,7 @@ import {
 } from "../../../src/lib/api/chunk-upload.js";
 import { setAuthToken } from "../../../src/lib/db/auth.js";
 import { setOrgRegion } from "../../../src/lib/db/regions.js";
-import { ApiError } from "../../../src/lib/errors.js";
+import { ApiError, ResolutionError } from "../../../src/lib/errors.js";
 import { mockFetch, useTestConfigDir } from "../../helpers.js";
 
 useTestConfigDir("api-chunk-upload-");
@@ -155,6 +155,38 @@ describe("assemble requests", () => {
     ]);
   });
 
+  test("assembleArtifactBundle names the project when the org has no such project (CLI-344)", async () => {
+    respondWith({ error: "One or more projects are invalid" }, 400);
+
+    const error = await assembleArtifactBundle("my-org", {
+      checksum: "abc",
+      chunks: ["c1"],
+      projects: ["my-projct"],
+    }).catch((e: unknown) => e);
+
+    expect(error).toBeInstanceOf(ResolutionError);
+    const { message } = error as ResolutionError;
+    expect(message).toContain(
+      "Project 'my-projct' was not found in organization 'my-org'.",
+    );
+    expect(message).toContain("sentry project list my-org/");
+  });
+
+  test("assembleArtifactBundle keeps other 400s as ApiError", async () => {
+    respondWith(
+      { error: "You need to specify a release together with a dist" },
+      400,
+    );
+
+    await expect(
+      assembleArtifactBundle("my-org", {
+        checksum: "abc",
+        chunks: ["c1"],
+        projects: ["my-project"],
+      }),
+    ).rejects.toBeInstanceOf(ApiError);
+  });
+
   test("assembleArtifactBundle rejects an unknown assemble state", async () => {
     respondWith({ state: "bogus", missingChunks: [] });
 
 
@@ -60,7 +60,13 @@ import { CONFIG_DIR_ENV_VAR } from "../../../src/lib/db/index.js";
 import { setOrgRegion } from "../../../src/lib/db/regions.js";
 import { withEnv } from "../../../src/lib/env.js";
 import { classifySilenced } from "../../../src/lib/error-reporting.js";
-import { ApiError, EXIT, isUserError } from "../../../src/lib/errors.js";
+import {
+  ApiError,
+  EXIT,
+  isUserError,
+  ResolutionError,
+  ValidationError,
+} from "../../../src/lib/errors.js";
 import type { SentryDeploy, SentryRelease } from "../../../src/types/index.js";
 import { mockFetch, useTestConfigDir } from "../../helpers.js";
 
@@ -174,6 +180,83 @@ describe("createRelease", () => {
     expect(release.version).toBe("1.0.0");
     expect(release.projects).toHaveLength(1);
   });
+
+  /** Answer the create request with a 400 and `body`. */
+  const reject400 = (body: unknown) => {
+    globalThis.fetch = mockFetch(
+      async () =>
+        new Response(JSON.stringify(body), {
+          status: 400,
+          headers: { "Content-Type": "application/json" },
+        }),
+    );
+  };
+
+  test("names the projects Sentry rejected (CLI-2G3)", async () => {
+    reject400({ projects: ["Invalid project ids or slugs"] });
+
+    const error = await createRelease("test-org", {
+      version: "1.0.0",
+      projects: ["web", "api"],
+    }).catch((e: unknown) => e);
+
+    expect(error).toBeInstanceOf(ResolutionError);
+    const { message } = error as ResolutionError;
+    expect(message).toContain(
+      "One of the projects 'web', 'api' was not found in organization 'test-org'.",
+    );
+    expect(message).toContain("sentry project list test-org/");
+    expect(classifySilenced(error)).toBe("user_input_error");
+  });
+
+  test("explains a release that exists for projects the token cannot access", async () => {
+    reject400({
+      projects: ["You do not have permission to one of the projects: web"],
+    });
+
+    const error = await createRelease("test-org", {
+      version: "1.0.0",
+      projects: ["api"],
+    }).catch((e: unknown) => e);
+
+    expect(error).toMatchObject({
+      name: "ValidationError",
+      field: "release.projects",
+    });
+    expect((error as ValidationError).message).toContain(
+      "Release '1.0.0' already exists in organization 'test-org' for projects your auth token cannot access: web.",
+    );
+    expect(classifySilenced(error)).toBe("user_validation");
+  });
+
+  test("names a release version Sentry rejects", async () => {
+    reject400({ version: ["Release with name latest is not allowed"] });
+
+    const error = await createRelease("test-org", {
+      version: "latest",
+      projects: ["web"],
+    }).catch((e: unknown) => e);
+
+    expect(error).toMatchObject({
+      name: "ValidationError",
+      field: "release.version",
+    });
+    expect((error as ValidationError).message).toContain(
+      "Sentry does not allow the release version 'latest'.",
+    );
+    expect(classifySilenced(error)).toBe("user_validation");
+  });
+
+  test("keeps other 400s as ApiError so they are captured", async () => {
+    reject400({ projects: ["This field is required."] });
+
+    const error = await createRelease("test-org", { version: "1.0.0" }).catch(
+      (e: unknown) => e,
+    );
+
+    expect(error).toBeInstanceOf(ApiError);
+    expect(classifySilenced(error)).toBeNull();
+  });
 });
 
 // =============================================================================
 
@@ -387,6 +387,15 @@ describe("classifySilenced", () => {
     },
   );
 
+  test.each(["release.version", "release.projects"])(
+    "silences release create ValidationError with field '%s' (CLI-2G3)",
+    (field) => {
+      expect(classifySilenced(new ValidationError("rejected", field))).toBe(
+        "user_validation",
+      );
+    },
+  );
+
   test("silences ResolutionError (user provided a value that wasn't found)", () => {
     expect(
       classifySilenced(