diff --git a/README.md b/README.md
index 00a8879..efdf16a 100644
--- a/README.md
+++ b/README.md
@@ -97,17 +97,18 @@ Run `zerogpu <command> --help` for command-level help.
 
 #### `login`
 
-Sign in to ZeroGPU. Prompts for your API key (masked input) and project ID, validates them, and persists the credentials.
+Sign in to ZeroGPU. Prompts for your API key (masked input), validates it, and persists the credential. The project is derived from the API key; pass `--project-id` only if you want to pin a specific project.
 
 ```bash
 zerogpu login
+zerogpu login --api-key zgpu-api-xxxxxxxx
 zerogpu login --api-key zgpu-api-xxxxxxxx --project-id 4ed3e5bb-c2ed-4d4a-8a66-2b161a27fd1a
 ```
 
 | Option | Description |
 |---|---|
 | `--api-key <key>` | Provide the API key directly (skips the prompt). |
-| `--project-id <id>` | Provide the project ID directly (skips the prompt). |
+| `--project-id <id>` | _(optional)_ Pin a specific project ID; derived from the API key if omitted. |
 
 #### `status`
 
diff --git a/docs/ADDING_COMMANDS.md b/docs/ADDING_COMMANDS.md
index 29ce337..3677062 100644
--- a/docs/ADDING_COMMANDS.md
+++ b/docs/ADDING_COMMANDS.md
@@ -22,14 +22,19 @@ This guide explains how to add a new CLI command to the ZeroGPU CLI.
    - Add the import alongside the other `register...` imports.
    - Call `registerXxxCommand(program)` inside `buildProgram()`.
 
-3. **Auth-gated commands** should follow this pattern:
+3. **Auth-gated commands** should follow this pattern. Only the API key is
+   required; the project is derived from it. Send `x-project-id` only when a
+   project ID is configured:
    ```ts
    const apiKey = getApiKey();
-   const projectId = getProjectId();
-   if (!apiKey || !projectId) {
+   const projectId = getProjectId(); // optional
+   if (!apiKey) {
      console.error("You're not fully signed in yet. Run 'zerogpu login' ...");
      process.exit(1);
    }
+   // headers:
+   //   "x-api-key": apiKey.apiKey,
+   //   ...(projectId ? { "x-project-id": projectId.projectId } : {}),
    ```
 
 4. **Error handling**:
diff --git a/docs/DOCUMENTATION.md b/docs/DOCUMENTATION.md
index 4a95a75..a8e830a 100644
--- a/docs/DOCUMENTATION.md
+++ b/docs/DOCUMENTATION.md
@@ -75,10 +75,10 @@ Credentials are persisted to a local config file and `ZEROGPU_API_KEY` is added
 | Variable | Purpose |
 |---|---|
 | `ZEROGPU_API_KEY` | API key. Used as fallback if no config file is present. Written by `zerogpu login`. |
-| `ZEROGPU_PROJECT_ID` | Project ID. Used as fallback if no config file is present. |
+| `ZEROGPU_PROJECT_ID` | _(optional)_ Project ID. Used as fallback if no config file is present. The backend derives the project from the API key, so this is only needed to pin a specific project. |
 
 ### Resolution order
-For every request the CLI resolves credentials by checking the **config file first**, then the corresponding **environment variable**. If either credential is missing, the command exits with code `1` and prompts you to run `zerogpu login`.
+For every request the CLI resolves credentials by checking the **config file first**, then the corresponding **environment variable**. If the API key is missing, the command exits with code `1` and prompts you to run `zerogpu login`. The project ID is optional and sent (as `x-project-id`) only when present.
 
 ---
 
@@ -128,7 +128,7 @@ zerogpu login [--api-key <key>] [--project-id <id>]
 | Flag | Type | Required | Description |
 |---|---|---|---|
 | `--api-key <key>` | string | optional | Provide the API key non-interactively. Must start with `zgpu-api-`. If omitted, the CLI shows a masked prompt. |
-| `--project-id <id>` | string (UUID) | optional | Provide the Project ID non-interactively. Must be a valid UUID v4 string. If omitted, the CLI prompts plainly. |
+| `--project-id <id>` | string (UUID) | optional | Pin a specific Project ID. Must be a valid UUID v4 string. If omitted, the project is derived from the API key (the CLI does not prompt for it). |
 
 **Examples**
 ```bash
@@ -624,7 +624,7 @@ All inference commands POST to:
 POST https://api.zerogpu.ai/v1/responses
 Content-Type: application/json
 x-api-key:    <ZEROGPU_API_KEY>
-x-project-id: <ZEROGPU_PROJECT_ID>
+x-project-id: <ZEROGPU_PROJECT_ID>   # optional; sent only when a project is configured
 ```
 
 The request body is:
@@ -663,7 +663,7 @@ It picks the first `content` entry whose `type === "output_text"` (falling back
 
 | Symptom | Likely cause | Fix |
 |---|---|---|
-| `You're not fully signed in yet.` | Missing config + missing env var | Run `zerogpu login`, or export `ZEROGPU_API_KEY` and `ZEROGPU_PROJECT_ID`. |
+| `You're not fully signed in yet.` | Missing config + missing env var | Run `zerogpu login`, or export `ZEROGPU_API_KEY` (`ZEROGPU_PROJECT_ID` is optional). |
 | `That doesn't look like a valid API key` | Key doesn't start with `zgpu-api-` | Copy the full key from the ZeroGPU console. |
 | `Project ID must be a UUID.` | Pasted a name/slug | Use the UUID shown next to the project in the console. |
 | `Request failed with status 401` | Bad/revoked key or wrong project | Re-run `zerogpu login`. |
diff --git a/src/commands/chat.ts b/src/commands/chat.ts
index 2af1e70..d588812 100644
--- a/src/commands/chat.ts
+++ b/src/commands/chat.ts
@@ -21,9 +21,9 @@ export function registerChatCommand(program: Command): void {
         const apiKey = getApiKey();
         const projectId = getProjectId();
 
-        if (!apiKey || !projectId) {
+        if (!apiKey) {
           console.error(
-            "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+            "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
           );
           process.exit(1);
         }
@@ -41,7 +41,7 @@ export function registerChatCommand(program: Command): void {
             headers: {
               "content-type": "application/json",
               "x-api-key": apiKey.apiKey,
-              "x-project-id": projectId.projectId,
+              ...(projectId ? { "x-project-id": projectId.projectId } : {}),
             },
             body: JSON.stringify(body),
           });
diff --git a/src/commands/chatThinking.ts b/src/commands/chatThinking.ts
index b448d8b..d59d70c 100644
--- a/src/commands/chatThinking.ts
+++ b/src/commands/chatThinking.ts
@@ -19,9 +19,9 @@ export function registerChatThinkingCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -33,7 +33,7 @@ export function registerChatThinkingCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/classifyIab.ts b/src/commands/classifyIab.ts
index 19eaf17..12efcde 100644
--- a/src/commands/classifyIab.ts
+++ b/src/commands/classifyIab.ts
@@ -19,9 +19,9 @@ export function registerClassifyIabCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -33,7 +33,7 @@ export function registerClassifyIabCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/classifyIabEnriched.ts b/src/commands/classifyIabEnriched.ts
index 784b9a8..7c6fdfd 100644
--- a/src/commands/classifyIabEnriched.ts
+++ b/src/commands/classifyIabEnriched.ts
@@ -19,9 +19,9 @@ export function registerClassifyIabEnrichedCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -33,7 +33,7 @@ export function registerClassifyIabEnrichedCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/classifyStructured.ts b/src/commands/classifyStructured.ts
index 7649f2e..9755fd1 100644
--- a/src/commands/classifyStructured.ts
+++ b/src/commands/classifyStructured.ts
@@ -23,9 +23,9 @@ export function registerClassifyStructuredCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -46,7 +46,7 @@ export function registerClassifyStructuredCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/classifyZeroShot.ts b/src/commands/classifyZeroShot.ts
index 5175e8c..6ffcdce 100644
--- a/src/commands/classifyZeroShot.ts
+++ b/src/commands/classifyZeroShot.ts
@@ -37,9 +37,9 @@ export function registerClassifyZeroShotCommand(program: Command): void {
         const apiKey = getApiKey();
         const projectId = getProjectId();
 
-        if (!apiKey || !projectId) {
+        if (!apiKey) {
           console.error(
-            "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+            "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
           );
           process.exit(1);
         }
@@ -66,7 +66,7 @@ export function registerClassifyZeroShotCommand(program: Command): void {
             headers: {
               "content-type": "application/json",
               "x-api-key": apiKey.apiKey,
-              "x-project-id": projectId.projectId,
+              ...(projectId ? { "x-project-id": projectId.projectId } : {}),
             },
             body: JSON.stringify({
               model: MODEL,
diff --git a/src/commands/extractEntities.ts b/src/commands/extractEntities.ts
index a18b2f4..22e8035 100644
--- a/src/commands/extractEntities.ts
+++ b/src/commands/extractEntities.ts
@@ -43,9 +43,9 @@ export function registerExtractEntitiesCommand(program: Command): void {
         const apiKey = getApiKey();
         const projectId = getProjectId();
 
-        if (!apiKey || !projectId) {
+        if (!apiKey) {
           console.error(
-            "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+            "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
           );
           process.exit(1);
         }
@@ -76,7 +76,7 @@ export function registerExtractEntitiesCommand(program: Command): void {
             headers: {
               "content-type": "application/json",
               "x-api-key": apiKey.apiKey,
-              "x-project-id": projectId.projectId,
+              ...(projectId ? { "x-project-id": projectId.projectId } : {}),
             },
             body: JSON.stringify({
               model: MODEL,
diff --git a/src/commands/extractJson.ts b/src/commands/extractJson.ts
index 7e55885..6784f38 100644
--- a/src/commands/extractJson.ts
+++ b/src/commands/extractJson.ts
@@ -23,9 +23,9 @@ export function registerExtractJsonCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -46,7 +46,7 @@ export function registerExtractJsonCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/extractPii.ts b/src/commands/extractPii.ts
index ca90e75..9d3ba9e 100644
--- a/src/commands/extractPii.ts
+++ b/src/commands/extractPii.ts
@@ -35,9 +35,9 @@ export function registerExtractPiiCommand(program: Command): void {
         const apiKey = getApiKey();
         const projectId = getProjectId();
 
-        if (!apiKey || !projectId) {
+        if (!apiKey) {
           console.error(
-            "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+            "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
           );
           process.exit(1);
         }
@@ -59,7 +59,7 @@ export function registerExtractPiiCommand(program: Command): void {
             headers: {
               "content-type": "application/json",
               "x-api-key": apiKey.apiKey,
-              "x-project-id": projectId.projectId,
+              ...(projectId ? { "x-project-id": projectId.projectId } : {}),
             },
             body: JSON.stringify({
               model: MODEL,
diff --git a/src/commands/generateFollowups.ts b/src/commands/generateFollowups.ts
index d104847..84a68a9 100644
--- a/src/commands/generateFollowups.ts
+++ b/src/commands/generateFollowups.ts
@@ -19,9 +19,9 @@ export function registerGenerateFollowupsCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -33,7 +33,7 @@ export function registerGenerateFollowupsCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/login.ts b/src/commands/login.ts
index 6d56e31..ccde69f 100644
--- a/src/commands/login.ts
+++ b/src/commands/login.ts
@@ -2,7 +2,7 @@ import { spawn } from "node:child_process";
 import { Command } from "commander";
 import { validateApiKey, validateProjectId } from "../lib/auth.js";
 import { writeConfig, readConfig } from "../lib/config.js";
-import { promptMasked, promptPlain } from "../lib/prompt.js";
+import { promptMasked } from "../lib/prompt.js";
 import { upsertEnvExport } from "../lib/shellEnv.js";
 
 interface LoginOptions {
@@ -49,21 +49,21 @@ export function registerLoginCommand(program: Command): void {
     .option("--api-key <key>", "Provide your API key directly (skips the prompt).")
     .option(
       "--project-id <id>",
-      "Provide your ZeroGPU project ID directly (skips the prompt).",
+      "Optionally pin a ZeroGPU project ID. If omitted, the project is derived from your API key.",
     )
     .action(async (options: LoginOptions) => {
       let rawKey = options.apiKey;
-      let rawProjectId = options.projectId;
+      const rawProjectId = options.projectId;
 
-      if (!rawKey || !rawProjectId) {
+      if (!rawKey) {
         const opened = await openBrowser(DASHBOARD_URL);
         if (opened) {
           console.log(
-            `Opening ${DASHBOARD_URL} in your browser — grab your Project ID and API Key from there.`,
+            `Opening ${DASHBOARD_URL} in your browser — grab your API Key from there.`,
           );
         } else {
           console.log(
-            `Tip: open ${DASHBOARD_URL} in your browser to grab your Project ID and API Key.`,
+            `Tip: open ${DASHBOARD_URL} in your browser to grab your API Key.`,
           );
         }
       }
@@ -84,32 +84,34 @@ export function registerLoginCommand(program: Command): void {
         process.exit(1);
       }
 
-      if (!rawProjectId) {
-        try {
-          rawProjectId = await promptPlain("Please enter your ZeroGPU project ID: ");
-        } catch {
-          console.error("Login cancelled. No changes were made.");
+      // Project ID is optional — the backend derives the project from the API key.
+      // Validate and store it only when the caller explicitly provided one.
+      let projectId: string | undefined;
+      if (rawProjectId) {
+        const projectResult = validateProjectId(rawProjectId);
+        if (!projectResult.ok) {
+          console.error(
+            `That doesn't look like a valid project ID — ${projectResult.reason}`,
+          );
+          console.error(
+            "It should be a UUID like 4ed3e5bb-c2ed-4d4a-8a66-2b161a27fd1a. Please try again.",
+          );
           process.exit(1);
         }
-      }
-
-      const projectResult = validateProjectId(rawProjectId ?? "");
-      if (!projectResult.ok) {
-        console.error(
-          `That doesn't look like a valid project ID — ${projectResult.reason}`,
-        );
-        console.error(
-          "It should be a UUID like 4ed3e5bb-c2ed-4d4a-8a66-2b161a27fd1a. Please try again.",
-        );
-        process.exit(1);
+        projectId = projectResult.key;
       }
 
       const existing = readConfig();
-      writeConfig({
-        ...existing,
-        apiKey: result.key,
-        projectId: projectResult.key,
-      });
+      // Re-login clears any previously pinned project ID; it is restored only
+      // when --project-id is passed this run, so login without it returns to the
+      // unpinned (derive-from-key) state.
+      const next = { ...existing, apiKey: result.key };
+      if (projectId) {
+        next.projectId = projectId;
+      } else {
+        delete next.projectId;
+      }
+      writeConfig(next);
 
       const env = upsertEnvExport("ZEROGPU_API_KEY", result.key);
       process.env["ZEROGPU_API_KEY"] = result.key;
diff --git a/src/commands/redactPii.ts b/src/commands/redactPii.ts
index ab5e8ea..a7e0062 100644
--- a/src/commands/redactPii.ts
+++ b/src/commands/redactPii.ts
@@ -19,9 +19,9 @@ export function registerRedactPiiCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -33,7 +33,7 @@ export function registerRedactPiiCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,
diff --git a/src/commands/summarize.ts b/src/commands/summarize.ts
index 50ea64d..93f85b9 100644
--- a/src/commands/summarize.ts
+++ b/src/commands/summarize.ts
@@ -16,9 +16,9 @@ export function registerSummarizeCommand(program: Command): void {
       const apiKey = getApiKey();
       const projectId = getProjectId();
 
-      if (!apiKey || !projectId) {
+      if (!apiKey) {
         console.error(
-          "You're not fully signed in yet. Run 'zerogpu login' to set your API key and project ID.",
+          "You're not fully signed in yet. Run 'zerogpu login' to set your API key.",
         );
         process.exit(1);
       }
@@ -30,7 +30,7 @@ export function registerSummarizeCommand(program: Command): void {
           headers: {
             "content-type": "application/json",
             "x-api-key": apiKey.apiKey,
-            "x-project-id": projectId.projectId,
+            ...(projectId ? { "x-project-id": projectId.projectId } : {}),
           },
           body: JSON.stringify({
             model: MODEL,