diff --git a/comfy_execution/jobs.py b/comfy_execution/jobs.py
index fcd7ef735..3fbcc3eb0 100644
--- a/comfy_execution/jobs.py
+++ b/comfy_execution/jobs.py
@@ -3,6 +3,7 @@ Job utilities for the /api/jobs endpoint.
 Provides normalization and helper functions for job status tracking.
 """
 
+import uuid
 from typing import Optional
 
 from comfy_api.internal import prune_dict
@@ -19,6 +20,26 @@ class JobStatus:
     ALL = [PENDING, IN_PROGRESS, COMPLETED, FAILED, CANCELLED]
 
 
+def validate_job_id(value) -> str:
+    """Validate a client-supplied job (prompt) id.
+
+    Job ids must be UUIDs in the canonical lowercase hyphenated form. The id
+    is stored and compared verbatim everywhere downstream — history keys,
+    websocket events, /interrupt matching, and the assets ``job_ids`` filter
+    (a String(36) column matched exactly) — so accepting another spelling
+    would either rewrite the client's id behind its back or mint a job whose
+    outputs the filter can never find. Rejecting loudly beats both.
+
+    Returns the id unchanged. Raises ValueError when the value is not a
+    string in canonical UUID form.
+    """
+    if not isinstance(value, str):
+        raise ValueError(f"job id must be a string, got {type(value).__name__}")
+    if str(uuid.UUID(value)) != value:
+        raise ValueError("job id must be a UUID in canonical lowercase hyphenated form")
+    return value
+
+
 # Media types that can be previewed in the frontend
 PREVIEWABLE_MEDIA_TYPES = frozenset({'images', 'video', 'audio', '3d', 'text'})
 
diff --git a/openapi.yaml b/openapi.yaml
index c27ed7adf..58614103a 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -896,6 +896,11 @@ components:
                     additionalProperties: true
                     description: The workflow graph to execute
                     type: object
+                prompt_id:
+                    description: Optional client-supplied job id. Must be a UUID in canonical lowercase hyphenated form; it is echoed back in the response. Omitted or null means the server generates one.
+                    format: uuid
+                    nullable: true
+                    type: string
                 workflow_id:
                     description: UUID identifying the cloud workflow entity to associate with this job
                     type: string
diff --git a/server.py b/server.py
index a85c1e591..cc3b33a5c 100644
--- a/server.py
+++ b/server.py
@@ -8,7 +8,7 @@ import time
 import nodes
 import folder_paths
 import execution
-from comfy_execution.jobs import JobStatus, get_job, get_all_jobs
+from comfy_execution.jobs import JobStatus, get_job, get_all_jobs, validate_job_id
 import uuid
 import urllib
 import json
@@ -942,7 +942,21 @@ class PromptServer():
 
             if "prompt" in json_data:
                 prompt = json_data["prompt"]
-                prompt_id = str(json_data.get("prompt_id", uuid.uuid4()))
+                client_prompt_id = json_data.get("prompt_id")
+                if client_prompt_id is None:
+                    # Absent or explicit null: the server mints the id.
+                    prompt_id = str(uuid.uuid4())
+                else:
+                    try:
+                        prompt_id = validate_job_id(client_prompt_id)
+                    except ValueError:
+                        error = {
+                            "type": "invalid_prompt_id",
+                            "message": "prompt_id must be a valid UUID",
+                            "details": "prompt_id must be a UUID string in canonical lowercase hyphenated form; omit it to let the server generate one",
+                            "extra_info": {}
+                        }
+                        return web.json_response({"error": error, "node_errors": {}}, status=400)
 
                 partial_execution_targets = None
                 if "partial_execution_targets" in json_data:
diff --git a/tests-unit/assets_test/test_prompt_id_enforcement.py b/tests-unit/assets_test/test_prompt_id_enforcement.py
new file mode 100644
index 000000000..fb961beae
--- /dev/null
+++ b/tests-unit/assets_test/test_prompt_id_enforcement.py
@@ -0,0 +1,69 @@
+"""POST /prompt enforces canonical-UUID job ids at creation time.
+
+Lives in assets_test because it uses this suite's booted-server fixture and
+because the invariant exists for the assets pipeline: the GET /api/assets
+``job_ids`` filter matches stored job ids exactly, so a job minted with a
+non-canonical id would produce assets the filter can never find.
+
+The prompt bodies here are intentionally invalid workflows — prompt_id
+validation happens before workflow validation, so a rejected id returns
+``invalid_prompt_id`` while an accepted id falls through to the ordinary
+workflow-validation error (proving it cleared the id check).
+"""
+import requests
+
+
+def _post_prompt(http: requests.Session, api_base: str, body: dict) -> requests.Response:
+    return http.post(api_base + "/prompt", json=body, timeout=30)
+
+
+def _error_type(r: requests.Response) -> str:
+    return r.json()["error"]["type"]
+
+
+def test_non_uuid_prompt_id_rejected(http: requests.Session, api_base: str):
+    r = _post_prompt(http, api_base, {"prompt": {}, "prompt_id": "not-a-uuid"})
+    assert r.status_code == 400, r.text
+    assert _error_type(r) == "invalid_prompt_id"
+
+
+def test_non_string_prompt_id_rejected(http: requests.Session, api_base: str):
+    # Previously str()-coerced (123 became the job id "123"); must now be a 400,
+    # not a 500 from uuid.UUID choking on a non-string.
+    r = _post_prompt(http, api_base, {"prompt": {}, "prompt_id": 123})
+    assert r.status_code == 400, r.text
+    assert _error_type(r) == "invalid_prompt_id"
+
+
+def test_non_canonical_uuid_rejected(http: requests.Session, api_base: str):
+    # Parseable as a UUID, but not the canonical lowercase form: rejected
+    # loudly rather than silently rewritten (downstream lookups match the
+    # stored id exactly).
+    r = _post_prompt(
+        http,
+        api_base,
+        {"prompt": {}, "prompt_id": "AAAAAAAA-BBBB-4CCC-8DDD-EEEEEEEEEEEE"},
+    )
+    assert r.status_code == 400, r.text
+    assert _error_type(r) == "invalid_prompt_id"
+
+
+def test_canonical_uuid_accepted(http: requests.Session, api_base: str):
+    # The id clears validation; the empty workflow then fails ordinary prompt
+    # validation, proving the request got past the id check.
+    r = _post_prompt(
+        http,
+        api_base,
+        {"prompt": {}, "prompt_id": "aaaaaaaa-bbbb-4ccc-8ddd-eeeeeeeeeeee"},
+    )
+    assert r.status_code == 400, r.text
+    assert _error_type(r) != "invalid_prompt_id"
+
+
+def test_null_prompt_id_not_rejected(http: requests.Session, api_base: str):
+    # Explicit null means "server generates" and must not be rejected as an
+    # invalid id. (The minted id itself is not observable here because the
+    # workflow is invalid; unit tests cover validate_job_id directly.)
+    r = _post_prompt(http, api_base, {"prompt": {}, "prompt_id": None})
+    assert r.status_code == 400, r.text
+    assert _error_type(r) != "invalid_prompt_id"
diff --git a/tests/execution/test_jobs.py b/tests/execution/test_jobs.py
index 814af5c13..30e47071d 100644
--- a/tests/execution/test_jobs.py
+++ b/tests/execution/test_jobs.py
@@ -1,5 +1,7 @@
 """Unit tests for comfy_execution/jobs.py"""
 
+import pytest
+
 from comfy_execution.jobs import (
     JobStatus,
     is_previewable,
@@ -10,9 +12,50 @@ from comfy_execution.jobs import (
     get_outputs_summary,
     apply_sorting,
     has_3d_extension,
+    validate_job_id,
 )
 
 
+class TestValidateJobId:
+    """validate_job_id guards job creation: POST /prompt rejects ids it raises on."""
+
+    def test_canonical_form_passes_through(self):
+        cid = "a1b2c3d4-e5f6-7a89-b0c1-d2e3f4a5b6c7"
+        assert validate_job_id(cid) == cid
+
+    @pytest.mark.parametrize(
+        "variant",
+        [
+            "A1B2C3D4-E5F6-7A89-B0C1-D2E3F4A5B6C7",          # uppercase
+            "{a1b2c3d4-e5f6-7a89-b0c1-d2e3f4a5b6c7}",        # braced
+            "urn:uuid:a1b2c3d4-e5f6-7a89-b0c1-d2e3f4a5b6c7",  # URN
+            "a1b2c3d4e5f67a89b0c1d2e3f4a5b6c7",              # bare hex
+            " a1b2c3d4-e5f6-7a89-b0c1-d2e3f4a5b6c7 ",        # padded
+        ],
+    )
+    def test_non_canonical_spellings_rejected(self, variant):
+        # uuid.UUID parses all of these, but accepting them would silently
+        # rewrite the client's id (history keys, websocket events, and the
+        # assets job_ids filter all match the stored form exactly).
+        with pytest.raises(ValueError):
+            validate_job_id(variant)
+
+    @pytest.mark.parametrize(
+        "bad",
+        ["", "not-a-uuid", "prompt-123", "a1b2c3d4-e5f6-7a89-b0c1", "None"],
+    )
+    def test_non_uuid_strings_rejected(self, bad):
+        with pytest.raises(ValueError):
+            validate_job_id(bad)
+
+    @pytest.mark.parametrize("bad", [123, 1.5, True, None, ["a"], {"id": "x"}])
+    def test_non_strings_rejected(self, bad):
+        # uuid.UUID raises AttributeError/TypeError on non-strings; the helper
+        # must normalize those to ValueError so callers need one except clause.
+        with pytest.raises(ValueError):
+            validate_job_id(bad)
+
+
 class TestJobStatus:
     """Test JobStatus constants."""