diff --git a/src/runloop_api_client/_constants.py b/src/runloop_api_client/_constants.py
index 4fe93f3ea..d6361c8ad 100644
--- a/src/runloop_api_client/_constants.py
+++ b/src/runloop_api_client/_constants.py
@@ -12,3 +12,12 @@
 
 INITIAL_RETRY_DELAY = 1.0
 MAX_RETRY_DELAY = 60.0
+
+# Maximum allowed size (in bytes) for individual entries in `file_mounts` when creating Blueprints
+# NOTE: Empirically, ~131,000 is the maximum command length after
+# base64 encoding; 98,250 is the pre-encoded limit that stays within that bound.
+# We measure size in bytes using UTF-8 encoding; base64 output is ASCII.
+FILE_MOUNT_MAX_SIZE_BYTES = 98_250
+
+# Maximum allowed total size (in bytes) across all `file_mounts` when creating Blueprints
+FILE_MOUNT_TOTAL_MAX_SIZE_BYTES = 786_000 * 10  # ~10 mb
diff --git a/src/runloop_api_client/_utils/__init__.py b/src/runloop_api_client/_utils/__init__.py
index dc64e29a1..80df24dba 100644
--- a/src/runloop_api_client/_utils/__init__.py
+++ b/src/runloop_api_client/_utils/__init__.py
@@ -1,3 +1,4 @@
+# isort: skip_file
 from ._sync import asyncify as asyncify
 from ._proxy import LazyProxy as LazyProxy
 from ._utils import (
diff --git a/src/runloop_api_client/_utils/_validation.py b/src/runloop_api_client/_utils/_validation.py
new file mode 100644
index 000000000..56ec6f76b
--- /dev/null
+++ b/src/runloop_api_client/_utils/_validation.py
@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+from typing import List, Optional
+
+
+class ValidationNotification:
+    """Collects validation errors without raising exceptions.
+
+    This follows the notification pattern: validations append errors, and callers
+    decide how to react (e.g., surface all messages at once or abort).
+    """
+
+    def __init__(self) -> None:
+        self._errors: List[str] = []
+        self._causes: List[Optional[Exception]] = []
+
+    def add_error(self, message: str, cause: Optional[Exception] = None) -> None:
+        self._errors.append(message)
+        self._causes.append(cause)
+
+    def has_errors(self) -> bool:
+        return len(self._errors) > 0
+
+    @property
+    def errors(self) -> List[str]:
+        # Return a copy to avoid external mutation
+        return list(self._errors)
+
+    def error_message(self) -> str:
+        # Join with semicolons to present multiple issues succinctly
+        return "; ".join(self._errors)
diff --git a/src/runloop_api_client/resources/blueprints.py b/src/runloop_api_client/resources/blueprints.py
index 34f98b498..4bb8cbace 100644
--- a/src/runloop_api_client/resources/blueprints.py
+++ b/src/runloop_api_client/resources/blueprints.py
@@ -1,4 +1,5 @@
 # File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
+# isort: skip_file
 
 from __future__ import annotations
 
@@ -15,6 +16,7 @@
 )
 from .._types import NOT_GIVEN, Body, Omit, Query, Headers, NotGiven, SequenceNotStr, omit, not_given
 from .._utils import maybe_transform, async_maybe_transform
+from .._utils._validation import ValidationNotification
 from .._compat import cached_property
 from .._resource import SyncAPIResource, AsyncAPIResource
 from .._response import (
@@ -23,6 +25,7 @@
     async_to_raw_response_wrapper,
     async_to_streamed_response_wrapper,
 )
+from .._constants import FILE_MOUNT_MAX_SIZE_BYTES, FILE_MOUNT_TOTAL_MAX_SIZE_BYTES
 from ..pagination import SyncBlueprintsCursorIDPage, AsyncBlueprintsCursorIDPage
 from .._exceptions import RunloopError
 from ..lib.polling import PollingConfig, poll_until
@@ -50,6 +53,57 @@ class BlueprintRequestArgs(TypedDict, total=False):
 __all__ = ["BlueprintsResource", "AsyncBlueprintsResource", "BlueprintRequestArgs"]
 
 
+def _format_bytes(num_bytes: int) -> str:
+    """Format a byte count in a human-friendly way (KB/MB/GB).
+
+    Uses binary units (1024). Avoids decimals when exact.
+    """
+    if num_bytes < 1024:
+        return f"{num_bytes} bytes"
+    for factor, unit in ((1 << 30, "GB"), (1 << 20, "MB"), (1 << 10, "KB")):
+        if num_bytes >= factor:
+            value = num_bytes / factor
+            if float(value).is_integer():
+                return f"{int(value)} {unit}"
+            return f"{value:.1f} {unit}"
+    return f"{num_bytes} bytes"
+
+
+def _validate_file_mounts(file_mounts: Optional[Dict[str, str]] | Omit) -> ValidationNotification:
+    """Validate file_mounts are within size constraints: returns validation failures.
+
+    Currently enforces a maximum per-file size to avoid server-side issues with
+    large inline file contents. Also enforces a maximum total size across all
+    file_mounts.
+    """
+
+    note = ValidationNotification()
+
+    if file_mounts is omit or file_mounts is None:
+        return note
+
+    total_size_bytes = 0
+    for mount_path, content in file_mounts.items():
+        # Measure size in bytes using UTF-8 encoding since payloads are JSON strings
+        size_bytes = len(content.encode("utf-8"))
+        if size_bytes > FILE_MOUNT_MAX_SIZE_BYTES:
+            over = size_bytes - FILE_MOUNT_MAX_SIZE_BYTES
+            note.add_error(
+                f"file_mount '{mount_path}' is {_format_bytes(over)} over the limit "
+                f"({_format_bytes(size_bytes)} / {_format_bytes(FILE_MOUNT_MAX_SIZE_BYTES)}). Use object_mounts instead."
+            )
+        total_size_bytes += size_bytes
+
+    if total_size_bytes > FILE_MOUNT_TOTAL_MAX_SIZE_BYTES:
+        total_over = total_size_bytes - FILE_MOUNT_TOTAL_MAX_SIZE_BYTES
+        note.add_error(
+            f"total file_mounts size is {_format_bytes(total_over)} over the limit "
+            f"({_format_bytes(total_size_bytes)} / {_format_bytes(FILE_MOUNT_TOTAL_MAX_SIZE_BYTES)}). Use object_mounts instead."
+        )
+
+    return note
+
+
 class BlueprintsResource(SyncAPIResource):
     @cached_property
     def with_raw_response(self) -> BlueprintsResourceWithRawResponse:
@@ -144,6 +198,10 @@ def create(
 
           idempotency_key: Specify a custom idempotency key for this request
         """
+        note = _validate_file_mounts(file_mounts)
+        if note.has_errors():
+            raise ValueError(note.error_message())
+
         return self._post(
             "/v1/blueprints",
             body=maybe_transform(
@@ -758,6 +816,10 @@ async def create(
 
           idempotency_key: Specify a custom idempotency key for this request
         """
+        note = _validate_file_mounts(file_mounts)
+        if note.has_errors():
+            raise ValueError(note.error_message())
+
         return await self._post(
             "/v1/blueprints",
             body=await async_maybe_transform(
diff --git a/tests/api_resources/test_blueprints.py b/tests/api_resources/test_blueprints.py
index 30ae21d3e..9826f0b67 100644
--- a/tests/api_resources/test_blueprints.py
+++ b/tests/api_resources/test_blueprints.py
@@ -108,6 +108,28 @@ def test_streaming_response_create(self, client: Runloop) -> None:
 
         assert cast(Any, response.is_closed) is True
 
+    @parametrize
+    def test_create_rejects_large_file_mount(self, client: Runloop) -> None:
+        # 98,250 bytes + 1 byte (pre-encoded limit to stay within ~131,000 b64'd)
+        too_large_content = "a" * (98_250 + 1)
+        with pytest.raises(ValueError, match=r"over the limit"):
+            client.blueprints.create(
+                name="name",
+                file_mounts={"/tmp/large.txt": too_large_content},
+            )
+
+    @parametrize
+    def test_create_rejects_total_file_mount_size(self, client: Runloop) -> None:
+        # Eighty files at per-file max (98,250) equals current total limit; add 1 byte to exceed
+        per_file_max = 98_250
+        file_mounts = {f"/tmp/{i}.txt": "a" * per_file_max for i in range(80)}
+        file_mounts["/tmp/extra.txt"] = "x"
+        with pytest.raises(ValueError, match=r"total file_mounts size .* over the limit"):
+            client.blueprints.create(
+                name="name",
+                file_mounts=file_mounts,
+            )
+
     @parametrize
     def test_method_retrieve(self, client: Runloop) -> None:
         blueprint = client.blueprints.retrieve(
@@ -536,6 +558,28 @@ async def test_streaming_response_create(self, async_client: AsyncRunloop) -> No
 
         assert cast(Any, response.is_closed) is True
 
+    @parametrize
+    async def test_create_rejects_large_file_mount(self, async_client: AsyncRunloop) -> None:
+        # 98,250 bytes + 1 byte (pre-encoded limit to stay within ~131,000 b64'd)
+        too_large_content = "a" * (98_250 + 1)
+        with pytest.raises(ValueError, match=r"over the limit"):
+            await async_client.blueprints.create(
+                name="name",
+                file_mounts={"/tmp/large.txt": too_large_content},
+            )
+
+    @parametrize
+    async def test_create_rejects_total_file_mount_size(self, async_client: AsyncRunloop) -> None:
+        # Eighty files at per-file max (98,250) equals current total limit; add 1 byte to exceed
+        per_file_max = 98_250
+        file_mounts = {f"/tmp/{i}.txt": "a" * per_file_max for i in range(80)}
+        file_mounts["/tmp/extra.txt"] = "x"
+        with pytest.raises(ValueError, match=r"total file_mounts size .* over the limit"):
+            await async_client.blueprints.create(
+                name="name",
+                file_mounts=file_mounts,
+            )
+
     @parametrize
     async def test_method_retrieve(self, async_client: AsyncRunloop) -> None:
         blueprint = await async_client.blueprints.retrieve(
diff --git a/uv.lock b/uv.lock
index 2e395a445..4425156fb 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1006,7 +1006,7 @@ wheels = [
 
 [[package]]
 name = "runloop-api-client"
-version = "0.65.0"
+version = "0.66.1"
 source = { editable = "." }
 dependencies = [
     { name = "anyio" },