diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 97f05b2a..ebcf4606 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,12 +1,14 @@
 name: CI
 on:
   push:
-    branches-ignore:
-      - 'generated'
-      - 'codegen/**'
-      - 'integrated/**'
-      - 'stl-preview-head/**'
-      - 'stl-preview-base/**'
+    branches:
+      - '**'
+      - '!integrated/**'
+      - '!stl-preview-head/**'
+      - '!stl-preview-base/**'
+      - '!generated'
+      - '!codegen/**'
+      - 'codegen/stl/**'
   pull_request:
     branches-ignore:
       - 'stl-preview-head/**'
@@ -17,7 +19,7 @@ jobs:
     timeout-minutes: 10
     name: lint
     runs-on: ${{ github.repository == 'stainless-sdks/steel-python' && 'depot-ubuntu-24.04' || 'ubuntu-latest' }}
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
+    if: (github.event_name == 'push' || github.event.pull_request.head.repo.fork) && (github.event_name != 'push' || github.event.head_commit.message != 'codegen metadata')
     steps:
       - uses: actions/checkout@v6
 
@@ -36,7 +38,7 @@ jobs:
         run: ./scripts/lint
 
   build:
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
+    if: (github.event_name == 'push' || github.event.pull_request.head.repo.fork) && (github.event_name != 'push' || github.event.head_commit.message != 'codegen metadata')
     timeout-minutes: 10
     name: build
     permissions:
diff --git a/.gitignore b/.gitignore
index 95ceb189..3824f4c4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .prism.log
+.stdy.log
 _dev
 
 __pycache__
diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 6db19b95..4ad3fef3 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "0.17.0"
+  ".": "0.18.0"
 }
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ab379643..2c7ba907 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,47 @@
 # Changelog
 
+## 0.18.0 (2026-04-23)
+
+Full Changelog: [v0.17.0...v0.18.0](https://github.com/steel-dev/steel-python/compare/v0.17.0...v0.18.0)
+
+### Features
+
+* **internal:** implement indices array format for query and form serialization ([5e7b0a2](https://github.com/steel-dev/steel-python/commit/5e7b0a243b6d3fd6299b60695e57ef709096e265))
+
+
+### Bug Fixes
+
+* **client:** preserve hardcoded query params when merging with user params ([a2ee006](https://github.com/steel-dev/steel-python/commit/a2ee00611edec4afc18dbe55707afcff37738b18))
+* **deps:** bump minimum typing-extensions version ([3815a72](https://github.com/steel-dev/steel-python/commit/3815a72732d737d7403f91fbec741c95dd5f89d5))
+* ensure file data are only sent as 1 parameter ([3d0485c](https://github.com/steel-dev/steel-python/commit/3d0485c4ab6faf9bf4cdf46b80f8bd5267a8b033))
+* **pydantic:** do not pass `by_alias` unless set ([f9686b4](https://github.com/steel-dev/steel-python/commit/f9686b47001a452f8591d9d65c7dd8c70dad5204))
+* sanitize endpoint path params ([abb806c](https://github.com/steel-dev/steel-python/commit/abb806c6ada0cb749bf734ada489eeff02738673))
+
+
+### Performance Improvements
+
+* **client:** optimize file structure copying in multipart requests ([fe678f2](https://github.com/steel-dev/steel-python/commit/fe678f2ed6cd4eac844a0c260c62e4fba9a84cbb))
+
+
+### Chores
+
+* **ci:** skip lint on metadata-only changes ([a90bcd7](https://github.com/steel-dev/steel-python/commit/a90bcd7ec396cded3006818beffe1c1f8780f6d1))
+* **internal:** more robust bootstrap script ([c6d65ae](https://github.com/steel-dev/steel-python/commit/c6d65ae060526ed85fc7e27040a921cad3c00196))
+* **internal:** tweak CI branches ([35a8acc](https://github.com/steel-dev/steel-python/commit/35a8acc7fd278d27398979892a10832291f74e9d))
+* **internal:** update gitignore ([4f0a06c](https://github.com/steel-dev/steel-python/commit/4f0a06c5eb8b1b88597e4d562de24284d4c9e2d3))
+* **tests:** bump steady to v0.19.4 ([fc582c4](https://github.com/steel-dev/steel-python/commit/fc582c4da1253d3851af25e753398ed6139cdac6))
+* **tests:** bump steady to v0.19.5 ([9927574](https://github.com/steel-dev/steel-python/commit/9927574b27a5d281732032176c9da052e7da5ea0))
+* **tests:** bump steady to v0.19.6 ([81717f3](https://github.com/steel-dev/steel-python/commit/81717f34046cb49e43a25db03d8b5bb153429284))
+* **tests:** bump steady to v0.19.7 ([4d06b71](https://github.com/steel-dev/steel-python/commit/4d06b71429d4341474d75d915d17bd597ebd42ec))
+* **tests:** bump steady to v0.20.1 ([793aca8](https://github.com/steel-dev/steel-python/commit/793aca80bd008a31fe47a9fff8380f2a9ab513f4))
+* **tests:** bump steady to v0.20.2 ([0be42f6](https://github.com/steel-dev/steel-python/commit/0be42f6789832bc33a87b8f5ce0a374cce1a4508))
+* **tests:** bump steady to v0.22.1 ([1fe72c6](https://github.com/steel-dev/steel-python/commit/1fe72c6a2ef34388e815315acbf93026c5727d5d))
+
+
+### Refactors
+
+* **tests:** switch from prism to steady ([791de4d](https://github.com/steel-dev/steel-python/commit/791de4d8baf003a3600e7e7a9422a36fc6729c12))
+
 ## 0.17.0 (2026-03-16)
 
 Full Changelog: [v0.16.0...v0.17.0](https://github.com/steel-dev/steel-python/compare/v0.16.0...v0.17.0)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index fbd86d24..c0e917b1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -85,7 +85,7 @@ $ pip install ./path-to-wheel-file.whl
 
 ## Running tests
 
-Most tests require you to [set up a mock server](https://github.com/stoplightio/prism) against the OpenAPI spec to run the tests.
+Most tests require you to [set up a mock server](https://github.com/dgellow/steady) against the OpenAPI spec to run the tests.
 
 ```sh
 $ ./scripts/mock
diff --git a/pyproject.toml b/pyproject.toml
index 8219a00b..c2b157d5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "steel-sdk"
-version = "0.17.0"
+version = "0.18.0"
 description = "The official Python library for the steel API"
 dynamic = ["readme"]
 license = "Apache-2.0"
@@ -11,7 +11,7 @@ authors = [
 dependencies = [
   "httpx>=0.23.0, <1",
   "pydantic>=1.9.0, <3",
-  "typing-extensions>=4.10, <5",
+  "typing-extensions>=4.14, <5",
   "anyio>=3.5.0, <5",
   "distro>=1.7.0, <2",
   "sniffio",
diff --git a/scripts/bootstrap b/scripts/bootstrap
index b430fee3..fe8451e4 100755
--- a/scripts/bootstrap
+++ b/scripts/bootstrap
@@ -4,7 +4,7 @@ set -e
 
 cd "$(dirname "$0")/.."
 
-if [ -f "Brewfile" ] && [ "$(uname -s)" = "Darwin" ] && [ "$SKIP_BREW" != "1" ] && [ -t 0 ]; then
+if [ -f "Brewfile" ] && [ "$(uname -s)" = "Darwin" ] && [ "${SKIP_BREW:-}" != "1" ] && [ -t 0 ]; then
   brew bundle check >/dev/null 2>&1 || {
     echo -n "==> Install Homebrew dependencies? (y/N): "
     read -r response
diff --git a/scripts/mock b/scripts/mock
index bcf3b392..feebe5ed 100755
--- a/scripts/mock
+++ b/scripts/mock
@@ -19,34 +19,34 @@ fi
 
 echo "==> Starting mock server with URL ${URL}"
 
-# Run prism mock on the given spec
+# Run steady mock on the given spec
 if [ "$1" == "--daemon" ]; then
   # Pre-install the package so the download doesn't eat into the startup timeout
-  npm exec --package=@stainless-api/prism-cli@5.15.0 -- prism --version
+  npm exec --package=@stdy/cli@0.22.1 -- steady --version
 
-  npm exec --package=@stainless-api/prism-cli@5.15.0 -- prism mock "$URL" &> .prism.log &
+  npm exec --package=@stdy/cli@0.22.1 -- steady --host 127.0.0.1 -p 4010 --validator-query-array-format=comma --validator-form-array-format=comma --validator-query-object-format=brackets --validator-form-object-format=brackets "$URL" &> .stdy.log &
 
-  # Wait for server to come online (max 30s)
+  # Wait for server to come online via health endpoint (max 30s)
   echo -n "Waiting for server"
   attempts=0
-  while ! grep -q "✖  fatal\|Prism is listening" ".prism.log" ; do
+  while ! curl --silent --fail "http://127.0.0.1:4010/_x-steady/health" >/dev/null 2>&1; do
+    if ! kill -0 $! 2>/dev/null; then
+      echo
+      cat .stdy.log
+      exit 1
+    fi
     attempts=$((attempts + 1))
     if [ "$attempts" -ge 300 ]; then
       echo
-      echo "Timed out waiting for Prism server to start"
-      cat .prism.log
+      echo "Timed out waiting for Steady server to start"
+      cat .stdy.log
       exit 1
     fi
     echo -n "."
     sleep 0.1
   done
 
-  if grep -q "✖  fatal" ".prism.log"; then
-    cat .prism.log
-    exit 1
-  fi
-
   echo
 else
-  npm exec --package=@stainless-api/prism-cli@5.15.0 -- prism mock "$URL"
+  npm exec --package=@stdy/cli@0.22.1 -- steady --host 127.0.0.1 -p 4010 --validator-query-array-format=comma --validator-form-array-format=comma --validator-query-object-format=brackets --validator-form-object-format=brackets "$URL"
 fi
diff --git a/scripts/test b/scripts/test
index dbeda2d2..19acc916 100755
--- a/scripts/test
+++ b/scripts/test
@@ -9,8 +9,8 @@ GREEN='\033[0;32m'
 YELLOW='\033[0;33m'
 NC='\033[0m' # No Color
 
-function prism_is_running() {
-  curl --silent "http://localhost:4010" >/dev/null 2>&1
+function steady_is_running() {
+  curl --silent "http://127.0.0.1:4010/_x-steady/health" >/dev/null 2>&1
 }
 
 kill_server_on_port() {
@@ -25,7 +25,7 @@ function is_overriding_api_base_url() {
   [ -n "$TEST_API_BASE_URL" ]
 }
 
-if ! is_overriding_api_base_url && ! prism_is_running ; then
+if ! is_overriding_api_base_url && ! steady_is_running ; then
   # When we exit this script, make sure to kill the background mock server process
   trap 'kill_server_on_port 4010' EXIT
 
@@ -36,19 +36,19 @@ fi
 if is_overriding_api_base_url ; then
   echo -e "${GREEN}✔ Running tests against ${TEST_API_BASE_URL}${NC}"
   echo
-elif ! prism_is_running ; then
-  echo -e "${RED}ERROR:${NC} The test suite will not run without a mock Prism server"
+elif ! steady_is_running ; then
+  echo -e "${RED}ERROR:${NC} The test suite will not run without a mock Steady server"
   echo -e "running against your OpenAPI spec."
   echo
   echo -e "To run the server, pass in the path or url of your OpenAPI"
-  echo -e "spec to the prism command:"
+  echo -e "spec to the steady command:"
   echo
-  echo -e "  \$ ${YELLOW}npm exec --package=@stainless-api/prism-cli@5.15.0 -- prism mock path/to/your.openapi.yml${NC}"
+  echo -e "  \$ ${YELLOW}npm exec --package=@stdy/cli@0.22.1 -- steady path/to/your.openapi.yml --host 127.0.0.1 -p 4010 --validator-query-array-format=comma --validator-form-array-format=comma --validator-query-object-format=brackets --validator-form-object-format=brackets${NC}"
   echo
 
   exit 1
 else
-  echo -e "${GREEN}✔ Mock prism server is running with your OpenAPI spec${NC}"
+  echo -e "${GREEN}✔ Mock steady server is running with your OpenAPI spec${NC}"
   echo
 fi
 
diff --git a/src/steel/_base_client.py b/src/steel/_base_client.py
index 278ca503..6db34d23 100644
--- a/src/steel/_base_client.py
+++ b/src/steel/_base_client.py
@@ -540,6 +540,10 @@ def _build_request(
                 files = cast(HttpxRequestFiles, ForceMultipartDict())
 
         prepared_url = self._prepare_url(options.url)
+        # preserve hard-coded query params from the url
+        if params and prepared_url.query:
+            params = {**dict(prepared_url.params.items()), **params}
+            prepared_url = prepared_url.copy_with(raw_path=prepared_url.raw_path.split(b"?", 1)[0])
         if "_" in prepared_url.host:
             # work around https://github.com/encode/httpx/discussions/2880
             kwargs["extensions"] = {"sni_hostname": prepared_url.host.replace("_", "-")}
diff --git a/src/steel/_compat.py b/src/steel/_compat.py
index 786ff42a..e6690a4f 100644
--- a/src/steel/_compat.py
+++ b/src/steel/_compat.py
@@ -2,7 +2,7 @@
 
 from typing import TYPE_CHECKING, Any, Union, Generic, TypeVar, Callable, cast, overload
 from datetime import date, datetime
-from typing_extensions import Self, Literal
+from typing_extensions import Self, Literal, TypedDict
 
 import pydantic
 from pydantic.fields import FieldInfo
@@ -131,6 +131,10 @@ def model_json(model: pydantic.BaseModel, *, indent: int | None = None) -> str:
     return model.model_dump_json(indent=indent)
 
 
+class _ModelDumpKwargs(TypedDict, total=False):
+    by_alias: bool
+
+
 def model_dump(
     model: pydantic.BaseModel,
     *,
@@ -142,6 +146,9 @@ def model_dump(
     by_alias: bool | None = None,
 ) -> dict[str, Any]:
     if (not PYDANTIC_V1) or hasattr(model, "model_dump"):
+        kwargs: _ModelDumpKwargs = {}
+        if by_alias is not None:
+            kwargs["by_alias"] = by_alias
         return model.model_dump(
             mode=mode,
             exclude=exclude,
@@ -149,7 +156,7 @@ def model_dump(
             exclude_defaults=exclude_defaults,
             # warnings are not supported in Pydantic v1
             warnings=True if PYDANTIC_V1 else warnings,
-            by_alias=by_alias,
+            **kwargs,
         )
     return cast(
         "dict[str, Any]",
diff --git a/src/steel/_files.py b/src/steel/_files.py
index b3386203..77e66753 100644
--- a/src/steel/_files.py
+++ b/src/steel/_files.py
@@ -3,8 +3,8 @@
 import io
 import os
 import pathlib
-from typing import overload
-from typing_extensions import TypeGuard
+from typing import Sequence, cast, overload
+from typing_extensions import TypeVar, TypeGuard
 
 import anyio
 
@@ -17,7 +17,9 @@
     HttpxFileContent,
     HttpxRequestFiles,
 )
-from ._utils import is_tuple_t, is_mapping_t, is_sequence_t
+from ._utils import is_list, is_mapping, is_tuple_t, is_mapping_t, is_sequence_t
+
+_T = TypeVar("_T")
 
 
 def is_base64_file_input(obj: object) -> TypeGuard[Base64FileInput]:
@@ -121,3 +123,51 @@ async def async_read_file_content(file: FileContent) -> HttpxFileContent:
         return await anyio.Path(file).read_bytes()
 
     return file
+
+
+def deepcopy_with_paths(item: _T, paths: Sequence[Sequence[str]]) -> _T:
+    """Copy only the containers along the given paths.
+
+    Used to guard against mutation by extract_files without copying the entire structure.
+    Only dicts and lists that lie on a path are copied; everything else
+    is returned by reference.
+
+    For example, given paths=[["foo", "files", "file"]] and the structure:
+        {
+            "foo": {
+                "bar": {"baz": {}},
+                "files": {"file": <content>}
+            }
+        }
+    The root dict, "foo", and "files" are copied (they lie on the path).
+    "bar" and "baz" are returned by reference (off the path).
+    """
+    return _deepcopy_with_paths(item, paths, 0)
+
+
+def _deepcopy_with_paths(item: _T, paths: Sequence[Sequence[str]], index: int) -> _T:
+    if not paths:
+        return item
+    if is_mapping(item):
+        key_to_paths: dict[str, list[Sequence[str]]] = {}
+        for path in paths:
+            if index < len(path):
+                key_to_paths.setdefault(path[index], []).append(path)
+
+        # if no path continues through this mapping, it won't be mutated and copying it is redundant
+        if not key_to_paths:
+            return item
+
+        result = dict(item)
+        for key, subpaths in key_to_paths.items():
+            if key in result:
+                result[key] = _deepcopy_with_paths(result[key], subpaths, index + 1)
+        return cast(_T, result)
+    if is_list(item):
+        array_paths = [path for path in paths if index < len(path) and path[index] == "<array>"]
+
+        # if no path expects a list here, nothing will be mutated inside it - return by reference
+        if not array_paths:
+            return cast(_T, item)
+        return cast(_T, [_deepcopy_with_paths(entry, array_paths, index + 1) for entry in item])
+    return item
diff --git a/src/steel/_qs.py b/src/steel/_qs.py
index ada6fd3f..de8c99bc 100644
--- a/src/steel/_qs.py
+++ b/src/steel/_qs.py
@@ -101,7 +101,10 @@ def _stringify_item(
                     items.extend(self._stringify_item(key, item, opts))
                 return items
             elif array_format == "indices":
-                raise NotImplementedError("The array indices format is not supported yet")
+                items = []
+                for i, item in enumerate(value):
+                    items.extend(self._stringify_item(f"{key}[{i}]", item, opts))
+                return items
             elif array_format == "brackets":
                 items = []
                 key = key + "[]"
diff --git a/src/steel/_utils/__init__.py b/src/steel/_utils/__init__.py
index dc64e29a..1c090e51 100644
--- a/src/steel/_utils/__init__.py
+++ b/src/steel/_utils/__init__.py
@@ -1,3 +1,4 @@
+from ._path import path_template as path_template
 from ._sync import asyncify as asyncify
 from ._proxy import LazyProxy as LazyProxy
 from ._utils import (
@@ -23,7 +24,6 @@
     coerce_integer as coerce_integer,
     file_from_path as file_from_path,
     strip_not_given as strip_not_given,
-    deepcopy_minimal as deepcopy_minimal,
     get_async_library as get_async_library,
     maybe_coerce_float as maybe_coerce_float,
     get_required_header as get_required_header,
diff --git a/src/steel/_utils/_path.py b/src/steel/_utils/_path.py
new file mode 100644
index 00000000..4d6e1e4c
--- /dev/null
+++ b/src/steel/_utils/_path.py
@@ -0,0 +1,127 @@
+from __future__ import annotations
+
+import re
+from typing import (
+    Any,
+    Mapping,
+    Callable,
+)
+from urllib.parse import quote
+
+# Matches '.' or '..' where each dot is either literal or percent-encoded (%2e / %2E).
+_DOT_SEGMENT_RE = re.compile(r"^(?:\.|%2[eE]){1,2}$")
+
+_PLACEHOLDER_RE = re.compile(r"\{(\w+)\}")
+
+
+def _quote_path_segment_part(value: str) -> str:
+    """Percent-encode `value` for use in a URI path segment.
+
+    Considers characters not in `pchar` set from RFC 3986 §3.3 to be unsafe.
+    https://datatracker.ietf.org/doc/html/rfc3986#section-3.3
+    """
+    # quote() already treats unreserved characters (letters, digits, and -._~)
+    # as safe, so we only need to add sub-delims, ':', and '@'.
+    # Notably, unlike the default `safe` for quote(), / is unsafe and must be quoted.
+    return quote(value, safe="!$&'()*+,;=:@")
+
+
+def _quote_query_part(value: str) -> str:
+    """Percent-encode `value` for use in a URI query string.
+
+    Considers &, = and characters not in `query` set from RFC 3986 §3.4 to be unsafe.
+    https://datatracker.ietf.org/doc/html/rfc3986#section-3.4
+    """
+    return quote(value, safe="!$'()*+,;:@/?")
+
+
+def _quote_fragment_part(value: str) -> str:
+    """Percent-encode `value` for use in a URI fragment.
+
+    Considers characters not in `fragment` set from RFC 3986 §3.5 to be unsafe.
+    https://datatracker.ietf.org/doc/html/rfc3986#section-3.5
+    """
+    return quote(value, safe="!$&'()*+,;=:@/?")
+
+
+def _interpolate(
+    template: str,
+    values: Mapping[str, Any],
+    quoter: Callable[[str], str],
+) -> str:
+    """Replace {name} placeholders in `template`, quoting each value with `quoter`.
+
+    Placeholder names are looked up in `values`.
+
+    Raises:
+        KeyError: If a placeholder is not found in `values`.
+    """
+    # re.split with a capturing group returns alternating
+    # [text, name, text, name, ..., text] elements.
+    parts = _PLACEHOLDER_RE.split(template)
+
+    for i in range(1, len(parts), 2):
+        name = parts[i]
+        if name not in values:
+            raise KeyError(f"a value for placeholder {{{name}}} was not provided")
+        val = values[name]
+        if val is None:
+            parts[i] = "null"
+        elif isinstance(val, bool):
+            parts[i] = "true" if val else "false"
+        else:
+            parts[i] = quoter(str(values[name]))
+
+    return "".join(parts)
+
+
+def path_template(template: str, /, **kwargs: Any) -> str:
+    """Interpolate {name} placeholders in `template` from keyword arguments.
+
+    Args:
+        template: The template string containing {name} placeholders.
+        **kwargs: Keyword arguments to interpolate into the template.
+
+    Returns:
+        The template with placeholders interpolated and percent-encoded.
+
+        Safe characters for percent-encoding are dependent on the URI component.
+        Placeholders in path and fragment portions are percent-encoded where the `segment`
+        and `fragment` sets from RFC 3986 respectively are considered safe.
+        Placeholders in the query portion are percent-encoded where the `query` set from
+        RFC 3986 §3.3 is considered safe except for = and & characters.
+
+    Raises:
+        KeyError: If a placeholder is not found in `kwargs`.
+        ValueError: If resulting path contains /./ or /../ segments (including percent-encoded dot-segments).
+    """
+    # Split the template into path, query, and fragment portions.
+    fragment_template: str | None = None
+    query_template: str | None = None
+
+    rest = template
+    if "#" in rest:
+        rest, fragment_template = rest.split("#", 1)
+    if "?" in rest:
+        rest, query_template = rest.split("?", 1)
+    path_template = rest
+
+    # Interpolate each portion with the appropriate quoting rules.
+    path_result = _interpolate(path_template, kwargs, _quote_path_segment_part)
+
+    # Reject dot-segments (. and ..) in the final assembled path.  The check
+    # runs after interpolation so that adjacent placeholders or a mix of static
+    # text and placeholders that together form a dot-segment are caught.
+    # Also reject percent-encoded dot-segments to protect against incorrectly
+    # implemented normalization in servers/proxies.
+    for segment in path_result.split("/"):
+        if _DOT_SEGMENT_RE.match(segment):
+            raise ValueError(f"Constructed path {path_result!r} contains dot-segment {segment!r} which is not allowed")
+
+    result = path_result
+    if query_template is not None:
+        result += "?" + _interpolate(query_template, kwargs, _quote_query_part)
+    if fragment_template is not None:
+        result += "#" + _interpolate(fragment_template, kwargs, _quote_fragment_part)
+
+    return result
diff --git a/src/steel/_utils/_utils.py b/src/steel/_utils/_utils.py
index eec7f4a1..771859f5 100644
--- a/src/steel/_utils/_utils.py
+++ b/src/steel/_utils/_utils.py
@@ -86,8 +86,9 @@ def _extract_items(
     index += 1
     if is_dict(obj):
         try:
-            # We are at the last entry in the path so we must remove the field
-            if (len(path)) == index:
+            # Remove the field if there are no more dict keys in the path,
+            # only "<array>" traversal markers or end.
+            if all(p == "<array>" for p in path[index:]):
                 item = obj.pop(key)
             else:
                 item = obj[key]
@@ -176,21 +177,6 @@ def is_iterable(obj: object) -> TypeGuard[Iterable[object]]:
     return isinstance(obj, Iterable)
 
 
-def deepcopy_minimal(item: _T) -> _T:
-    """Minimal reimplementation of copy.deepcopy() that will only copy certain object types:
-
-    - mappings, e.g. `dict`
-    - list
-
-    This is done for performance reasons.
-    """
-    if is_mapping(item):
-        return cast(_T, {k: deepcopy_minimal(v) for k, v in item.items()})
-    if is_list(item):
-        return cast(_T, [deepcopy_minimal(entry) for entry in item])
-    return item
-
-
 # copied from https://github.com/Rapptz/RoboDanny
 def human_join(seq: Sequence[str], *, delim: str = ", ", final: str = "or") -> str:
     size = len(seq)
diff --git a/src/steel/_version.py b/src/steel/_version.py
index 06fdcfc6..a9604d8d 100644
--- a/src/steel/_version.py
+++ b/src/steel/_version.py
@@ -1,4 +1,4 @@
 # File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 __title__ = "steel"
-__version__ = "0.17.0"  # x-release-please-version
+__version__ = "0.18.0"  # x-release-please-version
diff --git a/src/steel/resources/extensions.py b/src/steel/resources/extensions.py
index 6f20af7a..5a3dc284 100644
--- a/src/steel/resources/extensions.py
+++ b/src/steel/resources/extensions.py
@@ -7,8 +7,9 @@
 import httpx
 
 from ..types import extension_update_params, extension_upload_params
+from .._files import deepcopy_with_paths
 from .._types import Body, Omit, Query, Headers, NotGiven, FileTypes, omit, not_given
-from .._utils import extract_files, maybe_transform, deepcopy_minimal, async_maybe_transform
+from .._utils import extract_files, path_template, maybe_transform, async_maybe_transform
 from .._compat import cached_property
 from .._resource import SyncAPIResource, AsyncAPIResource
 from .._response import (
@@ -79,11 +80,12 @@ def update(
         """
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "url": url,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -91,7 +93,7 @@ def update(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return self._put(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             body=maybe_transform(body, extension_update_params.ExtensionUpdateParams),
             files=files,
             options=make_request_options(
@@ -145,7 +147,7 @@ def delete(
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
         return self._delete(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -197,7 +199,7 @@ def download(
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
         return self._get(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -233,11 +235,12 @@ def upload(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "url": url,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -307,11 +310,12 @@ async def update(
         """
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "url": url,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -319,7 +323,7 @@ async def update(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return await self._put(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             body=await async_maybe_transform(body, extension_update_params.ExtensionUpdateParams),
             files=files,
             options=make_request_options(
@@ -373,7 +377,7 @@ async def delete(
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
         return await self._delete(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -425,7 +429,7 @@ async def download(
         if not extension_id:
             raise ValueError(f"Expected a non-empty value for `extension_id` but received {extension_id!r}")
         return await self._get(
-            f"/v1/extensions/{extension_id}",
+            path_template("/v1/extensions/{extension_id}", extension_id=extension_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -461,11 +465,12 @@ async def upload(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "url": url,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
diff --git a/src/steel/resources/files.py b/src/steel/resources/files.py
index a2e72050..a5b2fc7f 100644
--- a/src/steel/resources/files.py
+++ b/src/steel/resources/files.py
@@ -7,8 +7,9 @@
 import httpx
 
 from ..types import file_upload_params
+from .._files import deepcopy_with_paths
 from .._types import Body, Omit, Query, Headers, NoneType, NotGiven, FileTypes, omit, not_given
-from .._utils import extract_files, maybe_transform, deepcopy_minimal, async_maybe_transform
+from .._utils import extract_files, path_template, maybe_transform, async_maybe_transform
 from .._compat import cached_property
 from .._resource import SyncAPIResource, AsyncAPIResource
 from .._response import (
@@ -98,7 +99,7 @@ def delete(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return self._delete(
-            f"/v1/files/{path}",
+            path_template("/v1/files/{path}", path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -132,7 +133,7 @@ def download(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "application/octet-stream", **(extra_headers or {})}
         return self._get(
-            f"/v1/files/{path}",
+            path_template("/v1/files/{path}", path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -169,11 +170,12 @@ def upload(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "path": path,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -257,7 +259,7 @@ async def delete(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return await self._delete(
-            f"/v1/files/{path}",
+            path_template("/v1/files/{path}", path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -291,7 +293,7 @@ async def download(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "application/octet-stream", **(extra_headers or {})}
         return await self._get(
-            f"/v1/files/{path}",
+            path_template("/v1/files/{path}", path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -328,11 +330,12 @@ async def upload(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "path": path,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
diff --git a/src/steel/resources/profiles.py b/src/steel/resources/profiles.py
index 3e149507..1a7f838e 100644
--- a/src/steel/resources/profiles.py
+++ b/src/steel/resources/profiles.py
@@ -7,8 +7,9 @@
 import httpx
 
 from ..types import profile_create_params, profile_update_params
+from .._files import deepcopy_with_paths
 from .._types import Body, Omit, Query, Headers, NotGiven, FileTypes, omit, not_given
-from .._utils import extract_files, maybe_transform, deepcopy_minimal, async_maybe_transform
+from .._utils import extract_files, path_template, maybe_transform, async_maybe_transform
 from .._compat import cached_property
 from .._resource import SyncAPIResource, AsyncAPIResource
 from .._response import (
@@ -80,13 +81,14 @@ def create(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "user_data_dir": user_data_dir,
                 "dimensions": dimensions,
                 "proxy_url": proxy_url,
                 "user_agent": user_agent,
-            }
+            },
+            [["userDataDir"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["userDataDir"]])
         # It should be noted that the actual Content-Type header that will be
@@ -140,13 +142,14 @@ def update(
         """
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "user_data_dir": user_data_dir,
                 "dimensions": dimensions,
                 "proxy_url": proxy_url,
                 "user_agent": user_agent,
-            }
+            },
+            [["userDataDir"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["userDataDir"]])
         # It should be noted that the actual Content-Type header that will be
@@ -154,7 +157,7 @@ def update(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return self._patch(
-            f"/v1/profiles/{id}",
+            path_template("/v1/profiles/{id}", id=id),
             body=maybe_transform(body, profile_update_params.ProfileUpdateParams),
             files=files,
             options=make_request_options(
@@ -208,7 +211,7 @@ def get(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._get(
-            f"/v1/profiles/{id}",
+            path_template("/v1/profiles/{id}", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -270,13 +273,14 @@ async def create(
 
           timeout: Override the client-level default timeout for this request, in seconds
         """
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "user_data_dir": user_data_dir,
                 "dimensions": dimensions,
                 "proxy_url": proxy_url,
                 "user_agent": user_agent,
-            }
+            },
+            [["userDataDir"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["userDataDir"]])
         # It should be noted that the actual Content-Type header that will be
@@ -330,13 +334,14 @@ async def update(
         """
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "user_data_dir": user_data_dir,
                 "dimensions": dimensions,
                 "proxy_url": proxy_url,
                 "user_agent": user_agent,
-            }
+            },
+            [["userDataDir"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["userDataDir"]])
         # It should be noted that the actual Content-Type header that will be
@@ -344,7 +349,7 @@ async def update(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return await self._patch(
-            f"/v1/profiles/{id}",
+            path_template("/v1/profiles/{id}", id=id),
             body=await async_maybe_transform(body, profile_update_params.ProfileUpdateParams),
             files=files,
             options=make_request_options(
@@ -398,7 +403,7 @@ async def get(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._get(
-            f"/v1/profiles/{id}",
+            path_template("/v1/profiles/{id}", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
diff --git a/src/steel/resources/sessions/captchas.py b/src/steel/resources/sessions/captchas.py
index 638dc058..abca5e2d 100644
--- a/src/steel/resources/sessions/captchas.py
+++ b/src/steel/resources/sessions/captchas.py
@@ -5,7 +5,7 @@
 import httpx
 
 from ..._types import Body, Omit, Query, Headers, NotGiven, omit, not_given
-from ..._utils import maybe_transform, async_maybe_transform
+from ..._utils import path_template, maybe_transform, async_maybe_transform
 from ..._compat import cached_property
 from ..._resource import SyncAPIResource, AsyncAPIResource
 from ..._response import (
@@ -81,7 +81,7 @@ def solve(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return self._post(
-            f"/v1/sessions/{session_id}/captchas/solve",
+            path_template("/v1/sessions/{session_id}/captchas/solve", session_id=session_id),
             body=maybe_transform(
                 {
                     "page_id": page_id,
@@ -131,7 +131,7 @@ def solve_image(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return self._post(
-            f"/v1/sessions/{session_id}/captchas/solve-image",
+            path_template("/v1/sessions/{session_id}/captchas/solve-image", session_id=session_id),
             body=maybe_transform(
                 {
                     "image_x_path": image_x_path,
@@ -172,7 +172,7 @@ def status(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return self._get(
-            f"/v1/sessions/{session_id}/captchas/status",
+            path_template("/v1/sessions/{session_id}/captchas/status", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -238,7 +238,7 @@ async def solve(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return await self._post(
-            f"/v1/sessions/{session_id}/captchas/solve",
+            path_template("/v1/sessions/{session_id}/captchas/solve", session_id=session_id),
             body=await async_maybe_transform(
                 {
                     "page_id": page_id,
@@ -288,7 +288,7 @@ async def solve_image(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return await self._post(
-            f"/v1/sessions/{session_id}/captchas/solve-image",
+            path_template("/v1/sessions/{session_id}/captchas/solve-image", session_id=session_id),
             body=await async_maybe_transform(
                 {
                     "image_x_path": image_x_path,
@@ -329,7 +329,7 @@ async def status(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return await self._get(
-            f"/v1/sessions/{session_id}/captchas/status",
+            path_template("/v1/sessions/{session_id}/captchas/status", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
diff --git a/src/steel/resources/sessions/files.py b/src/steel/resources/sessions/files.py
index ea7b2822..d8424abe 100644
--- a/src/steel/resources/sessions/files.py
+++ b/src/steel/resources/sessions/files.py
@@ -6,8 +6,9 @@
 
 import httpx
 
+from ..._files import deepcopy_with_paths
 from ..._types import Body, Omit, Query, Headers, NoneType, NotGiven, FileTypes, omit, not_given
-from ..._utils import extract_files, maybe_transform, deepcopy_minimal, async_maybe_transform
+from ..._utils import extract_files, path_template, maybe_transform, async_maybe_transform
 from ..._compat import cached_property
 from ..._resource import SyncAPIResource, AsyncAPIResource
 from ..._response import (
@@ -78,7 +79,7 @@ def list(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return self._get(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -115,7 +116,7 @@ def delete(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return self._delete(
-            f"/v1/sessions/{session_id}/files/{path}",
+            path_template("/v1/sessions/{session_id}/files/{path}", session_id=session_id, path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -149,7 +150,7 @@ def delete_all(
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return self._delete(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -186,7 +187,7 @@ def download(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "application/octet-stream", **(extra_headers or {})}
         return self._get(
-            f"/v1/sessions/{session_id}/files/{path}",
+            path_template("/v1/sessions/{session_id}/files/{path}", session_id=session_id, path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -220,7 +221,7 @@ def download_archive(
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         extra_headers = {"Accept": "application/zip", **(extra_headers or {})}
         return self._get(
-            f"/v1/sessions/{session_id}/files.zip",
+            path_template("/v1/sessions/{session_id}/files.zip", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -260,11 +261,12 @@ def upload(
         """
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "path": path,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -272,7 +274,7 @@ def upload(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return self._post(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             body=maybe_transform(body, file_upload_params.FileUploadParams),
             files=files,
             options=make_request_options(
@@ -328,7 +330,7 @@ async def list(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return await self._get(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -365,7 +367,7 @@ async def delete(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return await self._delete(
-            f"/v1/sessions/{session_id}/files/{path}",
+            path_template("/v1/sessions/{session_id}/files/{path}", session_id=session_id, path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -399,7 +401,7 @@ async def delete_all(
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         extra_headers = {"Accept": "*/*", **(extra_headers or {})}
         return await self._delete(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -436,7 +438,7 @@ async def download(
             raise ValueError(f"Expected a non-empty value for `path` but received {path!r}")
         extra_headers = {"Accept": "application/octet-stream", **(extra_headers or {})}
         return await self._get(
-            f"/v1/sessions/{session_id}/files/{path}",
+            path_template("/v1/sessions/{session_id}/files/{path}", session_id=session_id, path=path),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -470,7 +472,7 @@ async def download_archive(
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         extra_headers = {"Accept": "application/zip", **(extra_headers or {})}
         return await self._get(
-            f"/v1/sessions/{session_id}/files.zip",
+            path_template("/v1/sessions/{session_id}/files.zip", session_id=session_id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -510,11 +512,12 @@ async def upload(
         """
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
-        body = deepcopy_minimal(
+        body = deepcopy_with_paths(
             {
                 "file": file,
                 "path": path,
-            }
+            },
+            [["file"]],
         )
         files = extract_files(cast(Mapping[str, object], body), paths=[["file"]])
         # It should be noted that the actual Content-Type header that will be
@@ -522,7 +525,7 @@ async def upload(
         # multipart/form-data; boundary=---abc--
         extra_headers = {"Content-Type": "multipart/form-data", **(extra_headers or {})}
         return await self._post(
-            f"/v1/sessions/{session_id}/files",
+            path_template("/v1/sessions/{session_id}/files", session_id=session_id),
             body=await async_maybe_transform(body, file_upload_params.FileUploadParams),
             files=files,
             options=make_request_options(
diff --git a/src/steel/resources/sessions/sessions.py b/src/steel/resources/sessions/sessions.py
index d54ca834..e66c8b84 100644
--- a/src/steel/resources/sessions/sessions.py
+++ b/src/steel/resources/sessions/sessions.py
@@ -17,7 +17,7 @@
 )
 from ...types import session_list_params, session_create_params, session_events_params, session_computer_params
 from ..._types import Body, Omit, Query, Headers, NotGiven, SequenceNotStr, omit, not_given
-from ..._utils import required_args, maybe_transform, async_maybe_transform
+from ..._utils import path_template, required_args, maybe_transform, async_maybe_transform
 from .captchas import (
     CaptchasResource,
     AsyncCaptchasResource,
@@ -238,7 +238,7 @@ def retrieve(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._get(
-            f"/v1/sessions/{id}",
+            path_template("/v1/sessions/{id}", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -658,7 +658,7 @@ def computer(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return self._post(
-            f"/v1/sessions/{session_id}/computer",
+            path_template("/v1/sessions/{session_id}/computer", session_id=session_id),
             body=maybe_transform(
                 {
                     "action": action,
@@ -709,7 +709,7 @@ def context(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._get(
-            f"/v1/sessions/{id}/context",
+            path_template("/v1/sessions/{id}/context", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -751,7 +751,7 @@ def events(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._get(
-            f"/v1/sessions/{id}/events",
+            path_template("/v1/sessions/{id}/events", id=id),
             options=make_request_options(
                 extra_headers=extra_headers,
                 extra_query=extra_query,
@@ -795,7 +795,7 @@ def live_details(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._get(
-            f"/v1/sessions/{id}/live-details",
+            path_template("/v1/sessions/{id}/live-details", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -828,7 +828,7 @@ def release(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return self._post(
-            f"/v1/sessions/{id}/release",
+            path_template("/v1/sessions/{id}/release", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -1045,7 +1045,7 @@ async def retrieve(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._get(
-            f"/v1/sessions/{id}",
+            path_template("/v1/sessions/{id}", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -1465,7 +1465,7 @@ async def computer(
         if not session_id:
             raise ValueError(f"Expected a non-empty value for `session_id` but received {session_id!r}")
         return await self._post(
-            f"/v1/sessions/{session_id}/computer",
+            path_template("/v1/sessions/{session_id}/computer", session_id=session_id),
             body=await async_maybe_transform(
                 {
                     "action": action,
@@ -1516,7 +1516,7 @@ async def context(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._get(
-            f"/v1/sessions/{id}/context",
+            path_template("/v1/sessions/{id}/context", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -1558,7 +1558,7 @@ async def events(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._get(
-            f"/v1/sessions/{id}/events",
+            path_template("/v1/sessions/{id}/events", id=id),
             options=make_request_options(
                 extra_headers=extra_headers,
                 extra_query=extra_query,
@@ -1602,7 +1602,7 @@ async def live_details(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._get(
-            f"/v1/sessions/{id}/live-details",
+            path_template("/v1/sessions/{id}/live-details", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
@@ -1635,7 +1635,7 @@ async def release(
         if not id:
             raise ValueError(f"Expected a non-empty value for `id` but received {id!r}")
         return await self._post(
-            f"/v1/sessions/{id}/release",
+            path_template("/v1/sessions/{id}/release", id=id),
             options=make_request_options(
                 extra_headers=extra_headers, extra_query=extra_query, extra_body=extra_body, timeout=timeout
             ),
diff --git a/tests/test_client.py b/tests/test_client.py
index 410bcac6..22baf136 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -392,6 +392,30 @@ def test_default_query_option(self) -> None:
 
         client.close()
 
+    def test_hardcoded_query_params_in_url(self, client: Steel) -> None:
+        request = client._build_request(FinalRequestOptions(method="get", url="/foo?beta=true"))
+        url = httpx.URL(request.url)
+        assert dict(url.params) == {"beta": "true"}
+
+        request = client._build_request(
+            FinalRequestOptions(
+                method="get",
+                url="/foo?beta=true",
+                params={"limit": "10", "page": "abc"},
+            )
+        )
+        url = httpx.URL(request.url)
+        assert dict(url.params) == {"beta": "true", "limit": "10", "page": "abc"}
+
+        request = client._build_request(
+            FinalRequestOptions(
+                method="get",
+                url="/files/a%2Fb?beta=true",
+                params={"limit": "10"},
+            )
+        )
+        assert request.url.raw_path == b"/files/a%2Fb?beta=true&limit=10"
+
     def test_request_extra_json(self, client: Steel) -> None:
         request = client._build_request(
             FinalRequestOptions(
@@ -1241,6 +1265,30 @@ async def test_default_query_option(self) -> None:
 
         await client.close()
 
+    async def test_hardcoded_query_params_in_url(self, async_client: AsyncSteel) -> None:
+        request = async_client._build_request(FinalRequestOptions(method="get", url="/foo?beta=true"))
+        url = httpx.URL(request.url)
+        assert dict(url.params) == {"beta": "true"}
+
+        request = async_client._build_request(
+            FinalRequestOptions(
+                method="get",
+                url="/foo?beta=true",
+                params={"limit": "10", "page": "abc"},
+            )
+        )
+        url = httpx.URL(request.url)
+        assert dict(url.params) == {"beta": "true", "limit": "10", "page": "abc"}
+
+        request = async_client._build_request(
+            FinalRequestOptions(
+                method="get",
+                url="/files/a%2Fb?beta=true",
+                params={"limit": "10"},
+            )
+        )
+        assert request.url.raw_path == b"/files/a%2Fb?beta=true&limit=10"
+
     def test_request_extra_json(self, client: Steel) -> None:
         request = client._build_request(
             FinalRequestOptions(
diff --git a/tests/test_deepcopy.py b/tests/test_deepcopy.py
deleted file mode 100644
index 8b0c8442..00000000
--- a/tests/test_deepcopy.py
+++ /dev/null
@@ -1,58 +0,0 @@
-from steel._utils import deepcopy_minimal
-
-
-def assert_different_identities(obj1: object, obj2: object) -> None:
-    assert obj1 == obj2
-    assert id(obj1) != id(obj2)
-
-
-def test_simple_dict() -> None:
-    obj1 = {"foo": "bar"}
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-
-
-def test_nested_dict() -> None:
-    obj1 = {"foo": {"bar": True}}
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-    assert_different_identities(obj1["foo"], obj2["foo"])
-
-
-def test_complex_nested_dict() -> None:
-    obj1 = {"foo": {"bar": [{"hello": "world"}]}}
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-    assert_different_identities(obj1["foo"], obj2["foo"])
-    assert_different_identities(obj1["foo"]["bar"], obj2["foo"]["bar"])
-    assert_different_identities(obj1["foo"]["bar"][0], obj2["foo"]["bar"][0])
-
-
-def test_simple_list() -> None:
-    obj1 = ["a", "b", "c"]
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-
-
-def test_nested_list() -> None:
-    obj1 = ["a", [1, 2, 3]]
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-    assert_different_identities(obj1[1], obj2[1])
-
-
-class MyObject: ...
-
-
-def test_ignores_other_types() -> None:
-    # custom classes
-    my_obj = MyObject()
-    obj1 = {"foo": my_obj}
-    obj2 = deepcopy_minimal(obj1)
-    assert_different_identities(obj1, obj2)
-    assert obj1["foo"] is my_obj
-
-    # tuples
-    obj3 = ("a", "b")
-    obj4 = deepcopy_minimal(obj3)
-    assert obj3 is obj4
diff --git a/tests/test_extract_files.py b/tests/test_extract_files.py
index 6197514b..97ae9499 100644
--- a/tests/test_extract_files.py
+++ b/tests/test_extract_files.py
@@ -35,6 +35,15 @@ def test_multiple_files() -> None:
     assert query == {"documents": [{}, {}]}
 
 
+def test_top_level_file_array() -> None:
+    query = {"files": [b"file one", b"file two"], "title": "hello"}
+    assert extract_files(query, paths=[["files", "<array>"]]) == [
+        ("files[]", b"file one"),
+        ("files[]", b"file two"),
+    ]
+    assert query == {"title": "hello"}
+
+
 @pytest.mark.parametrize(
     "query,paths,expected",
     [
diff --git a/tests/test_files.py b/tests/test_files.py
index ae0e733c..722fb751 100644
--- a/tests/test_files.py
+++ b/tests/test_files.py
@@ -4,7 +4,8 @@
 import pytest
 from dirty_equals import IsDict, IsList, IsBytes, IsTuple
 
-from steel._files import to_httpx_files, async_to_httpx_files
+from steel._files import to_httpx_files, deepcopy_with_paths, async_to_httpx_files
+from steel._utils import extract_files
 
 readme_path = Path(__file__).parent.parent.joinpath("README.md")
 
@@ -49,3 +50,99 @@ def test_string_not_allowed() -> None:
                 "file": "foo",  # type: ignore
             }
         )
+
+
+def assert_different_identities(obj1: object, obj2: object) -> None:
+    assert obj1 == obj2
+    assert obj1 is not obj2
+
+
+class TestDeepcopyWithPaths:
+    def test_copies_top_level_dict(self) -> None:
+        original = {"file": b"data", "other": "value"}
+        result = deepcopy_with_paths(original, [["file"]])
+        assert_different_identities(result, original)
+
+    def test_file_value_is_same_reference(self) -> None:
+        file_bytes = b"contents"
+        original = {"file": file_bytes}
+        result = deepcopy_with_paths(original, [["file"]])
+        assert_different_identities(result, original)
+        assert result["file"] is file_bytes
+
+    def test_list_popped_wholesale(self) -> None:
+        files = [b"f1", b"f2"]
+        original = {"files": files, "title": "t"}
+        result = deepcopy_with_paths(original, [["files", "<array>"]])
+        assert_different_identities(result, original)
+        result_files = result["files"]
+        assert isinstance(result_files, list)
+        assert_different_identities(result_files, files)
+
+    def test_nested_array_path_copies_list_and_elements(self) -> None:
+        elem1 = {"file": b"f1", "extra": 1}
+        elem2 = {"file": b"f2", "extra": 2}
+        original = {"items": [elem1, elem2]}
+        result = deepcopy_with_paths(original, [["items", "<array>", "file"]])
+        assert_different_identities(result, original)
+        result_items = result["items"]
+        assert isinstance(result_items, list)
+        assert_different_identities(result_items, original["items"])
+        assert_different_identities(result_items[0], elem1)
+        assert_different_identities(result_items[1], elem2)
+
+    def test_empty_paths_returns_same_object(self) -> None:
+        original = {"foo": "bar"}
+        result = deepcopy_with_paths(original, [])
+        assert result is original
+
+    def test_multiple_paths(self) -> None:
+        f1 = b"file1"
+        f2 = b"file2"
+        original = {"a": f1, "b": f2, "c": "unchanged"}
+        result = deepcopy_with_paths(original, [["a"], ["b"]])
+        assert_different_identities(result, original)
+        assert result["a"] is f1
+        assert result["b"] is f2
+        assert result["c"] is original["c"]
+
+    def test_extract_files_does_not_mutate_original_top_level(self) -> None:
+        file_bytes = b"contents"
+        original = {"file": file_bytes, "other": "value"}
+
+        copied = deepcopy_with_paths(original, [["file"]])
+        extracted = extract_files(copied, paths=[["file"]])
+
+        assert extracted == [("file", file_bytes)]
+        assert original == {"file": file_bytes, "other": "value"}
+        assert copied == {"other": "value"}
+
+    def test_extract_files_does_not_mutate_original_nested_array_path(self) -> None:
+        file1 = b"f1"
+        file2 = b"f2"
+        original = {
+            "items": [
+                {"file": file1, "extra": 1},
+                {"file": file2, "extra": 2},
+            ],
+            "title": "example",
+        }
+
+        copied = deepcopy_with_paths(original, [["items", "<array>", "file"]])
+        extracted = extract_files(copied, paths=[["items", "<array>", "file"]])
+
+        assert extracted == [("items[][file]", file1), ("items[][file]", file2)]
+        assert original == {
+            "items": [
+                {"file": file1, "extra": 1},
+                {"file": file2, "extra": 2},
+            ],
+            "title": "example",
+        }
+        assert copied == {
+            "items": [
+                {"extra": 1},
+                {"extra": 2},
+            ],
+            "title": "example",
+        }
diff --git a/tests/test_utils/test_path.py b/tests/test_utils/test_path.py
new file mode 100644
index 00000000..55e06b12
--- /dev/null
+++ b/tests/test_utils/test_path.py
@@ -0,0 +1,89 @@
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from steel._utils._path import path_template
+
+
+@pytest.mark.parametrize(
+    "template, kwargs, expected",
+    [
+        ("/v1/{id}", dict(id="abc"), "/v1/abc"),
+        ("/v1/{a}/{b}", dict(a="x", b="y"), "/v1/x/y"),
+        ("/v1/{a}{b}/path/{c}?val={d}#{e}", dict(a="x", b="y", c="z", d="u", e="v"), "/v1/xy/path/z?val=u#v"),
+        ("/{w}/{w}", dict(w="echo"), "/echo/echo"),
+        ("/v1/static", {}, "/v1/static"),
+        ("", {}, ""),
+        ("/v1/?q={n}&count=10", dict(n=42), "/v1/?q=42&count=10"),
+        ("/v1/{v}", dict(v=None), "/v1/null"),
+        ("/v1/{v}", dict(v=True), "/v1/true"),
+        ("/v1/{v}", dict(v=False), "/v1/false"),
+        ("/v1/{v}", dict(v=".hidden"), "/v1/.hidden"),  # dot prefix ok
+        ("/v1/{v}", dict(v="file.txt"), "/v1/file.txt"),  # dot in middle ok
+        ("/v1/{v}", dict(v="..."), "/v1/..."),  # triple dot ok
+        ("/v1/{a}{b}", dict(a=".", b="txt"), "/v1/.txt"),  # dot var combining with adjacent to be ok
+        ("/items?q={v}#{f}", dict(v=".", f=".."), "/items?q=.#.."),  # dots in query/fragment are fine
+        (
+            "/v1/{a}?query={b}",
+            dict(a="../../other/endpoint", b="a&bad=true"),
+            "/v1/..%2F..%2Fother%2Fendpoint?query=a%26bad%3Dtrue",
+        ),
+        ("/v1/{val}", dict(val="a/b/c"), "/v1/a%2Fb%2Fc"),
+        ("/v1/{val}", dict(val="a/b/c?query=value"), "/v1/a%2Fb%2Fc%3Fquery=value"),
+        ("/v1/{val}", dict(val="a/b/c?query=value&bad=true"), "/v1/a%2Fb%2Fc%3Fquery=value&bad=true"),
+        ("/v1/{val}", dict(val="%20"), "/v1/%2520"),  # escapes escape sequences in input
+        # Query: slash and ? are safe, # is not
+        ("/items?q={v}", dict(v="a/b"), "/items?q=a/b"),
+        ("/items?q={v}", dict(v="a?b"), "/items?q=a?b"),
+        ("/items?q={v}", dict(v="a#b"), "/items?q=a%23b"),
+        ("/items?q={v}", dict(v="a b"), "/items?q=a%20b"),
+        # Fragment: slash and ? are safe
+        ("/docs#{v}", dict(v="a/b"), "/docs#a/b"),
+        ("/docs#{v}", dict(v="a?b"), "/docs#a?b"),
+        # Path: slash, ? and # are all encoded
+        ("/v1/{v}", dict(v="a/b"), "/v1/a%2Fb"),
+        ("/v1/{v}", dict(v="a?b"), "/v1/a%3Fb"),
+        ("/v1/{v}", dict(v="a#b"), "/v1/a%23b"),
+        # same var encoded differently by component
+        (
+            "/v1/{v}?q={v}#{v}",
+            dict(v="a/b?c#d"),
+            "/v1/a%2Fb%3Fc%23d?q=a/b?c%23d#a/b?c%23d",
+        ),
+        ("/v1/{val}", dict(val="x?admin=true"), "/v1/x%3Fadmin=true"),  # query injection
+        ("/v1/{val}", dict(val="x#admin"), "/v1/x%23admin"),  # fragment injection
+    ],
+)
+def test_interpolation(template: str, kwargs: dict[str, Any], expected: str) -> None:
+    assert path_template(template, **kwargs) == expected
+
+
+def test_missing_kwarg_raises_key_error() -> None:
+    with pytest.raises(KeyError, match="org_id"):
+        path_template("/v1/{org_id}")
+
+
+@pytest.mark.parametrize(
+    "template, kwargs",
+    [
+        ("{a}/path", dict(a=".")),
+        ("{a}/path", dict(a="..")),
+        ("/v1/{a}", dict(a=".")),
+        ("/v1/{a}", dict(a="..")),
+        ("/v1/{a}/path", dict(a=".")),
+        ("/v1/{a}/path", dict(a="..")),
+        ("/v1/{a}{b}", dict(a=".", b=".")),  # adjacent vars → ".."
+        ("/v1/{a}.", dict(a=".")),  # var + static → ".."
+        ("/v1/{a}{b}", dict(a="", b=".")),  # empty + dot → "."
+        ("/v1/%2e/{x}", dict(x="ok")),  # encoded dot in static text
+        ("/v1/%2e./{x}", dict(x="ok")),  # mixed encoded ".." in static
+        ("/v1/.%2E/{x}", dict(x="ok")),  # mixed encoded ".." in static
+        ("/v1/{v}?q=1", dict(v="..")),
+        ("/v1/{v}#frag", dict(v="..")),
+    ],
+)
+def test_dot_segment_rejected(template: str, kwargs: dict[str, Any]) -> None:
+    with pytest.raises(ValueError, match="dot-segment"):
+        path_template(template, **kwargs)