diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 60b11b0..2711a2f 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/**'
@@ -17,9 +19,9 @@ jobs:
     timeout-minutes: 10
     name: lint
     runs-on: ${{ github.repository == 'stainless-sdks/zeroentropy-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
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install Rye
         run: |
@@ -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:
@@ -44,7 +46,7 @@ jobs:
       id-token: write
     runs-on: ${{ github.repository == 'stainless-sdks/zeroentropy-python' && 'depot-ubuntu-24.04' || 'ubuntu-latest' }}
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install Rye
         run: |
@@ -61,14 +63,18 @@ jobs:
         run: rye build
 
       - name: Get GitHub OIDC Token
-        if: github.repository == 'stainless-sdks/zeroentropy-python'
+        if: |-
+          github.repository == 'stainless-sdks/zeroentropy-python' &&
+          !startsWith(github.ref, 'refs/heads/stl/')
         id: github-oidc
-        uses: actions/github-script@v8
+        uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8.0.0
         with:
           script: core.setOutput('github_token', await core.getIDToken());
 
       - name: Upload tarball
-        if: github.repository == 'stainless-sdks/zeroentropy-python'
+        if: |-
+          github.repository == 'stainless-sdks/zeroentropy-python' &&
+          !startsWith(github.ref, 'refs/heads/stl/')
         env:
           URL: https://pkg.stainless.com/s
           AUTH: ${{ steps.github-oidc.outputs.github_token }}
@@ -81,7 +87,7 @@ jobs:
     runs-on: ${{ github.repository == 'stainless-sdks/zeroentropy-python' && 'depot-ubuntu-24.04' || 'ubuntu-latest' }}
     if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install Rye
         run: |
diff --git a/.github/workflows/publish-pypi.yml b/.github/workflows/publish-pypi.yml
index ee48e47..50d291c 100644
--- a/.github/workflows/publish-pypi.yml
+++ b/.github/workflows/publish-pypi.yml
@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install Rye
         run: |
diff --git a/.github/workflows/release-doctor.yml b/.github/workflows/release-doctor.yml
index 50373dd..166ad75 100644
--- a/.github/workflows/release-doctor.yml
+++ b/.github/workflows/release-doctor.yml
@@ -12,7 +12,7 @@ jobs:
     if: github.repository == 'zeroentropy-ai/zeroentropy-python' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch' || startsWith(github.head_ref, 'release-please') || github.head_ref == 'next')
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Check release environment
         run: |
diff --git a/.gitignore b/.gitignore
index 95ceb18..3824f4c 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 ee49ac2..fd0ccba 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "0.1.0-alpha.11"
+  ".": "0.1.0-alpha.12"
 }
\ No newline at end of file
diff --git a/.stats.yml b/.stats.yml
index a9365b5..8a4fe5a 100644
--- a/.stats.yml
+++ b/.stats.yml
@@ -1,4 +1,4 @@
 configured_endpoints: 15
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/zeroentropy%2Fzeroentropy-5ff1b24060b908d169910a7366213eeb423b52d14b03cc16902923eb0ffac71a.yml
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/zeroentropy/zeroentropy-6d6fee8f2524057cf8e4329638eb7c97e2befc3054e867c6ab0931bea37bc6ec.yml
 openapi_spec_hash: fc50a1765bfbfe9a3da7ea9001e479d4
 config_hash: e56152e1ee1a9273241d925702077e49
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 54812b2..83afcc9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,55 @@
 # Changelog
 
+## 0.1.0-alpha.12 (2026-05-13)
+
+Full Changelog: [v0.1.0-alpha.11...v0.1.0-alpha.12](https://github.com/zeroentropy-ai/zeroentropy-python/compare/v0.1.0-alpha.11...v0.1.0-alpha.12)
+
+### Features
+
+* **internal:** implement indices array format for query and form serialization ([7afdedd](https://github.com/zeroentropy-ai/zeroentropy-python/commit/7afdedd66e2dff463aef37287e84596240d9ec3a))
+* support setting headers via env ([203afd7](https://github.com/zeroentropy-ai/zeroentropy-python/commit/203afd7d695864ce3d7175d00d9f0a2bf987a03e))
+
+
+### Bug Fixes
+
+* **client:** preserve hardcoded query params when merging with user params ([bb86cc5](https://github.com/zeroentropy-ai/zeroentropy-python/commit/bb86cc51decf47d411f9959dee26996fd2ec501b))
+* **deps:** bump minimum typing-extensions version ([f4373a8](https://github.com/zeroentropy-ai/zeroentropy-python/commit/f4373a8d663cb8d2ca4212f77eade5269b518d1e))
+* ensure file data are only sent as 1 parameter ([ca9efb2](https://github.com/zeroentropy-ai/zeroentropy-python/commit/ca9efb2c1f815a83b97e922cbf361f0fedd1cbc5))
+* **pydantic:** do not pass `by_alias` unless set ([74ac1d3](https://github.com/zeroentropy-ai/zeroentropy-python/commit/74ac1d3f8dac62dc11de9550d88838d984301ca5))
+* sanitize endpoint path params ([a5a6035](https://github.com/zeroentropy-ai/zeroentropy-python/commit/a5a60354eabb5dce3a6a8c519311cd8590995179))
+* use correct field name format for multipart file arrays ([6dbcfc6](https://github.com/zeroentropy-ai/zeroentropy-python/commit/6dbcfc65563a368ddb47e38f4939281a96946499))
+
+
+### Performance Improvements
+
+* **client:** optimize file structure copying in multipart requests ([62638d4](https://github.com/zeroentropy-ai/zeroentropy-python/commit/62638d4e38f6dbe92abf136d98b105265eb3557f))
+
+
+### Chores
+
+* **ci:** skip lint on metadata-only changes ([ca93b8a](https://github.com/zeroentropy-ai/zeroentropy-python/commit/ca93b8ae56e0dddb8d49ecc21125c2a1751594c6))
+* **ci:** skip uploading artifacts on stainless-internal branches ([d6bb8a8](https://github.com/zeroentropy-ai/zeroentropy-python/commit/d6bb8a8306064729e4739209011106a6cc5e224f))
+* **internal:** codegen related update ([22e2e37](https://github.com/zeroentropy-ai/zeroentropy-python/commit/22e2e3741320284fb678722a90d5febb6e423434))
+* **internal:** codegen related update ([d04e826](https://github.com/zeroentropy-ai/zeroentropy-python/commit/d04e82642f9257d413e2787d8d9841ed414a3cb0))
+* **internal:** codegen related update ([6d2c530](https://github.com/zeroentropy-ai/zeroentropy-python/commit/6d2c530d2206b6ed32b7619750321b77fca83f51))
+* **internal:** more robust bootstrap script ([b6b18e4](https://github.com/zeroentropy-ai/zeroentropy-python/commit/b6b18e43b1c4d1c20c112eb410ada7de1d83ec96))
+* **internal:** reformat pyproject.toml ([0588f9b](https://github.com/zeroentropy-ai/zeroentropy-python/commit/0588f9b51128f9de5f21a1e10451a25837a48f93))
+* **internal:** tweak CI branches ([536468f](https://github.com/zeroentropy-ai/zeroentropy-python/commit/536468f4e298fb16e55818729b42fbc906a9d21f))
+* **internal:** update gitignore ([3009ce0](https://github.com/zeroentropy-ai/zeroentropy-python/commit/3009ce0201a8f99cb608182617796dc71e17a910))
+* **test:** do not count install time for mock server timeout ([ed8f79f](https://github.com/zeroentropy-ai/zeroentropy-python/commit/ed8f79f5b4c05cde5e12eb4c6a8137cc6b7dde74))
+* **tests:** bump steady to v0.19.4 ([814a31b](https://github.com/zeroentropy-ai/zeroentropy-python/commit/814a31b0de576a870d04e41ef0b52da726741311))
+* **tests:** bump steady to v0.19.5 ([f9bdfb4](https://github.com/zeroentropy-ai/zeroentropy-python/commit/f9bdfb48058be235113b6c19054a15e670d7c32b))
+* **tests:** bump steady to v0.19.6 ([9433bde](https://github.com/zeroentropy-ai/zeroentropy-python/commit/9433bde7d0d7953ad3f9a7327bf1468f30f6d600))
+* **tests:** bump steady to v0.19.7 ([e2ddebd](https://github.com/zeroentropy-ai/zeroentropy-python/commit/e2ddebd1af39af87aea48cc211b2b3520002f02c))
+* **tests:** bump steady to v0.20.1 ([4942675](https://github.com/zeroentropy-ai/zeroentropy-python/commit/494267565119547a125bc548f258bda341c1925c))
+* **tests:** bump steady to v0.20.2 ([73a8f04](https://github.com/zeroentropy-ai/zeroentropy-python/commit/73a8f04822fb42ddcbc63946b99801ca12c8e87e))
+* **tests:** bump steady to v0.22.1 ([9c3fcb6](https://github.com/zeroentropy-ai/zeroentropy-python/commit/9c3fcb69b97fdb396880759e5bea46f54e0dde99))
+
+
+### Refactors
+
+* **tests:** switch from prism to steady ([5465c72](https://github.com/zeroentropy-ai/zeroentropy-python/commit/5465c7207251fa90b2281a7429c2f0f078632b2e))
+
 ## 0.1.0-alpha.11 (2026-03-03)
 
 Full Changelog: [v0.1.0-alpha.10...v0.1.0-alpha.11](https://github.com/zeroentropy-ai/zeroentropy-python/compare/v0.1.0-alpha.10...v0.1.0-alpha.11)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 7918a64..fd0d5fb 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 729d5d1..002ab2d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "zeroentropy"
-version = "0.1.0-alpha.11"
+version = "0.1.0-alpha.12"
 description = "The official Python library for the ZeroEntropy 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",
@@ -168,7 +168,7 @@ show_error_codes = true
 #
 # We also exclude our `tests` as mypy doesn't always infer
 # types correctly and Pyright will still catch any type errors.
-exclude = ['src/zeroentropy/_files.py', '_dev/.*.py', 'tests/.*']
+exclude = ["src/zeroentropy/_files.py", "_dev/.*.py", "tests/.*"]
 
 strict_equality = true
 implicit_reexport = true
diff --git a/scripts/bootstrap b/scripts/bootstrap
index b430fee..fe8451e 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 0b28f6e..feebe5e 100755
--- a/scripts/mock
+++ b/scripts/mock
@@ -19,23 +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 [ "$1" == "--daemon" ]; then
-  npm exec --package=@stainless-api/prism-cli@5.15.0 -- prism mock "$URL" &> .prism.log &
+  # Pre-install the package so the download doesn't eat into the startup timeout
+  npm exec --package=@stdy/cli@0.22.1 -- steady --version
 
-  # Wait for server to come online
+  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 via health endpoint (max 30s)
   echo -n "Waiting for server"
-  while ! grep -q "✖  fatal\|Prism is listening" ".prism.log" ; do
+  attempts=0
+  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 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 dbeda2d..19acc91 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/zeroentropy/_base_client.py b/src/zeroentropy/_base_client.py
index ddeaa86..e79904f 100644
--- a/src/zeroentropy/_base_client.py
+++ b/src/zeroentropy/_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/zeroentropy/_client.py b/src/zeroentropy/_client.py
index b7fd70d..198d788 100644
--- a/src/zeroentropy/_client.py
+++ b/src/zeroentropy/_client.py
@@ -19,7 +19,11 @@
     RequestOptions,
     not_given,
 )
-from ._utils import is_given, get_async_library
+from ._utils import (
+    is_given,
+    is_mapping_t,
+    get_async_library,
+)
 from ._compat import cached_property
 from ._version import __version__
 from ._streaming import Stream as Stream, AsyncStream as AsyncStream
@@ -94,6 +98,15 @@ def __init__(
         if base_url is None:
             base_url = f"https://api.zeroentropy.dev/v1"
 
+        custom_headers_env = os.environ.get("ZEROENTROPY_CUSTOM_HEADERS")
+        if custom_headers_env is not None:
+            parsed: dict[str, str] = {}
+            for line in custom_headers_env.split("\n"):
+                colon = line.find(":")
+                if colon >= 0:
+                    parsed[line[:colon].strip()] = line[colon + 1 :].strip()
+            default_headers = {**parsed, **(default_headers if is_mapping_t(default_headers) else {})}
+
         super().__init__(
             version=__version__,
             base_url=base_url,
@@ -292,6 +305,15 @@ def __init__(
         if base_url is None:
             base_url = f"https://api.zeroentropy.dev/v1"
 
+        custom_headers_env = os.environ.get("ZEROENTROPY_CUSTOM_HEADERS")
+        if custom_headers_env is not None:
+            parsed: dict[str, str] = {}
+            for line in custom_headers_env.split("\n"):
+                colon = line.find(":")
+                if colon >= 0:
+                    parsed[line[:colon].strip()] = line[colon + 1 :].strip()
+            default_headers = {**parsed, **(default_headers if is_mapping_t(default_headers) else {})}
+
         super().__init__(
             version=__version__,
             base_url=base_url,
diff --git a/src/zeroentropy/_compat.py b/src/zeroentropy/_compat.py
index 786ff42..e6690a4 100644
--- a/src/zeroentropy/_compat.py
+++ b/src/zeroentropy/_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/zeroentropy/_files.py b/src/zeroentropy/_files.py
index cc14c14..76da9e0 100644
--- a/src/zeroentropy/_files.py
+++ b/src/zeroentropy/_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]:
@@ -97,7 +99,7 @@ async def async_to_httpx_files(files: RequestFiles | None) -> HttpxRequestFiles
     elif is_sequence_t(files):
         files = [(key, await _async_transform_file(file)) for key, file in files]
     else:
-        raise TypeError("Unexpected file type input {type(files)}, expected mapping or sequence")
+        raise TypeError(f"Unexpected file type input {type(files)}, expected mapping or sequence")
 
     return files
 
@@ -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/zeroentropy/_models.py b/src/zeroentropy/_models.py
index 29070e0..8c5ab26 100644
--- a/src/zeroentropy/_models.py
+++ b/src/zeroentropy/_models.py
@@ -25,7 +25,9 @@
     ClassVar,
     Protocol,
     Required,
+    Annotated,
     ParamSpec,
+    TypeAlias,
     TypedDict,
     TypeGuard,
     final,
@@ -79,7 +81,15 @@
 from ._constants import RAW_RESPONSE_HEADER
 
 if TYPE_CHECKING:
+    from pydantic import GetCoreSchemaHandler, ValidatorFunctionWrapHandler
+    from pydantic_core import CoreSchema, core_schema
     from pydantic_core.core_schema import ModelField, ModelSchema, LiteralSchema, ModelFieldsSchema
+else:
+    try:
+        from pydantic_core import CoreSchema, core_schema
+    except ImportError:
+        CoreSchema = None
+        core_schema = None
 
 __all__ = ["BaseModel", "GenericModel"]
 
@@ -396,6 +406,76 @@ def model_dump_json(
             )
 
 
+class _EagerIterable(list[_T], Generic[_T]):
+    """
+    Accepts any Iterable[T] input (including generators), consumes it
+    eagerly, and validates all items upfront.
+
+    Validation preserves the original container type where possible
+    (e.g. a set[T] stays a set[T]).  Serialization (model_dump / JSON)
+    always emits a list — round-tripping through model_dump() will not
+    restore the original container type.
+    """
+
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls,
+        source_type: Any,
+        handler: GetCoreSchemaHandler,
+    ) -> CoreSchema:
+        (item_type,) = get_args(source_type) or (Any,)
+        item_schema: CoreSchema = handler.generate_schema(item_type)
+        list_of_items_schema: CoreSchema = core_schema.list_schema(item_schema)
+
+        return core_schema.no_info_wrap_validator_function(
+            cls._validate,
+            list_of_items_schema,
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                cls._serialize,
+                info_arg=False,
+            ),
+        )
+
+    @staticmethod
+    def _validate(v: Iterable[_T], handler: "ValidatorFunctionWrapHandler") -> Any:
+        original_type: type[Any] = type(v)
+
+        # Normalize to list so list_schema can validate each item
+        if isinstance(v, list):
+            items: list[_T] = v
+        else:
+            try:
+                items = list(v)
+            except TypeError as e:
+                raise TypeError("Value is not iterable") from e
+
+        # Validate items against the inner schema
+        validated: list[_T] = handler(items)
+
+        # Reconstruct original container type
+        if original_type is list:
+            return validated
+        # str(list) produces the list's repr, not a string built from items,
+        # so skip reconstruction for str and its subclasses.
+        if issubclass(original_type, str):
+            return validated
+        try:
+            return original_type(validated)
+        except (TypeError, ValueError):
+            # If the type cannot be reconstructed, just return the validated list
+            return validated
+
+    @staticmethod
+    def _serialize(v: Iterable[_T]) -> list[_T]:
+        """Always serialize as a list so Pydantic's JSON encoder is happy."""
+        if isinstance(v, list):
+            return v
+        return list(v)
+
+
+EagerIterable: TypeAlias = Annotated[Iterable[_T], _EagerIterable]
+
+
 def _construct_field(value: object, field: FieldInfo, key: str) -> object:
     if value is None:
         return field_get_default(field)
diff --git a/src/zeroentropy/_qs.py b/src/zeroentropy/_qs.py
index ada6fd3..4127c19 100644
--- a/src/zeroentropy/_qs.py
+++ b/src/zeroentropy/_qs.py
@@ -2,17 +2,13 @@
 
 from typing import Any, List, Tuple, Union, Mapping, TypeVar
 from urllib.parse import parse_qs, urlencode
-from typing_extensions import Literal, get_args
+from typing_extensions import get_args
 
-from ._types import NotGiven, not_given
+from ._types import NotGiven, ArrayFormat, NestedFormat, not_given
 from ._utils import flatten
 
 _T = TypeVar("_T")
 
-
-ArrayFormat = Literal["comma", "repeat", "indices", "brackets"]
-NestedFormat = Literal["dots", "brackets"]
-
 PrimitiveData = Union[str, int, float, bool, None]
 # this should be Data = Union[PrimitiveData, "List[Data]", "Tuple[Data]", "Mapping[str, Data]"]
 # https://github.com/microsoft/pyright/issues/3555
@@ -101,7 +97,10 @@ 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/zeroentropy/_types.py b/src/zeroentropy/_types.py
index 80afed5..8674e80 100644
--- a/src/zeroentropy/_types.py
+++ b/src/zeroentropy/_types.py
@@ -47,6 +47,9 @@
 ModelT = TypeVar("ModelT", bound=pydantic.BaseModel)
 _T = TypeVar("_T")
 
+ArrayFormat = Literal["comma", "repeat", "indices", "brackets"]
+NestedFormat = Literal["dots", "brackets"]
+
 
 # Approximates httpx internal ProxiesTypes and RequestFiles types
 # while adding support for `PathLike` instances
diff --git a/src/zeroentropy/_utils/__init__.py b/src/zeroentropy/_utils/__init__.py
index dc64e29..1c090e5 100644
--- a/src/zeroentropy/_utils/__init__.py
+++ b/src/zeroentropy/_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/zeroentropy/_utils/_path.py b/src/zeroentropy/_utils/_path.py
new file mode 100644
index 0000000..4d6e1e4
--- /dev/null
+++ b/src/zeroentropy/_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/zeroentropy/_utils/_utils.py b/src/zeroentropy/_utils/_utils.py
index eec7f4a..199cd23 100644
--- a/src/zeroentropy/_utils/_utils.py
+++ b/src/zeroentropy/_utils/_utils.py
@@ -17,11 +17,11 @@
 )
 from pathlib import Path
 from datetime import date, datetime
-from typing_extensions import TypeGuard
+from typing_extensions import TypeGuard, get_args
 
 import sniffio
 
-from .._types import Omit, NotGiven, FileTypes, HeadersLike
+from .._types import Omit, NotGiven, FileTypes, ArrayFormat, HeadersLike
 
 _T = TypeVar("_T")
 _TupleT = TypeVar("_TupleT", bound=Tuple[object, ...])
@@ -40,25 +40,45 @@ def extract_files(
     query: Mapping[str, object],
     *,
     paths: Sequence[Sequence[str]],
+    array_format: ArrayFormat = "brackets",
 ) -> list[tuple[str, FileTypes]]:
     """Recursively extract files from the given dictionary based on specified paths.
 
     A path may look like this ['foo', 'files', '<array>', 'data'].
 
+    ``array_format`` controls how ``<array>`` segments contribute to the emitted
+    field name. Supported values: ``"brackets"`` (``foo[]``), ``"repeat"`` and
+    ``"comma"`` (``foo``), ``"indices"`` (``foo[0]``, ``foo[1]``).
+
     Note: this mutates the given dictionary.
     """
     files: list[tuple[str, FileTypes]] = []
     for path in paths:
-        files.extend(_extract_items(query, path, index=0, flattened_key=None))
+        files.extend(_extract_items(query, path, index=0, flattened_key=None, array_format=array_format))
     return files
 
 
+def _array_suffix(array_format: ArrayFormat, array_index: int) -> str:
+    if array_format == "brackets":
+        return "[]"
+    if array_format == "indices":
+        return f"[{array_index}]"
+    if array_format == "repeat" or array_format == "comma":
+        # Both repeat the bare field name for each file part; there is no
+        # meaningful way to comma-join binary parts.
+        return ""
+    raise NotImplementedError(
+        f"Unknown array_format value: {array_format}, choose from {', '.join(get_args(ArrayFormat))}"
+    )
+
+
 def _extract_items(
     obj: object,
     path: Sequence[str],
     *,
     index: int,
     flattened_key: str | None,
+    array_format: ArrayFormat,
 ) -> list[tuple[str, FileTypes]]:
     try:
         key = path[index]
@@ -75,9 +95,11 @@ def _extract_items(
 
         if is_list(obj):
             files: list[tuple[str, FileTypes]] = []
-            for entry in obj:
-                assert_is_file_content(entry, key=flattened_key + "[]" if flattened_key else "")
-                files.append((flattened_key + "[]", cast(FileTypes, entry)))
+            for array_index, entry in enumerate(obj):
+                suffix = _array_suffix(array_format, array_index)
+                emitted_key = (flattened_key + suffix) if flattened_key else suffix
+                assert_is_file_content(entry, key=emitted_key)
+                files.append((emitted_key, cast(FileTypes, entry)))
             return files
 
         assert_is_file_content(obj, key=flattened_key)
@@ -86,8 +108,9 @@ 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]
@@ -105,6 +128,7 @@ def _extract_items(
             path,
             index=index,
             flattened_key=flattened_key,
+            array_format=array_format,
         )
     elif is_list(obj):
         if key != "<array>":
@@ -116,9 +140,12 @@ def _extract_items(
                     item,
                     path,
                     index=index,
-                    flattened_key=flattened_key + "[]" if flattened_key is not None else "[]",
+                    flattened_key=(
+                        (flattened_key if flattened_key is not None else "") + _array_suffix(array_format, array_index)
+                    ),
+                    array_format=array_format,
                 )
-                for item in obj
+                for array_index, item in enumerate(obj)
             ]
         )
 
@@ -176,21 +203,6 @@ 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/zeroentropy/_version.py b/src/zeroentropy/_version.py
index 5361556..933f906 100644
--- a/src/zeroentropy/_version.py
+++ b/src/zeroentropy/_version.py
@@ -1,4 +1,4 @@
 # File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 __title__ = "zeroentropy"
-__version__ = "0.1.0-alpha.11"  # x-release-please-version
+__version__ = "0.1.0-alpha.12"  # x-release-please-version
diff --git a/tests/test_client.py b/tests/test_client.py
index 6582560..00abf52 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -429,6 +429,30 @@ def test_default_query_option(self) -> None:
 
         client.close()
 
+    def test_hardcoded_query_params_in_url(self, client: ZeroEntropy) -> 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: ZeroEntropy) -> None:
         request = client._build_request(
             FinalRequestOptions(
@@ -1330,6 +1354,30 @@ async def test_default_query_option(self) -> None:
 
         await client.close()
 
+    async def test_hardcoded_query_params_in_url(self, async_client: AsyncZeroEntropy) -> 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: ZeroEntropy) -> None:
         request = client._build_request(
             FinalRequestOptions(
diff --git a/tests/test_deepcopy.py b/tests/test_deepcopy.py
deleted file mode 100644
index 8300d0e..0000000
--- a/tests/test_deepcopy.py
+++ /dev/null
@@ -1,58 +0,0 @@
-from zeroentropy._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 aedd4be..9fd07af 100644
--- a/tests/test_extract_files.py
+++ b/tests/test_extract_files.py
@@ -4,7 +4,7 @@
 
 import pytest
 
-from zeroentropy._types import FileTypes
+from zeroentropy._types import FileTypes, ArrayFormat
 from zeroentropy._utils import extract_files
 
 
@@ -35,6 +35,12 @@ 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",
     [
@@ -62,3 +68,24 @@ def test_ignores_incorrect_paths(
     expected: list[tuple[str, FileTypes]],
 ) -> None:
     assert extract_files(query, paths=paths) == expected
+
+
+@pytest.mark.parametrize(
+    "array_format,expected_top_level,expected_nested",
+    [
+        ("brackets", [("files[]", b"a"), ("files[]", b"b")], [("items[][file]", b"a"), ("items[][file]", b"b")]),
+        ("repeat", [("files", b"a"), ("files", b"b")], [("items[file]", b"a"), ("items[file]", b"b")]),
+        ("comma", [("files", b"a"), ("files", b"b")], [("items[file]", b"a"), ("items[file]", b"b")]),
+        ("indices", [("files[0]", b"a"), ("files[1]", b"b")], [("items[0][file]", b"a"), ("items[1][file]", b"b")]),
+    ],
+)
+def test_array_format_controls_file_field_names(
+    array_format: ArrayFormat,
+    expected_top_level: list[tuple[str, FileTypes]],
+    expected_nested: list[tuple[str, FileTypes]],
+) -> None:
+    top_level = {"files": [b"a", b"b"]}
+    assert extract_files(top_level, paths=[["files", "<array>"]], array_format=array_format) == expected_top_level
+
+    nested = {"items": [{"file": b"a"}, {"file": b"b"}]}
+    assert extract_files(nested, paths=[["items", "<array>", "file"]], array_format=array_format) == expected_nested
diff --git a/tests/test_files.py b/tests/test_files.py
index 84448cb..2db1a07 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 zeroentropy._files import to_httpx_files, async_to_httpx_files
+from zeroentropy._files import to_httpx_files, deepcopy_with_paths, async_to_httpx_files
+from zeroentropy._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 [entry for _, entry in extracted] == [file1, 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_models.py b/tests/test_models.py
index ed031c7..7a7f354 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -1,7 +1,8 @@
 import json
-from typing import TYPE_CHECKING, Any, Dict, List, Union, Optional, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Union, Iterable, Optional, cast
 from datetime import datetime, timezone
-from typing_extensions import Literal, Annotated, TypeAliasType
+from collections import deque
+from typing_extensions import Literal, Annotated, TypedDict, TypeAliasType
 
 import pytest
 import pydantic
@@ -9,7 +10,7 @@
 
 from zeroentropy._utils import PropertyInfo
 from zeroentropy._compat import PYDANTIC_V1, parse_obj, model_dump, model_json
-from zeroentropy._models import DISCRIMINATOR_CACHE, BaseModel, construct_type
+from zeroentropy._models import DISCRIMINATOR_CACHE, BaseModel, EagerIterable, construct_type
 
 
 class BasicModel(BaseModel):
@@ -961,3 +962,56 @@ def __getattr__(self, attr: str) -> Item: ...
     assert model.a.prop == 1
     assert isinstance(model.a, Item)
     assert model.other == "foo"
+
+
+# NOTE: Workaround for Pydantic Iterable behavior.
+# Iterable fields are replaced with a ValidatorIterator and may be consumed
+# during serialization, which can cause subsequent dumps to return empty data.
+# See: https://github.com/pydantic/pydantic/issues/9541
+@pytest.mark.parametrize(
+    "data, expected_validated",
+    [
+        ([1, 2, 3], [1, 2, 3]),
+        ((1, 2, 3), (1, 2, 3)),
+        (set([1, 2, 3]), set([1, 2, 3])),
+        (iter([1, 2, 3]), [1, 2, 3]),
+        ([], []),
+        ((x for x in [1, 2, 3]), [1, 2, 3]),
+        (map(lambda x: x, [1, 2, 3]), [1, 2, 3]),
+        (frozenset([1, 2, 3]), frozenset([1, 2, 3])),
+        (deque([1, 2, 3]), deque([1, 2, 3])),
+    ],
+    ids=["list", "tuple", "set", "iterator", "empty", "generator", "map", "frozenset", "deque"],
+)
+@pytest.mark.skipif(PYDANTIC_V1, reason="this is only supported in pydantic v2")
+def test_iterable_construction(data: Iterable[int], expected_validated: Iterable[int]) -> None:
+    class TypeWithIterable(TypedDict):
+        items: EagerIterable[int]
+
+    class Model(BaseModel):
+        data: TypeWithIterable
+
+    m = Model.model_validate({"data": {"items": data}})
+    assert m.data["items"] == expected_validated
+
+    # Verify repeated dumps don't lose data (the original bug)
+    assert m.model_dump()["data"]["items"] == list(expected_validated)
+    assert m.model_dump()["data"]["items"] == list(expected_validated)
+
+
+@pytest.mark.skipif(PYDANTIC_V1, reason="this is only supported in pydantic v2")
+def test_iterable_construction_str_falls_back_to_list() -> None:
+    # str is iterable (over chars), but str(list_of_chars) produces the list's repr
+    # rather than reconstructing a string from items. We special-case str to fall
+    # back to list instead of attempting reconstruction.
+    class TypeWithIterable(TypedDict):
+        items: EagerIterable[str]
+
+    class Model(BaseModel):
+        data: TypeWithIterable
+
+    m = Model.model_validate({"data": {"items": "hello"}})
+
+    # falls back to list of chars rather than calling str(["h", "e", "l", "l", "o"])
+    assert m.data["items"] == ["h", "e", "l", "l", "o"]
+    assert m.model_dump()["data"]["items"] == ["h", "e", "l", "l", "o"]
diff --git a/tests/test_utils/test_path.py b/tests/test_utils/test_path.py
new file mode 100644
index 0000000..c5d6b2a
--- /dev/null
+++ b/tests/test_utils/test_path.py
@@ -0,0 +1,89 @@
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from zeroentropy._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)