---
title: "Configuration reference"
description: "Every section and key in Kiln's configuration file, with types, defaults, constraints, and environment equivalents."
---

> Documentation Index
> Fetch the complete documentation index at: https://kiln.wbxdocs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Configuration reference

The sections below list every configuration key. Values in the **Default** column come from the code and may differ from values written in the example files. The examples are ready-to-use starting points, not a record of every built-in default.

## File format and loading

Configuration comes in TOML or JSONC. The two formats carry identical keys and structure; the file extension picks the parser:

- `.toml` is parsed as TOML.
- `.json` and `.jsonc` are parsed as JSONC: `//` line comments, `/* */` block comments, and trailing commas in objects and arrays are stripped first, then the result is parsed as standard JSON. Those characters inside string literals are left alone.
- Any other extension is rejected at startup, with the accepted extensions named in the error.

`configs/examples/kiln.toml` and `configs/examples/kiln.jsonc` in the repository are the same configuration written both ways. Keep local, private configuration in `configs/local.toml`, which is already gitignored.

The path is passed with `-config` and is mandatory; without it the process exits with code `2`:

```bash
kiln -config /etc/kiln/kiln.toml
```

> **Which relative paths get rewritten**
>
> Only `packager.keys_file` resolves relative paths **against the directory holding the config file**. Every other path key (`server.data_dir`, `epg.cache_dir`, `auth.token_private_key_file`, and so on) is used as written, so relative values resolve **against the process working directory**. Under systemd or in a container the working directory is rarely what you assume, so prefer absolute paths for anything path-shaped.

### When configuration is read and validated

Configuration is read once at startup and never reloaded; edits require a restart. Loading runs in a fixed order:

1. **Parse**

   Parse by extension. Syntax errors fail immediately.
2. **Apply environment overrides**

   Apply the `KILN_*` overrides, which at this stage outrank the matching keys in the file. See [Environment variables](/en/reference/env/).
3. **Fill defaults**

   Fill in defaults for empty or non-positive values and normalize `[[channels]]`: lowercase `ingress`, force `restart_on_failure` for DASH channels, and set `on_demand` when neither `on_demand` nor `autostart` was set.
4. **Resolve and load the key file**

   Resolve `packager.keys_file` to an absolute path, then read and validate it in full. A failure here is a startup failure.
5. **Validate**

   Run every structural check. Any failure prints the reason and exits with code `1`.

Resource adaptation runs after validation. Based on detected memory and CPU ceilings it overrides `server.memory_limit_mb`, `packager.inflight_bytes`, `packager.max_segment_bytes`, `packager.start_segments`, `packager.prefetch_segments`, `epg.max_refresh_concurrency`, and `epg.max_source_bytes` — **only downward, never upward**. The startup log prints the resulting values.

### What lives in the config file and what lives in the database

`[[channels]]`, `[[proxies]]`, and `[[egress.rules]]` seed the SQLite database under `server.data_dir` only while the corresponding table is empty. After that the admin surface owns them, and editing the config file no longer overwrites them. `egress.default`, `egress.playlist_policy`, `egress.docker_proxy_host`, and `server.public_base_url` are written with insert-if-absent semantics, so they too only apply on first run. `[[upstreams]]`, `[auth]`, and the remaining global keys are always taken from the file, though the username and password of an entry in `[[auth.users]]` can be changed from the admin surface and stored as a database override.

## `[server]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `listen` | string | `"0.0.0.0:8080"` | HTTP listen address. Environment equivalent: `KILN_LISTEN`. |
| `public_base_url` | string | `"http://127.0.0.1:8080"` | Externally reachable base URL used to build playback and other outward links. A trailing slash is trimmed. Environment equivalent: `KILN_PUBLIC_BASE_URL`. |
| `data_dir` | string | `"./data"` | Data directory holding SQLite, auto-generated signing keys, and the default EPG cache. Created with mode `0750` at startup. Environment equivalent: `KILN_DATA_DIR`. |
| `resource_mode` | string | `"auto"` | Resource adaptation mode; one of `auto`, `performance`, `constrained`. Anything else fails validation. Environment equivalent: `KILN_RESOURCE_MODE`. |
| `read_timeout_sec` | int | `15` | HTTP read timeout in seconds. Non-positive values fall back to the default. |
| `write_timeout_sec` | int | `0` | HTTP write timeout in seconds; `0` means no write timeout. A write timeout cuts long-lived streaming responses, so leave it at `0` unless you specifically need one. |
| `idle_timeout_sec` | int | `120` | HTTP idle timeout in seconds. Non-positive values fall back to the default. |
| `memory_limit_mb` | int | `0` | Go soft memory target in MiB; `0` leaves it unset. Negative or absurdly large values fail validation. Applied only when `GOMEMLIMIT` is unset. |

### The three resource modes

- `auto` first picks an internal profile from effective memory (compact below 256 MiB, balanced below 512 MiB, standard below 1 GiB, configured values kept at 1 GiB and above), then applies a CPU ceiling independently to pipeline depth and EPG concurrency.
- `constrained` skips detection and applies the tightest compact budget outright.
- `performance` opts out of adaptation entirely; configured values are used as written.

In every mode, adaptation only lowers configured values; it never raises a smaller value. See [Resource adaptation](/en/guide/operations/#resource-adaptation) for profile budgets, CPU ceilings, and the fixed Lite budget.

## `[logging]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `level` | string | `"info"` | Log level. Recognizes `debug`, `info`, `warn`, `error` and common aliases; unrecognized values fall back to `info`. Environment equivalent: `KILN_LOG_LEVEL`. |
| `format` | string | `"text"` | Output format. `json` selects the structured handler; anything else is treated as console text. Environment equivalent: `KILN_LOG_FORMAT`. |
| `color` | string | `"auto"` | Coloring policy. `always` forces color, `never` disables it, anything else behaves as `auto`, which colors only when the output is a terminal and `NO_COLOR` is unset. Only meaningful for `text`. Environment equivalent: `KILN_LOG_COLOR`. |

## `[auth]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `token_private_key` | string | `""` | Inline Ed25519 private key PEM. Environment equivalent: `KILN_TOKEN_PRIVATE_KEY`. |
| `token_public_key` | string | `""` | Inline Ed25519 public key PEM, used only to cross-check the private key. A mismatch fails startup. Environment equivalent: `KILN_TOKEN_PUBLIC_KEY`. |
| `token_private_key_file` | string | `""` | Path to a private key PEM, read when the inline private key is empty. Environment equivalent: `KILN_TOKEN_PRIVATE_KEY_FILE`. |
| `token_public_key_file` | string | `""` | Path to a public key PEM, read when the inline public key is empty. Environment equivalent: `KILN_TOKEN_PUBLIC_KEY_FILE`. |
| `token_issuer` | string | `"kiln"` | `iss` claim of the session JWT. |
| `token_audience` | string | `"kiln"` | `aud` claim of the session JWT. |
| `token_ttl_hours` | int | `24` | Session JWT lifetime in hours. Non-positive values fall back to the default. |
| `login_rate_per_min` | int | `20` | Per-minute rate limit on the login endpoint. Non-positive values fall back to the default. |
| `users` | array | Required | User table, see `[[auth.users]]` below. An empty table fails validation. |

Signing keys are resolved in order: inline private key, private key file, `{data_dir}/auth/ed25519.pem`. In the last case, if the file does not exist Kiln generates a key pair and writes `{data_dir}/auth/ed25519.pem` and `{data_dir}/auth/ed25519.pub.pem`. At least one of `token_private_key`, `token_private_key_file`, or `server.data_dir` must therefore be set, or validation fails. Key material must be valid Ed25519 PEM; a wrong length or a public key that does not match the private key fails startup.

## `[[auth.users]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `username` | string | Required | Login name. Empty or duplicated names fail validation. |
| `password_hash` | string | Required | bcrypt password hash. Empty fails validation. |
| `role` | string | Required | Role; empty fails validation. `admin` is the only privileged role and reaches every admin endpoint. Every other value is treated as a restricted role. |
| `channel_ids` | array of string | `[]` | Channel allowlist for a restricted role. Empty means all channels. `admin` is not constrained by it. |

Changing a username or password in the admin console writes an override row keyed by the original config username; the config file itself is never rewritten. See [Authentication](/en/guide/auth/) for the full model.

## `[security]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `play_require_auth` | bool | `true` | Whether playback endpoints require credentials. The key is optional; omitting it means `true`. |
| `allowed_hosts` | array of string | `[]` | Explicit exemptions for outbound hosts that resolve to loopback or private addresses. Upstream and channel declarations do not add entries; list each private hostname or IP here. |
| `public_hosts` | array of string | `[]` | Inbound `Host` allowlist. Empty means no restriction; `*` allows everything. `/healthz` and `/readyz` from a loopback client are always exempt. |
| `cors_origins` | array of string | `[]` | Allowed cross-origin origins. Empty means no CORS headers are emitted at all. `*` is echoed back literally only when it is the single entry; otherwise the matched origin is echoed with `Vary: Origin`. |
| `max_playlist_bytes` | int64 | `8388608` | Byte ceiling for a fetched playlist. Non-positive values fall back to the default. |
| `max_body_bytes` | int64 | `1048576` | Byte ceiling for admin request bodies. Non-positive values fall back to the default. A few bulk endpoints multiply it. |

> **Turning playback auth off belongs in debugging environments only**
>
> An explicit `play_require_auth = false` disables playback authentication on its own; no environment variable is needed. `KILN_PLAY_OPEN` still outranks the file: `1`, `true`, or `TRUE` forces authentication off, while `0`, `false`, or `FALSE` forces it on. Never turn it off on a deployment reachable from anywhere else.

Outbound requests also pass a fixed guard: link-local, unspecified, multicast, and cloud metadata addresses are always refused, while loopback or private addresses are reachable only when explicitly listed in `security.allowed_hosts`.

## `[packager]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `engine` | string | `"auto"` | Default packaging engine; one of `auto`, `native`, `ffmpeg`. Anything else fails validation. `auto` prefers native and falls back to ffmpeg for unsupported sources, `native` never falls back, `ffmpeg` always remuxes. When left empty, the value comes from `KILN_DEFAULT_PACKAGER_ENGINE`. |
| `keys_file` | string | `""` | Path to the global `kid:key` catalog. Relative paths resolve against the config file's directory. |
| `playlist_size` | int | `8` | Segments retained in the output playlist. Non-positive values fall back to the default. |
| `ll_hls` | bool | `false` | Enables CMAF parts, delta playlists, and blocking reload. The example config turns it on, but the underlying default is off. |
| `part_target_ms` | int | `500` | LL-HLS part target in milliseconds. Must be between `100` and `5000`, otherwise validation fails. |
| `start_segments` | int | `3` | Segments prepared during cold start. Non-positive values fall back to the default. Resource adaptation may lower it. |
| `prefetch_segments` | int | `3` | Segments prefetched in steady state. Non-positive values fall back to the default. Resource adaptation may lower it. |
| `max_segment_bytes` | int64 | `33554432` | Byte ceiling for a single segment; anything larger is treated as a fault. Non-positive values fall back to the default. Resource adaptation may lower it. |
| `grace_sec` | int | `30` | Grace period, in seconds, during which a segment remains fetchable after leaving the playlist. Non-positive values fall back to the default. |
| `primary_track_hold_sec` | int | `12` | How far, in media seconds, audio may run ahead of video. Non-positive values fall back to the default. It bounds playlist window advancement, not A/V sync. |
| `stall_timeout_sec` | int | `180` | Fails a publication whose manifest keeps updating while nothing reaches the playlist. `-1` disables it; `0` falls back to the default. An unreachable upstream is a different case and keeps retrying. |
| `inflight_bytes` | int64 | `100663296` | Segment memory budget shared across all channels, in bytes. Non-positive values fall back to the default. Resource adaptation may lower it. |

### The inflight_bytes trade-off

This value is what decides peak memory. A single 4K segment is tens of megabytes, so the budget is counted in bytes rather than segments — otherwise memory would become a function of the source bitrate. What it buys is cold-start latency and nothing else: in steady state each track pulls one segment per refresh and never approaches the budget. Lowering it reduces resident memory at the cost of how quickly a 4K channel produces its first playlist.

### How keys_file is loaded and validated

`keys_file` is read and validated in full once at startup. A single bad line fails the boot:

- One `kid:key` pair per line. Blank lines and lines starting with `#` are ignored.
- A missing colon, an empty `kid`, or an empty `key` is reported with the line number.
- `kid` must be 32 hexadecimal characters and may be written in dashed UUID form; dashes are removed before comparison.
- `key` must be 32 hexadecimal characters and must not contain dashes.
- A repeated `kid` is ignored when the key matches and rejected when it does not.
- The file must yield at least one pair.

Both `kid` and `key` are normalized by removing dashes and lowercasing. Keys never appear in the admin API, and edits require a restart. Any DASH channel that is not disabled requires a non-empty global catalog, or validation fails. See [Media engine](/en/guide/media-engine/) for engine selection and media handling.

## `[ffmpeg]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `mode` | string | `"native"` | Execution mode. `native` runs a local binary, `docker` lets Kiln launch a given image. Anything else fails validation. |
| `binary` | string | `"ffmpeg"` | Executable name or path used in `native` mode. |
| `docker_image` | string | `"kiln:local"` | Image used in `docker` mode; ignored otherwise. |
| `hls_time` | int | `2` | Target segment duration in seconds for remuxed output. Non-positive values fall back to the default. |
| `hls_list_size` | int | `8` | Segments retained in remuxed output playlists. When unset or non-positive, the default is `4` if `low_latency` is `true` and `8` otherwise. |
| `log_level` | string | `"error"` | Log level passed to ffmpeg. |
| `prefer_height` | int | `0` | Global preferred video height; `0` means no preference. A positive per-channel `prefer_height` overrides it. |
| `low_latency` | bool | `false` | Picks the default for `hls_list_size`: `4` when `true`, `8` otherwise. An explicit `hls_list_size` always wins. |
| `max_starts` | int | `0` | Ceiling on concurrent ffmpeg launches; non-positive values behave as `1`. It covers the launch only, not the readiness wait, so a slow source does not block other channels' cold start. |

## `[observe]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `enabled` | bool | `true` | Whether `/metrics` is served and OTLP export runs. The key is optional; omitting it means `true`. An explicit `false` makes `/metrics` return 404 and skips exporter setup entirely. |
| `otlp_endpoint` | string | `""` | OTLP/HTTP trace export endpoint; empty disables export. When set it must be an absolute `http` or `https` URL with a host, otherwise validation fails. |
| `otlp_insecure` | bool | `false` | Whether insecure transport is allowed for export. |
| `trace_sample_ratio` | float | `1` | Sampling ratio, constrained to `0` through `1`. Non-positive values are rewritten to `1` while defaults are filled in. |
| `service_name` | string | `"kiln"` | `service.name` reported to OTLP. |

## `[debug.pprof]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `enabled` | bool | `false` | Whether the dedicated pprof server starts. When off, no listener is created at all. |
| `listen` | string | `"127.0.0.1:6060"` | pprof listen address. When enabled it must parse as host and port, and the host must be a loopback IP, otherwise validation fails. |

pprof uses its own listener and mux and is never mounted on the public server. Turn it on only while collecting CPU, heap, block, or mutex profiles. See [Troubleshooting](/en/guide/troubleshooting/) for diagnostic workflows.

## `[epg]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `cache` | bool | `true` | Whether the guide cache is used. Omitting the key means enabled. |
| `cache_dir` | string | `"{data_dir}/epg"` | Cache directory. `memory` or `:memory:` switches to an in-memory store; any other value is treated as a directory path. |
| `refresh_interval_min` | int | `360` | Refresh interval in minutes. Non-positive values fall back to the default. |
| `max_refresh_concurrency` | int | `0` | Ceiling on sources refreshed concurrently; `0` means unlimited (every source in one pass). Negative values fail validation. Resource adaptation may lower it, including tightening `0` into a concrete number. |
| `max_source_bytes` | int64 | `67108864` | Byte ceiling per source; larger payloads are rejected. Non-positive values fall back to the default. Resource adaptation may lower it. |
| `default_timezone` | string | `"UTC"` | Timezone used when a source declares none. Must be a loadable timezone name, otherwise validation fails. |
| `serve_timezone` | string | `"keep"` | Output timezone policy. Only `keep` is accepted, meaning timestamps are served as-is. |
| `sources` | array | `[]` | Guide sources, see `[[epg.sources]]` below. |

## `[[epg.sources]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `id` | string | Required | Source identifier. Empty or duplicated values fail validation. |
| `name` | string | `""` | Display name. |
| `url` | string | `""` | XMLTV address. When set it must be an absolute `http` or `https` URL with a host. |
| `timezone` | string | `""` | Timezone for this source; must be loadable when set. Empty falls back to `epg.default_timezone`. |
| `proxy` | string | `"direct"` | Outbound route: `direct`, `auto`, or a `[[proxies]]` id. Unknown values fail validation. Empty is filled in as `direct`. |
| `enabled` | bool | `false` | Whether the source participates in refreshes. |

See [EPG](/en/guide/epg/) for channel matching and built-in source behavior.

## `[[proxies]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `id` | string | Required | Route identifier, referenced by `egress.default`, `[[egress.rules]]`, and `[[epg.sources]]`. `direct` is reserved and means no proxy. |
| `name` | string | `""` | Display name. |
| `url` | string | Required | Proxy address. Schemes are limited to `http`, `https`, `socks5`, `socks5h` — for example `http://127.0.0.1:7890` or `socks5h://127.0.0.1:7891`. |
| `disabled` | bool | `false` | Whether the route is inactive. A disabled route takes no traffic, and decisions pointing at it fall back to direct. |

## `[egress]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `default` | string | `"direct"` | Route used when no rule matches. Must be `direct` or a defined `[[proxies]]` id, otherwise validation fails. |
| `playlist_policy` | string | `"rewrite"` | Playlist URL rewriting policy; one of `rewrite`, `passthrough`, `auto`. Anything else fails validation. `rewrite` always rewrites to a Kiln-relative URL, `passthrough` always keeps the original, `auto` rewrites only when a proxy was actually used. |
| `docker_proxy_host` | string | `"host.docker.internal"` | Applies only when `ffmpeg.mode = "docker"`; the hostname the spawned ffmpeg container uses to reach the Kiln process. It does not affect `[[proxies]].url` — Kiln never rewrites a route address. |
| `rules` | array | `[]` | Routing rules, see `[[egress.rules]]` below. |

## `[[egress.rules]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `id` | string | `""` | Rule identifier. When empty, seeding generates `rule-1`, `rule-2`, and so on. |
| `priority` | int | `0` | Match order; lower runs first and the first match wins. |
| `kind` | string | `"host_suffix"` | Match type; one of `host_suffix`, `host_exact`, `host_regex`, `channel_id`, `url_regex`. Empty behaves as `host_suffix`. |
| `pattern` | string | `""` | Match input. Except for `channel_id`, a rule with an empty pattern is skipped. Regex kinds compile with Go's regexp syntax; a compile failure counts as no match. |
| `proxy` | string | Required | Route used on a match. Must be `direct` or a defined `[[proxies]]` id; empty or unknown fails validation. |
| `disabled` | bool | `false` | Whether the rule is inactive. |

See [Outbound proxying](/en/guide/proxy/) for route selection, rewriting policies, and container scenarios.

## `[[upstreams]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `id` | string | Required | Upstream identifier referenced by a channel's `upstream`. Empty fails validation. |
| `base_url` | string | Required | Upstream base address. Must parse as an absolute URL; empty or malformed fails validation. A host that resolves to a loopback or private address must also be listed explicitly in `security.allowed_hosts`. |
| `headers` | table | `{}` | Fixed headers added to requests toward this upstream. |

## `[[channels]]`

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `id` | string | Required | Channel identifier. Empty, `.`, `..`, path separators, control characters, and duplicates all fail validation. |
| `title` | string | `""` | Display name. |
| `group` | string | `""` | Group label used in playlists and the admin UI. |
| `logo_url` | string | `""` | Logo address. |
| `epg_id` | string | `""` | Identifier matched exactly against the channel id in XMLTV. |
| `epg_name` | string | `""` | Name used when matching XMLTV channels by name; falls back to `title` when empty. |
| `epg_source` | string | `""` | Restricts matching to one `[[epg.sources]]` id. |
| `source_url` | string | `""` | Absolute source address. When set it must be `http` or `https`, carry a host, and contain no fragment; `upstream` and `path` are then unnecessary. |
| `upstream` | string | Conditional | The `[[upstreams]]` id to use. Required and must exist when `source_url` is absent, otherwise validation fails. |
| `path` | string | Conditional | Path appended to the upstream `base_url`. Required when `source_url` is absent. |
| `ingress` | string | `"hls"` | Source type; `hls` or `dash`, lowercased before validation. Anything else fails validation. |
| `disabled` | bool | `false` | Whether the channel is inactive. A disabled DASH channel no longer requires the global key catalog. |
| `on_demand` | bool | `true` | Whether the channel pulls on demand. When both `on_demand` and `autostart` are `false`, defaults set `on_demand` to `true`. |
| `autostart` | bool | `false` | Whether the channel starts pulling at boot. |
| `idle_timeout_sec` | int | `90` | Seconds a session survives with no viewers. Non-positive values fall back to the default. |
| `max_viewers` | int | `0` | Concurrent viewer ceiling; `0` means unlimited. |
| `user_agent` | string | `""` | User-Agent used when fetching this channel. |
| `headers` | table | `{}` | Fixed headers added when fetching this channel. |
| `restart_on_failure` | bool | `false` | Whether the session restarts after a failure. Channels with `ingress = "dash"` are forced to `true`. |
| `prefer_height` | int | `0` | Preferred video height; `0` inherits `ffmpeg.prefer_height`. |
| `preferred_audio_languages` | array of string | `[]` | Audio language preference, applied in order. A non-empty `selection.audio.preferred_languages` takes precedence. |
| `packager` | string | `""` | Packaging engine for this channel; empty inherits `packager.engine`. When set it must be `auto`, `native`, or `ffmpeg`. |
| `selection` | table | `{}` | Fine-grained track selection, see below. |

### `[channels.selection]`

Selection is split into video, audio, and subtitles. Each group has a `mode` and a `track` selector.

| Key | Type | Default | Description |
| --- | --- | --- | --- |
| `video.mode` | string | `""` | One of `auto`, `cap`, `exact`; empty behaves as `auto`. `exact` requires at least one field in `video.track`, otherwise validation fails. |
| `video.max_height` | int | `0` | Video height ceiling. A positive value overrides `prefer_height`. |
| `video.max_frame_rate` | string | `""` | Frame rate ceiling. Currently only length- and control-character-validated; it does not yet influence selection. |
| `video.track` | table | `{}` | Video track selector, fields below. |
| `audio.mode` | string | `""` | One of `auto`, `prefer`, `only`; empty behaves as `auto`. `only` requires at least one field in `audio.track`, otherwise validation fails. |
| `audio.preferred_languages` | array of string | `[]` | Audio language preference. Empty falls back to the channel-level `preferred_audio_languages`. |
| `audio.track` | table | `{}` | Audio track selector. |
| `subtitles.mode` | string | `""` | One of `auto`, `off`, `prefer`, `only`; empty behaves as `auto`. Both `prefer` and `only` require at least one field in `subtitles.track`. |
| `subtitles.track` | table | `{}` | Subtitle track selector. |

All three selectors share the same fields, and any non-empty field counts as specified:

| Key | Type | Description |
| --- | --- | --- |
| `key` | string | Unique track key. |
| `adaptation_set_id` | string | DASH AdaptationSet id. |
| `representation_id` | string | DASH Representation id. |
| `language` | string | Language code. |
| `role` | string | Track role. |
| `codec` | string | Codec identifier. |
| `height` | int | Video height; only a positive value counts as specified. |
| `frame_rate` | string | Frame rate. |

Every string field is limited to 512 characters and must contain no control characters, otherwise validation fails. With the `ffmpeg` engine, a `subtitles.mode` of `prefer` or `only` fails validation outright; only `auto` and `off` are available. See [Fine-grained track selection](/en/guide/channels/#fine-grained-track-selection) for practical guidance.

Source: https://kiln.wbxdocs.com/en/reference/config/index.mdx