ConfigKit/docs/config.md

# Configuration Input Contract

This document owns ConfigKit's currently implemented source-input, YAML syntax,
schema, typed-decoding, loading, metadata, redaction, rendering, and ownership
contracts. The parser, owned document representation, and decoder remain private
behind the public loading lifecycle.

## Sources

ConfigKit exposes opaque sources through these constructors:

| Constructor | Current contract |
| --- | --- |
| `File(path)` | Requires a nonempty path, opens a regular file for each consumption, and closes only the library-owned file handle. Missing, inaccessible, unreadable, directory, and other nonregular inputs fail. |
| `Bytes(label, data)` | Requires a nonempty, non-secret diagnostic label and snapshots bytes during construction. Later caller mutation cannot affect the source. |
| `Reader(label, input)` | Requires a nonempty, non-secret label and non-nil reader. The reader is claimed once across all copies of the source, consumed but never closed, and cannot be reused. |
| `NoSource()` | Selects defaults-only loading, creates no source record, and marks no paths explicit. |

The zero `Source` is invalid. Constructor argument failures are retained inside the
opaque source and become safe structured diagnostics when the source is consumed.
File sources can observe file changes between consumptions; byte sources cannot.

Each source is limited to 8 MiB. A load using imports or an active profile is limited
to 64 MiB across all accepted sources, 1,024 accepted sources, 64 active import
edges, and 1,000,000 input nodes in aggregate. Glob expansion separately permits
1,000,000 visited directory entries, including nonmatches. Reads request at most the
remaining allowance plus one byte so excess data is rejected rather than silently
truncated. Data returned together with a read error is not accepted as partial input.

## Single-source loading

`Load[T](source, options...)` accepts zero or one `Options[T]`. The configuration
root `T` must satisfy the application schema contract below. With no options it
starts from the Go zero value. A `Definition[T].Defaults` factory, when supplied,
runs exactly once after successful source parsing, schema construction, optional
schema-version checking, and selected-profile loading. Its result is detached before
input is applied. Resolver-provided mutable values are detached before secret lookup
and validation, and a final detached copy prevents equal-valued aliases introduced
by validators from escaping into the returned configuration. The publication copy
preserves already validated secret state without rereading the environment.

The current lifecycle is:

```text
validate options → parse the selected source → traverse/compose enabled imports
  → build schema → check the optional application version → apply a selected profile
  → apply explicit CLI overrides → create and clone defaults → strictly decode
  → recorded application resolution
  → resolve typed secret references → semantic validation → classify secrets
  → snapshot configuration and metadata
```

`File`, `Bytes`, and `Reader` each contribute the first base source with deterministic
ID `source-1`; enabled imports continue in depth-first order as `source-2` onward.
`NoSource` skips parsing, supplies an absent version to a configured version checker,
contributes no source, and continues through defaults, decode, and projection. A
missing file remains an error; it is never interpreted as no input.

The current `Options` boundary accepts a defaults factory, `VersionRule`, resolver,
semantic validator, explicit path and glob imports, one selected profile, and
filesystem context. It also accepts explicit CLI override records, static secret
tags, and a dynamic path predicate. `NoSource` rejects imports, active profiles,
and filesystem context as invalid combinations but accepts overrides. Input
containing a reserved `_configkit` mapping key is rejected while imports are
disabled.

Every source, options, schema, version, defaults, decode, resolution, or validation
failure returns a nil result and a `*LoadError`. Successful results contain caller-owned `Config`, detached
`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Mutating
the returned configuration or inspection records cannot alter ConfigKit's private
snapshot. ConfigKit performs no logging. `Result.WriteEffective` exposes only the
frozen redacted projection.

### Application schema versions

`VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
decoding, the checker receives a `Version` preserving absent, explicit null, or
string input. Any other input kind is a `version.type` error and the checker does not
run. ConfigKit does not interpret the text or impose a universal version path.

The checker reports safe, value-free errors and warnings through `Validation.Error`
and `Validation.Warning`; ConfigKit resolves primary and related paths to exact or
nearest input locations. Reported errors fail loading. If the checker returns an
error without reporting one, ConfigKit emits a generic `version.hook_failed`
diagnostic and suppresses the returned error text. Warnings alone are retained on a
successful result. Callback panics remain programming failures.

### Layered metadata

`Result.Metadata` describes every final canonical path. Exact surviving input paths
are explicit and have a base or profile origin; omitted paths have a default origin.
An explicit parent mapping does not make omitted children explicit. Null state and
available source coordinates are retained. `History` records the Go-zero or application
default baseline followed by exact base or profile definitions, including
equal-valued input. Profile replacement removes displaced descendants from effective
metadata while retaining their base definition and the parent replacement operation
in history. All metadata queries and returned nested records are detached.

History is chronological: a path can have a synthetic default record, its exact base
definition, a profile replacement, and a recorded resolution operation. A descendant
query uses the ancestor's path for a whole-container or whole-sequence replacement;
the descendant can therefore retain history after `Lookup` stops reporting it as
effective. Equal-valued input is still an explicit base definition, while a derived
value for absent input remains non-explicit.

A mapping assembled from the selected file and imports has one composite base origin
whose locations retain source traversal order. A mapping also visited by a profile
has a profile-layer composite origin with the lower contributing locations followed
by the profile location; an empty mapping is still an explicit container even though
it contributes no children. Resolution origins are synthetic: they have layer
`resolution`, no fabricated file location, and a `derived_default` or `normalization`
operation. Reasons and dependency paths retain first occurrence order with duplicates
removed. Source records remain ordered as selected source, depth-first imports, the
selected profile, then synthetic CLI records in supplied order.

### Secret classification and effective rendering

Secret classification combines `configkit:"secret"` field tags with the optional
pure `Definition.Secret(Path) bool` predicate. Tags on concrete structs inside
supported dynamic values are retained in typed projections, including superseded
defaults and pre-resolution values. A tagged or predicate-classified
container makes all descendants sensitive. ConfigKit evaluates final and historical
paths, so replacing or deleting a definition does not erase its classification. The
predicate can add sensitivity but cannot declassify a tagged path.

Recorded resolution dependencies propagate sensitivity to their derived target.
Propagation reaches a fixed point, so record/map iteration order does not affect the
result and dependency cycles terminate. A dependency on a classified descendant or
on a container that contains one is treated conservatively as sensitive. Applications
must classify secret-producing transformations whose dependencies are not recorded.

`Result.WriteEffective` writes the frozen resolved snapshot as deterministic redacted
YAML. Mappings sort by decoded key string, sequences retain order, indentation is two
spaces with noncompact sequence indentation, line endings are LF, the document has no
start or end marker, and output ends with a newline. Nil pointers/maps/slices remain
`null`; non-nil empty maps and sequences remain `{}` and `[]`. A secret root is
replaced by the quoted string `'[REDACTED]'`. Redacted output is intended for
inspection and is not promised to be reloadable.

Rendering uses only a safe adapter tree built from the already completed canonical
typed projection. It does not invoke application YAML marshal hooks or text codecs;
all text-codec projection has succeeded before `Load` returns. Caller mutation of
`Result.Config` cannot change later output. Writer failures, including short writes,
are returned and may leave partial redacted output; internal dumper failures are
translated to a fixed safe error. There is no unredacted rendering option and
ConfigKit performs no implicit logging.

`Result.Config` is the explicit trusted boundary and contains ordinary typed values,
including secrets. Metadata records, diagnostics, and load errors contain no
configuration values. Explanation snapshots deliberately expose non-secret
configuration history after applying the redaction contract below. Applications
must still keep source labels, resolution reasons, and diagnostic messages value-free
because ConfigKit cannot infer and sanitize secrets embedded in arbitrary application
prose.

### Historical-value explanation

`Result.Explain(path)` returns a detached `Explanation` for any known effective or
historical path. Its effective value and each chronological before/after value are
`ValueSnapshot` records containing deterministic YAML text rather than mutable Go or
parser values. `Present=false` with empty YAML means absence or deletion; explicit
null is present and renders as `null`. The history starts with the canonical typed
application-default or Go-zero baseline, then includes only relevant base sources in
traversal order, the selected profile, supplied CLI records, and audited net
resolution operations. Overlapping parent/child resolution records are rendered
as one net subtree event with combined reasons and dependencies, rather than
pretending that the recorded assignments ran in lexical order. Explicit equal
writes remain visible. When input replay
differs from the final typed input, a `ChangeDecoding` event records the transition
before domain resolution. Its origin retains the decoded path's source/default
attribution; its reason identifies static-default retention and canonical decoding.
This built-in inspection operation is not accepted by `Resolution.Record`.

Base, profile, and CLI history preserves the policy-validated input representation,
including a superseded value that could not have decoded into the final Go field.
Baseline, effective, and resolution values use canonical typed projection. A
decoding event bridges replayed input and canonical typed values, so intermediate
null/scalar replacements cannot incorrectly imply permanent default removal. Mapping
queries reconstruct each source or override contribution, while sequences remain
atomic: a descendant history identifies its presence before and after the containing
sequence replacement without implying element-wise merging. Removed paths remain
explainable; unknown and malformed paths return false.

Every snapshot applies the same historical tag, predicate, and recorded-dependency
sensitivity closure as effective rendering before YAML is exposed. A redacted scalar
is the quoted marker `'[REDACTED]'`, and a container snapshot has `Redacted=true` if
it contains a masked descendant. Typed `Secret` snapshots contain only the configured
environment name. `SecretReference` reports that name only for a successfully
resolved effective reference, and is omitted when its path is classified. Credential
values are never retained in explanation history.

`Result.WriteExplanation(output, path)` renders the safe structured result with the
canonical JSON-pointer path, canonical-versus-input representation labels, effective
source, composite locations, chronological values, resolution reasons and
dependencies, and successful secret-reference lookup attribution. Control characters
in paths, source identifiers and labels, reasons, dependencies, and reference names
are escaped. An unknown path returns a fixed safe error before writing anything;
writer errors and short writes are returned and may leave partial output. There is no
unredacted mode.

Retained history consists of immutable source applications and audited subtree
deltas, rather than a full tree copy per field. Shared nodes count once toward a
4,000,000-node retained-history limit; exceeding it fails loading with
`explain.limit_exceeded`. Replaying one requested path is performed on demand, and
the returned YAML strings and nested records cannot mutate later explanations.

Application commands should render the separate `SecretsReport` immediately after
an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
`WriteEffective`, or `WriteExplanation` on the later load result as appropriate.
ConfigKit does not infer bootstrap outcomes from a secret reference and does not
bundle these independent reports into a command framework.

## Typed secret references

Credential-bearing fields use the opaque `Secret` type and store an environment
variable name in YAML, such as `api_key: OPENROUTER_API_KEY`. `SecretEnv(name)`
constructs the same unresolved reference for application defaults or recorded
resolution. Names must match `[A-Za-z_][A-Za-z0-9_]*`; they are public
configuration and remain visible through `EnvName`, canonical projection, history,
and effective output unless the path is separately classified for redaction.

A non-pointer `Secret` is required. An absent, null, or invalid reference, or an
unset or empty environment value, fails with a safe `secret.*` diagnostic at the
configuration path. `*Secret` is optional only when nil; every non-nil reference
must resolve to a nonempty value. Secret references work in typed structs, maps,
pointers, slices, and arrays. A dynamic `any` value does not establish secret
semantics and cannot contain `Secret`; applications must use a typed slot.

ConfigKit resolves only the effective names after recorded application resolution
and before semantic validation. Each unique name is read with `os.LookupEnv` once
per load. Validation can inspect the credential through the deliberately named
`Value()` accessor and distinguish success with `IsResolved()`. Later loads read
the environment again, and cloned defaults are rebuilt from names so a previously
resolved result cannot retain a stale credential. `Load` never writes to the
environment.

Credential bytes are held only in the per-load lookup state and the trusted returned
configuration, and are returned only by `Value`. Secret formatting uses a safe
public-name representation, JSON marshals only the name, and projection, metadata,
histories, diagnostics, effective output,
and resolution snapshots never retain the credential. Validation remains read-only:
in addition to ordinary projected-value comparison, ConfigKit checks the private
resolved state against this load's lookup result. Applications must keep their
validation messages value-free and must not include `Value()` in diagnostics.

For example, a typed pipeline field is declared as:

```go
type PipelineConfig struct {
    APIKey configkit.Secret `yaml:"api_key"`
}
```

The executable `ExampleSecretEnv` demonstrates loading and inspecting the safe
reference state without printing its value.

## Explicit secrets-directory bootstrap

`LoadSecretsDir(directory)` is a separate, opt-in startup operation. It is never
called by `Load`, is not a YAML option, and does not participate in configuration
validation. Applications choose the native directory path and should invoke the
operation once before starting concurrent work. The selected path may itself be a
symlink; ConfigKit resolves it to an existing canonical directory before examining
its immediate entries.

The bootstrap enumerates at most 1,024 entries and sorts them by filename. Every
entry must have an ASCII environment-variable name and be a regular, nonsymlink
file. Metadata for all entries is validated before file content is read or process
state is changed. Existing environment variables win, including variables whose
value is empty; their files are not opened. Each eligible file is limited to 1 MiB,
and eligible content is limited to 8 MiB in total. ConfigKit removes exactly one
terminal LF or CRLF, preserves all other whitespace, accepts an empty value, and
rejects NUL bytes.

All eligible files are read successfully before application begins. Presence is
checked again immediately before each `os.Setenv`: a variable populated since
preflight is skipped. A write failure stops application without rollback, so prior
writes remain and later eligible entries are `not_applied`. This operation is not
atomic and does not provide compare-and-set semantics for concurrent environment
mutation.

`SecretsReport.Entries` returns detached, filename-ordered `SecretFileResult`
records with `loaded`, `skipped_existing`, `not_applied`, or `failed` status.
`ExistingEmpty` distinguishes an empty existing winner. `SecretsReport.WriteTo`
renders only quoted names, canonical paths, statuses, and that empty-state flag;
it never retains or renders file contents. Failures return the non-nil partial
report with a `*SecretsError` containing detached `secrets.*` diagnostics and no
raw filesystem or environment error chain. Applications should retain the report
alongside the later configuration result and explicitly present blocked and failed
files when appropriate; ConfigKit stores no implicit global report.

### Recorded application resolution

An optional `Definition.Resolve` callback runs after successful typed decoding. It
receives the working configuration and a per-load `Resolution` context. The context's
`IsExplicit` query uses surviving input presence, so explicit false, zero, empty, and
null are never confused with absence or static defaults.

`Default` assigns and records a derived value only when its path is not explicit.
`Normalize` always assigns and records. For transformations performed directly by
application code, `Resolution.Record` declares a derived-default or normalization
operation; a recorded container covers changes to its descendants. Records require
a schema-valid path, valid kind, safe nonempty reason, and schema-valid dependency
paths. Repeated records at one path are coalesced: reasons and dependencies retain
their first occurrence, the last helper value is authoritative, and any normalization
makes the combined operation a normalization.

ConfigKit projects the configuration before and after the callback. Every net change
must have an exact or ancestor record. Helper-assigned values must agree with the
final value at their declared path, which supports ordinary map-entry copy/writeback
while detecting forgotten writeback or unrelated targets. Invalid paths, kinds,
values, records, helper targets, and unrecorded mutations fail the load. Equal-valued
derived assignments may still record application intent even when there is no
structural change.

`Resolution.Error` and `Resolution.Warning` attach exact or nearest available input
locations. Reported errors fail the load; warnings survive on a successful result. A
returned resolver error adds one generic `resolve.hook_failed` error only when the
callback did not report an error, and its text is never exposed. Callback panics are
programming failures. Resolution and validation contexts are callback-scoped; using
a nil, zero, or retained context outside its active callback panics with fixed,
value-free text.

Successful records update effective origins and append detached metadata history
without changing input explicitness. Removed paths leave effective metadata but
retain history. ConfigKit privately snapshots each recorded before/after subtree for
later safe explanation; caller mutations of `Result.Config` cannot alter those
snapshots.

### Semantic validation

An optional `Definition.Validate` callback runs after successful resolution and
receives the resolved configuration plus a per-load `Validation` context. It can
collect multiple independent `validate.error` and `validate.warning` reports.
Warnings continue the lifecycle and are returned on success; any error returns a nil
result with the full safe diagnostic and source report.

Primary and related paths must be valid for the application schema. Omitted declared
fields and missing string-map or dynamic keys are valid report targets; unknown
struct fields and malformed paths produce `validate.invalid_path`. ConfigKit attaches
the exact surviving input location when one exists, otherwise the nearest input
container location, or no primary location for defaults-only synthetic state.

Validation is read-only. ConfigKit projects the configuration immediately before and
after the callback and returns `validate.mutation` at every detected net-change path,
including nested mutable state. A value that can no longer be projected is also a
mutation failure. No mutated or otherwise invalid configuration is published.
Callback panics remain programming failures, and retained contexts use the same
fixed closed-context panic as version and resolution callbacks.

Only schema-bound exported state participates in loading, resolution audit,
validation audit, metadata, and snapshots. Ignored or unexported ordinary fields are
zero in returned detached values. Applications retain responsibility for domain
rules and runtime initialization; validators should not start clients or depend on
live services as a substitute for configuration validation.

## Strict document composition

ConfigKit strictly composes parsed base documents when `Options.Imports` is true.
Application defaults do not participate in this operation.

| Existing node | Incoming node | Result |
| --- | --- | --- |
| Absent | Absent | Absent |
| Absent | Present | Retain the incoming node |
| Present | Absent | Retain the existing node |
| Mapping | Mapping | Recurse by mapping key |
| Any other pair of present nodes | Any other pair of present nodes | `compose.conflict` |

Every non-mapping overlap is a conflict, even when values are equal. Nulls and
sequences are present atomic values, so they also conflict when repeated; sequences
are never joined by index. Disjoint entries under shared mappings compose, making
stable keyed mappings the natural representation for independently contributed
entries.

An empty mapping contributes no child definitions. Two empty mappings compose, and
an empty mapping neither clears nor conflicts with another mapping's children.
Shared mapping containers retain all contributing value locations as their composite
base origin. A conflict reports the first claim as its primary location and every
remaining claim as related locations, in deterministic source and declaration
traversal order. Traversal order never authorizes replacement. Composition clones
the resulting tree and its ordered claim records rather than mutating or aliasing an
input document.

## Selected profiles

`Options.Profile` selects at most one overlay by name. Names must match
`[a-zA-Z0-9_-]+`; they are not paths. The empty name and exact lowercase name
`default` select no profile, and `profiles/default.yml` is never loaded implicitly.
Every other name resolves only to `<configuration-root>/profiles/<name>.yml`, with
no extension fallback, case normalization, inheritance, or profile stack. A selected
missing profile is an error.

The profile is a separately policy-validated YAML mapping. It is loaded after the
base import graph is composed and its schema version is checked, but before defaults
and typed decoding. Profile bytes, nodes, and source records count toward the same
aggregate limits as base inputs. Its canonical path must remain within the configured
root and must not duplicate a file already present in the base graph.

Overlay is intentionally distinct from strict composition:

| Existing lower node | Incoming profile node | Result |
| --- | --- | --- |
| Absent | Value | Retain the profile value |
| Value | Absent | Retain the lower value |
| Mapping | Mapping | Recurse by mapping key |
| Any other pair of present nodes | Any other pair of present nodes | Replace the entire lower node |

Scalars, sequences, and null are atomic. Thus a profile sequence replaces rather
than extends a base sequence, `[]` clears it, null is an explicit replacement, and
cross-kind replacement is allowed when the final application schema accepts it.
Replacing a subtree removes all displaced descendants from effective presence and
origin data; history retains the displaced definitions and identifies the parent
replacement. Surviving profile paths have profile origins, while recursively shared
mapping containers retain their contributing locations.

A profile may not contain `_configkit` anywhere, contribute the configured schema
version, or replace an ancestor of that version with a scalar or null. These rules
apply even if the supplied version text would be unchanged. Profiles cannot mask a
base parse, composition, or version failure because those checks run first.

## Explicit CLI overrides

`Options.Overrides` accepts only records the application knows were explicitly set
by its CLI parser. Each `Override` supplies a non-root mapping `Path`, a detached
`Value`, and a required safe display `Flag` such as `--workers`; ConfigKit has no CLI
parser dependency. Parser defaults must not be submitted as overrides, and `Flag`
must never contain the raw argument or configured value. General environment
variable overlays are an explicit non-goal: environment access is reserved for
typed secret references.

Overrides apply after the selected profile and before typed defaults/decoding, so
the input precedence is application defaults < composed base < selected profile <
CLI. Recorded resolution, typed secret lookup, and semantic validation follow the
effective override tree. Overrides also work with `NoSource`.

Values are converted directly into ConfigKit's owned tree without YAML parsing or
string coercion. Supported values are `nil`, built-in strings and booleans, all
built-in signed and unsigned integer widths except `uintptr`, finite `float32` and
`float64`, and recursive `[]any`/`map[string]any`. Strings stay strings; duration and
secret fields therefore receive string values. Nil is explicit null; empty strings,
false, and zero remain intentional; empty sequences replace whole lists; and mapping
values recursively overlay existing mappings. Map keys are sorted canonically and
sequence order is preserved.

All records are detached and validated before any is applied. Root and explicit
sequence-index paths are invalid; parsed numeric tokens remain mapping keys. A path
may create missing mapping ancestors but cannot traverse an existing scalar, null,
or sequence. Duplicate and ancestor/descendant-overlapping records are rejected.
Neither paths nor values may create `_configkit`, and no record may overlap the
configured schema-version path. Unknown fields and effective type mismatches remain
ordinary strict-decoder errors.

At most 1,024 records are accepted, and each record creates one synthetic CLI
source after file/profile sources in supplied order. Override sources share the
1,024-source ceiling. A record has an 8 MiB string-byte limit; override strings and
source bytes share the 64 MiB aggregate limit. Synthetic mapping keys and values
count toward the existing 1,000,000-node and depth-256 ceilings. Cycles, named or
unsupported types, nonfinite floats, and limit excesses fail with safe `override.*`
diagnostics. Diagnostics and provenance identify the safe `Flag` source, never raw
arguments or values.

Override origins are explicit CLI origins. History retains replaced base/profile
definitions and records equal-value replacements as well as structural changes.
Caller-owned maps and slices are snapshotted during loading and must not be mutated
concurrently with the call. The executable `ExampleOverride` shows translating a
parser's explicit-set state into records while preserving an explicitly supplied
zero.

## Explicit imports

Imports are opt-in. With `Options.Imports` enabled, a document may reserve its root
`_configkit` mapping for an `imports` sequence. ConfigKit removes that mapping before
application decoding. Each sequence item is a mapping with exactly one nonempty
string `path` or `glob` plus an optional boolean `optional`; all other shapes and
fields are `import.invalid`. A `_configkit` key below the root is invalid. With
imports disabled, any occurrence remains `import.disabled`.

Path declarations use slash-separated relative paths. Absolute paths and network
URLs are invalid. Each path resolves from its declaring file's canonical directory,
and files are traversed depth-first in declaration order: a file contributes its own
body before its imports. Order stabilizes source records and diagnostics but never
permits replacement. Required missing paths fail; `optional: true` suppresses only
target nonexistence, not invalid paths, containment failures, directories,
permissions, parse errors, or composition errors.

Glob declarations use the slash-based Go `path.Match` grammar: `*`, `?`, character
classes, and backslash escapes are supported, while recursive `**` syntax is invalid.
Backslashes escape pattern characters rather than acting as directory separators.
ConfigKit validates the complete pattern before filesystem traversal, walks only
directories selected by preceding segments, and reads wildcard directories in
bounded batches. Every examined entry consumes the separate visit budget, including
nonmatches. Matches are sorted lexicographically by slash-relative spelling within
each declaration before depth-first source traversal. A required zero-match glob is
`import.missing`; `optional: true` suppresses only zero matches. Invalid patterns,
walk failures, matched directories, invalid documents, escapes, and repeated targets
remain errors. Final candidates are bounded by the remaining source limit before
their contents are accumulated.

The configuration root defaults to a selected file's canonical directory. A file
source may supply `FilesystemContext.Root` to widen that root but may not supply
`BaseDir`. Whenever byte and reader sources supply filesystem context, they must
provide both `BaseDir` and `Root` as absolute existing directories; their diagnostic
labels, including URI-looking labels, never imply filesystem paths. The base and
selected file must be inside the canonical root. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
post-open regular-file checks remain in place under the stable-input assumption.
ConfigKit canonicalizes symlinks,
checks containment with relative paths, and opens the canonical root-relative target
through a library-owned root handle. Contained relative or absolute symlink targets
are accepted; escapes are rejected. Canonical paths define identity, so hard links at
distinct paths remain distinct sources.

An active-stack repeat is `import.cycle` and reports the complete cycle's import
sites. A previously visited canonical path is `import.repeated` and reports both
inclusion routes, including diamonds and aliases. Each accepted file is read once.
Imported fragments may contain their own root directives, but may not claim the
application's schema-version path or replace one of its ancestors; the version must
come from the selected source. No directory is scanned implicitly, including one
named `conf.d`, and ConfigKit performs no remote fetching.

## YAML documents

The private YAML boundary currently accepts exactly one nonempty document whose root
is a mapping. Comments and block or flow collections are supported. It rejects:

- empty input or an empty document;
- every additional document, including an empty one;
- duplicate decoded mapping keys;
- non-string mapping keys;
- explicit tags, anchors, aliases, and merge keys;
- a quoted or plain mapping key equal to `<<`; and
- input exceeding the fixed depth or node limits.

Mapping key comparison is exact: it performs no case folding or Unicode
normalization. The private document tree retains declaration order, exact validated
number spelling, source identifiers, and separate key/value line and column
locations. It contains no upstream parser nodes or types.

Root depth is zero. Input depth is limited to 256 and each parsed input tree to
1,000,000 nodes, counting mapping keys as input nodes. The upstream parser also has
a private calibrated depth guard; passing that guard never bypasses ConfigKit's tree
limits.

## Scalars

Scalar meaning is determined by ConfigKit rather than upstream coercion:

| Input form | Meaning |
| --- | --- |
| Quoted, literal, or folded scalar | String |
| Plain `true` or `false` | Boolean |
| Plain `null`, `~`, or an empty mapping value | Null |
| JSON decimal number | Number, retaining its exact spelling |
| Date/time-looking or other ordinary plain text | String |

JSON numbers permit an optional minus, a zero or nonzero decimal integer without a
leading zero, an optional fraction with at least one digit, and an optional decimal
exponent. Hexadecimal, octal, digit separators, leading plus, leading/trailing-dot
forms, incomplete exponents, and non-finite values are rejected. Legacy boolean
words (`yes`, `no`, `on`, and `off`) and non-lowercase variants of the reserved
boolean/null words are rejected unless quoted.

Mapping keys use the same scalar classification and must classify as strings. Quote
keys that otherwise classify as a boolean, number, or null.

## Application schema binding

ConfigKit builds a private reflection descriptor graph for each schema operation;
it keeps no global mutable descriptor cache. A configuration root must be a struct
or string-keyed map. Nested supported shapes are:

- booleans, strings, signed and unsigned integers, and finite floating-point types;
- `time.Duration`, typed `Secret` references, and paired
  `encoding.TextMarshaler`/`TextUnmarshaler` scalars;
- pointers, structs, string-keyed maps, slices, fixed arrays, and `any`; and
- recursive type definitions whose eventual application values are acyclic.

Named scalar types retain their Go identity. `uintptr`, complex numbers, functions,
channels, unsafe pointers, non-string map keys, and nonempty interfaces are not
configuration types. A custom text scalar must provide both directions on its value
or pointer method set. Building a descriptor only inspects method sets; it never
executes application methods or parser-specific hooks.

Exported struct fields use an explicit `yaml` name or the lowercase Go field name.
Unexported fields and `yaml:"-"` are ignored. `omitempty` is accepted but does not
change binding or projection. An explicit `yaml:",inline"` flattens a struct
or pointer-to-struct, or designates at most one string-keyed inline map for otherwise
unmatched names. Ambiguous names, named inline fields, multiple inline maps, and
unsupported or malformed tag options are schema errors. Embedded fields are ordinary
named fields unless explicitly inlined. `configkit:"secret"` is the only current
ConfigKit field tag and marks its bound field or container for secret classification
and redacted inspection.

Schema path traversal uses these same bindings. Explicit key segments address
mappings, explicit index segments address sequences, and parsed numeric pointer
tokens become indices only in a sequence. Sequence indices cannot have leading
zeroes; fixed-array indices are bounds checked. Dynamic `any` and inline-map entries
remain schema-valid for application-defined descendants, but dynamic values cannot
introduce typed secret references.

Descriptors are reused by typed decoding, projection, and cloning so those operations
cannot acquire different field bindings.

### Canonical module options

Applications with a known module catalog represent each configured item as an object
with an explicit module identifier and an options mapping. Ordinary typed structs,
including a small union of optional option fields, use the same strict binding above;
application validation decides which fields and combinations apply to each module.
String shorthand, a library module registry, and custom structural decoder hooks are
not supported alternatives.

When a runtime-selected catalog genuinely requires dynamic options,
`map[string]any` retains ConfigKit's canonical dynamic scalars. In particular,
integers are `int64`; application validation owns allowed keys, value constraints,
range checks, and conversion to runtime `int` or catalog objects. ConfigKit does not
provide generic subtree conversion, and consumers must not add a YAML marshal/decode
round trip to exercise supported behavior. The
[consumer adoption guide](consumers/README.md) contains compiled typed and dynamic
examples.

## Defaults and typed decoding

The private decoder starts from an independently owned clone of the application's
typed defaults and applies only explicitly present input nodes. Defaults therefore
never become explicit, while equal-valued input remains explicit. Successful decode
produces detached typed state, baseline and final canonical trees, an input-presence
index, and default/input origins for the later public loading lifecycle.

| Configured input | Effect on the typed baseline |
| --- | --- |
| Absent field | Retain the default |
| Explicit false, zero, or empty string | Replace the default |
| Partial mapping or string-keyed map | Recurse and retain omitted default children or entries |
| Empty mapping | Retain existing mapping children |
| Sequence, including an empty sequence | Replace the entire sequence |
| Pointer object | Recurse into a non-nil default; otherwise allocate an object whose omitted children have ordinary Go zero values |
| Allowed null | Replace the value with nil and suppress its default |

Null is allowed only for pointers, maps, slices, and dynamic `any` values. It is an
error for scalars, structs, and arrays. New map entries and sequence elements begin
from their Go zero values; ConfigKit does not invent entry-level defaults.

Decoding is strict at every typed depth. Unknown struct fields, type mismatches,
invalid nulls, numeric overflow, and arrays with the wrong length are errors at the
original semantic path and source coordinates. String-keyed maps accept arbitrary
keys but preserve the strict schema of their values; inline maps accept unmatched
keys according to their element descriptor. Multiple independent field failures are
collected without publishing the partially decoded object.

Booleans and strings are not coerced. Integer destinations require integer syntax
and a value in the destination type's range. Float destinations accept finite JSON
numbers representable by their destination precision. Dynamic numbers become
`int64` for integer syntax and `float64` for fractional or exponent syntax, with
range and finiteness checks. Named scalar types retain their identity. Durations
decode only from strings accepted by `time.ParseDuration`; integers are rejected.
Paired text scalars decode only from strings through `encoding.TextUnmarshaler`, and
codec failures are classified without exposing application error text or input
values.

This decoder is available only through `Load`; ConfigKit exposes no generic document
conversion API.

## Typed projection and ownership

ConfigKit can internally project a schema-valid application value directly into its
owned canonical tree without YAML serialization. Projection includes every bound
field, including ordinary zero values; `omitempty` has no effect. Struct fields keep
binding order, dynamic map keys sort by decoded string, and sequences keep their
order. Integers retain exact base-10 values, finite floats use their destination
precision, durations use `time.Duration.String`, and paired text scalars use their
canonical marshaled text.

Nil pointers, maps, slices, and dynamic values project as null. Non-nil empty maps
and slices remain empty containers, preserving the distinction. Recursive acyclic
objects are supported. Cyclic values, non-finite floats, unsupported dynamic concrete
types, invalid codec output, and depth/node-limit violations fail with safe schema
classifications that omit codec error text.

The same descriptor graph creates detached typed copies. Pointers, maps, slices,
arrays, nested dynamic values, and shared references are recursively copied; shared
pointer identity is not preserved as configuration meaning. Paired text scalars are
rebuilt by unmarshaling their canonical text into fresh values, detaching private
backing storage. Ignored and unexported struct fields are zeroed rather than copied.

Private structural comparison operates on canonical trees. Mapping additions,
removals, and descendant replacements retain semantic paths. A sequence is atomic:
any sequence change reports the sequence path and never invents correspondence
between old and new indices. Cross-kind and null/container changes replace the
affected path.

These mechanisms support decoding, resolution auditing, metadata,
and rendering. They are not a public generic conversion or cloning API.

## Safe failures

Source failures use stable `source.*` codes and YAML failures use stable `yaml.*`
codes declared by the root package. Diagnostics contain safe classifications,
semantic paths when applicable, and available source coordinates. Raw parser,
conversion, reader-error, and input text is not forwarded. Source labels and
diagnostic messages are application-visible and therefore must never contain
secrets; terminal control characters are escaped by `WriteDiagnostics`.