# Package `promptkit`

## Purpose

This guide helps Go consumers assemble Promptkit and choose the main
preparation or execution workflow. The declarations and GoDoc in the
[root package](../../doc.go) own exact field, option, serialization,
concurrency, ownership, failure, and cancellation semantics. The
[framework format reference](../formats.md) owns prompt, profile, and schema
file contracts.

Import the package as:

```go
import "gitea.maximumdirect.net/eric/promptkit"
```

The following Go fragments are illustrative and omit surrounding package,
import, and error-handling code. Use the maintained examples for complete
programs.

## Construct An Engine

Create an engine with
[`NewEngine`](../../engine.go). A directory-backed setup supplies a prompt
directory and may supply profile and schema directories:

```go
engine, err := promptkit.NewEngine(promptkit.Config{
	PromptDir:  "prompts",
	ProfileDir: "profiles",
	SchemaDir:  "schemas",
})
```

Options support single-file or `fs.FS` sources, in-memory profiles,
engine-scoped backends, and injected artifact or model clients. Consult the
[constructor and option GoDoc](../../engine.go) for composition, precedence,
validation, and default transport behavior. Source discovery, format
validation, and profile precedence are defined by the
[framework format reference](../formats.md).

## Prepare Without Model Execution

[`Engine.Prepare`](../../engine.go) resolves the selected prompt and profile,
loads inputs and any structured-output schema, and renders messages without
calling a model client:

```go
prepared, err := engine.Prepare(ctx, promptkit.RunRequest{
	PromptID: "meeting.summary",
	Inputs: map[string]promptkit.ArtifactRef{
		"note": promptkit.Inline("Synthetic meeting notes"),
	},
})
```

The maintained
[offline preparation example](../../examples/go-library/prepare/main.go)
shows a complete runnable setup with a prompt file, in-memory profile, and
inline input. Exact request requirements and prepared-result fields belong to
the [`RunRequest` and `PreparedRun` GoDoc](../../types.go).

## Execute And Validate

[`Engine.Run`](../../engine.go) performs the same preparation, invokes the
configured model client, classifies the generated artifact, and validates the
content. A completed content check may return `ValidationFailed` in the result;
an operational inability to validate returns an error.

The maintained
[offline execution example](../../examples/go-library/run/main.go) injects a
deterministic model client and exercises `Run` without credentials, network
access, or paid calls. It is intentionally separate from the preparation
example so each workflow and its small prompt fixture can be copied and run on
its own.

Use the [`RunResult` and `ValidationResult` GoDoc](../../types.go) for the
returned data and the `Engine.Run` GoDoc for failure and cancellation
semantics. The
[OpenAI-compatible integration contract](../integrations/openai-compatible-chat.md)
owns the built-in client's outbound HTTP behavior.

## Inputs, Profiles, And Overrides

Use `File`, `Inline`, or `InlineWithURI` to construct request inputs. A request
can select a profile explicitly or use the prompt's default profile, and can
replace execution settings or the complete output contract.

The [public value GoDoc](../../types.go) defines nil, empty, zero, replacement,
copy, and credential behavior. The
[framework format reference](../formats.md) defines how those request values
interact with prompt definitions, file-backed profiles, built-ins, schemas,
and framework defaults.

For programmatic profiles,
[`OpenAICompatibleProfile`](../../profiles.go) converts ordinary
OpenAI-compatible settings into a value accepted by `WithProfiles`.

### Set A Per-Run Session And Reasoning

Supply a direct session ID when one prompt should be correlated with a
consumer-managed conversation or workflow without changing prompt variables:

```go
reasoning := "high"
result, err := engine.Run(ctx, promptkit.RunRequest{
	PromptID: "meeting.summary",
	SessionID: "conversation-42",
	Inputs: map[string]promptkit.ArtifactRef{
		"note": promptkit.Inline("Synthetic meeting notes"),
	},
	Execution: &promptkit.ExecutionTargetOverride{
		ReasoningEffort: &reasoning,
	},
})
```

A nil reasoning pointer inherits the selected profile, a pointer to a
nonblank string replaces it, and a pointer to a blank string disables
reasoning for that run. Session IDs are correlation metadata, not credentials;
use stable, non-secret values that are safe to expose to collaborators and
providers. The
[`RunRequest` and `ExecutionTargetOverride` GoDoc](../../types.go) owns the
exact normalization, precedence, error, copying, and exposure contract.

### Register A Custom Backend

Choose the smallest configuration that fits how the endpoint will be reused.

#### Use An Endpoint-Only Profile

Put the endpoint directly on an in-memory profile when only that profile needs
it and shared backend identity or capacity policy is unnecessary:

```go
engine, err := promptkit.NewEngine(promptkit.Config{
	PromptDir: "prompts",
},
	promptkit.WithProfiles(promptkit.Profile{
		ID:       "local-summary",
		Endpoint: "http://localhost:8000/v1",
		Model:    "example-model",
	}),
)
```

Endpoint-only profiles have an empty backend ID and remain unrestricted by
backend capacity policy.

#### Use The Conventional Local Backend

Use `LocalBackend` when profiles should share the conventional `local`
identity, endpoint, and concurrency limit:

```go
engine, err := promptkit.NewEngine(promptkit.Config{
	PromptDir: "prompts",
},
	promptkit.WithBackend(
		promptkit.LocalBackend("http://localhost:8000/v1", 2),
	),
	promptkit.WithProfiles(promptkit.Profile{
		ID:        "local-summary",
		BackendID: promptkit.BackendLocal,
		Model:     "example-model",
	}),
)
```

The helper is explicit: it does not pre-register a backend or read environment
variables. Supplying a positive limit leaves queue capacity omitted, so normal
backend registration selects the existing default waiting capacity of 1024.
The returned value still enters the engine through `WithBackend`.

#### Configure A Complete Backend

Use a keyed `Backend` value for authentication, extra request parameters, an
explicit queue capacity, a custom ID, or multiple local endpoints:

```go
noWaiting := 0
engine, err := promptkit.NewEngine(promptkit.Config{
	PromptDir: "prompts",
},
	promptkit.WithBackend(promptkit.Backend{
		ID:               "local-gpu",
		Endpoint:         "http://gpu-host:8000/v1",
		APIKeyEnv:        "LOCAL_GPU_API_KEY",
		ExtraParams:      map[string]any{"provider_option": "enabled"},
		ConcurrencyLimit: 2,
		QueueCapacity:    &noWaiting,
	}),
	promptkit.WithProfiles(promptkit.Profile{
		ID:        "gpu-summary",
		BackendID: "local-gpu",
		Model:     "example-model",
	}),
)
```

Use distinct custom IDs when registering multiple local endpoints.
Registrations belong to one engine and custom IDs cannot replace built-ins.
The [`Backend`, `LocalBackend`, and `WithBackend` GoDoc](../../backends.go)
defines exact construction, validation, copying, uniqueness, concurrency, and
request-default behavior.

Both file-backed and in-memory profiles select a registration through
`backend` or `Profile.BackendID`. Profile and request endpoint overrides retain
that routing and capacity identity. `PreparedRun.SelectedBackendID`,
`RunResult.SelectedBackendID`, and the effective `ExecutionTarget.BackendID`
expose it to consumers and injected model clients. Endpoint-only profiles
remain supported and expose an empty backend ID.

### Limit Backend Concurrency

Set `Backend.ConcurrencyLimit` when a backend needs protection from too many
simultaneous model calls. Leaving `QueueCapacity` nil, as in the local-backend
example above, selects the default waiting capacity of 1024.

To accept no waiting backlog beyond the active calls, provide an explicit
zero:

```go
noWaiting := 0
backend := promptkit.Backend{
	ID:               "local-gpu",
	Endpoint:         "http://gpu-host:8000/v1",
	ConcurrencyLimit: 2,
	QueueCapacity:    &noWaiting,
}
```

The pointer distinguishes an explicit zero from omission. Keep using keyed
`Backend` literals so additive configuration fields remain source-compatible.
Capacity belongs to one engine and the selected backend ID; endpoint-only
profiles and custom backends without a configured limit remain unrestricted.
Exact validation, defaulting, ownership, and concurrency semantics belong to
the [`Backend` GoDoc](../../backends.go).

## Credentials

File-backed profiles name an environment variable; in-memory profiles can
require a direct request key. Direct keys are request-scoped and are excluded
from supported JSON values and the package's `String` and `GoString`
summaries. The exact precedence and redaction guarantees belong to
[`RunRequest`, `GenerateRequest`, and the profile GoDoc](../../types.go).

## Protect Files And Generated Data

The default artifact reader opens a `File` reference as a caller-selected
operating-system path. It does not constrain paths to an application root,
impose an inbound request-size policy, or establish an untrusted-input security
boundary. Applications must validate and restrict untrusted paths and payloads
before constructing a request, or inject an artifact reader that enforces
their filesystem, authorization, and size policies.

Rendered messages, input and output artifact bodies, raw model output, and
validation diagnostics can contain sensitive data. API-key redaction does not
sanitize those values. Treat prepared values, results, collaborator requests,
errors, and logs according to the application's data-access, retention, and
secret-handling policies.

## Extension Interfaces

Inject an [`LLMClient` or `ArtifactReader`](../../types.go) when the built-in
behavior does not fit the application. Their GoDoc defines concurrent use,
context handling, ownership of copied values, nil responses, and preservation
of collaborator errors. Implementations must honor cancellation, safely manage
copies they retain, avoid unsafe logging of content or credentials, and enforce
the application policy that motivated the injection.

## Handle Errors

Use `errors.Is` with the
[public error sentinels and operation GoDoc](../../engine.go). The declarations
distinguish invalid construction, invalid requests, absent sources,
source-loading failures, collaborator failures, and operational validation
failures. Specific request conditions may also match the broader
`ErrInvalidRequest`, and injected collaborator identities are preserved where
documented. Invalid or duplicate backend registrations match
`ErrInvalidConfig`; selecting an unknown backend matches `ErrProfileLoad`.

When a limited backend has admitted all active and waiting calls, handle
`ErrCapacityExceeded` separately from request errors and provider failures:

```go
result, err := engine.Run(ctx, request)
if errors.Is(err, promptkit.ErrCapacityExceeded) {
	// Apply application policy: shed work, report overload, or retry later.
}
```

A rejected call returns no partial result and does not invoke the model
client. Promptkit does not prescribe retries or map this error to an HTTP
status; those choices remain with the consuming application. The
[`Engine.Run` and error GoDoc](../../engine.go) owns exact error and
cancellation identities.

## Application Boundary

Promptkit is an importable library. It does not own a command, inbound HTTP
API, process configuration, or deployment policy. Applications map the root
package's results and errors into those concerns, including inbound size and
trust policy.