diff --git a/convert.go b/convert.go
index 745f913..28e79ae 100644
--- a/convert.go
+++ b/convert.go
@@ -4,6 +4,7 @@ import (
 	"reflect"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
 )
 
 func toDomainRunRequest(req RunRequest) (domain.RunRequest, error) {
@@ -133,7 +134,7 @@ func toDomainExecutionTargetOverride(override *ExecutionTargetOverride) (*domain
 	if override == nil {
 		return nil, nil
 	}
-	extraParams, err := copyPublicJSONMap(override.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(override.ExtraParams)
 	if err != nil {
 		return nil, err
 	}
diff --git a/docs/internal/llm.md b/docs/internal/llm.md
index e441508..d1fbc72 100644
--- a/docs/internal/llm.md
+++ b/docs/internal/llm.md
@@ -36,6 +36,10 @@ caller's client. Generation then:
 5. performs the outbound request under the applicable deadlines; and
 6. decodes the first response choice and token usage.
 
+`internal/llm` owns the set of reserved OpenAI-compatible request fields used
+when validating extra parameters. Backend registration consumes the same rule
+without making the model client depend on registry configuration.
+
 The implementation has no retry loop, tool-call support, provider catalog,
 inbound HTTP behavior, or durable session store.
 
diff --git a/docs/internal/overview.md b/docs/internal/overview.md
index 9df10b2..675608a 100644
--- a/docs/internal/overview.md
+++ b/docs/internal/overview.md
@@ -14,17 +14,18 @@ contributor workflow and validation.
 | Root `promptkit` package | Provides the supported engine facade, source, backend-registration, and injection options, public request and result values, profile construction, extension interfaces, value conversion, redacted formatting, and public error mapping. | [Package GoDoc](../../doc.go), [backend API](../../backends.go), [engine assembly](../../engine.go) |
 | `examples/go-library/prepare` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, and `Prepare`. It is not a public library package. | [Example program](../../examples/go-library/prepare/main.go) |
 | `examples/go-library/run` | Demonstrates an offline downstream consumer using a prompt file, in-memory profile, inline input, an injected deterministic model client, and `Run`. It is not a public library package. | [Example program](../../examples/go-library/run/main.go) |
-| `internal/backend` | Constructs each engine's immutable registry from the built-in OpenRouter definition and consumer additions, validates and defensively copies definitions, and owns the shared reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
+| `internal/backend` | Constructs each engine's immutable registry from the built-in OpenRouter definition and consumer additions, validates and defensively copies definitions through the shared JSON-value package, and consumes the LLM-owned OpenAI-compatible reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
 | `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation. | [Domain declarations](../../internal/domain/domain.go) |
 | `internal/defaults` | Defines application-neutral framework constants and constructs the default execution target. It contains no CLI, server, or inbound HTTP limits. | [Framework defaults](../../internal/defaults/defaults.go) |
 | `internal/filecatalog` | Provides deterministic YAML discovery and path helpers for operating-system filesystems and `fs.FS` sources. | [File catalog](../../internal/filecatalog/catalog.go) |
+| `internal/jsonvalue` | Validates and deeply copies JSON-compatible extra-parameter trees while preserving supported concrete value types. | [JSON values](../../internal/jsonvalue/jsonvalue.go) |
 | `internal/promptdef` | Loads strictly decoded, validated prompt definitions from filesystem and `fs.FS` sources, including version selection and contained file-backed message content. | [Framework formats](../formats.md), [prompt-definition repository](../../internal/promptdef/filesystem_repository.go) |
 | `internal/profile` | Loads strictly decoded, validated execution profiles, including backend selection, from filesystem and `fs.FS` sources and composes repositories with error-preserving fallback. | [Framework formats](../formats.md), [profile repositories](../../internal/profile/filesystem_repository.go) |
 | `internal/profile/builtin` | Embeds the built-in profile catalog, whose entries select OpenRouter, and combines it with an optional primary repository. | [Built-in catalog](../formats.md#built-in-profile-catalog), [repository](../../internal/profile/builtin/repository.go) |
 | `internal/prompt` | Renders prompt messages from Go templates with artifact, variable, session, and cache-control data. | [Go-template renderer](../../internal/prompt/go_renderer.go) |
 | `internal/artifact` | Resolves ordinary inline and unrestricted caller-selected file references into copied artifacts with metadata and hashes. | [Internal sources and validation](sources.md) |
 | `internal/validate` | Validates basic, JSON, and JSON Schema output using operating-system filesystem or `fs.FS` schema sources. | [Framework formats](../formats.md#schemas), [internal sources and validation](sources.md) |
-| `internal/llm` | Defines the internal generation boundary and implements outbound OpenAI-compatible chat requests from resolved execution targets, including response decoding, authentication, and deadline handling. | [Internal model client](llm.md) |
+| `internal/llm` | Defines the internal generation boundary and implements outbound OpenAI-compatible chat requests from resolved execution targets, including response decoding, authentication, deadline handling, and ownership of the OpenAI-compatible reserved request-field policy. | [Internal model client](llm.md) |
 | `internal/usecase` | Resolves backend, profile, and request settings and coordinates preparation and execution across internal sources, rendering, artifact loading, generation, validation, and optional repair. | [Internal runner](runner.md) |
 
 The root package assembles these internal components without exposing their
diff --git a/docs/policy/architecture.md b/docs/policy/architecture.md
index c114392..1885716 100644
--- a/docs/policy/architecture.md
+++ b/docs/policy/architecture.md
@@ -27,6 +27,8 @@ The implemented internal components consist of:
   constructs the default execution target;
 - `internal/filecatalog`, which discovers YAML files and provides source-path
   helpers for filesystem and `fs.FS` consumers;
+- `internal/jsonvalue`, which validates and defensively copies JSON-compatible
+  extra-parameter trees;
 - `internal/promptdef`, which loads and validates prompt definitions from
   filesystem and `fs.FS` sources;
 - `internal/profile`, which loads, validates, and overlays execution profiles
@@ -76,9 +78,11 @@ downstream consumers, including Scriptorium
         narrow injected abstractions
 ```
 
-The backend registry depends on the domain model, has no mutation API after
-construction, and shares its OpenAI-compatible reserved request-field rule
-with the model client. The facade coordinates internal components and adapts
+The backend registry depends on the domain model and shared JSON-value
+validation, has no mutation API after construction, and consumes the
+OpenAI-compatible reserved request-field rule owned by the model client. The
+model client does not depend on registry configuration. The facade coordinates
+internal components and adapts
 the supported public extension interfaces to narrow internal abstractions.
 Internal components must not depend on consumers or on Scriptorium.
 
diff --git a/docs/roadmap/backends.md b/docs/roadmap/backends.md
deleted file mode 100644
index 8116c00..0000000
--- a/docs/roadmap/backends.md
+++ /dev/null
@@ -1,182 +0,0 @@
-# Extensible LLM Backend Registry
-
-**Status:** Complete.
-
-## Purpose
-
-This roadmap defines the scope and target end state for formal
-OpenAI-compatible backend support in Promptkit. It establishes the behavioral
-boundary and policy choices for the work.
-
-This document is planning material, not a description of current behavior.
-Current public contracts remain owned by Go declarations and GoDoc, framework
-file formats by the [format reference](../formats.md), and outbound HTTP
-behavior by the
-[OpenAI-compatible integration contract](../integrations/openai-compatible-chat.md).
-
-## Motivation
-
-Execution profiles currently carry backend connection and authentication
-details alongside model and generation settings. This repeats values such as
-the OpenRouter endpoint and `OPENROUTER_API_KEY` across profiles, leaves the
-built-in catalog implicitly tied to one backend, and gives consumers no
-explicit extension point for naming other OpenAI-compatible services.
-
-Promptkit should distinguish:
-
-- a **backend**, which identifies reusable connection, authentication, and
-  limited backend-wide request defaults; and
-- a **profile**, which selects a model and its reusable generation settings.
-
-That distinction should let consumers configure OpenRouter, OpenAI,
-local-network services, or other OpenAI-compatible deployments without
-duplicating backend defaults in every profile or replacing Promptkit's model
-client.
-
-## Scope
-
-The work introduces an engine-scoped registry of OpenAI-compatible backend
-definitions.
-
-Each backend definition will provide:
-
-- a stable, non-empty backend ID;
-- a default OpenAI-compatible base endpoint;
-- an optional environment-variable name for its API key; and
-- limited JSON-compatible request defaults when they are genuinely
-  backend-wide rather than model-specific.
-
-Promptkit will provide a small built-in backend catalog, initially containing
-OpenRouter. Downstream consumers will be able to register additional backend
-IDs during engine construction. Registration will be explicit and local to an
-engine; it will not mutate package-global state.
-
-Profiles will be able to name a backend. A profile that selects a backend may
-override its default endpoint and other supported defaults while continuing to
-own its model and generation settings. Existing endpoint-based custom profiles
-will remain supported without requiring a backend ID.
-
-The built-in model profile catalog will use the built-in OpenRouter backend
-instead of repeating OpenRouter connection and credential defaults in every
-profile.
-
-## Resolution And Precedence
-
-Effective execution settings will resolve in this order:
-
-1. application-neutral framework defaults;
-2. selected backend defaults, when a backend is named;
-3. selected profile values; and
-4. explicit per-run overrides.
-
-A profile endpoint will override its selected backend's endpoint. A per-run
-endpoint override will continue to take precedence over both.
-
-Credential selection will preserve direct request credentials as the highest
-precedence. An explicit per-run environment-variable override will take
-precedence over profile credential configuration, which will take precedence
-over the backend's default environment-variable name. Backend definitions and
-profiles will contain credential-source metadata only, never resolved secret
-values.
-
-Backend request defaults, profile request values, and per-run request
-overrides will follow one deterministic replacement rule. Resolution must not
-introduce implicit deep merging whose result depends on map iteration or
-incidental representation.
-
-## Registration And Validation
-
-Backend registration will be deterministic and validated during engine
-construction.
-
-- Built-in backend IDs are reserved and cannot be replaced by consumers.
-- Consumer registrations may add only new IDs.
-- Duplicate consumer IDs are invalid, including duplicates introduced through
-  repeated configuration.
-- Backend IDs, endpoints, credential-source metadata, and request defaults
-  must be validated before an engine is returned.
-- Selecting an unknown backend is an error associated with the profile or
-  request boundary that selected it.
-- A profile without a backend must continue to provide the connection details
-  required by the current endpoint-based path.
-- A backend may omit credential requirements so unauthenticated local-network
-  services remain supported.
-
-Failures will retain error identities appropriate to the existing engine
-configuration, profile-loading, and per-run validation boundaries.
-
-## Public And Extension Boundaries
-
-The effective backend ID will be observable in prepared and completed run
-metadata and will be available to injected model clients. This gives consumers
-and extensions a stable routing identity without requiring them to infer a
-backend from an endpoint URL.
-
-The existing public model-client injection boundary will remain supported.
-Because the initial registry supports only OpenAI-compatible backends,
-Promptkit does not need backend-specific transport factories or multiple
-protocol implementations in this scope. The built-in client will continue to
-send the resolved execution target to the selected OpenAI-compatible endpoint.
-
-Backend definitions are configuration values, not live service objects. They
-will not own mutable connections, credentials, health state, or process
-lifecycle.
-
-## Compatibility
-
-The target end state preserves the existing ways consumers configure and run
-Promptkit:
-
-- Profiles with explicit endpoints and no backend ID continue to work.
-- Profile endpoint overrides remain supported.
-- Direct request API keys and environment-variable overrides retain their
-  precedence.
-- Consumers may continue injecting a custom model client.
-- Model IDs and generation settings remain profile concerns.
-- Prompt selection, rendering, validation, artifacts, and synchronous
-  `Prepare` and `Run` behavior remain unchanged except for exposing the
-  resolved backend identity.
-
-## Non-Goals
-
-This scope does not include:
-
-- non-OpenAI-compatible protocols or provider-specific SDKs;
-- backend-specific concurrency limits or admission queues;
-- asynchronous task submission or durable jobs;
-- model discovery, live model catalogs, or provider capability probing;
-- retries, rate limiting, failover, load balancing, or health checks;
-- automatic backend selection based on model names or endpoint inspection;
-- global mutable registration;
-- consumer replacement of built-in backend IDs;
-- credential discovery beyond configured direct values and environment
-  variables; or
-- provider-specific application, deployment, or security policy.
-
-The dependent concurrency and bounded-queue work remains a separate future
-unit after backend identity and resolution are stable.
-
-## Target End State
-
-This roadmap reaches its target end state when:
-
-- Promptkit has an immutable, engine-scoped registry of validated
-  OpenAI-compatible backend definitions;
-- OpenRouter is available as a built-in backend with its endpoint and
-  credential environment-variable default;
-- consumers can register additional unique backend IDs without modifying
-  Promptkit;
-- profiles can select a backend and optionally override its endpoint;
-- existing endpoint-only profiles remain valid;
-- backend, profile, and per-run values resolve through documented,
-  deterministic precedence;
-- secret values remain outside backend and profile definitions;
-- the effective backend identity is available in preparation, execution, and
-  injected-client values;
-- the built-in model profiles use the OpenRouter backend rather than duplicate
-  its connection defaults;
-- the built-in OpenAI-compatible client and injected clients continue to work
-  through the existing generation boundary; and
-- current-state GoDoc, format, consumer, integration, architecture, and
-  internal documentation describe the implemented behavior without relying on
-  this roadmap.
diff --git a/docs/roadmap/future.md b/docs/roadmap/future.md
index 9caeac9..c32de70 100644
--- a/docs/roadmap/future.md
+++ b/docs/roadmap/future.md
@@ -35,9 +35,10 @@ consumers.
 
 ### Backend-specific concurrency management
 
-Extend the selected
-[LLM backend registry](backends.md) with optional per-backend concurrency limits
-and bounded, buffered admission queues. Promptkit could then route simultaneous
+Extend the
+[LLM backend registry](../consumers/pkg-promptkit.md#register-a-custom-backend)
+with optional per-backend concurrency limits and bounded, buffered admission
+queues. Promptkit could then route simultaneous
 generation requests according to backend capacity while containing accidental
 runaway submission. Downstream consumers would continue invoking synchronous
 `Run` calls, including concurrently from multiple goroutines, and each
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
deleted file mode 100644
index 70e5e0f..0000000
--- a/docs/roadmap/implementation.md
+++ /dev/null
@@ -1,505 +0,0 @@
-# Extensible LLM Backend Registry Implementation Plan
-
-## Purpose
-
-This document is the decision-complete implementation plan for the
-[extensible LLM backend registry](backends.md). It is written for a coding
-agent that will implement each stage in order.
-
-This is planning material, not a description of current behavior. During
-implementation, update current-state GoDoc and documentation only as the
-corresponding behavior lands. The feature roadmap owns the intended end state
-and policy choices; this document owns API shape, package changes, sequencing,
-tests, and completion gates.
-
-## Implementation Rules
-
-- Complete the stages in order. Do not begin a later stage while an earlier
-  stage's completion gate is unmet.
-- Preserve unrelated working-tree changes and do not broaden the feature into
-  concurrency control, queues, retries, provider discovery, or
-  non-OpenAI-compatible transports.
-- Keep the registry immutable and scoped to one `Engine`. Do not add
-  package-global registration or mutation after construction.
-- Keep secrets out of backend definitions, profiles, prepared values, results,
-  hashes, logs, and JSON. Backends store only an environment-variable name.
-- Preserve endpoint-only profiles and custom `LLMClient` injection.
-- Follow the architecture, documentation, and testing policies under
-  `docs/policy/`. In particular, keep the public API in the root package,
-  place implementation packages under `internal/`, update the component
-  inventory when the new package lands, and assign each behavior to one
-  durable test owner.
-- Use deterministic, offline tests. No stage may require a real provider,
-  credential, or network service.
-
-## Fixed Design
-
-### Public API
-
-Add the following root-package declarations:
-
-```go
-const BackendOpenRouter = "openrouter"
-
-type Backend struct {
-	ID          string
-	Endpoint    string
-	APIKeyEnv   string
-	ExtraParams map[string]any
-}
-
-func WithBackend(backend Backend) Option
-```
-
-`Backend` and `WithBackend` have these contracts:
-
-- `Backend` configures one OpenAI-compatible backend and has no stable JSON
-  representation.
-- `ID` is trimmed, non-empty, case-sensitive, and is the stable registry key.
-- `Endpoint` is trimmed and must be an absolute `http` or `https` URL with a
-  host. Paths are allowed; user information, query strings, and fragments are
-  rejected because this value is a base endpoint rather than a complete
-  request URL.
-- `APIKeyEnv` is optional. When present, it is trimmed and must match the
-  portable environment-variable form `[A-Za-z_][A-Za-z0-9_]*`.
-- `ExtraParams` follows the existing `Profile.ExtraParams` JSON-value rules:
-  non-empty string keys, finite numbers, JSON-compatible scalar and container
-  values, no cycles, and no collisions with `model`, `session_id`, `messages`,
-  `temperature`, `max_tokens`, `top_p`, `service_tier`, `reasoning_effort`, or
-  `response_format`. It is deeply copied during `NewEngine`.
-- An empty `ExtraParams` map means that the backend supplies no request
-  defaults.
-- Each `WithBackend` call adds one registration. Calls with different IDs
-  accumulate in option order; a repeated ID is an error rather than a
-  replacement. This is an explicit additive exception to the last-option-wins
-  categories documented on `Option`.
-- `BackendOpenRouter` is reserved. Consumers cannot replace it.
-- There is no public enumerate, lookup, remove, replace, or post-construction
-  registration API in this scope.
-
-Extend the existing public values as follows:
-
-| Value | Addition | Contract |
-| --- | --- | --- |
-| `Profile` | `BackendID string` | Optional backend selection. `Endpoint` is required only when `BackendID` is blank. |
-| `OpenAICompatibleProfileConfig` | `BackendID string` | Copied to `Profile.BackendID`. |
-| `ExecutionTarget` | `BackendID string \`json:"backend_id,omitempty"\`` | Effective routing identity supplied to injected clients; empty for endpoint-only profiles. |
-| `PreparedRun` | `SelectedBackendID string \`json:"selected_backend_id,omitempty"\`` | Equals the effective target backend ID. |
-| `RunResult` | `SelectedBackendID string \`json:"selected_backend_id,omitempty"\`` | Carries the prepared backend identity through execution. |
-
-Do not add a backend field to `RunRequest` or `ExecutionTargetOverride`.
-Backend selection remains a profile concern. Endpoint overrides do not change
-the selected backend identity.
-
-Trim `Profile.BackendID` and YAML `backend` values at their respective
-conversion/load boundaries. Treat an all-whitespace value as absent and expose
-only the trimmed value in effective targets and metadata.
-
-These exported struct fields are intentionally additive. Adding fields can
-break downstream unkeyed composite literals even though keyed literals and
-ordinary field access remain source-compatible. Accept that narrow risk; use
-and document keyed literals, and do not add parallel wrapper types or a second
-configuration path solely to preserve unkeyed literals.
-
-### Profile Format
-
-Add the optional strict-YAML field:
-
-```yaml
-backend: openrouter
-```
-
-The profile connection rule becomes:
-
-- `model` remains required;
-- at least one of `backend` or `endpoint` is required;
-- when both are present, the profile endpoint overrides the backend endpoint;
-  and
-- a profile with only an endpoint continues through the legacy path and has an
-  empty effective backend ID.
-
-Do not infer a backend from a model name or endpoint. Do not validate whether a
-backend ID exists while decoding a profile; registry membership is
-engine-scoped and is checked when the selected profile is prepared.
-
-### Registry And Internal Boundaries
-
-Add `internal/backend` as the cohesive owner of:
-
-- the immutable backend registry;
-- backend-definition validation and defensive copying;
-- the built-in OpenRouter definition; and
-- the internal not-found error used when a selected ID is absent.
-
-Add an internal domain backend value containing `ID`, `Endpoint`, `APIKeyEnv`,
-and `ExtraParams`. The registry constructor accepts consumer additions, installs
-built-ins first, rejects all collisions, and returns a fully constructed
-read-only registry. Lookup returns a defensive value so callers cannot mutate
-registry-owned maps.
-
-Use these internal declarations:
-
-```go
-var ErrBackendNotFound = errors.New("backend not found")
-
-func NewRegistry(additions []domain.Backend) (*Registry, error)
-func (r *Registry) GetBackend(id string) (domain.Backend, error)
-```
-
-`internal/usecase` owns the narrow resolver interface it consumes:
-
-```go
-type BackendResolver interface {
-	GetBackend(string) (domain.Backend, error)
-}
-```
-
-The concrete registry implements this interface. Supply it explicitly to
-`Runner` through both runner constructors and from the root engine assembly.
-A nil resolver must never panic; if an internally constructed runner selects a
-backend without a resolver, preparation fails as a profile-load failure.
-
-The built-in definition is exactly:
-
-| Field | Value |
-| --- | --- |
-| ID | `openrouter` |
-| Endpoint | `https://openrouter.ai/api/v1` |
-| API key environment variable | `OPENROUTER_API_KEY` |
-| Extra parameters | none |
-
-Define `OpenRouterID` as a constant in `internal/backend` and define the public
-`BackendOpenRouter` constant from it so there is one canonical literal. This
-root-to-internal implementation import does not expose an internal type in a
-public signature.
-
-### Resolution
-
-Resolve an execution target in this order:
-
-1. framework defaults;
-2. the selected backend, if any;
-3. the selected profile; and
-4. the request's `ExecutionTargetOverride`.
-
-The layers behave as follows:
-
-| Setting | Backend layer | Profile layer | Request layer |
-| --- | --- | --- | --- |
-| Backend ID | Supplies selected ID | Selects the backend | Cannot change it |
-| Endpoint | Supplies default | Non-empty value replaces backend | Non-empty value replaces profile/backend |
-| Model and generation fields | Not supplied | Existing profile behavior | Existing override behavior |
-| API-key environment name | Supplies default | Non-empty `api_key_env` replaces it | Non-empty `APIKeyEnv` replaces it |
-| Extra parameters | Non-empty map replaces prior map | Non-empty map replaces complete backend map | Non-empty map replaces complete profile/backend map |
-
-Never deep-merge `ExtraParams` maps. A non-empty higher-precedence map replaces
-the complete lower-precedence map. Preserve the existing distinction between a
-nil or empty override map and a supplied non-empty replacement.
-
-Credential handling remains:
-
-1. a non-blank `RunRequest.APIKey` wins and suppresses environment lookup;
-2. a non-blank request `APIKeyEnv` wins over profile and backend metadata;
-3. file-profile `api_key_env` wins over the backend default;
-4. backend `APIKeyEnv` is used when no higher layer supplies a credential
-   source; and
-5. a profile with `APIKeyRequired: true` clears an inherited backend
-   environment name and requires a direct key unless the request explicitly
-   supplies its own `APIKeyEnv`.
-
-Only the environment-variable name appears in effective targets. Preserve the
-current just-in-time environment lookup and missing-variable behavior.
-
-### Errors
-
-Do not add a public backend-specific error sentinel.
-
-| Failure | Required public identity |
-| --- | --- |
-| Invalid consumer backend definition | `ErrInvalidConfig` from `NewEngine` |
-| Consumer ID duplicates another consumer ID | `ErrInvalidConfig` |
-| Consumer ID collides with `BackendOpenRouter` | `ErrInvalidConfig` |
-| Selected profile names an unknown backend | `ErrProfileLoad`, not `ErrProfileNotFound` or `ErrInvalidRequest` |
-| File profile lacks both backend and endpoint | `ErrProfileLoad` |
-| In-memory profile lacks both backend and endpoint | `ErrInvalidConfig` |
-| Effective credential environment variable is unset | Existing `ErrAPIKeyEnvMissing` and `ErrInvalidRequest` identities |
-| Other invalid per-run effective settings | Existing `ErrInvalidRequest` |
-
-Error text must include the offending backend ID or field where useful, but
-exact prose is not a compatibility contract. Preserve wrapped internal causes
-where the existing error boundary permits it.
-
-### Metadata And Transport
-
-- Set `ExecutionTarget.BackendID`, `PreparedRun.SelectedBackendID`, and
-  `RunResult.SelectedBackendID` from the normalized selected profile backend.
-- Endpoint-only profiles leave all three values empty.
-- Pass `BackendID` through `GenerateRequest.Target` and repair targets so an
-  injected client can route or observe it.
-- The built-in OpenAI-compatible client must not serialize `backend_id` as a
-  provider request field. It continues to use the effective endpoint,
-  credential, typed generation fields, and extra parameters.
-- Backend selection does not change rendered-prompt hashes, prompt hashes,
-  session IDs, validation, artifacts, timing, or synchronous `Prepare` and
-  `Run` behavior.
-
-## Stage 1 — Registry Foundation
-
-**Status:** Complete.
-
-### Goal
-
-Introduce the internal domain and immutable registry without changing the
-public configuration or profile format.
-
-### Work
-
-1. Add the internal backend definition to `internal/domain`.
-2. Create `internal/backend` with:
-   - a registry constructor that installs OpenRouter and accepts additional
-     internal definitions;
-   - validation for IDs, endpoints, environment-variable names, JSON values,
-     and reserved request fields;
-   - deterministic built-in and consumer collision handling;
-   - immutable lookup with defensive map copying; and
-   - an internal recognizable not-found error.
-3. Reuse or extract one validation rule for OpenAI-compatible reserved request
-   fields so registry validation and `internal/llm` payload construction cannot
-   drift. Do not maintain duplicate reserved-key lists.
-4. Keep JSON-value copying type-preserving. Do not use a marshal/unmarshal
-   round trip that silently changes integer or container types.
-5. Update `docs/policy/architecture.md` and `docs/internal/overview.md` only
-   after the package exists, describing `internal/backend` as an implemented
-   immutable configuration registry rather than a runtime service manager.
-
-### Tests
-
-`internal/backend` owns focused package tests for:
-
-- the exact OpenRouter built-in values;
-- successful unique consumer additions;
-- built-in and consumer duplicate rejection;
-- blank and whitespace-normalized IDs;
-- invalid or non-HTTP(S) endpoints;
-- invalid environment-variable names;
-- invalid, cyclic, non-finite, empty-key, and reserved-key extra parameters;
-- not-found lookup; and
-- mutation isolation of input maps and returned values.
-
-Use one representative table per validation family rather than one test per
-branch. A race-specific registry test is unnecessary if the registry is
-immutable and the final repository race suite exercises concurrent reads.
-
-### Completion Gate
-
-- No public declaration or profile behavior has changed.
-- The registry has no mutating method after construction.
-- The built-in definition contains no credential value.
-- Focused backend tests and the complete repository validation pass.
-
-## Stage 2 — Profile Selection And Effective Resolution
-
-**Status:** Complete.
-
-### Goal
-
-Make the built-in OpenRouter backend selectable by file and in-memory profiles,
-migrate the built-in profiles, and expose effective backend identity.
-
-### Work
-
-1. Add `BackendID` to internal execution profiles and targets and add the YAML
-   key `backend`.
-2. Change file and public in-memory profile validation to require `model` plus
-   at least one of `backend` or `endpoint`. Preserve all existing numeric and
-   extra-parameter validation.
-3. Add `BackendID` to `Profile` and
-   `OpenAICompatibleProfileConfig`, and add `BackendOpenRouter`.
-4. Add backend identity to the public and internal execution target, prepared
-   run, and run result exactly as specified under **Public API**.
-5. Add the `BackendResolver` dependency immediately after the profile
-   repository in both runner constructors and update all internal call sites
-   and test helpers explicitly.
-6. Resolve the selected backend after profile loading and before effective
-   target resolution. Map lookup failure to `ErrProfileLoad`.
-7. Implement the exact endpoint, credential, and whole-map precedence rules
-   under **Resolution**. Keep backend ID independent from endpoint overrides.
-8. Construct the built-in-only registry in `NewEngine` and inject it into the
-   runner.
-9. Replace `endpoint` and `api_key_env` in every
-   `internal/profile/builtin/assets/*.yaml` profile with
-   `backend: openrouter`. Retain each profile's model and generation settings.
-10. Update conversions and copying so backend IDs survive every public/internal
-    boundary and registry-owned maps remain isolated.
-11. Update the GoDoc for all changed public declarations.
-12. Update `docs/formats.md`, `docs/internal/sources.md`, and
-    `docs/internal/runner.md` with the implemented profile field, conditional
-    endpoint rule, built-in selection, and effective resolution behavior.
-
-### Tests
-
-Assign test ownership as follows:
-
-- `internal/profile` tests own strict YAML decoding and the conditional
-  backend-or-endpoint validation matrix.
-- `internal/profile/builtin` tests own the invariant that every built-in
-  profile selects `openrouter` and no longer repeats its endpoint or
-  credential environment name.
-- `internal/usecase` tests own backend/profile/request precedence, unknown-ID
-  error wrapping, endpoint override identity, credential precedence including
-  `APIKeyRequired`, and whole-map replacement.
-- Root external-package contract tests own the new fields, JSON names and
-  omission behavior, public copying, and public error identities.
-- One assembled engine test must prove a built-in profile prepares with the
-  expected backend ID, endpoint, and environment-variable name without making
-  an HTTP call.
-
-Do not duplicate every profile-parser validation case at the engine boundary.
-
-### Completion Gate
-
-- All built-in and endpoint-only profiles prepare successfully.
-- A selected unknown backend matches public `ErrProfileLoad`.
-- Backend identity reaches prepared values, results, generation targets, and
-  repair targets.
-- No backend ID or secret is added to the outbound provider payload.
-- Current-state format and internal documentation match the implemented
-  built-in behavior.
-- The complete repository validation passes.
-
-## Stage 3 — Consumer Registration
-
-**Status:** Complete.
-
-### Goal
-
-Expose the engine-scoped extension point for additional unique
-OpenAI-compatible backend IDs.
-
-### Work
-
-1. Add `Backend` and `WithBackend` exactly as specified under **Public API**,
-   preferably in a cohesive root `backends.go` file.
-2. Convert, validate, normalize, and deeply copy public backend values during
-   `NewEngine`. Store pending additions in `engineOptions`; do not mutate a
-   registry from option application.
-3. After all options have applied, construct one immutable registry from the
-   built-in and pending consumer definitions. Map every construction failure
-   to `ErrInvalidConfig`.
-4. Make unique `WithBackend` calls additive. Reject duplicate IDs even when
-   they arrive through separate calls, and document this exception on
-   `Option` and `WithBackend`.
-5. Make registered backends available equally to directory, `fs.FS`, single
-   file, in-memory, prompt-default, and explicit request profile selection.
-6. Preserve the last-valid-option-wins behavior of every existing option
-   category.
-7. Update `docs/consumers/pkg-promptkit.md` with one minimal custom backend and
-   profile example. Keep exact field-by-field API detail in GoDoc and link to
-   it rather than duplicating it.
-
-### Tests
-
-Root public contract tests own:
-
-- one custom backend used by an in-memory profile;
-- one custom backend used by a file-backed profile;
-- multiple unique `WithBackend` calls accumulating;
-- duplicate consumer and built-in ID failures matching `ErrInvalidConfig`;
-- invalid public values and nested-map mutation isolation;
-- endpoint-only compatibility with no `WithBackend` option;
-- profile endpoint and request endpoint overrides retaining the custom backend
-  ID; and
-- an injected `LLMClient` observing the effective custom backend ID and
-  settings.
-
-Use an injected client or `httptest.Server`; never contact the configured
-external endpoint. Keep lower-level registry validation cases in
-`internal/backend`.
-
-### Completion Gate
-
-- Consumers can add only new IDs and cannot mutate or replace built-ins.
-- Registration is engine-local and two engines can use different definitions
-  for the same consumer ID without interference.
-- Existing option-category contract tests still pass unchanged except for the
-  documented additive backend case.
-- The consumer guide and GoDoc describe the implemented extension point.
-- The complete repository validation passes.
-
-## Stage 4 — Contract Hardening And Documentation Completion
-
-**Status:** Complete.
-
-### Goal
-
-Audit the completed feature across public, profile, use-case, and transport
-boundaries and make the current-state documentation self-sufficient.
-
-### Work
-
-1. Review the preceding tests as a suite. Remove redundant cases and retain a
-   lean precedence matrix that would catch layer-order, credential, copying,
-   compatibility, and error-identity regressions.
-2. Add or adjust one built-in-client transport test proving that effective
-   backend defaults reach the configured endpoint while `backend_id` is not
-   serialized. Use `httptest.Server` and synthetic credentials.
-3. Confirm public stable-JSON round trips for `ExecutionTarget`,
-   `PreparedRun`, and `RunResult`, including empty backend omission for legacy
-   profiles.
-4. Confirm concurrent `Prepare` and `Run` calls can read the immutable registry
-   under the race detector. Do not add concurrency limiting or a queue.
-5. Update all affected current-state owners:
-   - public GoDoc for the exact API and errors;
-   - `docs/formats.md` for profile YAML and built-ins;
-   - `docs/consumers/pkg-promptkit.md` for construction and use;
-   - `docs/integrations/openai-compatible-chat.md` for resolved endpoint,
-     credentials, request defaults, and the non-serialized routing identity;
-   - `docs/internal/overview.md`, `docs/internal/sources.md`,
-     `docs/internal/runner.md`, and `docs/internal/llm.md` for package
-     responsibilities and data flow; and
-   - `docs/policy/architecture.md` only to keep its implemented component
-     inventory and dependency description accurate.
-6. Search current-state documentation for obsolete statements that every
-   profile requires an endpoint or that every built-in repeats OpenRouter
-   connection settings. Update the canonical owner and replace duplicates with
-   links.
-7. Keep future concurrency and queue behavior only in
-   [the future feature catalog](future.md); do not imply that this feature
-   implements either capability.
-
-### Final Validation
-
-Run the complete maintainer sequence from the repository root:
-
-```sh
-go test ./...
-go test -race ./...
-go vet ./...
-go build ./...
-go run ./examples/go-library/prepare
-gofmt -l $(git ls-files '*.go')
-git diff --check
-```
-
-The formatting command must produce no paths. Follow every changed Markdown
-link and confirm its target exists. Inspect the final diff for raw credentials,
-global mutable state, generated workspace files, local `replace` directives,
-and unrelated changes.
-
-### Completion Gate
-
-- Every target-end-state item in [the feature roadmap](backends.md) is
-  implemented and protected at its owning boundary.
-- The public API is limited to the fixed declarations in this plan.
-- Endpoint-only profiles and injected clients remain compatible.
-- Built-in profiles obtain OpenRouter endpoint and credential metadata only
-  through the registry.
-- Current-state documentation no longer relies on either roadmap to explain
-  implemented behavior.
-- All final validation commands pass offline.
-
-## Open Questions
-
-None. The API shape, validation, precedence, compatibility behavior, error
-identities, package ownership, test ownership, and staging required for
-implementation are fixed by this plan.
diff --git a/engine_test.go b/engine_test.go
index 00753d0..4d5c8a8 100644
--- a/engine_test.go
+++ b/engine_test.go
@@ -2357,6 +2357,11 @@ func TestExtraParamsTypedNestedValuesAreCopiedAcrossPublicBoundary(t *testing.T)
 }
 
 func TestRunRejectsInvalidExtraParams(t *testing.T) {
+	cyclicMap := map[string]any{}
+	cyclicMap["self"] = cyclicMap
+	cyclicSlice := []any{nil}
+	cyclicSlice[0] = cyclicSlice
+
 	tests := []struct {
 		name        string
 		extraParams map[string]any
@@ -2368,42 +2373,10 @@ func TestRunRejectsInvalidExtraParams(t *testing.T) {
 		{name: "nan", extraParams: map[string]any{"bad": math.NaN()}},
 		{name: "positive infinity", extraParams: map[string]any{"bad": math.Inf(1)}},
 		{name: "negative infinity", extraParams: map[string]any{"bad": math.Inf(-1)}},
-	}
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			fake := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}}
-			engine := newContractEngineWithOptions(t, frameworkSchemaDir, promptkit.WithLLMClient(fake))
-
-			_, err := engine.Run(context.Background(), promptkit.RunRequest{
-				PromptID: frameworkMarkdownSummaryPromptID,
-				Inputs: map[string]promptkit.ArtifactRef{
-					"transcript": promptkit.Inline("Rin opens the gate."),
-					"glossary":   promptkit.Inline("gate: A guarded passage."),
-				},
-				Execution: &promptkit.ExecutionTargetOverride{ExtraParams: tc.extraParams},
-			})
-			if !errors.Is(err, promptkit.ErrInvalidRequest) {
-				t.Fatalf("expected ErrInvalidRequest, got %v", err)
-			}
-			if len(fake.requests) != 0 {
-				t.Fatalf("expected invalid request to fail before LLM call, got %d requests", len(fake.requests))
-			}
-		})
-	}
-}
-
-func TestRunRejectsCyclicExtraParams(t *testing.T) {
-	cyclicMap := map[string]any{}
-	cyclicMap["self"] = cyclicMap
-	cyclicSlice := []any{nil}
-	cyclicSlice[0] = cyclicSlice
-
-	tests := []struct {
-		name        string
-		extraParams map[string]any
-	}{
-		{name: "map", extraParams: cyclicMap},
-		{name: "slice", extraParams: map[string]any{"cycle": cyclicSlice}},
+		{name: "cyclic map", extraParams: cyclicMap},
+		{name: "cyclic slice", extraParams: map[string]any{"cycle": cyclicSlice}},
+		{name: "malformed JSON number", extraParams: map[string]any{"value": json.Number("+1")}},
+		{name: "empty nested key", extraParams: map[string]any{"nested": map[string]any{"": true}}},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
diff --git a/internal/backend/registry.go b/internal/backend/registry.go
index f3a6486..da23af1 100644
--- a/internal/backend/registry.go
+++ b/internal/backend/registry.go
@@ -11,6 +11,8 @@ import (
 	"strings"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
 )
 
 const (
@@ -75,7 +77,7 @@ func (r *Registry) GetBackend(id string) (domain.Backend, error) {
 	if !ok {
 		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
 	}
-	extraParams, err := copyJSONMap(definition.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
 	if err != nil {
 		return domain.Backend{}, fmt.Errorf("copy backend %q: %w", id, err)
 	}
@@ -83,25 +85,6 @@ func (r *Registry) GetBackend(id string) (domain.Backend, error) {
 	return definition, nil
 }
 
-// IsReservedRequestField reports whether name is owned by the standard
-// OpenAI-compatible chat request rather than backend extra parameters.
-func IsReservedRequestField(name string) bool {
-	switch name {
-	case "model",
-		"session_id",
-		"messages",
-		"temperature",
-		"max_tokens",
-		"top_p",
-		"service_tier",
-		"reasoning_effort",
-		"response_format":
-		return true
-	default:
-		return false
-	}
-}
-
 func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 	definition.Endpoint = strings.TrimSpace(definition.Endpoint)
 	if err := validateEndpoint(definition.Endpoint); err != nil {
@@ -126,7 +109,7 @@ func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 		if key == "" {
 			return domain.Backend{}, fmt.Errorf("backend %q extra parameter key must not be empty", definition.ID)
 		}
-		if IsReservedRequestField(key) {
+		if llm.IsReservedOpenAIChatRequestField(key) {
 			return domain.Backend{}, fmt.Errorf(
 				"backend %q extra parameter %q collides with a reserved request field",
 				definition.ID,
@@ -135,7 +118,7 @@ func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 		}
 	}
 
-	extraParams, err := copyJSONMap(definition.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
 	if err != nil {
 		return domain.Backend{}, fmt.Errorf("backend %q extra parameters: %w", definition.ID, err)
 	}
diff --git a/internal/backend/registry_test.go b/internal/backend/registry_test.go
index 0119c0c..0911733 100644
--- a/internal/backend/registry_test.go
+++ b/internal/backend/registry_test.go
@@ -1,9 +1,7 @@
 package backend_test
 
 import (
-	"encoding/json"
 	"errors"
-	"math"
 	"strings"
 	"testing"
 
@@ -188,42 +186,13 @@ func TestNewRegistryValidatesEnvironmentVariableNames(t *testing.T) {
 	}
 }
 
-func TestNewRegistryValidatesExtraParameters(t *testing.T) {
-	cyclic := map[string]any{}
-	cyclic["self"] = cyclic
-
+func TestNewRegistryRejectsInvalidAndReservedExtraParameters(t *testing.T) {
 	tests := []struct {
 		name        string
 		extraParams map[string]any
 	}{
-		{name: "empty top-level key", extraParams: map[string]any{"": true}},
-		{name: "empty nested key", extraParams: map[string]any{"nested": map[string]int{"": 1}}},
-		{name: "non-string map key", extraParams: map[string]any{"nested": map[int]string{1: "one"}}},
 		{name: "unsupported value", extraParams: map[string]any{"value": make(chan int)}},
-		{name: "cyclic value", extraParams: map[string]any{"value": cyclic}},
-		{name: "NaN", extraParams: map[string]any{"value": math.NaN()}},
-		{name: "positive infinity", extraParams: map[string]any{"value": math.Inf(1)}},
-		{name: "unsafe integer", extraParams: map[string]any{"value": int64(1 << 53)}},
-		{name: "invalid JSON number", extraParams: map[string]any{"value": json.Number("not-a-number")}},
-	}
-	for _, key := range []string{
-		"model",
-		"session_id",
-		"messages",
-		"temperature",
-		"max_tokens",
-		"top_p",
-		"service_tier",
-		"reasoning_effort",
-		"response_format",
-	} {
-		tests = append(tests, struct {
-			name        string
-			extraParams map[string]any
-		}{
-			name:        "reserved key " + key,
-			extraParams: map[string]any{key: true},
-		})
+		{name: "reserved key", extraParams: map[string]any{"model": "override"}},
 	}
 
 	for _, tc := range tests {
diff --git a/internal/backend/json_copy.go b/internal/jsonvalue/jsonvalue.go
similarity index 73%
rename from internal/backend/json_copy.go
rename to internal/jsonvalue/jsonvalue.go
index cd64c99..c9878c1 100644
--- a/internal/backend/json_copy.go
+++ b/internal/jsonvalue/jsonvalue.go
@@ -1,4 +1,6 @@
-package backend
+// Package jsonvalue validates and defensively copies JSON-compatible value
+// trees used by public configuration and request boundaries.
+package jsonvalue
 
 import (
 	"encoding/json"
@@ -11,16 +13,18 @@ import (
 
 const maxSafeJSONInteger = 1<<53 - 1
 
-type jsonVisit struct {
+type visit struct {
 	typ reflect.Type
 	ptr uintptr
 }
 
-func copyJSONMap(src map[string]any) (map[string]any, error) {
+// CopyMap validates and deeply copies an extra-parameter map while preserving
+// compatible concrete map, slice, array, scalar, and number types.
+func CopyMap(src map[string]any) (map[string]any, error) {
 	if src == nil {
 		return nil, nil
 	}
-	copied, err := copyJSONValue(reflect.ValueOf(src), "extra_params", make(map[jsonVisit]struct{}))
+	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}))
 	if err != nil {
 		return nil, err
 	}
@@ -31,7 +35,7 @@ func copyJSONMap(src map[string]any) (map[string]any, error) {
 	return out, nil
 }
 
-func copyJSONValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
+func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
 	if !value.IsValid() {
 		return nil, nil
 	}
@@ -39,12 +43,15 @@ func copyJSONValue(value reflect.Value, path string, seen map[jsonVisit]struct{}
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copyJSONValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen)
 	}
 	if !value.CanInterface() {
 		return nil, fmt.Errorf("%s: value cannot be copied", path)
 	}
 	if number, ok := value.Interface().(json.Number); ok {
+		if _, err := json.Marshal(number); err != nil {
+			return nil, fmt.Errorf("%s: invalid JSON number", path)
+		}
 		f, err := strconv.ParseFloat(number.String(), 64)
 		if err != nil || math.IsNaN(f) || math.IsInf(f, 0) {
 			return nil, fmt.Errorf("%s: invalid JSON number", path)
@@ -75,28 +82,28 @@ func copyJSONValue(value reflect.Value, path string, seen map[jsonVisit]struct{}
 		if value.IsNil() {
 			return nil, nil
 		}
-		visit := jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-		if _, ok := seen[visit]; ok {
+		current := visit{typ: value.Type(), ptr: value.Pointer()}
+		if _, ok := seen[current]; ok {
 			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
 		}
-		seen[visit] = struct{}{}
-		defer delete(seen, visit)
-		return copyJSONValue(value.Elem(), path, seen)
+		seen[current] = struct{}{}
+		defer delete(seen, current)
+		return copyValue(value.Elem(), path, seen)
 	case reflect.Map:
-		return copyJSONMapValue(value, path, seen)
+		return copyMapValue(value, path, seen)
 	case reflect.Slice:
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copyJSONSequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen)
 	case reflect.Array:
-		return copyJSONSequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen)
 	default:
 		return nil, fmt.Errorf("%s: unsupported JSON value type %s", path, value.Type())
 	}
 }
 
-func copyJSONMapValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
+func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
 	if value.IsNil() {
 		return nil, nil
 	}
@@ -104,12 +111,12 @@ func copyJSONMapValue(value reflect.Value, path string, seen map[jsonVisit]struc
 		return nil, fmt.Errorf("%s: map key type %s is not supported", path, value.Type().Key())
 	}
 
-	visit := jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-	if _, ok := seen[visit]; ok {
+	current := visit{typ: value.Type(), ptr: value.Pointer()}
+	if _, ok := seen[current]; ok {
 		return nil, fmt.Errorf("%s: cyclic value is not supported", path)
 	}
-	seen[visit] = struct{}{}
-	defer delete(seen, visit)
+	seen[current] = struct{}{}
+	defer delete(seen, current)
 
 	keys := value.MapKeys()
 	sort.Slice(keys, func(i, j int) bool {
@@ -129,7 +136,7 @@ func copyJSONMapValue(value reflect.Value, path string, seen map[jsonVisit]struc
 		if name == "" {
 			return nil, fmt.Errorf("%s: map key must not be empty", path)
 		}
-		copied, err := copyJSONValue(value.MapIndex(key), path+"."+name, seen)
+		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen)
 		if err != nil {
 			return nil, err
 		}
@@ -164,22 +171,22 @@ func copyJSONMapValue(value reflect.Value, path string, seen map[jsonVisit]struc
 	return out, nil
 }
 
-func copyJSONSequenceValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
-	var visit jsonVisit
+func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+	var current visit
 	if value.Kind() == reflect.Slice {
-		visit = jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-		if _, ok := seen[visit]; ok {
+		current = visit{typ: value.Type(), ptr: value.Pointer()}
+		if _, ok := seen[current]; ok {
 			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
 		}
-		seen[visit] = struct{}{}
-		defer delete(seen, visit)
+		seen[current] = struct{}{}
+		defer delete(seen, current)
 	}
 
 	values := make([]any, value.Len())
 	preserveType := true
 	elementType := value.Type().Elem()
 	for i := 0; i < value.Len(); i++ {
-		copied, err := copyJSONValue(value.Index(i), fmt.Sprintf("%s[%d]", path, i), seen)
+		copied, err := copyValue(value.Index(i), fmt.Sprintf("%s[%d]", path, i), seen)
 		if err != nil {
 			return nil, err
 		}
diff --git a/internal/jsonvalue/jsonvalue_test.go b/internal/jsonvalue/jsonvalue_test.go
new file mode 100644
index 0000000..8ca084f
--- /dev/null
+++ b/internal/jsonvalue/jsonvalue_test.go
@@ -0,0 +1,97 @@
+package jsonvalue_test
+
+import (
+	"encoding/json"
+	"math"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+)
+
+func TestCopyMapPreservesTypesAndIsolatesMutations(t *testing.T) {
+	nested := map[string]int{"limit": 2}
+	sequence := []string{"one", "two"}
+	input := map[string]any{
+		"count":    int64(7),
+		"number":   json.Number("-1.25e+2"),
+		"nested":   nested,
+		"sequence": sequence,
+	}
+
+	copied, err := jsonvalue.CopyMap(input)
+	if err != nil {
+		t.Fatalf("copy map: %v", err)
+	}
+	nested["limit"] = 99
+	sequence[0] = "changed"
+	input["added"] = true
+
+	if got, ok := copied["count"].(int64); !ok || got != 7 {
+		t.Fatalf("integer type or value changed: %#v", copied["count"])
+	}
+	if got, ok := copied["number"].(json.Number); !ok || got != "-1.25e+2" {
+		t.Fatalf("JSON number type or value changed: %#v", copied["number"])
+	}
+	if got := copied["nested"].(map[string]int)["limit"]; got != 2 {
+		t.Fatalf("nested map was not isolated: %d", got)
+	}
+	if got := copied["sequence"].([]string)[0]; got != "one" {
+		t.Fatalf("sequence was not isolated: %q", got)
+	}
+	if _, ok := copied["added"]; ok {
+		t.Fatalf("top-level map was not isolated: %#v", copied)
+	}
+}
+
+func TestCopyMapRejectsInvalidValues(t *testing.T) {
+	cyclicMap := map[string]any{}
+	cyclicMap["self"] = cyclicMap
+	cyclicSlice := []any{nil}
+	cyclicSlice[0] = cyclicSlice
+
+	tests := []struct {
+		name  string
+		value any
+	}{
+		{name: "empty nested key", value: map[string]int{"": 1}},
+		{name: "non-string map key", value: map[int]string{1: "one"}},
+		{name: "unsupported value", value: make(chan int)},
+		{name: "cyclic map", value: cyclicMap},
+		{name: "cyclic slice", value: cyclicSlice},
+		{name: "NaN", value: math.NaN()},
+		{name: "positive infinity", value: math.Inf(1)},
+		{name: "unsafe signed integer", value: int64(1 << 53)},
+		{name: "unsafe unsigned integer", value: uint64(1 << 53)},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if _, err := jsonvalue.CopyMap(map[string]any{"value": tc.value}); err == nil {
+				t.Fatal("expected validation error")
+			}
+		})
+	}
+}
+
+func TestCopyMapValidatesJSONNumberSyntaxAndRange(t *testing.T) {
+	for _, number := range []json.Number{"0", "-1", "1.25", "-1.25e+2"} {
+		t.Run("valid "+number.String(), func(t *testing.T) {
+			got, err := jsonvalue.CopyMap(map[string]any{"value": number})
+			if err != nil {
+				t.Fatalf("copy valid JSON number: %v", err)
+			}
+			if !reflect.DeepEqual(got["value"], number) {
+				t.Fatalf("JSON number changed: got %#v want %#v", got["value"], number)
+			}
+		})
+	}
+
+	for _, number := range []json.Number{"", "01", "+1", "1.", ".1", "1e9999", "not-a-number"} {
+		t.Run("invalid "+number.String(), func(t *testing.T) {
+			if _, err := jsonvalue.CopyMap(map[string]any{"value": number}); err == nil {
+				t.Fatal("expected invalid JSON number error")
+			}
+		})
+	}
+}
diff --git a/internal/llm/openai_compatible_client.go b/internal/llm/openai_compatible_client.go
index 8f9fa61..d98ddd5 100644
--- a/internal/llm/openai_compatible_client.go
+++ b/internal/llm/openai_compatible_client.go
@@ -14,7 +14,6 @@ import (
 	"time"
 	"unicode/utf8"
 
-	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
 	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 )
@@ -263,7 +262,7 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 		if key == "" {
 			return nil, errors.New("extra_params key must not be empty")
 		}
-		if backend.IsReservedRequestField(key) {
+		if IsReservedOpenAIChatRequestField(key) {
 			return nil, fmt.Errorf("extra_params key %q collides with reserved request field", key)
 		}
 		if _, err := json.Marshal(value); err != nil {
@@ -275,6 +274,25 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 	return out, nil
 }
 
+// IsReservedOpenAIChatRequestField reports whether name is owned by the
+// standard OpenAI-compatible chat request rather than extra parameters.
+func IsReservedOpenAIChatRequestField(name string) bool {
+	switch name {
+	case "model",
+		"session_id",
+		"messages",
+		"temperature",
+		"max_tokens",
+		"top_p",
+		"service_tier",
+		"reasoning_effort",
+		"response_format":
+		return true
+	default:
+		return false
+	}
+}
+
 type openAIChatRequestMessage struct {
 	Role    string `json:"role"`
 	Content any    `json:"content"`
diff --git a/internal/usecase/runner.go b/internal/usecase/runner.go
index 89f559f..76e476a 100644
--- a/internal/usecase/runner.go
+++ b/internal/usecase/runner.go
@@ -365,7 +365,7 @@ func mergeExecutionTarget(base domain.ExecutionTarget, override domain.Execution
 	if strings.TrimSpace(override.BackendID) != "" {
 		out.BackendID = override.BackendID
 	}
-	if override.Endpoint != "" {
+	if strings.TrimSpace(override.Endpoint) != "" {
 		out.Endpoint = override.Endpoint
 	}
 	if override.Model != "" {
diff --git a/json_copy.go b/json_copy.go
deleted file mode 100644
index 35e9b83..0000000
--- a/json_copy.go
+++ /dev/null
@@ -1,218 +0,0 @@
-package promptkit
-
-import (
-	"encoding/json"
-	"fmt"
-	"math"
-	"reflect"
-	"strconv"
-)
-
-const maxSafeJSONInteger = 1<<53 - 1
-
-type jsonVisit struct {
-	typ reflect.Type
-	ptr uintptr
-}
-
-func copyPublicJSONMap(src map[string]any) (map[string]any, error) {
-	if src == nil {
-		return nil, nil
-	}
-	copied, err := copyPublicJSONValue(reflect.ValueOf(src), "extra_params", make(map[jsonVisit]struct{}))
-	if err != nil {
-		return nil, err
-	}
-	out, ok := copied.(map[string]any)
-	if !ok {
-		return nil, fmt.Errorf("extra_params: expected object")
-	}
-	return out, nil
-}
-
-func copyPublicJSONValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
-	if !value.IsValid() {
-		return nil, nil
-	}
-	if value.Kind() == reflect.Interface {
-		if value.IsNil() {
-			return nil, nil
-		}
-		return copyPublicJSONValue(value.Elem(), path, seen)
-	}
-	if !value.CanInterface() {
-		return nil, fmt.Errorf("%s: value cannot be copied", path)
-	}
-	if number, ok := value.Interface().(json.Number); ok {
-		f, err := strconv.ParseFloat(number.String(), 64)
-		if err != nil || math.IsNaN(f) || math.IsInf(f, 0) {
-			return nil, fmt.Errorf("%s: invalid JSON number", path)
-		}
-		return number, nil
-	}
-
-	switch value.Kind() {
-	case reflect.Bool, reflect.String:
-		return value.Interface(), nil
-	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-		if value.Int() < -maxSafeJSONInteger || value.Int() > maxSafeJSONInteger {
-			return nil, fmt.Errorf("%s: integer is outside the JSON-safe range", path)
-		}
-		return value.Interface(), nil
-	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
-		if value.Uint() > maxSafeJSONInteger {
-			return nil, fmt.Errorf("%s: integer is outside the JSON-safe range", path)
-		}
-		return value.Interface(), nil
-	case reflect.Float32, reflect.Float64:
-		f := value.Convert(reflect.TypeOf(float64(0))).Float()
-		if math.IsNaN(f) || math.IsInf(f, 0) {
-			return nil, fmt.Errorf("%s: floating-point value must be finite", path)
-		}
-		return value.Interface(), nil
-	case reflect.Pointer:
-		if value.IsNil() {
-			return nil, nil
-		}
-		visit := jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-		if _, ok := seen[visit]; ok {
-			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
-		}
-		seen[visit] = struct{}{}
-		defer delete(seen, visit)
-		return copyPublicJSONValue(value.Elem(), path, seen)
-	case reflect.Map:
-		return copyPublicJSONMapValue(value, path, seen)
-	case reflect.Slice:
-		if value.IsNil() {
-			return nil, nil
-		}
-		return copyPublicJSONSequenceValue(value, path, seen)
-	case reflect.Array:
-		return copyPublicJSONSequenceValue(value, path, seen)
-	default:
-		return nil, fmt.Errorf("%s: unsupported JSON value type %s", path, value.Type())
-	}
-}
-
-func copyPublicJSONMapValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
-	if value.IsNil() {
-		return nil, nil
-	}
-	if value.Type().Key().Kind() != reflect.String {
-		return nil, fmt.Errorf("%s: map key type %s is not supported", path, value.Type().Key())
-	}
-
-	visit := jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-	if _, ok := seen[visit]; ok {
-		return nil, fmt.Errorf("%s: cyclic value is not supported", path)
-	}
-	seen[visit] = struct{}{}
-	defer delete(seen, visit)
-
-	type entry struct {
-		key   reflect.Value
-		name  string
-		value any
-	}
-	entries := make([]entry, 0, value.Len())
-	preserveType := true
-	elemType := value.Type().Elem()
-	iter := value.MapRange()
-	for iter.Next() {
-		key := iter.Key()
-		name := key.String()
-		copied, err := copyPublicJSONValue(iter.Value(), path+"."+name, seen)
-		if err != nil {
-			return nil, err
-		}
-		entries = append(entries, entry{key: key, name: name, value: copied})
-		if copied == nil {
-			if !canAssignNil(elemType) {
-				preserveType = false
-			}
-			continue
-		}
-		if !reflect.TypeOf(copied).AssignableTo(elemType) {
-			preserveType = false
-		}
-	}
-
-	if preserveType {
-		out := reflect.MakeMapWithSize(value.Type(), len(entries))
-		for _, entry := range entries {
-			if entry.value == nil {
-				out.SetMapIndex(entry.key, reflect.Zero(elemType))
-				continue
-			}
-			out.SetMapIndex(entry.key, reflect.ValueOf(entry.value))
-		}
-		return out.Interface(), nil
-	}
-
-	out := make(map[string]any, len(entries))
-	for _, entry := range entries {
-		out[entry.name] = entry.value
-	}
-	return out, nil
-}
-
-func copyPublicJSONSequenceValue(value reflect.Value, path string, seen map[jsonVisit]struct{}) (any, error) {
-	var visit jsonVisit
-	if value.Kind() == reflect.Slice {
-		visit = jsonVisit{typ: value.Type(), ptr: value.Pointer()}
-		if _, ok := seen[visit]; ok {
-			return nil, fmt.Errorf("%s: cyclic value is not supported", path)
-		}
-		seen[visit] = struct{}{}
-		defer delete(seen, visit)
-	}
-
-	values := make([]any, value.Len())
-	preserveType := true
-	elemType := value.Type().Elem()
-	for i := 0; i < value.Len(); i++ {
-		copied, err := copyPublicJSONValue(value.Index(i), fmt.Sprintf("%s[%d]", path, i), seen)
-		if err != nil {
-			return nil, err
-		}
-		values[i] = copied
-		if copied == nil {
-			if !canAssignNil(elemType) {
-				preserveType = false
-			}
-			continue
-		}
-		if !reflect.TypeOf(copied).AssignableTo(elemType) {
-			preserveType = false
-		}
-	}
-
-	if preserveType {
-		out := reflect.New(value.Type()).Elem()
-		if value.Kind() == reflect.Slice {
-			out = reflect.MakeSlice(value.Type(), value.Len(), value.Len())
-		}
-		for i, copied := range values {
-			if copied == nil {
-				out.Index(i).Set(reflect.Zero(elemType))
-				continue
-			}
-			out.Index(i).Set(reflect.ValueOf(copied))
-		}
-		return out.Interface(), nil
-	}
-
-	out := make([]any, len(values))
-	copy(out, values)
-	return out, nil
-}
-
-func canAssignNil(typ reflect.Type) bool {
-	switch typ.Kind() {
-	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
-		return true
-	default:
-		return false
-	}
-}
diff --git a/profiles.go b/profiles.go
index 13cda8a..ffc9bf5 100644
--- a/profiles.go
+++ b/profiles.go
@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
 	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
 )
 
@@ -81,7 +82,7 @@ func (r *memoryProfileRepository) GetProfile(_ context.Context, id string) (*dom
 }
 
 func toDomainProfile(publicProfile Profile) (domain.ExecutionProfile, error) {
-	extraParams, err := copyPublicJSONMap(publicProfile.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(publicProfile.ExtraParams)
 	if err != nil {
 		return domain.ExecutionProfile{}, err
 	}
diff --git a/public_contract_test.go b/public_contract_test.go
index 159b969..260aaa7 100644
--- a/public_contract_test.go
+++ b/public_contract_test.go
@@ -154,6 +154,7 @@ func TestCustomBackendFlowsThroughProfilesOverridesAndInjectedClient(t *testing.
 		promptkit.WithProfiles(
 			promptkit.Profile{ID: "backend-profile", BackendID: "custom", Model: "backend-model"},
 			promptkit.Profile{ID: "profile-endpoint", BackendID: "custom", Endpoint: "http://profile.example/v1", Model: "profile-model"},
+			promptkit.Profile{ID: "blank-profile-endpoint", BackendID: "custom", Endpoint: " \t ", Model: "profile-model"},
 		),
 		promptkit.WithLLMClient(client),
 	)
@@ -188,6 +189,16 @@ func TestCustomBackendFlowsThroughProfilesOverridesAndInjectedClient(t *testing.
 		t.Fatalf("profile endpoint override changed backend identity: %+v", prepared)
 	}
 
+	prepared, err = engine.Prepare(context.Background(), promptkit.RunRequest{
+		PromptID: "prompt", ProfileID: "blank-profile-endpoint",
+	})
+	if err != nil {
+		t.Fatalf("prepare blank profile endpoint: %v", err)
+	}
+	if prepared.SelectedBackendID != "custom" || prepared.EffectiveModelParams.Endpoint != "http://backend.example/v1" {
+		t.Fatalf("blank profile endpoint did not inherit backend endpoint: %+v", prepared)
+	}
+
 	prepared, err = engine.Prepare(context.Background(), promptkit.RunRequest{
 		PromptID: "prompt",
 		Execution: &promptkit.ExecutionTargetOverride{
@@ -208,6 +219,7 @@ func TestCustomBackendSupportsFileProfileAndBothSelectionPaths(t *testing.T) {
 		promptkit.WithProfileFS(fstest.MapFS{
 			"profile.yaml": &fstest.MapFile{Data: []byte(`id: file-profile
 backend: file-backend
+endpoint: "  "
 model: file-model
 `)},
 		}, "."),
@@ -303,6 +315,7 @@ func TestBackendRegistrationRejectsInvalidAndDuplicateDefinitions(t *testing.T)
 		{name: "invalid environment", backends: []promptkit.Backend{{ID: "custom", Endpoint: "http://example.test/v1", APIKeyEnv: "BAD-NAME"}}},
 		{name: "reserved extra parameter", backends: []promptkit.Backend{{ID: "custom", Endpoint: "http://example.test/v1", ExtraParams: map[string]any{"model": "override"}}}},
 		{name: "cyclic extra parameter", backends: []promptkit.Backend{{ID: "custom", Endpoint: "http://example.test/v1", ExtraParams: cycle}}},
+		{name: "malformed JSON number", backends: []promptkit.Backend{{ID: "custom", Endpoint: "http://example.test/v1", ExtraParams: map[string]any{"value": json.Number("01")}}}},
 		{name: "duplicate consumer id", backends: []promptkit.Backend{
 			{ID: " custom ", Endpoint: "http://one.example/v1"},
 			{ID: "custom", Endpoint: "http://two.example/v1"},