diff --git a/docs/formats.md b/docs/formats.md
index 3575041..cfe4b5c 100644
--- a/docs/formats.md
+++ b/docs/formats.md
@@ -166,7 +166,7 @@ extra_params:
 | --- | --- | --- |
 | `id` | yes | Profile identifier, trimmed before selection and publication. It must be non-empty after trimming and unique within one source after normalization. |
 | `backend` | unless `endpoint` is present | Backend registry ID. It is trimmed and registry membership is checked when the profile is prepared or inspected. |
-| `endpoint` | unless `backend` is present | Non-empty OpenAI-compatible base URL, including an API version path when required. When both connection fields are present, this overrides the backend endpoint without changing backend identity. |
+| `endpoint` | unless `backend` is present | OpenAI-compatible base URL, including an API version path when required. A nonempty value is trimmed and must be absolute HTTP or HTTPS with a host and without user information, a query, or a fragment. When both connection fields are present, this overrides the backend endpoint without changing backend identity. |
 | `model` | yes | Non-empty provider model name. |
 | `temperature` | no | Number from 0 through 2. |
 | `max_tokens` | no | Integer zero or greater. |
diff --git a/docs/integrations/openai-compatible-chat.md b/docs/integrations/openai-compatible-chat.md
index 5714b8d..aa4ca75 100644
--- a/docs/integrations/openai-compatible-chat.md
+++ b/docs/integrations/openai-compatible-chat.md
@@ -14,10 +14,16 @@ that produce these outbound settings.
 
 Generation sends an HTTP `POST` with `Content-Type: application/json`.
 Before the client is called, the engine resolves framework, backend, profile,
-and request values into one execution target. A non-empty endpoint from that
-target overrides the client's configured base URL. After trailing slashes are
-removed, `/chat/completions` is appended. Generation fails before sending when
-neither source supplies an endpoint.
+and request values into one execution target. Endpoint configuration is trimmed
+and must be an absolute HTTP or HTTPS URL with a host and without user
+information, a query, or a fragment. A non-empty endpoint from the target
+overrides the client's configured base URL. The final selected endpoint is
+validated again before transport.
+
+The completion URL is composed through parsed URL path operations. Nested base
+paths are retained, repeated trailing slashes are normalized, and the result
+has exactly one appended `/chat/completions` suffix. Generation fails before
+sending when neither source supplies a valid endpoint.
 
 The target's backend ID is routing metadata for prepared values, results, and
 injected clients. The built-in client does not derive the URL from that ID and
diff --git a/docs/internal/llm.md b/docs/internal/llm.md
index 2f917f6..301f306 100644
--- a/docs/internal/llm.md
+++ b/docs/internal/llm.md
@@ -25,16 +25,19 @@ and request precedence. The client uses its endpoint, credential metadata,
 generation fields, and extra parameters. `BackendID` remains routing metadata
 for the generation boundary and is not mapped into the provider payload.
 
-Construction validates the configured base URL and clones any supplied
-`http.Client` so Promptkit can apply its timeout default without mutating the
-caller's client. Generation then:
+Construction trims and validates a nonempty configured base URL and clones any
+supplied `http.Client` so Promptkit can apply its timeout default without
+mutating the caller's client. An empty configured base remains valid because a
+resolved request target may supply the endpoint. Generation then:
 
-1. validates shared execution-setting invariants and endpoint requirements;
+1. validates shared execution-setting invariants and the final selected base
+   endpoint;
 2. maps the internal request into the OpenAI-compatible chat payload;
 3. validates and merges extra parameters;
-4. resolves authentication;
-5. performs the outbound request under the applicable deadlines; and
-6. decodes the first response choice and token usage.
+4. composes `/chat/completions` through parsed URL path operations;
+5. resolves authentication;
+6. performs the outbound request under the applicable deadlines; and
+7. 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
@@ -65,6 +68,10 @@ configuration, invalid generation requests, request execution failures,
 non-success provider statuses, and malformed successful responses. Provider
 response bodies are not included in non-success errors.
 
+Invalid nonempty configured endpoints are configuration failures. A missing or
+invalid final selected endpoint is an invalid generation request and is
+rejected before transport.
+
 An `http.Client.Do` failure is represented by a redacting multi-cause error:
 the package request-failure sentinel and the exact returned transport error are
 both available through `errors.Is` and `errors.As`, while the rendered text
@@ -80,7 +87,8 @@ The
 [OpenAI-compatible client tests](../../internal/llm/openai_compatible_client_test.go)
 own configuration, client cloning, deterministic deadline precedence,
 authentication, request and response mapping, malformed data, error identity,
-cancellation, endpoint selection, and response-body suppression. The root
+cancellation, endpoint selection and composition, pre-transport rejection, and
+response-body suppression. The root
 transport contract tests also verify that resolved backend settings reach this
 client without serializing backend identity and that ordinary-run cancellation
 retains its public generation and context identities. All use local test
diff --git a/docs/internal/overview.md b/docs/internal/overview.md
index 4c93054..f9b1997 100644
--- a/docs/internal/overview.md
+++ b/docs/internal/overview.md
@@ -16,7 +16,7 @@ contributor workflow and validation.
 | `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 through the shared JSON-value package, and consumes the LLM-owned OpenAI-compatible reserved request-field rule. | [Backend registry](../../internal/backend/registry.go) |
 | `internal/capacity` | Owns engine-local bounded execution admission and FIFO model-generation permits for limited backend IDs, including cancellation-safe waiter removal and client wrapping. | [Internal capacity management](capacity.md) |
-| `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation, and owns source-neutral invariants for shared execution settings, session identifiers, and output contracts. Source parsing, required fields, source-specific normalization and defaulting, and boundary-specific error classification remain with their callers. | [Domain declarations](../../internal/domain/domain.go) |
+| `internal/domain` | Defines internal framework values for requests, artifacts, prompt definitions, profiles, execution targets, rendering, generation, and validation, and owns source-neutral invariants for shared execution settings, OpenAI-compatible base endpoints, session identifiers, and output contracts. Source parsing, required fields, other source-specific normalization, defaulting, and boundary-specific error classification remain with their callers. | [Domain declarations](../../internal/domain/domain.go), [endpoint invariant](../../internal/domain/endpoint.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 bounded JSON-compatible extra-parameter and prepared-schema trees while preserving supported concrete value types and rejecting cycles or excessive depth and work. | [JSON values](../../internal/jsonvalue/jsonvalue.go) |
diff --git a/docs/policy/architecture.md b/docs/policy/architecture.md
index 1f2f8f2..9774d6e 100644
--- a/docs/policy/architecture.md
+++ b/docs/policy/architecture.md
@@ -94,9 +94,10 @@ on consumers or on Scriptorium.
 
 `internal/domain` owns source-neutral invariants for values shared across
 multiple input and execution boundaries, including execution-setting bounds,
-session identifiers, and output-contract legality. Callers retain source
-parsing, required-field rules, source-specific normalization, defaulting,
-error classification, and other policy specific to their own boundary.
+OpenAI-compatible base endpoints, session identifiers, and output-contract
+legality. Callers retain source parsing, required-field rules, other
+source-specific normalization, defaulting, error classification, and policy
+specific to their own boundary.
 
 ## Repository And Consumer Boundary
 
diff --git a/engine_test.go b/engine_test.go
index b2e878e..632ab79 100644
--- a/engine_test.go
+++ b/engine_test.go
@@ -1857,7 +1857,7 @@ func TestPrepareWorksWithInMemoryProfilesWithoutProfileFiles(t *testing.T) {
 		SchemaDir: frameworkSchemaDir,
 	}, promptkit.WithProfiles(promptkit.Profile{
 		ID:       "memory-profile",
-		Endpoint: "http://memory-profile/v1",
+		Endpoint: "  https://memory-profile/nested/v1  ",
 		Model:    "memory-model",
 	}))
 	if err != nil {
@@ -1878,6 +1878,9 @@ func TestPrepareWorksWithInMemoryProfilesWithoutProfileFiles(t *testing.T) {
 	if prepared.EffectiveModelParams.Model != "memory-model" {
 		t.Fatalf("expected in-memory profile model, got %q", prepared.EffectiveModelParams.Model)
 	}
+	if prepared.EffectiveModelParams.Endpoint != "https://memory-profile/nested/v1" {
+		t.Fatalf("expected normalized in-memory profile endpoint, got %q", prepared.EffectiveModelParams.Endpoint)
+	}
 }
 
 func TestInMemoryProfilesOverrideBuiltInsAndProfileSources(t *testing.T) {
@@ -1961,6 +1964,79 @@ func TestWithProfilesRejectsInvalidExecutionSettings(t *testing.T) {
 	}
 }
 
+func TestEndpointValidationMapsPublicErrorCategories(t *testing.T) {
+	t.Run("backend registration is invalid configuration", func(t *testing.T) {
+		_, err := promptkit.NewEngine(
+			promptkit.Config{PromptDir: frameworkPromptDir},
+			promptkit.WithBackend(promptkit.Backend{ID: "invalid", Endpoint: "/v1"}),
+		)
+		if !errors.Is(err, promptkit.ErrInvalidConfig) {
+			t.Fatalf("expected ErrInvalidConfig, got %v", err)
+		}
+	})
+
+	t.Run("in-memory profile is invalid configuration", func(t *testing.T) {
+		_, err := promptkit.NewEngine(
+			promptkit.Config{PromptDir: frameworkPromptDir},
+			promptkit.WithProfiles(promptkit.Profile{
+				ID:       "invalid",
+				Endpoint: "https://provider.example/v1?mode=chat",
+				Model:    "model",
+			}),
+		)
+		if !errors.Is(err, promptkit.ErrInvalidConfig) {
+			t.Fatalf("expected ErrInvalidConfig, got %v", err)
+		}
+	})
+
+	t.Run("file profile is a profile-load failure", func(t *testing.T) {
+		profileFS := fstest.MapFS{
+			"profiles/invalid.yaml": &fstest.MapFile{Data: []byte(`
+id: invalid
+endpoint: 'https://provider.example/v1#chat'
+model: model
+`)},
+		}
+		engine, err := promptkit.NewEngine(
+			promptkit.Config{PromptDir: frameworkPromptDir},
+			promptkit.WithProfileFS(profileFS, "profiles"),
+		)
+		if err != nil {
+			t.Fatalf("construct engine: %v", err)
+		}
+		_, err = engine.InspectProfile(context.Background(), "invalid")
+		if !errors.Is(err, promptkit.ErrProfileLoad) {
+			t.Fatalf("expected ErrProfileLoad, got %v", err)
+		}
+	})
+
+	t.Run("request override is an invalid request", func(t *testing.T) {
+		engine, err := promptkit.NewEngine(
+			promptkit.Config{PromptDir: frameworkPromptDir},
+			promptkit.WithProfiles(promptkit.Profile{
+				ID:       "valid",
+				Endpoint: "https://provider.example/v1",
+				Model:    "model",
+			}),
+		)
+		if err != nil {
+			t.Fatalf("construct engine: %v", err)
+		}
+		_, err = engine.Prepare(context.Background(), promptkit.RunRequest{
+			PromptID:  frameworkMarkdownSummaryPromptID,
+			ProfileID: "valid",
+			Inputs: map[string]promptkit.ArtifactRef{
+				"transcript": promptkit.Inline("Rin opens the gate."),
+				"glossary":   promptkit.Inline("gate: A guarded passage."),
+			},
+			Execution: &promptkit.ExecutionTargetOverride{Endpoint: "ftp://provider.example/v1"},
+		})
+		if !errors.Is(err, promptkit.ErrInvalidRequest) {
+			t.Fatalf("expected ErrInvalidRequest, got %v", err)
+		}
+	})
+}
+
 func TestOpenAICompatibleProfileRunsThroughNormalProfilePath(t *testing.T) {
 	fake := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}}
 	prof := promptkit.OpenAICompatibleProfile(promptkit.OpenAICompatibleProfileConfig{
diff --git a/internal/backend/registry.go b/internal/backend/registry.go
index 1382920..951a5b6 100644
--- a/internal/backend/registry.go
+++ b/internal/backend/registry.go
@@ -5,7 +5,6 @@ package backend
 import (
 	"errors"
 	"fmt"
-	"net/url"
 	"regexp"
 	"sort"
 	"strings"
@@ -109,10 +108,11 @@ func (r *Registry) CapacityPolicies() map[string]domain.BackendCapacityPolicy {
 }
 
 func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
-	definition.Endpoint = strings.TrimSpace(definition.Endpoint)
-	if err := validateEndpoint(definition.Endpoint); err != nil {
+	endpoint, err := domain.NormalizeOpenAICompatibleBaseEndpoint(definition.Endpoint)
+	if err != nil {
 		return domain.Backend{}, fmt.Errorf("backend %q endpoint: %w", definition.ID, err)
 	}
+	definition.Endpoint = endpoint
 
 	definition.APIKeyEnv = strings.TrimSpace(definition.APIKeyEnv)
 	if definition.APIKeyEnv != "" && !environmentVariableName.MatchString(definition.APIKeyEnv) {
@@ -182,31 +182,3 @@ func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 	definition.ExtraParams = extraParams
 	return definition, nil
 }
-
-func validateEndpoint(endpoint string) error {
-	if endpoint == "" {
-		return errors.New("must not be blank")
-	}
-	if strings.Contains(endpoint, "#") {
-		return errors.New("must not contain a fragment")
-	}
-
-	parsed, err := url.Parse(endpoint)
-	if err != nil {
-		return fmt.Errorf("must be a valid URL: %w", err)
-	}
-	scheme := strings.ToLower(parsed.Scheme)
-	if scheme != "http" && scheme != "https" {
-		return errors.New("must use http or https")
-	}
-	if !parsed.IsAbs() || parsed.Hostname() == "" {
-		return errors.New("must be absolute and include a host")
-	}
-	if parsed.User != nil {
-		return errors.New("must not contain user information")
-	}
-	if parsed.RawQuery != "" || parsed.ForceQuery {
-		return errors.New("must not contain a query string")
-	}
-	return nil
-}
diff --git a/internal/domain/endpoint.go b/internal/domain/endpoint.go
new file mode 100644
index 0000000..26a86ce
--- /dev/null
+++ b/internal/domain/endpoint.go
@@ -0,0 +1,39 @@
+package domain
+
+import (
+	"errors"
+	"net/url"
+	"strings"
+)
+
+// NormalizeOpenAICompatibleBaseEndpoint trims and validates a source-neutral
+// OpenAI-compatible provider base endpoint.
+func NormalizeOpenAICompatibleBaseEndpoint(endpoint string) (string, error) {
+	endpoint = strings.TrimSpace(endpoint)
+	if endpoint == "" {
+		return "", errors.New("endpoint must not be blank")
+	}
+	if strings.Contains(endpoint, "#") {
+		return "", errors.New("endpoint must not contain a fragment")
+	}
+
+	parsed, err := url.Parse(endpoint)
+	if err != nil {
+		return "", errors.New("endpoint must be a valid URL")
+	}
+	parsed.Scheme = strings.ToLower(parsed.Scheme)
+	if parsed.Scheme != "http" && parsed.Scheme != "https" {
+		return "", errors.New("endpoint must use http or https")
+	}
+	if !parsed.IsAbs() || parsed.Hostname() == "" {
+		return "", errors.New("endpoint must be absolute and include a host")
+	}
+	if parsed.User != nil {
+		return "", errors.New("endpoint must not contain user information")
+	}
+	if parsed.RawQuery != "" || parsed.ForceQuery {
+		return "", errors.New("endpoint must not contain a query string")
+	}
+
+	return parsed.String(), nil
+}
diff --git a/internal/domain/endpoint_test.go b/internal/domain/endpoint_test.go
new file mode 100644
index 0000000..cb396e5
--- /dev/null
+++ b/internal/domain/endpoint_test.go
@@ -0,0 +1,47 @@
+package domain
+
+import "testing"
+
+func TestNormalizeOpenAICompatibleBaseEndpoint(t *testing.T) {
+	tests := []struct {
+		name     string
+		endpoint string
+		want     string
+		wantErr  bool
+	}{
+		{name: "http host", endpoint: "http://provider.example", want: "http://provider.example"},
+		{name: "https nested path and whitespace", endpoint: "  HTTPS://provider.example/api/openai/v1  ", want: "https://provider.example/api/openai/v1"},
+		{name: "IPv4 host and port", endpoint: "http://127.0.0.1:8080/v1", want: "http://127.0.0.1:8080/v1"},
+		{name: "IPv6 host and port", endpoint: "https://[::1]:8443/v1", want: "https://[::1]:8443/v1"},
+		{name: "repeated trailing slashes", endpoint: "https://provider.example/v1///", want: "https://provider.example/v1///"},
+		{name: "blank", endpoint: " \t\n ", wantErr: true},
+		{name: "relative path", endpoint: "/api/v1", wantErr: true},
+		{name: "scheme relative", endpoint: "//provider.example/v1", wantErr: true},
+		{name: "missing host", endpoint: "https:///v1", wantErr: true},
+		{name: "unsupported scheme", endpoint: "ftp://provider.example/v1", wantErr: true},
+		{name: "user information", endpoint: "https://user:secret@provider.example/v1", wantErr: true},
+		{name: "query", endpoint: "https://provider.example/v1?mode=chat", wantErr: true},
+		{name: "empty query", endpoint: "https://provider.example/v1?", wantErr: true},
+		{name: "fragment", endpoint: "https://provider.example/v1#chat", wantErr: true},
+		{name: "empty fragment", endpoint: "https://provider.example/v1#", wantErr: true},
+		{name: "malformed URL", endpoint: "https://provider.example/%zz", wantErr: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := NormalizeOpenAICompatibleBaseEndpoint(tc.endpoint)
+			if tc.wantErr {
+				if err == nil {
+					t.Fatalf("expected endpoint error, got %q", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("normalize endpoint: %v", err)
+			}
+			if got != tc.want {
+				t.Fatalf("normalized endpoint = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
diff --git a/internal/llm/openai_compatible_client.go b/internal/llm/openai_compatible_client.go
index c666565..48ab417 100644
--- a/internal/llm/openai_compatible_client.go
+++ b/internal/llm/openai_compatible_client.go
@@ -51,9 +51,11 @@ type OpenAICompatibleClient struct {
 }
 
 func NewOpenAICompatibleClient(cfg OpenAICompatibleConfig) (*OpenAICompatibleClient, error) {
-	baseURL := strings.TrimSpace(cfg.BaseURL)
-	if baseURL != "" {
-		if _, err := url.ParseRequestURI(baseURL); err != nil {
+	baseURL := ""
+	if strings.TrimSpace(cfg.BaseURL) != "" {
+		var err error
+		baseURL, err = domain.NormalizeOpenAICompatibleBaseEndpoint(cfg.BaseURL)
+		if err != nil {
 			return nil, fmt.Errorf("%w: invalid base URL: %v", ErrInvalidConfig, err)
 		}
 	}
@@ -75,7 +77,7 @@ func NewOpenAICompatibleClient(cfg OpenAICompatibleConfig) (*OpenAICompatibleCli
 	}
 
 	return &OpenAICompatibleClient{
-		baseURL:      strings.TrimRight(baseURL, "/"),
+		baseURL:      baseURL,
 		defaultModel: cfg.Model,
 		httpClient:   client,
 	}, nil
@@ -86,14 +88,18 @@ func (c *OpenAICompatibleClient) Generate(ctx context.Context, req domain.Genera
 		return nil, fmt.Errorf("%w: %v", ErrInvalidRequest, err)
 	}
 
-	endpoint := strings.TrimSpace(req.Target.Endpoint)
-	if endpoint == "" {
-		endpoint = c.baseURL
+	selectedEndpoint := req.Target.Endpoint
+	if strings.TrimSpace(selectedEndpoint) == "" {
+		selectedEndpoint = c.baseURL
 	}
-	if endpoint == "" {
-		return nil, fmt.Errorf("%w: endpoint is required", ErrInvalidRequest)
+	endpoint, err := domain.NormalizeOpenAICompatibleBaseEndpoint(selectedEndpoint)
+	if err != nil {
+		return nil, fmt.Errorf("%w: invalid endpoint: %v", ErrInvalidRequest, err)
+	}
+	endpoint, err = url.JoinPath(endpoint, defaults.OpenAIChatCompletionsPath)
+	if err != nil {
+		return nil, fmt.Errorf("%w: invalid endpoint path: %v", ErrInvalidRequest, err)
 	}
-	endpoint = strings.TrimRight(endpoint, "/") + defaults.OpenAIChatCompletionsPath
 
 	wireReq, err := openAIChatRequestFromGenerateRequest(req, c.defaultModel)
 	if err != nil {
diff --git a/internal/llm/openai_compatible_client_test.go b/internal/llm/openai_compatible_client_test.go
index 0a64eb5..25f9378 100644
--- a/internal/llm/openai_compatible_client_test.go
+++ b/internal/llm/openai_compatible_client_test.go
@@ -64,14 +64,21 @@ func assertDeadlineNear(t *testing.T, deadline, before, after time.Time, duratio
 }
 
 func TestNewOpenAICompatibleClientRejectsInvalidBaseURL(t *testing.T) {
-	_, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{
-		BaseURL: "://invalid",
-	})
-	if err == nil {
-		t.Fatal("expected invalid configuration error")
-	}
-	if !errors.Is(err, ErrInvalidConfig) {
-		t.Fatalf("expected ErrInvalidConfig, got %v", err)
+	for _, endpoint := range []string{
+		"://invalid",
+		"/v1",
+		"https:///v1",
+		"ftp://provider.example/v1",
+		"https://user@provider.example/v1",
+		"https://provider.example/v1?mode=chat",
+		"https://provider.example/v1#chat",
+	} {
+		t.Run(endpoint, func(t *testing.T) {
+			_, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{BaseURL: endpoint})
+			if !errors.Is(err, ErrInvalidConfig) {
+				t.Fatalf("expected ErrInvalidConfig, got %v", err)
+			}
+		})
 	}
 }
 
@@ -1312,6 +1319,94 @@ func TestOpenAICompatibleClientAllowsEmptyConfiguredBaseURL(t *testing.T) {
 	}
 }
 
+func TestOpenAICompatibleClientComposesCompletionURL(t *testing.T) {
+	tests := []struct {
+		name    string
+		baseURL string
+		wantURL string
+	}{
+		{name: "HTTP host", baseURL: "http://provider.example", wantURL: "http://provider.example/chat/completions"},
+		{name: "HTTPS host", baseURL: "https://provider.example", wantURL: "https://provider.example/chat/completions"},
+		{name: "nested path", baseURL: "https://provider.example/api/openai/v1", wantURL: "https://provider.example/api/openai/v1/chat/completions"},
+		{name: "trailing slash", baseURL: "https://provider.example/v1/", wantURL: "https://provider.example/v1/chat/completions"},
+		{name: "repeated trailing slashes", baseURL: "  https://provider.example/api/v1///  ", wantURL: "https://provider.example/api/v1/chat/completions"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			var selectedURL string
+			client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{
+				BaseURL: tc.baseURL,
+				Model:   "m",
+				HTTPClient: &http.Client{Transport: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+					selectedURL = req.URL.String()
+					return &http.Response{
+						StatusCode: http.StatusOK,
+						Header:     make(http.Header),
+						Body:       io.NopCloser(strings.NewReader(`{"choices":[{"message":{"content":"ok"}}]}`)),
+						Request:    req,
+					}, nil
+				})},
+			})
+			if err != nil {
+				t.Fatalf("construct client: %v", err)
+			}
+
+			_, err = client.Generate(context.Background(), domain.GenerateRequest{
+				Prompt: domain.RenderedPrompt{Messages: []domain.RenderedMessage{{Role: "user", Content: "hi"}}},
+			})
+			if err != nil {
+				t.Fatalf("generate: %v", err)
+			}
+			if selectedURL != tc.wantURL {
+				t.Fatalf("selected URL = %q, want %q", selectedURL, tc.wantURL)
+			}
+		})
+	}
+}
+
+func TestOpenAICompatibleClientRejectsInvalidSelectedEndpointBeforeTransport(t *testing.T) {
+	invalidEndpoints := []string{
+		"/v1",
+		"https:///v1",
+		"ftp://provider.example/v1",
+		"https://user@provider.example/v1",
+		"https://provider.example/v1?mode=chat",
+		"https://provider.example/v1#chat",
+		"https://sensitive-endpoint.example/%zz",
+	}
+	for _, endpoint := range invalidEndpoints {
+		t.Run(endpoint, func(t *testing.T) {
+			transportCalls := 0
+			client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{
+				BaseURL: "https://configured.example/v1",
+				Model:   "m",
+				HTTPClient: &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+					transportCalls++
+					return nil, errors.New("transport must not be called")
+				})},
+			})
+			if err != nil {
+				t.Fatalf("construct client: %v", err)
+			}
+
+			_, err = client.Generate(context.Background(), domain.GenerateRequest{
+				Prompt: domain.RenderedPrompt{Messages: []domain.RenderedMessage{{Role: "user", Content: "hi"}}},
+				Target: domain.ExecutionTarget{Endpoint: endpoint},
+			})
+			if !errors.Is(err, ErrInvalidRequest) {
+				t.Fatalf("expected ErrInvalidRequest, got %v", err)
+			}
+			if strings.Contains(err.Error(), endpoint) {
+				t.Fatalf("error exposed selected endpoint %q: %v", endpoint, err)
+			}
+			if transportCalls != 0 {
+				t.Fatalf("transport calls = %d, want 0", transportCalls)
+			}
+		})
+	}
+}
+
 func TestOpenAICompatibleClientRequiresEndpointWhenUnsetEverywhere(t *testing.T) {
 	client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{
 		BaseURL: "",
diff --git a/internal/profile/filesystem_repository.go b/internal/profile/filesystem_repository.go
index bee533a..e8761d7 100644
--- a/internal/profile/filesystem_repository.go
+++ b/internal/profile/filesystem_repository.go
@@ -132,7 +132,7 @@ func loadProfile(ctx context.Context, fsys fs.FS, root string, id string) (*doma
 		if err != nil {
 			return nil, fmt.Errorf("%w: %s: %v", ErrInvalidProfile, relPath, err)
 		}
-		if err := validateProfile(prof); err != nil {
+		if err := normalizeAndValidateProfile(prof); err != nil {
 			if errors.Is(err, ErrRawAPIKeyNotAllowed) {
 				return nil, fmt.Errorf("%w: %s", err, relPath)
 			}
@@ -256,13 +256,21 @@ func requireYAMLStreamEnd(decoder *yaml.Decoder) error {
 	return errors.New("profile file must contain exactly one YAML document")
 }
 
-func validateProfile(p *domain.ExecutionProfile) error {
+func normalizeAndValidateProfile(p *domain.ExecutionProfile) error {
 	if strings.TrimSpace(p.ID) == "" {
 		return errors.New("id is required")
 	}
-	if strings.TrimSpace(p.BackendID) == "" && strings.TrimSpace(p.Endpoint) == "" {
+	p.Endpoint = strings.TrimSpace(p.Endpoint)
+	if strings.TrimSpace(p.BackendID) == "" && p.Endpoint == "" {
 		return errors.New("backend or endpoint is required")
 	}
+	if p.Endpoint != "" {
+		endpoint, err := domain.NormalizeOpenAICompatibleBaseEndpoint(p.Endpoint)
+		if err != nil {
+			return err
+		}
+		p.Endpoint = endpoint
+	}
 	if strings.TrimSpace(p.Model) == "" {
 		return errors.New("model is required")
 	}
diff --git a/internal/profile/repository_test.go b/internal/profile/repository_test.go
index 8bfb9a9..5d289c3 100644
--- a/internal/profile/repository_test.go
+++ b/internal/profile/repository_test.go
@@ -63,7 +63,7 @@ func TestFilesystemRepository_GetProfile(t *testing.T) {
 			wantErr      bool
 		}{
 			{name: "backend only", connection: "backend: '  openrouter  '", wantBackend: "openrouter"},
-			{name: "endpoint only", connection: "endpoint: http://localhost:8000/v1", wantEndpoint: "http://localhost:8000/v1"},
+			{name: "endpoint only", connection: "endpoint: '  https://localhost:8000/nested/v1  '", wantEndpoint: "https://localhost:8000/nested/v1"},
 			{name: "both", connection: "backend: openrouter\nendpoint: http://localhost:8000/v1", wantBackend: "openrouter", wantEndpoint: "http://localhost:8000/v1"},
 			{name: "neither", wantErr: true},
 			{name: "blank backend", connection: "backend: '   '", wantErr: true},
@@ -351,6 +351,43 @@ model: second
 	})
 }
 
+func TestProfileRepositoriesRejectInvalidEndpoints(t *testing.T) {
+	tests := []struct {
+		name        string
+		endpoint    string
+		withBackend bool
+	}{
+		{name: "relative", endpoint: "/v1"},
+		{name: "missing host", endpoint: "https:///v1"},
+		{name: "unsupported scheme", endpoint: "ftp://provider.example/v1"},
+		{name: "user information", endpoint: "https://user@provider.example/v1"},
+		{name: "query", endpoint: "https://provider.example/v1?mode=chat"},
+		{name: "fragment", endpoint: "https://provider.example/v1#chat"},
+		{name: "backend with invalid override", endpoint: "/v1", withBackend: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			backend := ""
+			if tc.withBackend {
+				backend = "backend: openrouter\n"
+			}
+			repo := NewFSRepository(fstest.MapFS{
+				"profiles/invalid.yaml": profileMapFile(fmt.Sprintf(
+					"id: invalid-endpoint\nmodel: model\n%sendpoint: %q\n",
+					backend,
+					tc.endpoint,
+				)),
+			}, "profiles")
+
+			_, err := repo.GetProfile(context.Background(), "invalid-endpoint")
+			if !errors.Is(err, ErrInvalidProfile) {
+				t.Fatalf("expected ErrInvalidProfile, got %v", err)
+			}
+		})
+	}
+}
+
 func TestProfileRepositoriesValidateExtraParams(t *testing.T) {
 	const validProfile = `
 id: selected-profile
diff --git a/internal/usecase/execution_settings_test.go b/internal/usecase/execution_settings_test.go
index a0bc7d7..912d046 100644
--- a/internal/usecase/execution_settings_test.go
+++ b/internal/usecase/execution_settings_test.go
@@ -31,3 +31,53 @@ func TestRunnerPrepareExecutionRejectsInvalidExecutionSettings(t *testing.T) {
 		t.Fatalf("expected ErrInvalidRequest, got %v", err)
 	}
 }
+
+func TestRunnerPrepareExecutionValidatesAndNormalizesRequestEndpoints(t *testing.T) {
+	newRunner := func() *Runner {
+		return NewRunner(
+			&fakePromptRepo{def: promptDef(domain.FormatText, domain.ValidationNone, 0)},
+			&fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}},
+			nil,
+			defaultArtifactReader(),
+			defaultRenderer(),
+			&fakeLLM{forbid: true},
+			nil,
+			nil,
+		)
+	}
+
+	invalidEndpoints := []string{
+		"/v1",
+		"https:///v1",
+		"ftp://provider.example/v1",
+		"https://user@provider.example/v1",
+		"https://provider.example/v1?mode=chat",
+		"https://provider.example/v1#chat",
+	}
+	for _, endpoint := range invalidEndpoints {
+		t.Run(endpoint, func(t *testing.T) {
+			_, err := newRunner().PrepareExecution(context.Background(), domain.RunRequest{
+				PromptID:  "p",
+				ProfileID: "exec",
+				Inputs:    singleInputRef(),
+				Execution: &domain.ExecutionTargetOverride{Endpoint: endpoint},
+			})
+			if !errors.Is(err, ErrInvalidRequest) {
+				t.Fatalf("expected ErrInvalidRequest, got %v", err)
+			}
+		})
+	}
+
+	prepared, err := newRunner().PrepareExecution(context.Background(), domain.RunRequest{
+		PromptID:  "p",
+		ProfileID: "exec",
+		Inputs:    singleInputRef(),
+		Execution: &domain.ExecutionTargetOverride{Endpoint: "  https://provider.example/nested/v1  "},
+	})
+	if err != nil {
+		t.Fatalf("prepare normalized endpoint: %v", err)
+	}
+	if got := prepared.Details().EffectiveModelParams.Endpoint; got != "https://provider.example/nested/v1" {
+		t.Fatalf("effective endpoint = %q", got)
+	}
+}
diff --git a/internal/usecase/profile_inspection.go b/internal/usecase/profile_inspection.go
index f3ddd86..3d84026 100644
--- a/internal/usecase/profile_inspection.go
+++ b/internal/usecase/profile_inspection.go
@@ -57,14 +57,19 @@ func (r *Runner) resolveProfileSelection(
 	}, nil
 }
 
-func validateResolvedExecutionTarget(target domain.ExecutionTarget) error {
-	if strings.TrimSpace(target.Endpoint) == "" {
-		return errors.New("execution endpoint is required")
+func normalizeResolvedExecutionTarget(target domain.ExecutionTarget) (domain.ExecutionTarget, error) {
+	endpoint, err := domain.NormalizeOpenAICompatibleBaseEndpoint(target.Endpoint)
+	if err != nil {
+		return domain.ExecutionTarget{}, fmt.Errorf("execution endpoint: %w", err)
 	}
+	target.Endpoint = endpoint
 	if strings.TrimSpace(target.Model) == "" {
-		return errors.New("execution model is required")
+		return domain.ExecutionTarget{}, errors.New("execution model is required")
 	}
-	return domain.ValidateExecutionTargetSettings(target)
+	if err := domain.ValidateExecutionTargetSettings(target); err != nil {
+		return domain.ExecutionTarget{}, err
+	}
+	return target, nil
 }
 
 // InspectProfile resolves one explicit profile without prompt or execution work.
@@ -87,7 +92,8 @@ func (r *Runner) InspectProfile(
 		return nil, err
 	}
 	target, _ := resolveExecutionTarget(selection.backend, selection.profile, nil)
-	if err := validateResolvedExecutionTarget(target); err != nil {
+	target, err = normalizeResolvedExecutionTarget(target)
+	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrProfileLoad, err)
 	}
 	target.APIKey = ""
diff --git a/internal/usecase/runner.go b/internal/usecase/runner.go
index c63d127..ff88113 100644
--- a/internal/usecase/runner.go
+++ b/internal/usecase/runner.go
@@ -317,7 +317,8 @@ func (r *Runner) resolvePreparation(
 
 	effectiveModel, targetPresence := resolveExecutionTarget(selection.backend, selection.profile, req.Execution)
 	effectiveModel.APIKey = req.APIKey
-	if err := validateResolvedExecutionTarget(effectiveModel); err != nil {
+	effectiveModel, err = normalizeResolvedExecutionTarget(effectiveModel)
+	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrInvalidRequest, err)
 	}
 	if err := validateAPIKey(effectiveModel.APIKeyEnv, effectiveModel.APIKey, effectiveModel.APIKeyRequired); err != nil {
diff --git a/profiles.go b/profiles.go
index 04b649c..1f7248e 100644
--- a/profiles.go
+++ b/profiles.go
@@ -100,19 +100,27 @@ func toDomainProfile(publicProfile Profile) (domain.ExecutionProfile, error) {
 		APIKeyRequired:  publicProfile.APIKeyRequired,
 		ExtraParams:     extraParams,
 	}
-	if err := validatePublicProfile(prof); err != nil {
+	if err := normalizeAndValidatePublicProfile(&prof); err != nil {
 		return domain.ExecutionProfile{}, err
 	}
 	return prof, nil
 }
 
-func validatePublicProfile(prof domain.ExecutionProfile) error {
+func normalizeAndValidatePublicProfile(prof *domain.ExecutionProfile) error {
 	if strings.TrimSpace(prof.ID) == "" {
 		return errors.New("id is required")
 	}
-	if strings.TrimSpace(prof.BackendID) == "" && strings.TrimSpace(prof.Endpoint) == "" {
+	prof.Endpoint = strings.TrimSpace(prof.Endpoint)
+	if strings.TrimSpace(prof.BackendID) == "" && prof.Endpoint == "" {
 		return errors.New("backend or endpoint is required")
 	}
+	if prof.Endpoint != "" {
+		endpoint, err := domain.NormalizeOpenAICompatibleBaseEndpoint(prof.Endpoint)
+		if err != nil {
+			return err
+		}
+		prof.Endpoint = endpoint
+	}
 	if strings.TrimSpace(prof.Model) == "" {
 		return errors.New("model is required")
 	}
diff --git a/types.go b/types.go
index f00722b..87f17e5 100644
--- a/types.go
+++ b/types.go
@@ -297,7 +297,8 @@ type ExecutionTarget struct {
 	// empty for endpoint-only profiles. It is supplied to injected LLMClient
 	// implementations as part of the effective target.
 	BackendID string `json:"backend_id,omitempty"`
-	// Endpoint is the model-provider base URL.
+	// Endpoint is the normalized absolute HTTP or HTTPS model-provider base URL.
+	// It has a host and no user information, query, or fragment.
 	Endpoint string `json:"endpoint"`
 	// Model is the provider model identifier.
 	Model string `json:"model"`
@@ -396,7 +397,9 @@ type PromptInspection struct {
 // framework deadline when no higher-precedence value is present.
 type ExecutionTargetOverride struct {
 	// Endpoint replaces the profile or backend endpoint when non-empty without
-	// changing the effective BackendID.
+	// changing the effective BackendID. Preparation trims it and requires an
+	// absolute HTTP or HTTPS URL with a host and no user information, query, or
+	// fragment.
 	Endpoint string
 	// Model replaces the profile model when non-empty.
 	Model string
@@ -457,7 +460,8 @@ type Profile struct {
 	BackendID string
 	// Endpoint is the model-provider base URL. It is required only when
 	// BackendID is blank and otherwise overrides the backend endpoint when
-	// non-blank.
+	// non-blank. WithProfiles trims it and requires an absolute HTTP or HTTPS URL
+	// with a host and no user information, query, or fragment.
 	Endpoint string
 	// Model is the required non-blank provider model identifier.
 	Model string