Implement support for OpenRouter sticky routing via a session_id variable

docs/cli.md

@@ -83,6 +83,7 @@ Notes:
 
 - `--input name=path` maps prompt input names to local file paths.
 - `--var name=value` maps template variable names to values.
+- If a prompt defines `session_id: "{{ .session_id }}"`, pass the OpenRouter sticky-routing value with `--var session_id=<value>`.
 - Both flags can be repeated.
 - Both flags also support comma-separated batches, for example:
   - `--input transcript=./t.md,glossary=./g.yml`

docs/config.md

@@ -104,6 +104,7 @@ Field reference:
 - `version` (required): prompt version.
 - `default_profile` (optional): profile ID used when request does not provide `profile_id`.
 - `description` (optional): prompt description.
+- `session_id` (optional): Go-template string for OpenRouter sticky-routing `session_id`; rendered from request vars.
 - `inputs` (optional list): expected named inputs.
 - `messages` (required list): prompt message templates.
 - `output` (required object): output contract.
@@ -150,6 +151,14 @@ messages:
 
 Use cache control on stable reusable prompt content. Dynamic per-run inputs before the cache-controlled message change the provider cache key.
 
+Example prompt-level session ID:
+
+```yaml
+session_id: "{{ .session_id }}"
+```
+
+When configured, `session_id` is rendered with the same variable context as messages. The rendered value is trimmed, omitted when empty, and rejected if longer than 256 characters. CLI callers pass the value through `--var session_id=<value>`; HTTP callers pass it through `"vars": {"session_id": "<value>"}`.
+
 `output` fields:
 
 - `format` (required): `text`, `markdown`, or `json`.
@@ -205,7 +214,7 @@ Profile rules:
 
 Current outbound request behavior:
 
-- The OpenAI-compatible client currently serializes: `model`, `messages`, `temperature`, `max_tokens`, `top_p`, `service_tier`, and optional `response_format` for `json_schema` prompts.
+- The OpenAI-compatible client currently serializes: `model`, optional `session_id`, `messages`, `temperature`, `max_tokens`, `top_p`, `service_tier`, and optional `response_format` for `json_schema` prompts.
 - Messages without `cache_control` serialize with string `content`.
 - Messages with `cache_control` serialize as a single text content-block array containing `cache_control`.
 - `reasoning_effort` and `extra_params` are parsed and carried in effective settings, but are not currently serialized into outbound chat-completions requests.

docs/integrations/openai-compatible-chat.md

@@ -26,6 +26,7 @@ Example:
 Serialized JSON fields:
 
 - `model` (required after fallback resolution)
+- `session_id` (only when the rendered prompt includes a non-empty session ID)
 - `messages` (rendered prompt messages)
 - `temperature` (only when non-zero)
 - `max_tokens` (only when non-zero)
@@ -35,6 +36,8 @@ Serialized JSON fields:
 
 `service_tier` is provider-specific. OpenRouter currently documents request values such as `flex` and `priority`; Scriptorium forwards any non-empty configured value and lets the backend validate support.
 
+`session_id` is rendered from prompt YAML using request variables and serialized as a top-level JSON request field. Scriptorium does not send an `x-session-id` header. Empty rendered session IDs are omitted, and values longer than 256 characters are rejected before the HTTP request.
+
 Messages without prompt cache control serialize with string `content`:
 
 ```json
@@ -140,7 +143,7 @@ The following fields may exist in profile/effective settings but are not current
 - `reasoning_effort`
 - `extra_params`
 
-The client also does not serialize top-level `cache_control` or `session_id`.
+The client also does not serialize top-level `cache_control`.
 
 No built-in retries, tool-calls, or multi-request payload modes are implemented in this client.
 

docs/internal/adapters.md

@@ -146,5 +146,5 @@ Behavior highlights:
 - Adapter packages do not own runner decision logic.
 - External request/response strictness is part of contract stability.
 - Prepared-render output never includes resolved API key values.
-- Outbound OpenAI-compatible request includes only currently serialized fields (`model`, `messages`, optional `temperature`, `max_tokens`, `top_p`, optional `service_tier`, optional `response_format`).
+- Outbound OpenAI-compatible request includes only currently serialized fields (`model`, optional `session_id`, `messages`, optional `temperature`, `max_tokens`, `top_p`, optional `service_tier`, optional `response_format`).
-- Outbound cache control is message-level only; no top-level cache-control/session fields are serialized.
+- Outbound cache control is message-level only; no top-level cache-control field is serialized.

docs/roadmap/cache.md

@@ -129,7 +129,6 @@ These are intentionally out of scope for the initial feature:
 - Image, tool, or non-text content blocks.
 - Provider-specific automatic prompt caching toggles.
 - Top-level OpenRouter `cache_control`.
-- Top-level OpenRouter `session_id`.
 - General-purpose serialization of `extra_params`.
 - Provider-specific validation profiles for cache-control limits.
 

docs/roadmap/implementation.md

@@ -4,7 +4,7 @@
 
 This plan implements the feature defined in `docs/roadmap/cache.md`.
 
-The implementation target is message-level Anthropic/OpenRouter-style cache control for rendered prompt messages, plus cache usage observability. Do not implement deferred roadmap items such as `content_blocks`, top-level `cache_control`, `session_id`, or general `extra_params` serialization.
+The implementation target is message-level Anthropic/OpenRouter-style cache control for rendered prompt messages, plus cache usage observability. Do not implement deferred roadmap items such as `content_blocks`, top-level `cache_control`, or general `extra_params` serialization.
 
 Follow the policy documents under `docs/policy/` while implementing:
 
@@ -182,7 +182,7 @@ Request serialization decisions:
 - Keep string content for messages without cache control.
 - Use content blocks only when `RenderedMessage.CacheControl` is non-nil.
 - Do not introduce `content_blocks` prompt syntax.
-- Do not serialize top-level `cache_control`, top-level `session_id`, or `extra_params`.
+- Do not serialize top-level `cache_control` or `extra_params`.
 - Split request and response wire message structs. The current `openAIChatMessage` is used in both request and response paths; changing request content to `any` should not make response decoding less strict than necessary.
 
 Use request wire types equivalent to:

internal/domain/domain.go

@@ -47,6 +47,11 @@ const (
 	CacheControlEphemeral CacheControlType = "ephemeral"
 )
 
+const (
+	// SessionIDMaxLength is OpenRouter's documented maximum session_id length.
+	SessionIDMaxLength = 256
+)
+
 // CacheControl describes provider cache metadata attached to prompt content.
 type CacheControl struct {
 	Type CacheControlType `yaml:"type" json:"type"`
@@ -98,6 +103,7 @@ type PreparedRun struct {
 	OutputContract       OutputContract        `json:"output_contract"`
 	StructuredOutput     *StructuredOutputSpec `json:"structured_output,omitempty"`
 	InputHashes          map[string]string     `json:"input_hashes,omitempty"`
+	SessionID            string                `json:"session_id,omitempty"`
 	RenderedPromptHash   string                `json:"rendered_prompt_hash"`
 	Messages             []RenderedMessage     `json:"messages"`
 	StartTime            time.Time             `json:"start_time,omitempty"`
@@ -128,6 +134,7 @@ type PromptDefinition struct {
 	Version        string                  `yaml:"version"`
 	DefaultProfile string                  `yaml:"default_profile"`
 	Description    string                  `yaml:"description"`
+	SessionID      string                  `yaml:"session_id" json:"session_id,omitempty"`
 	Inputs         []PromptInput           `yaml:"inputs"`
 	Templates      []PromptMessageTemplate `yaml:"templates"`
 	OutputFormat   OutputFormat            `yaml:"output_format"`
@@ -189,7 +196,8 @@ type OutputContract struct {
 
 // RenderedPrompt represents the prompt after template application.
 type RenderedPrompt struct {
-	Messages []RenderedMessage `json:"messages"`
+	SessionID string            `json:"session_id,omitempty"`
+	Messages  []RenderedMessage `json:"messages"`
 }
 
 // RenderedMessage is a single message in a rendered prompt.

internal/domain/prepared_run_test.go

@@ -102,3 +102,39 @@ func TestPreparedRunJSONIncludesMessageCacheControlOnlyWhenPresent(t *testing.T)
 		t.Fatalf("expected second message to omit cache_control, got %#v", decoded.Messages[1])
 	}
 }
+
+func TestPreparedRunJSONIncludesSessionIDOnlyWhenPresent(t *testing.T) {
+	prepared := PreparedRun{
+		PromptID:          "prompt.id",
+		SelectedProfileID: "local-fast",
+		EffectiveModelParams: ExecutionTarget{
+			Endpoint: "http://llm/v1",
+			Model:    "gpt-test",
+		},
+		SessionID:          "session-123",
+		RenderedPromptHash: "rendered-hash",
+		Messages:           []RenderedMessage{{Role: "user", Content: "Summarize this."}},
+	}
+
+	b, err := json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal(b, &decoded); err != nil {
+		t.Fatalf("unmarshal failed: %v", err)
+	}
+	if decoded["session_id"] != "session-123" {
+		t.Fatalf("expected session_id in prepared run JSON, got %#v", decoded["session_id"])
+	}
+
+	prepared.SessionID = ""
+	b, err = json.Marshal(prepared)
+	if err != nil {
+		t.Fatalf("marshal failed: %v", err)
+	}
+	if strings.Contains(string(b), "session_id") {
+		t.Fatalf("expected empty session_id to be omitted, got %s", b)
+	}
+}

internal/format/prepared_run.go

@@ -96,6 +96,9 @@ func (textPreparedRunFormatter) Format(prepared *domain.PreparedRun) ([]byte, er
 	if prepared.PromptHash != "" {
 		fmt.Fprintf(&b, "prompt_hash: %s\n", prepared.PromptHash)
 	}
+	if prepared.SessionID != "" {
+		fmt.Fprintf(&b, "session_id: %s\n", prepared.SessionID)
+	}
 	fmt.Fprintf(&b, "rendered_prompt_hash: %s\n", prepared.RenderedPromptHash)
 
 	target := prepared.EffectiveModelParams

internal/format/prepared_run_test.go

@@ -89,6 +89,19 @@ func TestTextFormatterIncludesMessageCacheControlBeforeContent(t *testing.T) {
 	}
 }
 
+func TestTextFormatterIncludesSessionIDWhenPresent(t *testing.T) {
+	prepared := samplePreparedRun()
+	prepared.SessionID = "session-123"
+
+	out, err := FormatPreparedRun(prepared, PreparedRunFormatText)
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	if !strings.Contains(string(out), "session_id: session-123\n") {
+		t.Fatalf("expected session_id in text output, got:\n%s", out)
+	}
+}
+
 func TestTextFormatterOmitsEmptyCacheControlTTL(t *testing.T) {
 	prepared := samplePreparedRun()
 	prepared.Messages = []domain.RenderedMessage{
@@ -116,6 +129,7 @@ func TestTextFormatterOmitsEmptyCacheControlTTL(t *testing.T) {
 
 func TestJSONFormatterEmitsValidJSONAndIncludesPreparedRunFields(t *testing.T) {
 	prepared := samplePreparedRun()
+	prepared.SessionID = "session-123"
 
 	out, err := FormatPreparedRun(prepared, PreparedRunFormatJSON)
 	if err != nil {
@@ -139,6 +153,9 @@ func TestJSONFormatterEmitsValidJSONAndIncludesPreparedRunFields(t *testing.T) {
 	if decoded["rendered_prompt_hash"] != "rendered-hash" {
 		t.Fatalf("expected rendered_prompt_hash in json output, got %#v", decoded["rendered_prompt_hash"])
 	}
+	if decoded["session_id"] != "session-123" {
+		t.Fatalf("expected session_id in json output, got %#v", decoded["session_id"])
+	}
 	if _, ok := decoded["effective_model_params"]; !ok {
 		t.Fatalf("expected effective_model_params in json output, got %#v", decoded)
 	}

internal/llm/openai_compatible_client.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"gitea.maximumdirect.net/eric/scriptorium/internal/defaults"
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
@@ -169,6 +170,12 @@ func openAIChatRequestFromGenerateRequest(req domain.GenerateRequest, defaultMod
 	wireReq := openAIChatRequest{
 		Model: model,
 	}
+	if sessionID := strings.TrimSpace(req.Prompt.SessionID); sessionID != "" {
+		if n := utf8.RuneCountInString(sessionID); n > domain.SessionIDMaxLength {
+			return openAIChatRequest{}, fmt.Errorf("session_id length %d exceeds maximum %d", n, domain.SessionIDMaxLength)
+		}
+		wireReq.SessionID = sessionID
+	}
 
 	wireReq.Messages = make([]openAIChatRequestMessage, 0, len(req.Prompt.Messages))
 	for _, msg := range req.Prompt.Messages {
@@ -200,6 +207,7 @@ func openAIChatRequestFromGenerateRequest(req domain.GenerateRequest, defaultMod
 
 type openAIChatRequest struct {
 	Model          string                     `json:"model"`
+	SessionID      string                     `json:"session_id,omitempty"`
 	Messages       []openAIChatRequestMessage `json:"messages"`
 	Temperature    *float64                   `json:"temperature,omitempty"`
 	MaxTokens      *int                       `json:"max_tokens,omitempty"`

internal/llm/openai_compatible_client_test.go

@@ -181,7 +181,7 @@ func TestOpenAICompatibleClientSerializesCacheControlledMessageAsContentBlock(t
 		t.Fatalf("expected no error, got %v", err)
 	}
 
-	for _, forbidden := range []string{"cache_control", "session_id", "extra_params"} {
+	for _, forbidden := range []string{"cache_control", "extra_params"} {
 		if _, exists := observedBody[forbidden]; exists {
 			t.Fatalf("expected top-level %s to be omitted, got %#v", forbidden, observedBody[forbidden])
 		}
@@ -262,6 +262,96 @@ func TestOpenAICompatibleClientOmitsEmptyCacheControlTTL(t *testing.T) {
 	}
 }
 
+func TestOpenAICompatibleClientSerializesSessionID(t *testing.T) {
+	var observedBody map[string]any
+	var observedSessionHeader string
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		observedSessionHeader = r.Header.Get("x-session-id")
+		defer r.Body.Close()
+		if err := json.NewDecoder(r.Body).Decode(&observedBody); err != nil {
+			t.Fatalf("failed to decode request body: %v", err)
+		}
+		_, _ = w.Write([]byte(`{"choices":[{"message":{"content":"ok"}}]}`))
+	}))
+	defer ts.Close()
+
+	client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{BaseURL: ts.URL + "/v1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Generate(context.Background(), domain.GenerateRequest{
+		Prompt: domain.RenderedPrompt{
+			SessionID: " session-123 ",
+			Messages:  []domain.RenderedMessage{{Role: "user", Content: "hi"}},
+		},
+		Target: domain.ExecutionTarget{Model: "model"},
+	})
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	if observedBody["session_id"] != "session-123" {
+		t.Fatalf("expected top-level session_id, got %#v", observedBody["session_id"])
+	}
+	if observedSessionHeader != "" {
+		t.Fatalf("did not expect x-session-id header, got %q", observedSessionHeader)
+	}
+}
+
+func TestOpenAICompatibleClientOmitsEmptySessionID(t *testing.T) {
+	var observedBody map[string]any
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		defer r.Body.Close()
+		if err := json.NewDecoder(r.Body).Decode(&observedBody); err != nil {
+			t.Fatalf("failed to decode request body: %v", err)
+		}
+		_, _ = w.Write([]byte(`{"choices":[{"message":{"content":"ok"}}]}`))
+	}))
+	defer ts.Close()
+
+	client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{BaseURL: ts.URL + "/v1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Generate(context.Background(), domain.GenerateRequest{
+		Prompt: domain.RenderedPrompt{
+			SessionID: "   ",
+			Messages:  []domain.RenderedMessage{{Role: "user", Content: "hi"}},
+		},
+		Target: domain.ExecutionTarget{Model: "model"},
+	})
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	if _, exists := observedBody["session_id"]; exists {
+		t.Fatalf("expected empty session_id to be omitted, got %#v", observedBody["session_id"])
+	}
+}
+
+func TestOpenAICompatibleClientRejectsTooLongSessionID(t *testing.T) {
+	client, err := NewOpenAICompatibleClient(OpenAICompatibleConfig{
+		BaseURL: "http://example.com/v1",
+		Model:   "model",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Generate(context.Background(), domain.GenerateRequest{
+		Prompt: domain.RenderedPrompt{
+			SessionID: strings.Repeat("x", domain.SessionIDMaxLength+1),
+			Messages:  []domain.RenderedMessage{{Role: "user", Content: "hi"}},
+		},
+	})
+	if err == nil {
+		t.Fatal("expected invalid request error")
+	}
+	if !errors.Is(err, ErrInvalidRequest) {
+		t.Fatalf("expected ErrInvalidRequest, got %v", err)
+	}
+}
+
 func TestOpenAICompatibleClientParsesCacheUsage(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, _ = w.Write([]byte(`{

internal/prompt/go_renderer.go

@@ -6,7 +6,9 @@ import (
 	"errors"
 	"fmt"
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
+	"strings"
 	"text/template"
+	"unicode/utf8"
 )
 
 var (
@@ -50,6 +52,11 @@ func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefini
 		},
 	}
 
+	sessionID, err := renderSessionID(definition.SessionID, funcs, vars)
+	if err != nil {
+		return nil, err
+	}
+
 	var renderedMessages []domain.RenderedMessage
 
 	for i, tmplMsg := range definition.Templates {
@@ -82,10 +89,33 @@ func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefini
 	}
 
 	return &domain.RenderedPrompt{
-		Messages: renderedMessages,
+		SessionID: sessionID,
+		Messages:  renderedMessages,
 	}, nil
 }
 
+func renderSessionID(raw string, funcs template.FuncMap, vars map[string]string) (string, error) {
+	if strings.TrimSpace(raw) == "" {
+		return "", nil
+	}
+
+	tmpl, err := template.New("session_id").Funcs(funcs).Option("missingkey=error").Parse(raw)
+	if err != nil {
+		return "", fmt.Errorf("%w: session_id: %v", ErrInvalidTemplate, err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, vars); err != nil {
+		return "", fmt.Errorf("%w: session_id: %w", ErrRenderFailure, err)
+	}
+
+	sessionID := strings.TrimSpace(buf.String())
+	if n := utf8.RuneCountInString(sessionID); n > domain.SessionIDMaxLength {
+		return "", fmt.Errorf("%w: session_id length %d exceeds maximum %d", ErrRenderFailure, n, domain.SessionIDMaxLength)
+	}
+	return sessionID, nil
+}
+
 func cloneCacheControl(in *domain.CacheControl) *domain.CacheControl {
 	if in == nil {
 		return nil

internal/prompt/renderer_test.go

@@ -3,6 +3,7 @@ package prompt
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
@@ -155,6 +156,78 @@ func TestGoRenderer_Render(t *testing.T) {
 		}
 	})
 
+	t.Run("rendering session id from vars", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "  {{ .session_id }}  ",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, map[string]string{
+			"tone":       "concise",
+			"session_id": "agent-session-123",
+		})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.SessionID != "agent-session-123" {
+			t.Fatalf("unexpected session id: %q", res.SessionID)
+		}
+	})
+
+	t.Run("empty rendered session id is omitted", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "   ",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		res, err := renderer.Render(ctx, def, inputs, vars)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if res.SessionID != "" {
+			t.Fatalf("expected empty session id, got %q", res.SessionID)
+		}
+	})
+
+	t.Run("missing session id var fails rendering", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "{{ .session_id }}",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, vars)
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+	})
+
+	t.Run("too long rendered session id fails rendering", func(t *testing.T) {
+		def := &domain.PromptDefinition{
+			SessionID: "{{ .session_id }}",
+			Inputs:    []domain.PromptInput{{Name: "transcript", Required: true}},
+			Templates: []domain.PromptMessageTemplate{
+				{Role: "system", Content: "Speak in a {{.tone}} tone."},
+			},
+		}
+
+		_, err := renderer.Render(ctx, def, inputs, map[string]string{
+			"tone":       "concise",
+			"session_id": strings.Repeat("x", domain.SessionIDMaxLength+1),
+		})
+		if !errors.Is(err, ErrRenderFailure) {
+			t.Fatalf("expected ErrRenderFailure, got %v", err)
+		}
+	})
+
 	t.Run("inserting required input artifact", func(t *testing.T) {
 		def := &domain.PromptDefinition{
 			Inputs: []domain.PromptInput{{Name: "transcript", Required: true}},

internal/promptdef/filesystem_repository.go

@@ -29,6 +29,7 @@ type promptDefinitionFile struct {
 	Version        string                   `yaml:"version"`
 	DefaultProfile *string                  `yaml:"default_profile"`
 	Description    string                   `yaml:"description"`
+	SessionID      string                   `yaml:"session_id"`
 	Inputs         []promptInputFile        `yaml:"inputs"`
 	Messages       []promptMessageFile      `yaml:"messages"`
 	Output         promptOutputContractFile `yaml:"output"`
@@ -274,6 +275,7 @@ func normalizePromptDefinition(raw *promptDefinitionFile, sourcePath string) (*d
 		Version:        version,
 		DefaultProfile: defaultProfile,
 		Description:    strings.TrimSpace(raw.Description),
+		SessionID:      strings.TrimSpace(raw.SessionID),
 		Inputs:         inputs,
 		Templates:      templates,
 		OutputFormat:   raw.Output.Format,

internal/promptdef/repository_test.go

@@ -96,6 +96,16 @@ func TestFilesystemRepository_GetPromptDefinition(t *testing.T) {
 		}
 	})
 
+	t.Run("valid session id template", func(t *testing.T) {
+		p, err := repo.GetPromptDefinition(ctx, "valid-session-id", "")
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if p.SessionID != "{{ .session_id }}" {
+			t.Fatalf("expected trimmed session_id template, got %q", p.SessionID)
+		}
+	})
+
 	t.Run("valid nested file-backed prompt resolves content file relative to nested YAML", func(t *testing.T) {
 		nestedDir := filepath.Join(tmpDir, "dnd", "recap")
 		if err := os.MkdirAll(nestedDir, 0o755); err != nil {

internal/promptdef/testdata/valid_session_id.yaml

@@ -0,0 +1,10 @@
+id: valid-session-id
+version: "1.0.0"
+session_id: "  {{ .session_id }}  "
+messages:
+  - role: user
+    content: Hello.
+output:
+  format: markdown
+  validation_mode: basic
+  repair_attempts: 0

internal/usecase/runner.go

@@ -90,7 +90,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 	}
 
 	genResp, err := r.llm.Generate(ctx, domain.GenerateRequest{
-		Prompt:           domain.RenderedPrompt{Messages: prepared.Messages},
+		Prompt:           domain.RenderedPrompt{SessionID: prepared.SessionID, Messages: prepared.Messages},
 		Target:           prepared.EffectiveModelParams,
 		StructuredOutput: prepared.StructuredOutput,
 	})
@@ -233,6 +233,7 @@ func (r *Runner) Prepare(ctx context.Context, req domain.RunRequest) (*domain.Pr
 		OutputContract:       effectiveContract,
 		StructuredOutput:     structuredOutput,
 		InputHashes:          inputHashes,
+		SessionID:            renderedPrompt.SessionID,
 		RenderedPromptHash:   hashRenderedPrompt(*renderedPrompt),
 		Messages:             renderedPrompt.Messages,
 		StartTime:            start,
@@ -424,6 +425,11 @@ func resolveOutputContract(def *domain.PromptDefinition, override *domain.Output
 
 func hashRenderedPrompt(p domain.RenderedPrompt) string {
 	var b strings.Builder
+	if p.SessionID != "" {
+		b.WriteString("session_id=")
+		b.WriteString(p.SessionID)
+		b.WriteString("\n---\n")
+	}
 	for _, msg := range p.Messages {
 		b.WriteString(msg.Role)
 		b.WriteByte('\n')

internal/usecase/runner_test.go

@@ -160,7 +160,7 @@ func TestRunnerPrepareWithExplicitProfileSelection(t *testing.T) {
 		"a://t": {Body: []byte("transcript"), Hash: hashString("transcript")},
 		"a://g": {Body: []byte("glossary"), Hash: hashString("glossary")},
 	}}
-	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{Messages: []domain.RenderedMessage{{Role: "system", Content: "sys"}, {Role: "user", Content: "usr"}}}}
+	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{SessionID: "session-123", Messages: []domain.RenderedMessage{{Role: "system", Content: "sys"}, {Role: "user", Content: "usr"}}}}
 	llmClient := &fakeLLM{forbid: true}
 
 	runner := NewRunner(promptRepo, execRepo, reader, renderer, llmClient, nil)
@@ -198,6 +198,9 @@ func TestRunnerPrepareWithExplicitProfileSelection(t *testing.T) {
 	if len(prepared.Messages) != 2 {
 		t.Fatalf("expected two messages, got %d", len(prepared.Messages))
 	}
+	if prepared.SessionID != "session-123" {
+		t.Fatalf("expected prepared session id, got %q", prepared.SessionID)
+	}
 	if llmClient.calls != 0 {
 		t.Fatalf("prepare should not call llm, calls=%d", llmClient.calls)
 	}
@@ -632,6 +635,45 @@ func TestHashRenderedPromptIncludesCacheControlWhenPresent(t *testing.T) {
 	}
 }
 
+func TestHashRenderedPromptIncludesSessionIDWhenPresent(t *testing.T) {
+	withoutSession := domain.RenderedPrompt{Messages: []domain.RenderedMessage{
+		{Role: "system", Content: "sys"},
+		{Role: "user", Content: "usr"},
+	}}
+	withSession := domain.RenderedPrompt{
+		SessionID: "session-123",
+		Messages: []domain.RenderedMessage{
+			{Role: "system", Content: "sys"},
+			{Role: "user", Content: "usr"},
+		},
+	}
+	alsoWithSession := domain.RenderedPrompt{
+		SessionID: "session-123",
+		Messages: []domain.RenderedMessage{
+			{Role: "system", Content: "sys"},
+			{Role: "user", Content: "usr"},
+		},
+	}
+	otherSession := domain.RenderedPrompt{
+		SessionID: "session-456",
+		Messages: []domain.RenderedMessage{
+			{Role: "system", Content: "sys"},
+			{Role: "user", Content: "usr"},
+		},
+	}
+
+	sessionHash := hashRenderedPrompt(withSession)
+	if sessionHash == hashRenderedPrompt(withoutSession) {
+		t.Fatal("expected session_id to change rendered prompt hash")
+	}
+	if sessionHash != hashRenderedPrompt(alsoWithSession) {
+		t.Fatal("expected identical session_id to produce stable hash")
+	}
+	if sessionHash == hashRenderedPrompt(otherSession) {
+		t.Fatal("expected session_id value changes to affect rendered prompt hash")
+	}
+}
+
 func TestRunnerRunSuccessful(t *testing.T) {
 	promptRepo := &fakePromptRepo{def: promptDef(domain.FormatMarkdown, domain.ValidationBasic, 0)}
 	execRepo := &fakeExecutionProfileRepo{profiles: map[string]*domain.ExecutionProfile{"exec": defaultExecutionProfile()}}
@@ -639,7 +681,7 @@ func TestRunnerRunSuccessful(t *testing.T) {
 		"a://t": {Body: []byte("transcript"), Hash: hashString("transcript")},
 		"a://g": {Body: []byte("glossary"), Hash: hashString("glossary")},
 	}}
-	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{Messages: []domain.RenderedMessage{{Role: "system", Content: "sys"}, {Role: "user", Content: "usr"}}}}
+	renderer := &fakeRenderer{rendered: &domain.RenderedPrompt{SessionID: "session-123", Messages: []domain.RenderedMessage{{Role: "system", Content: "sys"}, {Role: "user", Content: "usr"}}}}
 	llmClient := &fakeLLM{resp: &domain.GenerateResponse{Content: "# recap", Usage: domain.TokenUsage{TotalTokens: 7}}}
 
 	runner := NewRunner(promptRepo, execRepo, reader, renderer, llmClient, nil)
@@ -686,6 +728,9 @@ func TestRunnerRunSuccessful(t *testing.T) {
 	if llmClient.lastReq.Target.TimeoutSeconds != 90 {
 		t.Fatalf("expected timeout propagation, got %d", llmClient.lastReq.Target.TimeoutSeconds)
 	}
+	if llmClient.lastReq.Prompt.SessionID != "session-123" {
+		t.Fatalf("expected session id to be sent to llm, got %q", llmClient.lastReq.Prompt.SessionID)
+	}
 }
 
 func TestRunnerRunAndPrepareResolveSameProfileAndEffectiveSettings(t *testing.T) {