Add variable composition and Handlebars template rendering

docs/reference/advanced/managed-variables/configuration-reference.md

@@ -14,7 +14,8 @@
 | `json_schema` | JSON Schema for validation (optional) |
 | `description` | Human-readable description (optional) |
 | `aliases` | Alternative names that resolve to this variable (optional, for migrations) |
-| `example` | JSON-serialized example value, used as template in UI (optional) |
+| `example` | JSON-serialized example value, used as starting point when creating versions in the UI (optional) |
+| `template_inputs_schema` | JSON Schema for template `{{placeholder}}` inputs (optional, set automatically when template inputs are declared via `logfire.template_var()`) |
 
 **LabeledValue** — A label with an inline serialized value:
 

docs/reference/advanced/managed-variables/index.md

@@ -14,6 +14,7 @@ Managed variables are a way to externalize runtime configuration from your code.
 - **Observability-integrated**: Every variable resolution creates a span, and using the context manager automatically sets baggage so downstream operations are tagged with which label and version was used
 - **Versions and labels**: Create immutable version snapshots of your variable's value, and assign labels (like `production`, `staging`, `canary`) that point to specific versions
 - **Rollouts and targeting**: Control what percentage of requests receive each labeled version, and route specific users or segments based on attributes
+- **Templates and composition**: Use `{{placeholder}}` Handlebars syntax in values that get rendered with runtime inputs, and compose variables from reusable fragments via `@{other_variable}@` references (see [Templates and Composition](templates-and-composition.md))
 
 ### Versions and Labels
 
@@ -112,6 +113,46 @@ With managed variables, you can iterate safely in production:
 - **Instant rollback**: If a version is causing problems, move the label back to the previous version in seconds, with no deploy required
 - **Full history**: Every version is immutable and preserved, so you can always see exactly what was served and when
 
+## Template Variables
+
+For AI applications, variables often contain prompt templates with placeholders that get filled in at runtime. **Template variables** support this natively with Handlebars `{{placeholder}}` syntax:
+
+!!! note "Install the variables extra for templates"
+    Template rendering requires the `pydantic-handlebars` package, which is installed by the `logfire[variables]` extra on Python 3.10 and later.
+
+    ```bash
+    pip install 'logfire[variables]'
+    ```
+
+```python
+from pydantic import BaseModel
+
+import logfire
+
+logfire.configure()
+
+
+class PromptInputs(BaseModel):
+    user_name: str
+    is_premium: bool = False
+
+
+prompt = logfire.template_var(
+    'system_prompt',
+    type=str,
+    default='Hello {{user_name}}!{{#if is_premium}} Welcome back, valued member.{{/if}}',
+    inputs_type=PromptInputs,
+)
+
+with prompt.get(PromptInputs(user_name='Alice', is_premium=True)) as resolved:
+    print(resolved.value)
+    #> Hello Alice! Welcome back, valued member.
+```
+
+Variables can also reference other variables using `@{variable_name}@` syntax, allowing you to compose values from reusable fragments that can be independently updated in the UI.
+
+For full details, see [Templates and Composition](templates-and-composition.md).
+
 ## How It Works
 
 Here's the typical workflow using the `AgentConfig` example from above:
@@ -231,8 +272,10 @@ This bypasses the rollout weights and directly resolves the value from the speci
 
 ### Variable Parameters
 
-| Parameter | Description                                                             |
-|-----------|-------------------------------------------------------------------------|
-| `name` | Unique identifier for the variable                                      |
+| Parameter | Description |
+|-----------|-------------|
+| `name` | Unique identifier for the variable |
 | `type` | Expected type for validation; can be a primitive type or Pydantic model |
-| `default` | Default value when no configuration is found (can also be a function)   |
+| `default` | Default value when no configuration is found (can also be a function) |
+
+For variables with Handlebars template rendering, use `logfire.template_var()` instead, which adds an `inputs_type` parameter. See [Templates and Composition](templates-and-composition.md).

docs/reference/advanced/managed-variables/templates-and-composition.md

@@ -0,0 +1,220 @@
+# Template Variables and Composition
+
+Managed variables can contain **Handlebars templates** (`{{placeholder}}`) and **composition references** (`@{other_variable}@`), enabling dynamic values that are assembled from multiple sources and rendered with runtime inputs.
+
+This is especially useful for AI applications where prompts are built from reusable fragments and personalized with request-specific data.
+
+!!! note "Install the variables extra"
+    Template rendering requires the `pydantic-handlebars` package, which is installed by the `logfire[variables]` extra on Python 3.10 and later:
+
+    ```bash
+    pip install 'logfire[variables]'
+    ```
+
+    Without this extra, `logfire.template_var()` raises an error immediately so your application does not silently use an unrendered template.
+
+## Template Variables
+
+A **template variable** is a variable whose value contains `{{placeholder}}` expressions that are rendered with typed inputs at resolution time. Define one with `logfire.template_var()` and call `.get(inputs)` to resolve and render in one step:
+
+```python
+from pydantic import BaseModel
+
+import logfire
+
+logfire.configure()
+
+
+class PromptInputs(BaseModel):
+    user_name: str
+    is_premium: bool = False
+
+
+prompt = logfire.template_var(
+    'system_prompt',
+    type=str,
+    default='Hello {{user_name}}!{{#if is_premium}} Thank you for being a premium member.{{/if}}',
+    inputs_type=PromptInputs,
+)
+
+with prompt.get(PromptInputs(user_name='Alice', is_premium=True)) as resolved:
+    print(resolved.value)
+    #> Hello Alice! Thank you for being a premium member.
+
+with prompt.get(PromptInputs(user_name='Bob')) as resolved:
+    print(resolved.value)
+    #> Hello Bob!
+```
+
+The full resolution pipeline is:
+
+1. **Resolve** — fetch the serialized value from the provider (or use the code default)
+2. **Compose** — expand any `@{variable_name}@` references (see [Composition](#variable-composition) below)
+3. **Render** — render `{{placeholder}}` Handlebars templates using the provided inputs
+4. **Deserialize** — validate and deserialize to the variable's type
+
+`logfire.template_var()` accepts the same parameters as `logfire.var()` plus an `inputs_type` parameter — a Pydantic `BaseModel` (or any type supported by `TypeAdapter`) describing the expected template inputs. It is used for type-safe `.get(inputs)` calls and generates a `template_inputs_schema` for validation.
+
+### Handlebars Syntax
+
+Template variables use [Handlebars](https://handlebarsjs.com/) syntax, powered by the [`pydantic-handlebars`](https://github.com/pydantic/pydantic-handlebars) library. The most common patterns:
+
+| Syntax | Description |
+|--------|-------------|
+| `{{field}}` | Insert a value |
+| `{{obj.nested}}` | Dot-notation access |
+| `{{#if field}}...{{/if}}` | Conditional block |
+| `{{#unless field}}...{{/unless}}` | Inverse conditional |
+| `{{#each items}}...{{/each}}` | Iterate over a list |
+| `{{#with obj}}...{{/with}}` | Change context |
+| `{{! comment }}` | Comment (not rendered) |
+
+### Structured Template Variables
+
+Template variables work with structured types too. Only string fields containing `{{placeholders}}` are rendered — other fields pass through unchanged:
+
+```python
+from pydantic import BaseModel
+
+import logfire
+
+logfire.configure()
+
+
+class UserContext(BaseModel):
+    user_name: str
+    tier: str
+
+
+class AgentConfig(BaseModel):
+    instructions: str
+    model: str
+    temperature: float
+
+
+agent_config = logfire.template_var(
+    'agent_config',
+    type=AgentConfig,
+    default=AgentConfig(
+        instructions='You are helping {{user_name}}, a {{tier}} customer.',
+        model='openai:gpt-4o-mini',
+        temperature=0.7,
+    ),
+    inputs_type=UserContext,
+)
+
+with agent_config.get(UserContext(user_name='Alice', tier='premium')) as resolved:
+    print(resolved.value.instructions)
+    #> You are helping Alice, a premium customer.
+    print(resolved.value.model)
+    #> openai:gpt-4o-mini
+```
+
+### Template Validation
+
+When a template variable is pushed to Logfire (via `logfire.variables_push()`), the `template_inputs_schema` is synced alongside the variable's JSON schema. The system validates that all `{{field}}` references in variable values (including values reachable through composition) are compatible with the declared schema.
+
+For example, if your `inputs_type` declares `user_name: str` and `is_premium: bool`, but a version value references `{{unknown_field}}`, the validation will flag this as an error.
+
+## Variable Composition {#variable-composition}
+
+**Composition** lets a variable's value reference other variables using `@{variable_name}@` syntax. When the variable is resolved, `@{ref}@` references are expanded by looking up the referenced variable and substituting its value.
+
+This is useful for building values from reusable fragments:
+
+```python
+import logfire
+
+logfire.configure()
+
+# A reusable instruction fragment
+safety_rules = logfire.var(
+    'safety_rules',
+    type=str,
+    default='Never share personal data. Always be respectful.',
+)
+
+# A prompt that includes the safety rules via composition
+agent_prompt = logfire.var(
+    'agent_prompt',
+    type=str,
+    default='You are a helpful assistant. @{safety_rules}@',
+)
+
+with agent_prompt.get() as resolved:
+    print(resolved.value)
+    #> You are a helpful assistant. Never share personal data. Always be respectful.
+```
+
+When `safety_rules` is updated in the Logfire UI, all variables that reference `@{safety_rules}@` automatically pick up the new value — no code changes or redeployment required.
+
+### Composition Control Flow
+
+The `@{}@` syntax supports a small Handlebars-compatible subset for composing variables. It supports simple references, dotted field reads, and block helpers that branch or iterate over a top-level referenced variable:
+
+| Syntax | Description |
+|--------|-------------|
+| `@{variable_name}@` | Insert a variable's value |
+| `@{variable.field}@` | Access a nested field |
+| `@{#if variable}@...@{else}@...@{/if}@` | Conditional on whether a variable is set |
+| `@{#each items}@...@{/each}@` | Iterate over a list variable |
+
+Block helper conditions and iterables must be top-level variable names. Use `@{#if user}@...@{user.active}@...@{/if}@` rather than `@{#if user.active}@`.
+
+### Composition Tracking
+
+Every `@{ref}@` expansion is recorded in the resolution result. You can inspect which variables were composed and their values:
+
+```python skip="true"
+with agent_prompt.get() as resolved:
+    for ref in resolved.composed_from:
+        print(f"  {ref.name}: version={ref.version}, label={ref.label}")
+```
+
+These composition details are also recorded as span attributes, so you can see the full composition chain in your Logfire traces.
+
+### Combining Templates and Composition
+
+Template variables and composition work together. A common pattern is to compose reusable fragments via `@{ref}@` and render runtime inputs via `{{}}`:
+
+```python skip="true"
+from pydantic import BaseModel
+
+import logfire
+
+logfire.configure()
+
+
+class ChatInputs(BaseModel):
+    user_name: str
+    language: str
+
+
+# Reusable fragment (no template inputs)
+tone_instructions = logfire.var(
+    'tone_instructions',
+    type=str,
+    default='Be friendly and concise.',
+)
+
+# Template variable that composes the fragment and renders inputs
+chat_prompt = logfire.template_var(
+    'chat_prompt',
+    type=str,
+    default='You are helping {{user_name}}. Respond in {{language}}. @{tone_instructions}@',
+    inputs_type=ChatInputs,
+)
+
+# Resolution: compose @{tone_instructions}@ first, then render {{user_name}} and {{language}}
+with chat_prompt.get(ChatInputs(user_name='Alice', language='French')) as resolved:
+    print(resolved.value)
+    # "You are helping Alice. Respond in French. Be friendly and concise."
+```
+
+### Cycle Detection
+
+The system detects circular references during validation. If variable A references `@{B}@` and variable B references `@{A}@`, `logfire.variables_validate()` reports the cycle, and `logfire.variables_push(strict=True)` fails instead of applying the invalid configuration. This prevents infinite loops during resolution.
+
+## Requirements
+
+`pydantic-handlebars` requires Python 3.10 or later. On Python 3.9, basic variable features (`logfire.var()` without templates or composition) still work, but template rendering is not available.