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When you instrument an agent with the Weave SDK, each span object (Turn, LLM, Tool, and SubAgent) exposes methods to attach custom metadata. Use these methods to stamp contextual information such as user IDs, tenants, experiment names, or environment labels onto agent spans, then filter and group agent activity by that metadata in the Weave UI. This metadata comes in two forms:
  • Attributes: Key-value properties of a span as a whole. Use set_attributes() (Python) or setAttributes() (TypeScript) to stamp attributes on a single span, or set session-wide attributes that apply to every span a session emits.
  • Events: Point-in-time markers that occur during a span’s lifetime, such as a permission prompt or a lifecycle transition. Use add_event() (Python) or addEvent() (TypeScript).
These methods mirror the OpenTelemetry (OTel) span API: set_attributes mirrors OTel’s Span.set_attributes, and add_event mirrors OTel’s Span.add_event. The Weave SDK emits OTel spans and stores all attributes, so they remain queryable in Weave.

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Set attributes on a span

Use set_attributes() (Python) or setAttributes() (TypeScript) to stamp arbitrary attributes on a single span. Pass a dictionary or object whether you have one key or many. The method returns the span, so you can chain calls. 
import weave

weave.init("[TEAM-NAME]/[PROJECT-NAME]")

with weave.start_session(agent_name="my-agent"):
    with weave.start_turn(user_message="What is the weather in Tokyo?") as turn:
        # Stamp attributes on this turn span
        turn.set_attributes({"user_id": "12345", "tenant": "acme", "env": "production"})
The same methods are available on every span class. For example, you can tag an individual tool call or LLM call: 
with weave.start_turn(user_message="What is the weather in Tokyo?") as turn:
    with turn.tool(name="get_weather") as tool:
        tool.set_attributes({"weave.display_name": "Weather lookup"})
Most attribute keys are arbitrary custom metadata, but Weave reserves two prefixes for special handling. Keys under weave.* map to built-in Weave fields, and keys under gen_ai.* map to OpenTelemetry GenAI semantic-convention fields. In the preceding example, weave.display_name is a reserved key that sets the span’s display name in the Agents and Traces UI. For arbitrary metadata that you want to filter and group by, use your own keys such as user_id or tenant, which Weave stores as filterable custom attributes.

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Set attributes on every span in a session

Stamping attributes one span at a time works well for span-specific metadata, but some metadata applies to an entire session. To apply the same attributes to every span a session emits, pass attributes when you start the session. This is useful for propagating session-wide metadata such as an integration identity or a deployment environment. 
import weave

weave.init("[TEAM-NAME]/[PROJECT-NAME]")

session = weave.start_session(
    agent_name="my-agent",
    attributes={"weave.integration.name": "my-harness", "env": "production"},
)
# Every turn, LLM, tool, and sub-agent span in this session carries these attributes.
As with per-span attributes, use your own custom keys for session attributes. Set semantic-convention fields, such as the agent name, session name, or model, through their typed parameters (agent_name, session_name, model) rather than through attributes. Avoid keys under the reserved gen_ai.* and weave.* prefixes. Weave extracts those into typed fields during ingestion, so a custom value under a reserved key is unsupported.

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Record events on a span

Use add_event() (Python) or addEvent() (TypeScript) to record a marker at a specific point in time within a span’s lifetime. Unlike an attribute, which describes the span as a whole, an event captures something that happens at a moment during the span, such as a permission prompt, a context-compaction step, or a lifecycle transition like spawned, streaming, or finished. Each event takes a name and an optional dictionary or object of attributes. 
with weave.start_turn(user_message="Delete the temp files") as turn:
    # Record a marker when the agent requests permission
    turn.add_event("weave.permission_request", {"tool": "shell", "command": "rm -rf ./tmp"})

    # ... agent continues ...

    turn.add_event("finished")
In the UI, recorded events can be viewed by selecting the Spans tab, then choosing the associated span. In the span detail panel, select the Raw data tab and expand the events array in the JSON tree, as shown in the following image. The Spans tab with a span selected that has events. The detail panel on the right shows the Raw data tab with the events array expanded, listing the events recorded on the span.

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When you can set attributes and events

Set attributes and events while the span is recording, that is, after the span starts and before it ends. In Python, this is inside the with block. In TypeScript, this is after start*() and before end(). If you call set_attributes(), add_event(), or their TypeScript equivalents on a span that hasn’t started yet or has already ended, the call is a no-op and logs a warning that names the fix. The call is silent (no warning) only when OTel isn’t installed or Weave is disabled.
To attach attributes when logging completed agent activity in a single batch rather than during live execution, populate the span object’s declared fields directly and pass the object to log_turn or log_session. See Log agent activity in batches.

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View and filter attributes in the UI

Adding attributes is only the first step. Their value comes from using them to analyze agent activity. After you stamp attributes on agent spans, you can filter and group agent conversations by those attributes in the Agents tab of your Weave project. For details, see View agent activity.