> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ag-ui.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Key Changes

> What the 1.0 draft changes against the 0.x line — draft

<Warning>
  **Draft — not yet ratified.** This document is published for review. It
  describes the intended 1.0 behaviour, its wording is not final, and nothing
  here is covered by a compatibility promise until a version is frozen. Do not
  cite it as a stable reference.
</Warning>

This page lists the behavioural changes the 1.0 draft makes against the 0.x
line of the protocol, for reviewers who know 0.x and want the differences
rather than the whole document. It is informative: the linked pages are the
specification.

## Major changes

1. **The specification exists.** 0.x defined shapes; behaviour lived in the
   TypeScript client. The rules on these pages — sequencing, unknown versus
   malformed, warnings, attribution — are now normative, written in
   [BCP 14](https://datatracker.ietf.org/doc/html/bcp14) language, with the
   [schema](/spec/draft/schema) authoritative for structure and this
   document authoritative for behaviour.

2. **Runs report how they ended.** `RUN_FINISHED` carries an optional
   `outcome`: absent or success means success, and the interrupt outcome
   carries what the run is waiting for. With it come `Interrupt`, resume
   entries on the [run input](/spec/draft/basic/run-input), and the
   [interrupt–resume pattern](/spec/draft/basic/patterns/interrupt-resume).
   Every producer written before outcomes existed is already conformant.

3. **Subagents.** Delegated work is attributed with `subagentRunId` on the
   events that can carry it, optionally announced by `SUBAGENT_STARTED` and
   closed by `SUBAGENT_FINISHED` or `SUBAGENT_ERROR`, with
   [rules for ownership, nesting, parallelism and termination](/spec/draft/events/subagents).

4. **Reasoning replaces thinking.** The 0.x `THINKING_*` events are retired in
   favour of the [reasoning family](/spec/draft/events/reasoning): spans,
   streamed reasoning messages matched by `messageId`, and
   `REASONING_ENCRYPTED_VALUE` for provider artefacts a consumer stores and
   returns without reading. Retired shapes are translated at the
   [compatibility boundary](/spec/draft/basic/versioning#retired-shapes), not
   dropped.

5. **Activity events.** `ACTIVITY_SNAPSHOT` and `ACTIVITY_DELTA` carry
   [structured progress](/spec/draft/events/activity) as messages whose
   content is an object, amended by JSON Patch.

6. **Unknown versus malformed, normatively.** Unrecognised events, fields and
   union members survive translation to enforcement, which drops an unknown
   event and strips unknown members, warning as it goes; a malformed known
   value is fatal. Nothing is removed before
   the translators have had their chance, and both transports feed one
   [processing pipeline](/spec/draft/basic/processing), which also states the
   pipeline's one admitted narrowing for chunk fields.

7. **The chunked form has rules.** First chunks MUST carry what opening
   requires (`messageId` and role semantics for text, `toolCallId` and
   `toolCallName` for tool calls); later chunks MAY omit them; a continuation
   repeating an opener field with a conflicting value is fatal
   ([streaming pattern](/spec/draft/basic/patterns/streaming)).

8. **A binary wire.** The [HTTP + Protobuf binding](/spec/draft/basic/transports/http-protobuf)
   is specified — negotiated by media type, 4-byte length-prefixed frames,
   generated from the same schema, with cross-implementation parity pinned by
   a shared byte corpus.

## Minor changes

1. `tools` and `context` on the run input are optional: absent and empty mean
   the same thing ([Run input](/spec/draft/basic/run-input)).
2. Metadata merge semantics are normative: per key, last write wins, no
   recursion, per-family merge targets; the `ag-ui` key is reserved
   ([Metadata](/spec/draft/basic/metadata)).
3. Absent means absent: optional fields are omitted, never `null`
   ([The event model](/spec/draft/basic#absent-means-absent)).
4. `RUN_FINISHED` and `RUN_ERROR` MAY carry per-provider token usage.
5. Input messages MAY carry multimodal content parts (text, image, audio,
   video, document), by URL or inline data
   ([Run input](/spec/draft/basic/run-input)).
6. `TOOL_CALL_RESULT` is a message in its own right and does not reopen the
   call it answers ([Tool calls](/spec/draft/events/tool-calls)).
7. A late `RUN_ERROR` after `RUN_FINISHED` is admitted, reporting a failure
   that surfaced after success was already reported
   ([Runs and steps](/spec/draft/events/lifecycle)).
8. A consumer keeps a stream it rejects apart from a run that reports its own
   failure, and "treat the run as failed" is defined: surface it, never report
   success, with the API shape left to the implementation
   ([Runs and steps](/spec/draft/events/lifecycle#error-handling)).
9. The protocol version travels in-band: consumers declare theirs on
   `RunAgentInput.protocolVersion`, producers answer with their own on
   `RUN_STARTED.protocolVersion`, and absence identifies a pre-versioning
   peer ([Versioning](/spec/draft/basic/versioning#version-negotiation)).