Skip to main content
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 Do not cite it as a stable reference.
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 language, with the 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, and the interrupt–resume pattern. 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.
  4. Reasoning replaces thinking. The 0.x THINKING_* events are retired in favour of the reasoning family: 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, not dropped.
  5. Activity events. ACTIVITY_SNAPSHOT and ACTIVITY_DELTA carry structured progress 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, 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).
  8. A binary wire. The HTTP + Protobuf binding 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).
  2. Metadata merge semantics are normative: per key, last write wins, no recursion, per-family merge targets; the ag-ui key is reserved (Metadata).
  3. Absent means absent: optional fields are omitted, never null (The event model).
  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).
  6. TOOL_CALL_RESULT is a message in its own right and does not reopen the call it answers (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).
  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).
  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).