What is the biggest Interactions API breaking change?

Google is replacing the `outputs` array with a `steps` array that represents a structured timeline of each turn, and moving output formatting controls into a new polymorphic `response_format` (removing `response_mime_type`).

When do legacy Interactions API responses stop working?

Per Google's migration guide, legacy schema support is permanently removed on June 8, 2026. REST users cannot rely on `Api-Revision` to opt out after that date, and older Python/JS SDKs (1.x) will break for Interactions API calls on the same day.

What should REST API users do to migrate safely?

Google recommends opting in early with the `Api-Revision: 2026-05-20` header, updating code to consume `steps` and `response_format`, and validating against the new streaming event types before the May 26 default flip and the June 8 sunset.

Gemini Interactions API: Steps Schema, `response_format`, and a June 8, 2026 Legacy Sunset

Google published a migration guide for breaking changes coming to the Gemini v1beta Interactions API. The headline: the API is moving from a loosely structured outputs response shape to a steps timeline, consolidating output-format controls into a polymorphic response_format, and updating streaming event types. There is also a clear timeline: for REST API users the new schema becomes the default on May 26, 2026, and legacy behavior is removed on June 8, 2026.

Key Takeaways

outputs → steps. The response now uses a steps array representing a structured timeline of interaction turns.
response_mime_type removed. Output formatting moves into a polymorphic response_format object (or array).
Streaming event names changed. New SSE event types (e.g. interaction.created, step.delta) replace deprecated ones.
Deadlines are soon. REST schema defaults flip on May 26, 2026; legacy schema is removed June 8, 2026.

What’s Changing

1) Steps schema (`outputs` replaced by `steps`)

The guide describes a new steps array that acts as a timeline of interaction steps, replacing the older outputs array.

POST /interactions returns only output steps.
GET /interactions/{id} returns the full step timeline including the initial user_input step.

2) Output format configuration (`response_format`)

Instead of a single response_mime_type, the new approach uses response_format entries with a type discriminator (for example text, audio, or image) and format-specific configuration:

response_mime_type is removed.
MIME types are specified inside each response_format entry.
Image generation moves image_config out of generation_config and into the response_format entry for type: "image".

3) Streaming event types

Google lists new streaming event types and explicitly maps deprecated names to their replacements.

Deprecated	Replacement
`interaction.start`	`interaction.created`
`content.start`	`step.start`
`content.delta`	`step.delta`
`content.stop`	`step.stop`
`interaction.complete`	`interaction.completed`
`error`	`interaction.error`

Timeline (per Google’s guide)

Date	Phase	REST API users	Python/JS SDK users
May 7, 2026	Opt-in	Add `Api-Revision: 2026-05-20` to opt in	Upgrade to Python ≥2.0.0 / JS ≥2.0.0 to opt in automatically
May 26, 2026	Default flip	New schema becomes default; opt out temporarily with `Api-Revision: 2026-05-07`	Older 1.x SDKs keep returning legacy responses (until June 8)
June 8, 2026	Sunset	Legacy schema permanently removed; `Api-Revision` ignored	Python 1.x / JS 1.x SDKs break for Interactions API calls

Migration Checklist

Update your parsing logic to read from steps instead of outputs.
Update formatting config to use response_format rather than response_mime_type.
If you use streaming, update your SSE event handlers to the new event names.
REST users: opt in early with Api-Revision: 2026-05-20 to validate before May 26.

Sources

Google migration guide: https://ai.google.dev/gemini-api/docs/interactions-breaking-changes-may-2026

Key Takeaways

What’s Changing

1) Steps schema (outputs replaced by steps)

2) Output format configuration (response_format)