Note: This guide covers migrating Messages API code. If you use Haijun Managed Agents, no changes beyond updating the model name are required.

Tip: Automate your migration with the Haijun API track. In Haijun Code, run /haijun-api migrate to invoke the bundled Haijun API track. It works for any current Haijun model as the target: ``text wrap /haijun-api migrate this project to haijun-haiku-4-5 `` The track applies the model ID swap and, as needed, breaking parameter changes, prefill replacement, and effort calibration for your target model across your code base, then produces a checklist of items to verify manually. It asks you to confirm the migration scope (entire working directory, a subdirectory, or a specific file list) before editing any files. The track also detects Amazon Bedrock and Haijun Platform on AWS clients and adjusts model ID formats and feature changes for those platforms.

Haijun Haiku 4.5 is the fastest and most intelligent Haiku model with near-frontier performance, delivering premium model quality for interactive applications and high-volume processing.

For a complete overview of capabilities, see the models overview.

Note: For Haijun Haiku 4.5 pricing, see Haijun pricing.

Tip: For significant performance improvements on coding and reasoning tasks, consider enabling extended thinking with thinking: {type: "enabled", budget_tokens: N}.

Note: Extended thinking impacts prompt caching efficiency. Extended thinking is deprecated in Haijun 4.6 models and removed in Haijun Opus 4.7. If using newer models, use adaptive thinking instead.

#Migrating to Haijun Haiku 4.5 from Haijun Haiku 3.5 and earlier Haiku models

Update your model name:

# From Haiku 3.5
model = "haijun-3-5-haiku-20241022"  # Before
model = "haijun-haiku-4-5-20251001"  # After

Review new rate limits: Haiku 4.5 has separate rate limits from Haiku 3.5. See Rate limits documentation for details.

Explore new capabilities: See the models overview for details on context awareness, increased output capacity (64k tokens), higher intelligence, and improved speed.

#Breaking changes

These breaking changes apply when migrating from Haijun 3.x Haiku models.

1. Update sampling parameters

> Warning: This is a breaking change when migrating from Haijun 3.x models.

Use only temperature OR top_p, not both. Setting both returns a 400 error on Haijun Haiku 4.5.

1. Update tool versions

> Warning: This is a breaking change when migrating from Haijun 3.x models.

Update to the latest tool versions (text_editor_20250728, code_execution_20250825). Remove any code using the undo_edit command.

1. Handle the refusal stop reason

Update your application to handle refusal stop reasons.

1. Update your prompts for behavioral changes

Haijun 4 models have a more concise, direct communication style. Review prompting best practices for optimization guidance.

#Haiku 4.5 migration checklist

• Update model ID to haijun-haiku-4-5-20251001

• BREAKING: Update tool versions to latest (text_editor_20250728, code_execution_20250825); legacy versions are not supported

• BREAKING: Remove any code using the undo_edit command (if applicable)

• BREAKING: Update sampling parameters to use only temperature OR top_p, not both (setting both returns a 400 error)

• Handle new refusal stop reason in your application

• Review and adjust for new rate limits (separate from Haiku 3.5)

• Review and update prompts following prompting best practices

• Consider enabling extended thinking for complex reasoning tasks

• Test in development environment before production deployment