Claude Haiku 4.5 vs DeepSeek V3.1 Terminus for Persona Consistency

What Persona Consistency demands: maintaining a consistent character, holding to backstory and style across turns, and resisting prompt-injection or unauthorized persona changes (see our benchmark definition: "Maintains character and resists injection"). Primary evidence here is our persona_consistency task score: Claude Haiku 4.5 = 5, DeepSeek V3.1 Terminus = 4. Supporting signals that matter are faithfulness (sticking to source/backstory), long_context (keeping state over long histories), tool_calling (correctly sequencing agent actions without breaking persona), and structured_output (format fidelity when persona must output constrained formats). In our tests Haiku leads on faithfulness (5 vs 3), long_context is equal (5 vs 5), and tool_calling favors Haiku (5 vs 3). DeepSeek’s strength is structured_output (5 vs 4), which helps when persona behavior must conform to strict schemas. With no external benchmark provided for this task, our internal persona_consistency score and related proxies are the basis for the verdict.

Where Claude Haiku 4.5 shines (based on scores):

• Long-running roleplay customer support: Haiku’s persona_consistency 5 and faithfulness 5 keep persona details and refusal boundaries intact across 30K+ token histories (context_window 200,000).
• Agentic assistants that must follow a persona while calling tools: Haiku’s tool_calling 5 and persona_consistency 5 reduce the risk of persona drift when invoking functions.
• Safety-sensitive character responses: higher safety_calibration (2 vs 1) and faithfulness make Haiku less likely to accept harmful recharacterizations. Where DeepSeek V3.1 Terminus shines (based on scores):
• Strict JSON or schema-limited persona outputs: structured_output 5 (vs Haiku 4) makes DeepSeek better when the persona must emit exact machine-readable formats.
• Budgeted deployments: DeepSeek’s cost ($0.21 input / $0.79 output per mTok) is substantially lower than Haiku’s ($1 / $5), so it’s attractive when a 1-point persona gap is acceptable.
• Multilingual roleplay: both models score 5 on multilingual; DeepSeek remains viable for non-English persona tasks while saving cost. Concrete numeric differences referenced: persona_consistency 5 vs 4; faithfulness 5 vs 3; tool_calling 5 vs 3; structured_output 4 vs 5; context windows 200,000 vs 163,840; costs $1/$5 vs $0.21/$0.79 per mTok.

For Persona Consistency, choose Claude Haiku 4.5 if you need the strongest adherence to a character, robust resistance to prompt injection, and seamless agent/tool interactions despite higher cost. Choose DeepSeek V3.1 Terminus if strict structured outputs or lower runtime costs matter more than the final 1-point gain in persona fidelity.