Custom model instructions (write your own instructions for a specific engine or model)

What it is

Free-text instructions you write once and have Omniscio automatically add to the prompts of a specific AI engine or model — so you can steer Codex one way, Claude another, or one model differently from its siblings, without retyping the guidance into every session.

What it does

You build a list of rules. Each rule is:

• a block of instruction text (up to ~8,000 characters), and
• a set of targets — who the rule applies to.

A target is either:

• a whole engine / family — pick "Codex", "Claude", "Gemini", etc. (a checkbox per engine), which applies the text to every model of that engine; or
• a specific model id — typed into the "specific model ids" field (comma-separated, e.g. gpt-5.5-codex, claude-opus-4-8), which applies the text to just those models.

When you start a session, it receives every rule whose engine or model matches that session, concatenated together. That means one rule can cover a single model, a whole family, or several models at once — and you can layer a broad "all Codex" rule with a model-specific add-on, and both apply.

Matching uses the session's effective model — the model you picked for the session, or the engine's default when you didn't pick one — so a rule targeting a specific model still applies to a plain, no-override launch of that model.

Where to find it

Where it lives

Settings → Accounts → "Custom model instructions" (a section near the bottom of the Accounts page, above account deletion). It works on desktop and over mobile Web Access.

How it behaves

How the text reaches the model

It rides the same mechanism Omniscio already uses for all of its built-in instructions, so it reaches every engine:

• Claude-family engines (Claude, plus the Claude-compatible providers) get it as part of the real system prompt (--append-system-prompt).
• Codex (on a recent version) gets it through Codex's OWN system-prompt channel — the developerInstructions slot on each session — a real, standing system prompt layered on top of Codex's own, the closest external analog to --append-system-prompt. An older Codex (which lacks that slot) falls back to the first-message channel below, so nothing regresses.
• Other external engines (Gemini and similar) have no system-prompt slot, so it rides the invisible first-message context — the same channel that already delivers Omniscio's other instructions to those engines. It is sent silently before your first message; it does not appear in the chat transcript.

Good to know

• Off until you use it. With no rules, nothing changes — sessions behave exactly as before. An empty rule (blank text) is inert; delete a rule to remove it.
• Not applied to the Ask Omniscio helper. The built-in Ask helpers keep their own fixed persona and never absorb your model instructions.
• It's a request to a model, not a guarantee — treat the instructions as a strong preference, like any system-prompt guidance.
• The dedicated Codex / Gemini / Claude sidebar projects (__codex__ / __gemini__ / __claude__) work fully. These used to be a known gap: a session started inside one received none of Omniscio's injected first-message context — custom instructions, project docs, the AI-coaching profile, the dev-pipeline reminder — because the injection code mistook their __…__ folder name for a purely-virtual project. As of 2026-08-15 those three sentinels are excluded from that check (they resolve to real per-engine home dirs — ~/Codex, ~/Gemini, ~/Claude), so a session there now receives your custom instructions and the rest of the standing context exactly like a session in a normal project folder. (The clickable "Spawned by …" provenance note was never part of this gap: it is a system row, not injected prompt context, and has no virtual-project gate — so a session spawned into these projects always kept its parent link.)

For agents

Under the hood

• Stored as the customPromptRules setting (a list of { id, text, targets[] }).
• Injected as the custom-instructions fragment of the shared engine system-prompt bundle, which is what makes it reach every engine identically.
• Contract: engine-prompt-bundle-contract (the single source of truth for engine-injected prompt fragments and their anti-drift guard).

Related

Which engines exist and how they are chosen is on the AI providers page. The instructions Omniscio already ships to every session, which these rules sit alongside rather than replace, are described on default agent instructions.