fix(pi): accept an array systemPrompt and keep the documentation-paragraph relocation

[iceteaSA]

Summary

Pi-compatible hosts (oh-my-pi) pass context.systemPrompt as an array of segments; buildAnthropicRequest only handled a string, so the prompt was dropped. This accepts the array form.

The array is joined with \n\n and routed through the same path the string branch uses (splitPiSystemPrompt → prependCachedPromptBlock), so Pi's documentation paragraph still moves out of top-level system[] into the first user message's cached block. An earlier shape of this change pushed each segment straight into system[], which would have reintroduced the OAuth billing rejection that relocation exists to avoid — the new test pins byte-equivalence between the array and string paths, and it fails if the array takes a separate branch.

Credit: original patch by randomvariable (CortexKit Discord, 2026-09-03), who could not open the PR themselves. Reviewed, reworked to share the relocation path, and re-authored here with Co-authored-by.

Changes

• packages/pi/src/convert.ts — context.systemPrompt: string | readonly string[]; array segments are trimmed, empties dropped (isNonEmptyText, shared with the string branch), joined, and handed to the existing relocation path. The SDK type declares string; the array form is a host divergence, noted at the branch.
• packages/pi/src/tests/convert.test.ts — array happy paths, empty/whitespace segments, empty array, and the relocation contract: arrayBody.system and arrayBody.messages equal the string path's output; the docs paragraph is not in system[].

Verification

• packages/pi: 98 pass / 0 fail · root typecheck 0 · format/biome clean.
• Red-first: the relocation test failed on the per-segment shape (docs paragraph at system[4]); reverting to that shape reddens it again.

Base: upstream/main 360b68e (v1.22.0).

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Summary by cubic

Fixes buildAnthropicRequest dropping Pi-compatible hosts' array systemPrompt values, which previously only handled strings. Array segments are now joined and routed through the same docs-paragraph relocation path as string prompts, keeping Pi's documentation paragraph out of top-level system[] to avoid OAuth billing rejection.

• Normalizes array segments by trimming and dropping empty/whitespace-only entries before joining with \n\n.
• Adds tests covering array happy paths, empty/whitespace segments, empty arrays, and byte-equivalence between array and string paths.

^{Written for commit 7da8919. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant Host as Pi Host (oh-my-pi)
    participant Convert as buildAnthropicRequest
    participant Split as splitPiSystemPrompt
    participant Cache as Cache Control Logic
    participant API as Anthropic API

    Note over Host,API: System Prompt Normalization Flow

    Host->>Convert: context.systemPrompt (string | readonly string[])

    alt Array systemPrompt
        Convert->>Convert: Filter non-empty segments (isNonEmptyText)
        Convert->>Convert: Trim each segment
        Convert->>Convert: Join with '\n\n'
    else String systemPrompt
        Convert->>Convert: Pass through as-is
    end

    Convert->>Split: normalizedSystemPrompt
    Split-->>Convert: { systemText, cachedBlock }

    alt systemText exists
        Convert->>Convert: Push text block to system[]
    end

    alt cachedBlock exists
        Convert->>Convert: Prepend cached prompt block to first user message
        Convert->>Cache: Add cache_control: { type: 'ephemeral' }
    end

    Convert->>API: Request body with system[] + messages[]
    
    Note over API: Documentation paragraph NOT in system[]<br/>(avoids OAuth billing rejection)

    alt Happy Path (valid request)
        API-->>Convert: 200 OK
    else Documentation paragraph in system[]
        API-->>Convert: 400/401 (billing rejection)
    end

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