Fixes #18226

Root cause

The asymmetry is in what Ollama forwards to llama-server. All API surfaces decode tool schemas into typed structs (api/types.go): ToolProperty has no pattern field, so a top-level string pattern is silently dropped during decoding (never compiled → 200 OK). But Items is any, so anything nested under array → items survives verbatim — including pattern values containing the ECMAScript identity escapes \/ and \-. In native chat mode, llama.cpp compiles tool schemas into the tool-call grammar and its regex dialect rejects those identity escapes → 400 Failed to initialize samplers: failed to parse grammar. This matches the issue's table exactly, including why Claude Code's Artifact tool (writes[].path pattern) breaks interactive mode against Ollama.

Fix

Unescape \/ → / and \- → - in every schema pattern value at the Ollama → llama-server boundary (llm/llama_server.go): applied to body["tools"], the chat response_format json_schema, and the completion json_schema (format schemas are raw at all depths, so escaped patterns failed there too). Semantics-preserving in every regex dialect (identity escapes carry no meaning); \\ is consumed as a unit so escaped-backslash patterns are untouched; other escapes and lookaheads pass through.

Notes kept out of scope deliberately: the top-level pattern dropping by the typed decode is a pre-existing schema-subset limitation (widening it is a separate decision), and the macOS mlxrunner xgrammar path is noted as a follow-up.

Tests

llm/llama_server_test.go:

• TestLlamaServerChatRequestNormalizesToolPatterns — both placements from the issue plus the Claude Code pattern, with plain-pattern and escaped-backslash invariants
• TestLlamaServerChatResponseFormatNormalizesPatterns
• TestUnescapePatternLiterals
• 2 new cases in the existing TestLlamaServerCompletionRequestFormat

go test ./llm/... passes, plus ./anthropic/ and ./api/; go vet ./llm/ and gofmt -l clean.