{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://llm-token-heatmap.venuiti.com/schemas/activation-diff.schema.json",
  "title": "LLM Token Heatmap Activation Diff",
  "description": "On-disk payload produced by `compare_activations(trace_a, trace_b, ...)`. Carries per-step `delta` records, one per (step, layer, submodule), plus alignment metadata describing how the two source traces were matched up (`token_id` vs `position`). Consumed by the CLI `diff` subcommand and the diff-mode UI. One file == one (trace_a, trace_b) pair.",
  "type": "object",
  "required": ["schema_version", "alignment", "steps"],
  "additionalProperties": false,
  "properties": {
    "schema_version": {
      "type": "string",
      "description": "Semver of this schema. Bumped independently of the main trace and activation schemas. Consumers must hard-fail on unknown major.",
      "pattern": "^[0-9]+\\.[0-9]+\\.[0-9]+$",
      "examples": ["1.0.0"]
    },
    "alignment": {
      "$ref": "#/$defs/Alignment",
      "description": "How the two source traces were aligned, including any per-step mismatches that the comparator flagged."
    },
    "steps": {
      "type": "array",
      "description": "Per-step diff records, ordered by the aligned `step` index. Length equals the number of successfully aligned steps; unmatched steps from either side are listed in `alignment.mismatches` instead.",
      "items": { "$ref": "#/$defs/DiffStep" }
    }
  },
  "$defs": {
    "Alignment": {
      "type": "object",
      "description": "Records the alignment mode applied by `compare_activations`, the tokenizer fingerprints of the two source traces, and any positions the comparator could not align.",
      "required": [
        "mode",
        "tokenizer_a_fingerprint",
        "tokenizer_b_fingerprint",
        "mismatches"
      ],
      "additionalProperties": false,
      "properties": {
        "mode": {
          "type": "string",
          "enum": ["token_id", "position", "auto"],
          "description": "Alignment strategy used. `token_id` zips traces on step index, requiring identical token ids; `position` zips on `decoded_text_offset`, tolerating different tokenizers; `auto` picks `token_id` when the fingerprints match, else `position`."
        },
        "tokenizer_a_fingerprint": {
          "type": "string",
          "description": "`activation_metadata.tokenizer_fingerprint` of trace A. Equal to `tokenizer_b_fingerprint` whenever `mode == \"token_id\"` (or when `mode == \"auto\"` resolved to id-based alignment)."
        },
        "tokenizer_b_fingerprint": {
          "type": "string",
          "description": "`activation_metadata.tokenizer_fingerprint` of trace B."
        },
        "mismatches": {
          "type": "array",
          "description": "Steps that could not be aligned (e.g. divergent token ids under `token_id` mode, or non-overlapping decoded-text offsets under `position` mode). Empty array when the two traces aligned cleanly.",
          "items": { "$ref": "#/$defs/AlignmentMismatch" }
        }
      }
    },
    "AlignmentMismatch": {
      "type": "object",
      "description": "A single step where alignment failed. At least one of `step_a` or `step_b` is present, allowing the consumer to render the unmatched step on the correct side of the diff UI.",
      "required": ["reason"],
      "additionalProperties": false,
      "properties": {
        "step_a": {
          "type": ["integer", "null"],
          "minimum": 0,
          "description": "Zero-indexed step in trace A that could not be aligned, or null if the unmatched step is on the B side."
        },
        "step_b": {
          "type": ["integer", "null"],
          "minimum": 0,
          "description": "Zero-indexed step in trace B that could not be aligned, or null if the unmatched step is on the A side."
        },
        "reason": {
          "type": "string",
          "description": "Human-readable explanation (e.g. `\"token_id_divergence\"`, `\"offset_gap\"`, `\"trailing_steps_in_a\"`)."
        }
      }
    },
    "DiffStep": {
      "type": "object",
      "description": "Diff record for a single aligned step. Carries the cross-tokenizer alignment fields from both source traces so consumers can re-render the corresponding token on either side without revisiting the originals.",
      "required": [
        "step",
        "token_id_a",
        "token_id_b",
        "decoded_text_offset_a",
        "decoded_text_offset_b",
        "delta"
      ],
      "additionalProperties": false,
      "properties": {
        "step": {
          "type": "integer",
          "minimum": 0,
          "description": "Zero-indexed step in the aligned output."
        },
        "token_id_a": {
          "type": "integer",
          "minimum": 0,
          "description": "Selected `token_id` from trace A at this step."
        },
        "token_id_b": {
          "type": "integer",
          "minimum": 0,
          "description": "Selected `token_id` from trace B at this step. Equal to `token_id_a` under `token_id` alignment; may differ under `position` alignment."
        },
        "decoded_text_offset_a": {
          "type": "integer",
          "minimum": 0,
          "description": "Character offset of this step's decoded token in trace A's decoded text."
        },
        "decoded_text_offset_b": {
          "type": "integer",
          "minimum": 0,
          "description": "Character offset of this step's decoded token in trace B's decoded text."
        },
        "delta": {
          "type": "array",
          "description": "Per (layer, submodule) deltas. Ordering matches the producer's iteration over the captured submodules of trace A.",
          "items": { "$ref": "#/$defs/LayerDelta" }
        }
      }
    },
    "LayerDelta": {
      "type": "object",
      "description": "Difference between trace A and trace B for one (layer, submodule) at one aligned step.",
      "required": ["layer", "submodule", "l2", "cosine", "top_changed_neurons"],
      "additionalProperties": false,
      "properties": {
        "layer": {
          "type": "integer",
          "minimum": 0,
          "description": "Zero-indexed decoder layer index."
        },
        "submodule": {
          "type": "string",
          "description": "Submodule name (e.g. `resid_pre`, `mlp.down_proj`). MUST appear in both source traces' `activation_metadata.captured_submodules`."
        },
        "l2": {
          "type": "number",
          "minimum": 0,
          "description": "L2 norm of the activation difference vector `(a - b)` at this (layer, submodule)."
        },
        "cosine": {
          "type": "number",
          "minimum": -1,
          "maximum": 1,
          "description": "Cosine similarity between trace A's and trace B's activation vectors. `1.0` means identical direction; `-1.0` means opposite."
        },
        "top_changed_neurons": {
          "type": "array",
          "description": "Neurons with the largest absolute delta at this (layer, submodule), sorted by descending `|delta|`.",
          "items": { "$ref": "#/$defs/TopChangedNeuron" }
        }
      }
    },
    "TopChangedNeuron": {
      "type": "object",
      "required": ["index", "delta"],
      "additionalProperties": false,
      "properties": {
        "index": {
          "type": "integer",
          "minimum": 0,
          "description": "Zero-indexed neuron position within the layer's hidden dimension."
        },
        "delta": {
          "type": "number",
          "description": "Signed delta `(a_value - b_value)` at this neuron index."
        }
      }
    }
  }
}