diff --git a/src/lib/data/codecErrors.ts b/src/lib/data/codecErrors.ts
new file mode 100644
index 00000000..928bdb9f
--- /dev/null
+++ b/src/lib/data/codecErrors.ts
@@ -0,0 +1,126 @@
+import * as zarr from "zarrita";
+
+/**
+ * A dataset whose chunks use a codec gridlook has no decoder for behaves
+ * exactly like a dataset that is merely slow or unreachable: the repository
+ * opens, every variable is listed, and the failure only arrives on the first
+ * chunk read. zarrita raises a precise error at that point — it names the
+ * codec — but the message reaches the reader under a "Could not fetch data"
+ * heading that points at the network, which is the wrong place to look.
+ *
+ * This module classifies those errors and rewrites them into something that
+ * says which codec is missing. It changes no behaviour and adds no data path:
+ * an unsupported dataset still fails, it just says why. Naming the codec is
+ * also what makes the gap reportable — an unsupported codec is invisible until
+ * someone runs into a dataset that uses it.
+ *
+ * Errors are matched both structurally and by message text, because the grid
+ * data worker flattens errors to a plain string before posting them back to
+ * the main thread (see `src/lib/grids/gridData.worker.ts`), so by the time one
+ * reaches the UI its class and fields are gone.
+ */
+
+export type TCodecErrorExplanation = {
+  heading: string;
+  detail: string;
+};
+
+const UNKNOWN_CODEC = /^Unknown codec:\s*(\S+)/;
+const CODEC_PIPELINE = /^Failed to (?:de|en)code chunk via codec "([^"]+)"/;
+const UNSUPPORTED_DATA_TYPE = /^Unknown or unsupported dataType:\s*(\S+)/;
+
+/** The `: reason` the worker appends when flattening a wrapped error. */
+const FLATTENED_CAUSE = /(?:^|\s)—\s(.+)$/;
+
+function messageOf(error: unknown) {
+  if (error instanceof Error) {
+    return error.message;
+  }
+  return typeof error === "string" ? error : "";
+}
+
+function causeMessageOf(error: unknown) {
+  if (error instanceof Error && error.cause instanceof Error) {
+    return error.cause.message;
+  }
+  // Across the worker boundary the cause survives only as flattened text.
+  return messageOf(error).match(FLATTENED_CAUSE)?.[1];
+}
+
+function unknownCodecName(error: unknown) {
+  if (zarr.isZarritaError(error, "UnknownCodecError")) {
+    return error.codec;
+  }
+  return messageOf(error).match(UNKNOWN_CODEC)?.[1];
+}
+
+function failingCodecName(error: unknown) {
+  if (zarr.isZarritaError(error, "CodecPipelineError")) {
+    return error.codec;
+  }
+  return messageOf(error).match(CODEC_PIPELINE)?.[1];
+}
+
+function unsupportedDataType(error: unknown) {
+  return messageOf(error).match(UNSUPPORTED_DATA_TYPE)?.[1];
+}
+
+function explainUnknownCodec(codec: string): TCodecErrorExplanation {
+  return {
+    heading: `Unsupported codec: ${codec}`,
+    detail:
+      `This dataset's chunks are compressed with "${codec}", which gridlook ` +
+      `has no decoder for. The data and the connection are fine — only the ` +
+      `codec is missing. Quote that codec name when reporting it.`,
+  };
+}
+
+function explainCodecFailure(
+  codec: string,
+  cause: string | undefined
+): TCodecErrorExplanation {
+  return {
+    heading: `Codec failed: ${codec}`,
+    detail: cause
+      ? `Decoding a chunk with "${codec}" failed: ${cause}`
+      : `Decoding a chunk with "${codec}" failed. The codec is available but ` +
+        `rejected this dataset's chunks.`,
+  };
+}
+
+function explainUnsupportedDataType(dataType: string): TCodecErrorExplanation {
+  const isFloat16 = dataType.startsWith("float16");
+  return {
+    heading: `Unsupported data type: ${dataType}`,
+    detail: isFloat16
+      ? `This variable is stored as ${dataType}, which this browser cannot ` +
+        `represent. Chrome 135, Firefox 129 or Safari 26 and newer support it.`
+      : `This variable is stored as ${dataType}, which gridlook cannot read.`,
+  };
+}
+
+/**
+ * Recognise a codec or data-type failure and describe it in the reader's
+ * terms. Returns `undefined` for everything else, so callers fall back to
+ * whatever they showed before.
+ */
+export function explainCodecError(
+  error: unknown
+): TCodecErrorExplanation | undefined {
+  const missingCodec = unknownCodecName(error);
+  if (missingCodec) {
+    return explainUnknownCodec(missingCodec);
+  }
+
+  const brokenCodec = failingCodecName(error);
+  if (brokenCodec) {
+    return explainCodecFailure(brokenCodec, causeMessageOf(error));
+  }
+
+  const dataType = unsupportedDataType(error);
+  if (dataType) {
+    return explainUnsupportedDataType(dataType);
+  }
+
+  return undefined;
+}
diff --git a/src/lib/grids/gridData.worker.ts b/src/lib/grids/gridData.worker.ts
index af94fb7d..964dff9b 100644
--- a/src/lib/grids/gridData.worker.ts
+++ b/src/lib/grids/gridData.worker.ts
@@ -6,6 +6,7 @@ import {
   type TGridDataWorkerRequest,
   type TGridDataWorkerResponse,
 } from "@/lib/grids/gridDataWorkerProtocol.ts";
+import { flattenErrorMessage } from "@/utils/errorHandling.ts";
 
 const workerScope = self as unknown as DedicatedWorkerGlobalScope;
 
@@ -35,7 +36,7 @@ workerScope.onmessage = async (event: MessageEvent<TGridDataWorkerRequest>) => {
     const response: TGridDataWorkerResponse = {
       requestId,
       type: GridDataWorkerMessageType.ERROR,
-      message: error instanceof Error ? error.message : String(error),
+      message: flattenErrorMessage(error),
     };
     workerScope.postMessage(response);
   }
diff --git a/src/ui/common/useLog.ts b/src/ui/common/useLog.ts
index db616910..4e37544d 100644
--- a/src/ui/common/useLog.ts
+++ b/src/ui/common/useLog.ts
@@ -1,17 +1,26 @@
 import { ToastType, useToast } from "./useToast.ts";
 
+import { explainCodecError } from "@/lib/data/codecErrors.ts";
 import { getErrorMessage, toNormalizedError } from "@/utils/errorHandling.ts";
 
+// A codec or data-type failure is worth reading and worth writing down, so it
+// stays up longer than a transient error.
+const EXPLAINED_ERROR_DURATION = 12000;
+
 export function useLog() {
   const { addToast } = useToast();
 
   function logError(maybeError: unknown, context?: string) {
     const error = toNormalizedError(maybeError);
     console.error(context, error, error?.stack);
-    const prefix = context ?? "Error";
+    // A recognised codec problem describes itself better than the call site
+    // can: "Could not fetch data" sends the reader to the network, which is
+    // not where the problem is.
+    const explanation = explainCodecError(maybeError);
+    const prefix = explanation?.heading ?? context ?? "Error";
     addToast(prefix, {
-      detail: `${getErrorMessage(error)}`,
-      duration: 4000,
+      detail: explanation?.detail ?? `${getErrorMessage(error)}`,
+      duration: explanation ? EXPLAINED_ERROR_DURATION : 4000,
       type: ToastType.DANGER,
     });
   }
diff --git a/src/utils/errorHandling.ts b/src/utils/errorHandling.ts
index 852b4ff6..ac7afac1 100644
--- a/src/utils/errorHandling.ts
+++ b/src/utils/errorHandling.ts
@@ -37,3 +37,20 @@ export function getErrorMessage(error: unknown) {
   // strip quotation marks added by JSON.stringify
   return errorMessage.replace(/^"(.*)"$/, "$1");
 }
+
+/**
+ * Flatten an error to a string that survives `postMessage`.
+ *
+ * zarrita reports a codec that threw as a `CodecPipelineError` naming the
+ * codec, with the reason it threw — a checksum mismatch, a truncated chunk —
+ * on `cause`. Structured-cloning an Error keeps neither the subclass nor the
+ * cause, so a worker that posts back only `error.message` drops the reason
+ * before anyone can read it. Appending the cause keeps both halves.
+ */
+export function flattenErrorMessage(error: unknown): string {
+  if (!(error instanceof Error)) {
+    return String(error);
+  }
+  const cause = error.cause instanceof Error ? error.cause.message : undefined;
+  return cause ? `${error.message} — ${cause}` : error.message;
+}
diff --git a/tests/unit/lib/data/codecErrors.test.ts b/tests/unit/lib/data/codecErrors.test.ts
new file mode 100644
index 00000000..7fa055f0
--- /dev/null
+++ b/tests/unit/lib/data/codecErrors.test.ts
@@ -0,0 +1,153 @@
+import { describe, expect, it } from "vitest";
+import { get, open } from "zarrita";
+
+import "@/lib/data/codecs.ts";
+import { explainCodecError } from "@/lib/data/codecErrors.ts";
+import { flattenErrorMessage } from "@/utils/errorHandling.ts";
+
+/**
+ * Every error fed to `explainCodecError` here is one zarrita actually threw,
+ * not a hand-written stand-in, so the patterns stay tied to the real messages.
+ *
+ * Each case is checked twice: once as thrown, and once after the round trip
+ * through `flattenErrorMessage` plus `new Error(...)` that the grid data
+ * worker performs, which strips the error's class and its fields.
+ */
+
+const V2MetadataKey = {
+  FILL_VALUE: "fill_value",
+  ZARR_FORMAT: "zarr_format",
+} as const;
+
+function base64Bytes(encoded: string) {
+  return Uint8Array.from(atob(encoded), (character) => character.charCodeAt(0));
+}
+
+/** What `gridData.worker.ts` posts back and `gridDataWorkerClient.ts` rebuilds. */
+function throughWorker(error: unknown) {
+  return new Error(flattenErrorMessage(error));
+}
+
+function v2Store(dtype: string, compressor: unknown, chunk: Uint8Array) {
+  const metadata = new TextEncoder().encode(
+    JSON.stringify({
+      chunks: [4],
+      compressor,
+      dtype,
+      filters: [],
+      order: "C",
+      shape: [4],
+      [V2MetadataKey.FILL_VALUE]: null,
+      [V2MetadataKey.ZARR_FORMAT]: 2,
+    })
+  );
+  return new Map([
+    ["/.zarray", metadata],
+    ["/0", chunk],
+  ]);
+}
+
+async function readV2(compressor: unknown, chunk: Uint8Array) {
+  const array = await open.v2(v2Store("<f4", compressor, chunk), {
+    kind: "array",
+  });
+  return await get(array);
+}
+
+function caught(promise: Promise<unknown>) {
+  return promise.then(
+    () => undefined,
+    (error: unknown) => error
+  );
+}
+
+describe("an unregistered codec", () => {
+  async function unknownCodecError() {
+    return await caught(readV2({ id: "quantize" }, new Uint8Array(16)));
+  }
+
+  it("names the codec in the heading", async () => {
+    const explanation = explainCodecError(await unknownCodecError());
+    expect(explanation?.heading).toBe("Unsupported codec: numcodecs.quantize");
+  });
+
+  it("says the network is not the problem", async () => {
+    const explanation = explainCodecError(await unknownCodecError());
+    expect(explanation?.detail).toContain("numcodecs.quantize");
+    expect(explanation?.detail).toContain("connection are fine");
+  });
+
+  it("is still recognised after the worker flattens it", async () => {
+    const explanation = explainCodecError(
+      throughWorker(await unknownCodecError())
+    );
+    expect(explanation?.heading).toBe("Unsupported codec: numcodecs.quantize");
+  });
+});
+
+describe("a registered codec that rejects the chunk", () => {
+  // A fletcher32 chunk whose trailing checksum does not match its payload.
+  const CORRUPT_FLETCHER32 = "AACAPwAAAEAAAEBAAACAQAAAAAA=";
+
+  async function codecPipelineError() {
+    return await caught(
+      readV2({ id: "fletcher32" }, base64Bytes(CORRUPT_FLETCHER32))
+    );
+  }
+
+  it("names the codec that failed", async () => {
+    const explanation = explainCodecError(await codecPipelineError());
+    expect(explanation?.heading).toBe("Codec failed: numcodecs.fletcher32");
+  });
+
+  it("keeps the reason zarrita put on the cause", async () => {
+    const explanation = explainCodecError(await codecPipelineError());
+    expect(explanation?.detail).toContain("checksum mismatch");
+  });
+
+  it("keeps the reason across the worker boundary", async () => {
+    // Without `flattenErrorMessage` the cause is dropped by structured clone
+    // and the reader is told only that some codec failed.
+    const explanation = explainCodecError(
+      throughWorker(await codecPipelineError())
+    );
+    expect(explanation?.heading).toBe("Codec failed: numcodecs.fletcher32");
+    expect(explanation?.detail).toContain("checksum mismatch");
+  });
+});
+
+describe("a data type this browser cannot represent", () => {
+  async function unsupportedDataTypeError() {
+    // zarrita maps float16 onto `globalThis.Float16Array`, which older
+    // browsers do not have.
+    const original = Reflect.get(globalThis, "Float16Array");
+    Reflect.deleteProperty(globalThis, "Float16Array");
+    try {
+      const store = v2Store("<f2", null, new Uint8Array(8));
+      return await caught(open.v2(store, { kind: "array" }));
+    } finally {
+      Reflect.set(globalThis, "Float16Array", original);
+    }
+  }
+
+  it("names the data type and the browsers that support it", async () => {
+    const explanation = explainCodecError(await unsupportedDataTypeError());
+    expect(explanation?.heading).toBe("Unsupported data type: float16");
+    expect(explanation?.detail).toContain("Safari 26");
+  });
+});
+
+describe("errors that are not codec problems", () => {
+  const UNRELATED = [
+    new Error("Failed to fetch chunk bytes from example.invalid"),
+    new TypeError("Failed to fetch"),
+    new Error("Cannot convert a BigInt value to a number"),
+    new Error("Not found: /some/path"),
+    "a bare string",
+    undefined,
+  ];
+
+  it.each(UNRELATED)("passes through %s", (error) => {
+    expect(explainCodecError(error)).toBeUndefined();
+  });
+});
diff --git a/tests/unit/utils/errorHandling.test.ts b/tests/unit/utils/errorHandling.test.ts
new file mode 100644
index 00000000..fa349740
--- /dev/null
+++ b/tests/unit/utils/errorHandling.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+
+import { flattenErrorMessage } from "@/utils/errorHandling.ts";
+
+describe("flattenErrorMessage", () => {
+  it("returns the message when there is no cause", () => {
+    expect(flattenErrorMessage(new Error("plain"))).toBe("plain");
+  });
+
+  it("appends the cause when there is one", () => {
+    const error = new Error("outer", { cause: new Error("inner") });
+    expect(flattenErrorMessage(error)).toBe("outer — inner");
+  });
+
+  it("stringifies a non-error", () => {
+    expect(flattenErrorMessage("oops")).toBe("oops");
+  });
+});