Kafka MCP

An MCP server that exposes Kafka debugging as tools an LLM can call. It speaks MCP over stdio by default, optionally serves streamable HTTP, and talks to Kafka with franz-go.

One process can connect to several Kafka clusters. In stdio mode, one endpoint is selected for the session. In HTTP mode, each endpoint has its own path, so a session is bound to one cluster by how it connects rather than by a parameter a caller could forget to send.

Install

Every release ships the same server four ways:

• Binary: kafka-mcp_<Os>_<arch> archives on the releases page. Put kafka-mcp on your PATH.

• MCP Bundle: kafka-mcp_<version>_<os>_<arch>.mcpb on the same page, for macOS on Apple Silicon, Linux (amd64, arm64) and Windows (amd64). Open it in a client that installs bundles, such as Claude Desktop. It asks for your configuration file and, when that file has several endpoints, which one to serve. Intel Macs use the binary or the image.

• Container image: ghcr.io/denizgursoy/kafka-mcp:<tag>. It starts with --server; for stdio, mount the file and pass --server=false:

  shCopy

  docker run -i --rm \
    --mount type=bind,src=$PWD/kafka-mcp.yaml,dst=/config/kafka-mcp.yaml,readonly \
    -e CONFIG_FILE=/config/kafka-mcp.yaml \
    ghcr.io/denizgursoy/kafka-mcp:latest --server=false --endpoint local
  

• MCP Registry: listed as io.github.denizgursoy/kafka-mcp in the official registry, with the image and the bundles, so a client that reads the registry can install it from there.

How releases reach the registry and the other catalogs is in PUBLISHING.md.

Configuration

kafka-mcp.{toml,yaml,yml,json} in the working directory, ~/.config/kafka-mcp/ or /etc configures the server. The http block is used only with --server.

http:
  address: ":8090"
  base_path: /kafka-mcp # optional; prefixes every endpoint and /healthz
output_dir: /var/tmp/kafka-mcp
clusters:
  prod:
    brokers:
      - kafka-1:9093
      - kafka-2:9093
    security:
      tls:
        enabled: true
        ca_file: /etc/kafka/ca.pem
        # cert_file: /etc/kafka/client.pem # optional mTLS; requires key_file
        # key_file: /run/secrets/client.key
      sasl:
        - scram:
            enabled: true
            algorithm: SCRAM-SHA-256 # or SCRAM-SHA-512
            user: kafka-mcp-readonly
            pass: "{env:KAFKA_PASSWORD}" # or password_file: /run/secrets/kafka
  preprod:
    brokers: kafka-preprod:9093
endpoints:
  prod-read:
    cluster: prod
    path: /mcp
    description: Production investigation and debugging
    read_only: true
  prod-write:
    cluster: prod
    path: /mcp/rw
    description: Approved production changes
    tools:
      copy_message: false
  preprod:
    cluster: preprod
    path: /mcp/preprod

clusters owns Kafka connection details: brokers, TLS and SASL. endpoints owns the policy and, in HTTP mode, the MCP route. Stdio selects an endpoint by name and does not use its path. Several endpoints may reference one cluster, so the example reuses one production connection at /kafka-mcp/mcp in read-only mode and /kafka-mcp/mcp/rw in writable mode. Paths are exact: /mcp does not capture /mcp/rw. description is optional and is reported by server_config so a caller knows what the endpoint is intended for.

http.base_path prefixes endpoint paths and the liveness route. A leading or trailing slash on an endpoint path is optional. Paths must be unique and may not be /, /healthz, or contain a query or fragment. When path is omitted, it defaults to /mcp/<endpoint-name>.

For compatibility, a file with no endpoints block still creates one endpoint per cluster at /mcp/<cluster-name>. Existing cluster-level read_only and tools values continue to apply as a lower bound during migration; an explicit endpoint cannot widen them. New configurations should put both fields under endpoints.

An endpoint may switch individual tools off, by name:

endpoints:
  prod-read:
    cluster: prod
    path: /mcp
    read_only: true
    tools:
      create_topic: false
      commit_offset: false

Cross-cluster writes are granted per endpoint. copy_message and produce_message may name a destination_cluster, and that cluster must be listed in the calling endpoint's destinations:

endpoints:
  prod-read:
    cluster: prod
    read_only: true
    destinations: [preprod]   # this session may copy into preprod, nowhere else

An endpoint with no destinations writes to no other cluster. A listed cluster configured read_only still refuses, and listing the endpoint's own cluster is an error, because that is governed by read_only. Names are checked at startup. A file with no endpoints block keeps its old behaviour: each cluster's endpoint lists every other cluster. Clusters that no endpoint serves are invisible to list_clusters, compare_clusters and destination_cluster.

A tool the map does not mention stays exposed, so the file states only what it withholds rather than relisting every tool and silently losing whatever is added later. Names are exact and lowercase, as listed by server_config. They are checked at startup: a name that is not a tool stops the server, because a typo would leave the tool it was meant to withhold exposed. server_config cannot be switched off, since it is how a session learns which cluster and policy it reached and which tools that endpoint has. The tools map only narrows an endpoint: read_only: true still withholds the writing tools regardless of what the map says.

A minimal local file:

clusters:
  local:
    brokers: localhost:19092
    schema_registry:
      urls: ["http://localhost:18081"]
endpoints:
  local:
    cluster: local
    path: /mcp/local

Run a custom configuration over stdio with CONFIG_FILE=/path/to/config.yaml go run ./cmd/server. If it defines several endpoints, select one with --endpoint <name>. Add --server to serve every configured endpoint over HTTP instead; --endpoint is not used in HTTP mode. Without CONFIG_FILE, chu discovers kafka-mcp.{toml,yaml,yml,json} first in the working directory, then in the operating system's user config directory (~/.config/kafka-mcp/ on Linux), and finally in /etc. It uses the first matching file rather than merging files. Its standard loader order is defaults, file, HTTP, then environment; environment overrides use the KAFKA_MCP_ prefix (for example, KAFKA_MCP_HTTP_ADDRESS=:9000 or KAFKA_MCP_HTTP_BASE_PATH=/kafka-mcp). Logging can be configured with LOG_LEVEL and LOG_PRETTY.

At least one cluster with a broker is required. http.address defaults to :8090, http.base_path defaults to the HTTP root, and output_dir defaults to the system temp directory. Exports are confined to that directory: output_file takes a new file name, never a path; an existing name is refused rather than overwritten.

brokers accepts either one address as a scalar or several addresses as a YAML list. Each address is passed to Kafka as a separate seed broker.

Keep secrets out of the file with {env:VAR} or password_file. Unknown fields are ignored by chu.

TLS is configured with twmb/tlscfg, using its TLS 1.2 minimum and recommended cipher suites. TLS uses the system trust store; ca_file adds a custom CA. For mTLS, supply both cert_file and key_file. security.sasl is a preference-ordered list: each entry enables either scram or plain, or uses oauth as shown below. For PLAIN, use plain: {enabled: true, user: alice, pass: "{env:KAFKA_PASSWORD}"}. Both accept optional zid (authorization identity) and password_file instead of pass; SCRAM also accepts is_token: true for delegation tokens. Algorithms are case-insensitive. Disabled entries are ignored; repeated mechanisms and entries enabling multiple mechanisms are rejected. Fallback negotiates a broker-supported mechanism; it does not retry invalid credentials. All Kafka connections, including message-reading sessions, use these settings.

Legacy per-cluster tls and sasl: {mechanism, user, password, password_file} remain supported, but cannot be combined with security on the same cluster.

For OAUTHBEARER client credentials, use this cluster security block:

security:
  tls:
    enabled: true
  sasl:
    - oauth:
        enabled: true
        token_url: https://identity.example.com/realms/apps/protocol/openid-connect/token
        client_id: kafka-mcp
        client_secret: "{env:KAFKA_CLIENT_SECRET}"
        scopes: [kafka]
        timeout: 10s
        # zid: optional-authorization-id
        # extensions: {tenant: example}

Alternatively, use oauth: {enabled: true, token: "{env:KAFKA_TOKEN}"} for a static token. Configure exactly one mode: static token, client credentials, or gcp. Client-credentials tokens are fetched on authentication, cached per cluster across admin and reading connections, and renewed near expiry using expires_in. Token requests use the current authentication context and a timeout (default 10 seconds). Static tokens are not renewed. The broker must support OAUTHBEARER and trust the identity provider. Token endpoint error bodies are never reported, only their HTTP status, because they can echo secrets.

oauth.proxy sends token requests through an http, https, socks5 or socks5h proxy; when unset, HTTPS_PROXY, HTTP_PROXY and NO_PROXY apply.

Broker security.tls settings apply to Kafka connections only. The token endpoint uses oauth.tls, which can also present a client certificate (RFC 8705). With a certificate, client_secret is optional: when empty, client_id goes in the request body and the certificate authenticates the client.

    - oauth:
        enabled: true
        token_url: https://identity.example.com/token
        client_id: kafka-mcp
        scopes: [kafka]
        tls:
          cert_file: /etc/certs/oauth/client-cert.pem
          key_file: /etc/certs/oauth/client-key.pem
          ca_file: /etc/certs/oauth/ca.pem # optional; system roots otherwise

oauth.gcp gets Google Cloud access tokens with an X.509 client certificate (workload identity federation over mTLS), optionally impersonating a service account. Use Google's external_account credential file; its credential_source is ignored, because the certificate below is used instead.

    - oauth:
        enabled: true
        gcp:
          enabled: true
          credentials_file: /etc/certs/gcp/wif-credentials.json
          cert_file: /etc/certs/kafka/client-cert.pem
          key_file: /etc/certs/kafka/client-key.pem
          format: managed_kafka # Google Managed Service for Apache Kafka; default raw
          # proxy: http://proxy:3128 # default: oauth.proxy, then the environment
          # lifetime: 1h             # impersonated token lifetime
          # refresh_before: 5m       # refresh this long before expiry
          # scopes: [https://www.googleapis.com/auth/cloud-platform]

audience, token_url and service_account_impersonation_url may be set directly and override the credential file. managed_kafka requires a service account to impersonate. A failed refresh keeps using the cached token until it actually expires.

In oauth.tls and oauth.gcp, cert, key, ca (and credentials for gcp) may be given inline instead of as _file, not both; inline keys and credentials accept {env:VAR}. Files are re-read on each token refresh, so a rotated certificate applies without a restart.

insecure_skip_verify: true disables server certificate and hostname verification. It is accepted under security.tls (brokers), schema_registry.tls, oauth.tls (token endpoint) and oauth.gcp (Google STS and impersonation), and defaults to false. It exposes the connection to man-in-the-middle attacks: prefer ca_file, and do not use it in production.

A cluster that cannot be reached at startup is still served, and list_clusters reports it as disconnected. One cluster being down must not block debugging the others.

Message formats

Messages are decoded to JSON for every tool that shows or searches them, by the first rule that applies:

1. The topic has a format in topic_formats.
2. The bytes carry a Schema Registry header (Confluent wire format) and the cluster has a schema_registry: Avro, Protobuf (with message index and referenced schemas) or JSON Schema.
3. Otherwise JSON, then UTF-8 text, then base64.

clusters:
  prod:
    brokers: kafka-1:9093
    schema_registry:
      urls: ["https://schema-registry:8081"]
      user: kafka-mcp                     # or bearer_token: "{env:SR_TOKEN}"
      password: "{env:SR_PASSWORD}"       # or password_file
      # tls: {enabled: true, ca_file: /etc/sr/ca.pem}
    topic_formats:                        # topics whose messages carry no schema id
      "orders.*":                         # exact name or glob; longest match wins
        value:
          format: protobuf
          proto_files: [shop/order.proto]
          import_paths: [./protos]        # or descriptor_set: ./orders.binpb
          message_type: shop.Order
      events:
        value: {format: avro, schema_file: ./schemas/event.avsc}
      metrics:
        key: {format: text}
        value: {format: msgpack}

Formats are avro, protobuf, json, msgpack, text and binary. Schema files and .proto sources are loaded at startup, so a missing file or type stops the server. A message that names a schema but cannot be decoded is returned as base64 with decode_error saying why, never silently. JSON output has sorted keys; Avro longs beyond 2^53 keep their exact value; unset Protobuf fields are shown with their defaults.

Each rendered message reports format, schema_id and message_type for the value, and key_format / key_schema_id when the key is not plain text.

server_config reports the endpoint name, exact path, description, policy and the cluster it serves, and never the password. Its authentication and sasl_user describe the first configured option; sasl_options lists all configured identities in preference order, not the mechanism negotiated by an individual broker connection. It also reports schema_registry URLs and topic_formats.

Authentication and browser clients

http.auth_token makes every MCP request carry Authorization: Bearer <token>; requests without it get 401. It accepts {env:VAR}. /healthz and CORS preflights need no token. Without it, the endpoints have no authentication of their own, which is safe only where nothing untrusted can reach the listener.

Set it with an environment variable, which keeps the secret out of the config file entirely:

export KAFKA_MCP_HTTP_AUTH_TOKEN="$(openssl rand -hex 32)"
CONFIG_FILE=kafka-mcp.yaml kafka-mcp --server

Every config key can be overridden this way: KAFKA_MCP_ followed by the key path in upper case with _ between levels, so http.auth_token is KAFKA_MCP_HTTP_AUTH_TOKEN. To keep the token in the file but the value in the environment under a name of your choice, reference it instead:

http:
  auth_token: "{env:MY_MCP_TOKEN}"   # the server refuses to start if MY_MCP_TOKEN is unset

Clients then send the token on every request:

curl -X POST http://localhost:8090/mcp/local \
  -H "Authorization: Bearer $KAFKA_MCP_HTTP_AUTH_TOKEN" \
  -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"t","version":"1"}}}'

In the client configuration used under Connecting a client, add the header to each remote entry. {env:...} is the client's own substitution, so the token stays out of that file too:

{
  "mcp": {
    "kafka-local": {
      "type": "remote",
      "url": "http://localhost:8090/mcp/local",
      "headers": { "Authorization": "Bearer {env:KAFKA_MCP_HTTP_AUTH_TOKEN}" }
    }
  }
}

In a container, pass it like any other secret:

docker run -e KAFKA_MCP_HTTP_AUTH_TOKEN="$TOKEN" \
  -e KAFKA_MCP_HTTP_CORS_ALLOW_ORIGINS="https://mcp-client.example" \
  -v "$PWD/kafka-mcp.yaml:/etc/kafka-mcp.yaml" -e CONFIG_FILE=/etc/kafka-mcp.yaml \
  -p 8090:8090 kafka-mcp

A command-line client sends no Origin header and is unaffected by CORS. A request that does carry an Origin is refused with 403 unless that origin is in allow_origins. This happens before any tool runs, because CORS on its own only hides the response from the page and the request itself would still execute. No origin is allowed by default, so no web page the user visits can drive the tools. To use a browser client, name its origin:

http:
  address: ":8090"
  cors:
    allow_origins: ["https://mcp-client.example"]
    allow_methods: [GET, POST, DELETE, OPTIONS]
    allow_headers: [content-type, accept, authorization, cache-control, last-event-id, mcp-session-id, mcp-protocol-version]
    expose_headers: [Mcp-Session-Id]
    allow_private_network: true
    max_age: 600

The same list from the environment is comma-separated: KAFKA_MCP_HTTP_CORS_ALLOW_ORIGINS="https://a.example,https://b.example". Setting it to * restores the old allow-everything behaviour.

http.cors is ada's CORS middleware configuration, read straight from the file, so every option that middleware has is available here. Each key is optional and keeps its own default, so setting allow_origins alone does not drop the rest. The defaults are the values above with no allow_origins. allow_origins: ["*"] restores the old allow-everything behaviour.

Three of them are load-bearing for the MCP transport. allow_methods needs GET, POST and DELETE: requests are posted, the event stream is a GET, and a client ends its session with DELETE. allow_headers needs mcp-session-id and mcp-protocol-version, which the client sends from the second request onwards, and a header missing there fails the whole preflight rather than being dropped. Mcp-Session-Id must stay in expose_headers: the session id arrives on the initialize response, and a page that cannot read it cannot make a second call.

allow_private_network answers Chrome's Private Network Access preflight, which a page on a public address must pass before it may reach a server on a private or loopback address; it defaults to on, and is granted only on a preflight the rest of the policy already allowed. Setting allow_credentials together with a wildcard allow_origins is refused by the middleware at startup unless unsafe_wildcard_origin_with_allow_credentials is also set, which it should not be.

An allowed origin can drive every tool with the server's Kafka credentials, so list only pages that should have that power, set auth_token (a browser client then has to send it), and set read_only: true on endpoints that should not write. A less obvious path such as /mcp/rw is not authentication.

Connecting a client

For the default stdio transport, configure the client to launch the binary. A configuration with one endpoint needs no arguments:

{
  "mcp": {
    "kafka-local": {
      "type": "local",
      "command": ["kafka-mcp"],
      "environment": {"CONFIG_FILE": "/path/to/kafka-mcp.yaml"}
    }
  }
}

When the file contains several endpoints, add the endpoint selection to the command, for example "command": ["kafka-mcp", "--endpoint", "prod-read"]. The process refuses to start without it so it cannot silently connect a session to the wrong cluster or permission policy.

To use HTTP, start kafka-mcp --server and register one remote client entry per endpoint:

{
  "mcp": {
    "kafka-local": {
      "type": "remote",
      "url": "http://localhost:8090/mcp/local"
    },
    "kafka-prod-read": {
      "type": "remote",
      "url": "http://localhost:8090/mcp",
      "enabled": false
    }
  }
}

When the server sets http.auth_token, add "headers": {"Authorization": "Bearer {env:KAFKA_MCP_HTTP_AUTH_TOKEN}"} to each entry; see Authentication and browser clients.

The key becomes the tool prefix, so these appear as kafka-local_list_topics and kafka-prod-read_list_topics. Name entries after both the cluster and the endpoint policy: the prefix is the clearest signal of what a call can hit and change. Use server_config to confirm rather than trusting the client-side name.

Each enabled cluster costs context: these tools are roughly 8k tokens of definitions. Enable only what you need, and put production behind an agent:

{
  "tools": { "kafka-prod*": false },
  "agent": {
    "kafka-prod": {
      "description": "Debugging against production Kafka.",
      "tools": { "kafka-prod*": true }
    }
  }
}

Permissions

Three layers, and only one of them is real security:

What a read-only endpoint exposes

read_only: true does more than refuse a write: the endpoint does not list the tools whose only purpose is to write. add_partitions, alter_topic_config, commit_offset, create_topic, delete_consumer_group, delete_records and delete_topic are absent from tools/list on a read-only endpoint, so a client never sees a tool it could not have used, and their preview cannot describe a change this endpoint would never apply.

A writable endpoint can withhold individual tools too, with its tools map. That is the same mechanism seen from the client: the tool is not registered, so it is absent from tools/list and from server_config.

copy_message and produce_message stay, because read_only protects the cluster being written to and the destination is chosen per call. Copying a message out of a read-only production cluster, or seeding a writable preprod cluster from a protected session, is exactly what they are for. The destination must be in the endpoint's destinations, and both refuse outright when the destination is the read-only cluster itself.

Hiding a tool decides what is advertised, not what is permitted: both tools still refuse at the point of mutation, so a registration mistake cannot turn into a write. server_config reports the tools the endpoint actually exposes, which is how a session can tell the two cases apart.

Giving two people different permissions

Kafka enforces permissions against the SASL principal, so two people running the same server with different credentials get different rights. Adding partitions requires ALTER on the topic:

# ali may read but not reshape topics
rpk acl create --allow-principal User:ali \
  --operation read,describe --topic orders

# deniz may also add partitions
rpk acl create --allow-principal User:deniz \
  --operation read,describe,alter --topic orders

Each points CONFIG_FILE at their own file, differing only in security.sasl[0].scram.user and the password. When ali calls add_partitions, the broker refuses:

not authorized to add partitions to "orders": the broker refused this request.
Adding partitions requires ALTER permission on the topic for the principal
this server connects as

ali cannot bypass that by editing config or rebuilding the binary, because the decision is made by Kafka rather than by this server. On a cluster without ACLs, read_only: true is the available protection.

Audit logging

Every tool call is logged. One slog record per call, on the server's own log stream, so nothing extra has to be configured:

{"level":"INFO","msg":"tool call","tool":"produce_message","outcome":"ok",
 "duration_ms":12,"endpoint":"prod-write","cluster":"prod","read_only":false,
 "principal":"kafka-mcp-rw","session":"QY7MZM...","client":"claude-code",
 "client_version":"1.0.0","confirm":true,"item_count":1,"targets":"orders"}

The tools that change a cluster — add_partitions, alter_topic_config, commit_offset, create_topic, copy_message, delete_consumer_group, delete_records, delete_topic, produce_message — are logged at INFO. Everything else is logged at DEBUG, because reads are constant and change nothing, so recording them at the same level would bury the writes among them. Raise the log level to see them.

Two fields carry most of the weight. confirm separates a real write from a preview, and targets names the topic, group, partition and offset each item pointed at, so a record says which topic was touched rather than only that some topic was.

Message content is never logged. produce_message and copy_message carry arbitrary payloads, and an audit log is usually readable by more people than the data it describes, so keys, values and headers are left out. The audit code has no field to unmarshal them into, so content cannot reach a log even by mistake.

None of the recorded identities is a person, and the server cannot make one up. What each actually means:

For attribution to a person, give each person their own endpoint and SASL credentials, as above: the principal in the record is then the answer to who acted. A shared credential cannot be made to answer it.

Tools

Batch operations

describe_topic, sample_messages, get_message, get_schema, consumer_lag, describe_consumer_group, open_transactions, add_partitions, alter_topic_config, create_topic, commit_offset, delete_consumer_group, delete_records, delete_topic, copy_message and produce_message take their target only as a required items array. There is no single-target form: one operation is an items array of length one.

{"items": [{"topic": "orders"}]}
{"items": [{"topic": "orders"}, {"topic": "payments"}]}

Everything naming or shaping an operation lives on the item, so each field and its description exist in exactly one place. What governs the whole call stays at the top level, which in practice means confirm.

The message-heavy tools accept at most 20 items; lag, schema and administrative tools accept at most 100.

Every response is the same envelope. Results stay in input order, each entry carrying index and either result or error, followed by succeeded, failed, applied and atomic: false. An item's failure is data in the response rather than an error for the call, so it never hides the items that worked. Batch writes preview every item first, then apply the valid ones only when the top-level confirm is true. They are not transactions: Kafka cannot roll back a topic, partition, offset or produced message after a later item fails. Duplicate write targets are refused before anything changes, except in produce_message, where two identical items mean two messages rather than a mistake.

list_clusters

Lists the clusters this server's endpoints serve, with whether each is reachable, whether it is configured read_only, and whether this session may write to it (writable: its own cluster unless the endpoint is read-only, another only when listed in destinations). Takes no parameters. Available from every endpoint, so a session can discover what copy_message, produce_message and compare_clusters may target.

{"clusters": [
  {"name": "prod", "connected": true, "read_only": true, "writable": false},
  {"name": "preprod", "connected": true, "read_only": false, "writable": true}
], "count": 2}

connected is checked when you call, not recorded at startup, so a cluster that has since gone down is reported honestly. Only the name, reachability and writability are reported: broker addresses and credentials are deliberately not, because this tool is reachable from every endpoint.

compare_clusters

Compares the topics of other clusters against the one this endpoint serves, and reports what differs. Use it to find what preproduction has that production does not, or to check whether two environments still match.

Item fields:

{"here": "preprod", "there": "prod",
 "here_cluster": {"name": "preprod", "brokers": 1, "topics": 12},
 "there_cluster": {"name": "prod", "brokers": 3, "topics": 11},
 "only_here": [{"topic": "orders-v2", "partitions": 6, "replication_factor": 1,
                "configs": {"retention.ms": "604800000"}}],
 "only_there": [],
 "differing": [{"topic": "orders", "differences": ["partitions"],
                "here": {"partitions": 1}, "there": {"partitions": 12}}],
 "in_both": 11}

only_here and only_there name the direction, which is decided by the endpoint you call: "here" is always the cluster this endpoint serves. Entries carry the partition count, replication factor and explicitly-set configs of the cluster that has the topic, so they can be passed straight to create_topic.

This tool creates and changes nothing. To create the missing topics, hand the chosen entries to create_topic, which previews them against the broker first and warns that a partition count can never be reduced.

differing is usually the more valuable half: a topic that exists on both sides with a different partition count or retention.ms is the common reason a bug reproduces in one environment and not the other. Only configs a topic sets for itself are compared, because two clusters may carry different broker defaults and comparing inherited values would report every topic as different.

A difference is not necessarily a mistake. A topic missing from production is often deliberate, so the report says what differs, never what is correct.

Topic listings come from the Kafka client's metadata cache, which is a few seconds old, so a topic created moments earlier may still appear in only_there. Repeat the comparison rather than creating it twice.

list_topics

Lists the topics on the cluster, sorted by name, each with its partition count, replication factor, the configs it sets for itself, and its size on disk.

The predicate sees:

return topic.indexOf('orders') >= 0
return size_bytes > 10e9                       // the topics filling the disk
return partitions > 6
return configs['cleanup.policy'] === 'compact'
return replication_factor === 1 && !internal

{"name": "list_topics", "arguments": {"script": "return partitions > 6"}}

{"topics": [{"topic": "orders", "partitions": 12, "replication_factor": 3,
             "configs": {"retention.ms": "604800000"}, "size_bytes": 48318382080}],
 "count": 1}

Filtering is JavaScript only, as it is for search_messages: a name match is return topic.indexOf('orders') >= 0. The predicate can also answer what a substring never could — which topics have more than six partitions, only one replica, or a compacted cleanup policy.

Only configs a topic sets for itself are reported. Inherited cluster defaults are excluded, because including them would make every topic look configured.

A topic whose predicate throws, or is cut short by the timeout, is counted in script_errors rather than listed, so a broken filter is never mistaken for an empty cluster.

Sizes come from DescribeLogDirs, which needs describe on the cluster. When the brokers do not answer it, every size_bytes is -1 and warnings says why. Sizes are bytes on disk after compression.

describe_topic

Reports a topic's partitions, offset ranges, message count, size on disk, time span and full configuration. Use it before searching to see how much data a search would read and how far back the topic can hold data at all.

Item fields:

{"results": [{"index": 0, "result": {
   "topic": "orders", "partition_count": 1, "message_count": 3,
   "size_bytes": 2048, "replicated_size_bytes": 6144,
   "partitions": [{"partition": 0, "start_offset": 0, "end_offset": 3, "message_count": 3, "size_bytes": 2048}],
   "configs": [{"key": "cleanup.policy", "value": "delete", "source": "DYNAMIC_TOPIC_CONFIG", "is_default": false},
               {"key": "retention.ms", "value": "604800000", "source": "DEFAULT_CONFIG", "is_default": true}]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

configs lists every topic config as the string Kafka reports, where -1 means unlimited. is_default is true when the value is inherited rather than set on the topic. Two entries decide whether a message can still exist at all: retention.ms (how long messages are kept) and cleanup.policy (compact keeps only the latest message per key).

size_bytes is one copy of the topic, per partition and in total; replicated_size_bytes counts every replica, which is what the brokers' disks hold. Both are omitted, with a warning, when the brokers do not report log dirs.

sample_messages

Reads a small sample of the newest messages and reports what they look like: value formats, field paths with their types, key statistics, which value fields carry the message key, and the schemas the values were written with. Avro, Protobuf and JSON Schema values are decoded first, so their field paths are reported like JSON. Use it before searching to decide how to search, and before producing to find the schema to write against.

Item fields:

{"results": [{"index": 0, "result": {
   "value_formats": {"json": 20, "text": 0, "binary": 0},
   "json_fields": [{"path": "payload.amount", "types": ["number"], "present": 20, "example": "500"}],
   "key_stats": {"present": 20, "absent": 0, "unique": 20, "all_unique": true},
   "key_in_value": ["payload.orderId"],
   "schemas": [{"format": "avro", "schema_id": 7, "message_type": "shop.Order", "count": 20}],
   "value_bytes": {"min": 180, "p50": 412, "max": 9020},
   "sampled_ranges": [{"partition": 0, "start": 980, "end": 1000}]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

key_in_value naming a field means the key is that identifier, so searching the key alone is the precise, cheap lookup. value_formats also counts avro, protobuf, json_schema, msgpack, null and undecodable — values that named a schema but could not be decoded, which is a configuration problem, not binary data.

search_messages

Scans a bounded range of a topic, filtering messages with a JavaScript expression. Kafka has no server-side search, so this reads messages and filters them client-side; the result reports what was covered.

The script

Return true to keep a message. In scope:

return key === 'order-123'
return value.eventType === 'NEW' && value.payload.amount >= 500
return value.payload.cancelledAt === null      // present and null
return value.payload.cancelledAt === undefined // field absent
return headers['correlation-id'] === 'corr-999'
return /ORD-\d{4}/.test(value.payload.orderId)
return value.indexOf('ERROR') >= 0             // non-JSON topic: value is a string
return schema_id === 57                        // written with one schema
return value_bytes > 900000                    // close to max.message.bytes

Searching by key exactly is far more precise than searching the body: 123 also appears inside "amount": 1123, and those false positives can fill max_matches and hide the message wanted.

A script that throws on a message is counted in script_errors and the scan continues, so a broken script is distinguishable from a genuine absence of matches. Scripts run in a sandbox with no filesystem, network or host access, and are stopped if they exceed the search timeout. They are not bounded by memory: something like 'x'.repeat(1e12) can exhaust the server process. Scripts are trusted input; the blast radius is this server, not the cluster.

Scan order

Every partition is read together, one chunk deep at a time: the newest chunk of every partition, then the chunk behind it, and so on. A limited newest-first search therefore returns the newest matches in the topic, not the newest in whichever partition happened to be read first.

Kafka orders records within a partition and never across them, so matches are merged and reported by timestamp, with partition and offset breaking ties. Timestamps are set by the producer unless the topic uses LogAppendTime, so they can be skewed; it is still the only thing comparable between partitions.

One scan reads every partition through a single connection, so a wide topic costs no more connections than a narrow one. It does read more: a limited search on a 12-partition topic examines the newest chunk of all twelve rather than stopping inside the first. max_messages_scanned still bounds it, and may become the stopped_reason on a wide topic sooner than on a narrow one.

Parallelism

parallelism reads a single-partition topic with that many concurrent readers, which is what makes a full scan of one large partition fast. Readers take chunks newest first (or oldest first), and a max_matches stop is decided only on chunks completed in that order, so the matches returned are the same as a sequential search would return. Because readers finish out of order, scanned_ranges may list several runs for the partition. A multi-partition topic is already read in parallel across its partitions, so the setting does not apply there, and a range too small to divide is read by one reader.

It pays off for count_only, group_by, output_file and full scans of a single partition.

Result

{"topic": "orders", "match_count": 1,
 "matches": [{"partition": 0, "offset": 17, "key": "order-42", "value": "...", "encoding": "utf8"}],
 "scanned_messages": 120, "scanned_ranges": [{"partition": 0, "start": 0, "end": 120}],
 "stopped_reason": "range_exhausted", "complete": true}

complete is true only when the whole range was read. An empty match list means "not there" only if complete is true; otherwise check stopped_reason and narrow the search.

For large result sets, use count_only to learn how many matches exist, then output_file to write them out instead of returning them.

Grouping

group_by summarises instead of listing. It is an expression over the same variables as script that returns a bucket name. Every match in the range is counted per bucket, as with count_only, and no bodies are returned. null and undefined form the bucket null; an object is grouped by its JSON.

{"name": "search_messages", "arguments": {"topic": "orders-dlq",
  "group_by": "return headers['error-reason']", "from_timestamp": "2026-10-10T06:00:00Z"}}

{"topic": "orders-dlq", "match_count": 1840, "matches": [],
 "groups": [{"key": "timeout", "count": 1702, "example": {"partition": 2, "offset": 90311}},
            {"key": "bad_schema", "count": 138, "example": {"partition": 0, "offset": 4410}}],
 "complete": true}

Buckets are sorted by count. groups_truncated is true when more than max_groups existed. A message the expression throws on is counted in script_errors and not bucketed. group_by cannot be combined with output_file.

get_message

Reads messages at exact offsets, plus optional neighbours.

Item fields:

{"name": "get_message", "arguments": {"items": [
  {"topic": "orders", "partition": 0, "offset": 17, "context": 1}]}}

Schema-encoded values are decoded to JSON, with format, schema_id and message_type saying what they were. Values that could not be decoded and are not valid UTF-8 are base64 encoded, with encoding set to base64; decode_error explains a value that named a schema but could not be decoded.

{"partition": 0, "offset": 17, "key": "o-1",
 "value": "{\"amount\":42,\"id\":\"o-1\"}", "format": "avro",
 "schema_id": 7, "message_type": "shop.Order", "encoding": "utf8", "value_bytes": 12}

get_schema

Reads schemas from the cluster's Schema Registry, by subject or by the schema_id a message carries. Read the schema before producing to a schema-encoded topic: it names every field and enum a value needs, which a sampled message may not show.

Item fields, giving subject or id:

{"results": [{"index": 0, "result": {
   "schema_id": 7, "subject": "orders-value", "version": 3, "versions": [1, 2, 3],
   "type": "avro", "schema": "{...}", "references": [],
   "message_types": null, "used_by": null, "compatibility": "BACKWARD",
   "check": {"compatible": false, "messages": ["{errorType:'TYPE_MISMATCH', …}"]}}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

message_types lists Protobuf messages, which produce_message takes as message_type. Looking up an id fills used_by with the subject versions that use it. A subject lookup reports compatibility, the level in force, inherited from the registry's global level when the subject sets none. check_schema asks the registry whether the candidate could be registered as the subject's next version under that level. check.messages gives the registry's reasons when it could not. Nothing is registered. The call fails as a whole when the cluster has no schema_registry.

list_consumer_groups

Lists consumer groups with their state, member count and the topics they consume.

{"groups": [{"group": "payments", "state": "Stable", "members": 2, "topics": ["orders"]}], "count": 1}

A group in state Empty can still report lag: committed offsets outlive the consumers that made them. Kafka has no topic-to-group index, so filtering by topic describes every group on the cluster.

describe_consumer_group

Describes consumer groups in detail: who the members are, which partitions each one owns, and where the group stands on every partition. Use it to turn "a partition is stuck" into "this pod on this host is stuck".

sample_seconds (optional, at most 60) watches the group for that long before describing it, reading it every second, and adds observation:

"observation": {"seconds": 30, "samples": 31,
  "states": ["Stable", "PreparingRebalance", "CompletingRebalance", "Stable"],
  "joined": [{"member_id": "payments-7-b…", "client_id": "payments-7", "host": "/10.0.4.17"}],
  "left":   [{"member_id": "payments-7-a…", "client_id": "payments-7", "host": "/10.0.4.17"}],
  "unstable": true}

A member that restarts rejoins with a new member_id, so it appears in both left and joined with the same host. That is how a crash-looping consumer behind a rebalance storm is found. The call blocks for the window.

{"results": [{"index": 0, "result": {
   "group": "payments", "state": "Stable", "protocol_type": "consumer", "assignor": "cooperative-sticky",
   "coordinator": 1, "total_lag": 4200,
   "members": [{"member_id": "payments-7-…", "client_id": "payments-7", "host": "/10.0.4.17",
                "assignments": [{"topic": "orders", "partitions": [0, 1]}]}],
   "partitions": [{"topic": "orders", "partition": 0, "has_commit": true, "committed_offset": 812,
                   "end_offset": 5012, "lag": 4200, "member_id": "payments-7-…",
                   "client_id": "payments-7", "host": "/10.0.4.17"}]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

Partitions are the union of what members own and what the group has committed, so an Empty group still shows its positions. has_commit: false means the group owns a partition it has never committed on, so where it starts is decided by the consumer's auto.offset.reset, not by an offset.

open_transactions

Finds open transactions holding back read_committed consumers. A transactional producer that hangs or dies mid-transaction leaves the partition's last stable offset stuck, and every read_committed consumer stops there. In consumer_lag that looks exactly like a poison message.

{"results": [{"index": 0, "result": {
   "topic": "orders", "blocked": true,
   "partitions": [{"partition": 0, "last_stable_offset": 812, "high_watermark": 5012,
                   "unreadable_messages": 4200,
                   "producers": [{"producer_id": 2004, "producer_epoch": 0, "transaction_start_offset": 812,
                                  "transactional_id": "payments-writer-1", "state": "Ongoing",
                                  "started_at": "2026-10-02T08:14:03Z", "open_for": "41m12s", "timeout_ms": 900000}]}]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

The fix is in the producer: restart or fence the application named by transactional_id, or wait for timeout_ms, after which the broker aborts the transaction. Moving the consumer's offset does not help.

consumer_lag

Measures how far behind a topic's consumers are, how fast messages are produced and consumed, and when the backlog will clear.

Item fields:

Up to 4 sampling windows run at a time, so the call takes about sample_seconds for every 4 items.

{"results": [{"index": 0, "result": {
   "topic": "orders", "total_lag": 4200,
   "produce_rate": {"last_minute": {"messages": 3000, "per_second": 50, "per_minute": 3000, "per_hour": 180000}},
   "groups": [{"group": "payments", "state": "Stable", "members": 2, "lag": 4200,
               "consume_rate": {"per_second": 120, "sampled_seconds": 5},
               "drain_per_second": 70, "eta_seconds": 60, "eta_human": "1m 0s",
               "status": "draining"}]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

The two rates are measured differently, and the output says so:

• produce_rate — historical fact, from message timestamps, over the last second, minute and hour. window_truncated marks a topic younger than the window.
• consume_rate — a sample: the committed offset is read, then read again sample_seconds later. sample_inconclusive means nothing moved.

The backlog drains at the consume rate minus the produce rate. status says what the numbers mean: caught_up, draining (with an ETA), growing (never clears, with growing_by_per_minute), stalled, no_active_consumers, or not_measured. An ETA is only given when the lag is genuinely shrinking.

Every partition also reports start_offset, and offset_expired: true when the committed offset is below it. Retention has then deleted the group's position: the consumer resets by its auto.offset.reset instead of resuming, and the group carries a warning. produce_rate.partitions gives per-partition counts for the last minute and hour, which separates key skew from a slow consumer.

With measure_backlog_age, each lagging partition reports committed_timestamp (the next message to consume) and lag_seconds. Each group reports oldest_unconsumed_at, and the topic its retention_ms. retention_risk is set, with a warning, once the oldest unconsumed message has used half of the retention: unless the group catches up, retention deletes it before it is consumed.

cluster_health

Checks the cluster this endpoint serves in one call: brokers, controller, and every partition that is not fully healthy.

{"cluster_id": "…", "controller": 1, "healthy": false,
 "brokers": [{"id": 1, "host": "kafka-1", "port": 9092, "rack": "eu-1a", "controller": true, "leaders": 61}],
 "summary": {"topics": 40, "partitions": 182, "offline": 0, "under_replicated": 3, "under_min_isr": 1, "errored": 0},
 "problems": [{"topic": "orders", "partition": 4, "issues": ["under_replicated", "under_min_isr"],
               "leader": 1, "replicas": [1, 2, 3], "isr": [1], "min_insync_replicas": 2}],
 "warnings": []}

offline means the partition has no leader, so nothing can be read or written. under_replicated means a replica is out of sync. under_min_isr is the condition behind NOT_ENOUGH_REPLICAS: producers using acks=all fail until the in-sync replicas recover. A broker leading no partitions while others lead many is usually one that restarted and was never given leadership back.

A problem partition being moved between brokers carries reassigning: true with its adding_replicas and removing_replicas, and summary.reassigning counts them. An added replica is out of sync until it has copied the log, so under_replicated there is expected during the move rather than an outage.

Some Kafka-compatible brokers, Redpanda among them, do not report min.insync.replicas. The topics affected are listed in min_isr_unknown, and under_min_isr is not judged for them rather than guessed.

list_acls

Lists access control entries, for when a client fails with TOPIC_AUTHORIZATION_FAILED or GROUP_AUTHORIZATION_FAILED.

{"acls": [{"principal": "User:payments", "host": "*", "resource_type": "topic", "resource_name": "orders",
           "pattern_type": "literal", "operation": "read", "permission": "allow"}], "count": 1}

A deny overrides every allow. A broker without an authorizer fails with SECURITY_DISABLED, which means ACLs are not enforced at all.

server_config

Reports the effective configuration: endpoint name, exact path, description, cluster, brokers, authentication mechanism and principal, TLS, read-only state, export directory and the tools this endpoint exposes. Takes no parameters. The password is never reported.

tools is the list for this endpoint, not for the deployment: a read-only endpoint omits add_partitions, alter_topic_config, commit_offset, create_topic, delete_consumer_group, delete_records and delete_topic, because it does not register them, and any endpoint omits whatever its tools configuration switches off.

Use it when a result is surprising: an empty topic list means something very different on a local broker than on production.

add_partitions

Adds partitions to a topic. Irreversible — Kafka cannot reduce a partition count. Not exposed on a read-only endpoint.

Item fields:

Without confirm it reports what would happen: current and target counts, whether messages are keyed, which consumer groups will rebalance, and warnings.

Adding partitions changes which partition a key hashes to, so existing keys lose their ordering guarantee. A keyed topic therefore requires acknowledge_key_ordering as well. Requesting fewer partitions than the topic has is refused with an explanation rather than attempted.

alter_topic_config

Changes topic-level configuration: retention, cleanup policy, maximum message size and anything else Kafka allows per topic. Changes are incremental, so every key not named keeps its value. Not exposed on a read-only endpoint.

Item fields:

{"results": [{"index": 0, "result": {
   "topic": "orders", "applied": false, "messages_past_retention": 18000,
   "changes": [{"key": "retention.ms", "action": "set", "current": "604800000",
                "current_source": "DYNAMIC_TOPIC_CONFIG", "requested": "86400000"}],
   "warnings": ["18000 message(s) are already older than the new retention of 24h0m0s and become eligible for deletion as soon as it applies; Kafka cannot bring them back"]}}],
 "succeeded": 1, "failed": 0, "applied": 0, "atomic": false}

The preview asks the broker to validate the change, so an unknown key or an invalid value is refused before confirm. Shortening retention.ms reports how many messages are already past the new limit; changing cleanup.policy or setting retention.bytes is warned about. After applying, every key is re-read from the broker, which is the only way the inherited value of a deleted override is known.

create_topic

Creates a topic. Refuses a topic that already exists rather than adjusting it. Not exposed on a read-only endpoint.

Item fields: