Resources

mcjtag exposes 7 MCP resources that provide read-only access to target state. Resources use the jtag:// URI scheme and can be polled by MCP clients without invoking tool calls.

How Resources Work

Resources are distinct from tools in several ways:

• Read-only — they reflect current state without side effects
• No parameters — the URI fully identifies the resource (except for templated URIs)
• Polling-friendly — clients can read resources repeatedly to monitor changes
• Error handling — resources return a dict with an error key if something goes wrong, rather than raising exceptions

All resources require an active OpenOCD connection. If the server is not connected, every resource returns:

{"error": "Not connected to OpenOCD. Use connect or start_openocd first."}

---

Resource Reference

jtag://target/state

Current target execution state and program counter.

Returns:

Field	Type	Description
name	str	Target name (e.g. "stm32f1x.cpu")
state	str	Execution state: halted, running, reset, debug-running
pc	str | null	Program counter as hex (e.g. "0x08001234"), null if not halted
halted	bool	Whether the target is halted

{
  "name": "stm32f1x.cpu",
  "state": "halted",
  "pc": "0x08001234",
  "halted": true
}

---

jtag://registers/all

All CPU register values. The target must be halted for this resource to succeed.

Returns:

Field	Type	Description
registers	dict[str, str]	Map of register name to hex value
count	int	Number of registers returned

Register values are formatted with width-appropriate hex padding (e.g., 32-bit registers as 0x00000000).

{
  "registers": {
    "r0": "0x00000000",
    "r1": "0x20000100",
    "sp": "0x20004ff0",
    "lr": "0x080011a5",
    "pc": "0x08001234",
    "xPSR": "0x61000000"
  },
  "count": 21
}

---

jtag://flash/banks

Flash bank topology and metadata.

Returns:

Field	Type	Description
banks	list[dict]	List of flash bank descriptors
count	int	Number of flash banks

Each bank contains:

Field	Type	Description
index	int	Bank index (0-based)
name	str	Flash driver name
base	str	Base address (hex)
size	int	Total size in bytes
target	str	Associated target name

{
  "banks": [
    {
      "index": 0,
      "name": "stm32f1x",
      "base": "0x08000000",
      "size": 65536,
      "target": "stm32f1x.cpu"
    }
  ],
  "count": 1
}

---

jtag://jtag/chain

JTAG scan chain enumeration with TAP details.

Returns:

Field	Type	Description
taps	list[dict]	List of TAP descriptors
count	int	Number of TAPs discovered

Each TAP contains:

Field	Type	Description
name	str	TAP name (e.g. "stm32f1x.cpu")
idcode	str	JTAG IDCODE (hex)
ir_length	int	Instruction register length in bits
enabled	bool	Whether the TAP is enabled

{
  "taps": [
    {
      "name": "stm32f1x.cpu",
      "idcode": "0x1ba01477",
      "ir_length": 4,
      "enabled": true
    }
  ],
  "count": 1
}

---

jtag://svd/peripherals

List of peripherals from the currently loaded SVD file.

Returns:

Field	Type	Description
svd_loaded	bool	Whether an SVD file is loaded
svd_path	str	Path to the loaded SVD file (only if loaded)
peripherals	list[str]	Peripheral names (empty list if no SVD loaded)

If no SVD file has been loaded, svd_loaded will be false and peripherals will be an empty list:

{
  "svd_loaded": false,
  "peripherals": []
}

After loading an SVD file via svd_inspect(svd_path=...):

{
  "svd_loaded": true,
  "svd_path": "/path/to/STM32F103.svd",
  "peripherals": ["GPIOA", "GPIOB", "GPIOC", "USART1", "USART2", "SPI1", "I2C1", "TIM1", "RCC", "FLASH"]
}

---

jtag://svd/{peripheral}

Decoded register values for a specific peripheral. This is a templated resource — replace {peripheral} with the actual peripheral name.

URI examples:

• jtag://svd/GPIOA
• jtag://svd/USART1
• jtag://svd/RCC

Returns:

Field	Type	Description
peripheral	str	Peripheral name
registers	dict[str, dict]	Map of register name to decoded register

Each register contains:

Field	Type	Description
address	str	Register address (hex)
raw_value	str	Raw 32-bit value read from hardware (hex)
fields	list[dict]	Decoded bitfields

Each bitfield contains:

Field	Type	Description
name	str	Field name from SVD
value	int	Extracted value
width	int	Field width in bits
offset	int	Bit offset within the register
description	str	Field description from SVD

Requires a loaded SVD file. Returns {"error": "No SVD file loaded"} if no SVD is available.

{
  "peripheral": "GPIOA",
  "registers": {
    "CRL": {
      "address": "0x40010800",
      "raw_value": "0x44444444",
      "fields": [
        {
          "name": "MODE0",
          "value": 0,
          "width": 2,
          "offset": 0,
          "description": "Port mode bits"
        },
        {
          "name": "CNF0",
          "value": 1,
          "width": 2,
          "offset": 2,
          "description": "Port configuration bits"
        }
      ]
    }
  }
}

---

jtag://transport/info

Active transport protocol and debug adapter information.

Returns:

Field	Type	Description
transport	str	Active transport (e.g. "swd", "jtag")
adapter	str	Adapter/probe identifier
speed_khz	int | null	Adapter clock speed in kHz (null if unavailable)
available_transports	list[str]	All transports the adapter supports

{
  "transport": "swd",
  "adapter": "cmsis-dap",
  "speed_khz": 1000,
  "available_transports": ["swd", "jtag"]
}

---

Resources vs. Tools

Use resources when you want to passively observe state. Use tools when you need to perform actions or need parameterized queries.

Need	Use
Check if target is halted	Resource: jtag://target/state
Halt the target	Tool: target_control(action="halt")
See all register values	Resource: jtag://registers/all
Read specific registers	Tool: read_registers(names=["pc", "sp"])
Check what peripherals are available	Resource: jtag://svd/peripherals
Decode a specific register with bitfields	Tool: svd_inspect(peripheral="GPIOA", register="CRL")